Ferromagnetic ferroelectric material

Abstract

This ferromagnetic ferroelectric material has ferroelectricity and ferromagnetism, and contains a compound represented by any one of general formulae (1) to (3). Bi1-xA2+ xFe1-xM4+ xO3 ... (1) (In the formula, A is Mg, Ca, or Sr, and M is a 4d element or a 5d element that can form a tetravalent ion. x satisfies 0.02 ≤ x ≤ 0.15.) Bi1-xA+ xFe1-xM5+ xO3 ... (2) (In the formula, A is Na or K, and M is a 4d element or a 5d element that can form a pentavalent ion. x satisfies 0.02 ≤ x ≤ 0.15.) Bi1-2xA2+ 2xFe1-xM5+ xO3 ... (3) (In the formula, A is Mg, Ca, or Sr, and M is a 4d element or a 5d element that can form a pentavalent ion. x satisfies 0.02 ≤ x ≤ 0.15.)

Description

Ferromagnetic and ferroelectric

[0001] The present invention relates to a ferromagnetic ferroelectric material.

[0002] Multiferroic materials, which have both ferromagnetic and ferroelectric properties, are expected to be applied to next-generation low-power memory devices. If the correlation between ferromagnetic and ferroelectric properties is strong enough to enable magnetization reversal in response to electric polarization reversal, it is expected that ultra-low-power magnetic memory with electric field writing and magnetic readout (voltage-driven) will be realized.

[0003] As an example of a magnetic memory element using such a multiferroic material, Patent Document 1 discloses a BiFe １－ｘ A ｘ O ３ A magnetic memory element using a thin film of a multiferroic material represented by the formula (wherein A is Co or Mn) has been proposed.

[0004] Japanese Patent Application Laid-Open No. 2019-009304

[0005] The multiferroic material described in Patent Document 1 has a spontaneous magnetization of 0.03 μm, which is necessary for application to a low-power nonvolatile magnetic memory. Ｂ The problem is that the magnetic field is small, and the coercive force required for non-volatility is also small.

[0006] The present invention has been made in view of the above circumstances, and one of its objects is to provide a new ferromagnetic ferroelectric material having an increased coercive force.

[0007] One aspect of the present invention is a ferromagnetic ferroelectric material having ferromagnetic and ferroelectric properties. The ferromagnetic ferroelectric material includes a compound represented by any one of the following general formulas (1) to (3): Bi １－ｘ A ２＋ ｘ Fe １－ｘ M ４＋ ｘ O ３ ... (1) (In the formula, A is Mg, Ca or Sr, M is a 4d element or a 5d element that can become a tetravalent ion, and x satisfies 0.02≦x≦0.15.) Bi １－ｘ A ＋ ｘ Fe １－ｘM ５＋ ｘ O ３ ... (2) (In the formula, A is Na or K, M is a 4d element or a 5d element that can become a pentavalent ion, and x satisfies 0.02≦x≦0.15.) Bi １－２ｘ A ２＋ ２ｘ Fe １－ｘ M ５＋ ｘ O ３ ... (3) (In the formula, A is Mg, Ca, or Sr, M is a 4d element or a 5d element that can become a pentavalent ion, and x satisfies 0.02≦x≦0.15.)

[0008] Any combination of the above components, and any transformation of the present invention into a method, material, composite, etc., are also valid aspects of the present invention.

[0009] According to the present invention, a new ferromagnetic ferroelectric material having an increased coercive force can be provided.

