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Element-specic study of the temperature dependent magnetization of Co-Mn-Sb thin lms J. Schmalhorst, ∗ D. Ebke, M. Meinert, A. Thomas, and G. Reiss Thin Films and Physics of Nanostructures, Department of Physics, Bielefeld University, 33501 Bielefeld, Germany E. Arenholz Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA (Dated: January 13, 2009) Magnetron sputtered thin Co-Mn-Sb lms were investigated with respect to their element-specic magnetic properties. Stochiometric Co 1 Mn 1 Sb 1 crystallized in the C1 b structure has been pre- dicted to be half-metallic and is therefore of interest for spintronics applications. It should show a characteristic antiferromagnetic coupling of the Mn and Co magnetic moments and a transition temperature T C of about 480K. Although the observed transition temperature of our 20nm thick Co 32.4 Mn 33.7 Sb 33.8 , Co 37.7 Mn 34.1 Sb 28.2 and Co 43.2 Mn 32.6 Sb 24.2 lms is in quite good agreement with the expected value, we found a ferromagnetic coupling of the Mn and Co magnetic moments which indicates that the lms do not crystallize in the C1 b structure and are probably not fully spin-polarized. The ratio of the Co and Mn moments does not change up to the transition temper- ature and the temperature dependence of the magnetic moments can be well described by the mean eld theory. PACS numbers: 75.70.-i, 78.70.Dm, 85.75.-d I. INTRODUCTION Fully spin-polarized so called half-metallic ferromag- netic materials are of highest interest for spintronic ap- plications such as nonvolatile memories 1 or eld pro- grammable logic devices 2,3 . They are characterized by a gap in the density of states (DOS) for minority (or majority) electrons at the Fermi energy E F . This half- metallicity has been predicted theoretically for some half and full Heusler compounds 4–8 . Moreover, a strong inu- ence of the atomic structure on their electronic properties is expected. Full Heusler alloys X 2 YZ often crystallize in the L2 1 structure which consists of four interpenetrating fcc sublattices (the unit cell is that of a fcc lattice with four atoms as basis: X at (000) and ( 1 2 2 ), Y at ( 1 4 4 ) and Z at ( 4 4 4 ) in Wycko coordinates ). The C1 b struc- ture of half Heusler alloys XYZ is comparable to the L2 1 structure, but with a vacant site at ( 1 2 2 ) instead of X. The structural, magnetic and electronic properties of the half Heusler compound CoMnSb are controversial in the literature. Galanakis 9 assumed CoMnSb to crystal- lize in the C1 b structure (X=Co, Y=Mn and Z=Sb). Us- ing a full-potential version of the screened KKR Green function method he calculated a total CoMnSb spin mag- netic moment of almost 3μ B per formula unit (f.u.). The Mn (3.18μ B ) and the Co (-0.13μ B ) spin moments are antiferromagnetically aligned and half-metallicity with a large spin-down gap (∼ 1eV ) was proposed. Experimentally, however, a total magnetic moment of about 4μ B / f.u. was observed 10 , which could be ex- plained by Tobola et al. 11 by assuming a more com- plex structure (space group F d ¯ 3m) originally proposed by Senateur et al. 12 in 1972. The calculated mean Mn and Co magnetic moments are 3.51μ B and 0.48μ B , re- spectively for this structure and ferromagnetic coupling is expected. Although CoMnSb crystallized in this struc- ture should still be half-metallic, the spin-down gap is predicted to be very narrow in contrast to the large gap derived from calculations by Galanakis 9 for the C1 b structure. Assuming a disordered L2 1 structure - the two X sub- lattices were occupied by 50% Co and 50% vacant sites and the Y and Z sublattices by Mn and Sb, respectively - Kaczmarska et al. 13 could also reproduce a magnetic moment of nearly 4μ B / f.u. by KKR-CPA calcula- tions. Again a ferromagnetic coupling of Mn (3.60μ B ) and Co (0.30μ B ) was derived, but half-metallicity disap- peared in this case. Very recently, Ksenofontov et al. 14 used NMR and M¨ ssbauer spectroscopy to study bulk o CoMnSb, they assigned it to the space group F m ¯ and 3m proposed that the CoMnSb structure can be presented as an alternation of Co 2 MnSb and MnSb structural units with a Co moment of 0.55μ B and a mean Mn moment of 3.48μ B . Again, half-metallicity is not predicted for this structure type. To unambiguously determine the magnetic prop- erties of these intriguing systems, we investigated the temperature dependence of magnetic moments of thin Co 32.4 Mn 33.7 Sb 33.8 , Co 37.7 Mn 34.1 Sb 28.2 and Co 43.2 Mn 32.6 Sb 24.2 lms grown on a Cu / Ta / V buer in an element specic way using soft x-ray spectroscopy. II. EXPERIMENTAL The samples were deposited at room temperature by DC-magnetron sputtering on 200nm thick Si 3 N 4 membranes from Silson Ltd., UK. The membranes (3mm×3mm) were supported by a 200μm thick Si frame (10mm×10mm). The surface roughness of the

Element-specific study of the temperature dependent magnetization of Co–Mn–Sb thin films