High spin polarization in CoFeMnGe equiatomic quaternary Heusler alloy

Abstract

We report the structure, magnetic property and spin polarization of CoFeMnGe equiatomic quaternary Heusler alloy. The alloy was found to exist in the L2 1 structure with considerable amount of DO 3 disorder. Thermal analysis result indicated the Curie temperature is about 711K without any other phase transformation up to melting temperature. The magnetization value was close to that predicted by the Slater-Pauling curve. Current spin polarization of P = 0.70 ± 0.1 was deduced using point contact Andreev reflection (PCAR) measurements. Halfmetallic trend in the resistivity has also been observed in the temperature range of 5 K to 300 K. Considering the high spin polarization and Curie temperature, this material appears to be promising for spintronic applications.Key words: Heusler alloy, Mössbauer spectroscopy, Slater-Pauling rule, Spin Polarization, Half-metal. PACS: 75.50.Bp, 75.30.Cr, 75.47.Np, 76.80.+y, 75.76 The polycrystalline bulk sample of an equiatomic CoFeMnGe (CFMG) alloy was prepared by arc melting of stoichiometric quantities of constituent elements in an inert atmosphere. The ingot was melted several times to increase the chemical homogeneity and the final weight loss was less than 1%. To increase the homogeneity, the as-cast alloy was annealed under vacuum for 14 days at 1073 K and then quenched in cold water. The crystal structure was investigated by x-ray diffraction with Cu K α radiation at room temperature. 57 FeMössbauer spectra at room temperature were recorded using a constant acceleration spectrometer with 25 mCi 57 Co(Rh) radioactive source. The spectrometer was calibrated by using natural iron foil of 25 µm before measuring the sample. The obtained spectra were analyzed using PCMOS-II least-squares fitting program. Curie temperature and the structural transition temperature (if any) were probed by using the differential thermal analysis (DTA)with a heating rate of 20 K min -1 . Magnetization measurements at 3 K and 300 K were performed by using a vibrating sample magnetometer (VSM) attached to a physical property measurement system (PPMS, Quantum design). Current spin polarization measurements were done by using the point contact Andreev reflection (PCAR) technique. 19 Sharp Nb tips prepared by electrochemical polishing were used to make point contacts with the sample.Spin polarization of the conduction electrons was obtained by fitting the normalized conductance G(V)/G n curves to the modified Blonder-Tinkham-Klapwijk (BTK) model. 20 A multiple parameter least squares fitting was carried out to deduce spin polarization (P) using dimensionless interfacial scattering parameter (Z), superconducting band gap (∆) and P as variables. Fig. 1 shows the Rietveld refinement of powder XRD pattern recorded at room temperature. The lattice parameter of the alloy was found to be 5.75 Å, which is in agreement with the earlier reports on CFMG. 14 As one can see, superlattice reflections, (111) and (200), are very weak for this alloy. When Z element is from the same period of the periodic system...

Equiatomic quaternary Heusler alloys: A material perspective for spintronic applications

“…Bainsla et al [7,8] at 5 K were obtained for CoFeMnSi and CoFeMnGe respectively by Alijani et al [45]. The difference between the MS values for CoFeMnSi could be attributed to the large structural disorder observed by Bainsla et al [8,46].…”

“…To further investigate the crystal structure of CoFeMnZ (Z = Si and Ge) alloys, Bainsla et al [7,8,46] performed EXAFS measurements [46] and found large structural disorder in the case of CoFeMnSi, while CoFeMnGe was found to exist in well ordered Y-phase. Room temperature 57 Fe Mössbauer analysis also revealed a large structural disorder (DO3) for…”

“…Bainsla et al [7,8], by performing the differential thermal analysis (DTA) in the temperature range of 400 -1450 K. There was no structural transition in the DTA curves for both the alloys. The absence of (111) peak in the XRD pattern of CoFeMnAl indicates the presence of B2 disorder as shown by Alijani et al [45].…”

“…High values of TC facilitates in technological applications of these materials. Bainsla et al [7,8] In order to understand the magnetic properties further, the magnetic behavior of these alloys were studied using ab-inito calculations by Alijani et al [45]. [45] observed that the stability of the Co and Mn moments together with the Slater-Pauling rule dictates whether the moment at Fe sites are aligned parallel or antiparallel to Mn.…”

“…Since, the prediction of HMF nature in NiMnSb [6], Heusler alloys have received a special attention due to the reasons mentioned above. In the large family of Heusler alloys, generally Co based alloys show relatively high Curie temperature and high spin polarization [7][8][9][10][11][12][13][14][15][16].…”