Preparation And Characteristics Of Co₂Mn_1-xTi_xSi Heusler Alloy, And Broadening Mechanism Of FMR Spectral Lines

Heusler compounds X2YZ in some structures are half-metallic ferromagnets, which present a gap in the minority band and100%spin polarized electrons in the Fermi level. Such properties make them suitable materials for spintronics applications Recently in Heusler alloys Y atoms are partly substituted with other atoms therefore constituting quaternary alloys to increase or decrease the magnetization and regulate the energy band. In this study, the quaternary Heusler Co2Mn1-xTixSi alloy is reported and the transformation of characteristics with substitution of Mn with Ti and different annealing condition are studied in detail. To reveal the reason of FMR spectral broadening, we calculate the FMR spectra of Ni thin films with different thickness from Landau-Lifshitz-Gilbert equation to verify the formulas and the program.At first, polycrystalline ingots of the compounds were prepared by arc melting of stoichiometric amounts of high-purity elements (99.99%) in an argon atmosphere. XRD measurements indicate that the phase of the polycrystalline ingots did not separate into two Heusler compounds and not present pure phase. The obtained ingots were annealed subsequently to get pure phase.After melting, the polycrystalline ingots were annealed in two different ways. One is annealed in an evacuated quartz tube at1073K for8days and the other is doing this again after the annealed polycrystalline ingots were cooled down. The crystallographic structure was investigated by x-ray powder diffraction. The magnetic properties were investigated by a vibrating sample magnetometer. And the FMR spectra were investigated by a ferromagnetic resonance spectrometer. The obtained results shows as follows:(1) The phase separatation of annealed samples is not observed. The samples annealed at1073K for8days present L21structures and the secondary annealed samples present DO3structures. The lattice parameter increases for substitution with Ti in both samples as expected from the larger atomic radius of Ti than that of Mn. The lattice parameters of L21structures vary from5.66to5.73A and those of DO3structures vary from5.94to6.01A with the increase amount of Ti concentration.(2) The roomtemperature saturation magnetization of the samples with L21and DO3structures decreases with Ti content increase. To L21structure samples the magnetic moment decreases from4.8μB/f.u. to1.3μB/f.u. for x increasing from0to1.0. The Mn-enriched compounds show a Slater-Pauling behavior for the annealed polycrystalline samples but the saturation magnetization is lower than the theoretical one for Ti-enriched compounds. This is due to the decrease of Curie temperature for high-Ti substituted samples. To DO3structure samples the magnetic moment decreases from3.2μB/f.u. to1.0μB/f.u. for x increasing from0to1.0.(3) The temperature-dependent magnetization of Co2Mn1-xTixSi sample was measured. The obtained Curie temperature decreases on substituting Mn with Ti. Curie temperature of L21structures Co2Mn1-xTixSi decreases from947.5K to401.5K for x=0.0to0.9except Co2MnSi with its Curie temperature higher than the available temperature of the VSM. Curie temperature of DO3structure samples ranges from783.0K to369.8K. The Curie temperatures of Mn-enriched compounds varied linearly with the number of valence electrons but deviate linear behavior in Ti-enriched compounds.(4) Regarding to the magnetic damping characterization of the powder samples, FMR was performed to find Gilbert magnetic damping constant a. Due to the irregular shapes of powders, the intrinsic a cannot be revealed due to the ultra wide of FMR line width. By polishing ingots into smooth sheets, the FMR line-width has been decreased obviously but the obtained damping constant a is still one order of magnitude larger than that of having been reported value. To reveal the reason of spectral broadening, calculation and simulation of FMR spectra were studied. We have deduced the absorptive part X" of the high frequency susceptibility X(ω) as a function of the external magnetic field from Landau-Lifshitz-Gilbert equation. Then the derivative of X(ω) was computed numerically by a Matlab program. The calculated spectrum is compared with experimental curve. To verify the formulas and program, we prepared Ni thin films with different thickness from9nm to45nm. The simulated spectra fitted well with the experimental FMR spectra of Ni thin films while damping parameter a and g-factor can be obtained.By studying the equations and simulated with different parameters and using Kittel formula, we deduced that the demagnetizing field caused by the flaws plays a leading role in FMR spectrum broadening of Co2Mn1-xTixSi samples.