Quantum theory of nucleation in ferromagnets

Abstract

%'e consider a ferromagnet whose magnetization is opposite to an applied magnetic field. The rate of quantum nucleation is calculated for a film and is estimated for a bulk solid. The temperature corresponding to the crossover from thermal to quantum nucleation is estimated. The effect may be large enough to be observed in materials with high anisotropy constants.

Quantum decay of metastable states in small magnetic particles

“…The tunneling rate can be written as ⌫ϭAexp(ϪB), with the exponent B being determined by the imaginary time action [1][2][3]19,20 …”

“…As the temperature goes down, thermal transitions die out but a finite probability of quantum transitions remains. [1][2][3] In such transitions the uncertainty principle manifests itself on a scale where it has been rarely seen. As many as 10 6 spins can coherently tunnel out of a metastable magnetic state, placing this effect in the domain of macroscopic quantum tunneling ͑MQT͒.…”

“…In this paper we shall study the uniaxial case by direct calculation of the imaginary-time path integral for the magnetic moment. [1][2][3] For the problem at hand, both methods make a number of approximations; the exact solution is impossible to obtain. One may worry, therefore, how reliable their predictions are for the dependence of the tunneling rate on the magnitude and the orientation of the field.…”

“…The solution of this problem elucidates some important points with regard to the two models which have been discussed in literature: with the field parallel 3 and perpendicular [1][2][3] to the anisotropy axis. We will show that even a very small misalignment of the field with the above two orientations completely changes the results.…”

Field-dependent quantum nucleation of antiferromagnetic bubbles

“…4 Chudnovsky and Gunther studied the quantum nucleation of a thin ferromagnetic film in a magnetic field along the opposite direction to the easy axis at zero temperature by applying the instanton method. 5 Later, Ferrera and Chudnovsky extended the quantum nucleation to a finite temperature. 6 Kim studied the effect of an arbitrarily directed magnetic field on the quantum nucleation of magnetization.…”