DSpace at VNU: Magnetic properties of half-metallic semi Heusler Co1-xCuxMnSb compounds

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 311 (2007) 605–608 www.elsevier.com/locate/jmmm Magnetic properties of half-metallic semi Heusler Co1ÀxCuxMnSb compounds N.P Duonga,Ã, L.T Hunga, T.D Hiena, N.P Thuya,b, N.T Trungc,a, E Bruăckc a International Training Institute for Materials (ITIMS), Hanoi University of Technology, Dai Co Viet Road, Hanoi, Vietnam Faculty of Electronics and Telecommunication, College of Technology, Vietnam National University, Hanoi, 140 Xuan Thuy Road, Cau Giay, Hanoi, Vietnam c Van der Waals-Zeeman Instituut, Universiteit van Amsterdam, Valckenierstraat 65, Amsterdam, 1018 XE, Amsterdam, Netherlands b Received 26 May 2006; received in revised form August 2006 Available online 14 September 2006 Abstract A study of the half-metallic character of the semi Heusler alloys Co1ÀxCuxMnSb (0pxp0.9) is presented We investigated the saturation magnetization MS at temperatures from K to room temperature and the temperature dependence of the DC magnetic susceptibility w above Curie temperature TC The magnetic moments at K, for most compositions are very close to the quantized value of mB for Mn3+ ion, the compound with 90% Co substituted by Cu is still ferromagnetic with MS (5 K) ¼ 3.78 mB/f.u These results emphasize the role of Co atoms in maintaining the ferromagnetic order in the material The Curie temperature is decreased from 476 K to about 300 K as the Cu content increases from 0% to 90% Above TC, the wÀ1 vs T curves follow very well the Curie–Weiss law The effective moment meff and paramagnetic Curie temperature y are derived A comparison between the values of MS at K and meff shows a transition from localized to itinerant spin system in these compounds r 2006 Elsevier B.V All rights reserved PACS: 75.50.Cc; 75.10.Jm; 75.10.Lp; 75.20.Hr; 75.47.De Keywords: Co1ÀxCuxMnSb; Heusler alloys; Half-metals; Ferromagnets; Localized electrons; Itinerant electrons Introduction Ternary semi Heusler alloys XMnSb (X ¼ 3d, 4d or 5d) which have C1b cubic structure (space group F-43 m) are ferromagnets with Curie temperature between 500 and 700 K except for CuMnSb which orders antiferromagnetically below 55 K [1,2] Due to the high TC and predicted half-metallicity, these alloys attract increasing interest in the field of spintronics Half-metallic ferromagnets are characterized by the coexistence of metallic behavior for one direction of electron spin and insulating or semiconducting behavior (energy band-gap) for the other Therefore, the electron density of states is ideally 100% spin polarized at the Fermi level and conductivity is governed entirely by single spin electron carriers [3,4] The magnetic ÃCorresponding author Tel.: +84 8680787; fax: +84 8692963 E-mail address: duong@itims.edu.vn (N.P Duong) 0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2006.08.023 properties of half-metallic ferromagnets are manifested by quantization of the magnetic moment at K and a change of spin system from localized to itinerant behavior upon increasing temperature [5] Recently, by employing electronic band structure calculations, Galanakis et al [6] have shown that CoMnSb has the largest spin-minority gap ($1 eV) among the XMnSb compounds, this gap is about 0.5 eV in the case of NiMnSb The Fermi level in CoMnSb locates at the lower edge of the gap A change of the lattice parameter by 2% still preserves the gap and shifts the Fermi level toward the lower edge and the higher edge as increasing or decreasing the lattice constant, respectively In addition, the hybridization between the wave functions of 3d electrons