Definitive Spectroscopic Determination of the Transverse Interactions Responsible for the Magnetic Quantum Tunneling inMn12-Acetate

Abstract

We present detailed angle-dependent single crystal electron paramagnetic resonance (EPR) data for field rotations in the hard plane of the S=10 single molecule magnet Mn(12)-acetate. A clear fourfold variation in the resonance positions may be attributed to an intrinsic fourth-order transverse anisotropy (O(4)/(4)). Meanwhile, a fourfold variation of the EPR line shapes confirms a recently proposed model wherein disorder associated with the acetic acid of crystallization induces a locally varying quadratic (rhombic) transverse anisotropy [O (2)/(2) identical with E(S (2)/(x)-S(2)/(y))]. These findings explain most aspects of the magnetic quantum tunneling observed in Mn(12)-acetate.

Microwave emission from a crystal of molecular magnets: The role of a resonant cavity

“…(27). This was the assumption that led to the coherent behavior of the molecules resulting in superradiant emission.…”

“…With the assumptions that R is real, ψ = π/2 and using Eqs. (26,27), one has b = d dτ θ(τ )/ √ 2 ≡θ/ √ 2, and from Eq. (25) we obtain which is the equation of a damped pendulum (θ measured from the inverted position), being often discussed in coherent atom-field interactions.…”

“…Now we shall analyze if the observed radiation can be considered as superradiance demanding γ ≪ 1, or is it rather a maser effect, where the absence of phase relaxation is not crucial. Therefore we first assume that γ = 0, and see that equations (26) and (27) admit a simple constant of motion:…”

“…(27). As we assume a constantḂ 0 , we can introduce a constant dimensionless external field sweep rate v via…”

“…Experiments including magnetization measurements, 4,23 neutron 24 and EPR 25,26,27 studies on crystals of Mn 12 Ac and Fe 8 O suggest that the Hamiltonian responsible for the magnetic properties can be written as a sum of two terms:…”