Session A22: Focus Session: Current Driven Magnetization Dynamics I

Spin-Transfer Effect in Nanopillar Spin Valves With A Thick Polarizing Layer

Current-induced magnetization switching has been the object of intensive study, motivated in part by possible applications for non-volatile magnetic storage. To date, the majority of studies have focused on structures where the moments of the magnetic layers lie in plane. Recently, however, it has been predicted that significantly faster switching times, as well as lower switching currents could be achieved in a device where the magnetization of the polarizing layer is perpendicular to the plane of the free layer [1, 2]. To study this effect we have fabricated Py 60nm / Cu 40nm / Py 5nm nanopillar spin valves patterned as 80 nm by 150 nm ellipses. The magnetization of the thick polarizing layer can be readily coerced to high out-of-plane angles by applying a weak field perpendicular to plane, while the much thinner free layer is only weakly affected. Here we present a phase diagram for our devices, obtained through DC and microwave-frequency measurements as function of current and applied out-of-plane field. We will discuss the results in the context of switching with an out-of-plane polarizer. [1] A. D. Kent et al., Appl. Phys. Lett. 84, 3897 (2004). [2] K. J. Lee et al., Appl. Phys. Lett. 86, 022505 (2005).   

Spin-transfer-induced magnetization reversal in bilayer magnetic nanopillars at high fields: dependence on free layer thickness

Spin transfer in asymmetric Co/Cu/Co bilayer magnetic nanopillar junctions has been studied as a function of free (thin) Co layer thickness from 1.8 to 5.3 nm. In particular, the critical current for magnetization reversal in large magnetic fields applied perpendicular to junction surface has been measured. Junctions with submicron lateral size were fabricated using a nano-stencil process. Junctions resistances scale with lateral area and their in-plane magnetoresistance was found to be independent of free layer thickness. The critical current decreases linearly with decreasing free layer thickness and extrapolates to a finite critical current in the limit of zero thickness. This can be understood as either a decrease in efficiency of the spin-transfer torque and/or an interfacial contribution to the damping of the free magnetic layer.   

Linewidths for spin-transfer-driven precession in magnetic nanopillars as a function of the direction of applied magnetic field

In a magnetic multilayer spin valve, the spin-transfer torque from a spin-polarized DC current can drive the free-layer magnetic moment into steady-state precessional modes [1, 2]. We report measurements of how the linewidths of these modes depend on the angle and magnitude of an applied magnetic field, for devices in the nanopillar configuration with elliptical cross sections. We find that the field direction studied most commonly, in-plane along the magnetic easy axis of the ellipse, generally gives the largest linewidths, corresponding to the least coherent precession. As the field is rotated either in plane or out of plane, the linewidths can change dramatically, decreasing by a factor of 50 or more in some devices. We will discuss the temperature dependence of the linewidths for the field directions that give the minimum linewidths, and we will compare both the field-angle dependence and the temperature dependence to theoretical models. [1] S. I. Kiselev et al., Nature 425, 380 (2003). [2] J. C. Sankey et al., cond-mat/0505733.   

Phase Locking of Spin-Transfer Oscillators

DC current flowing through a nanometer-scale lithographic contact made to a continuous spin-valve multilayer induces stable magnetic precession of the free layer at GHz frequencies. The resonance frequency of the spin-transfer oscillator (STO) is a function of both the applied current and external magnetic field, and the resonance has linewidths on the order of MHz. To study the properties of this resonance, and to determine the suitability of STOs for communications applications, we measured the response of these oscillators to variations in the magnetic field, electric current, and spin-wave environments. We studied phase locking effects induced by injecting ac currents, and by applying ac magnetic fields near the precession frequency of the device. In addition, we have fabricated two nanocontact devices in close proximity on the same magnetic film, and looked at the interactions between the two devices. In each case, when the impressed ac signal is sufficiently close to the STO frequency, the device will phase lock. For ac currents and fields, the device locks to the external signal via injection locking, a general property of nonlinear oscillators, taking on the frequency and phase characteristics of the source. For interacting nanocontacts, the devices modify each other's resonances, and lock together at frequency slightly different from the individual resonances. I will review these phase locking results, and discuss the variations of the locking with the excited mode of the oscillator. I will also present results on mutual locking of STOs, discuss the relative roles of dipolar fields and spin-wave interactions in the locking mechanism, and comment on the possible uses of phase controlled, coherent STOs.   

Probing wavenumbers of current-induced excitations in point-contact experiements

The magnetic state of a ferromagnet can be altered by an electrical current. For instance, the current was shown to induce spin waves, precession, and reversal of magnetization in magnetic nanostructures. Today a variety of experimental techniques provide a vast amount of data on such current-induced excitations. A typical experiment usually exploits dc resistance measurements to detect the excitations. In addition, high-frequency techniques can provide valuable information on frequencies of the current-induced spin waves. Probing wavenumbers of the excitations, however, represents an experimental challenge. Point contacts were instrumental both for our original observation of current-induced excitations and in providing the first data on frequencies of the current-induced spin waves. In the present work we demonstrate that point-contact technique can also provide valuable information on the wavenumber of spin waves induced by the current. By varying the size of point contacts we have been able to control the size of the excitation volume and therefore the wavelength of current-induced spin waves. This leads to a technique with in situ sensitivity to wavenumbers of current-induced excitations. The detailed size-dependent measurements of the current-induced excitations display an interesting relation between current and voltage thresholds for such excitations.   

