Quantum tunneling of magnetization

Abstract

As early as 1955, L Weil found evidence that a fraction of single domain (SD) particles in a sample of nickel powder failed to reach their “blocking temperature” down to 1K. This observation was later interpreted by Bean and Livingston as possibly being a result of switching via a process of quantum tunneling. In the past few years, a theoretical framework for processes involving quantum tunneling of magnetization (QTM) has been developed, stimulated by the general interest in Macroscopic Quantum Tunneling (MQT) as well as the practical importance of QTM in setting a limit on the lifetime of information storage on magnetic tapes. In this article, we will discuss primarily QTM in SD ferromagnetic particles. We will also review the subjects of dissipation and switching via thermal activation: In order to analyze experiments at the low temperatures at which QTM might be taking place, it is essential to fully understand behavior in the better understood classical regime, at higher temperatures. Furthermore, dissipation plays a key role in switching at all temperatures. All relaxation processes thus far studied theoretically are so weak that they bring the system into the so-called “low-dissipation” regime, for which a complete theory of MQT at non-zero temperatures is lacking. We will discuss the very exciting experiments on small magnetic particles which are being carried out at IBM Research Laboratories, by summarizing the experimental results and comparing them with theory, noting the failure of QTM theory to account for the observations.