Femtosecond Laser-Excitation-Driven High Frequency Standing Spin Waves in Nanoscale Dielectric Thin Films of Iron Garnets

Abstract : We demonstrate that femtosecond laser pulses allow triggering high-frequency standing spin-wave modes in nanoscale thin films of a bismuth-substituted yttrium iron garnet. By varying the strength of the external magnetic field, we prove that two distinct branches of the dispersion relation are excited for all the modes. This is reflected in particular at a very weak magnetic field (∼33 mT) by a spin dynamics with a frequency up to 15 GHz, which is 15 times higher than the one associated with the ferromagnetic resonance mode. We argue that this phenomenon is triggered by ultrafast changes of the magnetic anisotropy via laser excitation of incoherent and coherent phonons. These findings open exciting prospects for ultrafast photo magnonics. The continuous demand for more energy efficient and faster data transport and processing devices has triggered intense research activity to carry information with other means than the electron charge, which is associated with an inevitable Joule heating in current semiconductor-based information technologies [1]. One of the most promising ways to achieve this goal is the use of the coherent collective excitation of spins in magnetic materials, commonly known as magnons or spin waves (SWs) [2-5]. These waves can propagate in both metallic and dielectric magnetic media without involving any charge transport, which avoids the Joule heating, and therefore reduces substantially the power consumption for data processing. Besides this energy efficiency, using SWs offers other important advantages for ultrafast nanoscale computation due to their broad high frequency range from GHz to THz and tunable wavelength down to the nanoscale [6]. From a fundamental point of view, SWs exhibit remarkably rich physics involving both dipolar and exchange interactions [6,7]. This is reflected by versatile dispersion relations containing magnetostatic and exchange modes, which can either be tuned by an external magnetic field (H ext) and/or the sample size [3,6]. In this context, intense research is being carried out to understand the generation, propagation, manipulation, and detection of SWs in a continuously growing field of modern magnetism called magnonics [2-5]. Because of its low magnetic damping constant, the yttrium iron garnet (YIG) has attracted a lot of attention in the field of magnonics [8]. Indeed, many important magnetostatic SWs based devices have been realized using YIG during the last decade [9-12]. However, due to the small saturation magnetization of YIG together with the restrictions imposed by the size of the microwave antennas on the excited SWs wavelength with the standard method of microwave magnetic fields, magnetostatic SWs frequencies in such devices did not exceed a few GHz at low field, which is a major limiting factor for ultrafast applications. On the other hand, due to the strong exchange interaction between spins, the exchange SWs mode can have higher frequencies compared to the magnetostatic one. The possibility of triggering such exchange modes requires a nonuniform dynamical field across the magnetic film thickness [13], which is very challenging to induce using microwave antennas [13-15]. Recently, femtosecond laser pulses have been used as an efficient stimulus to excite a coherent spin precession [16,17]. In particular, it was demonstrated that femtosecond laser pulses can excite exchange standing SWs (SSWs) in metallic [18,19] and semiconductor [20,21] ferromagnets via thermal processes. On the other hand, most studies on iron garnets have been dedicated to the excitation and control of the homogenous resonance mode (k ¼ 0, i.e., low frequency) via nonthermal excitation mechanisms [22-29]. An important question in this context concerns the possibility to take advantage of femtosecond laser pulses for triggering a high frequency SSW in dielectric thin films of iron garnets. In this Letter, we demonstrate femtosecond laser-excited high frequency even and odd SSWs in nanoscale films of Bi-substituted yttrium iron garnet (Bi-YIG). Bi-YIG materials have, in addition to a low damping [30], very large magneto-optical Faraday effects [31], which make them well adapted for new photo magnonics devices. By varying the strength of the external applied magnetic field H ext , we demonstrate two distinct branches of the dispersion relation PHYSICAL REVIEW LETTERS 123, 027202 (2019) 0031-9007=19=123(2)=027202(6) 027202-1
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Marwan Deb, Elena Popova, Michel Hehn, Niels Keller, Sébastien Petit-Watelot, et al.. Femtosecond Laser-Excitation-Driven High Frequency Standing Spin Waves in Nanoscale Dielectric Thin Films of Iron Garnets. Physical Review Letters, American Physical Society, 2019, 123 (2), ⟨https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.027202⟩. ⟨10.1103/PhysRevLett.123.027202⟩. ⟨hal-02184539⟩

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