B. J. Hickey

Characterization of spin valves fabricated on opaque substrates by optical ferromagnetic resonance

We have used a transmission line deposited on a transparent substrate to deliver an optically triggered magnetic field pulse to a spin valve structure deposited upon an opaque substrate. The ensuing ferromagnetic resonance oscillations have been studied in optical pump-probe experiments in which the probe passes through the transmission line substrate. The resonance frequencies have been modeled by solving the Landau–Lifshitz equation and are used in determining the anisotropy, exchange bias, and interlayer coupling parameters of the sample.

Room temperature magnetic stabilization of buried cobalt nanoclusters within a ferromagnetic matrix studied by soft x-ray magnetic circular dichroism

Single dusting layers of size-selected Co nanoclusters (NCs) of sizes ranging from 1.5–5.5 nm have been deposited by a gas-phase aggregation method in ultrahigh vacuum, and embedded within a NiFe matrix. Magnetic hysteresis loops have been obtained using soft x-ray magnetic circular dichroism, which shows that these Co NCs embedded in NiFe exhibit room temperature ferromagnetism with identical coercivity to the surrounding NiFe film. The strong local exchange field at the interface between NiFe and Co NCs, combined with the magnetic anisotropy of the NiFe film, allows stabilization of NC ferromagnetism which persists to room temperature.

In situ transport in alumina-based magnetic tunnel junctions during high-vacuum annealing

We have performed in situ transport measurements on CoFeB∕AlO∕Co spin-valve magnetic tunnel junctions during annealing up to 200u2009°C. For optimally oxidized junctions we observe a marked decrease in the parallel state junction resistance, which occurs at around 150u2009°C and continues throughout the anneal process until the temperature is reduced back below 150u2009°C. Spectroscopic measurements reveal a stronger bias dependence due to enhanced magnon-assisted tunneling, and show that features relating to weak fcc texture in the cobalt upper electrode fade after annealing at 200u2009°C. We attribute this to a possible amorphization of the cobalt electrode in close proximity to the barrier interface.

In situ transport measurements of plasma-oxidized MgO magnetic tunnel junctions during the annealing process

The postdeposition annealing of magnetic tunnel junctions has become standard practice in junction fabrication to improve barrier properties. We have performed tunneling measurements at elevated temperatures during the annealing process on plasma-oxidized, MgO barrier, magnetic tunnel junctions. The data were interpreted in terms of the Hartman model which accounts for an asymmetric barrier profile. The results could not be interpreted solely by the temperature smearing of the Fermi distribution and so the electrode/barrier/electrode structure had been altered during the annealing. While the barrier height at one interface improved, the other was degraded. Our experiments lead us to believe that this is due to the diffusion of Mn to the barrier/pinned electrode interface. We show that through transport and tunneling magnetoresistance measurements it is possible to monitor physical processes such as material migration during the annealing process.

Additional sub-gap conductance enhancement in nanoscale Andreev point contact junctions

We demonstrate an enhancement in the sub-gap conductance in Nb/Al/Cu nanoscale pinhole junctions, significantly larger than the conductance doubling expected from the point contact Andreev reflection process. Such an enhancement is not observed in standard micrometre-driven NbTi/Cu wire-tipped junctions where the expected sub-gap conductance is regained. This additional sub-gap conductance arises due to regions of the pinhole junction being driven into the normal state due to current-induced pair breaking, causing the Andreev reflection process to occur at different interfaces within the contact structure as the bias is increased. This work demonstrates, again, that a great deal of care must be taken in interpreting Andreev reflection measurements in nanoscale junctions, in particular with regard to extracting accurate spin polarization.

Temperature-driven band motion prior to the phase transition of an itinerant ferromagnet

Tunneling magnetoresistance spectroscopy measurements have been performed on spin-valve magnetic tunnel junctions where the free magnetic electrode is a Cu38Ni62 alloy with a Curie temperature of around 240K. These measurements allow us to follow the temperature dependence of the energy of the bottom of a minority-spin band close to the Fermi energy, which is partly responsible for the tunneling conductance. The temperature dependence of the tunneling spin polarization of the CuNi electrode has also been extracted using the Julliere formula. We show that the tunneling spin polarization as a function of temperature can be related to the temperature-dependent magnetization using an extension of the commonly applied Stearns model that incorporates the motion of the band close to the Fermi level. This model reproduces the nonlinear relation between tunneling spin polarization and magnetization obtained for Cu38Ni62.