Magnetic State Switching in FeGa Microstructures

This work demonstrates that magnetoelectric composite heterostructures can be designed at the length scale of 10 microns that can be switched from a magnetized state to a vortex state, effectively switching the magnetization off, using electric field induced strain. This was accomplished using thin film magnetoelectric heterostructures of Fe81.4Ga18.6 on a single crystal (011) [Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32 (PMN-32PT) ferroelectric substrate. The heterostructures were tripped from a multi-domain magnetized state to a flux closure vortex state using voltage induced strain in a piezoelectric substrate. FeGa heterostructures were deposited on a Si-substrate for SQUID magnetometry characterization of the magnetic properties. The magnetoelectric coupling of a FeGa continuous film on PMN-32PT was characterized using a MOKE magnetometer with bi-axial strain gauges, and magnetic multi-domain heterostructures were imaged using X-Ray Magnetic Circular Dichroism – Photoemission Electron Microscopy (XMCD-PEEM) during the transition to the vortex state. The domain structures were modelled using MuMax3, a micromagnetics code, and compared with observations. The results provide considerable insight into designing magnetoelectric heterostructures that can be switched from an “on” state to an “off” state using electric field induced strain.

Short-Range Berezinskii-Kosterlitz-Thouless Phase Characterization for the q-State Clock Model

Beyond the usual ferromagnetic and paramagnetic phases present in spin systems, the usual q-state clock model presents an intermediate vortex state when the number of possible orientations q for the system is greater than or equal to 5. Such vortex states give rise to the Berezinskii-Kosterlitz-Thouless (BKT) phase present up to the XY model in the limit q→∞. Based on information theory, we present here an analysis of the classical order parameters plus new short-range parameters defined here. Thus, we show that even using the first nearest neighbors spin-spin correlations only, it is possible to distinguish the two transitions presented by this system for q greater than or equal to 5. Moreover, the appearance at relatively low temperature and disappearance of the BKT phase at a rather fix higher temperature is univocally determined by the short-range interactions recognized by the information content of classical and new parameters.

Short-range Berezinskii-Kosterlitz-Thouless Phase Characterization for the q-state Clock Model

Beyond the usual ferromagnetic and paramagnetic phases present in spin systems, the usual q-state clock model, presents an intermediate vortex state when the number of possible orientations q for the system is equal to 5 or larger. Such vortex states give rise to the Berezinskii-Kosterlitz-Thouless (BKT) phase present up to the XY model in the limit q→∞. Based on information theory, we present here an analysis of the classical order parameters plus new short-range parameters defined here. Thus, we show that even using the first nearest neighbors spin-spin correlations only, it is possible to distinguish the two transitions presented by this system for q greater than or equal to 5. Moreover, the appearance at relatively low temperature and disappearance of the BKT phase at a rather fix higher temperature is univocally determined by the short-range interactions recognized by the information content of classical and new parameters.

Geometric frustration in polygons of polariton condensates creating vortices of varying topological charge

Vorticity is a key ingredient to a broad variety of fluid phenomena, and its quantised version is considered to be the hallmark of superfluidity. Circulating flows that correspond to vortices of a large topological charge, termed giant vortices, are notoriously difficult to realise and even when externally imprinted, they are unstable, breaking into many vortices of a single charge. In spite of many theoretical proposals on the formation and stabilisation of giant vortices in ultra-cold atomic Bose-Einstein condensates and other superfluid systems, their experimental realisation remains elusive. Polariton condensates stand out from other superfluid systems due to their particularly strong interparticle interactions combined with their non-equilibrium nature, and as such provide an alternative testbed for the study of vortices. Here, we non-resonantly excite an odd number of polariton condensates at the vertices of a regular polygon and we observe the formation of a stable discrete vortex state with a large topological charge as a consequence of antibonding frustration between nearest neighbouring condensates.

Magnetic Sans of Nanoparticles and Complex Systems

This chapter provides an overview on the magnetic SANS of nanoparticles and complex systems, which include ferrofluids, magnetic steels, and spin glasses and amorphous magnets. The underlying assumptions of the conventional particle-matrix-based model of magnetic SANS, which assumes uniformly magnetized domains, characteristic e.g., for superparamagnets, are discussed and we provide a complete specification of the micromagnetic boundary-value problem. First attempts to provide analytical expressions for the vortex-state-related magnetic SANS of thin circular discs are considered.

Micromagnetic simulation of taenite particles

<p></p><div> <div> <div>The cloudy zone (CZ), an intergrowth structure of Fe-rich and Ni-rich phases that forms during slow cooling of iron meteorites are potential recorders of  their parent body’s thermal and magnetic history. The ability of the cloudy zone’s principal magnetic minerals, taenite and tetrataenite, to reliably record ancient magnetic fields from the early solar system has, however, insufficiently been investigated. In this work we performed a series of micromagnetic simulations in order to assess the magnetic stability of taenite grains. Micromagnetic simulations allow to investigate the changes in the magnetic state in taenite as a function of the grain size: in ellipsoidal grains below 68 nm (equivalent sphere volume diameter, ESVD) a single domain state dominates.  At 68 nm (ESVD) a “flowering” state starts, and further increase in size (> 75 nm) gives rise to a single vortex state. Contrary to common conception, theoretical evaluation of relaxation times for taenite grains based on micromagnetics leads to values that exceed the age of solar system, which makes taenite, not just its ordered equivalent tetrataenite, a reliable paleomagnetic recorder.</div> </div> </div>