JP7817702B2 - Tetragonal thin film structure - Google Patents

Description

The present invention relates to a tetragonal thin film structure having a tetragonal thin film on a support substrate, and is suitable for application to a structure having, for example, a thin film of FeNiN or an _L10 type FeNi ordered alloy having an _L10 type ordered structure as the tetragonal thin film.

FeNi (iron-nickel) ordered alloys with an L1 ₀ ordered structure have high magnetic anisotropy and are expected to be used as rare earth or precious metal-free magnet materials and magnetic device materials for magnetic recording, etc. L1 ₀ ordered FeNi alloy thin films can be formed on STO (strontium titanate: SrTiO ₃ ) support substrates by denitrifying an FeNiN thin film prepared by MBE (molecular beam epitaxy) in nitrogen plasma (see Non-Patent Document 1).

K. Ito et al., "Epitaxial L10-FeNi films with high degree of order and large uniaxial magnetic anisotropy fabricated by denitriding FeNiN films", Applied Physics Letters. 116, 242404(2020)

However, when the manufacturing method disclosed in Non-Patent Document 1 is used to form an FeNiN thin film on a support substrate whose main surface has a (100) plane orientation, it has been confirmed that two types of variants are formed, in which the c-axes of the FeNiN intersect at 90 degrees. This is because the crystal structure on the (100) plane has four-fold symmetry around the central axis, and the FeNiN grows so that the c-axis intersects each edge of the cubic crystal structure, resulting in a multi-variant state. The dimensions of each variant in this case were approximately 5 nm.

When the crystal orientation axes are orthogonal to each other in a multi-variant state, the magnetic, electrical, and optical properties of the orientation directions with excellent properties are averaged with the properties of the orientation directions with poor properties, posing a problem in that the properties that would normally be obtained in a single-variant state cannot be utilized. For example, the magnetocrystalline anisotropy constant Ku becomes an averaged value between orthogonal variants, resulting in reduced magnetocrystalline anisotropy.

The term "variant" here refers to the crystal orientation of a tetragonal thin film when it is deposited on a support substrate. "Single variant" means that the crystal orientation is in only one axis, while "multi-variant" means that the crystal orientation is in multiple axis directions. The entire tetragonal thin film does not need to be "single variant," but if it is "multi-variant" and the dimensions of each variant are small, there will be no dominant variant, and the properties will average out, making it impossible to achieve good properties. For this reason, in order to obtain better properties, it is important to be able to obtain large dimensions for the single variant.

In view of the above, the present invention aims to provide a tetragonal thin film structure with superior properties.

In order to achieve the above object, a tetragonal thin film structure according to claim 1 comprises a support substrate (10) having at least a portion of a main surface with a (110) plane orientation and having a crystal structure belonging to a cubic or tetragonal system, and a tetragonal thin film (20) formed on the main surface of the support substrate with a (110) plane orientation and having a uniaxial in-plane orientation in a plane tangent to the main surface , wherein the support substrate has a crystal structure belonging to a cubic system, and the RMS mismatch difference, which is the difference in RMS mismatch between the (110) plane orientation of the main surface of the support substrate and the (110) plane orientation and the (101) plane orientation of the tetragonal thin film, is 1.9% or less .
A tetragonal thin film structure according to claim 8 comprises a support substrate (10) having at least a portion of a main surface with a (110) plane orientation and having a crystal structure belonging to a cubic or tetragonal system, and a tetragonal thin film (20) formed on the main surface of the support substrate with a (110) plane orientation and having uniaxial in-plane orientation in a plane contacting the main surface, wherein the support substrate is made of LaAlO3 _, the tetragonal thin film is a thin film of FeNiN or a thin film of an L10 type FeNi ordered alloy having an L10 type ordered structure, and has an island structure, and at least one plane of the outer surface of each island is a {111} plane.

In this way, by using a support substrate with a cubic or tetragonal crystal structure having at least a portion of its main surface oriented in the (110) plane, and depositing a tetragonal thin film on the support substrate, the crystals of the tetragonal thin film can be made single variant. This makes it possible to create a tetragonal thin film structure with superior properties.

Note that the reference symbols in parentheses attached to each component indicate an example of the correspondence between that component and the specific components described in the embodiments described below.

1 is a cross-sectional view of a tetragonal thin film structure according to one embodiment of the present invention. 1B is a top view of a tetragonal thin film of the tetragonal thin film structure shown in FIG. 1A. FIG. FIG. 1 is a cross-sectional view of a tetragonal thin film structure according to a comparative example. 2B is a top view of a tetragonal thin film of the tetragonal thin film structure shown in FIG. 2A. FIG. (a) is a perspective view of the crystal structure of STO, (b) and (c) are diagrams showing the crystal structures of STO(100) and STO(110), respectively, when viewed from the normal direction of each plane. (a) is a perspective view of the crystal structure of FeNiN, (b) is a perspective view in which the (110) plane is hatched, and (c) and (d) are views showing the crystal structures of FeNiN(100) and FeNiN(110), respectively, when each plane is viewed from the normal direction. FIG. 1 is a diagram showing a schematic diagram of the alignment between STO(100) and FeNiN(100). 1 is a diagram showing the lattice mismatch between each material and FeNiN(110) when the support substrate is made of MgO(110), STO(110), or LAO(110), and its RMS (root mean square) mismatch. (a) is a perspective view of the crystal structure of FeNiN, in which the (110) and (101) planes are hatched, (b) is a diagram showing the (110) orientation of FeNiN, and (c) is a diagram showing the (101) orientation of FeNiN. 1 is a table showing the lattice mismatch and RMS mismatch between each material and FeNiN(101) when the support substrate is made of MgO(110), STO(110), or LAO(110). FIG. 1 is a diagram showing the relationship between the cubic lattice constant and the RMS mismatch when the support substrate is made of MgO(110), STO(110), or LAO(110). FIG. 1 is a diagram showing the relationship between the cubic lattice constant and the difference in RMS mismatch between the (110) and (101) plane orientations. 1 is a table summarizing the results of in-plane orientation, in-plane (001) orientation rate, and in-plane (110) orientation rate in Comparative Example 1 and Examples 1 to 5. FIG. 1 shows X-ray diffraction patterns obtained when X-rays are incident so that the scattering vector of the X-rays is in the normal direction to the surface of the tetragonal thin film in Comparative Example 1 and Examples 1 to 3. FIG. 1 shows X-ray diffraction patterns obtained when X-rays are incident on the surface of a tetragonal thin film in Comparative Example 1 and Examples 1 to 3 so that the scattering vector of the X-rays is in one direction within the surface. 13. FIG. 14 shows X-ray diffraction patterns when X-rays are incident so that the scattering vector of the X-rays is perpendicular to the plane of the surface of the tetragonal thin film in Comparative Example 1 and Examples 1 to 3. 1 is a diagram summarizing the main peaks of FeNiN. FIG. 10 is a diagram showing the results of investigating the in-plane magnetic properties of a tetragonal thin film made into an _L10 type FeNi ordered alloy thin film. FIG. 10 is a cross-sectional view of a tetragonal thin film structure according to Examples 4 and 5. FIG. 10 is a diagram showing the results of cross-sectional observation of the LAO (100) substrate in Example 4 from the [1-10] axis direction using a transmission electron microscope (TEM). FIG. 20 is a diagram showing the results of reciprocal lattice pattern analysis by fast Fourier transform (FFT) for region XIX in FIG. 18. 10 is a diagram showing the results of investigating the in-plane magnetic properties at 300 K of a tetragonal thin film that is an L1 ₀ type FeNi ordered alloy thin film according to Example 4. FIG. 10 is a diagram showing the results of investigating the in-plane magnetic properties at 10 K of a tetragonal thin film that is an L1 ₀ type FeNi ordered alloy thin film according to Example 4. FIG. 10 is a table showing magnetic properties in Examples 4 and 5. 10 is a diagram showing the results of investigating the in-plane magnetic properties at 300 K of a tetragonal thin film that is an L1 ₀ type FeNi ordered alloy thin film according to Example 5. FIG. 1 is a diagram showing the crystal structure of MgO with a face-centered cubic lattice as the basic unit cell.

