Journal of Magnetism and Magnetic Materials 316 (2007) 321–324 Magnetisation reversal of epitaxial films of g 0 -Fe 4 N on Cu(1 0 0) D. Ecija a , E. Jimenez a , J. Camarero a,Ã , J.M. Gallego b , J. Vogel c , N. Mikuszeit a , N. Sacrista´n a , R. Miranda a a Departamento de Fı´sica de la Materia Condensada, Universidad Auto´noma de Madrid, Cantoblanco 28049, Spain b Instituto de Ciencia de Materiales de Madrid, Centro Superiror de Investigaciones Cientı´ficas, ICMM-CSIC, Madrid, Spain c Institut Ne´el-CNRS, B.P. 166, F-38042 Grenoble, France Available online 6 March 2007 Abstract Epitaxial single-phase g 0 -Fe 4 N(1 0 0) thin films have been grown on Cu(1 0 0). The growth mode and structure have been studied from the early stages up to 50 nm thick films by means of in situ scanning tunnelling microscopy, low-energy electron diffraction and Auger electron spectroscopy. The nitride films are single crystals with sharp interfaces with the substrate, atomically flat and grown layer by layer. The magnetic properties have been studied ex situ by high-resolution vectorial Kerr magnetometry. The films are ferromagnetic at room temperature. The magnetisation lies within the plane of the film. The hysteresis loops are non-consistent with a simple cubic symmetry. Two easy axes, close to the /100S directions, but at 811 from each other, are found. The angular dependence of the vector hysteresis loops reveals different magnetisation reversal behaviours around the two hard-axes directions and around the two easy-axes directions. The origin of these features is discussed. r 2007 Published by Elsevier B.V. Keywords: Magnetisation reversal; Iron nitride; Epitaxial growth; Kerr effect; STM 1. Introduction Certain magnetic iron nitrides have raised considerable interest during the last decades. For instance, g 0 -Fe 4 N presents excellent magnetic properties, such as large saturation magnetisation, low coercivity and chemical stability; which make it suitable for applications in high- density magnetic storage devices [1]. Furthermore, the interest in determining experimentally the complex proper- ties of FCC-Fe predicted as a function of the lattice parameter can be tested in g 0 -Fe 4 N. In the cubic crystalline structure of g 0 -Fe 4 N, the Fe atoms form a FCC sublattice, while the N atom is placed at the centre of the cubic cell, occupying one of the four octahedral hollow sites and stabilising the Fe FCC cage. The lattice parameter is a ¼ 3795 A ˚ , so the misfit with the Cu lattice (a ¼ 3615 A ˚ ) is 4.7%, and epitaxial growth should, in principle, be possible. Like other nitrides, g 0 -Fe 4 N is a metastable compound, in the sense that decay into N 2 and Fe is energetically favourable. The decay temperature is close to 750 K, setting the maximum temperature for epitaxial growth. A procedure to grow thin films of single-phase g 0 -Fe 4 N on Cu(1 0 0) at temperatures of 700 K has been found [2,3]. We describe here a study of the angular dependence of the magnetisation reversal of well-characterised, epitaxial films of g 0 -Fe 4 N grown on Cu(1 0 0). Different reversal beha- viours are found depending on the angle between the field and the anisotropy axes. The polar plot of the remanence displays ‘‘butterfly’’ shape behaviour, not expected for a simple cubic crystal structure. The results can be under- stood, if besides the biaxial anisotropy expected from the crystalline structure of the film, an additional uniaxial term is considered. 2. Experiments The iron nitride films were grown in an ultra-high vacuum (UHV) chamber with a base pressure in the range of 10 8 Pa, equipped with a home-made scanning ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.jmmm.2007.02.187 Ã Corresponding author. Tel.: +34 91 4973048; fax: +34 91 4973961. E-mail address: julio.camarero@uam.es (J. Camarero).