M. Frontzek

Frustration in R2PdSi3 (R = Tb,Er) compounds: spin-glass or magnetic short range order? Neutron diffraction studies

Measurements of macroscopic magnetic properties of the isostructural compounds Tb2PdSi3 and Er2PdSi3 show an additional phase transition below the ordering temperature. The behaviours of the two compounds are similar and are interpreted as spin-glass-like phase transitions in the literature. In this contribution, we present detailed neutron diffraction studies of the mechanism of the spin-glass-like phase transition on a microscopic scale. We find a fundamental difference between the two compounds in their magnetic structures and, therefore, the spin-glass-like phase transitions. In the Tb2PdSi3 compound an additional antiferromagnetic short range ordered phase is found within the long range ordered phase. The appearance of the short range order is linked to the spin-glass-like transition. In contrast, the Er2PdSi3 compound shows only long range order. However, the antiferromagnetic order undergoes a modification within the ordered state. The temperature of the transition matches the spin-glass-like transition.

Magnetic excitations of Er2PdSi3 studied by inelastic neutron scattering in fields up to 12 T

The series R2PdSi3 (R = heavy rare earth) has been found to exhibit rich magnetic phase diagrams with a large variety of magnetic states. This complex magnetic behavior results from the interplay between RKKY interaction, magneto-crystalline anisotropy based on crystalline-electric field (CEF) effects and geometric frustration due to the AlB2 derived hexagonal crystal structure. Within the series the Er2PdSi3 compound has been found to exhibit in zero field a simple CEF excitation spectrum with just one strong transition at 3.5 meV from the ground state to the first excited state. Neutron spectroscopy in magnetic fields up to 12 T has been performed on the cold-triple axis spectrometer PANDA at FRM II. In this study two transitions within the 3.5 meV excitation could be resolved showing a linear dependency in applied magnetic fields with an almost identical slope of μBgL for fields above 2 T. The results can be explained in a picture of two slightly different sets of CEF parameters for two inequivalent Er3+ sites due the crystallographic superstructure observed in all single crystals of type R2PdSi3.

A Generic Phase Diagram for R2PdSi3 (R = Heavy Rare Earth)?

The magnetic phase diagrams of hexagonal R2PdSi3 compounds have been found to feature an apparent similar phase when the field is applied along the respective magnetic easy axes. To elucidate this, neutron diffraction on single crystalline Tb2PdSi3 and Ho2PdSi3 in magnetic fields was performed. These two compounds have different magnetic structures in zero field. In Tb2PdSi3 the propagation vector is parallel to the hexagonal axis while in Ho2PdSi3 the propagation is in the basal plane. In both compounds the magnetic moment direction is perpendicular to the propagation vector. The application of a magnetic field along the respective easy axes destroys the zero field magnetic structure and leads to a ferrimagnetic phase with magnetic intensity on the same positions for both compounds. These findings and their connections to the crystallographic (super-)structure are presented and discussed.

Neutron diffraction study of magnetic structures in single crystal Ho2PdSi3 in magnetic fields up to 5 T

Due to the interplay between RKKY exchange interaction, crystalline-electric field (CEF) effects and geometric frustration due to the AlB2-derived hexagonal crystal structure, the study of R2PdSi3 (R = rare earth) has been found to be a challenging field in rare earth magnetism. In this contribution we present the results of neutron diffraction experiments on a Ho2PdSi3 single crystal in the magnetically ordered state at T = 1.6 K. The compound orders antiferromagnetically in the basal plane with the propagation vector ? = (1/7-?, 2?, 0) where ? ~ 0.01 r.l.u. In magnetic fields applied along the (10) magnetic hard axis, the structure persists up to the highest measured field of 5 T. The dependence of the intensities as a function of field suggests that the ? structure is a single-k multi-domain structure.