Physical Properties Of Half-metallic Heusler Alloys Co₂Mn_Xfe_1-xAl Films

Heusler alloy was founded in1903and named after the founder, German mining engineer and chemist Friedrich Heusler. Recently, Heusler attracts a considerable attention since it has an uniquehalf-metallic band. At the meanwhile, it is also a ferromagnetic compound. Half-metallic ferromagnets are a unique class of materials in which one spin band has metallic character while the other spin band is semiconducting. As a result such a ferromagnet possesses conduction electrons that are100%spin polarized. They are now being considered as the potential materials for applications in the field of spin-electronic and magneto resistive de-vices. On the applicable viewpoints, it is required that the ferromagnetic materials used as an electrode of magnetic tunnel junctions (MTJs) for tunneling magnetoresistance (TMR) have high Curie temperature (Tc) as well as high spin polarization. This qualification can make the devices work well at room temperature. Actually Heusler alloys belong to a group of ternary intermetallics with the stoichiometric composition X2YZ, where X and Y are transition metal atoms and Z is usually a sp-element, respectively. Among all full-Heusler alloys, Co2-based compounds are of particular interest since they have comparatively high Tc. Recently, some theoretical investigations have been done to an-alyze the Tc and its intrinsic physical mechanism. For example, Umetsu et al. have reported Tc of Co2MnAl with both B2and L21phases by temperature-dependent thermomagne-tization curves. Thoene et al. calculated the Tc values for a variety of Heusler compounds by mean-field approximation (MFA). The Curie points of Co2TiGa and Co2FeGa have been evaluated by the first-principles density functional theory combined with a linear response method. However, the crucial issue is still under debate because the experimental results from the material growth and characterizations are scarce.In this paper, the optical properties of Co2MnxFe1-xAl films have been investigated byRaman Scattering emperiment. The experiment data have been analysied to determine the Curie point of each film.(1) Stucture of Co2MnxFe1-xAl films has been investigated; The DCXRD, AFM and HR-TEM measurements have been conducted to confirm the degree of the order for the films. Since Co2MnxFe1-xAl belongs to full-Heusler alloys, and for full-Heusler alloys there are three typical structures:L21, B2and A2, respectively. The DCXRD patterns show (004) refelection which illustrates these set of films belong to either L2i or B2strucutre. AFM images also show the good quality of the Co2MnxFe1-xAl films. The fast Fourier transform (FFT) analysis of the HR-TEM images shows (111) and (002) diffraction pattersn, in which (111) diffraction is definitely from the distinct L21reflection. Based on all the structure analysis, the Co2MnxFe1-xAl films can be considered as having the L21strucutre.(2) The lattice patterns of Co2MnxFe1-xAl films have been investigated. The lattcie of the films has been studied, it consist of eight stacked body-centered cubic (bcc) lattice. And it belongs to the space group Fm3m.(3) The phonon modes have been studied through the Raman Spectrum.Based on the lattice structure information, we can derive the phonon modes as Γ=F2g+2F1u. We can found three Raman peaks in the Raman Spectrum located at about320,569and947cm-1. The320cm-1peak can be corresponded to the F2g Phonon. We can attribute the modes at569and947cm-1to the2F1u phonon modes.(4) The curie temperature of Co2MnxFe1-xAl films has been investigated. The temperature dependence of the Raman spectra is measured in the termperature range of87-873K. The fequency and intensity of the Raman mode at320cm-1is qualitatively maintained. While scattering peak of the films detected at569cm-1has an obvious increase in intensity and frequency. The Raman mode at947cm-1also has the similar trend. Further study has been made to the trend of the two phonon modes. We found there is an anomalous change in the vibration. It can be assigned to the phase transistion induced by the structure change. Then the Curie temperature of Co2MnxFe1-xAl films can be drived as below:828K for x=0,823K for x=0.3,818K for x=0.7and788K for x=1.(5) The vibration of Co2MnxFe1-xAl films’Curie temperature with Mn composition has been investigated.The vibration can be expressed as:Tc(x)=832-36.2x(0<x<1). The reason is the lattice constant of the films becomes bigger with a higher Mn composition. There fore it brings the pressure induced Tc enhancement effect.