Magnetoresistive effects in Co/Pd multilayers on self-assembled nanoparticles (invited)

The deposition of Co/Pd multilayers onto self-assembled spherical particles provides a system with unique magnetic properties. The magnetic caps have high perpendicular magnetic anisotropy, are single-domain, and strongly exchange decoupled, but in electrical contact with each other, thus enabling magnetotransport measurements. By applying an external magnetic field, the caps can be switched individually. We systematically studied the magnetoresistance on a two-dimensional cap array consisting of Co/Pd multilayers deposited on particles with a diameter of 200 nm. In the vicinity of the coercive field, a hysteretic resistance peak occurs. It can be explained with the random magnetization configuration of the magnetic caps leading to an increased spin-dependent scattering of the conduction electrons. The underlying mechanism might be comparable to the one causing giant magnetoresistance in granular alloys. For temperatures above 77 K, additional resistivity contributions with high saturation fields are observed, which are tentatively explained by the decreasing size of magnetically ordered parts of the caps with increasing temperature, resulting finally in superparamagnetic behavior in the contact area between neighboring caps