of Co and Mn is considerably larger than that in the case between Ni and Mn As a consequence, the Mn moment is reduced, while the Co moment becomes negative, resulting finally in a calculated total moment of mB [6] ARTICLE IN PRESS N.P Duong et al / Journal of Magnetism and Magnetic Materials 311 (2007) 605–608 (422) (331) (420) (400) CuMnSb (311) (222) (111) (220) In order to clarify the half-metallic character in CoMnSb, this paper deals with the magnetic properties of the Co1ÀxCuxMnSb series Replacing Co by Cu, which is slightly larger than Co, will not affect the crystallographic structure but the band filling and hybridization (200) 606 The polycrystalline Co1ÀxCuxMnSb samples (0pxp1, step 0.1) were prepared by arc-melting the constituent elements of 99.9% purity according to the stoichiometric formulae in a cold copper crucible and protective argon atmosphere To compensate for evaporation losses, an excess of wt% of Mn and Sb was added to the starting compositions The molten ingots were heat treated at 1050 K in vacuum for days and quenched in water in order to obtain good homogeneity Crystal structure and microstructure analysis were carried out by means of X-ray diffraction (XRD) with Cu-Ka radiation and scanning electron microscopy (SEM), respectively Magnetization at temperatures between 100 and 800 K is measured using a vibrating sample magnetometer in applied magnetic fields up to 1.4 T For magnetization measurements at K we employ a commercial SQUID magnetometer MPMS5S with maximal applied field of T Intensity (a.u.) Experimental Co0.1Cu0.9MnSb CoMnSb 20 30 40 50 60 70 80 Fig X-ray diffraction spectra of the Co1ÀxCuxMnSb compounds (x ¼ 0, 0.9 and 1) Results and discussion 6.10 Co1-xCuxMnSb 6.05 Lattice constant (Å) All Co1ÀxCuxMnSb samples are characterized by XRD measurements As typical examples, Fig shows the diagrams of compositions x ¼ 0, 0.9 and All the samples form in the cubic phase with space group C1b The development of the lattice parameter with variation of x is depicted in Fig A continuous expansion of the lattice occurs when x increases because the radius of a Cu ion (1.28 A˚) is slightly larger than that of a Co ion (1.25 A˚) The total change in lattice parameter is about 3.8% when x increases from to The magnetization curves for the Co1ÀxCuxMnSb compounds at K are shown in Fig The concentration dependence of the saturation magnetization MS is plotted in Fig The values of the magnetic moments per formula unit are slightly larger than mB/f.u for xp0.7 The maximum value of 4.25 mB/f.u is observed for x ¼ 0.1 and 0.2 This very small excess has also been reported in pervious works for CoMnSb, which can be attributed to a small contribution from Co moments [5,7,8] The ferromagnetic ordering in the material is preserved with Cu substitution up to x ¼ 0.9, with a still quite large MS ¼ 3.78 mB/f.u and TC near room temperature These results indicate that in this series the magnetic moments are mainly localized on the Mn atoms The variation of MS at 4.5 K as a function of Cu substitution x for Ni1ÀxCuxMnSb alloys was recently investigated by Ren et al [9] They show that, for x lower than 0.7, the magnetic moments of the Ni1ÀxCuxMnSb compounds remain almost unchanged However, when x is larger than 0.7, the values of the 6.00 5.95 5.90 5.85 0.0 0.2 0.4 0.6 Cu content (x) 0.8 1.0 Fig Lattice constant of the Co1ÀxCuxMnSb compounds as a function of Cu substitution formula moments start to drop significantly, showing a breaking of ferromagnetic order As already known, in semi Heusler alloys, the magnetic ordering is governed by the Mn–Mn interaction which proceeds via itinerant ARTICLE IN PRESS N.P