* Corresponding author. Fax: #81-426-77-2483. E-mail address: iwasa@comp.metro-u.ac.jp (K. Iwasa) Present address: NEC Corporation, Minato-ku, Tokyo 108- 8001, Japan. Physica B 281&282 (2000) 437}439 Crystal}lattice modulations associated with unusual magnetic structures in the low-carrier system CeSb Kazuaki Iwasa*, Yoshihito Arakaki, Masahumi Kohgi, Takashi Suzuki Department of Physics, Faculty of Science, Tokyo Metropolitan University, Minami-osawa 1-1, Hachioji, Tokyo 192-0397, Japan Tsukuba Institute of Science and Technology, Tsukuba, Ibaraki 300-0819, Japan Abstract X-ray di!raction studies have been performed in order to investigate the p}f mixing e!ect producing the unusual magnetic properties in the low-carrier system CeSb. Several X-ray satellite-peak patterns were observed below ¹ "17 K. The wave numbers and the intensities of the satellite peaks are in accordance with the model of periodic arrangement of short and long distances due to two kinds of magnetic layers in the structures reported by Rossat- Mignod et al. (J. Magn. Magn. Mater. 52 (1985) 111). The shorter distance is due to the strong p}f mixing e!ect, which is spatially restricted within the ferromagnetic layers of -like Ce ions, and the longer one due to the layers of normal Ce ions. 2000 Elsevier Science B.V. All rights reserved. Keywords: Low-carrier system; CeSb; p}f mixing e!ect; Crystal}lattice modulation Physical properties of low-carrier systems have been attractive in studies of strongly correlated electron sys- tems. Cerium monopnictides are such typical systems and show various long-period magnetic structures under magnetic "elds or under high pressures, despite their simple NaCl-type structures [1]. The magnetic structures of CeSb are constructed by the stacking of ferromagneti- cally ordered Ce-ion (0 0 1) layers with unusually large magnetic moments (&2 ) and paramagnetic ones. The recent neutron scattering works on CeP and CeAs revealed that their magnetic structures under high pres- sures also consist of two kinds of planes with ferromag- netic moment of about 2 and 0.7 [2]. The latter becomes paramagnetic in higher-temperature magnetic ordered phases. The polarized-neutron study on CeP revealed that the 2 moments is explained by a 4f- electron wave function of the -like crystal-"eld state [3]. Thus, this large moment in Ce-monopnictides is attributed to the -like state, and the small one to the state becoming a ground state in the paramagnetic region. The strong p}f mixing e!ect and the carriers with low density have been considered to yield these phe- nomena [4]. Phenomena on their crystal lattices are also remark- able. A sudden contraction of the crystal lattices in CeSb and CeBi occurred with decreasing temperatures through ¹ [5]. This phenomenon indicates that, owing to the mixing e!ect, the e!ective ionic size of Ce ions in the -like state is smaller than that in the state. Therefore, crystal}lattice modulations are expected to appear in the ordered phases of Ce-monopnictides where di!erent interlayer distances given by the -like and Ce layers are arranged periodically. Using a cleaved single-crystal sample of CeSb, we observe several X-ray satellite-peak patterns along the [0 0 ]-axis below ¹ "17 K as shown by marks in Fig. 1. In the AFP3, AFP4 and AFP5 phases, higher- harmonic peaks were also clearly detected. In the AFP1 and AFP2 phases, only one satellite peak was observed within the experimental accuracy. The data taken in the AFP6 and AF phases do not show any satellites. We introduce a following model based on the di!erent e!ective ionic sizes for the two Ce-ion states. The distance between the neighboring Ce-ion layers of the state is de"ned as d. The distance between neighboring -like Ce-ion layers is assumed to be shorter and is given as Physb1=17191=Ramesh=Venkatachala=BG 0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 0 5 0 - 9