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Search for high-temperature magneto-electrics which realize new energy-efficient memory devices

Date2021.8. 5


Research Associate
【strongly correlated electrons, pressure】

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1.What is magneto-electrics ? What is the magneto-electric effect ?

Materials which possess a magnetic order can be applied to magnetic memory devices. If magnetic materials also possess a spontaneous electric polarization, they are called magneto-electrics (Fig. 1(a)). Magneto-electrics attract much attention because they exhibit the phenomenon called "magneto-electric effect" which can be applied to new-type devices resolving energy problems (Fig. 1(b)).

  • 大学HP-研究紹介_谷口晴香_英語版_Fig1_磁気強誘電体-電気磁気効果.jpgFig. 1: (a) Definition of magneto-electrics. (b) Schematic figure of the magneto-electric effect. Yellow arrows indicate the crossing interaction in the magneto-electric effect.

Electrons in a material play an important role to decide both its magnetic properties (magnetization M and magnetic susceptibility x) and dielectric properties (electric polarization P and dielectric constant ε). As you know, each electron possesses a charge. This charge affects the dielectric properties of the material. Moreover, each electron can be regarded as a tiny magnet called "spin" because of its rotation. This is why electrons also affect the magnetic properties of the material. Normally, the dielectric properties can be controlled only by the electric field E, and the magnetic properties can be controlled only by the magnetic field H. In magneto-electrics, however, the electric field E can also control the magnetic properties and the magnetic field H can also control the dielectric properties, because there exists a coupling between charge and spin. This crossing phenomenon is called "magneto-electric effect" and applicable to a new energy-efficient magnetic memory device which can be handled by the electric field.

2.How to achieve the magneto-electric effect at room temperature ? 

Although magneto-electrics are useful as mentioned above, the transition temperature below which the material exhibits the magneto-electric effect is much lower than room temperature, below approximately 30 K in many magneto-electrics. This is why you do not see the devices made of magneto-electrics in your daily life. Expensive liquid helium is required to reach 30 K. If the transition temperature can be raised up to 77 K which is the boiling point of inexpensive liquid nitrogen (high-temperature magneto-electrics), the application of magneto-electrics to some devices becomes realistic. In our group, we are searching for new-type magneto-electrics in which the phenomenon called charge ordering (Fig. 2) is the key of the magneto-electric coupling.

  • 大学HP-研究紹介_谷口晴香_英語版_Fig2_電荷整列.jpgFig. 2: Schematic figure of charge ordering. Lattice points represent the position of transition metal ions which construct the material, and red circles represents electrons. Each figure illustrates the case in which the number of the electrons in a specific orbital is (a) 1/2, (b) 1/3, and (c) 1/4 of the number of the transition metal ions.

In the charge ordering state, electrons are fixed in a periodic geometric arrangement because of the Coulomb repulsion among electrons. Because the charge ordering commonly occurs at a relatively high temperature of approximately 100-200 K and is often associated by a magnetic ordering, it is a promising candidate of the origin of the high-temperature magneto-electric coupling.

3.Discovery of the magneto-electric effect in charge-ordered manganites

As a candidate of the magneto-electrics originated from a charge-ordering, we focused on the material CaMn1-xSbxO3 (Fig.3 (a)). In this compound, since the number of electrons depends on the substitution ratio x, we expected that the condition favorable to the charge ordering is induced by tuning the ratio x.

  • 大学HP-研究紹介_谷口晴香_英語版_Fig3_CaMnSbO3.jpgFig. 3: (a) Crystal structure of CaMn1-xSbxO3, (b) Effect of a magnetic field on the dielectric constantε' of CaMn0.85Sb0.15O3.

First, we synthesized polycrystalline samples of CaMn1-xSbxO3 with x = 0.15, investigated the temperature dependence of the local activation energy by measuring the electric resistance, and successfully observed an anomaly suggesting a charge ordering. Next, we measured the dielectric constant (Fig. 3(b)) and observed a broad peak at 113 K which indicates the existence of microscopic electric polarizations. Further, we discovered the remarkable suppression of the dielectric peak height by magnetic fields. In other words, we succeeded in inducing a magneto-electric effect above the boiling point of liquid nitrogen!

4.Toward the proposal of more useful materials

We are now investigating the mechanism of the magneto-electric effect in CaMn0.85Sb0.15O3. In detail, we test how the temperature of the dielectric peak and the magnitude of the magneto-electric effect change when the strength of the electron-electron interaction is tuned by changing the number of electrons or by applying a pressure. We found that the dielectric peak temperature is enhanced by increasing the number of electrons or expanding the lattice. From these results, we will propose more useful materials in the future.

<Our paper on this work>
Title:"Glassy dielectric anomaly and negative magneto-capacitance effect in electron-doped
Ca1-x Srx Mn0.85Sb0.15O3"
Authors:H. Taniguchi, H. Takahashi, A. Terui, K. Sadamitsu, Y. Sato, M. Ito, K. Nonaka, S. Kobayashi,
M. Matsukawa, R. Suryanarayanan, N. Sasaki, S. Yamaguchi and T. Watanabe
Article information:Journal of Applied Physics 127, 184105 (2020).