Sub-Nanosecond Magnetisation Dynamics

 

Context

A key parameter for the operation of spin electronic devices is the switching of the magnetization in a magnetic nanostructure between two stable magnetic configurations. In the case of a memory these states represent the logical ’0’ and ’1’. In order to be competitive, the corresponding frequency of this switching has to be of the order of GHz, which corresponds to the internal precession frequency of the magnetization. Thus all aspects of the magnetization dynamics, see Fig. 1, need to be considered in order to develop, characterize and optimize the devices.

Projects

New ’non_conventional’ reversal processes are currently being investigated, which are very promising to achieve the required sub_nanosecond reversal time. For this, ultra short excitation pulses with durations of a few hundred picoseconds are used, such as magnetic field pulses (FIMS), heat pulses (TAS) or spin polarized current pulses (CIMS). SPINTEC has recently developed several high frequency measurement techniques in order to capture the magnetization dynamics as a function of time (resolution of 1ps - 20 ps) as well as in the frequency domain (up to 26 GHz). Most techniques are based on a pump-probe scheme. In the case of the field induced reversal, the pump-part consists of an impedance matched coplanar transmission line, into which a current pulse is injected. This current pulse creates a field pulse at the position of the sample and induces either a reversal process or magnetization oscillations. The most direct way to capture the dynamics is to register the transmitted power using either a high bandwidth oscilloscope (20 GHz) for the time domain or a vector network analyser (20 GHz), for the frequency domain (inductive techniques, see Fig. 2). These techniques are adequate to investigate the ’small’ amplitude oscillations and will be applied to study the normal modes in magnetic nanostructures as a function of the underlying magnetization configuration. In order to capture the dynamics of the magnetization reversal, we use the time resolved magneto-optic Kerr effect (MOKE) or the time resolved magneto-resistive (MR) effect. An example for the precessional reversal in a continuous film, investigated by MOKE is given in Fig. 3. The schematic of the time resolved MR is shown in Fig. 4 for field pulse excitation. This method will be applied to investigate the reversal processes in response to heat or current pulses. Frequency domain studies using a spectrum analyzer for the effects related to the dynamics induced by a spin polarized current in CPP spin valves are described in the section of Noise in sub-micrometric magnetoresistive elements .

Team

-  Ursula Ebels
-  Jean Pierre Nozières
-  Bernard Dieny
-  Olivier Redon
-  Claire Baraduc
-  Christophe Thirion
-  Sébastien Petit

Collaborations

-  Pascal Xavier (IMEP/ENSERG, Grenoble)
-  Jean-Christophe Toussaint (LLN, Grenoble)
-  Thierry Fournier (Nanofab/CRTBT, Grenoble)

Main publications

-  Frequency domain studies of CoZr continuous thin films and FeNi wires using coplanar transmission lines; M. Kerekes, A. D. C. Viegas, D. Stanescu, U. Ebels, P. Xavier, G.Suran, accepted for pubication in J. Appl. Phys.
-  Domain wall propagation in continuous thin films initiated by precessional reversal; M. Kerekes, A. D. C. Viegasc, D. Stanescu, U. Ebels, P. Xavier, L.S. Dornelles, R. L. Sommer, submitted to JMMM.


Fig. 1
Landau-Lifschitz-Gilbert equation and the different dynamic processes

Fig. 2
Schematics of the inductive technique in the frequency and time domain

Fig. 3
(a) Schematics of the precessional reversal (b) Magnetisation trajectories (expreimental and simulated) for a FeCuNbSiB thin film in the first reversal regime and the third reversal regime.

Fig. 4
Schematics of the time resolved magneto-resistive technique, shown for a CIP spin valve system and for an excitation by a short field pulse.