Chaolin Zha

Spin-torque oscillator with tilted fixed layer magnetization

A spin-torque oscillator with a fixed layer magnetization tilted out of the film plane is capable of strong microwave signal generation in zero magnetic field. Through numerical simulations, we study the microwave signal generation as a function of drive current for two realistic tilt angles. The tilted magnetization of the fixed layer can be achieved by using a material with high out-of-plane magnetocrystalline anisotropy, such as L10 FePt.

Zero-field precession and hysteretic threshold currents in a spin torque nano device with tilted polarizer

Using nonlinear system theory and numerical simulations, we map out the static and dynamic phase diagrams in the zero applied field of a spin torque nano device with a tilted polarizer (TP). We find that for sufficiently large currents, even very small tilt angles (?>1?) will lead to steady free layer precession in zero field. Within a rather large range of tilt angles, 1?<?<19?, we find coexisting static states and hysteretic switching between these using only current. In a more narrow window (1?<?<5?) one of the static states turns into a limit cycle (precession). The coexistence of current-driven static and dynamic states in the zero magnetic field is unique to the TP device and leads to large hysteresis in the upper and lower threshold currents for its operation. The nano device with TP can facilitate the generation of large amplitude mode of spin torque signals without the need for cumbersome magnetic field sources and thus should be very important for future telecommunication applications based on spin transfer torque effects.

Pseudo spin valves based on L10 (111)-oriented FePt fixed layers with tilted anisotropy

We demonstrate magnetoresistance (MR) in excess of 4% in FePt/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111)-oriented FePt fixed layers with a 36° out-of-plane tilted magnetization. The high MR is achieved by increasing the spin polarization at the Cu interfaces, using thin CoFe, and optimizing the FePt growth and Cu interface quality using Ta and Ta/Pt underlayers. We observe well-separated switching of the FePt/CoFe fixed layer and the CoFe/NiFe free layer, suggesting that CoFe is rigidly exchange coupled to FePt and NiFe in the respective layers.

Nanowaveguides and couplers based on hybrid plasmonic modes

Experimental demonstration of silicon nanowires based hybrid plasmonic waveguides and couplers with subwavelength mode confinement at the near infrared wavelength lambda = 980 nm are presented. By ...

Continuously graded anisotropy in single (Fe53Pt47)100−xCux films

We report on continuously graded anisotropy. During deposition, a compositional gradient is achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)70Cu30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after annealing using the composition dependence of the low-anisotropy (A1) to high-anisotropy (L10) ordering temperature. The critical role of the annealing temperature on the resultant anisotropy gradient is investigated. Magnetic measurements support the creation of an anisotropy gradient in properly annealed films which exhibit both a reduced coercivity and moderate thermal stability. These results demonstrate that an anisotropy gradient can be realized, and tailored, in single continuous films without the need for multilayers.

Surface-energy triggered phase formation and epitaxy in nanometer-thick Ni1−xPtx silicide films

The formation of ultrathin silicide films of Ni1-xPtx at 450-850 degrees C is reported. Without Pt (x=0) and for t(Ni)= 4 nm, polycrystalline NiSi films form and agglomerate at lower temperatures f ...

Microwave generation of tilted-polarizer spin torque oscillator

Microwave frequency generation in a spin torque oscillator (STO) with a tilted fixed layer magnetization is studied using numerical simulation of the Landau–Lifshitz–Gilbert–Slonczewski equation. The dependence of the STO free layer precession frequency on drive current is determined as a function of fixed layer tilt angle. We find that zero-field STO operation is possible for almost all tilt angles, which allow for great freedom in choosing the detailed layer structure of the STO.

Nanostructured MnGa films on Si/SiO2 with 20.5 kOe room temperature coercivity

Nanostructured Mn67Ga33 films exhibiting high room temperature coercivity (HC = 20.5 kOe) have been prepared by sputtering onto thermally oxidized Si substrates. Both the morphology and the coercivity of the films can be tuned by varying the growth parameters. The low deposition rate film, sputtered at a reduced power and working pressure, demonstrates a discontinuous island-like growth and the highest HC. The large HC is linked to the presence of the high anisotropy DO22 Mn3Ga phase and the single domain character of the exchange isolated, dipolar interacting, single crystal islands.

First-order reversal curve analysis of graded anisotropy FePtCu films

The reversal mechanisms of graded anisotropy FePtCu films have been investigated by alternating gradient magnetometer (AGM) and magneto-optical Kerr effect (MOKE) measurements with first-order reversal curve (FORC) techniques. The AGM-FORC analysis, which clearly shows the presence of soft and hard components, is unable to resolve how these phases are distributed throughout the film thickness. MOKE-FORC measurements, which preferentially probe the surface of the film, reveal that the soft components are indeed located toward the top surface. Combining AGM-FORC with the inherent surface sensitivity of MOKE-FORC analysis allows for a comprehensive analysis of heterogeneous systems such as graded materials.

Improved magnetoresistance through spacer thickness optimization in tilted pseudo spin valves based on L10 (111)-oriented FePtCu fixed layers

Through optimization of the Cu spacer thickness, we demonstrate magnetoresistance (MR) up to 5% in FePtCu/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111) FePtCu fixed layers with a tilted m ...