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The magnetic phase diagram of the quasi two-dimensional Heisenberg antiferromagnet K 2 MnF 4

Physica B+C, ISSN: 0378-4363, Vol: 113, Issue: 3, Page: 380-390
1982
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  • Citations
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Article Description

We present measurements of the differential susceptibility of the quasi two-dimensional antiferromagnet K 2 MnF 4 as a function of temperature (3 K< T <60 K) and in external magnetic fields up to 142 kOe. The zero-field ordering temperature is found as T c ( H =0)=43.9±0.2 K. Anomalies are found in the χ( H ) curves at constant temperature near H =55 kOe, indicative for spin-flop transitions. This transition field remains equal to within 10% of its low-temperature value up to T ≈ 40 K. The transition line in the H, T phase diagram merges smoothly into a second-order line ending at T c ( H =0). In the present χ( T, H ) data no evidence is found for a phase boundary separating the spin-flop from the paramagnetic phase. It is not clear whether this is due to the fact that the associated anomalies in χ( T, H ) are just too small to be detected, or that the supposed spin-flop phase for H > 55 kOe does not correspond to an ordered phase but is an extension of the paramagnetic regime. The latter possibility arises because K 2 MnF 4 in its spin-flop phase might correspond to the two-dimensional planar model with cubic anisotropy. This cubic anisotropy may explain the fact that χ( H ) does not diverge at the “spin-flop” transition, since it would entail a splitting-up of the first-order spin-flop line into two second-order boundaries encompassing an intermediate phase and meeting in a tetracritical point. The analogy with the tetracritical behaviour of RbMnF 3, as recently reported, is discussed.

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