JP6723602B2 - Multiferroic material and memory using the same - Google Patents

Description

The present invention relates to a multiferroic material and a memory using the same.

The memory is an important component that is indispensable to electronic devices. As electronic devices are becoming smaller and higher in performance, memory is also required to have a larger capacity and a smaller size. Since a non-volatile memory generally stores information using a hysteresis phenomenon, a ferroelectric substance or a ferromagnetic substance exhibiting the hysteresis phenomenon is used as a memory material. However, if the usual ferroelectrics and ferromagnets are miniaturized to the utmost limit, it becomes impossible to retain information recording due to thermal fluctuation. Therefore, a memory cell containing at least several tens of crystal structures is required for recording single information (1 bit) of the memory, and therefore physical miniaturization of a memory using a ferroelectric or a ferromagnetic material is required. There are limits. As materials that may break through such physical limits, monomolecular dielectrics that show dielectric hysteresis with a single molecule and single molecule magnets that show magnetic hysteresis with a single molecule have been attracting attention (for example, non-patent literature). See Reference 1.). These molecules exhibit hysteresis by a mechanism completely different from that of conventional ferroelectrics and ferromagnets, and theoretically it is possible to configure one memory cell with one molecule. Therefore, the monomolecular dielectric and the monomolecular magnet are positioned as minute elements having extremely strong properties that cannot be further reduced, and it is expected that the recording density will be greatly improved.

On the other hand, a multiferroic substance, which has two or more properties among ferroelectric properties, ferromagnetism, and ferroelasticity, is attracting attention as one substance. Is being used (see, for example, Patent Document 1). When a multiferroic substance is used as a memory material, it can be used as a memory that uses polarization information and a memory that uses magnetic information. In addition, there is a possibility of realizing a multi-bit memory that stores polarization information and magnetic information at the same time. Therefore, by developing a single-molecule multiferroic substance that combines the properties of single-molecule dielectrics and single-molecule magnets, which are considered to be extremely small recording elements, in one molecule, the conventional recording limit is dramatically improved. The development of such memory materials is expected. In addition to memories, miniaturization of electronic devices such as sensors and actuators is also expected.

International publication 2007/135817 pamphlet

R. Sessoli et al., Nature, 1993, 365 volumes, p.141.

However, a unimolecular multiferroic material having properties such that it can be used as a molecular memory is not yet known.

An object of the present disclosure is to realize a single-molecule multiferroic material having a property that it can be used as a molecular memory and a molecular memory using the same.

One embodiment of the multiferroic material of the present disclosure has a communication hole, and a cluster skeleton having a clathrate on each of the open end side and the other open end side in the communication hole, and a clathrate part of the clathrate part. It has dysprosium ions clathrated on one side and exhibits dielectric and magnetic hysteresis with a single molecule.

One aspect of the multiferroic molecular memory of the present disclosure comprises the multiferroic material of the present disclosure, by applying an external electric field, it is possible to reverse the polarization direction by switching the inclusion position of dysprosium ions, It maintains the polarization direction when no external electric field is applied.

In one embodiment of the multiferroic molecular memory, by applying an external magnetic field, the uniaxial magnetic anisotropy of dysprosium ions can be used to invert the magnetization direction, and the magnetization direction can be changed in the state where no external magnetic field is applied. Can be held.

In one embodiment of the multiferroic molecular memory, the cluster skeleton has a substantially oblate spheroidal shape, and communication holes may be provided along the rotation axis.

In one embodiment of the multiferroic molecular memory, the cluster skeleton can be a polyoxometallate represented by the chemical formula P ₅ W ₃₀ O ₁₁₀ .

The multiferroic electronic device of the present disclosure comprises the multiferroic material of the present disclosure and uses at least one of dielectric hysteresis and magnetic hysteresis.

