J.-E. Wegrowe

Magnetic relaxation experiments constitute a unique method of determining the nature of fluctuations in dissipative magnetic systems. At high temperatures these fluctuations are thermal and strongly temperature dependent. At low temperatures, where quantum fluctuations dominate, magnetic relaxation becomes independent of temperature. Such behavior has been observed in many systems. In this review we emphasize the study of low temperature relaxation in ferromagnetic nanoparticles, layers, and multilayers (including ‘‘domain wall junctions’’), and large single crystals. The results of magnetic relaxation experiments are shown to agree with theoretical predictions of quantum tunneling of the magnetization. When dissipation becomes important, in large and complex systems, a time dependent WKB exponent needs to be introduced.

We present magnetization measurements of individual amorphous Co particles (300 nm\ifmmode\times\else\texttimes\fi{}200 nm\ifmmode\times\else\texttimes\fi{}30 nm) patterned by electron-beam lithography. The hysteresis loops show mainly two magnetization jumps corresponding to domain-wall nucleation and annihilation. The angular dependence of these magnetization jumps is measured and the temperature dependence (0.1--6 K) of the nucleation and the annihilation processes is investigated by two independent methods: switching field and switching time measurements. Finally we compare the results obtained on one particle with those of an array of about 2\ifmmode\times\else\texttimes\fi{}${10}^{7}$ particles. We show experimentally that the dynamical properties measured on individual particles are connected with an unusual 1/T relaxation of the array of particles. \textcopyright{} 1996 The American Physical Society.

Magnetization reversal triggered by spin injection is measured in electrodeposited Co/Cu/Co pillars (diameter about 60 nm). Two protocols are used. (i) switching of magnetization after a current pulse is monitored as a function of applied field. The maximum offset from the switching field at which irreversible switching occurs is a measure of the strength of the effect; and (ii) irreversible and reversible magnetization changes are observed while the current is ramped at fixed applied field. (i) and (ii) show that irreversible transitions occur only from antiparallel to parallel magnetic configurations and for electrons flow from the polarizer to the analyzer.

Magnetoresistance measurements in the CPP geometry have been performed on single electrodeposited Co nanowires exchange biased on one side by a sputtered amorphous ${\mathrm{GdCo}}_{1.6}$ layer. This geometry allows the stabilization of a single domain wall in the Co wire, the thickness of which can be controlled by an external magnetic field. Comparing magnetization, resistivity, and magnetoresistance studies of single Co nanowires, of ${\mathrm{GdCo}}_{1.6}$ layers, and of the coupled system, gives evidence for an additional contribution to the magnetoresistance when the domain wall is compressed. This contribution could be interpreted as the spin-dependent scattering within the domain wall when the wall thickness becomes smaller than the spin diffusion length.

It is shown that a pulsed current driven through Ni nanowires provokes an irreversible magnetization reversal at a field distant from the spontaneous switching field $H_{sw}$ by $\Delta H$ of as much as 40 % of $H_{sw}$. The state of the magnetization is assessed by magnetoresistive measurements carried out on single, isolated nanowires. The reversible part of the magnetization follows that of a uniform rotation. The switching occurs between the two states accessible otherwise by normal field ramping. $\Delta H$ is studied as a function of the angle between the applied field and the wire, and also of the direction of the pulsed current. The results are interpreted in terms of spin-flip transfer from the spin-polarized current to the magnetization, while the switching is approximated by a curling reversal mode.