Biquadratic antisymmetric exchange and the magnetic phase diagram of magnetoelectric CuFeO2_2

Biquadratic {\it antisymmetric} exchange terms of the form $- [C_{ij} e^{\alpha}_{ij}({\bf s}_i\times{\bf s}_j)_z]^2$, where ${\bf e}_{ij}$ is the unit vector connecting sites $i$ and $j$ and $\alpha = x,y$, due partially to magnetoelectric coupling effects, are shown to be responsible for the spin-flop helical phase in CuFeO$_2$ 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 Ba$_3$NbFe$_3$Si$_2$O$_{14}$. 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 $T_C\simeq45$ 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 $M_s\simeq0.5$ $\mu_B \cong const.$ below 3 T rises towards higher fields and converges asymptotically around 50 T with the magnetization at $T_C$. From the magnetostriction and magnetization data, we extract the uniaxial pressure dependences of $T_C$, using a new method presented here, and of $M_s$. 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 Cr$\text{}_{2}$O$\text{}_{3}$, 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