Spin current and spin transfer torque in ferromagnet/superconductor spin valves

In this thesis we make use of the Spin Transfer Torque effect from a continuous microwave current to induce and study the spin dynamics of an individual sub-100 nm nanostructure. The idea that an electrical current can carry a spin angular momentum was introduced in the 1980 with the advent of Spintronics. In 1996, Slonczewski and Berger predicted that, when flowing through a metal ferromagnet, such a spin polarized current can exert a torque on the magnetization. This effect is known as Spin transfer Torque (STT) and implies the possibility to manipulate the magnetization by means of an electrical current. One of the characteristics of the STT effect is that it allows for the magnetization dynamics to be excited and detected electrically. This makes the STT a perfect tool for the investigation of spin dynamics at the nano-scale. Indeed, the intrinsic local character of transport measurements permits a single nanostructure to be addressed and studied. This thesis is devoted to the study of the STT effect induced by a microwave current (AC STT). Our experiment focuses on two main aspects: the intrinsic dynamics of the magnetization and the role of a microwave current in assisting the magnetization reversal. We perform our measurements on Co/Cu nanowires electrodeposited in nanoporous polycarbonate templates. The electrical contact to a nanostructure is realized thanks to a home-made sample-holder, without the use of any lithographic technique. In the first part of the thesis a continuous microwave current is used to both drive and probe the magnetization dynamics of a pseudo spin valve nanostructure. Our measurements show that the magnetization dynamics in such a structure is extremely complex and arises from the excitation of both magnetic layers in the pseudo spin valve. In the dynamical spectrum we identify the fundamental modes of both magnetic layers, as confirmed by macrospin simulations. Higher frequency modes are attributed to spatially non-uniform spin wave excitations, in agreement with that observed by others in arrays of nanoelements. These results validate our technique for making samples and contacting them for AC STT-induced dynamical studies. The second part of this work is aimed at testing the effect of AC STT on the static switching field of magnetic nanoelements. The magnetoresistive curve of a pseudo spin valve structure is recorded with and without an additional microwave current. Our results prove the effectiveness of AC STT in assisting the magnetization reversal. We measured a reduction of the switching field up to 80 mT by injecting a microwave current of about 100 µA. This experiment suggests that the range of sensitivity of magnetoresistive devices could be tuned by simply injecting a continuous microwave current. This point is particularly interesting for the prospect of technological applications such as magnetic sensors.

There have been several recent experiments involving spin injection and accumulation in mesoscopic metal samples, using lateral spin valve structures. The results have been interesting from the perspectives of both fundamental physics and applications. The resistance change associated with spin accumulation, ▵R, has been predicted to scale inversely with sample volume and this inverse scaling has been observed over ten decades. A value of ▵R = 1 Ohm has been reported, at room temperature, in a thin film Al wire structure with transverse dimensions of roughly 100 nm. High values of ▵R have been observed in samples characterized by electrode interface resistances that vary by 6 decades. Results are discussed within the framework of Johnson-Silsbee theory, and factors that limit inverse scaling are identified. Lateral spin valves with dimensions of tens of nm may be competitive for device applications. Structures with ▵R of order 1 Ohm are superior to CPP spin valves of comparable dimensions. Prospects for improved performance, the plausibility of a lateral spin valve with output levels of 10 Ohms and output impedance of 50 Ohms, and relevance to hard drive read heads and integrated nonvolatile random access memory applications are discussed.

Features of the current spin-orbit induced magnetic dynamics in multilayer nanostructures with nonmagnetic heavy metal layers possessing by a strong spin-orbit interaction are studied. These structures include ferromagnetic (F) (antiferromagnetic AF)/normal metal (N) nanostructures based on both conductive and insulating magneticsand heavy normal metals (e. g., FeCoB/Ta, YIG/Pt, Nio/Pt). The spin Hall effect of the conversion of an incoming charge current into a transverse (with respect to the charge current) spin current induces a spin-transfer torque and magnetic dynamics including a magnetic precession and switching. The magneto-dynamic effect of a spin current pumping generation together with the inverse spin Hall effect of conversion of the spin current into the incoming charge current provide the influence of the magnetic dynamics on the incoming charge current. These feedforward and feedback between the incoming charge current and the magnetic dynamics can be the basis for the spin-orbit driven self-sustained auto-oscillations of a magnetic order in the nanostructures. It is shown that the considered magnetic nanostructures possess by properties of controlled microwave radiation attaining tens THz in the antiferromagnetic case. Magnetic-induced changes of the electric resistance in the mentioned nanostructure are considered.

