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Ultrafast optical control of magnetic anisotropy in metallic and dielectric thin films

Name
Kalashnikova
Surname
Alexandra
Scientific organization
Ioffe Institute
Academic degree
PhD
Position
senior scientist
Scientific discipline
Physics & Astronomy
Topic
Ultrafast optical control of magnetic anisotropy in metallic and dielectric thin films
Abstract
Femtosecond optical pulses provide unique opportunity to control the magnetic state of matter on pico- and subpicosecond timescale. Here we present the results of experimental studies demonstrating ultrafast control of magnetic anisotropy in metallic (Galfenol) and dielectric (iron garnets) thin magnetic films. We demonstrate that picosecond change of magnetic anisotropy is mediated by laser-induced heating or strain and triggers the magnetization precession in the studied films, which amplitude, frequency and initial phase can be tuned by external magnetic field.
Keywords
laser-induced dynamics, magnetic thin films, femtosecond magnetism, picosecond magnetoacoustics
Summary

Ultrafast optical control of magnetic anisotropy in metallic and dielectric thin films

A. M. Kalashnikova

Ferroics Physics Laboratory, Ioffe Institute, 194024 St. Petersburg, Russia

aabc30kalashnikova@mail.ioffe.ru

In recent years controlling magnetic state of matter by external stimuli, which duration is much shorter than the typical magnetic field pulses, became one of the primary goals in fundamental and applied magnetism. The most promising approaches allowing effective and fast control of magnetization utilize spin-polarized currents [1], picoseconds acoustic pulses [2] and, as the ultimate tool, femtosecond optical pulses [3,4]. All these stimuli, in particular, can trigger magnetization precession in magnetic bulk media and nanostructures. One of the effective and universal approaches to trigger the precession relies on changing magnetic anisotropy on the ultrafast time scale and, in general, can be realized in various media, e.g. metals, semiconductors and dielectrics. 

In this talk we present an overview of the activities of the Ferroics Physics laboratory in the field of ultrafast magnetism and focus on controlling magnetic anisotropy of thin metallic and dielectric films by femtosecond laser pulses [5,6]. We discuss two mechanisms, both of which rely on absorption of laser pulse and fast increase of lattice temperature, which is a very general process, occurring in most of media subjected to femtosecond laser pulses. We demonstrate experimentally that the ultrafast lattice heating leads to change of magnetic anisotropy either directly, or via inverse magnetostriction. Also we show that the thin magnetic films grown on low-symmetry substrates exhibit response to the femtosecond optical excitation combining various processes, which can be used for a fine tuning of the parameters of the excited precession. As the objects media for these studies we’ve chosen dielectric magnetic garnets, the model media in magnonics, and metallic alloy Galfenol, the model magnetostrictive material. The studies were carried out using the magneto-optical pump-probe technique described in details elsewhere [4].

100-nm thick film of metallic alloy Galfenol Fe0.81Ga0.19 was grown on (311) GaAs substrate by magnetron sputtering. We show [5] that excitation of the film by femtosecond laser pulse launches the precession of magnetization which frequency, amplitude and initial phase can be controlled by changing the strength of the external magnetic field. While the field dependence of the first two parameters is trivial, the control of the initial phase of precession in a wide range of 0-90o has been demonstrated in such kind of experiments for the first time, to the best of our knowledge. Detailed study of the laser-induced precession allowed us to distinguish two mechanisms responsible for the excitation. The first one relies on ultrafast lattice heating and picoseconds change of magnetocrystalline anisotropy, and has been known from earlier studies of laser-induced magnetization precession in metals. The second mechanism relies on strain occurring due to lattice expansion, being a result of the heating. We show that the efficiency of these two mechanisms in the low symmetry Galfenol film changes when the external magnetic strength is tuned, which leads to the observed control of the initial phase of the excited precession by the field. 

Triggering the magnetization precession via ultrafast change of the magnetic anisotropy, being a result of laser-induced lattice heating, has been well investigated in magnetic metals. However, the efficiency of this mechanism in thin magnetic dielectric films remains unclear. In order to investigate this issue we have studied the laser-induced magnetization dynamics  in a 5-um thick single crystalline iron garnet Y3Fe5O12 film grown on (210) gadolinium gallium garnet substrate by liquid phase epitaxy. Studies of the magnetization dynamics excited by femtosecond laser pulses [6] have shown that the precession of magnetization occurs via two distinct mechanisms. The first one is the ultrafast inverse Faraday effect (IFE), which is the well-known phenomenon in ultrafast magnetism of dielectrics [3,4]. However, along with the polarization dependent excitation of the precession, ascribed to IFE, we have also observed the polarization-independent excitation. Detailed study of this process allowed us to conclude that there is an ultrafast change of the growth-induced magnetic anisotropy induced by the femtosecond laser pulse. We argue that this process relies on ultrafast heating of the lattice. Importantly, the efficiencies of the precession excitation via IFE and via anisotropy change are comparable, but exhibit different dependences on the external magnetic field strength. This allows for tuning the parameters of the excited precession by changing magnetic field and laser pulse polarization.

In conclusion, we would like to stress that in both studies the low symmetry of the magnetic films allowed to realize the combined excitation of magnetization precession via different mechanisms. This opens a possibility of tuning the parameters of the excited precession, including its initial phase, by changing the exciting optical pulse properties or applied magnetic field. We beleave that such a flexibility of is interest for application in spintronics and magnonics. 

[1] D. C. Ralph, M. D. Stiles, Spin transfer torques, J. Magn. Magn. Mater. 320, 1190 (2008).
[2] A.V. Scherbakov, A.S. Salasyuk, A.V. Akimov, X. Liu, M. Bombeck, C. Bruggemann, D.R. Yakovlev, V.F. Sapega, J.K. Furdyna, and M. Bayer, Coherent magnetization precession in ferromagnetic (Ga,Mn)As induced by picosecond acoustic pulses, Phys. Rev. Lett. 105, 117204 (2010)
[3] A. Kirilyuk, A. V. Kimel, Th. Rasing, Ultrafast optical manipulation of magnetic order, Rev. Mod. Phys. Rev. Mod. Phys. 82, 2731 (2010).
[4] A. M. Kalashnikova, A. V. Kimel, R. V. Pisarev, Ultrafast opto-magnetism, Phys. Usp. 58, 969 (2015). 
[5] V. N. Kats, T. L. Linnik, A. S. Salasyuk, A. W. Rushforth, M. Wang, P. Wadley, A.V. Akimov, S. A. Cavill, V. Holly, A. M. Kalashnikova, and A. V. Scherbakov, Ultrafast change of magnetic anisotropy driven by laser-generated coherent and non-coherent phonons in metallic films, Phys. Rev. B (in print).
[6] L. A. Shelukhin, V. V. Pavlov, P. A. Usachev, P. Yu. Shamrai, R. V. Pisarev, and A. M. Kalashnikova, Ultrafast laser-induced changes of the magnetic anisotropy in iron garnet films (submitted); arXiv:1507.07437.