JP7590705B2 - Tunneling magnetoresistance sensor - Google Patents

Description

The present invention relates to a tunnel magnetoresistance (TMR) sensor.

In recent years, devices with magnetic sensors including tunnel magnetoresistance (TMR) elements have been used in fields such as medicine, non-destructive testing, and electric vehicles, where high sensitivity, low power consumption, and portability are essential (see, for example, Patent Document 1). In this magnetic measurement device, the magnetic sensor is composed of a tunnel magnetoresistance element array in which multiple tunnel magnetoresistance elements are arranged in a lattice to reduce noise such as 1/f noise.

Recently, research has been conducted on mutual device synchronization in spin Hall nano-oscillators and its advantages (see, for example, Non-Patent Document 1). In addition, in tunnel magnetoresistance elements, 1/f noise is the most dominant noise in the low frequency band, and it has been found that the magnetic fluctuation is related to the switching state of the magnetization orientation at the interface between the fixed layer and the free layer (see, for example, Non-Patent Document 2).

International Publication No. WO2012/032962

Awad, A. A. et al., “Long-range mutual synchronization of spin Hall nano-oscillators”. Nat. Phys., 2016, 13, p.292-299 Z. Q. Lei, et al., “Review of Noise Sources in Magnetic Tunnel Junction Sensors”, IEEE Transactions on Magnetics, March 2011, Volume 47, Issue 3, p.602-612

In a magnetic measurement device such as that described in Patent Document 1, the TMR response of the tunnel magnetoresistance element differs when the magnetic field changes from positive to negative and from negative to positive, which is known as hysteresis, and this causes measurement errors and reduces sensitivity.

The present invention was made with an eye on these issues, and aims to provide a tunnel magnetoresistance sensor that can reduce measurement errors due to hysteresis and increase sensitivity.

The inventors of the present invention came up with the idea that the magnetic fluctuations of the tunneling magnetoresistance element described in Non-Patent Document 2 are caused by the direction of internal magnetization, and that the maximum value of the output density always appears when the free layer of the tunneling magnetoresistance element is changing the direction of magnetization. Based on this idea, they believed that mutual synchronization of each tunneling magnetoresistance element in the tunneling magnetoresistance element array might reduce noise.

That is, in order to achieve the above-mentioned object, the tunneling magnetoresistance sensor of the present invention has a plurality of tunneling magnetoresistance elements each having a fixed layer whose magnetization direction is fixed, a free layer whose magnetization direction is changeable, a tunnel barrier layer arranged between the fixed layer and the free layer, a first electrode layer arranged on the opposite side of the tunnel barrier layer of the fixed layer, and a second electrode layer arranged on the opposite side of the tunnel barrier layer of the free layer, and each tunneling magnetoresistance element is arranged in a row or a lattice array, and between two adjacent tunneling magnetoresistance elements, the free layer and the second electrode layer are shared and each forms an independent island, and between adjacent islands, adjacent tunneling magnetoresistance elements share the first electrode layer to form a tunneling magnetoresistance element array, and among each of the tunneling magnetoresistance elements, the distance between the free layers of adjacent tunneling magnetoresistance elements that share the first electrode layer is 0.5 μm or less.

The tunneling magnetoresistance sensor according to the present invention has adjacent tunneling magnetoresistance elements that share a first electrode layer, i.e., adjacent tunneling magnetoresistance elements that do not share a free layer, and the distance between the free layers is close to each other at 0.5 μm or less, so that the tunneling magnetoresistance elements can be synchronized with each other and hysteresis can be reduced. This makes it possible to reduce measurement errors due to hysteresis and increase sensitivity.

In the tunneling magnetoresistance sensor according to the present invention, it is preferable that the distance between the free layers of adjacent tunneling magnetoresistance elements that share the first electrode layer is 0.35 μm or less. In this case, by bringing the free layers of adjacent tunneling magnetoresistance elements closer together, it is possible to further reduce measurement errors and increase sensitivity. The distance between the free layers of adjacent tunneling magnetoresistance elements varies greatly depending on the manufacturing method, but for example, when lithography and ion milling are used, it can be narrowed to about 0.2 μm to 0.35 μm.

In the tunneling magnetoresistance sensor according to the present invention, it is preferable that the first electrode layer is an upper electrode and the second electrode layer is a lower electrode. In this case, during manufacturing, stacking is performed starting from the second electrode layer of the lower electrode, making it easy to form the desired distance between the free layers of adjacent tunneling magnetoresistance elements.

The present invention provides a tunnel magnetoresistance sensor that can reduce measurement errors due to hysteresis and increase sensitivity.