[0010] BiFe ０．８０ Co ０．２０ O ３ (BFCO) and BiFeO ３Figure 2(a) shows the X-ray diffraction patterns of Samples 1-8 compared to BFCO (BFO). Figure 2(b) shows the temperature dependence of magnetic susceptibility of Sample 1. Figure 3(a) shows the magnetization curve of Sample 2 compared to BFCO at 300 K at multiple temperatures. Figure 3(b) shows the temperature dependence of magnetic susceptibility of Sample 2. Figure 4(a) shows the magnetization curve of Sample 3 compared to BFCO at 300 K at multiple temperatures. Figure 4(b) shows the temperature dependence of magnetic susceptibility of Sample 3. Figure 5(a) shows the magnetization curve of Sample 4 compared to BFCO at 300 K at multiple temperatures. Figure 5(b) shows the temperature dependence of magnetic susceptibility of Sample 4. Figure 6(a) shows the magnetization curve of Sample 5 compared to BFCO at 300 K at multiple temperatures. Figure 6(b) shows the temperature dependence of magnetic susceptibility of Sample 5. FIG. 7(a) shows the magnetization curves of Sample 6 at multiple temperatures compared to BFCO at 300 K. FIG. 7(b) shows the temperature dependence of the magnetic susceptibility of Sample 6. FIG. 8(a) shows the magnetization curves of Sample 7 at multiple temperatures. FIG. 7(b) shows the temperature dependence of the magnetic susceptibility of Sample 7. FIG. 9(a) shows the magnetization curves of Sample 8 at multiple temperatures. FIG. 9(b) shows the temperature dependence of the magnetic susceptibility of Sample 8. BiFeO ３ 11(a) shows the X-ray diffraction pattern of Sample 9 compared with that of (BFO). FIG. 11(a) shows the magnetization curves of Sample 9 at multiple temperatures. FIG. 11(b) shows the temperature dependence of the magnetic susceptibility of Sample 9.

[0011] (Ferromagnetic Ferroelectric) The ferromagnetic ferroelectric according to this embodiment is a multiferroic material in which ferromagnetism and ferroelectricity coexist. ３ In the formula (1), a part of Bi is substituted with Ca, Sr, Na or K, and a part of Fe is substituted with a 4d element or a 5d element that can become a tetravalent or pentavalent ion having d electrons. Specifically, the ferromagnetic ferroelectric includes a compound represented by any one of the following formulas (1) to (3): Bi １－ｘ A ２＋ ｘ Fe １－ｘ M ４＋ｘ O ３ ... (1) In formula (1), A is Mg, Ca, or Sr, and M is a 4d element or a 5d element that can become a tetravalent ion. x satisfies 0.02≦x≦0.15. Bi １－ｘ A ＋ ｘ Fe １－ｘ M ５＋ ｘ O ３ In formula (2), A is Na or K, M is a 4d element or a 5d element that can become a pentavalent ion, and x satisfies 0.02≦x≦0.15. Bi １－２ｘ A ２＋ ２ｘ Fe １－ｘ M ５＋ ｘ O ３ In formula (3), A is Mg, Ca, or Sr, M is a 4d element or a 5d element that can become a pentavalent ion, and x satisfies 0.02≦x≦0.15.

[0012] BiFeO is a parent material of the compound represented by any one of the above formulas (1) to (3). ３ So Bi's 6s ２ Ferroelectricity due to lone pair electrons and Fe with spin 5 / 2 ３＋ Even in antiferromagnetic materials, weak ferromagnetism can occur when adjacent spins are tilted to each other, but in BiFeO ３ does not generate spontaneous magnetization because the direction of the next nearest neighbor spins aligned in parallel has a cycloid modulation in which they rotate little by little with a period of 620 Å. ３＋ Part of Co ３＋ BiFe substituted with １－ｘ Co ｘ O ３ In this case, the cycloid modulation disappears and the spontaneous magnetization is about 0.03 μB due to the spin tilt angle. ３＋The present inventors predicted that the spin-orbit interaction contributes to the high spin state (see K. Lee et al., Phys. Rev. Mater. 6, 064401 (2022)), and came up with the idea of ​​substituting Fe with a 4d or 5d element with a large spin-orbit interaction to increase the spontaneous magnetization. ３＋ Some of the sites are filled with Mg, Ca, Sr, Na or K, and Fe ３＋ By substituting a part of the sites with the same amount of 4d elements or 5d elements, BiFe １－ｘ Co ｘ O ３ It was found that the coercive force can be increased more than that of the conventional method.

[0013] Therefore, the compound represented by any one of the above formulas (1) to (3) is BiFeO ３ It exhibits weak ferromagnetism while maintaining the ferroelectric structure of the BiFe alloy. １－ｘ Co ｘ O ３ This is advantageous for retaining information in a magnetic memory using the compound.

[0014] Element A is Fe from the viewpoint of electrical neutrality. ３＋ This is to maintain the condition of the

[0015] In formulas (1) to (3), x is 0.15 or less. If x exceeds 0.15, the ferroelectric rhombohedral phase in the compound is mixed with the paraelectric orthorhombic phase or the paraelectric cubic phase, which may impair the ferroelectricity of the compound. In addition, the orthorhombic and cubic phases do not exhibit weak ferromagnetism, so the magnetization decreases.

