Magnetocaloric effect in the high-temperature antiferromagnet YbCoC2

The magnetic $H$-$T$ phase diagram and magnetocaloric effect in the recently discovered high-temperature heavy-fermion compound YbCoC$_2$ have been studied. With the increase in the external magnetic field YbCoC$_2$ experiences the metamagnetic transition and then transition to the ferromagnetic state. The dependencies of magnetic entropy change -$\Delta S_m (T)$ have segments with positive and negative magnetocaloric effects for $\Delta H \leq 6$~T. For $\Delta H = 9$~T magnetocaloric effect becomes positive with a maximum value of -$\Delta S_m (T)$ is 4.1 J / kg K and a refrigerant capacity is 56.6 J / kg

Magnetic field dependence of the neutron spin resonance in CeB6

In zero magnetic field, the famous neutron spin resonance in the f-electron superconductor CeCoIn5 is similar to the recently discovered exciton peak in the non-superconducting CeB6. Magnetic field splits the resonance in CeCoIn5 into two components, indicating that it is a doublet. Here we employ inelastic neutron scattering (INS) to scrutinize the field dependence of spin fluctuations in CeB6. The exciton shows a markedly different behavior without any field splitting. Instead, we observe a second field-induced magnon whose energy increases with field. At the ferromagnetic zone center, however, we find only a single mode with a non-monotonic field dependence. At low fields, it is initially suppressed to zero together with the antiferromagnetic order parameter, but then reappears at higher fields inside the hidden-order phase, following the energy of an electron spin resonance (ESR). This is a unique example of a ferromagnetic resonance in a heavy-fermion metal seen by both ESR and INS consistently over a broad range of magnetic fields.Comment: 7 pages, 6 figures including one animation, accepted to Phys. Rev.

The Competition between Staggered Field and Antiferromagnetic Interactions in Cugeo3:Fe

The EPR spectra along different crystallographic axes for single crystals of CuGeO3 containing 1% of Fe impurity have been studied in the frequency range 60-360 GHz at temperatures 0.5-30 K. The analysis based on the Oshikawa-Affleck (OA) theory suggests that the temperature dependences of the line width and g-factor are formed as a result of the competition between interchain antiferromagnetic interactions and staggered Zeeman energy. It is found that staggered magnetic moments in CuGeO3:Fe are located predominantly along b axis.Comment: 9 pages, 4 figures; submitted to QSS04 symposiu

The spin-Peierls compound MEM(TCNQ)

The spin-Peierls state in the organic compound MEM(TCNQ)2 is studied by magnetisation measurements in magnetic fields up to 50\un{T}. The possible universality of the magnetic field vs. temperature magnetic phase diagram of spin-Peierls systems is investigated by comparing the results on MEM(TCNQ)2 with the corresponding data for the inorganic spin-Peierls compound \chem{CuGeO_3}. Although the transition line between the uniform and the dimerised phase follows the same dependence for both compounds, strong discrepancies are found for the phase boundary between the dimerised and the high-field magnetic phase

Angular Dependences of ESR Parameters in Antiferroquadrupolar Phase of CeB₆

Angular dependences of ESR line parameters (g-factor and linewidth Δ H) were experimentally explored in the antiferroquadrupolar phase of heavy fermion system CeB₆ at T=1.8 K. The data were obtained in two experimental geometries with different mutual directions of the wavevector k and the external magnetic field H at frequency of f=60 GHz. A g-factor anisotropy was found: while g-factors for [110] and [111] directions are close to each other (g ≈1.6), it is considerably higher (g ≈1.75) for [100]. The obtained angular dependence g(Θ) was compared with the theoretically predicted g-factor behavior for the Γp₈ state of Ce³⁺ ion in antiferroquadrupolar phase of CeB₆. It turns out that the experimental g-factor is considerably smaller at all angles than the theoretically calculated limits (2<g<2.2) and, moreover, it has a different symmetry. This result together with the strong linewidth anisotropy ΔH(Θ) demonstrates that for the relevant description of experimental data the theoretical model should take into account also the interaction of Ce³⁺ ion with itinerant 5d electrons