No abstract

A field-induced magnetisation process in the frustrated antiferromagnets is often much richer compared to the materials without competing interactions. The applied field tends to stabilise unusual spin configurations which frequently results in the appearance of magnetisation plateaux. Here we report a study into the field-induced magnetisation of the two frustrated rare earth tetraborides, HoB4 and NdB4. NdB4 shows a fractional magnetisation plateau occurring at M/M sat ≈  before saturating in a field of 33 kOe. On cooling down to 0.5 K the temperature dependent susceptibility of NdB4 shows an unconventional transition where the system returns to the zero field antiferromagnetic state from a higher-temperature ferrimagnetic state. We are able to reconstruct the magnetic phase diagram of NdB4 from the magnetisation, susceptibility and resistivity measurements for both H  c and H ⊥ c. For HoB4, the most interesting behaviour is found at the lowest temperature of 0.5 K, where the field dependent magnetisation demonstrates a new fractional -magnetisation plateau. Further insight into the relations between the exchange interactions and single ion effects is gained through high-field magnetisation measurements in both HoB4 and NdB4.

The development of short-and long-range magnetic order induced in a frustrated zigzag ladder compound SrDy 2 O 4 by an applied field is studied using neutron-diffraction techniques. In zero field, SrDy 2 O 4 lacks long-range magnetic order down to temperatures as low as 60 mK, and the observed powder-neutron-diffraction (PND) patterns are dominated by very broad diffuse scattering peaks. Single-crystal neutron diffraction reveals that the zero-field magnetic structure consists of a collection of antiferromagnetic chains running along the c axis and that there is very little correlation between the chains in the ab plane. In an applied magnetic field, the broad diffuse scattering features in PND are gradually replaced by much sharper peaks, however, the pattern remains rather complex, reflecting the highly anisotropic nature of SrDy 2 O 4 . Single-crystal neutron diffraction shows that a moderate field applied along the b axis induces an up-up-down magnetic order associated with a 1 3 -magnetization plateau, in which magnetic correlation length in the ab plane is significantly increased, but it nevertheless remains finite. The resolution-limited k = 0 peaks associated with a ferromagnetic arrangement appear in powder and single-crystal neutron-diffraction patterns in fields of 2.5 T and above.