Correlation between magnetism and spin-dependent transport in CoFeB alloys

We report a correlation between the spin polarization of the tunneling electrons (TSP) and the magnetic moment of amorphous CoFeB alloys. Such a correlation is surprising since the TSP involves s-like electrons close to the Fermi level (EF), while the magnetic moment mainly arises due to all d-electrons below EF. We show that probing the s and d-bands individually provides clear and crucial evidence for such a correlation to exist through s-d hybridization, and demonstrate the tuneability of the electronic and magnetic properties of CoFeB alloys.Comment: Accepted for publication in Physical Review Letters. Letter (4 pages) and Supplementary material (4 pages

Magnetism in Co80-xFexB20 : effect of crystallization

We report on the change in the structural and magnetic properties of magnetically soft ternary Co80-xFexB20 alloys as a function of composition, thickness, and annealing temperature. Compositions high in cobalt show a significant change in coercivity after annealing. This is explained using the random anisotropy model by relating the magnetic exchange length to the grain size of the crystallites. The presented results are a systematic study explaining trends seen in the transition from soft to hard magnetic behavior, providing insight into why the soft CoFeB alloys have been so successful recently in spintronic device

Mn diffusion and the thermal stability of tunneling spin polarization

We examine the role of Mn diffusion in the thermal stability of tunneling spin polarization P by directly measuring P of Al/AlOx/Co/FeMn and Al/AlOx/Co90Fe10/FeMn junctions using superconducting tunneling spectroscopy (STS). We confirm Mn diffusion in our junctions using x-ray photoelectron spectroscopy after an ultrahigh vacuum 500 °C anneal. Surprisingly, and in contrast to the current belief, no drop in P is observed using STS. Therefore, though Mn diffuses significantly, it cannot be solely responsible for the drop in tunneling magnetoresistance observed after postdeposition anneals above 300 °C. ©2005 American Institute of Physic

Rigid Exchange Coupling in Rare-Earth-Lean Amorphous Hard/Soft Nanocomposites

Electrification of vehicles and renewable energy is increasing the demand for permanent magnets, but the cost and scarcity of rare-earth metals is an obstacle. Creating nanocomposites of rigidly exchange-coupled hard and soft magnets, for which the magnetization reversal occurs as in a single magnetic-phase material, is a promising route toward rare-earth-lean permanent magnets with high energy products. The hard/soft exchange coupling is, however, often reduced due to rough interfaces and structural defects, resulting in exchange-spring behavior rather than rigid exchange coupling. Here, it is shown that artificially sandwiched hard and soft amorphous magnets produced by magnetron sputtering exhibit smooth interfaces, and the first order reversal curve (FORC) technique is used to show that the hard and the soft phases are rigidly exchange coupled. Micromagnetic simulations, using a random-anisotropy model, are used to predict the thickness limit of the rigid exchange coupling. A great advantage of amorphous hard/soft composites is the possibility to obtain a wide range of magnetic properties by finely tuning the composition of the individual phases

Control of speed and efficiency of ultrafast demagnetization by direct transfer of spin angular momentum

Since the discovery in 1996 that the magnetization of a nickel thin film is reduced within a few picoseconds after femtosecond-laser excitation, ultrafast demagnetization has attracted a thriving interest. This attraction is driven by the twofold challenge of understanding magnetization dynamics in a strongly out-of-equilibrium regime and controlling the magnetic properties of materials on the subpicosecond timescale, with potential applications in spintronics. In the past decade significant progress has been made in understanding the microscopic processes that govern ultrafast demagnetization. The discussion has been particularly focused on the role of angular-momentum conservation during the demagnetization process. Here, using the time-resolved magneto-optical Kerr effect, we demonstrate that interlayer transfer of spin angular momentum in specially engineered Co/Pt multilayers speeds up the demagnetization process, bypassing the mechanism responsible for the conservation of total angular momentum taking place in a single ferromagnetic layer. This new channel for spin-angular-momentum dissipation leads to a reduction of the demagnetization time of up to 25%, accompanied by an increase of the total demagnetization by almost the same amoun