[0016] Examples of 4d elements that can be tetravalent ions include molybdenum (Mo), ruthenium (Ru), and rhodium (Rh). Examples of 4d elements that can be pentavalent ions include niobium (Nb), molybdenum (Mo), rhodium (Rh), and palladium (Pd).

[0017] Examples of 5d elements that can be tetravalent ions include tungsten (W), rhenium (Re), osmium (Os), and iridium (Ir). Examples of 5d elements that can be pentavalent ions include tungsten (W) and rhenium (Re).

[0018] From the viewpoint of ensuring an improvement in coercivity, M in formula (1) is preferably Ru, Ir, W, Mo, or Rh. For the same reason, M in formulas (2) and (3) is preferably Nb, Re, Mo, or W.

[0019] The temperature range in which the ferromagnetic ferroelectric according to the embodiment exhibits weak ferromagnetism can be controlled by adjusting the substitution amount x. A person skilled in the art can control the temperature range in which the ferromagnetic ferroelectric according to the embodiment exhibits weak ferromagnetism to a desired value by adjusting the substitution amount x according to the types of element A and element M.

[0020] (Method for producing ferromagnetic ferroelectrics) A method for producing a ferromagnetic ferroelectric containing a compound represented by any one of the above formulas (1) to (3) will be described. Note that the method for producing a ferromagnetic ferroelectric is not limited to the following method.

[0021] First, oxides of Bi, A, Fe, and M are mixed in a stoichiometric ratio, and the mixture is sealed in a gold capsule, for example. Then, the mixture is treated using a cubic anvil-type high-pressure synthesis apparatus, for example, under predetermined pressure conditions (for example, about 6 GPa), a predetermined heating temperature (for example, about 1200°C), and a predetermined treatment time (for example, about 30 minutes), thereby obtaining a ferromagnetic ferroelectric material.

[0022] When forming a ferromagnetic ferroelectric thin film for the purpose of manufacturing a magnetic memory, the ferromagnetic ferroelectric thin film can be formed on a desired substrate by methods known to those skilled in the art, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD). Specific examples of PVD methods include pulsed laser deposition (PLD) and electron beam evaporation. Specific examples of CVD methods include metal organic (MO) CVD and mist CVD.

[0023] Examples of the present invention will be described below, but these examples are merely illustrative examples for suitably explaining the present invention and do not limit the present invention in any way.

[0024] The crystal structure was evaluated using X-ray diffraction (XRD) (2θ-ω measurement in the perpendicular direction) (Bruker; D8 Advance). The magnetization curve and the temperature dependence of magnetic susceptibility were measured using a SQUID magnetometer (Quantum Design; MPMS).

[0025] (Samples 1 to 8) Starting material Bi ２ O ３ , CaO, MgO, Na ２ O, SrO, Fe ２ O ３ , RuO ２ , IrO ２ , Nb ２ O ５ were mixed in a stoichiometric composition, and the mixture was sealed in a gold capsule. Then, using a cubic anvil-type high-pressure synthesis apparatus, heat treatment was performed for 30 minutes under conditions of 6 GPa and 1200°C to synthesize samples 1 to 8. The compositions of samples 1 to 3, 5, 7, and 8 are represented by the above formula (1) and are as follows: The composition of sample 6 is represented by the above formula (2) and is as follows: The composition of sample 4 is represented by the above formula (3) and is as follows: Sample 1: Bi ０．９０ Ca ０．１０ Fe ０．９０ Ru ０．１０ O ３ (x=0.10) Sample 2: Bi ０．９５ Ca ０．０５ Fe ０．９５ Ru ０．０５ O ３ (x=0.05) Sample 3: Bi ０．９０ Ca ０．１０ Fe ０．９０ Ir ０．１０ O ３ (x=0.10) Sample 4: Bi ０．８０ Ca ０．２０ Fe ０．９０ Nb ０．１０ O ３ (x=0.10) Sample 5: Bi ０．８５ Mg ０．１５ Fe ０．８５ Ru ０．１５ O３ (x=0.15) Sample 6: Bi ０．８５ Na ０．１５ Fe ０．８５ Nb ０．１５ O ３ (x=0.15) Sample 7: Bi ０．９０ Sr ０．１０ Fe ０．９０ Ru ０．１０ O ３ (x=0.10) Sample 8: Bi ０．８５ Sr ０．１５ Fe ０．８５ Ru ０．１５ O ３ (x=0.15)

