No Arabic abstract
Recently, Yb-based triangular lattice antiferromagnets have garnered significant interest as possible quantum spin liquid candidates. One example is YbMgGaO4, which showed many promising spin liquid features, but also possesses a high degree of disorder owing to site-mixing between the non-magnetic cations. To further elucidate the role of chemical disorder and to explore the phase diagram of these materials in applied field, we present neutron scattering and sensitive magnetometry measurements of the closely related compound, YbZnGaO4. Our results suggest a difference in magnetic anisotropy between the two compounds, and we use key observations of the magnetic phase crossover to motivate an exploration of the field- and exchange parameter-dependent phase diagram, providing an expanded view of the available magnetic states in applied field. This enriched map of the phase space serves as a basis to restrict the values of parameters describing the magnetic Hamiltonian with broad application to recently discovered related materials.
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.
We present a magnetic phase diagram of rare-earth pyrochlore $rm{Yb_2Ti_2O_7}$ in a $langle 111 rangle$ magnetic field. Using heat capacity, magnetization, and neutron scattering data, we show an unusual field-dependence of a first-order phase boundary, wherein a small applied field increases the ordering temperature. The zero-field ground state has ferromagnetic domains, while the spins polarize along $langle 111 rangle$ above 0.65T. A classical Monte Carlo analysis of published Hamiltonians does account for the critical field in the low T limit. However, this analysis fails to account for the large bulge in the reentrant phase diagram, suggesting that either long-range interactions or quantum fluctuations govern low field properties.
Low-temperature, high-field (H[-110] <= 7.5 T), neutron diffraction experiments on single-crystal Ce0.70Pr0.30B6 are reported. Two successive incommensurate phases are found to exist in zero field. The appearance, for H >= 4.6 T at T = 2 K, of an antiferromagnetic structure, k{AF} = (1/2, 1/2, 1/2), most likely due to an underlying antiferroquadrupolar order, is discussed in connection with recent x-ray diffraction experiments.
We present measurements of the magnetoresistivity RHOxx of URu2Si2 single crystals in high magnetic fields up to 60 T and at temperatures from 1.4 K to 40 K. Different orientations of the magnetic field have been investigated permitting to follow the dependence on Q of all magnetic phase transitions and crossovers, where Q is the angle between the magnetic field and the easy-axis c. We find out that all magnetic transitions and crossovers follow a simple 1/cos(Q) -law, indicating that they are controlled by the projection of the field on the c-axis.
We have measured the magnetic field dependence of the paramagnetic to the field-induced high temperature antiferroquadrupolar magnetically ordered phase transition is CeB6 from 0 to 60 T using a variety of techniques. It is found that the field-dependent phase separation line becomes re-entrant above 35 T and below 10 K. Measurements of resonant ultra-sound, specific heat and neutron diffraction show conclusively that the zero-field temperature-dependent phase transition is to a state with no ordered dipole moments, but with second order transition signatures in both sound attenuation and specific heat.