Embodiments of the present invention will be described below with reference to the accompanying drawings. Note that in the following embodiments, identical or equivalent parts will be denoted by the same reference numerals.

(First embodiment)
A tetragonal thin film structure according to a first embodiment will be described with reference to Figures 1A and 1B, in which the arrows indicate the direction of the c-axis.

As shown in Figure 1A, the tetragonal thin film structure of this embodiment is composed of a tetragonal thin film 20 formed on a support substrate 10.

The support substrate 10 has a cubic or tetragonal crystal structure, with at least a portion of the interface with the tetragonal thin film 20 having a (110) plane orientation. For example, STO(110), LAO(110), or MgO(110) can be used as the support substrate 10. The cubic lattice constant of STO is 0.3905 nm. The cubic lattice constant of LAO is 0.379 nm. The cubic lattice constant of MgO is 0.4213 nm.

Note that LAO is lanthanum aluminate (LaAlO ₃ ), and MgO is magnesium oxide. The (110) written after STO, LAO, and MgO means that when the support substrate 10 is formed from these, the plane orientation of the main surface of the support substrate 10 is (110). Here, (110) is mentioned, but the same applies to (100), (001), etc., which will be described later.

As shown in FIGS. 1A and 1B , the tetragonal thin film 20 is a thin film having a uniaxial in-plane orientation on one surface in contact with the main surface of the support substrate 10. The tetragonal thin film 20 has a uniaxial in-plane orientation, with the arrow indicating the [001] direction, a (001) orientation in the direction of the arrow, and a (110) orientation with respect to the interface with the support substrate 10. More specifically, the tetragonal thin film 20 does not have a (001) orientation in the vertical direction, and the in-plane (001) orientation rate, which is the proportion of components containing variants oriented in the same direction, is 60% or more, preferably 70% or more. Furthermore, the in-plane (110) orientation rate rotated 90° in-plane is 60% or more, preferably 70% or more. Examples of the tetragonal thin film 20 include thin films of materials containing elements Fe, Ni, and N, and thin films of L10 _- type FeNi ordered alloys having an _L10-type ordered structure.

An example of a thin film made of a material containing Fe, Ni, and N is FeNiN. In the case of FeNiN, the lattice constant along the a-axis of FeNiN(100) is 0.4002 nm, and the lattice constant along the c-axis of FeNiN(001) is 0.3713 nm. When the tetragonal thin film 20 is made of FeNiN, the thin film can be obtained by a method of depositing FeNi on the support substrate 10 in nitrogen plasma, or by a method of depositing Fe and Ni layers alternately and then performing a nitriding treatment. Furthermore, when the tetragonal thin film 20 is made of FeNiN, components other than FeNiN, such as Fe ₂ Ni ₂ N, FeNi, or oxides, may be included.

Furthermore, when the tetragonal thin film 20 is an _{L10 type} FeNi ordered alloy thin film having an L10 type ordered structure, a thin film of the _L10 type FeNi ordered alloy can be obtained by forming an FeNiN thin film and then performing a denitrification process. If the tetragonal thin film 20 is an _L10 type FeNi ordered alloy thin film, the tetragonal thin film structure becomes an FeNi ordered alloy structure including an FeNi superlattice. Such an FeNi ordered alloy structure is applicable to magnetic materials and device materials for magnetic recording, magnetic sensors, etc., and has excellent magnetic properties with high coercivity.

Here, we will compare the tetragonal thin film structure of this embodiment with the conventional case where a tetragonal thin film is formed on the main surface of STO(100). Here, we will use an example where the support substrate is STO and the tetragonal thin film is FeNiN. In the following explanation, the structure where a tetragonal thin film is formed on the main surface of STO(100) will be referred to as the comparative structure.

In a comparative structure in which a tetragonal thin film J20 is formed on an STO(100) support substrate J10 as shown in Figure 2A, two types of variants are formed in which the c-axes of the FeNiN intersect at 90°, as shown in Figure 2B. TEM (transmission electron microscope) observations revealed that the dimensions of each variant were small, measuring approximately 5 nm.

