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-Resolving the spin reorientation and crystal-field transitions in TmFeO3 with terahertz transient

• Kailin Zhang
• , Kai Xu
• , Xiumei Liu
• , Zeyu Zhang
• , Zuanming Jin
• , Xian Lin
• , Bo Li
• , Shixun Cao
•  &Guohong Ma

http://www.nature.com/articles/srep23648
Rare earth orthoferrites (RFeO3) exhibit abundant physical properties such as, weak macroscopic magnetization, spin reorientation transition, and magneto-optical effect, especially the terahertz magnetic response, have received lots of attention in recent years. In this work, quasi-ferromagnetic (FM) and quasi-antiferromagnetic (AFM) modes arising from Fe sublattice of TmFeO3 single crystal are characterized in a temperature range from 40 to 300 K, by using terahertz time-domain spectroscopy (THz-TDS). The magnetic anisotropy constants in ac-plane are estimated according to the temperature-dependent resonant frequencies of both FM and AFM modes. Here, we further observe the broad-band absorptions centered ~0.52, ~0.61, and ~1.15 THz below 110 K, which are reasonably assigned to a series of crystal-field transitions (R modes) of ground multiplets (6H3) of Tm3+ ions. Specially, our finding reveals that the spin reorientation transition at a temperature interval from 93 to 85 K is driven by magnetic anisotropy, however, which plays negligible role on the electronic transitions of Tm ions in the absence of applied magnetic fields.