Room temperature magnetoresistance switching of Permalloy thin films induced by iron nanoparticles Appl. Phys. Lett. 92, 093121 (2008); Negative resistance contribution of a domain-wall structure in a constricted geometry

We have studied the giant magnetoresistance ͑GMR͒ in magnetic multilayer point contacts of three different types. The first generation contacts were made by deposition with molecular-beam epitaxy ͑MBE͒ of an uncoupled Co/Cu multilayer on a pre-etched hole in a thin membrane. These devices exhibited a GMR, but its ratio was low and, as deduced from finite element calculations, in many cases was dominated by the resistance of the multilayer electrode. When corrected for this, the maximum point-contact GMR was 3%. The multilayer structure at some depth in the constriction was disrupted, as observed by transmission electron microscopy. This was identified as a cause of the low GMR, together with contamination and an oxide layer in the constriction, resulting from ex situ sample rotation. The second generation was fabricated by sputtering of a coupled Co/Cu multilayer before etching of the nanohole, giving a proper multilayer at the constriction. Further, the GMR signal from the electrode was shorted by a thick Cu cap. This did not bring the expected increase of the GMR (ratioр5%), indicating that the so-called dead layers and the quality of the interface between the GMR system and the contacting metal were limiting. This interface quality was strongly improved for the third generation of contacts by using in situ rotation, while the question of multilayer quality was avoided by shifting to granular Co/Au. Granular Co/Au in the constriction was obtained by growing a discontinuous Co layer by MBE. The maximum GMR ratio of the granular contacts was 14%, an improvement of a factor 3. These contacts displayed small jumps in the GMR, two-level fluctuations in the resistance time trace and ballistic transport, the latter being evident from phonon peaks in the point-contact spectrum of a high resistance contact. ͓S0163-1829͑99͒01537-4͔

We use nanometer-sized point contacts to a Co/Cu spin valve to study the giant magnetoresistance (GMR) of only a few Co domains. The measured data show strong device-to-device differences of the GMR curve, which we attribute to the absence of averaging over many domains. The GMR ratio decreases with increasing bias current. For one particular device, this is accompanied by the development of two distinct GMR plateaus, the plateau level depending on bias polarity and sweep direction of the magnetic field. We attribute the observed behavior to current-induced changes of the magnetization, involving spin transfer due to incoherent emission of magnons and self-field effects.

Influence of magnetic domain-wall width and shape on magnetoresistance measurementsWe study the magnetoresistance ͑MR͒ of Py/Py, Co/Py, Co/Co, Ni/Ni, and Co/Cu point contacts (PyϭpermalloyϭNi 80 Fe 20 ). These devices are narrow constrictions or channels ͑diameter, length Ϸ30 nm͒ between two thin film electrodes. Due to the small size of the constriction, which is comparable to a bulk domain-wall ͑DW͒ thickness, a DW can be caught in it. For almost all material combinations studied we find that low resistance contacts show an MR minimum at zero field (Hϭ0) of magnitude 0.4%-1.3%, for temperatures between 1.5 and 293 K. The minimum occurs for all field orientations with respect to the channel axis. When the contact resistance increases beyond the value set by a diameter-to-length ratio for the channel of about unity, the resistance minima at Hϭ0 evolve into a maximum/minimum combination as expected for a predominant anisotropic magnetoresistance ͑AMR͒ effect. We use micromagnetic calculations based on magnetostatic and exchange interactions to obtain the magnetization in the constriction. These calculations predict that, due to the finite channel length, there are two partial DWs at either side of the channel. For high resistance contacts this agrees with the observed AMR, which results from scattering in the homogeneously magnetized material in the channel. The MR minimum for low resistance contacts arises from the DWs, which cause a resistance decrease. We attribute this decrease to a change of spin-dependent diffuse scattering at the constriction boundary due to the DWs.

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