Session B43: Focus Session: Spin Transfer Effect II

Effect of Lower Demagnetizing Field on Switching Currents of Magnetic Nanopillars

\textbf{Katine et al.[1] derive an expression of the form Is $\propto $ [Heff+2$\pi $M] (where 2$\pi $ times the magnetization M comes from the demagnetizing field for the switching layer) for the switching current in a ferromagnetic/non-magnetic/ferromagnetic (F/N/F) nanopillar. If this equation correctly describes Is, then reducing the demagnetizing field should apparently reduce Is. The interfacial interaction between Co and Au favors a perpendicular orientation of M. Sandwiching a thin switching Co layer between two Au layers should, thus, reduce its demagnetizating field. To look for this effect we made nanopillars of Co/Au/Co(t)/Au with Co thickness t = 1, 2, 3 and 4 nm, and measured magnetoresistances and switching currents. We will compare the switching currents of these nanopillars with those for Co/Cu/Co/Cu nanopillars. [1] J.A, Katine et. al., Phys.Rev. Lett. 84, 3149 (2000).}   

The effect of temperature on the dynamics of spin transfer switching of a Py nanomagnet

Current-induced magnetization dynamics and switching have drawn much focus recently as the spin transfer effect shows promise for MRAM type applications. A recent experiment measuring the time-resolved dynamics of current-driven magnetization reversal at ambient temperature [1] has indicated that a current threshold exists delineating thermally activated and spin-torque driven regimes. To directly investigate the role of temperature on nanomagnet switching, we make time-resolved measurements of the switching dynamics in a temperature range from 4K to 300K. The switching is studied in IrMn/Py/Cu/Py exchange-biased spin-valve nanopillars of elliptical shape (130nm x 60nm). The exchange bias direction is set approximately at 45 degrees to the long axis of the nanopillar. In this geometry, the thermal distribution of the initial magnetization direction of the free Py layer does not have a strong effect on the initial phase of the coherent oscillations. This allows us to average over multiple time traces without loss of the oscillatory signal. We will report on this dynamic signal measured as a function of temperature and current magnitude in the spin-torque driven regime of switching. [1] R. H. Koch, J. A. Katine, and J. Z. Sun, Phys. Rev. Lett. 92, 088302 (2004).   

Thermal Effects on Precessional States in Nanomagnets Driven by DC Spin-Transfer Torques

A DC current passing through a nanoscale magnetic multilayer can excite steady-state microwave-frequency magnetization precession by transferring spin angular momentum from one layer to the other [1-3]. In frequency-domain measurements, the spectra generated by such excitations consist of peaks with a non-zero width in frequency, $\Delta f$, indicating that the oscillatory signal produced by the precessing magnet is not perfectly periodic. Here we measure the temperature ($T)$ dependence of $\Delta f$. We argue that at least two mechanisms contribute to $\Delta f$: thermal deflections of the magnetic moment within a precessional orbit (for which $\Delta f\sim T^{1/2})$ and thermally-activated transitions between different dynamical states (for which ln($\Delta f)\sim $1/$T)$. \newline [1] S. I. Kiselev, J. C. Sankey et al., Nature \textbf{425}, 380 (2003). \newline [2] W. H. Rippard et al., Phys. Rev. Lett. \textbf{92}, 027201 (2004). \newline [3] I. N. Krivorotov et al., in press, Science.   

Current Induced Switching by Spin Torque Including the Effects of Temperature

Recently it has been shown[1] that the magnetization $\bf{M}$ of submicron patterned thin-film ferromagnetic elements can be switched by passing a current from another (pinned) ferromagnetic layer. This phenomenon could lead to a new type of high-density non-volatile MRAM (magnetic random access memory). The incoming electrons deposit their spin angular momentum in the ferromagnet, where it adds to the local vector magnetization. Quantitatively, one adds a "spin torque" term to the Landau-Lifshitz equation for $d\bf{M}/dt$, proportional to the component of the pinned magnetization perpendicular to $\bf{M}$. The familiar Arrhenius-Neel reaction rate formula [$\propto \exp(-E_b/kT)$, where $E_b$ is a potential energy barrier] cannot be used for this problem, because the spin-torque is not conservative so a potential energy cannot be defined. We have gone back to the fundamental Fokker-Planck equation from which the Arrhenius-Neel result was derived and reformulated it including the spin torque. We obtain a simple differential equation for the energy distribution, which gives the expected $\exp(-E/kT)$ if the current vanishes. In the limit of small oscillations about an easy axis, the energy distribution can be approximated by a Boltzmann distribution with an elevated effective temperature, allowing the use of an Arrhenius-Neel-like rate formula. This picture gives an adequate accounting[2] of room-temperature telegraph noise rates, for example. However, at low temperature the distribution resulting from our theory is qualitatively unlike a Boltzmann distribution, and describes the statistics of recently-observed large-amplitude precessional states[3], which have possible applications to tunable gigahertz oscillators. In addition to these steady-state applications, the new Fokker-Planck equation can model the effects of nanosecond current pulses in high-speed MRAM. [1] F. J. Albert et al, Appl. Phys. Lett. \textbf{77}, 3809 (2000). [2] D. M. Apalkov and P. B. Visscher, cond-mat preprint 0405305. [3] S. I. Kiselev, Nature \textbf{425}, 380 (2003).   