As shown in (a) of Figure 3, STO has a cubic crystal structure. Therefore, the STO (100) used in the support substrate J10 in the comparative structure has four-fold symmetry, meaning that the same lattice structure is obtained every time it is rotated 90° around the central axis, as shown in (b) of Figure 3. In contrast, the STO (110) used in the support substrate 10 in the structure of this embodiment has two-fold symmetry, meaning that the same lattice structure is obtained every time it is rotated 180° around the central axis, as shown in (c) of Figure 3.

On the other hand, as shown in Figure 4(a), FeNiN has a tetragonal crystal structure, with a layer where Ni is arranged and a layer where Fe and N are arranged. As shown in Figure 4(c), FeNiN(100) has a crystal structure close to STO(100). Note that the crystal structure of FeNiN(110) viewed from the normal direction of the (110) plane shown in Figure 4(b) has a surface arrangement different from that of STO(100), as shown in Figure 4(d).

As shown in Figure 5, in a comparative structure using STO(100) as the support substrate J10, the axes of the STO(100) align with the a-axis of the FeNiN(100), allowing the FeNiN(100) to be epitaxially grown on the support substrate J10. Because STO(100) is a cubic crystal, the atomic arrangement and lattice constants of the mutually orthogonal axes are identical. Therefore, when growing FeNiN using a cubic support substrate J10 with a (100) plane orientation on its main surface, there are two axial directions that align with the FeNiN, i.e., two growth directions. Therefore, as shown in Figures 2A and 2B, two variants are created in which the c-axes of the FeNiN constituting the tetragonal thin film J20 intersect at 90°.

In contrast, in the structure of this embodiment, which uses STO(110) as the support substrate 10, only the c-axis of the STO(110) is compatible with FeNiN. Therefore, when growing FeNiN using a support substrate 10 that is cubic and has a (110) plane orientation, there is only one axial direction that is compatible with FeNiN, meaning that the growth direction is limited to only one direction. For this reason, as shown in Figures 1A and 1B, the c-axis of the FeNiN that makes up the tetragonal thin film 20 is a single variant, oriented in a single direction.

In this way, by using a support substrate 10 with a cubic or tetragonal crystal structure with a (110) plane orientation and depositing a tetragonal thin film 20 on the support substrate 10, the crystals of the tetragonal thin film 20 can be made single variant. This makes it possible to obtain a tetragonal thin film structure with superior properties. Note that the entire tetragonal thin film 20 does not need to be single variant; it is sufficient to obtain a single variant with large dimensions, for example, 30 nm or more. In other words, it is fine to have parts of other variants, as long as the dimensions of the single variant parts are sufficiently larger than the dimensions of the other parts.

Although the example shown here uses STO(110) as the support substrate 10, we will explain the variant reduction effect for each material by comparing it with MgO(110) and LAO(110).

Figure 6 shows the lattice mismatch and RMS mismatch between each material and FeNiN(110) when the support substrate 10 is composed of the cubic oxides MgO(110), STO(110), and LAO(110). FeNiN(110)[001] in the figure indicates that the mismatch was measured along the [001] axis in FeNiN(110). Similarly, FeNiN(110)[1-10] in the figure indicates that the mismatch was measured along the [1-10] axis in FeNiN(110). The RMS mismatch is the root mean square of the mismatch in FeNiN(110)[001] and the mismatch in FeNiN(110)[1-10]. The RMS mismatch is an index that indicates the overall degree of lattice mismatch in the [001] and [1-10] axis directions.

As shown in this figure, in FeNiN(110)[001], the absolute values of the mismatch are LAO(110), STO(110), and MgO(110), in descending order. Furthermore, in FeNiN(110)[1-10], the absolute values of the mismatch are large for MgO(110) and LAO(110), while the absolute value of the mismatch is small for STO(110). Regarding the RMS mismatch, STO(110) has the smallest value, followed by LAO(110), and the largest for MgO(110).

Whether MgO(110), STO(110), or LAO(110) shown here is used as the support substrate 10, a large single variant can be obtained. Furthermore, it is believed that, fundamentally, the smaller the absolute value of the mismatch in the [001] and [1-10] axial directions in FeNiN(110), the greater the effect of reducing variants.

However, in an experiment in which FeNiN was grown as a tetragonal thin film 20 using MgO(110), STO(110), and LAO(110) as the support substrate 10, it was found that LAO(110) had a greater effect in reducing variants than STO(110). This is thought to be because when a tetragonal thin film 20 is grown using the (110) plane orientation, the (101) plane can also grow.

As shown in Figure 7(a), in FeNiN, the (110) plane is indicated by solid hatching, and the (101) plane is indicated by dashed hatching. Figure 7(b) shows (110)-oriented FeNiN, as viewed from the STO[110] and FeNiN[110] directions. A support substrate 10 with a (110) plane orientation is placed below the FeNiN crystal structure, and the crystal structure is shown when FeNiN is grown on top of it with the (110) plane. When FeNiN is grown with the (110) orientation as shown in this figure, the c-axis is uniquely determined. On the other hand, (c) in Figure 7 shows (101)-oriented FeNiN, viewed from the STO[110] and FeNiN[010] directions. It shows the crystal structure when a support substrate 10 with a (110) orientation is placed below the FeNiN crystal structure, and FeNiN is grown on top of it along the (101) plane. As shown in this figure, when FeNiN grows with a (101) orientation, growth in two directions along the c-axis is possible, resulting in multi-variant structures.

For this reason, it is desirable that FeNiN not grow in the (101) direction. To prevent FeNiN from growing on the (101) plane, the RMS mismatch with FeNiN (101) must be large to a certain extent.

Figure 8 shows the lattice mismatch and RMS mismatch between each material and FeNiN(101) when the support substrate 10 is composed of the cubic oxides MgO(110), STO(110), and LAO(110). FeNiN(101)[010] in the figure indicates that the mismatch was measured along the [010] axis in FeNiN(101). Similarly, FeNiN(101)[-101] in the figure indicates that the mismatch was measured along the [-101] axis in FeNiN(101). The RMS mismatch is the root mean square of the mismatch in FeNiN(101)[010] and the mismatch in FeNiN(101)[-101]. The RMS mismatch is an index that indicates the overall degree of lattice mismatch in the [010] and [-101] axis directions.

