Linear response to a heat-driven spin torque

The existence of a heat-driven spin torque is demonstrated using Co/Cu/Co spin valves embedded in metallic nanowires. Heat currents flowing in one direction or its opposite were obtained by heating optically one end or the other of the nanowires. The spin torque associated with the heat-driven spin current pushes the magnetization out of equilibrium, resulting in a change of the magnetoresistance, which is detected using a charge current small enough not to cause heating or induced fields of any significance. The giant magnetoresistance response to this torque peaks with the magnetic susceptibility, whereas the spurious signal coming from the temperature dependence of the resistance produces merely a field independent baseline. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Current Induced Magnetization Dynamics in Electrodeposited Nanostructures

Elena Murè

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.

EPFL2010

Current susceptibility of magnetization in spin valves

Jean-Philippe Ansermet, Julie Dubois, Simon Granville, Haiming Yu

We observed the second harmonic voltage response of pseudo-spin valves subjected to currents oscillating at frequencies in the sub-kilohertz range. Peaks appear in its magnetic field dependence, with a signal-to-noise ratio equal to or greater than the magnetoresistance response of the same samples. This signal is interpreted as arising from the response of the magnetization to the spin torque induced by current when the magnetic layers are in non-collinear orientations. Thus, the method probes the current susceptibility of the magnetization which is shown to be sensitive to non-collinear states undetectable in the magnetoresistance.

2009

Thermal Spin Transfer Torque and Transverse Spin Relaxation in Spin Valves

Haiming Yu

Spin caloritronics, i.e., the addition of thermal effects to the electrical and magnetic properties of nanostructures, has recently seen a rapid development. It has been predicted that a heat current can exert a spin torque on the magnetization in a nanostructure, analogous to the well-known spin-transfer torque induced by an electrical current. In this thesis, I provide the experimental evidence for this effect in spin valves, showing the switching field change with heat current. I present measurements of the second harmonic voltage response of Co-Cu-Co pseudo-spin-valves deposited in the middle of Cu nanowires. I exploit the quasi-1D nature of the nanostructures to generate a heat current by way of asymmetric Joule heating in the Co layers. Both the magnitude of the second harmonic response of the spin valve and the field value of the maximum response are found to be dependent on the heat current. Both effects show that the magnetization dynamics of the pseudo-spin-valves is influenced by the heat current. Thus, the data provide a quantitative estimate of the thermal spin torque exerted on the magnetization of the Co layers. In the last chapter, I measured the second harmonic response to an oscillating current crossing an exchange-biased spin valve as a function of the angle between the magnetization vectors of the fixed and free layers. This signal characterizes the linear response of the magnetization to a torque. A diffusive model is shown to predict that this angular dependence is quite sensitive to the value of the transverse spin diffusion length λJ , i.e. the decay length of the spin polarization which is perpendicular to the magnetization. Our observations imply a value of about 6 nm for CoFe.

EPFL2011

Current Induced Magnetization Dynamics in Electrodeposited Nanostructures

Elena Murè

EPFL2010