Colossal magnetoelectric effect at the critical point in Ni3TeO6 with the experimental group of S.-W. Cheong (Nature Communications, accepted)
The manipulation of magnetic ordering with applied electric fields is of pressing interest for information storage applications and spintronics. Recently, such magnetoelectric control was realized in multiferroics – materials with coexisting magnetic and ferroelectric orders. However, their magnetoelectric switching is accompanied by significant hysteresis, resulting from large barrier, separating different ferroic states. Hysteresis prevents robust switching, unless the applied field overcomes a certain value (coercive field). We address the role of a switching barrier on magnetoelectric control, and identify a material, collinear antiferromagnetic and pyroelectric Ni3TeO6, in which magnetoelectric switching across spin-flop transition is possible without hysteresis. The barrier between two magnetic states in the vicinity of a spin-flop transition is almost flat, and thus small changes in external electric or magnetic fields allow to switch the ferroic state through an intermediate state in a continuous manner, resulting in a colossal magnetoelectric response. This colossal magnetoelectric effect resembles the large piezoelectric effect at the morphotropic phase boundary in ferroelectrics.
Multiferroic domain walls and vortices in hexagonal YMnO3
It was experimentally observed that the structural domain walls coincide with the antiferromagnetic ones in multiferroic YMnO3. Furthermore, the experiments revealed the pattern of hexagonal structural vortices, with the polarization changing sign six times around the vortex core. We formulated the phenomenological model, describing the interacting orders in this material, and used it to calculate the shape of domain walls and vortices. Our results exhaustively explain the experimental observations.
Solitonic lattice and Yukawa forces in the rare earth orthoferrite TbFeO3
The neutron diffraction experiments on TbFeO3 revealed the unusual phase with sharp Tb kinks separated by distance of ~350 Å, appearing under an applied magnetic field. Within our phenomenological theory we reproduced the experimental phase diagram and understood the mechanism stabilizing these large separations between the Tb domain walls.
Ferromagnetic insulator state and magnetodielectric anomalies in Fe doped FeTiO3
Experiments revealed a very unusual ferromagnetic insulator state in the Fe:FeTiO3. The substitution of magnetic Fe impurities instead of non-magnetic Ti in A-type antiferromagnetic FeTiO3 induces locally strong ferromagnetic interaction between the layers, thus frustrating the AFM ordering. This stabilizes a peculiar spin texture, magnetic polaron, which carries large magnetic moment. We formu
lated a model describing these magnetization textures, calculated a shape and magnetic moment, and a ferromagnetic interaction between the interacting poalrons, responsible for the onset of FM state. The holes, introduced by doping, enlarge the polarons due to double exchange and lead to non-trivial magnetoelectric effect. But the inversion symmetry in the bulk is not broken, and the contribution to the magnetoelectric coupling from the polarons centered on odd and even layers of Ti are cancelled, although the magnetodielectric anomalies must be observed.
Magnetoelectric switching in GdFeO3
Recent experiments demonstrated the mutual control of ferroelectricity and ferromagnetism in GdFeO3. However the effect was small, and clear understanding of physics involved was lacking. We developed the phenomenological model describing the interacting orders of Fe and Gd and calculated the dynamics of a switching process within the simplified quasi-1D model.
Excitonic magnetoabsorption in Cu2O
Cu2O is the first semiconductor discovered, and also the first material in which excitons were observed. However for more than half-century of studies, the interpretation of magnetoabsorption spectra remaine
dincomplete, and there was even no agreement about the number of excitonic states absorbing the light of different polarization. The group of Paul van Loosdrecht did high-resolution polarized magnetoabsorption measurements, motivating the theoretical study. We calculated the excitonic energies both in high and low magnetic field regime and derived the selection rules, which allowed to fit the experimental spectra. We found that two lines in the n=2 level closely overlap, which resolved the long-standing controversy about the number of levels.
Magnetic excitations in the low-temperature ferroelectric phase of multiferroic YMn2O5
Multiferroic materials could potentially allow for the ultrafast switching of magnetization by polarization or light. For understanding of these phenomena the study of elementary excitations combining the magnon and phonon character is crucial. We studied the YMn2O5 with a very complex unit cell containing 8 magnetic ions, and having incommensurate magnetic spiral ground state with a period close to 4. We constrained the possible couplings, calculated the ground state and electromagnon spectra, which allowed us to interpret the data from inelastic neutron scattering.