Influence of Electrode Structure on Switching Characteristics in Nanopillar Spin Valves

We examine the effect spin scattering within the electrodes of a spin-valve nanopillar has on spin torque and damping within the structure. Devices were fabricated with the free layer adjacent to either the top or bottom electrode and with Au or Pt top electrodes. Macrospin simulations, when compared to pulsed current switching experiments, indicate that gold electrode samples of either free layer orientation have similar switching parameters, while devices with the free layer adjacent to a top platinum electrode exhibit lower spin torque and larger damping than Au capped devices, in agreement with spin accumulation [1, 2] and spin pumping [3] models. However, by placing the free layer opposite a platinum cap, the largest values for spin torque and damping were achieved, which was an unexpected result. In addition, we will discuss how placement of the free layer close to the bottom electrode induces effects such as fixed layer switching and microwave excitations in zero effective field, which are not seen in the opposite configuration. [1] J. Manschot, A. Brataas, G. E. W. Bauer, Appl. Phys. Lett. \textbf{85}, 3250 (2004). [2] A. A. Kovalev, A. Brataas, G. E. W. Bauer, Phys. Rev. B \textbf{66}, 224424 (2002). [3] Y. Tserkovnyak, A. Brataas, G. E. W. Bauer, Phys. Rev. B. \textbf{67}, 140404 (2003).   

X-Ray Imaging of Spin Transfer Induced Magnetization Reversal

Magnetization switching by spin injection has been observed in giant magneto-resistance measurements, giving an insight into the temporal evolution of the magnetization. So far, however, the nanoscale magnetization distribution during the switching process has remained hidden. Here we report, for the first time, imaging the magnetic switching process using advanced pump-probe x-ray microscopy. We observe that the switching process is initiated and determined by the lateral motion of a magnetic vortex driven by the spin current. Motion pictures with 200 picosecond time resolution show that the switching process is based on the motion of a magnetic vortex, leading to C-like patterns which may decay later into a uniform magnetic state. Our measurements show the fundamental role played by the curled Oersted field which necessarily accompanies the spin injection current.   

Effects of current on the magnetization states of Permalloy nanodisks

We will describe expreimental evidence and theoretical model demonstrating that both the vortex and the single domain magnetic configurations of Ni80Fe20 circular nanodiscs can be achieved by the application of magnetic field or current flowing perpendicular to the disc plane. The magnetic configurations of Ni80Fe20/Cu/Ni80Fe20 trilayers have been determined by the response to a small current via the giant magnetoresistance. In addition, we report detection of the vortex state in a single magnetic layer, by exploiting a magnetoresitance arising from the suppression of spin-accumulation in the vortex state. Our analysis shows that this magnetoresistance effect becomes increasingly significant in small nanostructures.   

Spin-mixing effects on magnetic switching, probed by thermoelectric measurements

Current-Induced Magnetization switching has been observed in single nanowires of Co, Ni, Co-Cu and Ni-Cu multilayers. A novel thermoelectric measurement under high DC current is presented. It features a field dependence stronger than GMR. Its dependence on field orientation shows that it provides information complementary to GMR or magneto-thermoelectric power measurements. It is argued with a simple model that this measurement depends strongly on the difference of spin mixing rates of spin conversions going from ``up'' to ``down'' and conversely. Thus, this novel transport measurement provides information on electron-magnon collisions in magnetic nanostructures. The field dependence of this signal is about 10 times larger than the magnetoresistance of the same sample, be it AMR or GMR. In multilayers, the field dependence is shown to decay away when the thickness of the layers is larger than the spin-diffusion length.   

Universal spin pumping with rf magnetic fields

A new method for generating spin accumulation in metals or semiconductors is by application of an rf magnetic field [1], similar to the spin battery effect induced by a ferromagnet in resonance [2]. A dc spin accumulation is produced that is in general a small fraction of $\hbar\omega$, where $\omega$ is the rotation frequency of the rf field. When a resonant dc magnetic field is also applied the spin accumulation can be enhanced towards the universal value $\hbar\omega$. In addition, spin diffusion into an adjacent region without fields can dramatically enhance spin accumulation at the interface. We discuss the application of this method to produce spin accumulation in semiconductor or metal spin electronic devices without the necessity of either ferromagnetic electrodes or charge currents.\\ $[1]$ S. M. Watts {\it et al.}, submitted to PRL \\ $[2]$ A. Brataas {\it et al.}, Phys. Rev. \textbf{B} 66 (2002).   

Spin Transfer in Magnetic Nano Devices with Perpendicular Anisotropy

Spin transfer predicted by Slonczewski and Berger has attracted a great deal of attention in recent years. Experimental findings have proved spin transfer in current-perpendicular-to-plane (CPP) spin-valves (SV) and magnetic tunnel junctions (MTJs) with in-plane magnetization configuration. MRAM cells with perpendicular magnetic anisotropy may support high recording density than that using shape anisotropy with in plane magnetization configuration. However, there is no report on spin transfer in any magnetic nano device with perpendicular anisotropy. In this work, perpendicular magnetized spin transfer nano-devices with sub-200 nm dimensions: Si/SiO$_{2}$/bottom electrode/[CoFe2.5{\AA}/Pt15 {\AA}]$_{m}$/CoFe5 {\AA} /Cu30 {\AA} /[CoFe4.5 {\AA} /Pt23 {\AA}]$_{n}$/Top electrode, has been fabricated and tested. Two [CoFe/Pt]$_{n}$ multilayers with different coercivities are used as the free layer and fixed layer in nano-devices. CPP magnetoresistive (MR) loop tested under perpendicular magnetic field shows the GMR is around 0.47{\%}. The switching field for the free layer is around 200 Oe. Current induced magnetization switching was realized with a positive switching current 48 mA and a negative switching current -62 mA, respectively. Furthermore, field dependence of the switching current will also be demonstrated.