Giant Spin Model for Nanomagnets

A magnetic molecule, that behaves like a small nanomagnet, must have a well-defined magnetic anisotropy and a large ground state spin. A typical example is the above-mentioned octanuclear iron (III) oxo-hydroxo cluster (Fig. 3). 

The internal iron (III) ions are octahedrally coordinated by the two oxides and four hydroxo bridges. The outer iron(III) ions coordinate three nitrogens and three hydroxyls. Spin polarized neutron scattering showed that all Fe ions have a spin 5/2, six spins up and two down [23]. This rationalizes the S = 10 spin ground state that is in agreement with magnetization measurements. 

and Sz are the three components of the spin operator, D and E are the anisotropy constants which were determined via high-frequency electron paramagnetic resonance (D/kB 0.275 K and E/kB 0.046 K [10]), and the last term of the Hamiltonian describes the Zeeman energy associated with an applied field H. 

The tunneling probability P when sweeping the longitudinal field Hz at a constant rate over an avoided energy level crossing (Fig. 7) is given by 

m and rrC are the quantum numbers of the avoided level crossing, denote the tunnel splitting between the levels (Fig. 7), dHJdt is the constant field sweep rate, g 2, /uB is the Bohr magneton, and h is Planck s constant. 

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