C. L. Chien

Electric-field-assisted switching in magnetic tunnel junctions.

The advent of spin transfer torque effect accommodates site-specific switching of magnetic nanostructures by current alone without magnetic field. However, the critical current density required for usual spin torque switching remains stubbornly high around 10(6)-10(7) A cm(-2). It would be fundamentally transformative if an electric field through a voltage could assist or accomplish the switching of ferromagnets. Here we report electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities. These results represent a crucial step towards ultralow energy switching in magnetic tunnel junctions, and open a new avenue for exploring other voltage-controlled spintronic devices.

Granular magnetic solids (invited)

Granular metals consist of nanometer size metal granules embedded in an immiscible medium. They display a rich variety of physical properties as a result of their unique nanostructure and extra degrees of freedom. They are also suitable for the exploration of finite‐size effects, enhanced and tailored properties of fundamental interest, and for technological applications. Granular magnetic solids of elemental metals and alloys will be discussed. Single‐domain characteristics, superparamagnetic relaxation, enhanced ferromagnetic properties, granular alloys, and spin glasses are some of the topics covered.Granular metals consist of nanometer size metal granules embedded in an immiscible medium. They display a rich variety of physical properties as a result of their unique nanostructure and extra degrees of freedom. They are also suitable for the exploration of finite‐size effects, enhanced and tailored properties of fundamental interest, and for technological applications. Granular magnetic solids of elemental metals and alloys will be discussed. Single‐domain characteristics, superparamagnetic relaxation, enhanced ferromagnetic properties, granular alloys, and spin glasses are some of the topics covered.

Magnetic Properties of Epitaxial Mn-doped ZnO Thin Films

Epitaxial ZnO thin films doped with 7% Mn have been made by reactive rf magnetron sputtering onto (112_0) sapphire substrates at 400 °C. X-ray diffraction measurements reveal that the Zn0.93Mn0.07O film has a (0001) wurtzite single-crystal structure with a rocking curve width of 0.98°. UV–VIS absorption spectra show a band gap of 3.25 eV for pure ZnO films and 3.31 eV for the Zn0.93Mn0.07O film with states extending into the gap. The Auger electron spectroscopy shows homogeneous distribution of Mn in the film. The magnetic properties of the Zn0.93Mn0.07O film have been measured by a superconducting quantum interference device magnetometer at various temperatures with fields up to 5 T. No ferromagnetic ordering has been observed at temperature at 5 K. Instead, paramagnetic characteristics with a Curie–Weiss behavior have been observed.

Magnetic bistability and controllable reversal of asymmetric ferromagnetic nanorings.

Magnetization reversals through the formation of a vortex state and the rotation of an onion state are two processes with comparable probabilities for symmetric magnetic nanorings with a radius of about 50 nanometers. This magnetic bistability is the manifestation of the competition between the exchange energy and the magnetostatic energy in nanomagnets. The relative probability of the two processes in symmetric nanorings is dictated by the ring geometry and cannot be altered after fabrication. In this work, we report a novel type of nanorings--asymmetric nanorings. By tuning the asymmetry, we can control the fraction of the vortex formation process from about 40% to nearly 100% by utilizing the direction of the external magnetic field. The observed results have been accounted for by the dependence of the domain-wall energy on the local cross-section area for which we have provided theoretical calculations.

Intrinsic spin Seebeck effect in Au/YIG.

The acute magnetic proximity effects in Pt/YIG compromise the suitability of Pt as a spin current detector. We show that Au/YIG, with no anomalous Hall effect and a negligible magnetoresistance, allows the measurements of the intrinsic spin Seebeck effect with a magnitude much smaller than that in Pt/YIG. The experiment results are consistent with the spin polarized density functional calculations for Pt with a sizable and Au with a negligible magnetic moment near the interface with YIG.

Granular cobalt in a metallic matrix

Ultrafine face‐centered‐cubic Co particles in a conducting Cu matrix have been produced by annealing of sputtered metastable CoxCu1−x alloy films (0.10≤x≤0.80). The average size of the particles, and therefore the magnetic properties of these materials, can be easily controlled by the processing parameters. Single‐domain behavior results in coercivities in excess of 600 Oe, while the magnetization is determined by the choice of alloy composition. The evolution of the structural and magnetic properties have been studied as a function of annealing conditions, and are compared to those previously observed with Fe‐based materials.

Low-field magnetoresistance anisotropy in ultrathin Pr0.67Sr0.33MnO3 films grown on different substrates

We have conducted a comparative study of the strain effect on the anisotropic low-field magnetoresistance (LFMR) in ultrathin Pr0.67Sr0.33MnO3 films epitaxially grown on LaAlO3(LAO), NdGaO3(NGO), and SrTiO3(STO) substrates. Distinctive LFMR effects have been observed on films with compressive (on LAO), tensile (on STO), and nearly zero (on NGO) strains. The films with compressive strain show very large negative LFMR and MR hysteresis when a magnetic field is applied perpendicular to the film plane, while those with tensile strain show positive LFMR for the same field orientation. All samples show negative MR in a parallel magnetic field. These results can be qualitatively interpreted based on the strain-induced magnetic anisotropy.

Granular metal films as recording media

High‐density recording media require materials with a high magnetization and high coercivity as well as chemical stability, wear, and corrosion resistance. We explore the potential of granular metal films for recording media. Films of Fe granules about 150 A in size embedded in an amorphous SiO2 matrix exhibit coercivities as high as 3 kOe at low temperatures and 1.1 kOe at room temperature, and magnetizations of about 150 emu/g. The methods with which these materials are fabricated, the essential microstructure characterization, and magnetic measurements are described.

Granular Fe in a metallic matrix

Ultrafine Fe particles with coercivities in excess of 500 Oe have been obtained by rapid annealing of sputtered metastable FexCu1−x alloys. The enhanced magnetic properties of these phase‐separated materials are typical of single‐domain Fe grains, and can be controlled by the annealing temperature. The Cu matrix, however, is highly conducting, unlike conventional granular materials where the matrix is generally insulating. The evolution of the magnetic properties and microstructure during annealing are described.

Controllable High-Speed Rotation of Nanowires

We report a versatile method for executing controllable high-speed rotation of nanowires by AC voltages applied to multiple electrodes. The rotation of the nanowires can be instantly switched on or off with precisely controlled rotation speed (to at least 1800 rpm), definite chirality, and total angle of rotation. We have determined the torque due to the fluidic drag force on nanowire of different lengths. We also demonstrate a micromotor using a rotating nanowires driving a dust particle into circular motion. This method has been used to rotate magnetic and nonmagnetic nanowires as well as carbon nanotubes.