[en] We measured absorption of THz radiation in YFeO₃ single crystals at a temperature of 3 K in the magnetic field up to 17 T applied in all three crystallographic directions. Two spin-wave modes were observed at the Γ point with energies 1.2 meV (9.8 cm^–1) and 2.4 meV (19.3 cm^–1) in zero field. From the magnetic-field dependence of mode energies, we have refined the previously proposed model [S. E. Hahn et al., Phys. Rev. B 89, 014420 (2014)] and quantified the parameters of Dzyaloshinskii-Moriya interactions and single-ion anisotropies. Authors:

[en] This work examines the magnetic order and spin dynamics of a double-exchange model with competing ferromagnetic and antiferromagnetic Heisenberg interactions between the local moments. The Heisenberg interactions are periodically arranged in a Villain configuration in two dimensions with nearest-neighbour, ferromagnetic coupling J and antiferromagnetic coupling -ηJ. This model is solved at zero temperature by performing a 1/√S expansion in the rotated reference frame of each local moment. When η exceeds a critical value, the ground state is a magnetically frustrated, canted antiferromagnet. With increasing hopping energy t or magnetic field B, the local moments become aligned and the ferromagnetic phase is stabilized above critical values of t or B. In the canted phase, a charge-density wave forms because the electrons prefer to sit on lines of sites that are coupled ferromagnetically. Due to a change in the topology of the Fermi surface from closed to open, phase separation occurs in a narrow range of parameters in the canted phase. In zero field, the long-wavelength spin waves are isotropic in the region of phase separation. Whereas the average spin-wave stiffness in the canted phase increases with t or η, it exhibits a more complicated dependence on field. This work strongly suggests that the jump in the spin-wave stiffness observed in Pr_1-xCa_xMnO₃ with 0.3 ≤ x ≤ 0.4 at a field of 3 T is caused by the delocalization of the electrons rather than by the alignment of the antiferromagnetic regions

[en] This work develops a generalized technique for determining the static and dynamic properties of any non-collinear magnetic system. By rotating the spin operators into the local spin reference frame, we evaluate the zeroth, first, and second order terms in a Holstein-Primakoff expansion, and through a Green's functions approach, we determine the structure factor intensities for the spin-wave frequencies. To demonstrate this technique, we examine the spin-wave dynamics of the generalized Villain model with a varying interchain interaction. The new interchain coupling expands the overall phase diagram with the realization of two non-equivalent canted spin configurations. The rotational Holstein-Primakoff expansion provides both analytical and numerical results for the spin dynamics and intensities of these phases.

[en] We examine the spin-wave (SW) dynamics of Dy/Y multilayers in order to separate the contribution of the Dy-Y interface from that of bulk Dy. The SW frequencies and intensities of bulk Dy are determined analytically. When the Dy layers in a multilayer geometry are decoupled, the SW dispersion relations are discontinuous with discrete excitations. With a Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction coupling through the Y spacer, the discrete excitations become dispersive and the main SW branches split due to the multilayer geometry. Regardless of the strength of the intermediate RKKY interaction, the dispersion signature of the bulk remains.

[en] Cd₂Os₂O₇ crystallizes in the pyrochlore structure and undergoes a metal-insulator transition (MIT) near 226 K. We have characterized the MIT in Cd₂Os₂O₇ using x-ray diffraction, resistivity at ambient and high pressure, specific heat, magnetization, thermopower, Hall coefficient, and thermal conductivity. Both single crystals and polycrystalline material were examined. The MIT is accompanied by no change in crystal symmetry and a change in unit-cell volume of less than 0.05%. The resistivity shows little temperature dependence above 226 K, but increases by 3 orders of magnitude as the sample is cooled to 4 K. The specific heat anomaly resembles a mean-field transition and shows no hysteresis or latent heat. Cd₂Os₂O₇ orders magnetically at the MIT. The magnetization data are consistent with antiferromagnetic order, with a small parasitic ferromagnetic component. The Hall and Seebeck coefficients are consistent with a semiconducting gap opening at the Fermi energy at the MIT. We have also performed electronic structure calculations on Cd₂Os₂O₇. These calculations indicate that Cd₂Os₂O₇ is metallic, with a sharp peak in the density of states at the Fermi energy. We interpret the data in terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic order. A Slater transition -- unlike a Mott transition -- is predicted to be continuous, with a semiconducting energy gap opening much like a BCS gap as the material is cooled below T_MIT

[en] The high-temperature ferromagnet MnBi continues to receive attention as a candidate to replace rare-earth-containing permanent magnets in applications above room temperature. This is due to a high Curie temperature, large magnetic moments, and a coercivity that increases with temperature. The synthesis of MnBi also allows for crystals that are free of interstitial Mn, enabling more direct access to the key interactions underlying the physical properties of binary Mn-based ferromagnets. In this work, we use inelastic neutron scattering to measure the spin waves of MnBi in order to characterize the magnetic exchange at low temperature. Consistent with the spin reorientation that occurs below 140 K, we do not observe a spin gap in this system above our experimental resolution. A Heisenberg model was fit to the spin wave data in order to characterize the long-range nature of the exchange. It was found that interactions up to sixth nearest neighbor are required to fully parametrize the spin waves. Surprisingly, the nearest-neighbor term is antiferromagnetic, and the realization of a ferromagnetic ground state relies on the more numerous ferromagnetic terms beyond nearest neighbor, suggesting that the ferromagnetic ground state arises as a consequence of the long-ranged interactions in the system.

[en] When the Ni concentration exceeds about 18%, Mn-Ni alloys were expected to support two different noncollinear spin-density wave (SDW) phases. A triple-Q SDW with moments along the crystal diagonals was believed to appear in the fcc phase between T_N and T_t. Below T_t, the fct phase with c>a was believed to contain a double-Q SDW with moments in the ab plane and at 45 degree sign angles from the crystal axes. Based on resistivity, neutron-scattering, and susceptibility measurements, we show that the structural and magnetic phase transitions in a Mn_1-xNi_x alloy with x≅0.20 are actually distinct, with the structural phase transition at T_t≅250 K lying far above the magnetic transition at T_m≅125 K. A Hamiltonian which includes elastic, magnetoelastic, and noncollinearity energies is used to describe these two transitions. In the tetragonal phase between T_t and T_m, our model predicts a new SDW phase with moments tilted away from the crystal diagonals toward the ab plane. The energy gap in the spin-wave spectrum is predicted to change discontinuously at T_m. (c) 2000 The American Physical Society

[en] The spin-wave excitations of the geometrically frustrated triangular lattice antiferromagnet CuFeO₂ have been measured using high resolution inelastic neutron scattering. Antiferromagnetic interactions up to third nearest neighbors in the ab plane (J₁, J₂, J₃, with J₂/J₁≅0.44 and J₃/J₁≅0.57), as well as out-of-plane coupling (J_z, with J_z/J₁≅0.29) are required to describe the spin-wave dispersion relations, indicating a three-dimensional character of the magnetic interactions. Two energy dips in the spin-wave dispersion occur at the incommensurate wave vectors associated with multiferroic phase and can be interpreted as dynamic precursors to the magnetoelectric behavior in this system