Time dependence of spin currents in noncollinear magnetic multilayers:the diffusion equation approach

We used the time dependent diffusion equations to study the time evolution of spin torque in noncollinear magnetic multilayers. For 3d transition-metal ferromagnetic layers we find this torque build up in femtoseconds; it reach its steady state in about 75 femtoseconds after undergoing damped oscillations with a period of about 5 femtoseconds. In our approach the initial discontinuity of the spin current at the interface between noncollinear magnetic layers does not directly create spin torque; rather it is the source term that creates transverse spin accumulation and thereby removes the discontinuity in the spin current when steady state is achieved. In this view the spin torque comes from the transverse spin accumulation. We find the dependence of the spin torque on the angle between the magnetizations predicted by the diffusion equation is close to that found by using the Boltzmann equation [1]. Work supported by the National Science Foundation, Grant DMR 0131883. [1] Jianwei Zhang and P.M. Levy, Phys. Rev. B70, 184442(2004).   

Enhanced Magnetic Damping in Spin-Transfer Excitation

While magnetic damping is understood to play a fundamental role in spin-torque phenomena, little experimental work has been done to study the effect of varying the damping parameter $\alpha $. Recently, light terbium (Tb) doping in thin films of permalloy (Py) has been shown to increase $\alpha $ by several orders of magnitude [1]. To directly study the effect of increased $\alpha $ on spin-transfer systems, we have fabricated 0.05 um$^{2}$ Py/Cu/Py nanopillar spin valves with Tb-doping between 0 and 2{\%} in the free layer. We find that, while the GMR varies less than 20{\%}, the critical currents for reversibly switching the free layer (proportional to $\alpha )$ are two to three times larger in the 2{\%} Tb samples than in pure Py samples. This substantial increase is still considerably less than the increase in $\alpha $ observed in the bulk film measurements of similar composition samples, suggesting that processes other than intrinsic spin-orbital coupling can dominate $\alpha $ in spin-transfer nanopillars. The Tb doping also increases the critical current for the onset of processional dynamics. These results suggest one approach to reducing the negative impact of spin torque effects on nanoscale spin valve and tunnel junction read head sensors. [1] S. E. Russek, et al., J. Appl. Phys. 91, 8659 (2002).   

Landau-Lifshitz-Gilbert simulations for nanoscale ferromagnetic

We have studied the magnetization dynamics of several types of nanoscale ferromagentic heterostructures using stochastic simulations of the Landau-Liftshitz-Gilbert equation. Dynamic ``phase diagrams'' are obtained and contrasted using several forms of torque--a simple sine formula, Slonczewski's original spin-transfer torque formula[1] and the new more realistic spin-transfer torque formula appropriate for asymmetric geometries [2]. We also studied the importance of the spin-puming term[3]. For the case of ultra-thin spin valve structures, we obtain good agreement with recent experimental results for the dynamic phase diagram and the systematics of magnetization telegraph noise at finite temperature. [1]. J. C. Slonczewski, JMMM 247, 324-338(2002) [2]. Jiang Xiao et al., Phys. Rev. B 70, 172405(2004) [3]. Y. Tserkovnyak et al., Phys. Rev. L 88, 117601(2002)   

Spin-Transfer Effect in Py/Al/Py Nanopillar Spin Valves

In order to understand the relation between giant magnetoresistance (GMR) and spin transfer effects, we compare spin-transfer-induced switching in spin valve nanopillars with Al and Cu spacer layers. The spin valve trilayer consisted of a Py 20nm / Al (Cu) 12nm / Py 5.5nm (4.5nm) with a nominal configuration of a 40nm by 120nm ellipse. Measurements of resistance vs. applied field and resistance vs. DC current were taken. Our measurements show that the Al/Py interface exhibits drastically different spin-dependent scattering properties as compared to Cu/Py interface resulting in a significant decrease of GMR by factor of 10 in the Py/Al/Py spin valves. Surprisingly, this dramatic decrease of GMR is not accompanied by a large increase in the critical current for spin-transfer-induced switching. Our observations suggest that spin-transfer torque and GMR are not always simply inverse proportional to each other, and thus spin torque measurements can give information on spin-dependent transport properties of magnetic multilayers that is complementary to the GMR measurements.   

