ANTIFERROMAGNETIC PHASES IN NdCo2Si2

Magnetic characteristics of NdCo2Si2 have been studied by magnetic and neutron diffraction measurements using a single crystal. Three antiferromagnetic phases were found below 32 K. A simple collinear antiferromagnetic phase is stable in the range 0-15 K where a metamagnetic process is observed. Square-wave structures with the propagation vectors K = (0, 0, 0.928) and (0, 0, 0.785) appear for 15 K < T < 24 K and 24 K < T < 32 K (= TN) , respectively. Ternary compounds RCo2Si2 (R = rare earth) crystallize in the tetragonal ThCr2Si2 type structure, in which the same atoms lie in alternate layers stacked along the oaxis with a sequence of RSiCoSi ... Most of RCo2Si2 compounds have been reported to be antiferromagnetic [I-31. Their magnetic characteristics are very interesting in co~ec t i on with the layer structure. Recently, a metamagnetic magnetization process and appearance of successive magnetic transitions in PrCo2Si2 have been found [4]. The magnetic behaviour is similar to that obtained for the Ising system with competing exchange interactions [5]. In the present study, magnetic and neutron diffraction measurements on the single crystal NdCo2Si2 have been carried out. The temperature dependence of magnetic susceptibilities X, and X, along the cand a-axes is shown in figure 1. Three anomalies were observed at TI = 15 K, T2 = 24 K and TN = 32 K in X, vs. T curve. The rapid changes in X , at Ti and T2 suggest that the transitions are of first order. On the other hand, X , is almost temperature independent below TN. The compound exhibits large magnetic anisotropy, and it is believed to be due to the crystalline electric field effects. Fig. 1. Temperature dependence of magnetic susceptibilities along the c and a-axis of the NdCoaSiz single crystal. Figure 2 shows the magnetization curves along the c-axis at various temperatures. The magnetization in the c-plane is linear against applied field and very small. The value at 4.2 K is 0.1 p~/f .u . at 54 kOe (Fig. 2a). The easy direction of magnetization is the c-axis. There is no magnetic anisotropy in the cplane. At 4.2 K, a steplike magnetization process is seen along the c-axis; the magnetization increases rapidly around Hcl = 40 kOe and Hc2 = 58 kOe with increasing field. The large hysteresis loops observed indicate that the transitions are of first order. The magnetization at the maximum field in this study reaches to 0.66 p~/f .u . Leciejewicz et al. [3] have reported from neutron diffraction studies that only Nd atom is magnetic and has nearly theoretical ~ d ~ + moment, 3.27 ,ug, at 4.2 K. The magnetization obtained at the maximum field is much smaller than the full Nd moment. The spin configuration does not reach a ferromagnetic arrangement. Therefore, additional Fig. 2. Magnetization curves along the c-axis of the NdCo2Siz single crystal at (a) 4.2 K, (b) 16 K, (c) 20 K and (d) 35 K. Magnetization in the c-plane is only shown in (a). Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888197 C8 432 JOURNAL DE PHYSIQUE steplike increases in magnetization should appear at higher fields. The magnetizations just after the first and second abrupt increases are nearly equal to one fourteenth and one fifth of the full moment, respectively. Therefore, small fractions of Nd moments are reversed at the fields. The magnetization process along the c-axis changes drastically with temperature. For TI < T < T2, the transition at Hcl disappears, but the step at Hc2 remains. The magnetization at 16 K is given in figure 2b as an example. For T2 < T < TN, there is no more transition within this experimental field up to 60 kOe, but a precursor of metamagnetic transition is seen at high field (Fig. 2c). For T > TN, the magnetization curve becomes linear (Fig. 2d). Neutron diffraction measurements on the single crystal have been performed at the Research Reactor Institute, Kyoto University. Antiferromagnetic superlattice peaks were observed on lattice rows parallel to the c*-axis, but not on the c*-axis below TN. This fact suggests that the magnetic moments lie along the caxis and the propagation vector is given as k = (0, 0, k) [2 x/c] for each phase: magnetic structures consist of (001) ferromagnetic planes with moments modulated along the c-axis. The thermal variation of integrated intensity for the (1 0 1 k) magnetic superlattice peaks is shown in figure 3. As evidenced from the figure, there are three antiferromagnetic phases. Below TI = 15 K, a k = 1 phase is stable in which the magnetic unit cell is in accordance with the chemical one and the sequence of ferromagnetic planes is + + . At TI, the k = 1 phase disappears and a k = 0.928 phase appears and is stable up to T2 = 24 K. In this range, additional small superlattice peaks which correspond to the third harmonics were observed, indicating that the magnetic structure is a square-wave structure. A phase with k = 0.785 develops at T2 and persists up to TN = 32 K. Within the experimental accuracy, the magnitude of the propagation vectors may be considered as fraction numbers as k = 13/14 1: 0.928 and k = 11/14 1: 0.785. Therefore, it should particulary be mentioned that the magnetic cell may be fourFig. 3. Thermal variation of integral intensity of the (1 0 1 k) magnetic superlattice peaks of NdCozSiP. teen times as large as the chemical unit cell for both phases. The magnetic behaviour of NdCozSi2 is similar to that obtained for the Ising systems with competing exchange interactions [5]. Measurements of the magnetization at high fields and neutron diffraction under magnetic fields are now in progress for analysis of the magnetization process and for discussion on the possibility of the successive magnetic transitions. [I] Yakinthos, J. K., Routsi, Ch. and Ikonomou, P. F., J. Less-Common Met. 72 (1980) 205. [2] Kolenda, M., Szytda, A., and Zygmunt, A., Proc. Int. Conf. on Crystalline Electric Field and Structural Effects in f-Electron System (Wroclaw) 1981, p. 309. [3] Leciejewicz, J., Kolenda, M. and Szytula, A., Solid State Commun. 45 (1983) 145. [4] Shigeoka, T., Iwata, N., Fujii, H., Okamoto, T. and Hashimoto, Y., J. Magn. Magn. Mater. 70 (1987) 239. [5] Bak, P. and von Boehm, J., Phys. Rev. B 21 (1980) 5297.