As shown in this figure, in FeNiN(101)[010], the order of the absolute mismatch is STO(110), MgO(110), and LAO(110). In FeNiN(101)[-101], the order of the absolute mismatch is STO(110), LAO(110), and MgO(110). Furthermore, in terms of RMS mismatch, STO(110) has the smallest RMS mismatch, followed by LAO(110), and then MgO(110).

To grow FeNiN(110), a low RMS mismatch is preferable, and in that sense, STO(110) is preferred, as shown in Figure 6. However, to prevent the growth of FeNiN(101), a high RMS mismatch is preferable, and LAO(110) or MgO(110) are better than STO(110). Therefore, the experimental results show that using LAO(110) was more effective at reducing variants than using STO(110).

The relationship between the cubic lattice constant and the RMS mismatch was investigated for FeNiN(110) and FeNiN(101) when MgO(110), STO(110), and LAO(110) were used as the support substrate 10. The results shown in Figure 9 were obtained. The relationship between the cubic lattice constant and the RMS mismatch difference, i.e., the difference in the RMS mismatch between FeNiN(110) and FeNiN(101), was also investigated. The results shown in Figure 10 were obtained. The RMS mismatch difference is the RMS mismatch of the desired orientation minus the RMS mismatch of the undesired orientation. In this case, it is the FeNiN(110) RMS mismatch minus the FeNiN(101) RMS mismatch.

As shown in Figure 9, there appears to be a correlation between the cubic lattice constant and the RMS mismatch. In both FeNiN(110) and FeNiN(101) cases, the RMS mismatch is smallest near 0.3905 nm, the cubic lattice constant of STO(110). The RMS mismatch increases as the cubic lattice constant becomes smaller or larger than that. However, as the cubic lattice constant becomes smaller than that of STO(110), the RMS mismatch difference decreases. In other words, while the RMS mismatch difference is 1.97% for STO(110), it is nearly 0% for LAO(110).

If the RMS mismatch between FeNiN(110) and FeNiN(101) were originally the same, FeNiN(110) would be more likely to form than FeNiN(101) due to factors such as surface energy stability. Therefore, even if FeNiN(110) becomes somewhat larger, it is thought that FeNiN(110) was preferentially formed due to the smaller RMS mismatch difference between the (110) and (101) surface orientations.

Therefore, if a cubic crystal structure with a smaller RMS mismatch difference is used, the orientation can be controlled, and for example, FeNiN(110) is preferentially produced, resulting in a more single variant and a greater variant reduction effect. Specifically, if a cubic crystal structure with a lattice constant of 1.9% or less, which is smaller than the 1.97% in the case of STO(110), or 0.39 nm or less, which is smaller than 0.3905 nm in cubic crystal lattice constant, is used, a greater variant reduction effect than STO(110) can be achieved.

While STO(110), LAO(110), and MgO(110) are used as examples of materials for the support substrate 10 having a cubic crystal structure, this is not a limitation. Figures 9 and 10 show the RMS mismatch and RMS mismatch difference calculated from the cubic lattice constant, plotted as approximate curves. The calculation results for STO(110), LAO(110), and MgO(110) are also plotted. This approximate curve shows the RMS mismatch and RMS mismatch difference corresponding to the cubic lattice constant of a material. For materials other than STO(110), LAO(110), and MgO(110), the relationship between the cubic lattice constant and the RMS mismatch and RMS mismatch difference generally corresponds to this approximate curve. Therefore, even if a material other than STO(110), LAO(110), or MgO(110) is used, if it has a cubic crystal structure of 1.9% or less, which is smaller than the 1.97% in the case of STO(110), a greater variant reduction effect than STO(110) can be achieved.

Next, a comparative structure using STO(100) was used as Comparative Example 1, and FeNiN was grown using STO(110), MgO(110), and LAO(110) as Examples 1 to 3, respectively. Specifically, the temperature of the support substrate 10 was set to 350°C, and a 20-nm FeNiN film was epitaxially grown on the support substrate 10 as a tetragonal thin film 20 by MBE with simultaneous supply of Fe, Ni, and radio frequency (RF) _N2 . The _N2 flow rate into the chamber of the growth apparatus was 1.0 sccm, and the RF input was 280 W. Comparative Example 1 and Examples 1 to 3 will be described with reference to FIG. 11 . The results shown in FIG. 11 were obtained based on the X-ray diffraction results shown in FIGS. 12 to 14 , which will be described later.

(Comparative Example 1)
First, a comparative structure was fabricated and the FeNiN perpendicular orientation direction and in-plane (001) orientation rate were investigated. As a result, as shown in Figure 11, it was found that the (100) orientation was perpendicular to the substrate, and (001) was not perpendicular to the surface. The in-plane (001) orientation rate was 57.5%. In other words, it was confirmed that a multi-variant state existed, with two variants perpendicular to the [001] direction in the surface.

Example 1
In Example 1, FeNiN was formed as a tetragonal thin film 20 using a support substrate 10 composed of STO (110), and the FeNiN perpendicular orientation direction and in-plane (001) orientation rate were investigated. As a result, as shown in FIG. 11, it was found that the orientation direction perpendicular to the substrate was (110), and (001) was not perpendicular to the surface. The in-plane (001) orientation rate was 72.7%, which indicates a higher in-plane (001) orientation than in Comparative Example 1. Furthermore, the (110) in-plane orientation rate was 69.7%, indicating a (110) orientation.

Example 2
In Example 2, FeNiN was formed as a tetragonal thin film 20 using a support substrate 10 composed of MgO (110), and the FeNiN perpendicular orientation direction and in-plane (001) orientation rate were investigated. As a result, as shown in FIG. 11, it was found that the substrate orientation direction perpendicular to the plane was (110), and (001) was not perpendicular to the plane. The in-plane (001) orientation rate was 72.7%, indicating a higher (001) orientation than in Comparative Example 1. Furthermore, the in-plane (110) orientation rate was 69.7%, indicating a (110) orientation.

Example 3
In Example 3, FeNiN was formed as a tetragonal thin film 20 using a support substrate 10 composed of LAO (110), and the FeNiN perpendicular orientation direction and in-plane (001) orientation rate were investigated. As a result, as shown in FIG. 11 , it was found that the substrate orientation direction perpendicular to the plane was (110), and (001) was not perpendicular to the plane. The in-plane (001) orientation rate was 98.1%, indicating a higher (001) orientation than in Comparative Example 1. Furthermore, the in-plane (110) orientation rate was 95.9%, indicating a higher (110) orientation than in Examples 1 and 2. In other words, both the in-plane (001) orientation rate and the in-plane (110) orientation rate were high, confirming that a single variant with the largest dimensions was obtained among Examples 1 to 3.