Spin-transfer-induced magnetization reversal in bilayer nanopillars as a function of free layer thickness

Bilayer magnetic nanopillars have been studied as a function of free layer thickness at low temperature to test models of spin-transfer induced switching and the factors that control the critical current for reversal. In particular, the critical current for magnetization reversal in large magnetic fields applied perpendicular to the junction surface at $4.2$ K has been measured. Samples with lateral dimensions smaller than $100$ nm were fabricated using a nano-stencil mask process that produces large arrays of junctions. This has been combined with a thin film in-situ wedge growth mechanism that enables continuous variation of free layer thickness between $0$ and $4$ nm across the wafer. The final stack consists of a $12$ nm fixed Co layer, $10$ nm of Cu, followed by the thin free Co layer of variable thickness. The resistance and differential resistance of junctions were measured as a function of magnetic field ($0$ to $6$ T) and current density. A clear switching boundary is observed and the critical current for reversal increases linearly with magnetic field for H$>1.5$ T, even for junctions with free layer thicknesses well below $1$ nm. The slope $dI_c/dH$ has only a weak dependence on free layer thickness, in contrast to the linear dependence expected in the original model of spin-transfer. We discuss possible origins for this behavior. We also present magnetotransport and FMR studies of ultra-thin Co films ($<5$ nm), which shed light on their magnetic characteristics.   

Ab-initio theory of current induced switching

Current induced switching in Co/Cu/Co and Py/Cu/Py trilayers is described in terms of ab-initio determined magnetic twisting energies and corresponding sheet resistances. In viewing the twisting energy as an energy flux the characteristic time thereof is evaluated by means of the Landau-Lifshitz-Gilbert equation using ab-initio parameters. The obtained switching times are in very good agreement with available experimental data. It is found that the magnetoresistance viewed as a function of the current is essentially determined by the twisting energy as a function of the relative angle between the orientations of the magnetization in the magnetic slabs. For all ab-initio type calculations the fully relativistic screened Korringa-Kohn-Rostoker method and the corresponding Kubo-Greenwood equation in the context of density functional theory are applied.   

Effect of Asymmetric Leads on Switching Currents in Magnetic Nanopillars

Manschot et al.[1] predicted that the critical current for switching from parallel (P) to antiparallel (AP) magnetic states in ferromagnetic/nonmagnetic/ferromagnetic (F/N/F) nanopillars could be reduced by as much as an order of magnitude via asymmetric nonmagnetic leads with different effective resistances (resistivity times spin diffusion length). They proposed sandwiching the nanopillar between high effective resistance Cu (next to the free layer) and low effective resistance Pt (next to the fixed layer). Combining Pt with Ag$_{0.95}$Sn$_{0.05}$ (which has a higher effective resistance than Cu), we found a much more modest reduction in switching current between average data for AgSn/Co/Cu/Co/Pt and Pt/Co/Cu/Co/AgSn nanopillars. We'll compare our relative switching currents with calculations using a diffusive transport model based upon the Valet-Fert theory of current-perpendicular-to-plane magnetoresistance. [1] Jan Manschot et. al. Appl. Phys. Lett. 85, 3250 (2004).   

Tunable spin-torque in magnetic tunnel junctions with two fixed layers

We have fabricated nanoscale magnetic tunnel junctions (MTJs) with an additional copper spacer layer and fixed magnetic layer above the free layer of the standard MTJ structure. When this device is biased, the additional magnetic layer acts as a second source of spin-polarized electrons that exert torque on the free layer. Depending on the relative orientation of the two fixed layers, this torque can either oppose or augment the torque exerted by tunnel current. We have measured the T-dependant switching behavior of these devices which we can describe by a model that incorporates spin-torque effects and substantial ohmic heating at spin-torque switching current levels. We find that the spin-torque exerted by our MTJs is very comparable to that exerted by the spin valve. By adjusting the relative orientation of the fixed layers, we can either effectively double or nearly cancel out the net spin-torque on the free layer. This has significance for applications like spin-transfer switched MRAM, where spin-torque is exploited, and for magnetic sensing applications, where spin-torque is parasitic.   

Spin Transport and Spin Transfer in a Current-Perpendicular-to-the-Plane-Device with a Nanocontact Current Concentrator

CPP GMR devices allow the effective study of spin-dependent transport in nanomagnets. CPP devices may also be the only viable option for magnetic read head sensors for ultra-high density storage since the impedance level of nanoscaled magnetic tunnel junctions is likely to become too large to be effective. However, edge effects in nanoscale CPP device structures can substantially degrade the magnetoresistance, and the impedance level of even a nanoscale all-metallic CPP device may be considerably less than the optimum. We have developed a novel nanocontact technique whereby a $\sim $ 3 nm aluminum oxide barrier layer is inserted in the midst of the Cu spacer of a $\sim $ 100 nm diameter CPP spin valve. This barrier layer is patterned with e-beam lithography followed by an in-situ ion milling and deposition process to form an intentional metallic pinhole, $\sim $ 10-20 nm in diameter. This design concentrates the current flow away from the device edges, increases the device impedance, and may potentially decrease the current required for spin transfer switching of a nanomagnet.