Ultrafast Magnetoacoustics in Nickel Films

Abstract

We report about the existence of magneto-acoustic pulses propagating in a 200-nm-thick ferromagnetic nickel film excited with 120 fs laser pulses. They result from the coupling between the magnetization of the ferromagnetic film and the longitudinal acoustic waves associated to the propagation of the lattice deformation induced by the femtosecond laser pulses. The magneto-acoustic pulses are detected from both the front and back sides of the film, using the time-resolved magnetooptical Kerr technique, measuring both the time dependent rotation and ellipticity. We show that the propagating acoustic pulse couples efficiently to the magnetization and is strong enough to induce a precession of the magnetization. It is due to a transient change of the crystalline anisotropy associated to the lattice deformation. It is shown that the results can be interpreted by combining the concepts of acoustic pulse propagation and ultrafast magnetization dynamics.The technology of information and communication constantly needs to improve the speed and density of memory devices. Towards that goal, intense researches are being carried out for manipulating the spins in magnetic materials using various excitation methods like the use of external magnetic field pulses [1][2][3]. Alternatively, femtosecond laser pulses have been utilized to induce an ultrafast demagnetization via a sudden and abrupt change of the temperature of the magnetic material [4-10], or using the inverse Faraday effect [11,12]. This new field of magnetism, named "femtomagnetism" [13,14], uses photons to directly manipulate magnetic structures with a temporal resolution of a few femtosecond. The demagnetization can then be used to modify the anisotropy of the magnetic material [15] which leads to a reorientation of the magnetization vector followed by its precession and damping in the direction opposite to the initial one [10,16,17]. In spite of their versatility, due to the various laser wavelengths and pulse durations, magnetooptical methods are limited by the absorption depth of photons. For application purposes, it is a disadvantage for controlling devices at long distances, particularly in opaque materials like ferromagnetic metals.In the present work we explore an alternative way of controlling the magnetization, based on magnetoacoustic performed at room temperature in ferromagnetic films. It is known that strain pulses, corresponding to a lattice deformation of a material, can be generated with a laser pulse, a subject which has been extensively studied theoretically and experimentally [18][19][20], since the pioneering works of Thomsen et al. [21,22]. The relative amplitude of these strain pulses can be as large as 10 −3 . Such acoustic waves have been used for perturbing the magnetic properties of a dilute magnetic semiconductor material, at low temperature and with a very low efficiency [23]. Here we show that one can efficiently use the strain pulses that propagates over long distances in ferromagnetic metals at room temperature and induce very large ...

Surface-acoustic-wave-driven ferromagnetic resonance in (Ga,Mn)(As,P) epilayers

“…When not taken into account in the numerical calculation (e.g., at T = 40 K), the peak of acoustic attenuation is found about 5 dB cm −1 lower than otherwise, 094401-5 a sizable underestimation. The resonance field remains the same however, since c 11 , c 13 , and c 33 are all linear in A χ , whose resonance field is solely given by the micromagnetic parameters at the chosen temperature. Finally, we wish to address the quantitative agreement between predicted and measured effects.…”

“…(9) and (10)], contrary to simpler cases treated previously [11]. In fact, it is through the z decay that c 13 and c 33 constants are modified by the magnetostrictive interaction; they would otherwise be left unchanged.…”

“…In the case of precessional (resonant) switching, two features are necessary: sizable magnetoacoustic coupling (to trigger precession), and a highly nonlinear system (to force wide, noncircular precession needed for magnetization reversal). The first point has already been addressed in ferromagnetic metals, since the 1960s by ac electrical excitation of strain waves [7][8][9][10][11][12], and more recently by optical excitation of acoustic pulses [13,14]. The electrical approach consists in the radio-frequency (rf) excitation of a piezoelectric emitter.…”

Terahertz modulation of the Faraday rotation by laser pulses via the optical Kerr effect

“…The laser induced acoustic solitons are an example of the propagating optical inhomogeneities created by a transient stress. [15][16][17][18] In this case, the frequency of the Faraday modulation Ω is determined by the speed of sound and resides in a GHz range. An inhomogeneity moving with a relativistic speed can be created by an intense laser pulse, which due to the optical Kerr effect, induces a linear dichroic region.…”

Terahertz Sum-Frequency Excitation of a Raman-Active Phonon

“…Many control strategies take advantage of terahertz (THz) modes of the material. An ubiquitous example is lattice vibrations (phonons) whose control by femtosecond laser pulses [4,5] has enabled new pathways to permanent material modification [6], insulator-to-metal transitions [7,8], and magnetization manipulation [2,9].…”