Non-thermal light-induced spin dynamics in

YIG:Co films via the photomagnetic effect

A. Stupakiewicz1*

, M. Pashkevich1,2

, A. Maziewski1

1Laboratory of Magnetism, Faculty of Physics, University of Bialystok, 15-424, Bialystok,

Poland 2Scientific-Practical Materials Research Centre of the NASB, 220072, Minsk, Belarus

* and@uwb.edu.pl

Abstract We observed different frequencies of the magnetization precession

within GHz-THz range in Co-substituted yttrium iron garnet films by exciting lin-

early polarized femtosecond pulses. The photomagnetic effect can be useful for

non-thermally controlling of spin systems and ultrafast switching in magnetic de-

vices.

Introduction

Previous studies have revealed the fascinating role of electron-spin-based phe-

nomena in spintronic and magnonic devices operating in a wide frequency range.

The functionality of garnet systems has been shown to be very broad: for example,

the excitation of surface plasmons, the propagation of nonlinear spin-waves, the

photomagnetic effect [1], the observation of the inverse Faraday effect induced by

an ultrafast laser pulse [2], and many others. Optical non-contact methods offer

special ways of studying local spin-density effects with high spatial and temporal

resolution. A recent study on a Co-substituted yttrium-iron garnet (YIG:Co) thin-

film reported a 20 angle of the magnetization precession through non-thermal ex-

citation by linearly polarized femtosecond laser pulses via photoinduced magnetic

anisotropy [3]. Interesting light-induced magnetic properties have been observed

in an ultrathin Co layer deposited on a garnet film. In this bilayer, damped oscilla-

tions in the Faraday rotation transients were demonstrated, representing

precessional motion of the magnetization vector in both an ultrathin Co layer and

garnet of the bilayer, with a modulation resulting from two distinct frequencies

[4].

We observed in YIG:Co films the magnetization precession with different fre-

quencies: an unexpected field-independent high-frequency mode and a field-

dependent low-frequency ferromagnetic mode within 1-10 GHz values. The polar-

ization dependencies in a wide spectral range were measured.

� Springer International Publishing Switzerland 2015J.-Y. Bigot et al. (eds), Ultrafast Magnetism I, Springer Proceedings in Physics 159DOI 10.1007/978-3-319-07743-7_61

194

Results and discussion

Experimental studies were performed on garnet films with a composition of

Y2CaFe3.9Co0.1GeO12 on a gadolinium-gallium-garnet (001)-oriented substrate.

The surface of the garnet thin film was treated with a low energy oxygen ion beam

[5]. The saturation magnetization of garnets at room temperature was about 7 Gs.

We used ferromagnetic resonance with an X-band microwave source and magne-

to-optical magnetometer to study the magnetic anisotropy of our samples at room

temperature. The easy magnetization axis was slightly inclined from a <111>-type

crystallographic direction relative to the sample plane. To investigate the spin pre-

cession using femtosecond laser excitation we performed Faraday rotation meas-

urements on garnets using time-resolved tools with a magneto-optical pump-probe

method. The polarization plane of the pump beam was along the [100] and [010]

crystallographic axes, when we have maximal amplitude of magnetization preces-

sion in the garnet film. On the contrary, the polarization plane of the probe beam

was along [110] axis. We measured the Faraday rotation angle (proportional to the

out-of-plane component of the magnetization) of the probe pulses as a function of

the delay time and external in-plane magnetic field H.

Figure 1 shows the results for the magnetization precession as a function of the

delay time at H=0.75 kOe with different polarizations of pump light. We observed

that the precession of the magnetization with an opposite phase is triggered as

non-thermal characterization of ultrafast dynamics in garnets.

Fig. 1. Time-resolved Faraday rotation for linear pump polarizations along [100] and [010] axes

of pump laser pulses with in-plane magnetic field H=0.75 kOe.

At the initial timescale, up to few picoseconds, the high-frequency mode was

observed. This mode was independent from the external magnetic field at a wide

spectral range (800-1600 nm) of pump light. The low frequency mode increased

Non-thermal light-induced spin dynamics in YIG… 195

linearly with the amplitude of the applied magnetic field. In this case, the observa-

tions agree with the behavior for ferromagnetic spin resonance modes in garnet

films [4]. During pulses of about 20 ps, we simultaneously observed the relaxation

of the high-frequency precession and the excitation of the low-frequency preces-

sion of the magnetization. This excitation is consistent with an optically induced

magnetic anisotropy change in a garnet film [3]. Though qualitatively similar in

behavior, the ultrafast dynamic modification of the anisotropy differs from the

static one in the orientation of the light polarization, which raises a question about

the origins of the two modifications. In this case, one can expect that the ultrafast

photomagnetic anisotropy at YIG:Co may be connected with femtosecond excita-

tion of magnetic ions in tetrahedral sites, using light excitation of the exchange

resonance mode.

The experimental studies and analysis as demonstrated show the possibility of

non-thermally exciting and controlling the magnetization of ferrimagnets using

femtosecond laser pulses by the ultrafast photomagnetism. Our result allows for

the possibility of tunable manipulations of the magnetization switching with fre-

quencies from GHz up to THz values.

Acknowledgments Supported by SYMPHONY project operated within the Foundation for

Polish Science Team Programme co-financed by the EU European Regional Development Fund,

OPIE 2007–2013.

References

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196 A. Stupakiewicz et al.