Giant magnetoelectric coupling effect in multiferroic hexagonal ferrite

Multiferroicity refers to the orderly coexistence of ferroelectricity, ferromagnetism, and iron elasticity. The multi-ferric material and magnetoelectric coupling effect not only contains a wealth of basic physics problems, but also has important application prospects. It is a research hotspot in condensed matter physics and materials science in recent years. Multiferroic materials are divided into two major categories: composite materials and single-phase materials. The magnetoelectric coupling of composite materials is indirect coupling using interface effects. The magnetoelectric coupling of single-phase materials is an intrinsic bulk effect. Over the past decade or so, a wide variety of single-phase, multi-ferroic materials have been discovered. However, the magnetoelectric coupling effect (magnetic field controlled polarization or electric field controlled magnetism) of known single-phase multiferroic materials is generally weak, which greatly limits the application of single-phase multiferroic materials in future magnetoelectronic devices. . How to greatly improve the magnetoelectric coupling effect of single-phase materials has become a major challenge in this field.

Recently, Sun Yang Researcher (Quantum Design Product User), Associate Professor Chai Yizhen, and Ph.D. A huge magnetoelectric coupling effect is achieved, and a positive magnetic coupling coefficient of up to 33000 ps/m and an inverse magnetoelectric coupling coefficient of 32000 ps/m are obtained, creating a new world record of the magnetoelectric coupling effect of single-phase materials.

Figure 1. Hexagonal ferrite Positive magnetoelectric coupling effect at 10 K

Hexagonal ferrite is a kind of iron-based oxide with hexagonal crystal system, which can be further divided into M, W, X, Y, Z, and U-type hexagonal ferrites according to structural units. Due to the competition of various magnetic interactions, a rich non-collinear spiral magnetic structure can be produced by partial element replacement in hexagonal ferrite. For some specific helical magnetic structures, non-collinear spins can produce macroscopic polarization by inverse Dzyaloshinskii-Moriya interaction, resulting in a second type of multiferromagnetic and magnetoelectric coupling effect of magnetically ordered driving. In previous studies, although strong magnetoelectric coupling effects have been observed in some hexagonal ferrites, there is still a lack of clear understanding of how to further achieve huge magnetoelectric coupling effects in hexagonal ferrites. And ideas.

Figure 2. Hexagonal ferrite (x = 1.6) inverse magnetoelectric coupling effect at 10 K

In order to understand the Y-type hexagonal ferrite The physical origin of the coupling effect of the giant magneto-electricity, the PhD student (0.0≤x≤1.6) A series of single crystal samples were systematically studied for their macroscopic magnetic and magnetoelectric coupling effects as a function of Sr content. At the same time, Sun Yang's research team cooperated with Dr. Cao Huibo from the Oak Ridge National Laboratory in the United States to study the magnetic structure of this series of single crystal samples in detail using neutron scattering technology. A phase diagram of the conical helical magnetic structure in the system as a function of Sr content and applied magnetic field.

Figure 3. Relationship between spin cone symmetry and magnetoelectric coupling coefficient in hexagonal ferrite

The results show that the strength of the magnetoelectric coupling effect in hexagonal ferrite is closely related to the symmetry of the spin cone: when the symmetry of the spin cone is reduced from quadruple symmetry to double symmetry, it is driven by an external magnetic field. The spin cone can be flipped 180 degrees; at the same time, the polarization produced by the spin structure will also be reversed by 180 degrees. The magnetic anisotropy is regulated by element replacement so that this phase change occurs near the zero magnetic field, resulting in a large magnetoelectric coupling coefficient. Therefore, this study not only obtained the largest positive-reverse magnetoelectric coupling coefficient in single-phase materials, but also pointed out the direction of how to improve the magneto-electric coupling effect in multi-iron hexagonal ferrite.

The above research results were published in Nature Communications 8, 519 (2017). The work was supported by the National Natural Science Foundation of China (11534015, 11374347), the Ministry of Science and Technology (2016YFA0300701) and the Chinese Academy of Sciences project (XDB07030200).

Source: (State Key Laboratory of Magnetics, Institute of Physics, Chinese Academy of Sciences, the final interpretation right belongs to the official website of the State Key Laboratory of Magnetics, Institute of Physics, Chinese Academy of Sciences)

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