For Comparative Example 1 and Examples 1 to 3, X-rays were incident from multiple directions to examine the X-ray diffraction patterns. Specifically, X-rays were incident in two directions in which the scattering vector of the X-ray diffraction was normal to the surface of the tetragonal thin film 20, i.e., the normal to the main surface of the support substrate 10 (hereinafter referred to as the first direction), and in which the X-ray diffraction patterns were examined. The two in-plane directions are two orthogonal directions in the plane; hereinafter, one of these directions will be referred to as the second direction, and the other direction perpendicular to the second direction will be referred to as the third direction.

Figures 12 to 14 show X-ray diffraction patterns obtained when X-rays are incident in the first to third directions, respectively, for Comparative Example 1 and Examples 1 to 3. In Figure 12, X-rays are incident so that the scattering vector of the X-ray diffraction is aligned in the first direction, which is the normal direction to the surface of the tetragonal thin film 20, resulting in an X-ray diffraction pattern that reflects the crystalline structure in the normal direction to the surface of the tetragonal thin film 20. In Figures 13 and 14, X-rays are incident so that the scattering vector of the X-ray diffraction is aligned in the second and third directions, which are in-plane on the surface of the tetragonal thin film 20, resulting in X-ray diffraction patterns that reflect the crystalline structure in the in-plane directions on the surface of the tetragonal thin film 20. For Comparative Example 1, the second direction is the [001] direction of the substrate, and for the third direction, the diffraction intensity simply swaps between 002 and 200 due to substrate symmetry; therefore, the X-ray diffraction pattern for Comparative Example 1 is not shown in Figure 14. For Examples 1 to 3, if the second direction is the [001] direction of the substrate, then the third direction corresponds to the [110] direction of the substrate. X-ray diffraction measurements were performed by depositing an FeNiN film on a 10 mm square, 0.5 mm thick support substrate 10, and then irradiating it with CuKa radiation at a wavelength λ of 0.15418 nm in each of the first to third directions. The angle 2θ (degrees) was measured in 0.02° increments in the range of 38 to 72°.

First, the main X-ray diffraction peaks of FeNiN are shown in Figure 15. For example, (200) indicates that the peak occurs at an angle 2θ (degrees) of 45.317°. Regarding the angles 2θ (degrees) shown in this figure, we determined whether a peak appears within a 1.0° angle range nearby. The diffraction intensity background and error were determined, for example, in the range of 53-57°, where there are no peaks for FeNiN or the substrate. The peak intensity was calculated by averaging within a 0.1° range around the peak top. If the peak intensity is higher than the diffraction intensity background, it can be said that there is a direction oriented in the direction of that angle 2θ (degrees). Furthermore, because the diffraction intensity of the (001) and (110) peaks is weak, they were not used to determine the crystal structure.

13 shows the in-plane (001) orientation ratio in the surface of the tetragonal thin film 20. The in-plane (001) orientation ratio was calculated to determine the degree of in-plane orientation. The in-plane (001) orientation ratio was calculated by determining the peak intensities of peaks exhibiting fundamental reflections such as FeNiN(200) and FeNiN(002), and then summing these peak intensities to calculate the proportion of the (001) orientation direction. In other words, the in-plane (001) orientation ratio was calculated based on peaks exhibiting fundamental reflections such as FeNiN(200) and FeNiN(220), which are not due to diffraction intensity background or error and are not derived from the support substrate 10.
14 also shows the in-plane (110) orientation ratio within the surface of the tetragonal thin film 20. The in-plane (110) orientation ratio was calculated to determine the degree of in-plane orientation in the third direction, and Comparative Example 1 was excluded from the symmetry of the substrate. The in-plane (110) orientation ratio was calculated by determining the peak intensities of peaks exhibiting fundamental reflections such as FeNiN(220) and FeNiN(202), and then calculating the proportion of these peak intensities that were accounted for by the (220) orientation direction. In other words, the in-plane (110) orientation ratio was determined based on peaks exhibiting fundamental reflections such as FeNiN(200) and FeNiN(220), which are not due to diffraction intensity background or error and are not derived from the support substrate 10.

In the case of FeNiN, if the support substrate 10 is STO(100), it is preferable that a peak appears at an integer multiple n of (n00), for example (200), and if it is STO(110), it is preferable that a peak appears at an integer multiple n of (nn0), for example (220). Furthermore, if a peak appears at (202), which is not an integer multiple, it indicates that the material is in a multivariant state.

Looking at Figure 12, in Comparative Example 1, a peak appears in FeNiN(200), but no peak appears in FeNiN(002). This indicates that the a-axis is parallel to the direction perpendicular to the (100) plane, and that there is almost no c-axis perpendicular to the STO(100) plane.

Furthermore, in Examples 1 to 3, a peak appears at FeNiN (220), and no peak appears at angles 2θ (degrees) corresponding to other directions such as FeNiN (002). In particular, in Example 3, a large peak intensity appears at (220).

On the other hand, looking at Figure 13, in Comparative Example 1, two peaks appear, one for FeNiN(200) and one for FeNiN(002). This means that c-axes in different directions exist within the STO(100) plane, meaning that two types of variants have been created in which the FeNiN c-axes intersect at 90°, as shown in Figures 2A and 2B. Furthermore, since the intensities of the two peaks, FeNiN(200) and FeNiN(002), are roughly equal, it can be seen that the c-axes in the two directions appear roughly equally, and that each variant is occurring roughly equally.

In Examples 1 to 3, two peaks appear, one for FeNiN (200) and one for FeNiN (002), but there is a difference between the two peaks. This means that the orientation is in the direction corresponding to the peak with the greater intensity, and it can be seen that a single variant with large dimensions was obtained. A similar observation can be made in Figure 14. Furthermore, as shown in Figures 13 and 14, while the orientation rate in Comparative Example 1 was less than 60%, Examples 1 to 3 were all 60% or higher, and most were 70% or higher. In particular, when the support substrate 10 was made of LAO, a high in-plane (001) orientation rate of 95% or higher was obtained, indicating that a single variant with large dimensions was obtained.