1 is a side view showing a tunnel magnetoresistive sensor according to an embodiment of the present invention; 2 is a front view showing a layer configuration of a tunneling magnetoresistive element of the tunneling magnetoresistive sensor shown in FIG. 1 . FIG. 2A is a plan view showing the distribution of independent islands having a free layer and a lower electrode layer of a tunneling magnetoresistive element of the tunneling magnetoresistive sensor shown in FIG. 1, and FIG. 2B is a cross-sectional view taken along line A-A' in FIG. 1 is an optical microscope photograph in plan view showing a state in which an independent island has been manufactured in the tunnel magnetoresistive sensor according to the embodiment of the present invention. 1A is a graph showing magnetoresistance curves of a tunneling magnetoresistance sensor according to an embodiment of the present invention (island spacing is 0.3 μm) and a comparative tunneling magnetoresistance sensor having an island spacing of 5 μm; FIG. 1B is a graph showing an enlarged view of the area where the magnetic field in FIG. 1A is zero. 1 is a graph showing the relationship between the distance between free islands of a tunnel magnetoresistive sensor according to an embodiment of the present invention and the normalized TMR difference at zero field (Hc) and normalized TMR sensitivity (TMRS).

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
1 to 6 show a tunnel magnetoresistive sensor according to an embodiment of the present invention.
As shown in FIG. 1, a tunneling magnetoresistive sensor 10 has a plurality of tunneling magnetoresistive elements 11, which are arranged in a row.

As shown in FIG. 2, each tunnel magnetoresistance element 11 has a substrate 12, and an undercoat layer 13, a lower electrode layer 14, a free layer 15, a tunnel barrier layer 16, a fixed layer 17, a fixation promotion layer 18, and an upper electrode layer 19, which are stacked in this order on the substrate 12.

The substrate 12 may be made of any material, for example, Si or _SiO2 , as long as it is non-magnetic and can withstand the deposition of each layer and heat treatment. The undercoat layer 13 is provided on the surface of the substrate 12 to smooth out the surface roughness. The undercoat layer 13 may be made of any material, as long as it can smooth out the surface roughness of the substrate 12. The undercoat layer 13 preferably has a layer thickness of about 2 nm to 10 nm.

The lower electrode layer 14 is provided so as to contact the surface of the undercoat layer 13 opposite the substrate 12. The lower electrode layer 14 may be made of any conductive material, and may have a structure in which multiple metal materials are laminated. The lower electrode layer 14 preferably has a layer thickness of about 2 nm to 20 nm. The lower electrode layer 14 forms the second electrode layer.

The free layer 15 is provided on the surface of the lower electrode layer 14 opposite the undercoat layer 13. The free layer 15 has a structure in which a first ferromagnetic layer 21, a first non-magnetic layer 22, and a second ferromagnetic layer 23 are stacked in this order on the lower electrode layer 14. The magnetization direction of the first ferromagnetic layer 21 can be changed by the influence of external magnetic flux. The first ferromagnetic layer 21 is a soft magnetic material, such as NiFe, CoFeSiB, etc., and preferably has a layer thickness of about 30 nm to 200 nm.

The first non-magnetic layer 22 is provided to magnetically couple the first ferromagnetic layer 21 and the second ferromagnetic layer 23, and to separate the first ferromagnetic layer 21 from the crystal structure of the second ferromagnetic layer 23. The first non-magnetic layer 22 is preferably made of, for example, Ru or Ta, and has a layer thickness of about 0.2 to 1 nm. The second ferromagnetic layer 23 is capable of changing its magnetization direction under the influence of external magnetic flux. The second ferromagnetic layer 23 is preferably made of, for example, CoFeB, and has a layer thickness of about 1.4 nm to 5.0 nm.

The tunnel barrier layer 16 is provided on the surface of the free layer 15 opposite the lower electrode layer 14. The tunnel barrier layer 16 is made of an insulating material, such as MgO, Mg-Al-O, or AlOx. It is preferable that the tunnel barrier layer 16 has a layer thickness of, for example, about 1 nm to 10 nm.

The fixed layer 17 is provided on the surface of the tunnel barrier layer 16 opposite the free layer 15, sandwiching the tunnel barrier layer 16 between the free layer 15 and the fixed layer 17. The fixed layer 17 has a structure in which a third ferromagnetic layer 24, a second non-magnetic layer 25, and a fourth ferromagnetic layer 26 are stacked in this order on the tunnel barrier layer 16. The magnetization direction of the third ferromagnetic layer 24 is fixed. The third ferromagnetic layer 24 is preferably made of CoFeB, for example, and has a layer thickness of about 1.4 nm to 10 nm.

