X-ray imaging of magnetic normal modes driven by spin transfer torque in magnetic nanopillar devices

We have fabricated Py/Cu/CoFe magnetic nanopillars with 100 × 250 nm2 elliptical cross sections on top of suspended silicon nitride membranes, designed for performing x-ray transmission microscopy of magnetization dynamics. We are now studying these devices to image the magnetic normal modes that are excited by the spin transfer torque from a microwavefrequency current passing through the nanopillar. Summary of Research: A spin-polarized current passing through a ferromagnet can exert a torque on the ferromagnet that is strong enough to either reverse its magnetization or excite steady-state highfrequency (1-20 GHz) magnetic precession, depending on the device structure and applied magnetic field [1]. This spin-torque effect is under intensive study both because of its fundamental importance and because of potential applications in magnetic random access memory (MRAM) and high speed signal processing. One of the critical outstanding issues in Figure 1: Device schematic: nanopillar device on top of a suspended silicon nitride membrane. this field is the nature of the magnetization dynamics excited by the spin torque. Imaging [2, 3] of the spatial and temporal dependence of the magnetic dynamic modes with high resolution x-ray transmission microscopy can offer a direct view of any spatial inhomogeneities. X-ray transmission microscopy can selectively image a ferromagnetic element via x-ray magnetic circular dichroism (XMCD), by which the transmission of x-ray photons through a ferromagnetic material depends on the angle between the polarization of the photons and the magnetization. We start device fabrication with a Si substrate coated with 150 nm of low stress (film tension < 200 MPa) silicon nitride films on both sides, one of which will be made into suspended membranes to allow for x-ray transmission microscopy (Figure 1). We deposit the metallic multilayers of the structure (in nm): {Ta 3 / CuN 20}2 / Ta 3 / Cu 2 / Py 15 / Cu 4 / Co90Fe10 5 / Cu 2 / Pt 20 (where Py is the magnetic alloy permalloy, Ni81Fe19) in a sputtering system. E-beam lithography and ion milling are then used to define pillars with elliptical cross section of 100 × 250 nm2 (Figure 1). Multiple steps of photolithography and ion milling are used to make the contact leads, and silicon oxide is deposited by electron-beam evaporation to provide electrical isolation between top and bottom leads. To form the suspended nitride membrane, the Si substrate is etched anisotropically in heated KOH solution, using photolithographically-patterned silicon nitride on the back side of the substrate as the etch mask. The XMCD experiments are carried out at the scanning transmission x-ray microscope (STXM) at the Advanced Light Source of Lawrence Berkeley National Laboratory, which has a spatial resolution of 15 nm and temporal resolution of 70 ps. The x-ray is incident at 30° from the normal direction of the device plane (Figure 1), and its energy is tuned to the characteristic Co L3 resonance absorption edge. These experiment conditions allow detection of the in-plane component of the magnetization of the 5 nm thin CoFe layer in the nanopillar. A magnetic field is applied in the device plane along the short axis of the ellipse. PHYSICS & NANOSTRUCTURE PHYSICS 213 2011-2012 CNF RESEARCH ACCOMPLISHMENTS P H Y S Figure 2 shows the electrical characterization of the spintransfer-driven ferromagnetic resonance [4] (ST-FMR) spectrum of one sample as a function of microwave current frequency, f, and magnetic field, H. For x-ray studies, we excite the sample into a particular resonance mode using appropriate values of f and H (for example, point A at about 3 GHz and -20 Oe in Figure 2), and use the x-ray microscope to image the spatial configuration of the magnetization at eight different phases evenly spaced throughout one oscillation cycle. Magnetic contrast images are shown in Figure 3 relative to the average of all phases. The contrast images suggest the presence of both an approximately uniform center mode and a nonuniform edge mode — the edge precesses Figure 3: Magnetic contrast images corresponding to resonance mode A in Figure 2. (Grayscale corresponds to the normalized difference in x-ray transmission between one phase and the average of all phases, proportional to the component of the magnetic moment dynamics parallel to the short axis of the nanopillar cross-section.) Dotted ellipses are drawn to indicate the nanopillar location. Figure 2: ST-FMR spectra of one device at magnetic fields from -220 Oe to 220 Oe. at a different phase and amplitude than the center of the device. We are currently working on accumulating sufficient data from different devices, analyzing and modeling the magnetic normal modes, and comparing with micromagnetic simulations to understand the mode structures.