ChemInform Abstract: Magnetic Properties of Y(Co1-xNix)4Al Compounds

Journal of

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Journal of Alloys and Compounds 242 (1996) L5-L7

Letter

Magnetic properties of Y(COl_xNix)4A1compounds E. B u r z o a, S. B o d e a a, D. P o p a, V. P o p a, R. B a l l o u b, G h . B o r o d i c ~Faculty of Physics, Babes-Bolyai University, .3400 Cluj-Napoca, Romania bLaboratoire de Magndtisme Louis Ndel, 166X, 38042 Grenoble-Cedex, France :Institute of Isotopic and Molecular Technology, 3400 Cluj-Napoca, Romania Received 9 May t996

Abstract

Magnetic measurements were performed on Y(Co~_,Nix)4A1 compounds in the temperature range 4.2-750 K and fields up to 7 T. The magnetic behaviour of Ni and Co is analyzed with the spin fluctuation model.

Keywords: Yttrium transition metal compounds;Magneticordering;Spin fluctuationmodel

The RCo s and RNi s compounds, where R is a rare-earth or yttrium, crystallize in the CaCus-type structure. The above systems form solid solutions in all the composition range [1]. When replacing one cobalt atom by aluminium, the CaCus-type structure is preserved. A sensitive diminution of the Curie temperatures and cobalt moments is evidenced, compared with values determined in RCo s compounds [2-4]. In RNi s compounds, nickel has an exchange enhanced magnetic susceptibility [5]. Recent studies [6] show that the magnetic behaviour of YNi s is well described by the spin fluctuation model [7]. We studied the Y(COl_xNix)4A1 compounds in order to obtain information on the magnetic behaviour of Co and Ni in a system where the end series compounds show different magnetic behaviour. The Y(Co>xNix)4A1 samples were prepared by arc melting the constituent elements in a purified argon atmosphere and remelting several times to ensure good homogeneity. They were thermally treated under vacuum at 1000°C for one week. X-ray analysis showed the presence of only one phase having CaCu stype structure. The lattice parameters depend little on composition, Fig. 1. The ionic radii of Co 2+ and Ni 2+ are nearly the same, and consequently no changes in lattice constants are expected when substituting Ni for Co, as experimentally observed. Magnetic measurements were performed in the temperature range 4-750 K and external fields up to 7T. The spontaneous magnetizations Ms were ob0925-8388/96/$15.00 © i996 Elsevier Science S.A. All rights reserved PII S0925-8388(96)02387-0

tained from magnetization isotherms according to the saturation law M = M~(1 - a/I-I) + X'oH. We denote by X'o the field independent susceptibility, and a is the coefficient of magnetic hardness. In the paramagnetic range, the magnetic susceptibilities were obtained from their field dependences according to the relation ,)( =/Y.m "ff cM=H-1 by extrapolating the measured values to / _ / - 1 0. c denotes a presumed impurity content and M= is the corresponding saturation magnetization. By this method any possible alteration of X values as a result of the presence of small quantities of magnetic ordered impurities is avoided. YNi4AI, at 4.2 K, is in a paramagnetic state. At temperatures T > 10K the magnetic susceptibility follows a Curie-Weiss type dependence, Fig. 2. The above behaviour can be analyzed by the self-consistent

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E. Burzo et al. / Journal of Alloys and Compounds 242 (7996) L 5 - L 7

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only their mean values may be obtained. The mean cobalt moment at 4.2 K decreases from 1t.% (x = 0) to 0.17#B (x = 0.8). The diminution of the cobalt magnetic moment may be attributed to the variation of exchange interactions in the system when replacing cobalt by the non-magnetic nickel. To analyze this we computed the exchange fields at 4.2K, He×oh(CO), acting on cobalt. A linear dependence of the magnetization as a function of the Hc~ch(CO) values is shown in Fig. 4. Mco = VcoHexch(CO), with '/Co ~ (2 × 10°)-1#z/Oe

0 300

(1) 400

500

600

700

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T(K) Fig. 2, Thermal variations Y(Col_yi~)4A.I compounds,

of reciprocal

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renormalization (SCR) theory of spin fluctuations [7]. In the case of exchange enhanced paramagnetism, the amplitude of local spin fluctuations (S~oc) = 3kBT Eq Xq is a temperature dependent quantity. For small q values, the (S~oc) in YNi4A1 increase rapidly with temperature and reach an upper limit determined by the charge neutrality condition at a temperature T* ~ 10 K. For T > T* the above compound behaves as if having local moments. From the effective nickel moment Meff Ni ~ I/xB, an electronic configuration 3d 9.s is suggested for nickel. In this case only longitudinal spin fluctuations are present. The compounds having x ~