A. Kirste

Crossover in the Van Vleck paramagnet TmPO4

Magnetic anomalies caused by crossing of the lowest-lying energy levels (crossover) of Tm 3+ ion in the magnetic field along the tetragonal axis of a TmPO4 single-crystal near 30 T are investigated experimentally and theoretically. Measurements of the differential magnetic susceptibility dM=dH by the compensated pick-up coil method with different rates of up- and down-sweep of the magnetic field from 2 K to about 19 K have allowed studying temperature variations of width and shape of the dM=dH peak and hysteresis phenomena associated with relaxation processes. The influence of the misorientation effect and hyperfine coupling on the character of anomalies are explored. Comparison of experimental and theoretical data suggests a cooling of a sample, accompanied by an increase of spin-lattice relaxation times, when approaching the crossover in pulsed regime. r 2003 Elsevier Science B.V. All rights reserved.

Energy level crossing effects in the rare-earth zircons TmPO4 and PrVO4

High-field magnetization measurements using the single-turn coil technique not only give detailed information on the critical magnetic field of level crossing anomalies in the rare-earth zircons TmPO4 and PrVO4, but also provide strong evidence of hysteresis effects in up- and down-sweep of the external magnetic field.

Peculiarities of energy level crossing effects in a singlet paramagnet PrVO4

Abstract Magnetic anomalies caused by an energy level interaction in a high magnetic field are investigated experimentally and theoretically for a Van Vleck paramagnet PrVO 4 . Maxima of the differential susceptibility d M /d H associated with a crossing of the lowest-lying levels at the field H C ≈51.3 T are observed for various temperatures. The hysteresis is detected, which is ascribed to the relaxation processes. The magnetocaloric effect calculated on the assumption of adiabatic magnetization at pulsed fields shows the monotonous decrease of the temperature with increasing field up to H C and its subsequent increase. The problem of spin-lattice relaxation times in the vicinity of crossover is discussed. The great importance of the hyperfine coupling in the crossover effects for the PrVO 4 , which is among the enhanced nuclear magnets, is emphasized.

Spin dynamics in megagauss fields

Transient magnetic fields in the megagauss regime prove as a new tool to investigate detailed information on spin-lattice relaxation using the delayed population change of the spin levels in the presence of rapidly varying magnetic fields. The method is applied to HgSe and HgTe epitaxial layers in magnetic fields up to 150 T.

Spin-glass behaviour and dynamical process of high field magnetisation in disordered perovskite manganites

In single crystals of A-site disordered perovskite manganites R1−xAExMnO3 (R = Sm, Gd, Eu, AE = Ba, Sr, x = 0.5, 0.45), we have observed the spin-glass behaviour. Applying pulsed high magnetic fields, a metamagnetic phase transition was observed in some samples, whereas in other samples, the magnetisation was just a smoothly varying function of magnetic field up to the saturation. The difference between the two types of behaviour seems to arise from the difference in the average ionic radius of the A-site ions and the disorder in the A-site layers. In the metamagnetic transition, magnetisation hysteresis was found to be smaller as the field sweep rate is faster, which is contrary to the ordinary spin relaxation phenomena.

Magnetotransport in HgSe:Fe quantum‐dots

We report on magnetotransport measurements in intrinsically populated HgSe:Fe quantum‐dot systems in magnetic fields up to 12 T. Depending on the dot density we observe positive and negative differential magnetoresistance, the later similar to hopping conductivity in impurity bands.

MAGNETOOPTICAL EXPERIMENTS USING THE EXPLOSIVE DRIVEN FLUX COMPRESSION UP TO 1000T

We discuss three recent magnetooptical experiments using the explosive driven flux-compression technique that have been performed in cooperation of the Berlinian with the Russian group of VNIIEF, Sarov. We present an overview on the experimental techniques touching the frontiers of physics as well as a detailed discussion of the results obtained on the semiconducting materials GaN, GaAs and HgSe. Special emphasis will be laid on the interpretation in context with theoretical predictions and analysis that go beyond the ordinary k•p-formalism but are also valid in the limit of the HOFSTADTER butterfly.