The second non-magnetic layer 25 is provided to magnetically couple the third ferromagnetic layer 24 and the fourth ferromagnetic layer 26, and to separate the third ferromagnetic layer 24 from the crystal structure of the fourth ferromagnetic layer 26. The second non-magnetic layer 25 is preferably made of, for example, Ru or Ta, and has a thickness of about 0.2 to 1 nm. The magnetization direction of the fourth ferromagnetic layer 26 is fixed. The fourth ferromagnetic layer 26 is preferably made of, for example, CoFe, and has a thickness of about 1 nm to 10 nm.

The fixation promotion layer 18 is provided on the surface of the fixed layer 17 opposite the tunnel barrier layer 16 in order to promote fixation of the fourth ferromagnetic layer 26. The fixation promotion layer 18 is made of an antiferromagnetic material such as IrMn or PtMn, and has a layer thickness of about 5 nm to 20 nm.

The upper electrode layer 19 is provided on the surface of the fixation promotion layer 18 opposite the fixation layer 17. The upper electrode layer 19 may be made of any conductive material, and may have a structure in which multiple metal materials are laminated. The upper electrode layer 19 preferably has a layer thickness of about 2 nm to 20 nm. The upper electrode layer 19 forms the first electrode layer.

In a specific example shown in FIG. 2, the undercoat layer 13 is made of Ta and has a thickness of 5 nm. The lower electrode layer 14 is made of Ta/Ru, with the Ru layer having a thickness of 10 nm and the Ta layer having a thickness of 5 nm. The first ferromagnetic layer 21 of the free layer 15 is made of CoFeSiB and has a thickness of 100 nm. The first non-magnetic layer 22 of the free layer 15 is made of Ru and has a thickness of 0.8 nm. The second ferromagnetic layer 23 of the free layer 15 is made of CoFeB and has a thickness of 3 nm. The tunnel barrier layer 16 is made of MgO and has a thickness of 1.6 nm. The third ferromagnetic layer 24 of the fixed layer 17 is made of CoFeB and has a thickness of 3 nm. The second non-magnetic layer 25 of the fixed layer 17 is made of Ru and has a thickness of 0.85 nm. The fourth ferromagnetic layer 26 of the pinned layer 17 is made of CoFe and has a thickness of 5 nm. The pinning promotion layer 18 is made of IrMn and has a thickness of 6 nm. The upper electrode layer 19 is made of Ru/Ta, with the Ta layer being 5 nm thick and the Ru layer being 8 nm thick.

The tunnel magnetoresistance element 11 can be manufactured by depositing each layer on the substrate 12 using a physical vapor deposition method such as sputtering or molecular beam epitaxial growth (MBE). In addition, the tunnel magnetoresistance element 11 may be heat-treated after each layer is deposited in order to obtain the desired crystal structure. The temperature for the heat treatment is preferably 325°C to 450°C.

As shown in FIG. 1, the tunneling magnetoresistance elements 11 are grouped into pairs of two adjacent tunneling magnetoresistance elements 11, which share the free layer 15 and the lower electrode layer 14 between the two tunneling magnetoresistance elements 11 and form independent islands 31. In addition, between the adjacent islands 31, the adjacent tunneling magnetoresistance elements 11 share the upper electrode layer 19. In this way, each tunneling magnetoresistance element 11 constitutes a tunneling magnetoresistance element array.

As shown in FIG. 1 and FIG. 3(a), the tunnel magnetoresistance sensor 10 has a spacing between adjacent islands 31, i.e., a spacing between the free layers 15 of adjacent islands 31, of 0.5 μm or less. It is particularly preferable that the spacing between adjacent islands 31 is 0.1 μm to 0.35 μm. In a specific example shown in FIG. 3(a), the tunnel magnetoresistance sensor 10 has multiple tunnel magnetoresistance element arrays arranged in parallel at a predetermined interval. In the figure, a represents the spacing between adjacent islands 31, and b represents the spacing between multiple tunnel magnetoresistance element arrays. In addition, when there is no external magnetic field, the magnetization direction of the fixed layer 17 (the direction of the Pin arrow in the figure) is parallel to the hard magnetization axis, and the magnetization direction of the free layer 15 (the direction of the Free arrow in the figure) is parallel to the easy magnetization axis. Furthermore, as shown in FIG. 3(b), if the height of the island 31 in the film thickness direction is h, the ratio of the distance between adjacent islands 31 (groove width) a to the film thickness h can be expressed as Q (=h/a).

