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Register a new userExceptional field dependence of antiferromagnetic magnons in LiFePO$_4$
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Low-energy magnon excitations in magnetoelectric LiFePO$_4$ have been investigated by high-frequency high-field electron spin resonance spectroscopy in magnetic fields up to B = 58 T and frequencies up to f = 745 GHz. For magnetic fields applied along the easy magnetic axis, the excitation gap softens and vanishes at the spin-flop field of BSF = 32 T before hardening again at higher fields. In addition, for B smaller than BSF we observe a resonance mode assigned to excitations due to Dzyaloshinskii-Moriya (DM)-interactions, thereby evidencing sizable DM interaction of approx 150 micro eV in LiFePO4. Both the magnetisation and the excitations up to high magnetic fields are described in terms of a mean-field theory model which extends recent zero field inelastic neutron scattering results. Our results imply that magnetic interactions as well as magnetic anisotropy have a sizable quadratic field dependence which we attribute to significant magnetostriction.
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Spin-wave dispersions in the antiferromagnetic state of single crystal LiFePO$_4$ were determined by inelastic neutron scattering measurements. The dispersion curves measured from the (010) reflection along both {it a}$^ast$ and {it b}$^ast$ reciprocal-space directions reflect the anisotropic coupling of the layered Fe$^{2+}$ (S = 2) spin-system. The spin-wave dispersion curves were theoretically modeled using linear spin-wave theory by including in the spin-Hamiltonian in-plane nearest- and next-nearest-neighbor interactions ({it J}$_1$ and {it J}$_2$), inter-plane nearest-neighbor interactions ({it J}$_bot$) and a single-ion anisotropy ({it D}). A weak (010) magnetic peak was observed in elastic neutron scattering studies of the same crystal indicating that the ground state of the staggered iron moments is not along (010) direction, as previously reported from polycrystalline samples studies, but slightly rotated away from this axis.
We report the magnetic phase diagram of single-crystalline LiFePO$_4$ in magnetic fields up to 58~T and present a detailed study of magneto-elastic coupling by means of high-resolution capacitance dilatometry. Large anomalies at tn in the thermal expansion coefficient $alpha$ imply pronounced magneto-elastic coupling. Quantitative analysis yields the magnetic Gruneisen parameter $gamma_{rm mag}=6.7(5)cdot 10^{-7}$~mol/J. The positive hydrostatic pressure dependence $dT_{rm N}/dp = 1.46(11)$~K/GPa is dominated by uniaxial effects along the $a$-axis. Failure of Gruneisen scaling below $approx 40$~K, i.e., below the peak temperature in the magneto-electric coupling coefficient [onlinecite{toft2015anomalous}], implies several competing degrees of freedom and indicates relevance of recently observed hybrid excitations~[onlinecite{yiu2017hybrid}]. A broad and strongly magnetic-field-dependent anomaly in $alpha$ in this temperature regime highlight the relevance of structure changes. Upon application of magnetic fields $B||b$-axis, a pronounced jump in the magnetisation implies spin-reorientation at $B_{rm SF} = 32$~T as well as a precursing phase at 29~T and $T=1.5$~K. In a two-sublattice mean-field model, the saturation field $B_{rm sat,b} = 64(2)$~T enables the determination of the effective antiferromagnetic exchange interaction $J_{rm af} = 2.68(5)$~meV as well as the anisotropies $D_{rm b} = -0.53(4)$~meV and $D_{rm c} = 0.44(8)$~meV.
Spin waves in chiral magnetic materials are strongly influenced by the Dzyaloshinskii-Moriya interaction resulting in intriguing phenomena like non-reciprocal magnon propagation and magnetochiral dichroism. Here, we study the non-reciprocal magnon spectrum of the archetypical chiral magnet MnSi and its evolution as a function of magnetic field covering the field-polarized and conical helix phase. Using inelastic neutron scattering, the magnon energies and their spectral weights are determined quantitatively after deconvolution with the instrumental resolution. In the field-polarized phase the imaginary part of the dynamical susceptibility $chi(varepsilon, {bf q})$ is shown to be asymmetric with respect to wavevectors ${bf q}$ longitudinal to the applied magnetic field ${bf H}$, which is a hallmark of chiral magnetism. In the helimagnetic phase, $chi(varepsilon, {bf q})$ becomes increasingly symmetric with decreasing ${bf H}$ due to the formation of helimagnon bands and the activation of additional spinflip and non-spinflip scattering channels. The neutron spectra are in excellent quantitative agreement with the low-energy theory of cubic chiral magnets with a single fitting parameter being the damping rate of spin waves.
We present a dynamical mean-field study of antiferromagnetic magnons in one-, two- and three-orbital Hubbard model of square and bcc cubic lattice at intermediate coupling strength. Weinvestigate the effect of anisotropy introduced by an external magnetic field or single-ion anisotropy.For the latter we tune continuously between the easy-axis and easy-plane models. We also analyzea model with spin-orbit coupling in cubic site-symmetry setting. The ordered states as well as themagnetic excitations are sensitive to even a small breaking ofSU(2)symmetry of the model andfollow the expectations of spin-wave theory as well as general symmetry considerations.
We investigated effects of magnetic field H on antiferromagnetic (AF) structures in CeRh_{1-x}Co_xIn_5 by performing the elastic neutron scattering measurements. By applying H along the [1,-1,0] direction, the incommensurate AF state with the propagation vector of q_{h1}=(1/2,1/2,0.297) observed at H=0 is replaced by the commensurate AF state with the q_{c2} = (1/2, 1/2, 1/4) modulation above 2 T for x=0.23, while the AF states with the q_{c1}=(1/2,1/2,1/2) and q_{h2}=(1/2,1/2,0.42) modulations seen at H=0 change into a single q_{c1}-AF state above ~1.6 T for x=0.7. These results suggest the different types of AF correlation for Co concentrations of 0.23 and 0.7 in an applied magnetic field H.
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