arXiv subjects

J. Grollier

Publications and source records attributed to J. Grollier.

48 records · Page 3Linked to original sources

arXiv2008

High domain wall velocities due to spin currents perpendicular to the plane

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: 50 m/s for planar polarizer and 640 m/s for perpendicular polarizer for J = 5*10^6 A/cm^2. 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.

cond-mat.mtrl-sci↗

arXiv2008

Coupling efficiency for phase locking of a spin transfer oscillator to a microwave current

The phase locking behavior of spin transfer nano-oscillators (STNOs) to an external microwave signal is experimentally studied as a function of the STNO intrinsic parameters. We extract the coupling strength from our data using the derived phase dynamics of a forced STNO. The predicted trends on the coupling strength for phase locking as a function of intrinsic features of the oscillators i.e. power, linewidth, agility in current, are central to optimize the emitted power in arrays of mutually coupled STNOs.

cond-mat.mtrl-sci↗

arXiv2008

Impact of the electrical connection of N Spin Transfer Oscillators on their synchronization : an analytical study

Spin Transfer Nano-Oscillators (STNOs) are good candidates for applications to telecommunications, but their output power is very low, under the nW. A possible solution to overcome this problem is to synchronize an assembly of STNOs. For manufacturing purposes, large number of STNOs will have to be electrically connected. In this letter, we study analytically the impact of electrical connection on the synchronization of STNOs. Our analysis shows that the phase dynamics of the coupled STNOs can be described in the framework of the Kuramoto model. The conditions for successful synchronization of an assembly of STNOs are derived.

cond-mat.mtrl-sci↗

arXiv2007

Microwave excitations associated with a wavy angular dependence of the spin transfer torque : model and experiments

The spin transfer torque (STT) can lead to steady precession of magnetization without any external applied field in magnetic spin valve where the magnetic layer have very different spin diffusion length. This effect is associated with an unusual angular dependence of the STT, called "wavy" (WAD-STT), predicted in the frame of diffusive models of spin transfer. In this article, we present a complete experimental characterization of the magnetization dynamics in the presence of a WAD-STT. The results are compared to the prediction of the magnetization dynamics obtained by single domain magnetic simulations (macrospin approximation). The macrospin simulations well reproduced the main static and dynamical experimental features (phase diagram, R(I) curves, dependence of frequency with current and field) and suggest that the dynamical excitations observed experimentally are associated with a large angle out-of-plane precession mode. The present work validates the diffusive models of the spin transfer and underlines the role of the spin accumulation and the spin relaxation effects on the STT.

cond-mat.mtrl-sci↗

arXiv2007

Shaped angular dependence of the spin transfer torque and microwave generation without magnetic field

The generation of oscillations in the microwave frequency range is one of the most important applications expected from spintronics devices exploiting the spin transfer phenomenon. We report transport and microwave power measurements on specially designed nanopillars for which a non-standard angular dependence of the spin transfer torque (wavy variation) is predicted by theoretical models. We observe a new kind of current-induced dynamics that is characterized by large angle precessions in the absence of any applied field, as this is also predicted by simulation with such a wavy angular dependence of the torque. This type of non-standard nanopillars can represent an interesting way for the implementation of spin transfer oscillators since they are able to generate microwave oscillations without applied magnetic field. We also emphasize the theoretical implications of our results on the angular dependence of the torque.

cond-mat.mtrl-sci↗

arXiv2006

Microwave spectroscopy on magnetization reversal dynamics of nanomagnets with electronic detection

We demonstrate a detection method for microwave spectroscopy on magnetization reversal dynamics of nanomagnets. Measurement of the nanomagnet anisotropic magnetoresistance was used for probing how magnetization reversal is resonantly enhanced by microwave magnetic fields. We used Co strips of 2 um x 130 nm x 40 nm, and microwave fields were applied via an on-chip coplanar wave guide. The method was applied for demonstrating single domain-wall resonance, and studying the role of resonant domain-wall dynamics in magnetization reversal.

cond-mat.mtrl-sci↗

arXiv2005

Synchronization of spin-transfer oscillators driven by stimulated microwave currents

