Synopsis: Powering up Magnetization

New theoretical work identifies a dynamic form of multiferroic behavior, in which a time-varying electric polarization induces magnetization in a material.

https://physics.aps.org/synopsis-for/10.1103/PhysRevMaterials.1.014401

Michael Schirber

Multiferroics are materials that exhibit both a permanent magnetization (ferromagnetism) and a permanent electric polarization (ferroelectricity). In some cases, the magnetization and polarization are coupled, which could offer novel ways to control devices for spintronics and other applications. A new theoretical work describes a previously uncharacterized case of magnetoelectric coupling, in which a time-varying electric polarization induces a magnetization. This so-called “dynamical multiferroicity” may explain multiple phenomena, including a recent observation of light-driven magnetization.

Magnetic and electric fields are intimately related through Maxwell’s equations. A time-varying electric field in a coil, for example, generates a magnetic field. In certain multiferroic materials, the internal magnetization and polarization bear a similar connection to each other. One known example of this is terbium manganite, for which a spatially varying magnetization produces an electric polarization. Dominik Juraschek and colleagues at the Swiss Federal Institute of Technology (ETH) in Zurich showed that a reciprocal effect was possible in which a time-varying polarization produces a magnetization. Microscopically, one can imagine a material’s polarization made up of tiny rotating electric dipoles that, acting like nanoscale coils, generate localized magnetic fields.

This dynamical multiferroicity could explain a recent observation of a magnetization wave (or magnon) induced by terahertz light pulses in erbium ferrite. According to the team’s analysis, these pulses generate lattice oscillations (phonons) that produce the requisite time-varying electric polarization. The team predicts a similar phonon-mediated effect in a highly polarizable, nonmagnetic material, like strontium titanate, placed in a strong magnetic field. The researchers also consider the potential of dynamic engineering, in which one could create novel magnetoelectric states in a material by continuously driving its polarization with terahertz light.

This research is published in Physical Review Materials.

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Abstract-Theoretical investigation of nonlinear damping and nonlinear phase shift of spin-electromagnetic waves propagating in infinite multiferroics at sub-terahertz frequencies

I A Ustinova, M A Cherkasskii, A B Ustinov and B A Kalinikos

Department of Physical Electronics and Technology, St.Petersburg Electrotechnical University, 5 Popov Street, St.Petersburg 197376, Russia

I A Ustinova et al 2015 J. Phys.: Conf. Ser. 661 012050

http://dx.doi.org/10.1088/1742-6596/661/1/012050
http://iopscience.iop.org/article/10.1088/1742-6596/661/1/012050/meta


The nonlinear phase shift and nonlinear damping of spin-electromagnetic waves were theoretically studied for the first time in sub-terahertz frequency range in infinite homogeneous longitudinal magnetized multiferroics. The research was based on the solution of the Ginzburg-Landau equation. It is shown that the saturation of the phase shift occurs due to the nonlinear damping if the nonlinear damping coefficients exceed v1=108 s-1 and v2=109 s-1.

Multiferroic material displays a novel spin structure that allows light to travel in only one direction

Figure 1: Multiferroics have a screw spin structure (arrows) with clockwise (lower) and anticlockwise (upper) orientations that can control the propogation of light at terahertz and gigahertz frequencies. Credit: Youtarou Takahashi, RIKEN Center for Emergent Matter Science

http://phys.org/news/2014-10-multiferroic-material.html#jCp

A research team led by Youtarou Takahashi from the RIKEN Center for Emergent Matter Science has demonstrated a novel phenomenon called magnetochiral dichroism, which prevents light from propagating parallel or antiparallel to the direction of magnetization. The discovery, which was made in the multiferroic 'helimagnet' gallium-doped copper iron oxide, could lead to new possibilities in the control of light at gigahertz and terahertz frequencies.

Multiferroic materials exhibit both magnetic order and an electric polarization property called ferroelectricity. These properties are determined by the polarization of electron 'spin' in the multiferroic lattice. Scientists have recently taken a particular interest in multiferroics with spiral or helical spin structures, known as helimagnets. Theory predicts that the combination of spin helicity and magnetization in these materials could result in a novel form of magnetoelectric coupling called magnetochiral dichroism, in which helical or chiral electric and magnetic polarizations combine to form a dynamic electromagnetic field that can interact with light passing through the material.

The multiferroic mixed oxide has a longitudinal helical or 'conical screw' spin structure due to competition between exchange interactions and geometric frustration among iron ions (Fig. 1). The conical screw spin structure provides the electronic chirality and magnetization that are required for magnetochiral dichroism. The inclusion of gallium in the material, a non-magnetic element, helps to stabilize the spin structure, which is needed to drive the novel optical phenomenon.

The researchers examined the multiferroic material using terahertz spectroscopy and discovered that a reversal of electronic chirality or magnetization reversed the direction of magnetochiral dichroism. They also found that the propagation of light against the direction of the magnetochiral dichroism was severely inhibited, with a high absorbance or extinction coefficient at gigahertz and terahertz frequencies.

The results suggest that multiferroic materials such as gallium-doped copper iron oxide could be used to control the propagation of electromagnetic waves in new ways, with implications for the development of isolators and optical devices that only permit the transmission of light in one direction.

Takahashi believes that magnetochiral dichroism, which differs from other forms of directional dichroism such as magnetic circular dichroism, is likely to be a common property of multiferroic materials with conical screw spin structures and arises through collective optically active spin wave excitations known as electromagnon resonance.

"This multiferroic material has an extinction coefficient of up to 400 per cent," says Takahashi. "Our achievement opens a new door for applications in gigahertz and terahertz optics, such as isolator and controllable filter devices."

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More information: Kibayashi, S., Takahashi, Y., Seki, S. & Tokura, Y. "Magnetochiral dichroism resonant with electromagnons in a helimagnet." Nature Communications 5, 4583 (2014). DOI: 10.1038/ncomms5583

Read more at: http://phys.org/news/2014-10-multiferroic-material.html#jCp