Duong et al / Journal of Magnetism and Magnetic Materials 311 (2007) 605–608 4.5 607 50 x=0 4.0 x = 0.1 40 3.5 x = 0.2 x = 0.5 M (emu/g) M (μB/f.u.) 3.0 2.5 2.0 CoMnSb Co0.9Cu0.1MnSb 1.5 Co0.8Cu0.2MnSb 1.0 Co0.3Cu0.7MnSb x = 0.9 20 H = 0.1 T Co0.1Cu0.9MnSb 0.0 0.0 0.5 1.0 H (T) 1.5 300 2.0 Fig The magnetization curves of the Co1ÀxCuxMnSb compounds (x ¼ 0, 0.1, 0.2, 0.7, 0.8 and 0.9) measured at K 350 400 450 T (K) 500 550 600 Fig The thermomagnetic curves of the Co1ÀxCuxMnSb compounds (x ¼ 0, 0.1, 0.2, 0.5, 0.7 and 0.9) measured in kOe 4.5 800 4.0 700 3.5 600 500 3.0 TC (K) Ms (μB/f.u.) x = 0.7 10 Co0.2Cu0.8MnSb T=5K 0.5 30 2.5 400 300 2.0 1.5 Co1-x Cux MnSb Ni1-x Cux MnSb 200 Co1-xCuxMnSb 100 Ni1-xCuxMnSb 1.0 0.0 0.2 0.4 0.6 Cu content (x) 0.8 1.0 0.5 0.0 0.2 0.4 0.6 Cu content x 0.8 1.0 Fig The TC values as a function of Cu substitution of the Co1ÀxCuxMnSb compounds (’) of present work and of the Ni1ÀxCuxMnSb compounds (K) according to Ref [9] Fig The Cu concentration dependence of the saturation magnetization of the Co1ÀxCuxMnSb compounds at K (’) according to present work and of the Co1ÀxCuxMnSb compounds at 4.5 K (J) according to Ref [9] electrons in a broad conduction band [3] Obviously, Co plays an important role in maintaining the strength of the indirect exchange interactions between the Mn moments even at high levels of Cu substitution (xX0.7) The Curie temperature of the samples was determined from the M vs T curves in applied magnetic field of kOe (Fig 5) When the Cu concentration increases from to 0.9, the TC value of the Co1ÀxCuxMnSb compounds decreases slowly from 576 to $300 K in contrast to the case of the Ni1ÀxCuxMnSb compounds where TC drops drastically from 750 to 87 K as depicted in Fig The results for the Curie temperatures are consistent with the small reduction of the magnetic moments per formula unit at K, namely the Mn–Mn interaction remains strong when increasing x to 0.9 A plot of the inverse magnetic susceptibility wÀ1 as a function of temperature for the samples is presented in Fig showing the Curie–Weiss behavior above the Curie temperature From these experimental curves, corresponding values of effective moment meff and paramagnetic Curie temperature y were calculated The lattice and magnetic parameters of the Co1ÀxCuxMnSb compounds are listed in Table The TC, MS and lattice parameter values for CoMnSb are in good agreement with that found in literature [5,7,8] ARTICLE IN PRESS N.P Duong et al / Journal of Magnetism and Magnetic Materials 311 (2007) 605–608 608 14 x = 0.1 12 x = 0.4 x = 0.3 χ-1 (106 mol/m3) x = 0.5 10 x = 0.8 x = 0.9 H = 0.1 T 300 350 400 450 500 T (K) 550 600 Fig The temperature dependence of the inverse magnetic susceptibility of the Co1ÀxCuxMnSb compounds (x ¼ 0.1, 0.3, 0.4, 0.5, 0.8 and 0.9) Table Crystallographic and magnetic Co1ÀxCuxMnSb compounds properties Compounds Lattice TC constant (A˚) (K) M5S K y (K) (mB/f.u.) CoMnSb Co09Cu01MnSb Co08Cu02MnSb Co07Cu03MnSb Co06Cu04MnSb Co05Cu05MnSb Co03Cu07MnSb Co02Cu08MnSb Co01Cu09MnSb 5.859 – 5.886 – 5.905 5.950 6.049 – 6.067 4.05 4.26 4.26 4.20a 4.15a 4.11a 4.0 3.9 3.78 476.0 472.6 460.5 457.0 455.5 425.0 391.3 350.7 $300.0 of the ferromagnetic meff (mB) pc/ps 490–520 [5] 4.6–4.0 [5] 0.88–0.4 [5] 486.0 3.88 0.70 – – – 459.3 4.18 0.82 454.4 4.04 0.76 467.9 4.24 0.81 – – – 393.7 3.68 0.72 308.4 3.1 0.60 Symbols are defined in the text a Linearly interpolated values from data in Fig From the magnetic data, some comments can be made on the spin system of these materials The so-called Rhodes–Wohlfarth ratio pc/ps is expected to be equal to for local-moment ferromagnets and larger than for itinerant ferromagnets [10,11] Here, ps is the saturation moment in units of mB at K and pc is the effective moment per magnetic atom deduced from the Curie constant C ¼ m2B pc(pc+2)/3kB In previous experimental work, Otto et al have shown that for CoMnSb, NiMnSb and PtMnSb, the ratio pc/ps is less than unity [5] which can be understood neither by a localized nor by an itinerant model To explain this behavior, the authors proposed a simple molecular field model taking into account both local moments and spin-polarized itinerant electrons and showed that the ratio pc/pso1 is a characteristic of half-metallic ferromagnets Also in the case of the Co1ÀxCuxMnSb compounds, we found that all the values of the Rhodes–Wohlfarth ratio pc/ps are significantly smaller than unity (see Table 1) In conclusion, single-phase polycrystalline Co1ÀxCuxMnSb samples have been obtained by arc-melting method followed by an annealing process The saturation magnetization at K of the samples with xp0.9 is very close to mB per formula unit, indicating that the magnetic moments mainly localized on the Mn atoms in this compounds The Co atoms have a small contribution to the total magnetization but play an important role in the mechanism of the indirect exchange interaction between Mn moments as demonstrated by the room-temperature ferromagnetism in Co0.1Cu0.9MnSb Although the Curie temperature of CoMnSb (576 K) is considerably lower than that of NiMnSb (750 K), the Cu substitution leads to a slow decrease in TC of the Co1ÀxCuxMnSb series as compared to that of the Ni1ÀxCuxMnSb In the paramagnetic region, the temperature dependence of the inverse DC magnetic susceptibility of the Cu-substituted samples follows well the Curie–Weiss law The derived Rhodes– Wohlfarth ratios pc/pso1 indicate half-metallicity in these materials The results also imply a transition of the spin system from localized to itinerant character when the temperature increases from K upward Acknowledgment This work was supported by the State Program of Vietnam, Project no 811604 on Nano Science and Nano Technology and the VLIR-HUT Project of Hanoi University of Technology, Code AP05/Prj3/Nr02 References [1] C Hordequin, J Pierre, R Currat, J Magn Magn Mater 162 (1996) 75 [2] H Forster, G.B Johnston, D.A Wheeler, J Phys Chem Solids 29 (1968) 855 [3] R.A de Groot, F.M Mueller, P.G van Engen, K.H.J Buschow, Phys Rev Lett 50 (1983) 2024 [4] I.I Mazin, D.J Singh, Claudia Ambrosch-Draxl, Phys Rev B 441 (8) (1999) 59 [5] M.J Otto, R.A.M van Woerden, P.J van der Valk, J Wijngaard, C.F van Bruggen, C Haas, K.H.J Buschow, J Phys Condens Matter (1989) 2341 [6] I Galanakis, P.H Dederichs, N Papanikolaou, Phys Rev B 66 (2002) 134428 [7] M.G Natera, M.R.L.N Murthy, R.J Begum, N.S Satya Murthy, Phys Status Solidi a (1970) 959 [8] A Szytula, Z Dimitrijevic, T Todorovic, A Kolodziejczyk, J Szelag, A Wanic, Phys Status Solidi a (1972) 97 [9] S.K Ren, W.Q Zou, J Gao, X.L Jiang, F.M Zhang, Y.W Du, J Magn Magn Mater 288 (2005) 276 [10] P Rhodes, E.P Wohlfarth, Proc R Soc A 273 (1963) 247 [11] E.P Wohlfarth, J Magn Magn Mater (1978) 113  ... compounds (’) of present work and of the Ni1ÀxCuxMnSb compounds (K) according to Ref [9] Fig The Cu concentration dependence of the saturation magnetization of the Co1ÀxCuxMnSb compounds at K (’)... saturation magnetization at K of the samples with xp0.9 is very close to mB per formula unit, indicating that the magnetic moments mainly localized on the Mn atoms in this compounds The Co atoms... The magnetization curves for the Co1ÀxCuxMnSb compounds at K are shown in Fig The concentration dependence of the saturation magnetization MS is plotted in Fig The values of the magnetic moments