DOI: 10.17035/d.2018.0049660686
title: Ultrafast Magnetization Dynamics in a Nanoscale Three-Dimensional Cobalt Tetrapod Structure

description: The ultrafast magnetization dynamics from a single tetrapod structure was measured using a custom-built time-resolved magneto opticKerr effect (TR-MOKE) microscope based on two-colour collinear pump-probe technique. The second harmonic (λpump= 400 nm, fluence = 19 mJ/cm2, pulse width = 100 fs) of the fundamental laser beam from a mode-locked Ti-sapphire laser (Tsunami, Spectra Physics) was used to pump the sample, while the time-delayed fundamental beam (λprobe= 800 nm, fluence = 7 mJ/cm2, pulse width = 80 fs) was used to probe the polar Kerr rotation by using an optical bridge detection as a function of the time delay between the pump and probe beams. The optical bridge detector isolates the Kerr rotation and the total reflectivity signal to avoid breakthrough of one signal into another and the measurement is done using lock-in amplifiers in a phase sensitive manner to attain high sensitivity. The probe beam is tightly focused (spot diameter = 800 nm) using a microscope objective of numericalaperture of 0.65 at the junction of the tetrapod structure, while the pump beam is slightly defocused at the focal plane of the probe beam with a spot diameter of about 1 μm. The probe beam is carefully placed at the centre of the pump-beam and the junction of the tetrapod structure using an x-y-z piezoelectric scanning state with a feedback loop and a white-light illumination system. A static magnetic field with varying magnitude is applied at a small angle (~ 15°) to the normal direction of the substrate plane (as shown in Fig. 2(a)), the out-of-plane component of which defined as the bias field H.  Here, we provide the time-resolved reflectivity, ultrafast demagnetization and precessional dynamics data for three different values of Hobtained from the TR-MOKE setup.  Files:   Tr_Reflectivity.txt  Ultrafast_demagnetization_data at Field _3.92_kOe.txt  Tr_Kerr_rotation_H_3.40 kOe.txt  Tr_Kerr_rotation_H_3.92 kOe.txt  Tr_Kerr_rotation_H_4.50 kOe.txt   In the above files, the 1st column is time in seconds. For the reflectivityfile,the 2ndcolumn contains reflectivity data and for the other files it contains Kerr rotation data.   The fast Fourier transforms of the time-resolved Kerrr rotation data are presented as below. In the FFT data files, the 1stcolumn is the frequency in Hertz and 2ndcolumn is the power of spectra in arbitrary units.   Files:   FFT_whole_Tr_Kerr_rotation data_H_3.40 kOe.txt  FFT_whole_Tr_Kerr_rotation data_H_3.92 kOe.txt  FFT_whole_Tr_Kerr_rotation data_H_4.50 kOe.txt  FFT_Partial_Tr_Kerr_rotation data_H_3.40 kOe.txt  FFT_Partial_Tr_Kerr_rotation data_H_3.92 kOe.txt  FFT_Partial_Tr_Kerr_rotation data_H_4.50 kOe.txt  We have performed 3D micromagnetic simulations using MuMax3 software. In the simulation, we have considered a tetrapod structure made of four cobalt cylindrical legs of diameters similar to the experimental sample but due to the limited computational resources, we have considered the length of each cylindrical leg as 2.5 µm. The sample is discretized into cells of dimensions 25 × 25 × 25 nm3.The magnetic parameters used for the simulation are saturation magnetization Ms= 1400 emu/cc, anisotropy constant K=0, gyromagnetic ratio γ= 17.6 MHz/Oe and exchange stiffness constant Aex= 3.0 × 10-5erg/cm. The external bias field His applied according to the experimental configuration and a square pulsed field of 10 ps risetime, 200 ps width and peak amplitude of 20 Oe is applied perpendicular to the sample plane. The simulated magnetization dynamics data are acquired from a volume of 1 μm3 from the tetrapod junction.The FFT power spectra data of the simulated time-resolved magnetization are provided here.  Files:   FFT_Simulated_Tr_mag_data_H_3.40 kOe.txt  FFT_Simulated_Tr_mag_data_H_3.92 kOe.txt  FFT_Simulated_Tr_mag_data_H_4.50 kOe.txt