IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008 3887 EXAFS Studies and Magnetic Behavior of FeCuZr Ball-Milled Alloys Álvaro Martínez , Juan J. Romero , Aria F. Yang , German R. Castro , Vince G. Harris , JosephC. Woicik , Antonio Hernando , and Patricia Crespo Instituto de Magnetismo Aplicado (UCM-ADIF-CSIC) Las Rozas, Madrid 28230, Spain Instituto de Cerámica y Vidrio, CSIC C/Kelsen, Madrid CP:28049, Spain Center for Microwave Magnetic Materials and Integrated Circuits, Northeastern University. Boston, MA 02115 USA Sp-line European Synchrotron Radiation Facility, Grenoble, CEDEX 09 F-38043 France Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02115-5000 USA National Institute of Science and Technology, Gaithersburg, MD 20899 USA Metastable alloys of nominal composition (Fe Cu ) Zr at.%) have been synthesized by high energy ball milling. In spite of the high and positive enthalpy of mixing between Fe and Cu, nanocrystalline or amorphous alloys have been obtained depending on the Zr content. Alloys exhibit a ferromagnetic behavior with a Curie temperature below room temperature. The thermal dependence of the thermorremanence shows a anomalous increase above the Curie temperature. This behavior seems to be related with a magnetovolume effect. Extended X-ray absorption fine structure (EXAFS) measurements have been performed to explain this effect. Preliminary results seem to indicate an almost negligible thermal expansion at temperatures below Tc, while normal thermal expansion takes place at higher temperatures. Index Terms—Amorphous alloys, Curie’s temperature, extended X-ray absorption fine structure (EXAFS), high energy ball milling (HEBM), thermoremanence. I. INTRODUCTION I N RECENT years, there has been a new interest on alloys with high enthalpy of mixing. In order to overcome the im- miscibility, nonequilibrium processing techniques are required for producing mixing at an atomic level. Among them, high en- ergy ball milling (HEMB) has emerged as a powerful technique since it can extend regions of metastable solubility of immis- cible elements to obtain solid solution of elements with high enthalpy of mixing [1]. HEBM is a technique commonly used to obtain supersaturated solid solutions, alloys with high energy of mixing and alloys of combinations of elements which do not show appreciable solubility in their equilibrium phase diagrams [2]–[5]. The FeCu system is an example of a binary system with a very low solid miscibility at room temperature. The FeCu equilibrium phase diagram indicates a small miscibility at room temperature, only Fe Cu and Fe Cu has been obtained [6], however, by means of HEBM FeCu solid solutions have been obtained in almost all the compositional range. Fe Cu al- loys with (at.%) exhibit a face centred cubic (fcc) struc- ture while for higher Fe content the alloys exhibit a body cen- tred cubic (bcc) structure. It is worth to mention that equiatomic fcc-FeCu alloys are ferromagnetic, with a Curie temperature (Tc) of around 500 K, in spite of fcc-Cu and fcc-Fe not being ferromagnetic at their ground state. The ferromagnetic character observed in Fe Cu alloys seems to be related with magneto- volume effects, since the fcc lattice is expanded with respect to fcc-Cu and fcc-Fe [7]. Amorphous FeCu alloys have been not synthesized up to present date by high energy ball milling or by any other Digital Object Identifier 10.1109/TMAG.2008.2002480 Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. nonequilibrium technique. Amorphization of 3-D transition metals has been, traditionally, induced by adding metalloids such as boron or by high atomic volume elements such as Zr. Normally, amorphous alloys with around 80 at.% of 3-D metal can be obtained by HEBM. In this paper, amorphization of equiatomic fcc-FeCu alloys has been induced by adding Zr [8]. In this paper, the influence of Zr alloying on the magnetic properties of Fe Cu alloys is studied. Binary FeZr on the Fe rich side exhibit anomalous magnetic behavior at low temper- ature, as well as invar and re-entrant spin glass behavior. The magnetic properties are strongly dependent of the Fe content as well as on the local atomic order. Amorphous FeZr alloys have been produce by melt spinning up to an Fe concentration range of 90 at.%. By using other non equilibrium processing techniques, such as sputtering or mechanical alloying, different amorphization ranges are observed. For instance, by mechanical alloying amorphous alloys can be obtained for Fe concentrations ranging between 30–70 at.%. It will be shown that the magnetic properties change drastically with respect to Fe Cu alloys upon Zr addition. In particular, the alloys exhibit an anoma- lous magnetic behavior for temperatures above Curie temper- ature characterized by an spontaneous increase of the magneti- zation above Tc that seems to be associated with a change of the thermal expansion behavior in the ferromagnetic-paramagnetic transition temperature [9], [10]. In order to account for such be- havior the temperature dependence of the nearest-neighbor dis- tance has been studied by means of EXAFS in the K-edge of iron. II. EXPERIMENTAL DETAILS Metastable alloys of nominal composition (Fe Cu ) Zr ( at. %) have been synthesized by high energy ball milling (HEBM) in a planetary mill with hardened steel vials. To avoid oxidation of the powder upon the milling process, the vials were sealed under nitrogen atmosphere prior to the milling. The starting materials were Fe, Cu, and Zr in powder form, with 0018-9464/$25.00 © 2008 IEEE Authorized licensed use limited to: IEEE Xplore. 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