[0026] FIG. 1 shows the BiFe ０．８０ Co ０．２０ O ３ (BFCO) and BiFeO ３ Sample 1 (Bi) compared with (BFO) ０．９０ Ca ０．１０ Fe ０．９０ Ru ０．１０ O ３ ), Sample 2 (Bi ０．９５ Ca ０．０５ Fe ０．９５ Ru ０．０５ O ３ ), Sample 3 (Bi ０．９０ Ca ０．１０ Fe ０．９０ Ir ０．１０ O ３ ), Sample 4 (Bi ０．８０ Ca ０．２０ Fe ０．９０ Nb ０．１０ O ３ ), Sample 5 (Bi ０．８５ Mg ０．１５ Fe ０．８５ Ru ０．１５ O ３ ), Sample 6 (Bi ０．８５ Na ０．１５ Fe ０．８５ Nb ０．１５ O ３ ), Sample 7 (Bi ０．９０ Sr ０．１０ Fe ０．９０ Ru ０．１０ O ３ ), Sample 8 (Bi ０．８５ Sr ０．１５ Fe０．８５ Ru ０．１５ O ３ As shown in Figure 1, samples 1 to 8 retained the ferroelectric rhombohedral structure of BFO. ２＋ , Mg ２＋ , Na ＋ , Sr ２＋ By co-substitution with Ru ４＋ , Ir ４＋ , Nb ５＋ It was confirmed that the solid solution was successfully formed.

[0027] Figure 2(a) shows the magnetization curves of Sample 1 at multiple temperatures compared to BFCO at 300 K. Figure 2(b) shows the temperature dependence of the magnetic susceptibility of Sample 1. As shown in Figure 2(a), Sample 1 with 10% Ru substitution was confirmed to have the same spontaneous magnetization as BFCO and approximately three times the coercivity at room temperature. As shown in Figure 2(b), Sample 1 maintains weak ferromagnetism even at a high temperature of 400 K.

[0028] Figure 3(a) shows the magnetization curves of Sample 2 at multiple temperatures compared to BFCO at 300 K. Figure 3(b) shows the temperature dependence of the magnetic susceptibility of Sample 2. As shown in Figure 3(a), Sample 2 with 5% Ru substitution has the same spontaneous magnetization and coercivity as Sample 1 below 200 K, but the weak ferromagnetism disappears at 300 K. As shown in Figure 3(b), Sample 2 exhibits weak ferromagnetism below approximately 250 K.

[0029] Figure 4(a) shows the magnetization curves of Sample 3 at multiple temperatures compared to BFCO at 300 K. Figure 4(b) shows the temperature dependence of the magnetic susceptibility of Sample 3. As shown in Figure 4(a), Sample 3 with 10% Ir substitution was confirmed to have the same spontaneous magnetization as BFCO and about three times the coercivity. As shown in Figure 4(b), Sample 3 maintains weak ferromagnetism even at a high temperature of 400 K.

[0030] Figure 5(a) shows the magnetization curves of Sample 4 at multiple temperatures compared to BFCO at 300 K. As shown in Figure 5(a), Sample 4 with 10% Nb substitution was confirmed to have a spontaneous magnetization approximately 0.6 times that of BFCO and a coercive force approximately 10 times that of BFCO. As shown in Figure 5(b), Sample 5 maintains weak ferromagnetism even at 400 K.

[0031] Figure 6(a) shows the magnetization curves of Sample 5 at multiple temperatures compared to BFCO at 300 K. Figure 6(b) shows the temperature dependence of the magnetic susceptibility of Sample 5. As shown in Figure 6(a), Sample 5, in which 15% of Bi is substituted with Mg and 15% of Fe with Ru, was confirmed to have a spontaneous magnetization approximately twice that of BFCO. As shown in Figure 6(b), Sample 5 maintains weak ferromagnetism even at 400 K.

[0032] Figure 7(a) shows the magnetization curves of Sample 6 at multiple temperatures compared to BFCO at 300 K. Figure 7(b) shows the temperature dependence of the magnetic susceptibility of Sample 6. As shown in Figure 7(a), Sample 6, in which 15% of Bi is substituted with Na and 15% of Fe with Nb, was confirmed to have a spontaneous magnetization approximately 0.6 times that of BFCO and a coercive force approximately three times that of BFCO. As shown in Figure 7(b), Sample 6 maintains weak ferromagnetism even at 400 K.

