A group of scientists from the Moscow Institute of Physics and Technology (MIPT), the National Institute of Electronic Technology (MIET), and the Prokhorov Institute of General Physics have proposed a theoretical model that explains the unexpectedly high linear magnetoelectric effect in BiFeO3 (bismuth ferrite) observed in many experiments. The research team also proposed a method to further enhance the effect. The results of this study have been published in the journal Physical Review B.

A characteristic of bismuth ferrite is that in large samples, the spins of Fe3+iron ions are arranged in a cycloidal form. This spin structure can be destroyed by strong magnetic fields or mechanical stress. In the absence of cycloids, bismuth ferrite exhibits a large linear magnetoelectric effect, which is the focus of this study.

"The theoretical description proposed in the paper may be applicable to other multiferrous alloys such as BiFeO3. This will help predict the value of their magnetoelectric effects, making it easier to find new and promising industrial application materials," said Professor Anatoly Zvezdin, director of the MIPT Magnetic Heterostructure and Energy Conservation Information Technology Spintronics Physics Laboratory.

Polyiron and Magnetoelectric Effects

Polyferrous materials are materials that exhibit different iron sequences simultaneously, including magnetism, ferroelectricity, and/or ferroelasticity. Magnetoelectric (ME) effects can occur if there is an interaction between electrical and magnetic subsystems in a material.

Magnetoelectric effect refers to the occurrence of polarization under the influence of an external magnetic field and magnetization under the influence of an electric field. This allows electric fields to be used to control the magnetism of materials, while magnetic fields can be used to control the electrical properties of materials. If the ME effect value is very high (dozens or hundreds of times higher than the normal value), it is called a huge ME effect.

The main uses of the magnetoelectric effect are variable magnetic field and static magnetic field sensors. These sensors are used in navigation systems, electric motors, and also in car ignition systems. Compared to similar devices based on the Hall effect or magnetoresistance, sensors based on the ME effect have higher sensitivity (according to research, the sensitivity is up to 1 million times) and relatively low manufacturing costs.

The ME effect provides exciting possibilities for the application of multiferrous materials in new magnetic memories, such as read-only memories. The ME effect may also be used to manufacture high-precision microwave radiation devices and wirelessly transmit energy to small electronic devices.

Bismuth ferrite

The subject of this study is bismuth ferrite (BiFeO3), a very promising multiferrous material with promising practical applications. It is planned to create ultra energy efficient magnetoelectric memories.

In addition, bismuth ferrite exhibits magnetoelectric effects at room temperature, while in most other magnetoelectric materials, ME effects of this magnitude can only be observed at extremely low temperatures (below - 160 degrees Celsius). Bismuth ferrite is an antiferromagnetic material, which means that the magnetic moments of its magnetic sublattice (a structure formed by atoms with the same parallel spin) cancel out each other, and the total magnetization of the material is close to zero. However, the spatial arrangement of spins forms the same cycloidal spin structure.

In the 1980s, it was believed that this multiferrous material exhibited only secondary magnetoelectric effects (i.e., polarization was in secondary proportion to the applied magnetic field). The fact that linear magnetoelectric effects have long been "neglected" is related to spin cycloids: due to their structure, certain characteristics, such as magnetization and magnetoelectric effects, are "averaged" to zero. However, when bismuth ferrite is placed in a strong magnetic field (greater than a certain threshold), the structure is destroyed, accompanied by the appearance of a linear ME effect (when polarization is linearly proportional to the applied magnetic field).

Early experiments showed that the linear magnetoelectric effect value in bismuth ferrite was low (almost 1000 times lower than the actual value), but later experimental studies showed a larger ME effect and demonstrated that using it in layered structures can achieve record magnetoelectric effect values.

The author of this article has theoretically demonstrated the occurrence of the linear ME effect based on the Ginzburg Landau theory, and explained the greater experimental value of this effect before. As part of their theory, researchers have also shown that the ME effect can be enhanced in the presence of electrostatic fields.