The spin cycloid structure in BiFeO3. Credit: MIPT press-office
Electromagnetic effect allows to control magnetic polarization of a material by applying external electric field. Materials with such properties may be used to develop a new generation of ultra-energy efficient computer memoy . A team proposed a theoretical model that explains the unexpectedly high values of the linear magnetoelectric effect in BiFeO3 (bismuth ferrite) that have been observed in a number of experiments and also suggested a way of further enhancing the effect.
One particular feature of bismuth ferrite is that in bulk samples, spins of Fe3+ iron ions are arranged in the form of a cycloid. This spin structure can be destroyed by a strong magnetic field or mechanical stress. Without a spin cycloid, bismuth ferrite exhibits a large linear magnetoelectric effect. “The theoretical description presented in the paper may be applicable to other multiferroics similar to BiFeO3. This will help in predicting the value of their magnetoelectric effect, which, in turn, will make it easier to find new and promising materials for industrial applications,” says Prof. Anatoly Zvezdin.
Multiferroics are materials that simultaneously exhibit different ferroic orders, including magnetic, ferroelectric and/or ferroelastic. If there is an interaction between electric and magnetic subsystems in a material, a magnetoelectric (ME) effect may occur.
ME is when electric polarization occurs under the influence of an external magnetic field and magnetization occurs under the influence of an electric field. This allows an electric field to be used to control the magnetic properties of a material and a magnetic field to be used to control the electric properties. If the value of the ME effect is high (dozens or hundreds of times higher than normal), it is called a giant ME effect.
The main use of the magnetoelectric effect is in variable and static magnetic field sensors eg in navigation systems, electric motors, and also in vehicle ignition systems. Compared to similar devices based on the Hall effect or magnetoresistance, sensors based on the ME effect are more sensitive (up to one million times more sensitive) and they are also relatively cheap to manufacture.
The ME effect offers exciting possibilities for the use of multiferroics in new types of magnetic memory, e.g. ROM — read only memory. The ME effect could also potentially be used to create high-precision equipment for working with radiation in the microwave range, and to wirelessly transmit power to miniaturized electronic devices.
BiFeO3 is a highly promising multiferroic that is very promising in terms of its practical applications. It is planned to be used to create ultra energy-efficient magnetoelectric memory. It also exhibits a magnetoelectric effect at room temperature, while in most other magnetoelectrics an ME effect of this magnitude is only observed at extremely low temperatures (below -160C). Bismuth ferrite is an antiferromagnetic, which means that the magnetic moments of its magnetic sublattices (structures formed by atoms with the same parallel spins) cancel each other out, and the total magnetization of the material is close to zero. However, the spatial arrangement of the spins forms the same cycloidal spin structure.
In the 1980s it was thought that this multiferroic exhibited only a quadratic magnetoelectric effect (i.e. polarization is quadratically proportional to the applied magnetic field). The fact that the linear magnetoelectric effect “went unnoticed” for a long time had to do with the spin cycloid: due to the spin cycloid structure, certain characteristics, such as magnetization and the magnetoelectric effect “average out” to zero. However, when bismuth ferrite is placed in a strong magnetic field (greater than a certain critical value), the structure is destroyed and this is accompanied by the emergence of a linear ME effect (when polarization is linearly proportional to the applied field).
Early experiments indicated a low value of the linear magnetoelectric effect in bismuth ferrite (almost one thousand times lower than the actual value), however later experimental studies revealed a large ME effect and it was also demonstrated that by using it in layered structures, record values of the magnetoelectric effect can be achieved.
The authors of the paper developed a theoretical justification for the occurrence of the linear ME effect based on the Ginzburg-Landau theory and explained the previously large experimental value of the effect. As part of their theory, the researchers also showed that the ME effect could be enhanced in the presence of an electrostatic field. https://mipt.ru/english/news/scientists_from_mipt_explain_how_the_giant_magnetoelectric_effect_occurs_in_bismuth_ferrite
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