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Register a new userSpin-transfer switching and low-field precession in exchange-biased spin valve nano-pillars
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Using a three-dimensional focused-ion beam lithography process we have fabricated nanopillar devices which show spin transfer torque switching at zero external magnetic fields. Under a small in-plane external bias field, a field-dependent peak in the differential resistance versus current is observed similar to that reported in asymmetrical nanopillar devices. This is interpreted as evidence for the low-field excitation of spin waves which in our case is attributed to a spin-scattering asymmetry enhanced by the IrMn exchange bias layer coupled to a relatively thin CoFe fixed layer.
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We report magnetization and magetoresistance measurements in hybrid ferromagnetic metal/semiconductor heterostructures comprised of MnAs/(Ga,Mn)As bilayers. Our measurements show that the (metallic) MnAs and (semiconducting) (Ga,Mn)As layers are exchange coupled, re- sulting in an exchange biasing of the magnetically softer (Ga,Mn)As layer that weakens with layer thickness. Magnetoresistance measurements in the current-perpendicular-to-the-plane geometry show a spin valve effect in these self-exchange biased bilayers. Similar measurements in MnAs/p- GaAs/(Ga,Mn)As trilayers show that the exchange coupling diminishes with spatial separation between the layers.
Recently it has been predicted that a spin-polarized electrical current perpendicular-to-plane (CPP) directly flowing through a magnetic element can induce magnetization switching through spin-momentum transfer. In this letter, the first observation of current-induced magnetization switching (CIMS) in exchange-biased spin-valves (ESPVs) at room temperature is reported. The ESPVs show the CIMS behavior under a sweeping dc current with a very high critical current density. It is demonstrated that a thin Ruthenium (Ru) layer inserted between a free layer and a top electrode effectively reduces the critical current densities for the CIMS. An inverse CIMS behavior is also observed when the thickness of the free layer increases.
First-principles density-functional theory calculations show switching magnetization by 90 degree can be achieved in ultrathin BFO film by applying external electric-field. Up-spin carriers appear to the surface with positive field while down-spin ones to the negative field surface, arising from the redistribution of Fe-t2g orbital. The half-metallic behavior of Fe-3d states in the surface of R phase film makes it a promising candidate for AFM/FM bilayer heterostructure possessing electric-field tunable FM magnetization reversal and opens a new way towards designing spintronic multiferroics. The interface exchange-bias effect in this BFO/FM bilayer is mainly driven by the Fe-t2g orbital reconstruction, as well as spin transferring and rearrangement.
We present switching field distributions of spin-transfer assisted magnetization reversal in perpendicularly magnetized Co/Ni multilayer spin-valve nanopillars at room temperature. Switching field measurements of the Co/Ni free layer of spin-valve nanopillars with a 50 nm x 300 nm ellipse cross section were conducted as a function of current. The validity of a model that assumes a spin-current dependent effective barrier for thermally activated reversal is tested by measuring switching field distributions under applied direct currents. We show that the switching field distributions deviate significantly from the double exponential shape predicted by the effective barrier model, beginning at applied currents as low as half of the zero field critical current. Barrier heights extracted from switching field distributions for currents below this threshold are a monotonic function of the current. However, the thermally-induced switching model breaks down for currents exceeding the critical threshold.
We investigate numerically the transverse versus vortex phase diagram of head-to-head domain walls in Co/Cu/Py spin valve nano-stripes (Py: Permalloy), in which the Co layer is mostly single domain while the Py layer hosts the domain wall. The range of stability of the transverse wall is shifted towards larger thickness compared to single Py layers, due to a magnetostatic screening effect between the two layers. An approached analytical scaling law is derived, which reproduces faithfully the phase diagram.
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