Magnetic phase transitions of competing system CePtAl

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Physica B 237-238 (1997)212-214

Magnetic phase transitions of competing system CePtA1 H . K i t a z a w a a'*, S. N i m o r i a, J. T a n g a, F. I g a b, A. D r n n i a, T. M a t s u m o t o a, G . K i d o a aNational Research Institute for Metals (NRIM), 1-2-I Sengen, Tsukuba, lbaraki 305, Japan bHiroshima University, Higashi-Hiroshima 739, Japan

Abstract

Magnetic properties of CePtAI single crystals with orthorhombic TiNiSn-type crystal structure were investigated for each axis by measurements of magnetic susceptibility, high-field magnetization up to 30 T and magnetoresistance up to 10T. The pronounced anisotropy observed indicates that magnetism in CePtAI is dominated by a ferromagnetic component along the magnetically easy a-axis and that the magnetically hard axis changes from the c-axis (T > 39 K) to the b-axis (T < 39 K). The results are consistent with the magnetic structure model proposed by neutron diffraction. Keywords: CePtA1; Magnetization; Anisotropic exchange interactions

The ternary intermetallic compound CePtA1 with an orthorhombic TiNiSn-type crystal structure shows successive magnetic phase transitions at Tt = 5.9 K, T2 = 4.3 K and T 3 = 2.2 K. Recently, powder neutron diffraction measurement [1] revealed complex magnetic structures consisting of two coexisting, partly ferromagnetic and incommensurate magnetic propagation vectors: k l - - - 0 and k 3 = E0, 0.463(8), 0] for T2 < T < T1; and kl = 0 and k2 = [0, ½, 0 ] for T < T2. Magnetism in CePtAI is dominated by a ferromagnetic component (1.35#B at 1.5 K) oriented parallel to the a-axis. The experiments on CePtA1 single crystals presented in this paper illustrate the temperature and field dependence of magnetic anisotropy. CePtA1 single crystals were grown with the Czochralsky pulling method in a tetra-arc furnace. The high-field magnetization experiments up to 30 T were carried out with the extraction method using the brand-new 40 T-class hybrid magnet at NRIM. Magnetic susceptibility from 1.9 to 300 K

*Corresponding author.

was measured by a S Q U I D magnetometer (Quantum Design). The magnetoresistance up to 10 T was measured by a conventional 4 terminals DC method. The temperature dependence of the inverse magnetic susceptibility of CePtA1 shown in Fig. 1 for each crystal axis reveals a pronounced magnetic anisotropy which is mainly caused by crystallineelectric field interactions. The magnetically easy axis corresponds to the a-axis and the magnetically hard axis changes from the c-axis (T > 39 K) to the b-axis (T < 39 K). Between 10 and 300 K the inverse magnetic susceptibility along the a-axis follows a Curie-Weiss law with an effective magnetic moment/t~fe = 2.58#B (close to the free ion value for C e 3 + of 2.54/~B) and a Curie temperature O~, = 6.5 K indicating the existence of ferromagnetic interactions. Fig. 2 displays the magnetization process for each axis at 5 K (T2 < T < T1). Along the magnetically easy a-axis the magnetization Ma shows a spontaneous ferromagnetic moment of 1.0/~B/Ce. Along the magnetically hard b-axis the magnetization Mb is almost proportional to the magnetic field. In the magnetization curve Mc two

0921-4526/97/$17.00 (~) 1997Elsevier ScienceB.V. All rights reserved PII S092 1-4526(97)00 1 04-X

11, Kitazawa et al. /Physica B 237-238 (1997) 212-214

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tribution in the paramagnetic state (T > T1) and exhibits a minimum in the magnetically ordered state (T < T1). Ap/pb(O) and Ap/pc(O) are almost field independent above T1 and show a positive contribution below T~. Some anomalies in Ap/pd0 ) (marked by black triangles in Fig. 3) might indicate field-induced magnetic phase transitions. The observed pronounced anisotropy indicates that the magnetism in CePtA1 is dominated by a ferromagnetic component along magnetically easy a-axis and that the magnetic structures are unstable in a magnetic field applied along the caxis. Further experiments using single crystals are needed to establish the complex magnetic phase diagram of CePtA1 which appears to be caused by competing anisotropic exchange interactions in a network, where Ce-Ce neighbors (3.56 A) form chains parallel to the b-axis and Ce-Ce secondnearest neighbors (3.77 A) form chains along the a-axis.

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plateaus appear at around 5 and 25T. Fig. 3 displays the magnetoresistance Ap/p(O) at selected temperatures. Ap/pa(O) shows a large negative con-

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H. Kitazawa et al. / Physica B 237-238 (1997) 212-214

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References

[1] A. D6nni, H. Kitazawa, P. Fischer, J. Tang, M. Kohgi, Y. Endo and Y. Mofii, J. Phys.: Condens. Matter 7 (1995) 1663.