Muons and magnets

Abstract:

The magnetic properties of the two isostructural molecule-based magnets - Ni(NCS)2(thiourea)2, S=1 [thiourea=SC(NH2)2] and Co(NCS)2(thiourea)2, S=3/2 - are characterized using several techniques in order to rationalize their relationship with structural parameters and to ascertain magnetic changes caused by substitution of the spin. Zero-field heat capacity and muon-spin relaxation measurements reveal low-temperature long-range ordering in both compounds, in addition to Ising-like (D<0) single-ion anisotropy (DCo∼-100 K, DNi∼-10 K). Crystal and electronic structure, combined with dc-field magnetometry, affirm highly quasi-one-dimensional behavior, with ferromagnetic intrachain exchange interactions JCo≈+4 K and JNi∼+100 K and weak antiferromagnetic interchain exchange, on the order of J′∼-0.1 K. Electron charge- and spin-density mapping reveals through-space exchange as a mechanism to explain the large discrepancy in J-values despite, from a structural perspective, the highly similar exchange pathways in both materials. Both species can be compared to the similar compounds MCl2(thiourea)4, M = Ni(II) (DTN) and Co(II) (DTC), where DTN is known to harbor two magnetic-field-induced quantum critical points. Direct comparison of DTN and DTC with the compounds studied here shows that substituting the halide Cl- ion for the NCS- ion results in a dramatic change in both the structural and magnetic properties.

Abstract:

© 2021 American Physical Society. We report the results of muon-spin spectroscopy (μ+SR) measurements on the staggered molecular spin chain [pym-Cu(NO3)2(H2O)2] (pym = pyrimidine), a material previously described using sine-Gordon field theory. Zero-field μ+SR reveals a long range magnetically ordered ground state below a transition temperature TN=0.23(1) K. Using longitudinal-field (LF) μ+SR we investigate the dynamic response in applied magnetic fields 0<B<500 mT and find evidence for ballistic spin transport. Our LF μ+SR measurements on the chiral spin chain [Cu(pym)(H2O)4]SiF6·H2O instead demonstrate one-dimensional spin diffusion, and the distinct spin transport in these two systems suggests that additional anisotropic interactions play an important role in determining the nature of spin transport in S=1/2 antiferromagnetic chains.

Abstract:

We report the results of muon-spin spectroscopy (μ+SR) measurements on the staggered molecular spin chain [pym-Cu(NO3)2(H2O)2] (pym = pyrimidine), a material previously described using sine-Gordon field theory. Zero-field μ+SR reveals a long range magnetically ordered ground state below a transition temperature TN=0.23(1) K. Using longitudinal-field (LF) μ+SR we investigate the dynamic response in applied magnetic fields 0 < B < 500 mT and find evidence for ballistic spin transport. Our LF μ+SR measurements on the chiral spin chain [Cu(pym)(H2O)4]SiF6⋅H2O instead demonstrate one-dimensional spin diffusion, and the distinct spin transport in these two systems suggests that additional anisotropic interactions play an important role in determining the nature of spin transport in S=1/2 antiferromagnetic chains.

Abstract:

I review some general concepts in magnetism including the nature of magnetic exchange (direct, indirect and superexchange), and how exchange interactions play out in multiple spin systems. The nature of atomic orbitals and the way in which they interact with the spin system is also considered. Several examples are also treated, including the Jahn–Teller interaction and its role in the properties in layered manganites.