High power and low critical current density spin transfer torque nano-oscillators using MgO barriers with intermediate thickness.

Reported steady-state microwave emission in magnetic tunnel junction (MTJ)-based spin transfer torque nano-oscillators (STNOs) relies mostly on very thin insulating barriers [resulting in a resistance × area product (R × A) of ~1 Ωµm2] that can sustain large current densities and thus trigger large orbit magnetic dynamics. Apart from the low R × A requirement, the role of the tunnel barrier in the dynamics has so far been largely overlooked, in comparison to the magnetic configuration of STNOs. In this report, STNOs with an in-plane magnetized homogeneous free layer configuration are used to probe the role of the tunnel barrier in the dynamics. In this type of STNOs, the RF modes are in the GHz region with integrated matched output powers (P out ) in the range of 1-40 nW. Here, P o u t values up to 200 nW are reported using thicker insulating barriers for junctions with R × A values ranging from 7.5 to 12.5 Ωµm2, without compromising the ability to trigger self-sustained oscillations and without any noticeable degradation of the signal linewidth (Γ). Furthermore, a decrease of two orders of magnitude in the critical current density for spin transfer torque induced dynamics (J STT ) was observed, without any further change in the magnetic configuration.

Magnetic tunnel junctions with integrated thermometers for magnetothermopower measurements.

Magnetic tunnel junction (MTJ) micropillars were fabricated with integrated thermometers and a heater line (HL) for thermovoltage measurements. This novel thermometer configuration enabled a direct measurement of ΔT across the MTJ micropillar. The MTJ devices were patterned from a CoFeB/MgO/CoFeB stack, with a 1.2 nm to 1.6 nm MgO wedge across the wafer, resulting in resistance area products in the range of 0.7 kΩ · µm2 < R × A < 55 kΩ · µm2. This allowed the measurement of thermoelectric properties as a function of the tunnel barrier thickness. The thermometers showed a homogeneous heating behavior for all devices across the wafer. Combining the in-stack temperature measurements and finite element simulations the thermal profile across the MTJ structure and the thermopower were estimated with a noticeable improvement of the measurement accuracy. The studied MTJ structures showed tunneling magnetoresistance (TMR) ratios up to 125%, and tunneling magnetothermopower (TMTP) up to 35%.

Tunneling magnetothermopower in magnetic tunnel junction nanopillars.

We study tunneling magnetothermopower (TMTP) in CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars. Thermal gradients across the junctions are generated by an electric heater line. Thermopower voltages up to a few tens of µV between the top and bottom contact of the nanopillars are measured which scale linearly with the applied heating power and hence the thermal gradient. The thermopower signal varies by up to 10 µV upon reversal of the relative magnetic configuration of the two CoFeB layers from parallel to antiparallel. This signal change corresponds to a large spin-dependent Seebeck coefficient of the order of 100 µV/K and a large TMTP change of the tunnel junction of up to 90%.

Biased quasiballistic spin torque magnetization reversal.

We explore the ultrafast limit of spin torque magnetization reversal time. Spin torque precession during a spin torque current pulse and free magnetization ringing after the pulse is detected by time-resolved magnetotransport. Adapting the duration of the pulse to the precession period allows coherent control of the final orientation of the magnetization. In the presence of a hard axis bias field, we find optimum quasiballistic spin torque magnetization reversal by a single precessional turn directly from the initial to the reversed equilibrium state.

Magnetotransport properties depending on the nanostructure of Fe(3)O(4) nanowires.

We have studied the magnetic behaviour of Fe(3)O(4) nanowires (NWs) with two different diameter ranges, above 150 nm and below 60 nm, made by electrodeposition techniques into a polymeric template. The nanowires were characterized using various techniques, in particular Mössbauer and thermoelectrical power measurements. The stoichiometric distribution of Fe cations showed clearly the presence of the magnetite inverse spinel electronic structure. Structural analysis performed using high-resolution transmission electron microscopy revealed two kinds of nanowire morphologies depending on the size. For nanowires above 150 nm in diameter, a contiguous network of well-bound nanoparticles was obtained. Instead, with a diameter of 60 nm, a polycrystalline structure was observed. The largest nanowires presented a magnetoresistance (MR) greater than 10%, whereas the thinner nanowires had almost none.