We have simulated the non-linear dynamics of networks of spin-transfer oscillators. The oscillators are magnetically uncoupled but electrically connected in series. We use a modified Landau-Lifschitz- Gilbert equation to describe the motion of each oscillator in the presence of the oscillations of all the others. We show that the oscillators of the network can be synchronized not only in frequency but also in phase. The coupling is due to the microwave components of the current induced in each oscillator by the oscillations in all the other oscillators. Our results show how the emitted microwave power of spin-transfer oscillators can be considerably enhanced by current-induced synchronization in an electrically connected network. We also discuss the possible application of our synchronization mechanism to the interpretation of the surprisingly narrow microwave spectrum in some isolated spin-transfer oscillators.

cond-mat.mtrl-sci↗

arXiv2005

Unified description of bulk and interface-enhanced spin pumping

The dynamics of non-equilibrium spin accumulation generated in metals or semiconductors by rf magnetic field pumping is treated within a diffusive picture. The dc spin accumulation produced in a uniform system by a rotating applied magnetic field or by a precessing magnetization of a weak ferromagnet is in general given by a (small) fraction of hbar omega, where omega is the rotation or precession frequency. With the addition of a neighboring, field-free region and allowing for the diffusion of spins, the spin accumulation is dramatically enhanced at the interface, saturating at the universal value hbar omega in the limit of long spin relaxation time. This effect can be maximized when the system dimensions are of the order of sqrt(2pi D omega), where D is the diffusion constant. We compare our results to the interface spin pumping theory of A. Brataas et al. [Phys. Rev. B 66, 060404(R) (2002)].

cond-mat.mes-hall↗

arXiv2003

Magnetization reversal by injection and transfer of spin: experiments and theory

Reversing the magnetization of a ferromagnet by spin transfer from a current, rather than by applying a magnetic field, is the central idea of an extensive current research. After a review of our experiments of current-induced magnetization reversal in Co/Cu/Co trilayered pillars, we present the model we have worked out for the calculation of the current-induced torque and the interpretation of the experiments.

cond-mat.mtrl-sci↗

arXiv2003

Switching a spin-valve back and forth by current-induced domain wall motion

We have studied the current-induced displacement of a domain wall (DW) in the permalloy (Py) layer of a Co/Cu/Py spin valve structure at zero and very small applied field. The displacement is in opposite direction for opposite dc currents, and the current density required to move DW is only of the order of 10^6 A/cm^2. For H = 3 Oe, a back and forth DW motion between two stable positions is observed. We also discuss the effect of an applied field on the DW motion.

cond-mat.mtrl-sci↗

arXiv2002

Field dependence of magnetization reversal by spin transfer

We analyse the effect of the applied field (Happl) on the current-driven magnetization reversal in pillar-shaped Co/Cu/Co trilayers, where we observe two different types of transition between the parallel (P) and antiparallel (AP) magnetic configurations of the Co layers. If Happl is weaker than a rather small threshold value, the transitions between P and AP are irreversible and relatively sharp. For Happl exceding the threshold value, the same transitions are progressive and reversible. We show that the criteria for the stability of the P and AP states and the experimentally observed behavior can be precisely accounted for by introducing the current-induced torque of the spin transfer models in a Landau-Lifschitz-Gilbert equation. This approach also provides a good description for the field dependence of the critical currents.

cond-mat.mtrl-sci↗

arXiv2002

Switching the magnetic configuration of a spin valve by current induced domain wall motion

We present experimental results on the displacement of a domain wall by injection of a dc current through the wall. The samples are 1 micron wide long stripes of a CoO/Co/Cu/NiFe classical spin valve structure. The stripes have been patterned by electron beam lithography. A neck has been defined at 1/3 of the total length of the stripe and is a pinning center for the domain walls, as shown by the steps of the giant magnetoresistance curves at intermediate levels (1/3 or 2/3) between the resistances corresponding to the parallel and antiparallel configurations. We show by electric transport measurements that, once a wall is trapped, it can be moved by injecting a dc current higher than a threshold current of the order of magnitude of 10^7 A/cm^2. We discuss the different possible origins of this effect, i.e. local magnetic field created by the current and/or spin transfer from spin polarized current.

cond-mat.mtrl-sci↗