Single-ion anisotropy

The preferred spin orientation of a magnetic ion leads to its single-ion magnetic anisotropy (or magnetocrystalline anisotropy). The simplest way of describing this 

In the effective spin approximation, one circumvents the need to explicitly describe the unquenched orbital moments of a magnetic system by treating the system as a spin-only system. The effect of unquenched orbital moments is treated indirectly by introducing anisotropic g-factors. As a consequence, for a magnetic ion with nondegenerate magnetic orbital (e.g., Cu ), the SOC Hamiltonian Hso is replaced with the zero-field spin Hamiltonian H f 

---

The structurally related salts [M(Cp )2] [M (tds)2] (M = Fe, Mn, Cr M = Ni, Pt) and [Fe(Cp )2][Pt(tds)2] allowed a systematic study of the effect of a diversity of variables on the magnetic behavior of these compounds, such as the variation of the spin of the cation, the role of the single ion anisotropy, the effect of the variation of the size of atoms involved in the intermolecular contacts. Furthermore, the analysis of the intermolecular contacts in these compounds provided a reasonable interpretation of the intra and interchain magnetic coupling, and its relative strength within the series [44, 45]. 

Neutron diffraction and susceptibility measurements have shown that the main interaction along the chain is ferromagnetic and that this interaction occurs over a large temperature range157-159. Single-ion anisotropy favors an orientation of the spin within the plane perpendicular to the chain direction. While the difference in anisotropy is large between... 

In Table 7 are compiled the values of the intrachain constant J/k and the single-ion anisotropy parameter D/k. Although there are some discrepancies, recent measurements167, 168) seem to be in agreement with a value of D close to that of J. 

Reference (261a) reports uniaxial ([100] of pseudocell) antiferromagnetisra 81.5°K < T < 88.3°K, parasitic ferromagnetism due to canted spins T < 81.5°K as a result of different single-ion anisotropies, thermal hysteresis in the canted-spin uniaxial-spin transition, and an He 9000 oe for field-induced spin canting in the intermediate temperature range. 

The chemistry of polynuclear Co(ll) complexes is quite rich [32,161,181] and has been particularly exploited in the context of molecular magnetism [154, 182], Since this ion can possess up to three unpaired electrons (S = 3/2) and display large single-ion anisotropy, it is a suitable candidate for SMM synthesis. However, only two SMMs of Co(ll) have been reported to date, although the first identified single-chain magnet (SCM) [ 183] also contains Co(II). 

Although the Heisenberg and Ising models have been intensively used in theoretical works, the description of real materials often requires more complicated Hamiltonians. For example when single-ion anisotropy is relevant, a finite magnetic anisotropy has to be considered. In this case, the corresponding Hamiltonian can be written ... 

As this expression only rehes on the temperature dependence of the magnetic correlations, Eq. 44 is valid for any Ising-Hke chain independently of the domain wall structure. More specifically, in the case of narrow domain walls and for single-ion anisotropy this expression becomes (using the notations of Eqs. 7a and 23) ... 

---