Time-resolved Magneto-optical Kerr Effect Measurements Research Articles

Spin current studies in Fe∕Ag,Au∕Fe by ferromagnetic resonance and time-resolved magneto-optics

A precessing magnetization within a magnetic double layer acts as a peristaltic spin pump which transports spin momentum but no net electric charge. Crystalline Fe single layers Au∕12Fe∕GaAs(001) and double layers Fe12∕(Au,Ag)∕Fe16∕GaAs(001) were prepared by molecular beam epitaxy, where the integers represent the number of Fe atomic layers, and (Au,Ag) represents a set of gold and silver layers of different thicknesses. Ferromagnetic resonance (FMR) was used to investigate spin diffusion in thick Au layers in Au∕12Fe∕GaAs(001) samples. Time-resolved magneto-optical Kerr effect (TRMOKE) measurements are an ideal tool for investigating the propagation of spin currents in these structures. Spin currents generated by the bottom 16Fe layer propagated across the normal metal spacer and resulted in rf excitations in the top 12Fe film. Landau-Lifshitz-Gilbert equations of motion modified by spin pump and spin sink effects were used to interpret the FMR and TRMOKE measurements. The spin diffusion lengths in Au were Ag were found 34 and 170nm, respectively.

Dynamic properties of arrays of ferromagnetic rectangular bars

Dynamic properties of arrays of rectangular bars were studied employing network analyzer ferromagnetic resonance and time-resolved magneto-optic Kerr effect (MOKE) measurements. The bars were patterned on top of coplanar waveguides and oriented with their long axis either parallel or perpendicular to the external magnetic field. Orientation of the bars parallel to the field results in an increase in the resonant frequency by up to ∼2GHz, as well as the linewidth broadening by 0.3–1GHz. The shift is shown to be in agreement with results expected from the difference of demagnetizing factors. The general behavior of the linewidth corresponds to calculations including shape anisotropy; however, the experimentally measured effect is much stronger than expected from theory. Fourier transforms of the time-resolved MOKE signal reveal the presence of several oscillatory modes, associated with the dynamics in the central part and at the borders of ferromagnetic elements.

Ultrafast magnetization dynamics in high perpendicular anisotropy [Co∕Pt]n multilayers

We present time-resolved magneto-optical Kerr effect measurements of picosecond laser induced precessional dynamics in out-of-plane magnetized [Co(4Å)∕Pt(8Å)]n multilayer films. A fast precession of magnetization (up to 81GHz) emerges 6ps after pumping and decays within a 100ps time scale. The uniform precessional frequency was studied for varying external magnetic bias field and number of Co∕Pt repeats n. The variation of the precession frequency with external bias field is quantitatively understood using the macrospin model of the Landau-Lifshitz-Gilbert equation of motion [Phys. Z. Sowjetunion 8, 153 (1935); Phys. Rev. 100, 1243 (1955)], yielding a large perpendicular anisotropy of up to 1.0×107ergs∕cm3 for our samples. The precession frequency increases sharply with reduced n and almost saturates below n=5, suggesting a commensurate variation of the perpendicular anisotropy. A heavy damping of the precessional motion is observed with increasing n and can be explained by enhanced spin-orbit coupling in ultrathin films and interfacial effects.

Elastic and Magnetic Dynamics of Nanomagnet-Ordered Arrays Impulsively Excited by Subpicosecond Laser Pulses

This Letter reports on the first observation of elastic and magnetic dynamics of ordered arrays of permalloy nanodots excited by low-intensity 120 fs light pulses. The first order of the diffraction pattern, generated by the probe beam in a pump-probe configuration, is used for time-resolved reflectivity and time-resolved magneto-optical Kerr effect measurements. The nonadiabatical absorption of the pump triggers an acoustic standing wave, detected by the reflected probe signal, with a frequency related to the array wave vector. Instead, the magneto-optical signal exhibits, on the nanosecond time scale, the signature of the heat-exchange diffusion processes. In addition, a clear oscillation of the magnetic signal, at a frequency close to the frequency of the acoustic wave, is unambiguously detected. Finally, the interplay between the elastic and magnetic dynamics is analyzed and interpreted.

Probing local spin modes in exchange-biased MTJ elements

We have studied the local magnetization dynamics in exchange-biased magnetic tunnel junction (MTJ) elements, by time-resolved magneto-optical Kerr effect measurements. We have observed a strong spatial dependence of the frequency and damping of the precessional motion in the free layer. A local edge mode has been detected, as well as increased damping at the edges. The experimental results are in good agreement with micromagnetic simulations, in which the stray field from the pinned layer of the MTJ is included.

Time-resolved magnetization precession and reversal dynamics investigated using tunneling current and Kerr effect

Magnetization precession and reversal dynamics in permalloy (Py) layer was studied using ns-time-scale spin-polarized tunneling current probing and ps-time-scale Kerr microscopy. In the time-resolved magneto-optical Kerr effect measurement, the precessional frequency increased with increasing external static field. The signal was fitted well with the calculation using Landau–Lifshitz–Gilbert equation of motion with the damping parameter α of 0.008, which corresponded to the value obtained for the film measured using a ferromagnetic resonance.

Observation of the spin-charge thermal isolation of ferromagneticGa0.94Mn0.06Asby time-resolved magneto-optical measurements

The dynamics of magnetization under femtosecond optical excitation is studied in a ferromagnetic semiconductor Ga_{0.94}Mn_{0.06}As with a time-resolved magneto-optical Kerr effect measurement with two color probe beams. The transient reflectivity change indicates the rapid rise of the carrier temperature and relaxation to a quasi-thermal equilibrium within 1 ps, while a very slow rise of the spin temperature of the order of 500ps is observed. This anomalous behavior originates from the thermal isolation between the charge and spin systems due to the spin polarization of carriers (holes) contributing to ferromagnetism. This constitutes experimental proof of the half-metallic nature of ferromagnetic Ga_{0.94}Mn_{0.06}As arising from double exchange type mechanism originates from the d-band character of holes.

Magnetization reversal in the picosecond range measured with time-resolved magneto-optical Kerr effect

We have performed time-resolved magneto-optical Kerr effect measurements with picosecond resolution in a pump-probe experiment using a 30 ps pulsed dye laser (λ=577 nm). Two typical magneto-optical storage materials were investigated: a Gd13Tb12Fe75 amorphous film evaporated on a glass substrate and a Co/Pt multilayer on a Si substrate. Curie-point thermomagnetic writing has been performed in less than 2 ns with the Co/Pt multilayer, whereas with the amorphous film magnetization reversal could not be observed within 6 ns. These differences in switching times are likely the consequence of the different thermal conductivities of the substrates.