135 research outputs found
Theory of Magnetic-Field-Induced Polarization Flop in Spin-Spiral Multiferroics
The magnetic-field-induced 90-degree flop of ferroelectric polarization P in
a spin-spiral multiferroic material TbMnO3 is theoretically studied based on a
microscopic spin model. We find that the direction of the P-flop or the choice
of +Pa or -Pa after the flop is governed by magnetic torques produced by the
applied magnetic field H acting on the Mn spins and thus is selected in a
deterministic way, in contradistinction to the naively anticipated
probabilistic flop. This mechanism resolves a puzzle of the previously reported
memory effect in the P direction depending on the history of the magnetic-field
sweep, and enables controlled switching of multiferroic domains by externally
applied magnetic fields. Our Monte-Carlo analysis also uncovers that the
magnetic structure in the P||a phase under H||b is not a so-far anticipated
simple ab-plane spin cycloid but a conical spin structure.Comment: 7 pages, 4 figures, accepted for publication in Physical Review
Microwave magnetochiral effect in Cu2OSeO3
We theoretically find that in a multiferroic chiral magnet Cu2OSeO3, resonant
magnetic excitations are coupled to collective oscillation of electric
polarization, and thereby attain simultaneous activity to ac magnetic field and
ac electric field. Because of interference between these magnetic and electric
activation processes, this material hosts gigantic magnetochiral dichroism on
microwaves, that is, the directional dichroism at gigahertz frequencies in
Faraday geometry. The absorption intensity of microwave differs by as much as
30% depending on whether its propagation direction is parallel or antiparallel
to the external magnetic field.Comment: 5 pages, 5 figures, accepted for publication in Physical Review
Letter
Controlled creation of nanometric skyrmions using external magnetic fields
To exploit nanometric magnetic skyrmions as information carriers in
high-density storage devices, a method is needed that creates intended number
of skyrmions at specified places in the device preferably at a low energy cost.
We theoretically propose that using a system with a fabricated hole or notch,
the controlled creation of individual skyrmions can be achieved even when using
an external magnetic field applied to the entire specimen. The fabricated
defect turns out to work like a catalyst to reduce the energy barrier for the
skyrmion creation.Comment: 5 pages, 4 figure
Spin and Orbital States and Their Phase Transitions of the Perovskite-Type Ti Oxides: Weak coupling Approach
The magnetic phase diagram of the perovskite-type Ti oxides as a function of
the GdFeO3-type distortion is examined by using the Hartree-Fock analysis of a
multiband d-p Hamiltonian from a viewpoint of competitions of the spin-orbit
interaction, the Jahn-Teller (JT) level-splitting and spin-orbital
superexchange interactions. Near the antiferromagnetic (AFM)-to-ferromagnetic
(FM) phase boundary, A-type AFM [AFM(A)] and FM states accompanied by a certain
type of orbital ordering are lowered in energy at large JT distortion, which is
in agreement with the previous strong coupling study. With increasing the
GdFeO3-type distortion, their phase transition occurs. Through this magnetic
phase transition, the orbital state hardly changes, which induces nearly
continuous change in the spin coupling along the c-axis from negative to
positive. The resultant strong two-dimensionality in the spin coupling near the
phase boundary is attributed to the strong suppression of T_N and T_C, which is
experimentally observed. On the other hand, at small GdFeO3-type without JT
distortions, which correspond to LaTiO3, the most stable solution is not G-type
AFM [AFM(G)] but FM. Although the spin-orbit interaction has been considered to
be relevant at the small or no JT distortion of LaTiO3 in the literature, our
analysis indicates that the spin-orbit interaction is irrelevant to the AFM(G)
state in LaTiO3 and superexchange-type interaction dominates. On the basis of
further investigations on the nature of this FM state and other solutions, this
discrepancy is discussed in detail.Comment: 9 pages, 10 figure
Resonance modes and microwave driven translational motion of skyrmion crystal under an inclined magnetic field
We theoretically investigate the microwave-active resonance modes of a
skyrmion crystal on a thin-plate specimen under application of an external
magnetic field that is inclined from the perpendicular direction to the
skyrmion plane. In addition to the well-known breathing mode and two rotation
modes, we find novel resonance modes that can be regarded as combinations of
the breathing and rotation modes. Motivated by the previous theoretical work of
Wang [Phys. Rev. B {\bf 92}, 020403(R) (2015).], which demonstrated
skyrmion propagation driven by breathing-mode excitation under an inclined
magnetic field, we investigate the propagation of a skyrmion crystal driven by
these resonance modes using micromagnetic simulations. We find that the
direction and velocity of the propagation vary depending on the excited mode.
