Project annotation

Further development of magnetic data storage and data processing systems requires reducing heat losses appearing during remagnetization of nanoscale magnetic elements. Although great progress has been achieved in recent years, critical current of memory cell switching (information writing) still exceeds 10^6 A/cm^2. This leads to essential heating of magnetic devices. Therefore control of magnetization with an electric field rather than with electric current is attracting a lot of attention all over the world. Such an effect is called magnetoelectric coupling. Linear magnetoelectric coupling is allowed only in the medium with broken spatial and time symmetries. This essentially restricts the choice of materials allowing remagnetization induced by an electric field. A common disadvantage of all known magnetoelectric coupling mechanisms is their small magnitude. One way to improve magnetoelectric coupling is to create artificial material and nanostructures with controlled properties.

 Artificial systems provide wide range of opportunities for the creation of complicated magnetic structures which are extremely sensitive to an external electric field. The project focuses on the creation and investigation of artificial magnetoelectric materials. In particular, the project will aim to solve a number of new tasks, namely:

• Theoretical and experimental investigation of spin currents and magnetoelectric coupling in ferromagnetic nanostructures with non-collinear magnetic structure

• Theoretical and experimental investigation of magnetic proximity effects and magnetoelectric coupling in multilayer structures ferromagnetic insulator/ferromagnetic metal

• Theoretical investigation of magnetoelectric coupling effect in granular multiferroics – material in which ferromagnetic metal grains are embedded into a ferroelectric matrix

Expected results and their significance

1. Theoretical and experimental investigations of second harmonic generation (optical frequencies) by ferromagnetic multilayer structures with non-collinear magnetization distribution. A new method for spin current characterization will be developed. According to a modern theory, equilibrium spin curents are responsible for magnetoelectric coupling in systems with a non-collinear magnetic structure [1]. Thus, investigation of spin currents will give a solid contribution to understanding magnetoelectric effects and the proposed project is extremely important for numerous scientific groups working on spin currents and magnetoelectric phenomena

2. Investigation of magnon excitation by electric field [2,3] in ferromagnetic multilayer structures with non-collinear magnetization distribution. In fact, this will be the first observation of such stimulations in such artificial multiferroics. The project is aimed to discovering of magnetoelectric coupling in these structures. We will find magnetoelectric coupling strength and estimate the possibility of using of these materials in practice

3. Theoretical and experimental investigation of a magnetic proximity effect between ferromagnetic metal films separated by ferromagnetic insulator. We will specifically investigate the proximity effect as a function of an external electric field applied to the magnetic insulator layer. Magnetic leads are supposed to have non-collinear orientation of magnetic moments. Such an arrangement of leads magnetization creates an inhomogeneous domain wall in the insulator ferromagnetic layer. Due to magnetoelectric coupling, an electrical polarization appears in the insulating

magnetic spacer. To determine magnetoelectric coupling strength we will use the feromagnetic resonance method. The magnetoelectric coupling may be used for remagnetization of magnetic metal leads. The existence of a magnetic proximity effect in a ferromagnetic dielectric/ferromagnetic insulator system is indirectly confirmed by the injection of spin currents observed in such a system [4]

4. Theoretical investigation of magnetoelectric coupling in granular multiferroics – materials in which magnetic metal grains are embedded into (or placed in proximity of) a ferroelectric matrix. Note that only strictions mediated magnetoelectric coupling mechanisms were previously considered for granular multiferroics [5]. In the present project, we will study novel mechanism based on Coulomb interaction. The goal of the project is to estimate magnetoelectric coupling strength in granular multiferroic and to determine the prospects for its practical use. Magnetoelectric coupling in solids is currently a hot topic in the field of condensed matter physics. Thus, the scientific significance of the proposed project is high. The possibility of applying the results of these studies to specific devices is to be determined by the magnitude of the magnetoelectric coupling. 

REFERENCES

1. H. Katsura, N. Nagaosa, and A. V. Balatsky, Spin сurrent and мagnetoelectric еffect in noncollinear magnets, PRL 95, 057205 (2005).
2. A. Pimenov et al. Nature Phys. 2 97 (2006).
3. A.A.Mukhin, UFN 179, 904 (2009).
4. P. Hyde et al, Electrical detection of direct and alternating spin current injected from a ferromagnetic insulator into a ferromagnetic metal, Phys. Rev. B 89, 180404(R) (2014).
5. Ce-Wen Nan et al. Multiferroic magnetoelectric composites: Historical perspective, status, and future directions, J.Appl.Phys. 103, 031101, (2008).