[0033] Figure 8(a) shows the magnetization curve of Sample 7. Figure 8(b) shows the temperature dependence of the magnetic susceptibility of Sample 7. As shown in Figure 8(a), Sample 7, in which 10% of Bi was substituted with Sr and 10% of Fe with Ru, showed an increase in spontaneous magnetization at 100K or less compared to 200K and 300K. As shown in Figure 8(b), Sample 7 shows an increase in weak ferromagnetism at approximately 150K or less.

[0034] Figure 9(a) shows the magnetization curve of Sample 8. Figure 9(b) shows the temperature dependence of the magnetic susceptibility of Sample 8. As shown in Figure 9(a), Sample 8, in which 15% of Bi was substituted with Sr and 15% of Fe with Ru, had the same spontaneous magnetization as Sample 7 and about three times the coercivity at temperatures below 100K, but the weak ferromagnetism disappeared above 200K. As shown in Figure 9(b), Sample 8 exhibited weak ferromagnetism below approximately 150K.

[0035] (Sample 9) Starting material Bi ２ O ３ , CaO, Fe ２ O ３ , W.O. ２ , W.O. ３ These were mixed in a stoichiometric composition and the mixture was sealed in a gold capsule. Then, using a cubic anvil-type high-pressure synthesis apparatus, the mixture was heat-treated at 6 GPa and 1200°C for 30 minutes to synthesize Sample 9. The composition is expressed by the above formula (3) and is as follows: Sample 9: Bi０．９０ Ca ０．１０ Fe ０．９５ W ０．０５ O ３ (x=0.05)

[0036] FIG. 10 shows the structure of BiFeO ３ Sample 9 (Bi) compared with (BFO) ０．９０ Ca ０．１０ Fe ０．９５ W ０．０５ O ３ 10 shows the X-ray diffraction pattern of Sample 9. As shown in FIG. 10, Sample 9 retained the ferroelectric rhombohedral structure of BFO. ２＋ By co-substitution with W ５＋ It was confirmed that the solid solution was successfully formed.

[0037] Figure 11(a) shows the magnetization curves of the sample at multiple temperatures compared to BFCO at 300 K. Figure 11(b) shows the temperature dependence of the sample's magnetic susceptibility. As shown in Figure 11(a), Sample 9, in which 10% of Bi is replaced by Ca and 5% of Fe is replaced by W, was confirmed to have a spontaneous magnetization approximately 1.3 times that of BFCO. As shown in Figure 11(b), Sample 9 maintains weak ferromagnetism even at 400 K.

[0038] The present invention has been described above based on the embodiments. These embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the components and treatment processes, and that such modifications are also within the scope of the present invention.

[0039] The present invention can be used for ferromagnetic and ferroelectric materials.

Claims

1. A ferromagnetic ferroelectric having ferromagnetic and ferroelectric properties, comprising a compound represented by any one of the following general formulas (1) to (3): Bi １－ｘ A ２＋ ｘ Fe １－ｘ M ４＋ ｘ O ３ ... (1) (In the formula, A is Mg, Ca or Sr, M is a 4d element or a 5d element that can become a tetravalent ion, and x satisfies 0.02≦x≦0.15.) Bi １－ｘ A ＋ ｘ Fe １－ｘ M ５＋ ｘ O ３ ... (2) (In the formula, A is Na or K, M is a 4d element or a 5d element that can become a pentavalent ion, and x satisfies 0.02≦x≦0.15.) Bi １－２ｘ A ２＋ ２ｘ Fe １－ｘ M ５＋ ｘ O ３ ... (3) (In the formula, A is Mg, Ca, or Sr, M is a 4d element or a 5d element that can become a pentavalent ion, and x satisfies 0.02≦x≦0.15.) 2. The ferromagnetic ferroelectric according to claim 1, wherein the compound is a compound represented by formula (1), and M is Ru, Ir, W, Mo or Rh.

3. The ferromagnetic ferroelectric according to claim 1, wherein the compound is a compound represented by formula (2), and M is Nb, Re, Mo or W.

4. The ferromagnetic ferroelectric according to claim 1, wherein the compound is a compound represented by formula (3), and M is Nb, Re, Mo or W.

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