Furthermore, a film of FeNiN was formed on the main surface of the support substrate 10 composed of LAO (110), and then denitrification treatment was performed to convert the tetragonal thin film 20 into an _L10 type FeNi ordered alloy thin film, and the in-plane magnetic properties of the tetragonal thin film 20 were then investigated. Figure 16 shows the results, showing the magnetic properties in the direction parallel to the c-axis and the magnetic properties in the direction perpendicular to the c-axis. A sample was prepared in which an _L10 type FeNi ordered alloy thin film was formed as the tetragonal thin film 20 on the support substrate 10, and an external magnetic field was applied and swept to measure the change in M/ _Ms .

As shown in Figure 16, the magnetic properties are completely different in the directions parallel and perpendicular to the c-axis. The crystal structure of the _{L1 0} type FeNi ordered alloy is composed of a stacked structure of Fe layers and Ni layers, and has the property of being easily magnetized in the stacked direction. Therefore, in the direction parallel to the c-axis, i.e., the stacked direction of the Fe layers and Ni layers, even if a small magnetic field is applied, M/ _Ms becomes a large value, making it easy to magnetize, while in the direction perpendicular to the c-axis, M/ _Ms does not become a large value unless a large magnetic field is applied, making it difficult to magnetize. This shows that the L1 ₀ type FeNi ordered alloy thin film is made into a crystal with high in-plane orientation. Note that M represents magnetization and _Ms represents saturation magnetization.

Next, Examples 4 and 5 will be described. Examples 4 and 5 were obtained by using LAO (110) as the support substrate 10, forming an FeNi film on the LAO (110), nitriding it in an ammonia gas atmosphere to form an FeNiN film, and then denitriding it in a hydrogen gas atmosphere. Specifically, the temperature of the support substrate 10 was set to 400°C, and a 16.6 nm thick FeNi film was formed on the support substrate 10 by radio frequency (RF) sputtering using a 50:50 atm% FeNi alloy target in Example 4, and a 12.7 nm thick FeNi film was formed on the support substrate 10 in Example 4 and Example 5. The FeNi thin film samples were then nitrided in an ammonia gas atmosphere at 325°C for 20 hours, and then further nitrided at 375°C for 10 hours to form FeNiN. The FeNiN perpendicular orientation direction, in-plane (001) orientation ratio, and in-plane (110) orientation ratio were measured for Examples 4 and 5, and the results shown in FIG. 11 were obtained. Thereafter, denitrification treatment was carried out in a hydrogen gas atmosphere at 200° C. for 2 hours, thereby obtaining samples of Examples 4 and 5 in which a tetragonal thin film 20 of an _L10 type FeNi ordered alloy was formed on a support substrate 10.

Example 4
In Example 4, FeNiN was deposited as a tetragonal thin film 20 using a support substrate 10 composed of LAO (110) and nitrided. The FeNiN perpendicular orientation direction and in-plane (001) orientation rate were examined. As a result, as shown in FIG. 11 , the orientation direction perpendicular to the substrate was (110). The in-plane (001) orientation rate was 93.2%, indicating a higher (001) orientation rate compared to Comparative Example 1. Furthermore, the in-plane (110) orientation rate was 100%, indicating a higher (110) orientation rate compared to Examples 1 to 3. In other words, it was confirmed that both the in-plane (001) orientation rate and the in-plane (110) orientation rate were high values, and a higher orientation, i.e., a wider single variant region, was obtained than in Examples 1 and 2. In addition, since the denitrification reaction from FeNiN is a topotactic reaction that maintains the ordered structure of Fe and Ni, it is thought that the orientation ratio of _L10 type FeNi after denitrification from FeNiN is similar to that of FeNiN.

In Example 4, it was found that a tetragonal thin film 20 was formed on a support substrate 10, with multiple island-shaped structures 21 scattered thereon, as shown in Figure 17. Specifically, the denitrified sample was thinned using focused ion beam (FIB) processing to expose the cross-section, and high-resolution cross-sectional TEM observation of the sample was performed along the [1-10] axis of the LAO (110). As a result, it was found that the tetragonal thin film 20 had planes reflecting the island-shaped structures 21 being FeNi single crystals with (110), (111), and (11-1) orientations, and had at least one {111} plane, as shown in Figure 18. Note that in Figure 18, arrows and Miller indices are attached to the top and side surfaces of each particle constituting the tetragonal thin film 20 to make the crystal planes easier to understand. The notation {111} represents six equivalent planes: (111), (11-1), (1-11), (1-1-1), (-1-11), and (-1-1-1). The island-like structure 21 having at least one {111} plane means that its outer surface has one or more planes that fall into any of the six equivalent planes: (111), (11-1), (1-11), (1-1-1), (-1-11), and (-1-1-1). In this specification, the "-" in the Miller index notation refers to the bar that appears above the index immediately following it, as shown in FIG. 18, for example.

The tetragonal thin film 20 of Example 4 had a large number of FeNi island structures 21, each approximately 30 nm thick, formed on an LAO(110) support substrate 10, and these island structures 21 were scattered to form an archipelago. Note that "scattered" here includes cases where the multiple island structures 21 were arranged at a distance from each other, as well as cases where some island structures 21 were in contact with each other and other island structures 21 were arranged at a distance from each other. Furthermore, when viewed from the [1-10] axis direction of LAO(110), the multiple islands that made up the tetragonal thin film 20, i.e., the FeNi island structures 21, each had an outer surface, the upper surface opposite the support substrate 10, that was the (110) plane. Furthermore, the side surfaces adjacent to the top surfaces of the FeNi island structures 21 that make up the tetragonal thin film 20 are {111} planes, and at least one of the outer surfaces is a {111} plane.

