Tetrataenite and other Fe – Ni equilibrium phases produced by reduction of nanocrystalline NiFe 2 O 4 Enio Lima Jr a , Valderes Drago a, * , Paulo F.P. Fichtner b , Paulo H.P. Domingues c a Departamento de Fı ´sica, Universidade Federal de Santa Catarina, Campus Universitario, Floriano ´polis 88040900, Brazil b Departamento de Metalurgia, Universidade Federal do Rio Grande do Sul, Porto Alegre 04501970, Brazil c Instituto de Fı ´sica, Universidade Federal do Rio de Janeiro, Rio de Janeiro 21941972, Brazil Received 26 June 2003; accepted 30 August 2003 by H. Akai Abstract Various Fe – Ni alloys are formed by hydrogen reduction at low temperature of a nanometric NiFe 2 O 4 obtained through a wet chemical method. In particular, we have produced bulk quantities of the ordered tetrataenite phase which permitted the measurement of its ferromagnetic transition temperature at 567 8C for the first time in a synthetic sample. q 2003 Elsevier Ltd. All rights reserved. PACS: 75.30.Kz; 75.50.Bb; 76.80. þ y Keywords: A. Nanostructured powder; A. Fe–Ni alloys; B. Chemical synthesis; E. Mossbauer spectroscopy 1. Introduction In the Fe–Ni phase diagram, one of the alloys of great academic interest is the chemically ordered tetrataenite that can exist at low temperatures between 49 and 55% at. Fe [1]; it presents a tetragonal structure (superstructure L1 0 like CuAu), spatial group P4=mmm; with a ¼ 2.533 A ˚ and c ¼ 3.582 A ˚ [2]. The order – disorder transition temperature for tetrataenite was established at 320 8C [3]. Since below this temperature the auto-diffusion in the Fe–Ni system is insignificant in laboratory time periods, this phase cannot be produced by the simple annealing of the corresponding chemically disordered taenite phase. Therefore, significant fractions of tetrataenite are found only in meteoritic samples which have suffered a slow and very long cooling process [4]. In these cases, it appears in a spinodal equilibrium with the other disordered taenite and the low moment fcc phases in a somewhat complex microstructure as described in Ref. [1]. Small amounts of tetrataenite have been produced in the laboratory by electron irradiation [5], neutron irradiation [6], ion irradiation [7] and mechanical alloying [4], because these methods introduce great quantities of defects in the material with the consequent enhancement in diffusivity. Recently we presented a new method to produce Fe–Ni alloys at low temperature giving special attention to the tetrataenite phase [8]. This method consists of a cyclic oxy- reduction of micrometric iron grains capped with Ni by the electroless technique. In the present work, we have produced the Fe–Ni equilibrium phases by the reduction of nanocrystalline nickel ferrite (NiFe 2 O 4 ) under hydrogen flux at 320 8C. We had success in producing even the ordered tetrataenite phase at this temperature, because although its disorder transition temperature is 320 8C the kinetics of its disorder is very slow [9]. In the first part of this paper, we describe the synthesis and characterisation of the NiFe 2 O 4 produced. In the second part, we produced various Fe – Ni phases by reducing the Ni ferrite under hydrogen flux at 320 8C. In the third part, we investigate the microstructure of the alloys formed in this reducing process. The techniques used for this were Mo ¨ssbauer spectroscopy (MS), transmission electron microscopy (TEM) and thermomagnetic analyses (TMA). 0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2003.08.046 Solid State Communications 128 (2003) 345–350 www.elsevier.com/locate/ssc * Corresponding author. Tel.: þ55-483-316-835; fax: þ 55-483- 319-946. E-mail address: vdrago@fisica.ufsc.br (V. Drago).