~ ) Solid State Communications, Vol. No. 419-422, 1993. 85, 5, PP. Printed in Great Britain. 0038-1098/9356.00+.00 Pergamon Press Ltd MOSSBAUER INVESTIGATION OF THE PECULIAR MAGNETISM OF Tb(Mn 1-xFex)2 AT SMALL SUBSTITUTION OF Fe FOR Mn J.L. Oddou and C. Jeandey CENG, DRFMC/SPSMS/LIH, 85X, 38041 Grenoble Cedex, France R. Ballou and J. Deportes Laboratoire de Magn6tisme L. N6el, CNRS, 166X, 38042 Grenoble Cedex, France and B. Ouladdiaf Institut Laue-l_angevin, 156X, 38042 Grenoble Cedex, France (Received 6July 1992, in revised form 15 September 1992 by P. Burlet) M6ssbauer experiments have been performed on the Tb(Mn0.96Fe0.04)2 intermetallic compound from liquid nitrogen temperature down to liquid helium temperature. Above 34 K, the obtained spectra are interpreted in a two sites model with populations in the ratio 3:1. Below 34 K, each site splits into two equally populated subsites differing from each other by the quadrupolar splitting and isomer shift. At this temperature, one observes also a rotation of the hyperfine field of the site of population 1. These data are consistent with an earlier proposal of a collinear magnetic structure at high temperature where only one Mn atom over four is magnetic, with a reorientation of the corresponding moment below 34 K as has been shown up by neutron scattering. 1. INTRODUCTION Interesting magnetic behaviors, arising from the combined effect of a magnetic-non magnetic (M-NM) instability and a high magnetic frustration, characterize the RMn2 (R = Y, Sc, Th or Lanthanide) intermetallic compounds. Within the series, a critical value for the Mn- Mn spacing appears to exist below which Mn is not magnetic and above which a large Mn magnetic moment is stabilized [1]. In the later case the Mn-Mn interactions become dominant and stabilize antiferromagnetism. All the magnetic orderings are then subject to a geometric frustration inherent to the topology of the Mn packing which consists in a stacking of regular tetrahedra of Mn, connected either by alternating shared vertices and shared bases (generating an hexagonal phase) or by shared vertices (generating a cubic phase). Owing to the magnetic frustration, complex magnetic structures are stabilized [2]. On the other hand, many magnetic states with close energy may exist which, associated with the M-NM instability, leads to a high sensitivity of the magnetic behaviors with respect to applied magnetic field, applied hydrostatic pressure, small substitution of Lu, Sc for Tb or Y for example or else small substitution of Fe, Co for Mn. Initial magnetic ordered states are abruptly destabilized towards spin-glass states, very unusual non ordered states with heavy-fermion like behavior or "mixed" magnetic structures (ferrimagnetic structures and, at the limit, magnetic structures where magnetic and non-magnetic Mn ions coexist) [3, 4]. The case of small substitution of Fe for Mn is particularly well suited for a local investigation by M6ssbauer effect of the change in the magnetic properties induced by the "chemical" pressure so exerted. In fact MOssbauer experiments can help to elucidate the magnetic structure which can be proposed from neutron scattering experiments on Tb(Mnl-xFex)2 compounds. 2-EXPERIMENT A polycrystalline sample of the Tb(Mn0.96Fe0.04)2 compound was prepared from starting Tb and Mn elements of 99.99% purity and enriched in 57Fe iron. It was melted in a water-cooled copper crucible, under argon atmosphere, in an induction furnace allowing for a quasi levitation of the melt. After an annealing under vacuum (10 -7 mbar) at 800°C during 3 days, the obtained ingots were checked by the Debye-Scherrer X-ray method. No foreign phase other than the cubic C15 structure was detected. 57Fe M6ssbauer measurements were recorded at numerous temperatures between 4.2 K and 77 K on a 512 multichannel analyzer. The absorber was 18 mg/cm 2 of Tb(Mn0.96Fe0.04)2. The sample absorber was mounted in a conventional liquid helium horizontal transmission cryostat equipped with beryllium windows. Between 4.2 K and 50 K, absorber temperatures were obtained by electrical heating of the sample ; between 52 K and 77 K, they were obtained by pumping on liquid nitrogen bath (for these experiments, liquid nitrogen has been substituted for helium). Temperatures were measured with a thin foil Ni- manganin probe (Cryogenic Linear Temperature Sensor) and regulated to within 0.3 K by a conventional PID system. The source is 57Co in a rhodium matrix, kept at room temperature and moved in a constant acceleration mode with a maximum velocity calibrated at 4.78 mm/s. The spectra were analyzed in a least square procedure by full diagonalization of the Hamiltonian describing quadrupolar and magnetic interactions [5]. 419