Abstract
Although low-dimensional S = 1 antiferromagnets remain of great interest, difficulty in obtaining high-quality single crystals of the newest materials hinders experimental research in this area. Polycrystalline samples are more readily produced, but there are inherent problems in extracting the magnetic properties of anisotropic systems from powder data. Following a discussion of the effect of powder-averaging on various measurement techniques, we present a methodology to overcome this issue using thermodynamic measurements. In particular we focus on whether it is possible to characterise the magnetic properties of polycrystalline, anisotropic samples using readily available laboratory equipment. We test the efficacy of our method using the magnets [Ni(H2O)2(3,5-lutidine)4](BF4)2 and Ni(H2O)2(acetate)2(4-picoline)2, which have negligible exchange interactions, as well as the antiferromagnet [Ni(H2O)2(pyrazine)2](BF4)2, and show that we are able to extract the anisotropy parameters in each case. The results obtained from the thermodynamic measurements are checked against electron-spin resonance and neutron diffraction. We also present a density functional method, which incorporates spin–orbit coupling to estimate the size of the anisotropy in [Ni(H2O)2(pyrazine)2](BF4)2.
Highlights
The investigation of low-dimensional quantum magnets is a key thrust of condensed matter physics
We describe the effect of powder averaging and discuss to what extent the parameters in the Hamiltonian can be extracted from data, focusing on bulk thermodynamic measurements of susceptibility, magnetization and heat capacity that can, in principle, be performed using commonly available laboratory apparatus without the need to access equipment at a large user facility
In this paper we have presented an experimental method for extracting the anisotropy parameters of polycrystalline S = 1 magnets from thermodynamic data and applied it to the situation of magnetically-isolated, exchange-free systems as well as an extended material with antiferromagnetic exchange pathways
Summary
The investigation of low-dimensional quantum magnets is a key thrust of condensed matter physics. [1,2,3,4,5,6,7,8]) and are predicted to display vibrant phase diagrams arising from competing interactions and their interplay with singleion anisotropy These diagrams encompass quantum critical points [1, 2], nematic and supersolid states [5, 9, 10], as well as topologically interesting gapped and quantum paramagnetic phases [11,12,13,14]. The complication of powder-averaging leads to difficulties interpreting the results of bulk thermodynamic measurements. This issue is made worse if the magnitude of the anisotropy is on a similar energy scale to the strength of exchange interactions in the compound [18, 22, 24]
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