Vortex-like state observed in ferromagnetic contacts

Point-contacts (PC) offer a simple way to create high current densities, 10^9 A/cm^2 and beyond, without substantial Joule heating. We have shown recently (Nano Letters, 7 (2007) 927) that conductivity of nanosized PCs between a normal and ferromagnetic metals exhibits bi-stable hysteretic states versus both bias current and external magnetic field - the effect typical for spin-valve structures. Here we report that apart from the bi-stable state a third intermediate-resistance state is occasionally observed. We interpret this state as due to a spin-vortex in the PC, nucleated either by Oersted field of the bias current and/or by the circular geometry of PC. The observed three-level-states in the PC conductivity testify that the interface spins are both weakly coupled to the spins in the bulk and have depressed exchange interaction within the surface layer.Comment: 4 pgs., 4 figs. submitted to ICM-09 (July 26-21, Karlsruhe) V2: corrected typos, accepted for publication in J. Phys.: Conf. Serie

Spin Torques in Point Contacts to Exchange-Biased Ferromagnetic Films

Hysteretic magneto-resistance of point contacts formed between non-magnetic tips and single ferromagnetic films exchange-pinned by antiferromagnetic films is investigated. The analysis of the measured current driven and field driven hysteresis agrees with the recently proposed model of the surface spin-valve, where the spin orientation at the interface can be different from that in the bulk of the film. The switching in magneto-resistance at low fields is observed to depend significantly on the direction of the exchange pinning, which allows identifying this transition as a reversal of interior spins of the pinned ferromagnetic films. The switching at higher fields is thus due to a spin reversal in the point contact core, at the top surface of the ferromagnet, and does not exhibit any clear field offset when the exchange-pinning direction or the magnetic field direction is varied. This magnitude of the switching field of the surface spins varies substantially from contact to contact and sometimes from sweep to sweep, which suggests that the surface coercivity can change under very high current densities and/or due to the particular microstructure of the point contact. In contrast, no changes in the effect of the exchange biasing on the interior spins are observed at high currents, possibly due to the rapid drop in the current density away from nanometer sized point contact cores.Comment: 3 pages, 3 figs, presented on 11th Joint MMM-Intermag Conference, Jan. 18-22, 2010, Washington, US