85 research outputs found
Biquadratic antisymmetric exchange and the magnetic phase diagram of magnetoelectric CuFeO
Biquadratic {\it antisymmetric} exchange terms of the form , where is the
unit vector connecting sites and and , due partially to
magnetoelectric coupling effects, are shown to be responsible for the spin-flop
helical phase in CuFeO at low magnetic field and temperature. Usual
biquadratic {\it symmetric} exchange, likely due to magnetoelastic coupling, is
found to support the stability of axial magnetic states at higher fields in
this nearly-Heisenberg like stacked triangular antiferromagnet. A model
Hamiltonian which also includes substantial interplane and higher-neighbor
intraplane exchange interactions, reproduces the unique series of observed
commensurate and incommensurate periodicity phases with increasing applied
magnetic field in this highly frustrated system. The magnetic field-temperature
phase diagram is discussed in terms of a Landau-type free energy.Comment: 7 pages, 9 figure
Single domain magnetic helicity and triangular chirality in structurally enantiopure Ba3NbFe3Si2O14
A novel doubly chiral magnetic order is found out in the structurally chiral
langasite compound BaNbFeSiO. The magnetic moments are
distributed over planar frustrated triangular lattices of triangle units. On
each of these they form the same triangular configuration. This ferro-chiral
arrangement is helically modulated from plane to plane. Unpolarized neutron
scattering on a single crystal associated with spherical neutron polarimetry
proved that a single triangular chirality together with a single helicity is
stabilized in an enantiopure crystal. A mean field analysis allows discerning
the relevance on this selection of a twist in the plane to plane
supersuperexchange paths
Inversion symmetry breaking in noncollinear magnetic phase of a triangular lattice antiferromagnet CuFeO2
Magnetoelectric and magnetoelastic phenomena correlated with a phase
transition into noncollinear magnetic phase have been investigated for single
crystals of CuFeO2 with a frustrated triangular lattice. CuFeO2 exhibits
several long-wavelength magnetic structures related to the spin frustration,
and it is found that finite electric polarization, namely inversion symmetry
breaking, occurs with noncollinear but not at collinear magnetic phases. This
result demonstrates that the noncollinear spin structure is a key role to
induce electric polarization, and suggests that frustrated magnets which often
favor noncollinear configurations can be plausible candidates for
magnetoelectrics with strong magnetoelectric interaction.Comment: 15 page
Colossal magnetostriction and negative thermal expansion in the frustrated antiferromagnet ZnCr2Se4
A detailed investigation of ZnCr2Se4 is presented which is dominated by
strong ferromagnetic exchange but orders antiferromagnetically at T_N = 21 K.
Specific heat C and thermal expansion Delta L/L exhibit sharp first-order
anomalies at the antiferromagnetic transition. T_N is strongly reduced and
shifted to lower temperatures by external magnetic fields and finally is fully
suppressed suggesting a field induced quantum critical behavior close to 60
kOe. Delta L/L(T) is unusually large and exhibits negative thermal expansion
below 75 K down to T_N indicating strong frustration of the lattice.
Magnetostriction Delta L/L(H) reveals colossal values (0.5x10^{-3}) comparable
to giant magnetostriction materials. Electron-spin resonance, however, shows
negligible spin-orbital coupling excluding orbitally induced Jahn-Teller
distortions. The obtained results point to a spin-driven origin of the
structural instability at T_N explained in terms of competing ferromagnetic and
antiferromagnetic exchange interactions yielding strong bond frustration.Comment: 5 pages 4 figure
Complex-Orbital Order in Fe_3O_4 and Mechanism of the Verwey Transition
Electronic state and the Verwey transition in magnetite (Fe_3O_4) are studied
using a spinless three-band Hubbard model for 3d electrons on the B sites with
the Hartree-Fock approximation and the exact diagonalisation method.
Complex-orbital, e.g., 1/sqrt(2)[|zx> + i |yz>], ordered (COO) states having
noncollinear orbital moments ~ 0.4 mu_B on the B sites are obtained with the
cubic lattice structure of the high-temperature phase. The COO state is a novel
form of magnetic ordering within the orbital degree of freedom. It arises from
the formation of Hund's second rule states of spinless pseudo-d molecular
orbitals in the Fe_4 tetrahedral units of the B sites and ferromagnetic
alignment of their fictitious orbital moments. A COO state with longer
periodicity is obtained with pseudo-orthorhombic Pmca and Pmc2_1 structures for
the low-temperature phase. The state spontaneously lowers the crystal symmetry
to the monoclinic and explains experimentally observed rhombohedral cell
deformation and Jahn-Teller like distortion. From these findings, we consider
that at the Verwey transition temperature, the COO state remaining to be
short-range order impeded by dynamical lattice distortion in high temperature
is developed into that with long-range order coupled with the monoclinic
lattice distortion.Comment: 16 pages, 13 figures, 6 tables, accepted for publication in J. Phys.
Soc. Jp
Critical Scaling of the Magnetization and Magnetostriction in the Weak Itinerant Ferromagnet UIr
The weak itinerant ferromagnet UIr is studied by magnetization and
magnetostriction measurements. Critical behavior, which surprisingly extends up
to several Tesla, is observed at the Curie temperature K and is
analyzed using Arrott and Maxwell relations. Critical exponents are found that
do not match with any of the well-known universality classes. The
low-temperature magnetization below 3 T
rises towards higher fields and converges asymptotically around 50 T with the
magnetization at . From the magnetostriction and magnetization data, we
extract the uniaxial pressure dependences of , using a new method
presented here, and of . These results should serve as a basis for
understanding spin fluctuations in anisotropic itinerant ferromagnets.Comment: 4 pages, 3 figure
Magnetoelectric Effect in Magnetic Materials
Magnetoelectric effect in magnetic insulators is reviewed. Alter an intuitive explanation of the effect in antiferromagnetic CrO, the following topics are discussed on the experiments. Measurement and controlling of antiferromagnetic domains as well as antiferromagnetic domain wall motion. Expansion of the free energy of crystals in terms of magnetic and electric fields and polarizations. Information on the magnetic symmetry of crystals. Investigation of mechanisms of magnetoelectric effect. Excited states and optical observation
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