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arXiv:cond-mat/0009098

Intermediate spin approach to the tunneling of nano-magnets

S. E. Barnes

Published Sep 7, 2000• 0 citations
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Abstract

The theory for the quantum tunneling of nano-magnets is developed within the intermediate spin framework. Periodic magnetic effects are seen to reflect that associated with a change of flux by a single flux quantum $Φ_{0}$. Essential are Schrödinger cat wave functions which involve superpositions of different magnitudes of the applied magnetic field. For systems in which the tunneling paths are not co-planar the theory leads to essentially complex magnetic fields although the expectation values of the fields remain real. In general the ground states correspond to minimal uncertainty squeezed states. The degree of squeezing depends on the anisotropy parameters.

DOI
10.48550/arxiv.cond-mat/0009098

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The theory for the quantum tunneling of nano-magnets is developed within the intermediate spin framework. Periodic magnetic effects are seen to reflect that associated with a change of flux by a single flux quantum Φ0Φ_{0}Φ0​. Essential are Schrödinger cat wave functions which involve superpositions of different magnitudes of the applied magnetic field. For systems in which the tunneling paths are not co-planar the theory leads to essentially complex magnetic fields although the expectation values of the fields remain real. In general the ground states correspond to minimal uncertainty squeezed states. The degree of squeezing depends on the anisotropy parameters.
Quantum mechanical objects or nano-objects have been proposed as bits for information storage. While time-averaged properties of magnetic, quantum-mechanical particles have been extensively studied experimentally and theoretically, experimental investigations of the real time evolution of magnetization in the quantum regime were not possible until recent developments in pump–probe techniques. Here we investigate the quantum dynamics of effective spin systems by means of analytical and numerical treatments. Particular attention is paid to the quantum revival time and its relation to the magnetization tunneling. The quantum revival time has been initially defined as the recurrence time of a total wave-function. Here we show that the quantum revivals of wave-functions and expectation values in spin systems may be quite different which gives rise to a more sophisticated definition of the quantum revival within the realm of experimental research. Particularly, the revival times for integer spins coincide which is not the case for half-integer spins. Furthermore, the quantum revival is found to be shortest for integer ratios between the on-site anisotropy and an external magnetic field paving the way to novel methods of anisotropy measurements. We show that the quantum tunneling of magnetization at avoided level crossing is coherent to the quantum revival time of expectation values, leading to a connection between these two fundamental properties of quantum mechanical spins.
We investigate the reversal of magnetization and the coherence of tunneling when an external magnetic field is rotated instantaneously in systems of a few (N) spin 1/2 particles described by an anisotropic Heisenberg Hamiltonian at T=0. The temporal evolution is calculated by a numerically exact solution of the time-dependent Schrödinger equation, and the mean value in time of each spin component is computed as a function of the magnetic field. The correlation function and the spectrum are analyzed in terms of the macroscopic quantum coherence. Our calculations demonstrate that this model for small (N<11) magnetic particles exhibit collective tunneling of the magnetization only for some specific resonant values of the applied magnetic field, at variance with the Stoner–Wohlfarth model.
These lecture notes give an introduction to quantum magnetism. We start by a discussion of paramagnetism and the Weiss meanfield theory applied to the Ising model. In the following sections, we turn to the fully quantum‐mechanical Heisenberg Hamiltonian. First, we consider the limit of small clusters whose properties we analyze via exact diagonalization. Bulk quantum magnets are then within the framework of the linear spinwave theory. A short discussion of Monte Carlo techniques follows. Finally, we discuss aspects of entanglement in quantum spin systems.
The Aharonov-Casher effect is a quantum interference arising in mesoscopic rings involving particles with magnetic moments. The wave function acquires a phase due to an electric-field flux F enclosed by the ring and gives rise to flux-dependent oscillations in persistent spin currents. In the ground state of the spin-1/2 Heisenberg chain the period and amplitude of the oscillations are associated with the properties of the Fermi surface of the elementary excitations of the spin chain (spinons). We consider an antiferromagnetic spin-1/2 Heisenberg ring with a concentration x of impurity spins S. The calculations are performed within the framework of Bethe's ansatz. Due to the finite concentration of impurities the ground state consists of two (rather than one) Dirac seas, corresponding to spinons and 2S strings of rapidities. The persistent spin current reveals two periods of oscillation associated with the properties of the two Fermi seas. The interference pattern as a function of F is discussed for several external magnetic fields.
Starting from a phenomenological Hamiltonian originally written in terms of angular momentum operators we derive a new quantum angle-based Hamiltonian that allows for a discussion on the quantum spin tunneling. The study of the applicability of the present approach, carried out in calculations with a soluble quasi-spin model, shows that we are allowed to use our method in the description of physical systems such as the Mn12-acetate molecule, as well as the octanuclear iron cluster, Fe8, in a reliable way. With the present description the interpretation of the spin tunneling is seen to be direct, the spectra and energy barriers of those systems are obtained, and it is shown that they agree with the experimental ones.
It is constructed in this paper that the three-states superposition multimode functional superposition state light field ｜ψ~((3))_f〉_q is made up of the multimode vacuum state ｜{0_j}〉_q, and the two different multimode complex functional coherent states ｜{f~~((a)*)_f(x,y,z)}〉_q and ｜{f~~((b)*)_f(x,y,z)}〉_q which have rather different spatial intensity distribution characteristic according to the superposition principle of quantum state in quantum mechanics. The properties of equal-power higher-power amplitude-squeezing (Y-squeezing) of generalized magnetic field component of the state ｜ψ~((3))_f〉_q is studied firstly by utilizing the multimode squeezed state theory. It is shown that under a series of fixed conditions, the generalized magnetic-field component of the state ｜ψ~((3))_f〉_q can display the effect of generalized nonlinear equal-power higher-power Y-squeezing which changes periodically and alternatively; and the asymmetrical spatial distribution of the classical intensity and the classical amplitude, and the spatial distribution features of the initial classical phase of light-field in the state ｜ψ~((3))_f〉_q would have a direct influence upon the squeezed degree of equal-power higher-power Y-squeezing present by the generalized magnetic-field component in the state ｜ψ~((3))_f〉_q.