Ultrafast laser induced dynamics in ferromagnets : towards the control of the spin order from the femtosecond to the sub- nanosecond time scale

Time dependent mechanisms in magnetism cover a wide range of phenomena. The corresponding time scales embrace an extremely large window that can range from tens of years of logical bit lifetimes in data storage devices to the sub–picosecond rate of electronic interactions. From the technological point of view, the demand for increasing writing speed in memory devices boost the research towards the investigation of the fastest regime. However such a short timescale is connected to very fundamental processes at the microscopic and atomic level where our comprehension is still cloudy. The main limiting factor is the difficulty to experimentally access the correspondent temporal window. The study of the so–called ultrafast spin dynamics has been possible only recently, thanks to the development of femtosecond laser sources. The availability of ultrashort light pulses allows one to probe the evolution of the system properties via optical spectroscopies and with femtosecond resolution. The first pioneering work was performed by Beaurapiere and coworkers in 1996. They shined a nickel sample with intense femtosecond pulses and observed a quenching of the spin order occurring in less than a picosecond after the optical excitation. This process has been called ultrafast demagnetization. Nowadays, it is a well–known laser induced effect in common 3d magnetic metals, however its comprehension is still under debate after more than 16 years. The work of Beaurapiere et al. raised interesting opportunities also for technological applications. Femtosecond laser pulses revealed to be capable of manipulating the spin order at rates several order of magnitude faster than modern magnetic computer components. One of the most intriguing possibility is to optically reverse the direction of the magnetization resembling the writing mechanism of memory devices. From the first experimental demonstrations of nanosecond laser–assisted magnetic inversion in 1997, up to now, picosecond switching has been achieved only in few cases, for instance, in specific ferrimagnetic samples or in low temperature ferromagnetic semiconductors. In the present work we study laser induced spin dynamics in metallic ferromagnets. We exploit the pump–probe technique to detect the transient modification of the magneto-optical Kerr effect (MOKE) within an extent time window from few femtoseconds to hundreds of picoseconds. Three are the main purpose of the investigation: (i) disclosing the mechanisms underlying the ultrafast demagnetization process, (ii) developing a reliable experimental method to achieve the optical spin switching in the picosecond regime and (iii) studying optical contributions to transient MOKE signals. First of all, focusing our attention on the femtosecond regime, we characterize the ultrafast dynamics of the magnetic order for different laser intensities and ambient temperatures in metallic systems. In particular the investigated samples are 8 nm thin iron films expitaxially grown on MgO in our laboratory. The observed temperature dependence of the ultrafast demagnetization and the slower remagnetization allows us to ascribe the former effect to the electron–magnon interaction while the latter to the Elliot–Yafet type electron-phonon scattering. On the other hand, ultrafast spin transport phenomena have been investigated during my six months visit at the Spectroscopy of Solids and Interfaces (SSI) group of Professor Dr. T. Rasing at the Radboud University in Nijmegen (Netherlands). According to recent theories the demagnetization may take place as a consequence of majority spin draining from the irradiated area. Therefore, we have studied the effect of laser excitation in tunneling magnetoresistance microstructures with the main scope of finding a correlation between spin currents and ultrafast dynamics. A non negligible possible outcome of such studies, combining ultrafast dynamics and electronic devices, could be the generation of electro-optical devices exploiting femtosecond laser induced spin motion. However, our measurements are still in progress, thus only some preliminary results are shown and interpreted in the framework of the electronic superdiffusion theory. Secondly, in the picosecond regime, we demonstrate the possibility to change the magnetization direction in thin iron layers using only ultrashort laser pulses. The sample heating due to photon absorption, results in coherent spin waves. With a proper orientation and intensity of the external field, we show that it is possible to control the magnetic precessional motion. In this way we can reproducibly and repeatedly commute the magnetic vector between preferential directions in less than 100 ps. This rate is more than ten times higher than the working speed of modern storage magnetic devices. In addition, ferromagnetic iron is characterized by an in–plane biaxial anisotropy, with the interesting technological follow-up of allowing one to record two bits of information on the same spot. To conclude, we address the problem of optical artifacts in pump-probe magneto–optical experiments. The measured spin dynamics can contain a transient optical contribution not related to the true magnetic evolution that may lead to a wrong interpretation of the experimental data. This issue is of fundamental importance, since our knowledge about ultrafast phenomena is retrieved only via optical spectroscopies. In particular concerning the TR-MOKE technique, many experimental works have already demonstrated that the measured magneto–optical Kerr effect may not follow the transient spin behavior in the first hundreds of femtoseconds. However in the picosecond time scale no experimental evidence of optical contribution has been observed up to now. We show that, under particular condition, the Kerr signal might considerably differ from the genuine magnetic dynamics also for a pump and probe delay longer than 50 ps. We have investigated this issue in two benchmark systems, Fe and CrO2, that allowed us to to disentangle the magnetic information from purely optical one after a wide characterization of the TR-MOKE dynamics. This last work arises from a collaboration with the experimental groups of Prof. Dr. M. Münzenberg at the I. Physikalisches Institut of Georg–August–Universität Göttingen (Germany) and Prof Dr. A. Gupta from the department of chemistry of the University of Alabama (USA).

