Abstract
EuSe is a simple magnetic system that appears to show many complicated features. Under applied pressure it undergoes a transition from an antiferromagnet (AF) to a ferromagnet (FM). This transition provides a means of testing certain basic fundamentals of magnetic theory and an opportunity to explore the complexities of EuSe. Using the muon-spin rotation and relaxation technique (μSR), EuSe was measured at pressures ranging from ambient to 11 kbar. In ambient-pressure EuSe, muon data reveal two local fields, but show only a single field in the FM state formed under pressure. The μSR measurements appear to show a continuous transition at Tc, contrary to previous Mössbauer results that were interpreted as being evidence of a first-order transition. Values determined for the critical exponent, β, in AF and FM EuSe, differ and therefore appear to be a clear counterexample to the Universality Hypothesis. The values of β also are indicative of EuSe’s being a 2D magnet for pressures up to 11 kbar. The nature and values of the local fields seen by the muons is discussed and analyzed.
Highlights
The magnetic properties of EuSe offer a unique opportunity to provide answers to some fundamental questions in the theory of magnetic behavior of simple insulating magnetic systems
The μSR measurements appear to show a continuous transition at Tc, contrary to previous Mössbauer results that were interpreted as being evidence of a first-order transition
Values determined for the critical exponent, β, in AF and FM EuSe, differ and appear to be a clear counterexample to the Universality Hypothesis
Summary
The magnetic properties of EuSe offer a unique opportunity to provide answers to some fundamental questions in the theory of magnetic behavior of simple insulating magnetic systems. This seeming complexity can be understood in a simple way by hypothesizing that the [111] sheets of spins are strongly exchange-coupled by two-neighbor-exchange interactions, J1 and J2, but become de-coupled from each other when J2 ∼ −J1, forming a 2D magnetic system, in which the much weaker interactions between these sheets determine the stacking arrangements. An early Mössbauer study was interpreted as showing that EuSe undergoes a first-order magnetic phase transition at Tc. Several subsequent papers repeated this claim without providing any further direct evidence. A stong motivation for undertaking these studies and using μSR was to obtain clearer evidence of whether EuSe undergoes a first-order transition and if not, to measure its β
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