M. Brands

Multiple switching fields and domain wall pinning in single Co nanowires

We have measured the low temperature magnetoresistance (MR) of ferromagnetic cobalt nanowires of 30 nm thickness and widths between 32 and 700 nm. The wires are in situ capped with a 2 nm Pt layer to prevent oxidation. Single wires exhibit sharp resistance peaks at the coercive fields Hc—where Hc increases with decreasing wire width—which can be completely understood in terms of an in-plane switching process. Adding 2 × 2 µm2 nucleation pads at the wire ends clearly reduces the coercive fields of the wires. By fabricating wires with two different widths a two-step switching process can be achieved where the wider nanowire switches first. In this case, a single domain wall is introduced in the constriction between the wider and the narrower part of the wire. The MR can be studied in more detail and with higher accuracy by adding additional voltage leads close to the domain wall. The results are discussed to distinguish between contributions resulting from domain wall magnetoresistance and anisotropic magnetoresistance (AMR).

Spin accumulation in ferromagnetic/nonmagnetic devices

We have fabricated ferromagnetic/nonmagnetic (FM/NM) metal heterojunctions for the detection of the spin accumulation effect in different nonmagnetic metals. The polycrystalline heterojunctions are prepared by high resolution electron beam lithography (HR-EBL) and a special oblique evaporation technique. The ferromagnetic (FM) and the nonmagnetic (NM) metal are evaporated on top of each other in a single evaporation process to achieve a clean interface between the two metals. The spin accumulation effect is detected in nonmagnetic copper (Cu) and aluminum (Al), from which we determine the polarization of the interface between the ferromagnetic and nonmagnetic metal.

Absence of weak electron localization in ferromagnetic Co nanowires

Co nanowires are fabricated by standard electron beam lithography (EBL) and lift-off technique and by subsequent electron beam evaporation of Co. Some of the wires are covered with either Pt or C in order to prevent oxidation. Magentoresistance measurements are carried out in a He bath-cryostat with applied magnetic fields of up to B = 5.0 T in out-of-plane direction. Structural investigations reveal that the Co-wires have a polycrystalline morphology. Electron diffraction patterns indicate the predominance of hexagonal close packed (HCP) Co. Magnetic force microscopy (MFM) investigations show that all Co-wires are in a single-domain state at remanence with the magnetization lying in-plane along the long wire axis due to their strong shape anisotropy.

Absence of weak electron localization in carbon-covered Co-nanowires

We have investigated the low temperature resistance and magnetoresistance behavior of ferromagnetic cobalt wires in perpendicular external magnetic fields of up to B=5 T. Magnetic force microscopy investigations show that the cobalt wires are single-domain at remanence, if the wire width is smaller than about 800 nm. At low temperatures a logarithmic resistance increase is observed which is consistently explained as originating from enhanced electron-electron interaction (EEI) in two dimensions, while the magnetoresistance is dominated by the anisotropic magnetoresistance. Additional resistance corrections due to weak electron localization (WEL) are not observed within the accuracy of our measurements, which is discussed in the framework of recent experimental and theoretical works.