Magnetic phase diagram of CeCu2Ge2 up to 15 T: On the route to understand field-induced phase transitions
AbstractThe features of the magnetic (H,T) phase diagram of CeCu2Ge2 are similar to those of superconducting CeCu2Si2, but the nature of these phases and transitions is still controversial. For CeCu2Ge2 we present results on electrical transport, thermodynamic measurements (magnetization, magnetostriction), and elastic neutron diffraction for fields up to 15 T parallel to the  direction. Two magnetic phases AF1, AF2 and a third, yet unidentified ferrimagnetic phase AF3 exist below TN=4.2 K and in fields up to approximately 26 T. At temperatures below 2.5 K a first-order transition from AF1 to AF2 at around 7.8 T was found experimentally, characterized by a shift of the observed propagation vector from q1=(0.285−0.2850.543) to q2=(0.34−0.270.55). Above 12.5 T reflections belonging neither to the AF1 nor to the AF2 type were found. To interpret the macroscopic measurements and neutron data a mean-field simulation with the McPhase program was carried out, yielding a low-field double-q magnetic structure AF1 with q1±=(0.278±0.2780.556) that jumps to AF2 with q2±=(0.286±0.2860.545) at about 5 T (to be compared to the experimental value of 7.8 T). This transition is followed by a single-q structure AF3 with q3=(0.280.280.56) at 10 T (as compared to 12.5 T from experiment) that is stable up to saturation at 26 T. These calculations also reveal the principal dependence of the experimental magnetization and susceptibility published earlier. The predicted single-q structure was not detectable by neutrons because of limitations in the employed scattering geometry.