Macroscopic quantum coherence in ferrimagnets

Abstract

We study macroscopic quantum coherence (MQC) in small magnetic particles where the magnetization (in ferromagnets) or the Néel vector (in antiferromagnets) can tunnel between energy minima. We consider here the more general case of MQC in ferrimagnets by studying a model for a mesoscopic antiferromagnet with an uncompensated magnetic moment. Through semi-classical calculations we show that even a small moment has a drastic effect on MQC. In particular, there is a rapid crossover to a regime where the MQC tunnel splitting is equal to that obtained for a ferromagnet, even though the system is still an antiferromagnet for all other aspects. We calculate this tunnel splitting via instanton methods and compare it with numerical evaluations. As an application we re-examine the experimental evidence for MQC in ferritin and show that even though the uncompensated moment of ferritin is small it greatly modifies the MQC behavior. The excess spin allows us to extract values for experimental parameters without making any assumption about the classical attempt frequency, in contrast to previous fits. Finally, we also discuss the implications of our results for MQC in molecular magnets.

Manifestation of Spin Selection Rules on the Quantum Tunneling of Magnetization in a Single-Molecule Magnet

“…Transverse field resonances are understood by correctly orienting the Jahn-Teller axes of the individual manganese ions and including transverse dipolar fields. These factors are likely to be important for QTM in all single-molecule magnets.Observations of basic quantum phenomena in singlemolecule magnets (SMMs) [1][2][3][4][5][6][7][8][9][10][11][12] and their potential for quantum information technologies [3] have resulted in a much better understanding of magnetization dynamics in nanoscale systems. In the bottom-up approach of supramolecular chemistry, the quantum properties of SMMs are dictated by molecular composition and configuration.…”

Excess spin and the dynamics of antiferromagnetic ferritin

“…Measurements made on fully loaded natural ferritin (n = 4500) and partially loaded natural ferritin (n = 2000) are in good agreement with a 1/2 power law. [11][12][13] We can now compare the measured values of the excess spin and the MQC resonance frequency ν MQC as a functions of n. An antiferromagnet strongly coupled to an uncompensated moment can be described by the effective action 14,15…”

Spin-phase interference, coherent superposition, and quantum tunneling at excited levels in nanoantiferromagnets

“…The spin-phase interference between excited-level tunneling paths is not clearly shown for AFM particles. Moreover, previous works on AFM spin tunneling [9][10][11][12] have been confined to the system with biaxial symmetry, which has two energetically degenerate easy directions in the basal plane. The purpose of this paper is to study the quantum tunneling and spin-phase interference at excited states for AFM particles in the absence of an external magnetic field.…”

“…6, 7, and 10. However, the theoretical studies on AFM systems [9][10][11][12] have been focused on phase interference between two opposite winding ground-state tunneling paths in biaxial particles. The spin-phase interference between excited-level tunneling paths is not clearly shown for AFM particles.…”