In addition, it is found that a mode with a dominant counterclockwise-rotation
component drives much faster propagation of the skyrmion crystal than the
previously studied breathing mode. Our findings enable us to perform efficient
manipulation of skyrmions in nanometer-scale devices or in magnetic materials
with strong uniaxial magnetic anisotropy such as GaVS and
GaVSe, using microwave irradiation.Comment: 9+ pages, 9 figures, to be published in Physical Review
Selective Activation of an Isolated Magnetic Skyrmion in a Ferromagnet with Microwave Electric Fields
We theoretically reveal that pure eigenmodes of an isolated magnetic skyrmion
embedded in a ferromagnetic environment can be selectively activated using
microwave electric fields without exciting gigantic ferromagnetic resonances,
in contrast to conventional methods using microwave magnetic fields. We also
demonstrate that this selective activation of a skyrmion can efficiently drive
its translational motion in a ferromagnetic nanotrack under application of an
external magnetic field inclined from the normal direction. We find that a mode
with combined breathing and rotational oscillations induces much faster
skyrmion propagation than the breathing mode studied previously by Wang et al.
[Phys. Rev. B 92, 020403(R) (2015)].Comment: 5 pages, 4 figure
Theoretically predicted picosecond optical switching of spin chirality in multiferroics
We show theoretically with an accurate spin Hamiltonian describing the
multiferroic Mn perovskites that the application of the picosecond optical
pulse with a terahertz frequency can switch the spin chirality through
intensely exciting the electromagnons. There are four states with different
spin chiralities, i.e. clockwise and counterclockwise ab/bc-plane spin spirals,
and by tuning the strength, shape and length of the pulse, the switching among
these states can be controlled at will. Dynamical pattern formation during the
switching is also discussed.Comment: 4+ pages, 5 figure
Dynamical magnetoelectric phenomena of multiferroic skyrmions
Magnetic skyrmions, vortex-like swirling spin textures characterized by a
quantized topological invariant, realized in chiral-lattice magnets are
currently attracting intense research interest. In particular, their dynamics
under external fields is an issue of vital importance both for fundamental
science and for technical application. Whereas observations of magnetic
skyrmions had been limited to metallic magnets so far, their realization was
discovered also in a chiral-lattice insulating magnet Cu2OSeO3 in 2012.
Skyrmions in the insulator turned out to exhibit multiferroic nature with
spin-induced ferroelectricity. Strong magnetoelectric coupling between
noncollinear skyrmion spins and electric polarizations mediated by relativistic
spin-orbit interaction enables us to drive motion and oscillation of magnetic
skyrmions by application of electric fields instead of injection of electric
currents. Insulating materials also provide an environment suitable for
detection of pure spin dynamics through spectroscopic measurements owing to
absence of appreciable charge excitations. In this article, we review recent
theoretical and experimental studies on multiferroic properties and dynamical
magnetoelectric phenomena of magnetic skyrmions in insulators. We argue that
multiferroic skyrmions show unique coupled oscillation modes of magnetizations
and polarizations, so-called electromagnon excitations, which are both
magnetically and electrically active, and interference between the electric and
magnetic activation processes leads to peculiar magnetoelectric effects in a
microwave frequency regime.Comment: 27 pages, 26 figures, invited review article, to be published in
Journal of Physics: Condensed Matte
Theory of Magnetism-Driven Negative Thermal Expansion in Inverse Perovskite Antiferromagnets
Magnetism-induced negative thermal expansion (NTE) observed in inverse
perovskite antiferromagnets Mn3AN (A=Zn, Ga, etc.) is theoretically studied by
a classical spin model with competing bond-length-dependent exchange
interactions. We numerically reproduce the crystal-volume expansion upon
cooling triggered by a non-coplanar antiferromagnetic order and show that the
expansion occurs so as to maximize an energy gain of the nearest-neighbor
antiferromagnetic interactions. This mechanism is not specific to inverse
perovskite magnets and might also be expected in magnets with other crystal
structures. We propose other candidate crystal structures that might exhibit
NTE through this mechanism.Comment: 6 pages, 5 figure
Theory of magnetic switching of ferroelectricity in spiral magnets
We propose a microscopic theory for magnetic switching of electric
polarization (P) in the spin-spiral multiferroics by taking TbMnO3 and DyMnO3
as examples. We reproduce their phase diagrams under a magnetic field H_ex by
Monte-Carlo simulation of an accurate spin model and reveal that competition
among the Dzyaloshinskii-Moriya interaction, spin anisotropy, and spin exchange
is controlled by the applied H_ex, resulting in magnetic transitions
accompanied by reorientation or vanishing of P. We also discuss the relevance
of the proposed mechanisms to many other multiferroics such as LiCu2O2, MnWO4,
and Ni3V2O4Comment: 4+ pages, 4 figure
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