Coherent electric field manipulation of Fe3+ spins in PbTiO3

Magnetoelectrics, materials that exhibit coupling between magnetic and electric degrees of freedom, not only offer a rich environment for studying the fundamental materials physics of spin-charge coupling but also present opportunities for future information technology paradigms. We present results of electric field manipulation of spins in a ferroelectric medium using dilute ferric ion–doped lead titanate as a model system. Combining first-principles calculations and electron paramagnetic resonance (EPR), we show that the ferric ion spins are preferentially aligned perpendicular to the ferroelectric polar axis, which we can manipulate using an electric field. We also demonstrate coherent control of the phase of spin superpositions by applying electric field pulses during time-resolved EPR measurements. Our results suggest a new pathway toward the manipulation of spins for quantum and classical spintronics.

BaTiO3 nanoparticle-decorated hierarchical Ni nanowire assemblies for magnetoelectric applications

One dimensional multiferroic systems with spin-charge coupling at room temperature are highly useful in future multifunctional devices.

Optical control of competing exchange interactions and coherent spin-charge coupling in two-orbital Mott insulators

In order to have a better understanding of ultrafast electrical control of exchange interactions in multi-orbital systems, we study a two-orbital Hubbard model at half filling under the action of a time-periodic electric field. Using suitable projection operators and a generalized time-dependent canonical transformation, we derive an effective Hamiltonian which describes two different regimes. First, for a wide range of non-resonant frequencies, we find a change of the bilinear Heisenberg exchange J_{\textrm{ex}}Jex that is analogous to the single-orbital case. Moreover we demonstrate that also the additional biquadratic exchange interaction B_{\textrm{ex}}Bex can be enhanced, reduced and even change sign depending on the electric field. Second, for special driving frequencies, we demonstrate a novel spin-charge coupling phenomenon enabling coherent transfer between spin and charge degrees of freedom of doubly ionized states. These results are confirmed by an exact time-evolution of the full two-orbital Mott-Hubbard Hamiltonian.

Multiferroicity from charge ordering? A case study

Magnetoelectric multiferroics have a large applications potential. Among possible mechanisms of multiferroicity, ferroelectricity originating from charge ordering (CO) is particularly intriguing because it potentially combines large electric polarizations with strong magnetoelectric couplings – but example materials where this is realized are very difficult to find. I will present a case study of such materials, rare earth ferrites, which were recently found to be a non-example. After LuFe2O4 had been proposed to be a multiferroic due to ferroelectricity originating from Fe2+/Fe3+ CO below 320 K, it has become the generally accepted prototypical example of this mechanism, attracting increasing attention. The proposal had been made due to indications of a polar state by dielectric and pyroelectric measurements, and a reasonable model of CO based on the location of superstructure reflections. This model has not been verified though, and the spin-order has been unknown. I will present x-ray and neutron diffraction, and circular dichroism, measurements that allowed i) a full refinement of the CO crystal structure [1], ii) the determination of the spin structures in two nearly degenerate competing magnetic phases [2], and iii) the relation between these orderings [1]. The results reveal a very strong spin-charge coupling. Most importantly, the unambiguously determined arrangement of Fe2+ and Fe3+ ions excludes any CO-based ferroelectricity – suggesting that LuFe2O4 and other rare earth ferrites are not ferroelectric [3]. Time permitting I will also briefly discuss recent time-resolved diffraction experiments on magnetite, another (more likely) candidate material for CO-based ferroelectricity.