Dennis Meier

Magnetoelectric domain control in multiferroic TbMnO3

Visualizing ferroelectric domains Multiferroic materials support intertwined ferromagnetic and ferroelectric orders, with the magnetic field capable of controlling the electric order and vice versa. Matsubara et al. used second harmonic generation microscopy to visualize what happens to the ferroelectric domains in the multiferroic TbMnO3 when an externally applied magnetic field changes the direction of electric polarization by 90°. Unexpectedly, the domain walls, initially parallel to the polarization vector, did not change their shape or position. The resulting transition from neutral to charged domain walls may help in the development of future ferroelectric devices. Science, this issue p. 1112 Second harmonic generation microscopy visualizes the evolution of ferroelectric domains under an external magnetic field. The manipulation of domains by external fields in ferroic materials is of major interest for applications. In multiferroics with strongly coupled magnetic and electric order, however, the magnetoelectric coupling on the level of the domains is largely unexplored. We investigated the field-induced domain dynamics of TbMnO3 in the multiferroic ground state and across a first-order spin-flop transition. In spite of the discontinuous nature of this transition, the reorientation of the order parameters is deterministic and preserves the multiferroic domain pattern. Landau-Lifshitz-Gilbert simulations reveal that this behavior is intrinsic. Such magnetoelectric correlations in spin-driven ferroelectrics may lead to domain wall–based nanoelectronics devices.

Ferroic nature of magnetic toroidal order

Electric dipoles and ferroelectricity violate spatial inversion symmetry, and magnetic dipoles and ferromagnetism break time-inversion symmetry. Breaking both symmetries favours magnetoelectric charge-spin coupling effects of enormous interest, such as multiferroics, skyrmions, polar superconductors, topological insulators or dynamic phenomena such as electromagnons. Extending the rationale, a novel type of ferroic order violating space- and time-inversion symmetry with a single order parameter should exist. This existence is fundamental and the inherent magnetoelectric coupling is technologically interesting. A uniform alignment of magnetic vortices, called ferrotoroidicity, was proposed to represent this state. Here we demonstrate that the magnetic vortex pattern identified in LiCoPO4 exhibits the indispensable hallmark of such a ferroic state, namely hysteretic poling of ferrotoroidic domains in the conjugate toroidal field, along with a distinction of toroidal from non-toroidal poling effects. This consolidates ferrotoroidicity as fourth form of ferroic order.

Functional domain walls in multiferroics.

During the last decade a wide variety of novel and fascinating correlation phenomena has been discovered at domain walls in multiferroic bulk systems, ranging from unusual electronic conductance to inseparably entangled spin and charge degrees of freedom. The domain walls represent quasi-2D functional objects that can be induced, positioned, and erased on demand, bearing considerable technological potential for future nanoelectronics. Most of the challenges that remain to be solved before turning related device paradigms into reality, however, still fall in the field of fundamental condensed matter physics and materials science. In this topical review seminal experimental findings gained on electric and magnetic domain walls in multiferroic bulk materials are addressed. A special focus is put on the physical properties that emerge at so-called charged domain walls and the added functionality that arises from coexisting magnetic order. The research presented in this review highlights that we are just entering a whole new world of intriguing nanoscale physics that is yet to be explored in all its details. The goal is to draw attention to the persistent challenges and identify future key directions for the research on functional domain walls in multiferroics.

Hysteresis effects in the phase diagram of multiferroic GdMnO 3

We present high-resolution thermal expansion

New features in the phase diagram of TbMnO3

\ensuremath{\alpha}(T)

Nature of low-temperature phase transitions in CaMn7O12

and magnetostriction

Local dynamics of topological magnetic defects in the itinerant helimagnet FeGe

\mathrm{\ensuremath{\Delta}}L(H)∕L

Functional electronic inversion layers at ferroelectric domain walls

measurements of

Incompatible Magnetic Order in Multiferroic Hexagonal DyMnO3

{\mathrm{GdMnO}}_{3}

Second harmonic generation on incommensurate structures: The case of multiferroic MnWO4

, which develops an incommensurate antiferromagnetic order (ICAFM) below