Determining the anisotropy and exchange parameters of polycrystalline spin-1 magnets

Abstract

Although low-dimensional S=1 antiferromagnets remain of great interest, difficulty in obtaining highquality 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(H 2 O) 2 (3,5-lutidine) 4 ](BF 4 ) 2 and Ni(H 2 O) 2 (acetate) 2 (4-picoline) 2 , which have negligible exchange interactions, as well as the antiferromagnet [Ni(H 2 O) 2 (pyrazine) 2 ](BF 4 ) 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(H 2 O) 2 (pyrazine) 2 ](BF 4 ) 2 .

Magnetic ground state of the one-dimensional ferromagnetic chain compoundsM(NCS)2(thiourea)2(MCurley1, Scatena2, Williams3 et al. 2021Phys. Rev. MaterialsSelf Cite8130View full textAdd to dashboardCite

“…These observations are consistent with large FM primary exchange interactions and a considerably smaller secondary AFM exchange, leading to an AFM ground state. via gµ B µ 0 H sf = 2Sn J where n is the number of nearest interchain neighbours [47]. Taking g = 2.29 (1), from χ −1 (T ), and n = 4 we estimate J = 0.13(1) K.…”

Magneto-structural Correlations in Ni2+–Halide···Halide–Ni2+ Chains

“…Therefore, we define the z -axis to be along the c direction, with the x–y plane analogous to the a–b plane. We use a model containing SIA and paramagnetic contributions 33 and perform a global fit to the data for the field applied parallel and perpendicular to the z -axis, shown by solid lines in Figure 4 a. The fitting returns the following values: g xy = 2.12(1), g z = 2.13(2), and D = +8.7(3) K. We note that the fitted g z and g xy are the same within errors.…”

“…Single crystals of sufficient size to perform bulk magnetometry measurements of NiCl 2 (3,5-lut) 4 and NiBr 2 (3,5-lut) 4 are not currently available. Therefore, we use the methods of extracting the Hamiltonian parameters of polycrystalline samples described by Blackmore et al 33 …”

“…This is consistent with an S = 1 compound with easy-plane single-ion anisotropy and modest magnetic interactions between adjacent Ni 2+ ions. 33 In general, two critical fields can be identified in polycrystalline M ( H ) data of Ni 2+ antiferromagnets that are attributed to moments saturating when the field is parallel to the easy-plane g xy μ B μ 0 H sat xy = 2 SnJ and along the hard-axis g z μ B μ 0 H sat z = 2( nJ + D ) S , where n is the number of nearest neighbors. The lower saturation field occurs at μ 0 H sat xy = 2.3(2) T. The upper saturation field is estimated as the point at which d 2 M /d H 2 first crosses zero, which occurs at μ 0 H sat z = 11.1(5) T. Using g xy = 2.20(1) and g z = 2.08(1) obtained from ESR measurements (see below), values of J = 0.85(7) K and D = +6.1(5) K are extracted, consistent with the fit to χ( T ) presented above.…”

“… 32 To overcome this, we have successfully used additional information available from low-temperature, high-field magnetization measurements (namely spin-flop, metamagnetic, and saturation fields) to simultaneously extract J and D in the low-temperature state. 21 , 32 , 33 ESR is highly suited to characterizing the single-ion properties of quantum magnets. In our compounds, the size of the zero-field splitting requires magnetic fields that are higher than can found in commercially available ESR machines.…”