Abstract
Controlling the angular momentum of spins with very short external perturbations is a key issue in modern magnetism. For example it allows manipulating the magnetization for recording purposes or for inducing high frequency spin torque oscillations. Towards that purpose it is essential to modify and control the angular momentum of the magnetization which precesses around the resultant effective magnetic field. That can be achieved with very short external magnetic field pulses or using intrinsically coupled magnetic structures, resulting in a transfer of spin torque. Here we show that using picosecond acoustic pulses is a versatile and efficient way of controlling the spin angular momentum in ferromagnets. Two or three acoustic pulses, generated by femtosecond laser pulses, allow suppressing or enhancing the magnetic precession at any arbitrary time by precisely controlling the delays and amplitudes of the optical pulses. A formal analogy with a two dimensional pendulum allows us explaining the complex trajectory of the magnetic vector perturbed by the acoustic pulses.
Highlights
Institut de Physique et Chimie des Materiaux de Strasbourg, UMR 7504, CNRS, Universitede Strasbourg, BP 43, 23 rue du Loess, 67034 Strasbourg Cedex 02, France
To explain the effect of each particular sequence of acoustic pulses and the corresponding control of angular momentum, we make a formal analogy between the controlled motion of precession and a two dimensional pendulum subject to momentum kicks provided by the acoustic pulses
We show that a sequence of two or three acoustic pulses is well adapted for controlling the spin torques in ferromagnetic materials, resulting either in the suppression or the amplification of the magnetization precession
Summary
The preceding study shows that the control of the magnetization dynamics with acoustic pulses depends on the two delays T1(n2) and T2(m3 ) which values allow determining for example the cancellation of the precession as shown in Fig. 4b or its amplification. Their relative delays and amplitudes can serve the purpose of controlling the magnetization vector via the change of the magneto-elastic anisotropy. To precise this concept, which is of major importance for applications, we have studied the effect of unipolar and bipolar shaped acoustic pulses on the magnetization trajectory. The conditions of coherent control can be determined by choosing the delay such that: T 5 mTprec 1 dT for the amplification and T 5 (m 1 1/2)Tprec 1 dT for the suppression of the precession of magnetization
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