Classical and Quantum Magnetization Reversal Studied in Nanometer-Sized Particles and Clusters

Nanometer-sized magnetic particles have generated continuous interest since the late 1940s because the study of their properties has proved to be scientifically and technologically very challenging. In particular, it was recognized that the ferromagnetic state, with a given orientation of the particle moment, has a remanent magnetization if the particle is small enough. This was the starting point of huge permanent magnets and magnetic recording industries. However, despite intense activity during the last few decades, the difficulties in making nanoparticles of good enough quality have slowed the advancement of this field. As a consequence, for 50 years, these applications concentrated above and then near the micrometer scale. In the last decade, this has no longer been the case because of the emergence of new fabrication techniques that have led to the possibility of making small objects with the required structural and chemical qualities. In order to study these objects, new techniques such as magneticforce microscopy, magnetometry based on micro-Hall probes or micro-SQUIDs(superconducting quantum interference device) were developed. This led to a new understanding of the magnetic behavior of nanoparticles, which is now very important for the development of new fundamental theories of magnetism and in modeling new magnetic materials for permanent magnets or high density recording.