Neutron diffraction and ESR studies of pseudocubic Nd 0.77 Ba 0.23 MnO 3 and its anomalous critical behavior above T C V.A. Ryzhov a , A.V. Lazuta a, * , I.A. Kiselev a , Yu.P. Chernenkov a , O.P. Smirnov a , S.A. Borisov a , I.O. Troynchuk b , D.D. Khalyavin b a Petersburg Nuclear Physics Institute RAS, Gatchina, St Petersburg 188300, Russian Federation b Institute of Physics of Solids and Semiconductors, National Academy of Sciences, ul. P. Brovki 17, Minsk 220072, Belarus Received 17 June 2003; accepted 4 July 2003 by E.L. Ivchenko Abstract Structural neutron diffraction study and electron spin resonance (ESR) measurements are presented for insulating Nd 0.77 Ba 0.23 MnO 3 with the Curie temperature T C < 124 K. Its pseudocubic structure exhibits the definite distortions to a low symmetry. A detailed analysis of the data is given for a Pbnm space group in the temperature range 4.2–500 K. A systematic transformation of the crystalline structure, which is accompanied by the peculiarities in the temperature variations of structural parameters, is found. The ESR data are in agreement with the predictions for 3D isotropic ferromagnets in the paramagnetic critical region at t p < 0:2 # t , 1; t ¼ðT 2 T C Þ=T C : The main disagreement observed below t p is that the ESR linewidth reveals no the characteristic critical enhancement. The anomalous critical behavior, which is found in the present and our previous studies in this range, is consistent with the development of some structural changes. Different temperature treatments (slow/fast cooling/heating, with/without external magnetic field) of the sample, which usually reveal the possible hysteretic phenomena, have a little effect on the ESR spectra. These results present a challenge to the understanding of the magnetic phase transition in these manganites. q 2003 Elsevier Ltd. All rights reserved. PACS: 61.12 2 q; 75.40.Gb; 76.30. 2 v Keywords: A: Magnetically ordered materials; D: Phase transitions; E: Neutron scattering; E: Electron paramagnetic resonance 1. Introduction Nd 12x Ba x MnO 3 is a series of doped manganites which exhibits a transition from paramagnetic (P) to ferromagnetic (F) state for 0:2 # x # 0:35 ðT C , 120 KÞ [1]. For x $ 0:3; these compounds show metallic behavior below T C ; whereas for x , 0:3 they remain insulating in the magnetic ordered phase. A colossal magnetoresistance is observed for all x near T C : As the most of other Ba-doped manganites, the compounds have the very small structure distortions so that they are close to the cubic perovskites [1,2]. Due to this peculiarity, one may expect to find the P – F transition of the same second order as for the isotropic 3D ferromagnets, at least in the insulating region of doping. However, our investigations of the linear and nonlinear critical properties of the x ¼ 0:23 single crystal above T C < 124 K revealed a more complicated behavior. The conventional critical behavior was observed to proceed only for T p < 147 K # T # 2T C : For T p . T $ T C ; an anomalous behavior, which suggests the coexistence of two magnetic phases, was found. At the same time, neutron and X-ray diffraction showed that the sample remained pseudocubic at 293 and 110 K [3,4]. In this paper we present a comprehensive neutron diffraction study of the x ¼ 0:23 compound which reveals the small deviations of its structure from pseudocubic. The 0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1098(03)00617-3 Solid State Communications 128 (2003) 41–45 www.elsevier.com/locate/ssc * Corresponding author. Tel./fax: þ 7-812-713-1963. E-mail address: alexandr@vl9467.spb.edu (A.V. Lazuta).