It is a b-axis projection view of the single molecule|numerator multiferroic substance which concerns on one Embodiment. It is a c-axis projection view of the single molecule|numerator multiferroic substance which concerns on one Embodiment. It is a figure which shows an example of the polarization energy structure of the dysprosium clathrate cluster which concerns on one Embodiment. It is a figure which shows an example of the magnetic energy structure of the dysprosium clathrate cluster which concerns on one Embodiment. It is a temperature dependence of IR spectrum of DyPOM concerning one example. It is a plot which shows the temperature dependence of the complex component of the relative dielectric constant in DyPOM which concerns on an Example. It is an Arrhenius plot of the complex component of relative permittivity. It is a plot which shows the temperature dependence of polarization in DyPOM concerning one example. It is the figure which integrated and standardized the plot which shows the temperature dependence of remanent polarization in DyPOM concerning one example. It is a plot which shows the electric field dependence of polarization in DyPOM concerning one example. It is a plot which shows the temperature dependence of the alternating magnetic susceptibility in DyPOM concerning one example.

The unimolecular multiferroic substance of the present embodiment, a communication hole and a cluster skeleton having a clathrate portion provided on one open end side and the other open end side in the communicating hole, respectively, It has a dysprosium ion clathrated on one side and exhibits dielectric and magnetic hysteresis with a single molecule.

1A and 1B show an example of the single molecule multiferroic substance of the present embodiment. As shown in FIGS. 1A and 1B, the unimolecular multiferroic substance of the present embodiment has a dysprosium clathrate having a cluster skeleton 101 and a dysprosium ion (Dy ³⁺ ) 102 clathrated by the cluster skeleton 101. It is a cluster.

The cluster skeleton 101 has a communication hole 111, and has an inclusion portion (stable site) 113 capable of including dysprosium ions on one open end side and the other open end side of the communication hole 111, respectively. .. The cluster skeleton 101 can be a phosphorus-tungsten oxide molecule.

The dysprosium ion 102 is included in one of the two inclusion portions 113 of the cluster skeleton 101. The dysprosium clathrate cluster can take two different polarization directions depending on which clathrate part 113 the dysprosium ion is clathrated into. The inclusion of dysprosium ions in one of the inclusion portions 113 causes the polarization of the dysprosium ions 102 in the dysprosium inclusion cluster to form molecular polarization. By applying an external electric field, the dysprosium ion 102 can be moved to the inclusion portion 113 that is stable with respect to the external electric field. Therefore, since the magnitude and direction of molecular polarization can be controlled by an external electric field, the dysprosium inclusion cluster can be used as a multiferroic molecular memory using molecular polarization. In the case of the dysprosium clathrate cluster of the present embodiment, which is a multiferroic substance, the polarization can be controlled not only by the external electric field but also by the external magnetic field, both the external electric field and the external magnetic field, or the stress field.

The dysprosium inclusion cluster of this embodiment also functions as a single molecule magnet. The coordination form of dysprosium ions of the present embodiment causes dysprosium ions to have uniaxial magnetic anisotropy, whereby the magnetization direction and size can be fixed. The magnetization direction and size can be controlled by applying an external magnetic field. Therefore, the dysprosium inclusion cluster can perform the reversal of the magnetization direction independently of the reversal of the polarization direction by applying the external magnetic field, and the magnetization direction can be changed in the state where the external magnetic field is not applied. Since it can be retained, it can be applied to a multiferroic molecular memory or the like. In the case of the dysprosium clathrate cluster of the present embodiment, which is a multiferroic substance, the magnetization direction can be controlled not only by an external magnetic field but also by an external electric field, both the external electric field and the external magnetic field, or a stress field.

The dysprosium inclusion clathrate of the present embodiment is such that the magnitude and direction of molecular polarization depending on the inclusion position of dysprosium ions and the magnitude and direction of magnetization derived from the spin of dysprosium ions are inverted and held independently of each other. You can Therefore, the dysprosium clathrate cluster of this embodiment can function as a multi-bit molecular memory that stores polarization information and magnetic information.

The dysprosium inclusion cluster of this embodiment has the same dielectric hysteresis and magnetic hysteresis as those of the ferroelectric substance and the ferromagnetic substance. However, the dysprosium clathrate cluster of the present embodiment exhibits dielectric hysteresis and magnetic hysteresis even when a plurality of dysprosium clathrate clusters are not aggregated, unlike ordinary ferroelectric substances. Therefore, it is possible to make a single molecule multi-bit memory in which one molecule is one magnetic and dielectric memory cell.

The cluster skeleton 101 has a substantially flat spheroidal shape, and the communication hole 111 is provided along the rotation axis. Therefore, the cluster skeleton 101 becomes a crystal that forms an orthorhombic crystal structure (Pnma), and when it is a single crystal, the selectivity of the direction of molecular polarization can be increased. That is, when a single crystal is used, it is possible to reduce the selection of random directions and enhance the selectivity of unidirectionalization and vice versa.

