Magnetic field induced irreversibility in UNiAl

We report on low-temperature measurements of magnetization, magnetoresistiv ity, Hall resistivity, magnetostriction, and elastic moduli on a UNiAl sing le crystal in magnetic fields up to 14 T applied along the c axis of the he xagonal structure. UNiAl is an itinerant 5f-electron antiferromagnet below T-N = 19.3 K. At temperatures below 7 K (similar to0.3 T-N), it exhibits a first-order metamagnetic transition (MT) at a critical field mu H-0(c) appr oximate to 11.35 T between the low-field antiferromagnetic state and the hi gh-field ferromagnetic one. The transition is accompanied by a sharp anomal y in transport and lattice properties. A striking observation is that the " virgin" curves expressing the field dependence of the various electronic pr operties clearly differ from those observed on a sample that has once appea red in the high-field state. This irreversibility in bulk properties corres ponds to an irreversible change of the magnetic-structure propagation vecto r within the basal plane observed by neutron diffraction. The different sha pes of M(H), rho (H), and rho (H)(H) curves are strongly suggestive of fiel d-induced changes in the spectrum of spin fluctuations, which play a substa ntial role in the physics of UNiAl. This seems to be corroborated by the ef fect of "high-field annealing" on the low-temperature specific-heat data. T he irreversibility vanishes at temperatures T greater than or equal to 7 K where the MT loses its first-order character. (C) 2001 American Institute o f Physics.