L Journal of Alloys and Compounds 335 (2002) 70–76 www.elsevier.com / locate / jallcom Investigation of the crystal structure and magnetic properties of Nd Gd Mn Si compounds 1 2x x 2 2 a,b, a b * S. Kervan , Y. Elerman , M. Acet a Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Ankara, Turkey b ¨ Tieftemperaturphysik, Gerhard-Mercator Universitat Duisburg, D-47048 Duisburg, Germany Received 21 August 2001; accepted 24 August 2001 Abstract X-ray powder diffraction and DC magnetization studies were performed on the polycrystalline Nd Gd Mn Si (0#x#1) 12x x 2 2 compounds. Magnetic properties were examined between 5 and 700 K in applied fields up to 50 kOe. All compounds investigated crystallize in the ThCr Si -type structure with the space group I 4/ mmm. Substitution of Gd for Nd leads to a linear decrease in the lattice 2 2 ´ constants and the unit cell volume. In the x(T ) curves above room temperature, all samples show a clear peak coinciding with the Neel temperature due to antiferromagnetic coupling of the ferromagnetic Mn layers. At low temperatures below about 60 K, the rare earth sublattice orders and also the antiferromagnetic coupling of the ferromagnetic Mn layers becomes ferromagnetic. The samples with x 50.2 and 0.6 exhibit a spin reorientation phenomenon. In terms of the mean field two-sublattice model, the values of the intrasublattice coupling constant n and the intersublattice coupling constant n for the antiferromagnetic phase have been derived from the 11 12 temperature dependence of the magnetic susceptibility.  2002 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Rare earth compounds; X-ray diffraction; Magnetic measurements 1. Introduction magnetic properties of RMn Si compounds were studied 2 2 before [4–14]. All RMn Si compounds except LaMn Si 2 2 2 2 The ternary rare earth RT X compounds (R5rare are antiferromagnets. The NdMn Si and GdMn Si com- 2 2 2 2 2 2 earth, T53d transition metal, X5Si or Ge) have been pounds are antiferromagnetic below T 5382 and 453 K, N extensively studied because of physical properties ranging respectively. The magnetic moment of the Nd sublattice in from superconductivity to heavy fermion behavior [1]. NdMn Si orders and couples ferromagnetically with the 2 2 Most of these compounds crystallize in the simple body- magnetic moment of the Mn sublattice below 34 K, centered tetragonal ThCr Si -type structure with the space whereas the magnetic moment of the Gd sublattice in 2 2 group I 4/ mmm, in which the R, T and X atoms occupy the GdMn Si becomes ordered and antiferromagnetically 2 2 2a(0,0,0), 4d(0,1/2,1/4) and 4e(0,0,z) positions, respec- coupled with the magnetic moment of the Mn sublattice tively [2]. The stacking of atomic layers along the c-axis below 65 K [5–8]. This paper presents the influence of direction with the sequence T–X–R–X–T forms this substitution of Gd for Nd on the crystal structure and structure. The coupling between ferromagnetic Mn layers magnetic properties of Nd Gd Mn Si compounds. 12x x 2 2 a is sensitive to the intralayer Mn–Mn distance R , Mn–Mn a ˚ yielding ferromagnetic coupling for R . 2.85 A or Mn–Mn a ˚ antiferromagnetic coupling for R , 2.85 A [3]. 2. Experimental Mn–Mn Moreover, neutron diffraction studies in the RMn X 2 2 compounds revealed that the AFil structure composed of The Nd Gd Mn Si samples with x 50.0, 0.2, 0.4, 12x x 2 2 antiferromagnetically coupled ferromagnetic Mn layers 0.6, 0.8 and 1.0 were prepared by induction melting a ˚ along the c-axis is stable for R ,2.84 A [4]. The constituent elements under argon atmosphere in a water- Mn–Mn cooled copper boat. The purity of the elements were 99.9% for Nd and Gd, 99.98% for Mn and, 99.95% for Si. The *Corresponding author. Tel.: 190-312-212-6720; fax: 190-312-223- mass loss of Mn during the melting was compensated by 2395. E-mail address: kervan@science.ankara.edu.tr (S. Kervan). adding the 2% excess Mn. The polycrystalline ingots were 0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388(01)01818-7