Landau-Zener quantum tunneling in disordered nanomagnets

TL;DR

This paper investigates Landau-Zener quantum tunneling in disordered nanomagnets, revealing how disorder affects transition probabilities and magnetization dynamics, with implications for quantum control in nanoscale magnetic systems.

Contribution

It introduces a model with a giant-spin Hamiltonian including disorder, analyzing how disorder influences quantum tunneling and magnetization evolution in nanomagnets.

Findings

Disorder smooths transition probability distributions.

Coherent tunneling events depend on initial states and disorder.

Disorder suppresses the backward cascade, causing damping.

Abstract

We study Landau-Zener macroscopic quantum transitions in ferromagnetic metal nanoparticles containing on the order of 100 atoms. The model that we consider is described by an effective giant-spin Hamiltonian, with a coupling to a random transverse magnetic field mimicking the effect of quasiparticle excitations and structural disorder on the gap structure of the spin collective modes. We find different types of time evolutions depending on the interplay between the disorder in the transverse field and the initial conditions of the system. In the absence of disorder, if the system starts from a low-energy state, there is one main coherent quantum tunneling event where the initial-state amplitude is completely depleted in favor of a few discrete states, with nearby spin quantum numbers; when starting from the highest excited state, we observe complete inversion of the magnetization through a peculiar ``backward cascade evolution''. In the random case, the disorder-averaged transition probability for a low-energy initial state becomes a smooth distribution, which is nevertheless still sharply peaked around one of the transitions present in the disorder-free case. On the other hand, the coherent backward cascade phenomenon turns into a damped cascade with frustrated magnetic inversion.