55 Mn NMR study of the hexagonal Laves phase compound ThMn 2 : Coexistence of magnetic and nonmagnetic sites of Mn Saurav Giri, Hiroyuki Nakamura, and Masayuki Shiga Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan ~Received 11 December 1998! The magnetic ground state of the hexagonal Laves phase intermetallic compound ThMn 2 was investigated by 55 Mn nuclear magnetic resonance. Both paramagneticlike resonance at the zero-Knight-shift position and antiferromagnetic resonance at high frequencies were found to be consistent with the coexistence of magnetic and nonmagnetic Mn atoms, i.e., an antiferromagnetic state with partially nonmagnetic Mn sites, proposed from neutron diffraction experiments. The hyperfine field at the magnetic 6 h site was estimated to be 138 kOe, which is larger than that observed for another antiferromagnetic Laves phase compound YMn 2 ~114 kOe! despite the 6 h -site magnetic moment estimated by neutron diffraction experiments (1.6m B ) being smaller than the moment of YMn 2 (2.7m B ). Possible interpretations are discussed. @S0163-1829~99!05821-X# I. INTRODUCTION A series of the Laves phase intermetallic compounds R Mn 2 with R 5rare earth shows a wide variety of magnetic properties. 1 They form the cubic C 15 or the hexagonal C 14 structure depending on the R element or sample preparation procedures. Their magnetic properties are mainly character- ized by the instability of the Mn moment, and antiferromag- netic, nonmagnetic, and mixed ~partially antiferromagnetic! ground states have been found for the Mn sublattice. Con- cerning the Mn moment instability, in addition to the Mn-Mn interatomic distance, topological frustration of the antiferro- magnetic interactions and strong magnetic anisotropy have recently been discussed extensively. 2,3 Especially, the mag- netism on the network of regular tetrahedra found in the cubic Laves phase compound has attracted considerable at- tention since it is one of the typical three-demensional highly frustrated lattices. YMn 2 and its related compounds, where only the Mn sublattice can be magnetic, are good examples in itinerant electron frustrated systems. 4 In the case of pure YMn 2 , the frustration is released by ordering into a long- period helically modulated structure accompanied by a small crystal deformation. 5,6 An actinide-based compound ThMn 2 forms the hexagonal C 14 structure, 7 and only Mn atoms bear a magnetic moment as the same as YMn 2 . In the hexagonal structure, there are two nonequivalent crystallographic Mn sites (6 h and 2 a ). The tetrahedra of Mn atoms are connected with each other to form a chain along the sixfold c axes. The 6 h Mn atoms are located in c planes to form the kagome ´ lattice, the 2 a Mn and Th atoms lying in between the planes formed by the 6 h Mn atoms. The magnetic susceptibility of ThMn 2 ~Refs. 3 and 8! shows a maximum at around 115 K which corre- sponds to the antiferromagnetic Ne ´ el temperature, T N . The low-temperature specific heat is large even in the magneti- cally ordered state, 9 which is ascribed to large spin fluctua- tions in the nonmagnetic 2 a site revealed by neutron diffrac- tion measurements. Neutron diffraction study 3,10 confirmed that ThMn 2 orders antiferromagnetically with a first-order phase transition at T N 5115 K, accompanied by anomalous temperature dependences in both a and c lattice parameters. As the temperature decreases the c parameter contracts, while a expands. However, the spontaneous volume magne- tostriction below T N is not appreciable in contrast to the discontinuous large expansion of the other antiferromagnetic R Mn 2 compounds. The results also show that Mn atoms at the 2 a site are nonmagnetic and the magnetic structure is described by the propagation vector Q5( 1 3 1 3 0). Magnetic layers formed by 6 h Mn atoms are perpendicular to the c axis and coupled antiferromagnetically with each other. In each layer the magnetic moments lie in the c-plane kagome ´ lattice and are arranged as shown in Fig. 1. The proposed structure is spin noncolinear; a triangle made by parallelly coupled three Mn moments forms the 120° arrangement with neigboring triangles. The ordered moment was estimated to be (1.660.1) m B . The coexistence of magnetic and nonmag- netic Mn atoms, i.e., the mixed phase, has been found in some R Mn 2 compounds such as DyMn 2 and HoMn 2 . 11,12 Among hexagonal C 14 R Mn 2 compounds, a partially non- magnetic structure similar to ThMn 2 has been proposed for high-pressure synthesized hexagonal TbMn 2 although it is a collinear structure. 13 However, in these cases, the molecular FIG. 1. Alignments of 6 h Mn magnetic moments of ThMn 2 in the basal plane. PHYSICAL REVIEW B 1 JUNE 1999-I VOLUME 59, NUMBER 21 PRB 59 0163-1829/99/59~21!/13943~5!/$15.00 13 943 ©1999 The American Physical Society