Examples of the cluster skeleton 101 include a polyoxometallate (POM) molecule having a polyoxometallate skeleton and represented by the chemical formula P ₅ W ₃₀ O ₁₁₀ . This molecule is a Preyssler type POM.

The dysprosium inclusion cluster of this embodiment can be manufactured as follows, for example. First, [K _12.5 Na _1.5 [NaP ₅ W ₃₀ O ₁₁₀ ].15H ₂ O], which is a Placer-type POM encapsulating sodium ions, was prepared by a known method (for example, Inge Creaser et. al. Inorg. Chem, 32). , 1573-1578 (1993), etc.). Next, the Dy(NO ₃ ) ₃ aqueous solution is added dropwise to the obtained POM aqueous solution containing Na ions at 60 to 70° C. and mixed. Next, the mixed solution is treated at 160° C. for 24 hours and then recrystallized to obtain a placer type POM (DyPOM) containing dysprosium ions.

Since one inclusion part of the dysprosium clathrate cluster is empty, dysprosium ions can be moved to the side of the vacant inclusion part by applying an external electric field.

FIG. 2 is a diagram illustrating an example of the polarization energy structure of the dysprosium clathrate cluster of the present embodiment. In the dysprosium clathrate cluster, two clathrates (ion stable sites) inside are crystallographically equivalent. A unidirectional polarization P is formed by trapping the dysprosium ion at one site.

If the temperature is sufficiently low, it does not have thermal energy that jumps over the energy barrier U _E between one site and the other site, so that the transfer of dysprosium ions does not occur and the direction of polarization P is maintained. You can When the external electric field E is applied in this state, the dysprosium ions move to a more stable site, and the direction of the polarization P can be changed. Even if the external electric field to be applied is small when the temperature is high, the dysprosium ion can be easily moved to the other site by supplementing the thermal energy. When the temperature is further increased, the inter-site fluctuation of the dysprosium ion increases, and even when the external electric field is not applied, the dysprosium ion moves to the other site, and the polarization direction cannot be maintained.

In the dysprosium clathrate cluster of this embodiment, at room temperature, movement of dysprosium ions can be caused by applying an external electric field of about 100 V/cm to 1000 V/cm. Specifically, by applying an external electric field of 428 V/cm, migration of dysprosium ions can be caused, and a polarization moment of 21 μC/cm can be obtained at 285K. By increasing the magnitude of the external electric field, the molecular polarization can be controlled even at a lower temperature, but the applied external electric field is 1600 V/cm or less from the viewpoint of preventing reduction of the dysprosium inclusion cluster. Is preferred.

The dysprosium inclusion cluster of the present embodiment also functions as a single molecule magnet. FIG. 3 is a diagram illustrating an example of the magnetic energy structure of the dysprosium clathrate cluster of the present embodiment. The dysprosium ion of the dysprosium inclusion cluster causes uniaxial magnetic anisotropy depending on its coordination form. That is, when reversing a certain magnetic direction, it is necessary to exceed the energy barrier U _M. When the temperature is sufficiently low, there is no thermal energy that jumps over the energy barrier U _M between the one magnetization direction and the other magnetization direction, so that the magnetization reversal of the dysprosium ion does not occur and the direction of the magnetization M is changed. Can be maintained. When the external magnetic field H is applied in this state, the dysprosium ions are inverted to the more stable magnetization direction, and the direction of the magnetization M can be changed. When the temperature rises, even if the external magnetic field applied is small, the magnetization direction of the dysprosium ions can be easily reversed by complementing the thermal energy. When the temperature further rises, fluctuations in the magnetization direction of the dysprosium ions become large, and even when the external magnetic field is not applied, the magnetization reversal of the dysprosium ions occurs and the magnetization direction cannot be maintained.

In the dysprosium clathrate cluster of the present embodiment, in the case of 3K, the magnetization direction of dysprosium ions can be reversed by applying an external magnetic field of 100G or more. In addition, it is desirable to lower the temperature from the viewpoint of improving the residual magnetization and the coercive force. Specifically, at 0.5 K, the residual magnetization is about 25% of the saturation magnetization, and a coercive force of about 500 G is obtained.

