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Local spin anisotropy effects upon the magnetization and specific heat of dimer single molecule magnets | | Dmitri V. Efremov
; Richard A. Klemm
; | | Date: |
7 Sep 2004 | | Subject: | Mesoscopic Systems and Quantum Hall Effect; Statistical Mechanics | cond-mat.mes-hall cond-mat.stat-mech | | Abstract: | We present an exactly solvable model of equal spin $s_1$ dimer single molecule magnets. The spins within each dimer interact via the Heisenberg and the most general quadratic global and local (single-ion) anisotropic spin interactions, and with the magnetic induction ${f B}$. For antiferromagnetic couplings and $s_1>1/2$, the low temperature $T$ magnetization ${m M}({m B})$ exhibits $2s_1$ steps of universal height and midpoint slope, the $s$th step of which occurs at the non-universal level-crossing magnetic induction $B_{s,s_1}^{
m lc}( heta,phi)$, where $ heta,phi$ define the direction of ${m B}$. The specific heat $C_V$ exhibits zeroes as $T o0$ at these $B_{s,s_1}^{
m lc}( heta,phi)$ values, which are equally surrounded by universal peak pairs as $T o0$. The non-universal $B_{s,s_1}^{
m lc}( heta,phi)$ values lead to a rich variety of magnetization plateau behavior, the structure and anisotropy of which depend upon the various global and local anisotropic spin interaction energies. We solve the model exactly for $s_1=1/2$, 1, and 5/2, and present ${m M}({m B})$ and $C_V({m B})$ curves at low $T$ for these cases. For weakly anisotropic dimers, rather simple analytic formulas for ${m M}({m B})$ and $C_V({m B})$ at arbitrary $s_1$ accurately fit the exact solutions at sufficiently low $T$ or large $B$. An expression for $B_{s,s_1}^{
m lc}( heta,phi)$ accurate to second order in the four independent anisotropy energies is derived. Our results are discussed with regard to existing experiments on $s_1=5/2$ Fe$_2$ dimers, suggesting further experiments on single crystals of these and some $s_1=9/2$ Mn$_4$]$_2$ dimers are warranted. | | Source: | arXiv, cond-mat/0409168 | | Services: | Forum | Review | PDF | Favorites | |
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