Paramagnetic resonance and non-resonant microwave absorption in iron niobate G. Alvarez a, * , R. Font b , J. Portelles b , O. Raymond c , R. Zamorano d a Departamento de Materiales Meta ´licos y Cera ´micos, Instituto de Investigaciones en Materiales, Universidad Nacional Auto ´noma de Me´xico, Me´xico D.F. 04510, Mexico b Departamento de Fı ´sica Aplicada, Facultad de Fı ´sica, Universidad de la Habana, San La ´zaro y L. Vedado, La Habana, Cuba c Centro de Nanociencias y Nanotecnologı ´a, Universidad Nacional Auto ´noma de Me´xico, Km 107, Carretera Tijuana Ensenada., Ensenada, Baja California 22860, Mexico d Departamento de Ciencias de los Materiales, ESFM-IPN, U.P.A.L.M., Edificio 9, Me´xico D.F. 07738, Mexico article info Article history: Received 9 July 2008 Received in revised form 8 October 2008 Accepted 1 November 2008 Available online 13 November 2008 Keywords: Electron paramagnetic resonance (EPR) Mixed valency oxides Non-resonant microwave absorption abstract An electron paramagnetic resonance (EPR) study of FeNbO 4 powder samples in monoclinic phase (wolframite-type) at X-band (8.8–9.8 GHz), in the 90–300 K temperature range, is presented. For all the temperatures, the EPR spectrum shows a single line associated with Fe 3þ ions. Changes in the lineshape of the EPR spectrum, which can be attributed to Fe 2þ ions, are detected at low temperatures. This behavior can be ascribed to a strong magnetic dipolar interaction between Fe 2þ and Fe 3þ ions. The non- resonant microwave absorption techniques: magnetically-modulated microwave absorption spectros- copy (MAMMAS) and low-field microwave absorption spectroscopy (LFMAS), were used for a further knowledge on this material. MAMMAS response suggests also the presence of Fe 2þ ions, that originates a change in microwave absorption regime for T < T p (¼140 K), associated with the presence of short- range magnetic correlations. LFMAS spectra showed a linear behavior with positive slope and non- hysteretic traces. The profiles obtained by plotting the slope vs. temperature of the LFMAS line are similar to those detected by the MAMMAS technique, confirming that both types of measurement show the same processes of absorption. Ó 2008 Elsevier Masson SAS. All rights reserved. 1. Introduction In the last years, a considerable attention has been given for some types of ABO 4 oxides, e.g. FeTaO 4 , FeWO 4 and FeNbO 4 for applications as gas sensor, catalytic and photodetector technologies. Among these materials, the iron niobate FeNbO 4 (FN), has recently gained considerable attention [1–4]. This compound in particular has been investigated as a possible photoanode material, with potential applications in the conversion of solar energy [5]. Indeed, it is well known as the key precursor for the successful preparation of single-phase perovskite Pb(Fe 0.5 Nb 0.5 )O 3 [6], free of secondary pyrochlore or iron-oxide phases. FN, also known as ferrocolumbite, can be found in four struc- tural types identified as: wolframite-type (space group P2/c) or AlNbO 4 -type (space group C2/m) with a monoclinic structure; ixiolite or a-PbO 2 type (space group Pbcn) with orthorhombic structure and the rutile-type (space group P4 2 /mnm) with tetrag- onal structure. In the wolframite lattice, there are zigzag chains of edge-sharing NbO 6 and FeO 6 octahedral along the [001] direction, with ordering of Fe 3þ and Nb 5þ ions. Each chain accommodates either Fe or Nb. The Fe 3þ ion is of primary importance for the magnetic properties of this material: antiferromagnetic ordering at T N ¼ 38.5 K (Ne ´el temperature) is reported for FN with wolframite- type structure [2,7]. Additionally, of the several classes of compounds, the mixed valency oxides in which the transition metal ions are at identical lattice sites with different valence states have shown interesting properties. In the pure and stoichiometric monoclinic phase FN, the Fe and Nb ions are in þ3 and þ5 valence state, respectively. However, some physical behaviors observed in FN are due to oxygen deficiency which leads to the generation of a fraction of Fe 2þ ions. In recent works [1,4], the possible detection of mixed valence (Fe 2þ /Fe 3þ ) of Fe-ions in the Fe–O–Fe framework of the monoclinic phase FN, from electrical conductivity, magnetic susceptibility and 57 Fe Mo ¨ ssbauer studies, is broadly discussed. Electron paramagnetic resonance (EPR) is the most powerful spectroscopic method available to unambiguously determine the valence state of paramagnetic ions [8]. This technique allows also investigate the nature of magnetic phases in materials at different temperatures [9,10]. Recently, we have implemented two techniques to measure the non-resonant microwave absorption, as a function of temperature or dc applied magnetic field [11], which have been denominated as magnetically-modulated microwave absorption spectroscopy * Corresponding author. Tel.: þ52 55 5622 4641. E-mail address: memodin@yahoo.com (G. Alvarez). Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie 1293-2558/$ – see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.11.001 Solid State Sciences 11 (2009) 881–884