The dysprosium inclusion cluster of this embodiment can hold and change the direction of molecular polarization and the direction of magnetization independently. Further, since the polarization information can be retained even at a low temperature, it can be retained simultaneously with magnetic recording. Therefore, one molecule can function as a multi-bit molecular memory that can store four states.

It is possible to read out the molecular polarization from the dysprosium clathrate cluster, and to bring a member sensitive to at least one of an electric field and a magnetic field generated by the dysprosium clathrate cluster close to the dysprosium clathrate cluster. The magnitude and direction of the molecular polarization of the dysprosium clathrate cluster and the magnetization of the dysprosium clathrate cluster by the mechanical, optical or electromagnetic action caused by bringing a member sensitive to at least one of an electric field and a magnetic field close to the dysprosium clathrate cluster. It is possible to read the strength and the direction. Electromagnetic waves such as ferroelectrics, ferromagnets, multiferroics, electromagnetic coils, and lasers can be used as the member that senses at least one of an electric field and a magnetic field.

In the dysprosium inclusion cluster of the present embodiment, which is a single molecule multiferroic substance, one molecule can be configured to form one memory cell. Therefore, the storage density can be greatly improved. It is also possible that a plurality of molecules form one memory cell or the like.

Further, the dysprosium clathrate cluster of the present embodiment can be used not only for memories but also for various electronic devices using at least one of dielectric hysteresis and magnetic hysteresis.

Hereinafter, the molecular memory of the present invention will be described in more detail with reference to Examples. The following examples are merely examples, and do not limit the present invention.

<Preparation of Dysprosium inclusion cluster>
First, 0.10 mol of sodium tungstate dihydrate (Na ₂ WO ₄ .2H ₂ O) was dissolved in 30 ml of water heated to 60°C to 70°C. Subsequently, by heating 85% phosphoric acid (H ₃ PO ₄₎ solution 26.5mL at 120 ° C. was added a 24 hours for hydrothermal synthesis. Subsequently, 15 mL of water and 0.13 mol of potassium chloride (KCl) were added, filtered, and recrystallized to obtain sodium clathrate polyoxometalate (NaPOM).

Next, 1.23×10 ⁻⁴ mol of NaPOM was dissolved in 12 mL of water and heated to 60° C. to 70° C. After that, 2.46×10 ⁻⁴ mol of dysprosium nitrate (Dy(NO ₃ ) ₃ ) was added together with 3 mL of water, and the mixture was heated at 160° C. for 24 hours to perform hydrothermal synthesis. Subsequently, 5.3×10 ^-2 mol of potassium chloride (KCl) was added in 2 portions, and the precipitates generated in each were separately collected by filtration. Only the filter cake obtained in the second time was recrystallized to obtain crystals of dysprosium-inclusion polyoxometalate (DyPOM:K ₁₂ [DyP ₅ W ₃₀ O ₁₁₀ ].nH ₂ O).

<Infrared absorption spectrum>
The infrared absorption spectrum (IR spectrum) of the obtained DyPOM was measured. For the measurement, FT/IR-660Plus manufactured by JASCO Corporation was used, and the measurement was performed at various temperatures from 300K (room temperature) to 5K. As shown in FIG. 4, as the temperature became lower, the peaks near 1160 cm ⁻¹ , 1065 cm ⁻¹ and 1020 cm ⁻¹ shifted to the higher wave number side, and a new peak appeared near 1113 cm ⁻¹ . This indicates that the dysprosium ion that fluctuated between stable sites in the high temperature region slowly stopped at the stable site in the low temperature region.

<Dielectric constant>
The temperature dependence of the complex component of the relative dielectric constant of the obtained DyPOM was measured. An LCR meter (E4980A manufactured by Agilent) was used for the measurement. A DyPOM powder was formed into a pellet shape (thickness: 84 μm, area: 1.88 mm ² ), and gold paste was applied to both surfaces thereof to provide electrodes, which was used as a measurement sample. The applied voltage was 2 V and the measurement frequency was 100 Hz to 2 MHz.

