Session Z3: Response of Magnetism to Electric Fields and Light

Domains in multiferroics with magnetically induced ferroelectricity

Two types of multiferroics are distinguished. In the split-order-parameter multiferroics magnetic and ferroelectric order evolve independently while in the joint-order-parameter multiferroics the emergence of the spontaneous polarization is a direct consequence of the magnetic order. The latter type is particularly interesting because of the inherent giant magnetoelectric effects. In the joint-order-parameter multiferroics any magnetoelectric interaction is, at its root, an interaction of its magnetic and ferroelectric domains. Yet, very little is known about the topology of these domains. In my talk I will discuss the domain topology and its magnetoelectric manipulation in a variety of joint-order-parameter multiferroics: MnWO$_{4}$, RMn$_{2}$O$_{5}$, RMnO$_{3}$, CuO, CuCrO$_{2}$. Domains are resolved by optical second harmonic generation. Two types of unusual and fundamentally different domains will be distinguished: (i) hybrid-multiferroic domains in which hallmarks of magnetic and ferroelectric domains are inseparably entangled; (ii) incommensurate translation domains whose walls correspond to discontinuities in the incommensurate magnetic wave vector.   

Femtosecond response of exchange biased bilayers

Ultrafast heating by femtosecond laser pulses can decouple the ferromagnetic and antiferromagnetic layers in exchange biased bilayers, and induce a precession of the magnetization if a reverse magnetic field is applied. In Ni/FeF$_{2}$ bilayers, however, the ultrafast excitation produces novel magnetization dynamics that have not been observed before. An unexpected precession of the magnetization is initiated by a weak excitation, which does not decouple the layers, in reverse magnetic fields that exceed the exchange bias. The precession results from an abrupt change, as a function of the temperature, of the favorable orientation of frustrated spins at the interface. Another remarkable response is obtained when the laser heats the interface above the blocking temperature. The precession is then accompanied by reversal of the exchange bias. The reversal can be induced by a single excitation pulse, and shows that the antiferromagnet is also strongly affected by the optical perturbation. This non-trivial response cannot be extrapolated from the known slow dynamics of the bilayers, and provides important information on the physics of the interlayer coupling.   

Electric field modulation of magnetism in multiferroics

Multiferroics with coexistent ferroelectric and magnetic orders can provide an interesting laboratory to test unprecedented magnetoelectric responses and their possible applications. One such example is the dynamical and/or resonant coupling between the magnetic and electric dipoles in a solid. Here, as the examples of electric field modulation of magnetism in multiferroics, (1) the multiferroic domain wall dynamics and (2) the electric-dipole active magnetic responses are discussed with the review of recent experimental observations.   

Electrical control of exchange coupling in disordered multiferroics

The revival of the magnetoelectric (ME) effect [1] has vitally been boosted by recent intensified research on multiferroic materials [2], which promise to maximise the ME efficiency. While the primordial bilinear ME effect requires stringent symmetry properties, higher order ME effects are less demanding. In particular the biquadratic ME effect has recently attracted growing interest, $e.g$. in ferroelectromagnetic RE manganites, where it is related to the magnetocapacitance or magnetodielectric effect. In disordered systems with broken translational symmetry it is even dominating, while ME effects of lower order may be absent. In type I multiferroics, where magnetic and electric ordering have different origins, it controls the exchange interaction via quadratic spin-lattice interaction. This has been realized in the magnetic relaxor Pb(Fe$_{0.5}$Nb$_{0.5})$O$_{3}$, in quantum paraelectric EuTiO$_{3}$ and in the magnetoelectric multiglass Sr$_{0.98}$Mn$_{0.02}$TiO$_{3}$ [3]. We have measured nonlinear ME$_{E}$ effects in these `disordered multiferroics' using SQUID susceptometry [4] and interpret the \textit{EH}$^{2}$- and $E^{2}H^{2}$-type magnetoelectric (ME) effect in terms of electric field or polarization controlled exchange coupling. \\[4pt] [1] M. Fiebig, \textit{J. Phys}.\textit{ D: Appl. Phys.} \textbf{38}, R123 (2005) \\[0pt] [2] H. Schmid, \textit{Ferroelectrics }\textbf{221}, 9 (1999) \\[0pt] [3] V. V. Shvartsman \textit{et al.}, \textit{Phys. Rev. Lett}. \textbf{101}, 165704 (2008) \\[0pt] [4] P. Borisov \textit{et al., Rev. Sci. Instrum.} \textbf{78}, 106105 (2007)