Thermodynamic properties of ferromagnetic mixed-spin chain systems

Abstract

Using a combination of high-temperature series expansion, exact diagonalization and quantum Monte Carlo, we perform a complementary analysis of the thermodynamic properties of quasi-one-dimensional mixed-spin systems with alternating magnetic moments. In addition to explicit series expansions for small spin quantum numbers, we present an expansion that allows a direct evaluation of the series coefficients as a function of spin quantum numbers. Due to the presence of excitations of both acoustic and optical nature, the specific heat of a mixed-spin chain displays a double-peak-like structure, which is more pronounced for ferromagnetic than for antiferromagnetic intra-chain exchange. We link these results to an analytically solvable half-classical limit. Finally, we extend our series expansion to incorporate the single-ion anisotropies relevant for the molecular mixed-spin ferromagnetic chain material MnNi(NO${}_2$)${}_4$(ethylenediamine)${}_2$, with alternating spins of magnitude 5/2 and 1. Including a weak inter-chain coupling, we show that the observed susceptibility allows for an excellent fit, and the extraction of microscopic exchange parameters.

8 pages including 7 figures, submitted to Phys. Rev. B; series extended to 29th. QMC added

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• Numerical interchain mean-field theory for the specific heat of the bimetallic ferromagnetically coupled chain compound MnNi(NO${}_2$)${}_4$(en)${}_2$ (en = ethylenediamine)
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• Thermodynamics of the S=1 spin ladder as a composite S=2 chain model
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