FIG. 19 shows the results of reciprocal lattice pattern analysis by FFT on an XIX region having a size of 20 nm in the thickness direction relative to the support substrate 10 and 50 nm in the planar direction relative to the support substrate 10, as indicated by the dashed line in FIG. 18 . In FIG. 19 , Miller indices are added to some of the diffraction spot patterns of reflections indicated by white dots. In FIG. 19 , the white dots consist of diffraction spot patterns of basic reflections and diffraction spot patterns of superlattice reflections, with particularly bright white dots indicating diffraction spots of basic reflections, and the white dots indicated by the tips of the white arrows in the figure indicating diffraction spots of superlattice reflections. In Example 4, as shown in FIG. 19 , diffraction spots of 001 and 110 superlattice reflections were clearly observed, while diffraction spots of other superlattice reflections were not clearly observed. This result indicates that the orientation direction of the L1 ₀ -FeNi island structures 21 was uniform in an observation range of at least 20 nm × 50 nm, and a wide single variant region of at least 20 nm × 50 nm or more was obtained.

Next, the results of evaluating the magnetic properties of Example 4 at 300 K (Kelvin) and 10 K are shown in Figures 20 and 21.

The magnetic properties were measured using a vibrating sample magnetometer (VSM) at 300 K and a superconducting quantum interference device (SQUID) at 10 K. VSM stands for vibrating sample magnetometer, and SQUID stands for superconducting quantum interference device.

As shown in FIG. 20 , in Example 4, when an external magnetic field was applied in the direction parallel to the c-axis in-plane at 300 K, M/M _s was saturated by application of a smaller external magnetic field than when an external magnetic field was applied in the direction perpendicular to the c-axis in-plane or perpendicular to the substrate surface. This indicates that in Example 4, the axis of easy magnetization is in the in-plane c-axis direction. Here, the c-axis in-plane parallel direction refers to the c-axis direction in the L1 ₀ type FeNi thin film, i.e., the direction parallel to the LAO[001] direction, and the c-axis in-plane perpendicular direction refers to the direction perpendicular to the c-axis direction, i.e., the LAO[1-10] direction. Furthermore, the direction perpendicular to the substrate surface refers to the direction perpendicular to the L1 ₀ type FeNi thin film, i.e., the LAO[110] direction. In other words, in Example 4, it can be seen that an L1 ₀ type FeNi ordered alloy thin film with high magnetic orientation was formed, which was easily magnetized in the direction parallel to the c-axis, i.e., the stacking direction of the Fe layer and Ni layer. Furthermore, in Example 4, as shown in Figure 21, the M/ _Ms increased and saturated when an external magnetic field was applied in which the direction parallel to the c-axis in-plane was smaller than the direction perpendicular to the c-axis in-plane, and the magnetic orientation tended to be similar even at 10 K to that at 300 K.

As shown in Figure 22, Example 4 had an Ms of 1.36 T ( _tesla = Wb/ ^m2 ) at 300 K and 1.59 T at 10 K. The coercive force _Hc (unit: kA/m) at 300 K was 210, 50, and 30 in the direction parallel to the c-axis in-plane, the direction perpendicular to the c-axis in-plane, and the direction perpendicular to the substrate surface, respectively. Furthermore, the coercive force _Hc at 10 K was 280 in the direction parallel to the c-axis in-plane and 90 in the direction perpendicular to the c-axis in-plane. This demonstrates that Example 4 is a tetragonal thin film structure having high coercive force in the direction parallel to the c-axis in-plane at both 10 K and 300 K, and excellent magnetic orientation and coercive force characteristics.

Example 5
For Example 5, the FeNiN plane-perpendicular orientation direction and in-plane (001) orientation rate were investigated in the same manner as in Example 4. As a result, as shown in FIG. 11 , in Example 5, the plane-perpendicular substrate orientation direction was (110). The in-plane (001) orientation rate was 95.5%, which was a higher (001) orientation than in Comparative Example 1. Furthermore, in Example 5, the in-plane (110) orientation rate was 100%, which, like Example 4, showed a higher (110) orientation than in Examples 1 and 2, and it was confirmed that a higher orientation, i.e., a wider single variant region, was obtained.

The magnetic properties of Example 5 were evaluated at 300 K. As a result, as shown in FIG. 23 , it was found that, in Example 5, when an external magnetic field was applied that was smaller in the c-axis in-plane direction than in the c-axis in-plane direction, the M/ _Ms increased to a large value, saturating, and the easy axis of magnetization was in the c-axis in-plane direction. This indicates that, like Example 4, Example 5 forms an _L10- type FeNi ordered alloy thin film that is easily magnetized in the stacking direction of the Fe and Ni layers and has high magnetic orientation. Furthermore, as shown in FIG. 22 , Example 5 exhibited an _Ms of 1.32 T at 300 K, and _Hc values of 320, 60, and 90 in the c-axis in-plane direction, the c-axis in-plane direction, and the substrate surface direction, respectively. These results indicate that, like Example 4, Example 5 is a tetragonal thin film structure with high coercivity in the c-axis in-plane direction and excellent magnetic orientation and coercivity characteristics.

The reason that Examples 4 and 5 achieve coercivity equal to or greater than that of Examples 1 to 3 is thought to be because, while in Examples 1 to 3, the tetragonal thin film 20 is formed as a single continuous film, the tetragonal thin film 20 in Examples 4 and 5 has an island structure. Specifically, when an external magnetic field is applied in a direction different from the internal magnetization direction, the domain walls that form are less likely to move than in a continuous film due to the pinning effect of the nanometer-order uneven structure, making the internal magnetization less likely to change in response to the external magnetic field. It is presumed that the coercivity is further improved due to the pinning effect of this domain wall.

In addition, in Examples 4 and 5, the tetragonal thin film 20 has specific crystal planes such as (110) and (111) occupying large areas, and the group of island-shaped structures 21 is isolated in a form close to single crystal. Therefore, Examples 4 and 5 can be said to have a structure in which the generation of soft magnetic phases due to amorphous structures and crystal grain boundaries near the surface of the island-shaped structures 21 is suppressed. As a result, Examples 4 and 5 have a structure in which the proportion of soft magnetic components that serve as starting points for magnetization reversal in the tetragonal thin film 20 is reduced, and it is thought that high coercivity is achieved.

In the above, examples 4 and 5 were fabricated using LAO(110) as the support substrate 10 as a representative example, but this is not limiting. Even if a cubic or tetragonal crystal structure such as STO(110) or MgO(110) is used as the support substrate 10, it is expected that a tetragonal thin film structure with the same basic structure as in examples 4 and 5 will be obtained.