As shown in FIG. 5, in the temperature range of 200 K or higher, frequency dispersion was observed in which the peak of the complex component of the relative permittivity shifted depending on the frequency. This result indicates that the dysprosium ion is moving in the internal space of the POM molecule. FIG. 6 is an Arrhenius plot using the peak top temperature and the frequency of the complex component of the relative dielectric constant, the vertical axis of the graph is the natural logarithm of the frequency, and the horizontal axis is the reciprocal of the peak top temperature. The energy barrier obtained from the Arrhenius plot was 0.437 eV, and the blocking temperature at 10 Hz was 203K.

<Pyroelectric effect>
The pyroelectric effect of the obtained DyPOM was confirmed using an electrometer (Keithley 6517A). First, DyPOM was cooled, and an electric field of ±0.55 kV/cm was applied to perform poling treatment. Next, the pyroelectric current was measured while increasing the temperature at a rate of 0.5 K/min under a zero electric field. As shown in FIG. 7, a clear pyroelectric current peak was observed around 260K.

FIG. 8 is a graph (PT graph) showing the temperature dependence of a value (P/P _100K ) obtained by normalizing the integrated value of the pyroelectric current of DyPOM by the integrated value of the pyroelectric current at _100K . The absolute value of polarization started to decrease from about 230K and became 0 at about 295K. This temperature was defined as the ferroelectric development temperature.

<Dielectric hysteresis>
Dielectric hysteresis of the obtained DyPOM was confirmed using a ferroelectric tester (Precision LC manufactured by Radiant Co.). FIG. 9 is the dielectric hysteresis loop of the DyPOM sample. As the temperature increased from 200K to 285K, the dielectric hysteresis increased and the spontaneous polarization also increased. A ferroelectric hysteresis loop was observed when the maximum applied voltage was 0.4 kV/cm at 260K. This temperature coincided with the peak temperature observed in the pyroelectric current measurement.

From the above results, it has been clarified that the origin of spontaneous polarization is due to the bias of dysprosium ions in DyPOM, and DyPOM exhibits dielectric hysteresis as a single molecule. The DyPOM having such a ferroelectric hysteresis characteristic can be used as a molecular memory that holds polarization information.

<Magnetic Susceptibility>
In FIG. 10, the temperature dependence of the AC magnetic susceptibility of the obtained DyPOM was measured. For the measurement, a superconducting quantum interference magnetometer (manufactured by QUANTUM DESIGHN, MPMS-XL) was used, and at a temperature of 2K to 12K, the frequency was 1 Hz to 1000 Hz, and an alternating magnetic field of 2 Oe was applied. ing. In FIG. 10, frequency dispersion peculiar to the single molecule magnet appeared, and it was confirmed that the single molecule magnet was obtained. As described above, it was confirmed that DyPOM was a single molecule magnet, and it became clear that it was a molecular memory that holds magnetic information.

The obtained DyPOM can independently hold polarization information and magnetic information, and can be a multi-bit molecular memory that holds polarization information and magnetic information.

The unimolecular multiferroic substance of the present invention has characteristics required for a molecular memory, and is useful as a multibit molecular memory or the like.

101 Cluster skeleton 102 Dysprosium ion 111 Communication hole 113 Inclusion part

Claims (6)

A cluster skeleton having a communication hole, having a clathrate on each of the open end side and the other open end side in the communication hole, and a dysprosium ion clathrate to one of the clathrates. A multiferroic material that comprises and exhibits single molecule dielectric and magnetic hysteresis. Comprising the multiferroic material of claim 1,
A multiferroic molecular memory capable of reversing the polarization direction by exchanging the inclusion position of the dysprosium ion by applying an external electric field and retaining the polarization direction in the state where the external electric field is not applied. 3. The multi according to claim 2, wherein by applying an external magnetic field, the magnetization direction can be reversed by the uniaxial magnetic anisotropy of dysprosium ions, and the magnetization direction is maintained in a state where the external magnetic field is not applied. Ferroic molecular memory. The multiferroic molecular memory according to claim 2, wherein the cluster skeleton has a substantially oblate spheroidal shape, and the communication holes are provided along a rotation axis. The multiferroic molecular memory according to claim 2, wherein the cluster skeleton is a polyoxometallate represented by the chemical formula P ₅ W ₃₀ O ₁₁₀ . A multiferroic electronic device comprising the multiferroic material according to claim 1 and using at least one of dielectric hysteresis and magnetic hysteresis.