Antiferromagnets are promising candidates as active components in spintronic applications. They share features with ferrimagnets in that opposing spin orientations exist in two or more sublattices. Spin-transfer torque and spin pumping are essential ingredients in antiferromagnetic and ferrimagnet spintronics. This paper develops an out-of-equilibrium quantum theory of the spin dynamics of collinear magnets containing many spins coupled to normal metal reservoirs. At equilibrium, the spins are parallel or antiparallel to the easy axis. The theory, therefore, covers collinear antiferromagnets and ferrimagnets. We focus on the resulting semiclassical spin dynamics. The dissipation in the spin dynamics is enhanced due to spin pumping. Spin accumulations in the normal metals induce deterministic spin-transfer torques on the magnet. Additionally, each electron's discrete spin angular momentum causes stochastic fluctuating torques on the antiferromagnet or ferrimagnet. We derive these fluctuating torques. The fluctuation-dissipation theorem holds at high temperatures, including the effects of spin pumping. At low temperatures, we derive shot noise contributions to the fluctuations.

We explore routes to realize electrically driven Bose-Einstein condensation of magnons in insulating antiferromagnets. Even in insulating antiferromagnets, the localized spins can strongly couple to itinerant spins in adjacent metals via spin-transfer torque and spin pumping. We describe the formation of steady-state magnon condensates controlled by a spin accumulation polarized along the staggered field in an adjacent normal metal. Two types of magnons exist in antiferromagnets, which carry opposite magnetic moments. Consequently, and in contrast to ferromagnets, Bose-Einstein condensation can occur for either sign of the spin accumulation. This condensation may occur even at room temperature when the interaction with the normal metal is fast compared to the relaxation processes within the antiferromagnet. In antiferromagnets, the operating frequencies of the condensate are orders of magnitude faster than in ferromagnets.

The spin injection and accumulation in metallic lateral spin valves with transparent interfaces are studied using dc injection current. Unlike ac-based techniques, this allows the investigation of the effects of the direction and magnitude of the injected current. We find that the spin accumulation is reversed by changing the direction of the injected current, whereas its magnitude does not change. The injection mechanism for both current directions is thus perfectly symmetric, leading to the same spin injection efficiency for both spin types. This result is accounted for by a spin-dependent diffusion model. Joule heating increases considerably the local temperature in the spin valves when high-current densities are injected ($\ensuremath{\sim}80--105\text{ }\text{K}$ for $1--2\ifmmode\times\else\texttimes\fi{}{10}^{7}\text{ }\text{A}\text{ }{\text{cm}}^{\ensuremath{-}2}$), strongly affecting the spin accumulation.

A ferromagnet is shown to exert an exchange field on the spin accumulation localized in the semiconductor interface region of a Fe/MgO/Si tunnel contact. The proximity exchange coupling across the MgO modifies spin precession of the spin accumulation and thereby produces detectable signatures in the inverted Hanle effect: exchange-induced shifts of the inverted Hanle curves, hysteresis, and discontinuities at the coercive field of the ferromagnet, at which the exchange field is reversed. The proximity exchange field is locked antiparallel to the magnetization of the ferromagnet, has values up to 380 Oe, and can be controlled by a bias voltage across the tunnel contact.

Specular spin valves show enhanced giant magnetoresistive (GMR) ratio when compared to other, simpler, spin valve structures. The enhancement of GMR results from specular reflection in nano-oxide layers (NOLs) formed by the partial oxidation of the pinned and free layer. These oxides forming the NOL order antiferromagnetically (AFM) below a temperature T∼175 K. Here, we study the effects of the pinned layer magnetization and its domain structure on the AFM ordering of the NOL by performing field cooling measurements with different cooling fields (H0). We observe enhanced (reduced) exchange field and magnetoresistive ratio for H0>0(<0), i.e., parallel (antiparallel) to the pinned magnetization. These measurements allowed us to confirm the existence of a wide distribution of blocking temperatures (TB) in the NOL of specular spin valves, having a maximum at T≈175 K, and extending to NOL regions with TB as low as 15 K.

