Magnetoresistivity and Monte Carlo studies of magnetic phase transitions inC6Eu

The high-field magnetoresistivity ρ(H) of the antiferromagnetic first-stage graphite intercalation compound ${\mathrm{C}}_6$Eu has been measured with H⊥c^ and H∥c^. Both the longitudinal (J∥H) magnetoresistivity ${\rho }_l$($H_{\perp }$) and the transverse (J⊥H) magnetoresistivity ${\rho }_t$($H_{\perp }$) with H⊥c^ show distinct changes across the magnetic phase boundaries which occur at fields of 1.5, 8, 15, and 21.5 T at a temperature T=4.2 K. The phase transition at H=15 T was not observed previously by the pulsed magnetization measurements. A Monte Carlo simulation based on the Hamiltonian of Sakakibara and Date was carried out for the ${\mathrm{C}}_6$Eu system. The 15-T phase transition is explained as a transition from a ‘‘canted’’ to a ‘‘fan’’ state. The transverse magnetoresistivity ${\rho }_t$($H_{?}$) with H?c^ shows a clear anomaly at the field corresponding to the onset of the transition to the spin-aligned paramagnetic state. A magnetic phase diagram has been accurately determined based on the results of the magnetoresistivity measurements. The various spin configurations in the phase diagram are identified and the parameters of the Hamiltonian are determined using the results of the Monte Carlo simulation.