Memory effects in individual submicrometer ferromagnets

Abstract

We have used ballistic Hall micromagnetometry to study the magnetization of individual submicrometer nickel disks ͑80 nm high, 0.1-1.0 m diameter͒. At low temperatures, hysteresis loops of the disks no longer show inversion symmetry in a magnetic field, as if the time reversal symmetry were broken. Furthermore, the magnetization of the smallest disks can be ''frozen'' in two possible states that are characterized by hysteresis loops which are each other's inverse. At temperatures below 19.5 K a magnetic field as high as 2 T cannot switch between the states, proving that it is extremely difficult to fully polarize a small ferromagnetic particle. On the other hand, at slightly higher temperatures ͑only TϾ19.8 K), a field as low as 0.1 T appears to be enough to fully polarize the disks. We attribute this extraordinary behavior to the glass-liquid transition experienced by spins at the particle surface. ͓S0163-1829͑98͒06542-4͔

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

“…By measuring the electrical resistance of isolated Ni wires with diameters between 20 and 40 nm, Giordano and Hong studied the motion of magnetic domain walls [24,25]. Other low temperature techniques which may be adapted to single particle measurements are Hall probe magnetometry [26,27,28], magnetometry based on magnetoresistance [29,30,31] or spin-dependent tunneling with Coulomb blockade [32,33]. At the time of writing, the micro-SQUID technique allows the most detailed study of the magnetization reversal of nanometer-sized particles [34,35,36,37,38,39].…”

Three-Dimensional Magnetization Reversal Measurements in Nanoparticles

“…In the past few years, this has no longer been the case because of the emergence of new fabrication techniques which have allowed the fabrication of small objects with the required structural and chemical qualities. In order to study these mesoscopic objects, new techniques were developed such as magnetic force microscopy [3], magnetometry based on micro-Hall probes [4,5], micro-SQUIDs [6], or magnetometry based on spindependent tunneling with Coulomb blockade [7]. These techniques lead to a new understanding of the magnetic behavior of nanoparticles, which is very important for the development of new fundamental theories of magnetism and for modeling new magnetic materials for permanent magnets or high density recording.…”

Spectroscopy of discrete energy levels in ultrasmall metallic grains

“…However, independently of whether all open questions can be resolved or not, the fact that magnetic anisotropy causes each resonance energy in the tunnel spectrum to shift reproducibly by on the order of 0.1 meV as H is swept about the hysteresis loop has a very interesting potential application: it could be used as a tool to perform detailed studies of the dynamics of magnetization reversal in individual nm-scale grains, since the jumps occuring in the tunneling spectra upon reversal of the magnetization allow one to monitor precisely when, as function of ramped applied field or of waiting time, this magnetization reversal occurs. Thus this method would be complementary to magnetic force microscopy [174], Hall magnetometry [175], and SQUID techniques [176,177] for studying magnetization reversal.…”