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Evolution of Free Volume in Ultrasoft Magnetic FeZrN Films during Thermal Annealing N.G. Chechenin1, A. van Veen2, H. Schut2, A.R. Chezan1, D.O. Boerma1, T. Vystavel1, J.Th.M. De Hosson1 1 Materials Science Centre, University of Groningen, Groningen, The Netherlands; 2Interfaculty Reactor Institute, Delft University of Technology, Delft, The Netherlands ABSTRACT The thermal stability of nanocrystalline ultra-soft magnetic (Fe98Zr2)1-xNx films with x=0.10-0.25 was studied using high-resolution transmission electron microscopy (HRTEM), positron beam analysis (PBA) and thermal desorption spectrometry (TDS). The results demonstrate that grain growth during the heat treatment is accompanied by an increase of the free volume, by nitrogen reallocation and desorption. All this can drastically deteriorate the ultrasoft magnetic properties. The desorption starts already at slightly elevated temperatures, below 100 0C. However, most of the nitrogen leaves the sample at a temperature above 500 0C.

INTRODUCTION Ultrasoft magnetic films with a low coercive magnetic field of ~1-5 Oe, an induced uniaxial magnetic anisotropy of 10-50 Oe and a high saturation magnetization ~15-20 kG, are required for future high frequency applications in the GHz region. The grain size is a crucial parameter of the microstructure of a magnetic film: nanometer size of the grains is required for averaging out the magnetocrystalline anisotropy and for reduction of the intrinsic stray field [1,2]. In the FeXN type of films nitrogen serves as a grain refiner and the transition metal X= Ti, Cr, Zr, Ta, etc. as a nitrogen binder, favoring a high concentration of nitrogen content. This type of the films proved to give the required magnetic properties [3-8]. The thermal stability of these films is one of the points of concern. Often the films in this system cannot withstand a thermal treatment, and the soft magnetic properties deteriorate. We obtain excellent magnetic parameters in FeZrN films, deposited by DC reactive sputtering on different substrates at various temperatures. The main emphasis of this study concentrates on formation of nano-sized grains, on grain growth and evolution of the intergranular free space during a thermal treatment in the temperature range up to 600 0C. The microstructural evolution was investigated by x-ray diffraction (XRD), TEM and HRTEM (using an in-situ heating stage), PBA and TDS. TDS gives directly the amount of outgased nitrogen during heat treatment. Positrons proved to be a sensitive probe for the free volume in the nearamorphous and recrystallized films. Due to the nanometer size of the grains nearly all positrons diffuse to the grain boundaries and voids, and are trapped and annihilated there. The size and character of the free volume on an atomic scale was monitored by PBA during thermal annealing, where two competing processes are involved: the reduction of the free space by the J7.3.1 Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 22 Aug 2017 at 20:40:12, subject to the Cambridge C