W. G. Wang

Real-time evolution of tunneling magnetoresistance during annealing in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

We report the study of the real-time evolution of tunneling magnetoresistance (TMR) in CoFeB∕MgO∕CoFeB junctions during annealing at 380°C. The TMR quickly developed at the early stage of the annealing, with 200% magnetoresistance observed in less than 10min, followed by a slow approach to saturation. This evolution of TMR was correlated with the structural changes, including crystallization of amorphous CoFeB electrodes and improvement of barrier quality during the annealing.

Tunneling magnetoresistance in (001)-oriented FeCo∕MgO∕FeCo magnetic tunneling junctions grown by sputtering deposition

Magnetic tunneling junctions (MTJs) with MgO tunnel barrier have been fabricated on both oriented and nonoriented buffer layers on Si(001) substrate by magnetron sputtering. FeCo∕MgO∕FeCo MTJs fabricated on oriented buffer layers show larger tunneling magnetoresistance (TMR) value up to 84% without high temperature postannealing, whereas those MTJs on nonoriented buffer layers show 45% of TMR. The high-resolution transmission electron microscopy images reveal an excellent morphology and very coherent crystal structure with FeCo(001)[110]∕MgO(001)[100]∕FeCo(001)[110] orientation. The results indicate that high TMR can be achieved without high temperature postannealing by sputtering deposition on appropriate oriented buffer layers.

All-optical vector measurement of spin-orbit-induced torques using both polar and quadratic magneto-optic Kerr effects

We demonstrate that the magneto-optic-Kerr effect with normal light incidence can be used to obtain quantitative optical measurements of both components of spin-orbit-induced torque (both the antidamping and effective-field components) in heavy-metal/ferromagnet bilayers. This is achieved by analyzing the quadratic Kerr effect as well as the polar Kerr effect. The two effects can be distinguished by properly selecting the polarization of the incident light. We use this all-optical technique to determine the spin-orbit torques generated by a series of Pt/Permalloy samples, finding values in excellent agreement with spin-torque ferromagnetic resonance measurements.

Large spin Hall angle in vanadium film

We report a large spin Hall angle observed in vanadium films sputter-grown at room temperature, which have small grain size and consist of a mixture of body centered tetragonal (bct) and body centered cubic (bcc) structures. The spin Hall angle is as large as θV = −0.071 ± 0.003, comparable to that of platinum, θPt = 0.076 ± 0.007, and is much larger than that of bcc V film grown at 400 °C, θV_bcc = −0.012 ± 0.002. Similar to β-tantalum and β-tungsten, the sputter-grown V films also have a high resistivity of more than 200 μΩ∙cm. Surprisingly, the spin diffusion length is still long at 16.3 nm. This finding not only indicates that specific crystalline structure can lead to a large spin Hall effect but also suggests 3d light metals should not be ruled out in the search for materials with large spin Hall angle.

Properties of (Zn,Cr)Te semiconductor deposited at room temperature by magnetron sputtering

We report the fabrication of (Zn,Cr)Te films at room temperature by magnetron sputtering. Various structural and elemental characterizations revealed there was only a zinc blende phase from the ZnTe host and Cr atoms were distributed uniformly in these films. The magnetization measurement by superconducting quantum interference device magnetometer clearly showed that the samples were ferromagnetic at low temperatures with Curie temperature around 150K. The magnetic circular dichroism measurements confirmed that the observed ferromagnetism was originated from the interaction of substitutional Cr ions and ZnTe host. Transport measurement revealed typical semiconductor behaviors with the large negative magnetoresistance observed.

Tunneling magnetoresistance in FeCo/MgO/FeCo tunneling junctions depending on FeCo/MgO interface state

This article discusses the TMR, tunneling resistance, and I-V characteristic in FeCo/MgO/FeCo MTJs with various FeCo/MgO interface state by using wedged-film deposition technique.

Negative TMR in magnetic tunneling junctions with Zr oxide barrier

The magnetic tunnel junctions (MTJs) were prepared on Si [001] by using magnetron sputtering system. The structures of the MTJs are Si/FeNi/Cu/FeMn/Co/barrier/Co/Cu were the barrier are hybrids: AlO/sub x//ZrO/sub x/ and ZrO/sub x//AlO/sub x/. AlO/sub x/ as a barrier was also studied for comparison. Negative tunneling magnetoresistance (TMR) was observed in MTJs using hybrid barrier. The TMR shows a strong asymmetric bias dependence and changes from 8% at around zero bias to -4% at a high bias. The negative TMR at high bias is due to not only the energy dependence of spin-DOS but also the barrier shape of the MTJs. The mechanism for the negative TMR at high bias is different from that responsible for the inverse TMR in low bias range.