An Experimental Study of the Ising Chain Statistics under the Magnetic Field

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Abstract
The first experimental study of the statistics of a quasi-one-dimensional Ising system under the magnetic field H α , described by the Hamiltonian \begin{aligned} \mathscr{H}=-2J_{1} \sum _{\langle ij \rangle} S_{i}^{z}S_{j}^{z}-2J_{2} \sum _{\langle ik \rangle} S_{i}^{z}S_{k}^{z}-g_{\alpha}\mu _{\text{B}}H^{\alpha} \sum _{i} S_{i}^{\alpha}\hspace{1em}(\alpha=x,~y,~z) \label{e1} \end{aligned} has been performed, where J 1 and J 2 are the intra- and the inter-chain exchange constants, respectively. A single crystal of the compound (CH 3 ) 3 NHCoCl 3 ·2H 2 O has been used as a model sample of the ferromagnetic system with J 1 / k B =14.2 K and J 2 / k B =0.20 K. It has been revealed that the experimental values of the magnetic heat capacity under the field H α >2 J 2 / g z µ B (≈0.8 kOe) applied along the spin preferential axis are excellently reproduced by the values calculated for the isolated Ising chain under the longitudinal field ( α= z ; g z =6.54). For the temperature higher than 7 K (≈ J 1 /2 k B ), the experimental values of the magnetic heat capacity under the field along the spin hard axis have also agreed with the theoretical values for the isolated Ising chain under the transverse field ( α= y ; g y =3.90).