In the tunneling magnetoresistance sensor 10, the distance between the islands 31 in which adjacent tunneling magnetoresistance elements 11 share the free layer 15 and the lower electrode layer 14 is 0.5 μm or less, so that the tunneling magnetoresistance elements 11 are synchronized with each other and hysteresis can be reduced. This can reduce measurement errors due to hysteresis and increase sensitivity.

The layers of the tunneling magnetoresistance element 11 were deposited on the substrate 12 in an ultra-high vacuum chamber by magnetron sputtering to produce the tunneling magnetoresistance sensor 10 shown in FIG. 1. During the production, after the free layer 15 was deposited, lithography and ion milling were used to form grooves from the free layer 15 to the surface of the substrate 12, forming independent islands 31. The distance between adjacent islands 31 (a in FIG. 3(a)), i.e., the width of the formed groove, was 0.3 μm. FIG. 4 shows an optical microscope photograph in plan view after the islands 31 were formed and the resist was removed. The thin vertical groove in FIG. 4 is the groove a in FIG. 3(a). Also, Q (=h/a) shown in FIG. 3(b) was close to 1.

After each layer of the tunneling magnetoresistance element 11 was formed, a heat treatment was performed at 350° C. with a magnetic field applied. In this way, the tunneling magnetoresistance sensor 10 shown in FIG. 1 was manufactured. As a comparative example, a tunneling magnetoresistance sensor 10 in which the spacing between the islands 31 was 5 μm was also manufactured by the same method.

The magnetoresistance curves of the tunneling magnetoresistance sensor 10 with the island 31 spacing of 0.3 μm and the comparative tunneling magnetoresistance sensor with the island 31 spacing of 5 μm were measured by the four-terminal method. The measurement results are shown in FIG. 5(a). FIG. 5(b) shows an enlarged view of the area near zero magnetic field in FIG. 5(a). As shown in FIGS. 5(a) and (b), it was confirmed that the tunneling magnetoresistance sensor 10 with the island 31 spacing of 0.3 μm has a smaller deviation between the curve when the magnetic field changes from positive to negative and the curve when the magnetic field changes from negative to positive, and has a smaller hysteresis, compared to the comparative example with the island 31 spacing of 5 μm. This is thought to be because the tunneling magnetoresistance elements 11 of adjacent islands 31 are more strongly synchronized with each other by bringing the independent islands 31 closer together.

Based on FIG. 5, the relationship between the distance between islands 31 (Distance Between Free Islands) and the amount of deviation in the TMR ratio at zero magnetic field (Normalized TMR Difference at zero field; Hc) and the TMR sensitivity (Normalized TMR Sensitivity; TMRS) was determined and shown in FIG. 6. As shown in FIG. 6, it was confirmed that the smaller the distance between islands 31, the smaller the deviation in the TMR ratio and the smaller the hysteresis. It was also confirmed that this reduces the measurement error and therefore increases the TMR sensitivity.

REFERENCE SIGNS LIST 10 Tunneling magnetoresistance sensor 11 Tunneling magnetoresistance element 12 Substrate 13 Undercoat layer 14 Lower electrode layer 15 Free layer 21 First ferromagnetic layer 22 First non-magnetic layer 23 Second ferromagnetic layer 16 Tunnel barrier layer 17 Fixed layer 24 Third ferromagnetic layer 25 Second non-magnetic layer 26 Fourth ferromagnetic layer 18 Fixation promotion layer 19 Upper electrode layer 31 Island

Claims (3)

a plurality of tunnel magnetoresistance elements each including a fixed layer whose magnetization direction is fixed, a free layer whose magnetization direction is changeable, a tunnel barrier layer disposed between the fixed layer and the free layer, a first electrode layer disposed on the opposite side of the fixed layer to the tunnel barrier layer, and a second electrode layer disposed on the opposite side of the free layer to the tunnel barrier layer;
the tunneling magnetoresistive elements are arranged in a row or in a lattice array, the free layer and the second electrode layer are shared between two adjacent tunneling magnetoresistive elements , and each tunneling magnetoresistive element forms an independent island, and the first electrode layer is shared between adjacent islands by adjacent tunneling magnetoresistive elements, thereby constituting a tunneling magnetoresistive element array;
A tunneling magnetoresistive sensor comprising: adjacent tunneling magnetoresistive elements sharing the first electrode layer, the distance between the free layers being 0.5 μm or less. The tunneling magnetoresistance sensor according to claim 1, characterized in that the distance between the free layers of adjacent tunneling magnetoresistance elements that share the first electrode layer is 0.35 μm or less. 3. The tunneling magnetoresistive sensor according to claim 1, wherein the first electrode layer is an upper electrode, and the second electrode layer is a lower electrode.

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