In questa tesi viene descritta l’attività sperimentale svolta in tre anni di lavoro nel gruppo di spettroscopie ottiche ultraveloci della prof. C. Dallera e del dott. E. Carpene. Durante questo periodo sono stati realizzati principalmente esperimenti di pump-probe presso il laboratorio ULTRAS al Dipartimento di Fisica: sfruttando la sorgente laser ultraveloce è possibile perturbare un materiale incidendo su di esso con un fascio molto intenso, e misurarne quindi le proprietà su una scala temporale che va da 50 fs a 1 ns. In particolare il mio lavoro si è concentrato sulle dinamiche di spin in sistemi magnetici, principalmente campioni di ferro di spessore nanometrico cresciuti epitassialmente. A questo scopo un particolare setup sperimentale è stato allestito per misurare la variazione di polarizzazione della luce riflessa da un campione magnetico (effetto Kerr magneto-ottico, MOKE). Il primo argomento trattato nella tesi riguarda la demagnetizzazione sulla scala dei 100 fs. È stata fatto uno studio sistematico del processo in funzione della temperatura del campione e dell'intensità della perturbazione ottica in modo tale da identificarne il meccanismo sottostante. Siamo stati in grado escludere il coinvolgimento dell'interazione spin-fonone, avvalorando l’ipotesi (già pubblicata dal nostro gruppo) che il meccanismo sia di carattere magnonico. Sempre nell'ambito del problema dell’identificazione del processo responsabile della demagnetizzazione ultrarapida ho lavorato per 6 mesi presso la Radboud university a Nijmegen (Paesi Bassi) nel gruppo del prof. Rasing. Qui ho investigato un modello alternativo secondo cui gli elettroni fotoeccitati diffondono verso zone non irraggiate del sistema in maniera diversa a seconda del loro stato di spin. Per fare ciò è stato realizzato un esperimento di pump-probe su particolari sistemi TMR (Tunnelling MagnetoResistance) che combinasse misure ottiche ed elettriche. L’analisi dei dati sperimentali è tuttora in corso. Nel lavoro di tesi vengono successivamente trattate le dinamiche magnetiche sulla scala temporale dei picosecondi. In particolare viene dimostrata la possibilità di cambiare l'orientazione della magnetizzazione in film sottili di ferro sfruttando la sola luce laser. In speciali condizioni sperimentali di fluenze di laser, e campo magnetico esterno è possibile provocare un’oscillazione coerente di spin che porta la magnetizzazione in una nuova posizione di equilibrio in 100 ps. Un altro importante aspetto affrontato riguarda il significato delle misure MOKE: tali misure contengono informazioni sulle proprietà ottiche e magnetiche. I più recenti esperimenti del mio dottorato sono stati condotti su film ferromagnetici di CrO2 e sono stati volti proprio a sviluppare un metodo che ci ha permesso di separare l'informazioni ottica da quella magnetica. Questo risultato é di fondamentale importanza per l'interpretazione degli esperimenti condotti dal nostro e da altri gruppi.