59Co NMR study of the metamagnetic transition in Y1−xRxCo3 (R  Nd and Gd)

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59Co

Physica B 237-238 (1997) 592-593

NMR study of the metamagnetic transition in YI_xRxCo3 (R = Nd and Gd)

Masayuki Itoh a,*, Tadahiro Shibata a, Hirotaka Tanaka a, Katsuhiko Koui b, Tsuneaki Goto b a Department of Physics, Faculty of Science, Chiba University, Yayoi-cho, Inage-ku, Chiba 263, Japan b Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan

Abstract

We have performed 59CoNMR measurements to study microscopically itinerant metamagnetism in intermetallic compounds Yl-xRxCo3 (R = Nd and Gd). At 4.2 K, two metamagnetic transitions induced by the Nd exchange field are found to take place at x ,-0.35 and 0.85 in Yl-xNdxCo3, whereas one transition is observed at x ~ 0.25 in Yl-xGdxCo3. Keywords: NMR; Yl-xNdxCo3; Yt-xGdxCo3; Metamagnetism

Itinerant metamagnetism is one of the interesting phenomena in intermetallic compounds, and is theoretically interpreted to come from a characteristic structure of the density of states. YCo3, one of such intermetallic compounds, has a rhombohedral crystal structure of the PuNi3 type with three Co sites (3b, 6c and 18h) and two Y sites (3a and 6c). In YCo3 successive metamagnetic transitions due to three Co sites were observed in ultra-high magnetic fields [1]. Recently, the effect of the exchange field due to the rare-earth magnetic moment on the metamagnetic transition was studied from magnetization measurements in Yl-xNdxCo3 [2,3] and Yl-xGdxCo3 [4]. In this paper, we have performed 59Co NMR measurements to clarify microscopically the metamagnetic transition induced by the exchange field due to Nd and Gd magnetic moments in Yl-xRxCo3 ( R = N d and Gd) in zero external field. Polycrystalline samples of Yl-xRxCo3 were prepared by the arc melting in argon atmosphere as described in Refs. [2-4]. Frequency-swept 59Co NMR spectra were taken by using a super-heterodyne coherent pulsed spectrometer. * Corresponding author. 0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All fights reserved Pll S092 1-4526(97)00282-2

In YCo3 we observed three 59Co signals due to three Co sites (3b, 6c and 18h) in the domain at 4.2 K in zero external field, whereas four 59Co signals were observed because of magnetically nonequivalent two 18h (18hi and 18h2) sites in NdCo3, as are presented in Fig. 1. We follow the site assignment of 59Co NMR signals discussed in Ref. [5]. In mixed compounds Yl-xNdxCo3, frequency-swept 59Co NMR spectra were found to have broad peaks due to a distribution of the internal field Hn at each Co site. In the analysis of the NMR spectra in Yl-xNdxCo3, we reasonably assume that Hn at each Co site is expressed as Hn = Hn(Co) + Htr(Nd) +/-/dip + HLor + Ha where Hn(Co) is the hyperfine field due to a parent Co magnetic moment and neighboring Co moments, H~(Nd) is the transferred hyperfine field due to the nearest-neighbor Nd moments, Hdip is the dipole field, HLor is the Lorentz field, and Ha is the demagnetization field. A random distribution of the Nd atom leads to several satellite lines. Each line is assumed to have an area proportional to a binomial probability and to have Htr(Nd)= mHt° where m is the number occupied by Nd in the nearest-neighbor Y sites and /-/to ~ 5 kOe is estimated from the analysis for x < 0.3. We fitted the experimental data to the superposition

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of several Gaussians. A fitted result for x = 0.4 is presented in Fig. 1. Consequently, we obtain the x dependence of Hn(Co) as is shown in Fig. 2 . / / , ( C o ) at the 3b and 6c sites is found to increase rapidly at x ,-0.35, whereas Hn(Co) at the 18h site increases at x ~ 0.85. This x dependence of Hn(Co) indicates that successive metamagnetic transitions induced by the Nd exchange field take place at x ~ 0.35 and x ~ 0.85 at 4.2 K in Yl-xNdxCo3. From the similar analysis of 59Co NMR spectra in Yl-xGdxCo3, we found that all Co sites undergo metamagnetic transition at x ~ 0.25. Thus, we have confirmed metamagnetic transitions induced by the exchange field of Nd and Gd magnetic moments without applying an external

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field in Y1-xRxCo3 (R = Nd and Gd) from the microscopic point of view.

References [1] T. Goto, H. Aruga Katori, T. Sakakibara and M. Yamaguchi, Physica B 177 (1992) 255. [2] T. Goto, K. Koui, M.I. Bartashevich, H. Aruga Katori, M. Yamaguchi, I. Yamamoto and F. Sugaya, Physica B 193 (1994) 10. [3] K, Koui, M.I. Bartashevich, T. Goto, H. Aruga Katori and M. Yamaguchi, Physica B 201 (1994) 143. [4] H. Aruga Katori, T. Goto, I. Yu Gaidukova, R.Z. Levitin, A.S. Markosyan, I.S. Dubenko and A. Yu Sokolov, J. Phys.: Condens. Matter 6 (1994) 11 119. [5] H. Yoshie, T. Fujii, H. Nagai, A, Tsujimura and Y. Nakamura, J. Phys. Soc. Japan 54 (1985) 2725.