(Other embodiments)
Although the present disclosure has been described based on the above-described embodiment, it is not limited to the embodiment and includes various modifications and modifications within the equivalent range. In addition, it is not limited to magnetic materials, and various combinations and forms, as well as other combinations and forms including only one element, more than one element, or less than one element, are also within the scope and concept of the present disclosure.

For example, in the above embodiment, STO(110), LAO(110), and MgO(110) were given as examples of cubic or tetragonal crystal structures that make up the support substrate 10, but the support substrate 10 may be made of other materials. Even in this case, the effects described in the above embodiment can be obtained by forming a tetragonal thin film 20 on the (110) plane as the main surface of the support substrate 10. In particular, if the RMS mismatch difference is 1.9% or less, which is smaller than the 1.97% in the case of STO(110), or if the cubic lattice constant is 0.39 nm or less, which is smaller than 0.3905 nm, a greater variant reduction effect than STO(110) can be achieved.

In addition, in the above embodiment, the support substrate 10 having a main surface with a (110) plane orientation was described, but the entire surface of the support substrate 10 does not have to be a main surface with a (110) plane orientation. In other words, it is sufficient for the support substrate 10 to have a main surface with a (110) plane orientation at least at the interface in contact with the tetragonal thin film 20.

Furthermore, in the above embodiment, a cubic crystal material having a main surface with a (110) plane orientation was mainly described as the constituent material of the support substrate 10, but the same effects as described above can also be obtained when a tetragonal crystal material is used.

In addition, for cubic sodium chloride-type crystal structures composed of two elements, such as MgO, a face-centered cubic lattice is depicted as the basic unit cell, as shown in Figure 24, rather than the minimum unit cell, in order to uniformly describe it as a cubic perovskite-type crystal structure composed of three elements, such as STO.

(Features of the present invention)
[Claim 1]
A support substrate (10) having at least a portion of a main surface with a (110) plane orientation and having a crystal structure belonging to a cubic or tetragonal system;
a tetragonal thin film (20) formed on the main surface of the support substrate having a (110) plane orientation, the tetragonal thin film having a uniaxial in-plane orientation in a plane in contact with the main surface.
[Claim 2]
the support substrate has a cubic crystal structure,
2. The tetragonal thin film structure according to claim 1, wherein an RMS mismatch difference between the (110) plane orientation of the main surface of the support substrate and each of the (110) plane orientation and the (101) plane orientation of the tetragonal thin film is 1.9% or less.
[Claim 3]
3. The tetragonal thin film structure according to claim 1, wherein said support substrate is made of any one of _SrTiO3 , _LaAlO3 , and MgO.
[Claim 4]
4. The tetragonal thin film structure of claim 3, wherein said support substrate is made of _LaAlO3 .
[Claim 5]
5. A tetragonal thin film structure according to claim 1, wherein the (001) orientation rate in the plane in contact with the main surface of the tetragonal thin film is 60% or more.
[Claim 6]
6. A tetragonal thin film structure according to claim 1, wherein said tetragonal thin film is a thin film of a material containing Fe, Ni, and N as elements.
[Claim 7]
6. The tetragonal thin film structure according to claim 1, wherein said tetragonal thin film is a thin film of FeNiN or a thin film of an _L10 type FeNi ordered alloy having an _L10 type ordered structure.
[Claim 8]
A support substrate (10) having at least a portion of a main surface with a (110) plane orientation and having a crystal structure belonging to a cubic or tetragonal system;
a tetragonal thin film (20) formed on the main surface of the support substrate having a (110) plane orientation and having a uniaxial in-plane orientation in a plane in contact with the main surface,
The support substrate is made of _LaAlO3 ,
A tetragonal thin film structure, wherein the tetragonal thin film is a thin film of FeNiN or a thin film of an L1 ₀ type FeNi ordered alloy having an L1 ₀ type ordered structure.
[Claim 9]
9. A tetragonal thin film structure according to claim 1, wherein the tetragonal thin film has an island structure, and at least one of the outer surfaces of each island is a {111} plane.

10 Support substrate 20 Tetragonal thin film

Claims (8)

A support substrate (10) having at least a portion of a main surface with a (110) plane orientation and having a crystal structure belonging to a cubic or tetragonal system;
a tetragonal thin film (20) formed on the main surface of the support substrate having a (110) plane orientation and having a uniaxial in-plane orientation in a plane in contact with the main surface,
the support substrate has a cubic crystal structure,
A tetragonal thin film structure, wherein an RMS mismatch difference, which is a difference between the (110) plane orientation of the main surface of the support substrate and each of the (110) plane orientation and the (101) plane orientation of the tetragonal thin film, is 1.9% or less . 2. The tetragonal thin film structure according to claim 1, wherein said support substrate is made of any one of SrTiO ₃ , LaAlO ₃ and MgO. 3. The tetragonal thin film structure of claim 2 , wherein said support substrate is composed of _LaAlO3 . The tetragonal thin film structure of claim 1, wherein the (001) orientation rate in the plane tangent to the main surface of the tetragonal thin film is 60% or more. The tetragonal thin film structure of claim 1, wherein the tetragonal thin film is a thin film of a material containing Fe, Ni, and N as elements. 2. The tetragonal thin film structure according to claim 1, wherein the tetragonal thin film is a thin film of FeNiN or a thin film of an L1 ₀ type FeNi ordered alloy having an L1 ₀ type ordered structure. 7. A tetragonal thin film structure according to claim 1, wherein the tetragonal thin film has an island structure, and at least one of the outer surfaces of each island is a {111} plane. A support substrate (10) having at least a portion of a main surface with a (110) plane orientation and having a crystal structure belonging to a cubic or tetragonal system;
a tetragonal thin film (20) formed on the main surface of the support substrate having a (110) plane orientation and having a uniaxial in-plane orientation in a plane in contact with the main surface,
The support substrate is made of _LaAlO3 ,
The tetragonal thin film structure is a thin film of FeNiN or a thin film of an _L10 type FeNi ordered alloy having an _{L10 type} ordered structure, and has an island structure, and at least one face of the outer surface of each island is a {111} plane .