Magnetization reversal by a femtosecond laser pulse

Thermally induced magnetization switching (TIMS) of an overlayer by controlling the surface dipolar fields associated with the template microstructure was first demonstrated in Fe/MnAs/GaAs. Driven by temperature only, without applying external fields, the process has potential for applications in sensors and devices. The next logical step is driving it with an ultrafast optical laser pulse. Understanding the correlation between the laser induced MnAs phase modulations and the Fe magnetization reversal is important for designing devices, and was the goal of our time-resolved resonant x-ray scattering experiments at FERMI.

Figure 1. Temperature driven Fe magnetization switching (left). At 5°C only the ferromagnetic α-MnAs phase exists (a); the magnetization of Fe and MnAs are set parallel by a magnetic pulse, H_ext. Over the phase-coexistence temperature region (12-50 °C), the self-organized striped microstructure alternates ferromagnetic α and paramagnetic β phases (b); the dipolar fields extending above the surface switch the Fe magnetization locally. Cooling down, the MnAs recovers its initial configuration, while the Fe layer magnetization has reversed (c).
Laser driven Fe magnetization switching (right). Magnetization sensitive specular reflectivity of p-polarized FEL radiation tuned to the Fe-3p resonance. A magnetic pulse saturates the Fe magnetization in the high reflectivity state (M^Fe+). A single optical-laser pulse (F=10 mJ cm^-2, 100 fs duration) changes the reflectivity to the M^Fe- value, corresponding to the opposite Fe magnetization direction.

Data were collected at the DIPROI beamline using the IRMA reflectometer. The FEL probe, tuned to the Fe 3p→3d resonance (53.7 eV), was coupled to a 100 fs optical laser pump with adjustable delay and ~10 fs timing jitter. The absorption of a single optical pulse changes the resonant reflectivity locally, corresponding to the Fe magnetization reversal. We monitored the evolution of the MnAs microstructure in a time-resolved diffraction experiment, measuring the Bragg peaks originating from the regular modulation of α and β phases. Figure 2 shows that, within the phase coexistence region (16 and 34°C), there is a fast structural modification (1-20 ps), related to electron-lattice and spin-lattice energy exchange processes, resulting in the formation of a homogeneous paramagnetic β MnAs surface layer, followed by a slower heat diffusion driven process. Figure 3 summarizes the proposed mechanism correlating microstructure dynamics with laser-induced magnetization switching. The important results of this experiment are that a single laser pulse can reverse the Fe magnetization locally and that this laser-triggered process is not driven by the fast modifications induced into the MnAs template structure, but by the formation of the self-organized regular microstructure during the slower return to the equilibrium.

Figure 2. Time resolved diffraction at FERMI. (a) 1st and 2nd order Bragg peaks from the MnAs α/β microstructure, as a function of the pump-probe delay Δt. (b) 1st order intensity vs. Δt. The lines are double exponential decays with given t1 and t2 values.

Figure 3.  Laser driven Fe magnetization reversal sketch. The laser pulse excites the electrons (a), transferring the energy to the lattice within ps (b), promoting the MnAs α→β transition in the interaction volume. The transformation progresses by thermal diffusion (c, d). The Fe magnetization switches when the system crosses the phase-coexistence region during cool-down (e), and then remains antiparallel to the MnAs one at equilibrium (f).

This research was conducted by the following research team:

C. Spezzani¹, E. Ferrari^1,2, E. Allaria¹, F. Vidal^3,4, A. Ciavardini⁵, R. Delaunay^6,7,  F. Capotondi¹, E. Pedersoli¹, M. Coreno^1,5, C. Svetina^1,8, L. Raimondi¹, M. Zangrando¹,  R. Ivanov¹, I. Nikolov¹, A. Demidovich¹, M. B. Danailov¹, H. Popescu⁹, M. Eddrief^3,4,  G. De Ninno^1,10, M. Kiskinova¹, M. Sacchi^3,4,9
 

¹Elettra – Sincrotrone Trieste, Trieste, Italy
²Università degli Studi di Trieste, Dipartimento di Fisica,  Trieste, Italy.
³Sorbonne Universités, UPMC Univ Paris 06, Paris, France
⁴CNRS, UMR 7588, Institut des NanoSciences de Paris, Paris, France
⁵CNR – ISM, Monterotondo Scalo (RM), Italy
⁶Sorbonne Universités, UPMC Univ Paris 06, Paris, France
⁷CNRS, UMR 7614, LCPMR, Paris, France
⁸University of Trieste, Graduate School of Nanotechnology, Trieste, Italy
⁹Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, France
¹⁰Laboratory of Quantum Optics, University of Nova Gorica, Nova Gorica, Slovenia

Contact persons:
Maurizio Sacchi email: