Research Overview
Quantum Tunneling of the magnetization
Our research interests are focused on the study of how the microscopic laws of physics –quantum mechanics– manifest themselves at a macroscopic scale. Nanoscale magnetic systems are excellent candidates for these studies due to the fact that one can go from quantum to classical regimes by changing the size of the system under study. One approach to this is by decreasing the size of a magnet to nanometers. When the magnet becomes small enough its quantum properties play an important role. On the other hand, the bottom-up approach consists in taking a quantum (“small”) system and increase its size until arriving at the macroscopic scale. As the system becomes bigger and bigger, more degrees of freedom are available to interact with the environment and destroy its quantum properties.
In general, the size, shape, composition, orientation and other properties of the system, like anisotropy or intrinsic interactions (i.e. dipolar, hyperfine or exchange interactions), are the sources that govern the way in which a system behaves classically or quantum mechanically. An understanding of the interactions that destroy the quantum properties of a macroscopic system (i.e. decoherence) are of fundamental importance in Physics. Nanometer sized magnetic systems offer a great possibility to explore this topic.
We are particularly interested in chemically synthesized magnetic nanostructures and, in particular, materials known as single molecule magnets (SMMs). SMMs consist of a core of strongly exchange-coupled transition metal ions that collectively have a very large magnetic moment per molecule. SMM crystals have a number of advantages over other types of magnetic nanostructures. Most importantly, they are monodisperse—each molecule in the crystal has the same spin, orientation, magnetic anisotropy and atomic structure. They thus enable fundamental studies of properties intrinsic to magnetic nanostructures that have previously been inaccessible. One of the most important results obtained with these compounds is the observation of quantum tunneling between different states of the magnetization of the molecule, leading to a step-wise hysteresis magnetization curve with acceleration of the magnetic relaxation for fields where quantum tunneling is turned on. Such quantum effects are also widely studied in superconducting quantum interference devices (SQUIDs) and quantum dots.
Condensed matter systems like these, with discrete energy levels coupled to the environment in the solid, are important to understanding the border between quantum and classical physics, and the decoherence of quantum systems. From a technological point of view, these systems have several new potential applications. As an example, SMMs have recently been proposed for use in quantum computing and quantum information storage. They also represent the ultimate limit to classical magnetic information storage, with one bit per molecule and possible storage densities many orders of magnitude greater than present day magnetic media. Understanding the quantum properties in these materials and relations with their structural characteristics will enable the creation of new and more adequate molecular compounds useful for quantum information technology and prototypes to study fundamental aspects of nanoscale physics.
This is a rapidly advancing field with many recent fundamental discoveries. For example, recently a crossover between quantum and thermal regimes of magnetic relaxation has been observed and studied. Berry phase interference in magnetic quantum has also been seen, with minima in the tunnel rates for discrete values of an external transverse magnetic field. This allows turning on and off the quantum tunneling by slightly varying the magnitude of an applied magnetic field. Moreover, a very recent observation establishes that quantum magnetic states of neighboring molecules can be quantum entangled. These findings lead to a very fast grow of the field since 1996, involving close to 30 research groups around the world, both theoretical and experimental, working with different techniques of measurement. This includes dc and ac magnetometry, Electron Paramagnetic Resonance, Neutron scattering or Calorimetric experiments as well as some new high sensitivity techniques that have been developed in the field (i.e. micro-Hall effect or micro-SQUID magnetometry)I
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