Hard–soft spin valve structures have been grown by molecular beam epitaxy on MgO(001) substrates. The hard magnetic layer consists of (Co50Fe50)/Ir/(Co50Fe50) artificial ferrimagnetic (AFi) system, while a Fe/Co bilayer integrated in the buffer, has been used as a soft detection layer. The Fe has been grown at 500 °C giving rise to a monocrystalline layer with a body centered cubic structure. The spin valve structure presents a progressive evolution after successive annealing steps up to 350 °C. The total giant magnetoresistance (GMR) reaches its maximum (5.3%) after annealing at 250 °C, together with a good rigidity of the hard layer and a sharp switch of the magnetic moments. Such characteristics are reduced, but still interesting, after annealing at 300 °C. For annealing at higher temperature (350 °C) the total GMR signal and the coercive field of the AFi decrease dramatically and all the stack behaves like a single magnetic layer. Rutherford backscattering measurements were performed in order to investigate the changes in the morphology of CoFe/Ir interfaces and to correlate them to the magnetotransport properties.

We consider the motion of a sharp domain wall in magnetic nanowires with electric current. The width of the domain wall is much smaller than the electron wavelength, which is typical for magnetic semiconductors. We calculate the distributions of the spin density and the spin current related to different modes of the scattering states. The accumulated transverse components of the spin density and the spin current oscillate in the vicinity of the wall and they essentially affect its dynamics, whereas the longitudinal part of the spin current is responsible for another component of the spin torque creating a force for the current‐induced motion of the domain wall along the nanowire. We also analyze the dynamics of the sharp domain wall using the standard Landau–Lifshits–Gilbert formalism and the two‐component spin torque calculated for this model. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Time evolution of the spin accumulation and spin current in a magnetic domain wall was studied by solving the Boltzmann equation of the spin accumulation. Assuming the spatially homogeneous electric current density, we find that the time evolution of the longitudinal spin current is characterized by the momentum relaxation time of the conducting electrons. On the other hand, the time evolution of the transverse spin accumulation and spin current are characterized by the spin-flip relaxation time.

We consider long and narrow spin valves composed of a first magnetic layer with a single domain wall (DW), a normal metal spacer, and a second magnetic layer that is a planar or a perpendicular polarizer. For these structures, we study numerically DW dynamics taking into account the spin torques due to the perpendicular spin currents. We obtain high DW velocities: 5 m/s for planar polarizer and 80 m/s for perpendicular polarizer for I=0.01 mA. These values are much larger than those predicted and observed for DW motion due to the in-plane spin currents. The ratio of the magnitudes of the torques, which generate the DW motion in the respective cases, is responsible for these large differences.

We investigate the influence of Au capping layer on the spin accumulation in a lateral spin valve consisting of Permalloy and Cu wires. The nonlocal spin valve measurements show that the Au capping significantly suppresses the spin accumulation in the Cu wire, although it improves the electrical conductivity. This is understood as a result of the small spin-flip resistance of Au whereby absorbed spins are successively flipped.

Submicron Ni0.8Fe0.2/Cu/Co spin valves and Co/Cu pseudo spin valves with widths ranging from 200 to 0.5 μm have been realized by direct deposition and liftoff. The resulting configurations were structurally characterized by atomic force microscope and cross-sectional transmission electron microscopy. Transport and magnetic properties of arrays of these spin-valve structures were studied using superconducting quantum interference device magnetometry. Magnetic measurements carried out on several spin-valve structures showed that both coercivity and field at which maximum magnetoresistance (MR) occurs increase with decrease in widths of the spin valves. Comparison of the results obtained from the magnetometry and transport measurements indicated the existence of an offset between the coercive field and field of maximum MR. Resistance versus temperature measurements for varying linewidth show a minimal shift in peak temperature, indicating that processing effects are minimal.

We establish a brief relation between spin current and spin torque, including spin–orbit torque and spin transfer torque in 2D Rashba ferromagnets with nonuniform magnetic texture. Both electrically and thermally induced charge, heat, and spin current are investigated by the Luttinger’s mechanical method, and we derive the contributions of magnetization corresponding to the thermal spin current and the thermal spin torque. The novel transport currents are also found in this paper when the interplay between spin–orbit coupling and nonuniform magnetic texture is taken into account.