Asymmetric magnetization reversal of partially devitrified Co 66 Si 15 B 14 Fe 4 Ni 1 J.C. Martínez-García, J.A. García, M. Rivas * Departamento de Física de la Universidad de Oviedo, c/Calvo Sotelo s/n, 33007 Oviedo, Spain article info Article history: Available online 29 October 2008 PACS: 75.30.Hx 75.50.Kj 75.60.Ej Keyword: Magnetic Properties abstract Anomalous hysteresis has been observed in annealed Co 66 Si 15 B 14 Fe 4 Ni 1 amorphous ribbons consisting of asymmetrically distorted and horizontally shifted loops. Although the magnetic hysteresis loops present remarkable similarities with those produced by exchange bias uniaxial anisotropy, this is discarded in these samples, so the comprehension of the origin of these characteristics is an interesting topic of research. Transmission electron microscopy, selected area diffraction and X-ray diffraction experiments have been carried out in order to investigate the structural properties of the samples. A very low dilution of nanocrystallites, with sizes of about 20 nm, has been detected coexisting with some bigger crystals of around 0.4–1.0 lm. The hysteretic particularities and their evolution with different magnetic treatments are then analyzed as a consequence of the dipolar interaction between the crystalline particles and the residual amorphous matrix. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction In the last years nanomagnetic materials have attracted great interest because an appropriate control of their morphological characteristics allows having a broad range of magnetic hysteresis behaviors from soft to hard [1,2]. Among the different particular magnetic effects that can be achieved in these systems, the hori- zontal displacement of the hysteresis loops is a very interesting one because of its potential technological application, in particular, in alloys with very low magnetostriction it provides a promising way to get asymmetric giant magnetoimpedance curves, very con- venient to use this effect for magnetic sensors. There exist at least two kinds of systems in which hysteresis loops are horizontally shifted: the ferromagnetic–antiferromag- netic systems, in which the shift is originated by exchange bias anisotropy [3,4], and the hard-soft systems in which it is caused by the dipolar interactions between the different phases [5]. The study exposed in this paper is devoted to the second kind of phe- nomenon. The analysis of the asymmetrical magnetization in dif- ferent hard-soft magnetic systems will bring to light a variety of particularities that should lead to a better comprehension of the subject and finally allow the convenient control of the properties of the hysteresis. 2. Experimental techniques The samples used in this work were obtained from a commer- cial ribbon 25 lm thick of the amorphous Co 66 Si 15 B 14 Fe 4 Ni 1 , cut to pieces of 40 Â 5 mm. These samples were annealed in a quartz-tube preheated furnace, the temperature being continu- ously monitored by a thermocouple. The crystallization process of the alloy was studied by differential scanning calorimetry (DSC) with a TA Instruments MDSC-2920 calorimeter. The struc- tural details of the annealed samples were analyzed by X-ray diffraction (XRD), with a SIEMENS D-50 working with Brag-Brent- ano configuration and using the Cu K a1 radiation (k = 0.1541 nm), and by transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) performed with a JEOL-200 microscope using an acceleration voltage of 160 kV. Ion milling was used to prepare the thin foils for these studies. The magnetic hysteresis loops were obtained by means of a computer controlled inductive measurement system. 3. Results and discussion 3.1. Structural analysis The annealing temperature, T a = 520 °C, was chosen to be close to the onset of crystallization that appears at 540 °C as revealed the DSC analysis. The duration of the thermal treatments was rela- tively short, 20–90 min, with the intention to interrupt the crystal- lization process when only a few nuclei have been created. The result of this devitrification can be appreciated in the TEM micro- graphs of Fig. 1 in which we can see nanometric quasi-spherical particles of about 10 nm (Fig. 1(a)), as well as much bigger particles with sizes over 500 nm (Fig. 1(c)). Nice electron diffraction pat- terns (Fig. 1(b)) have been obtained from these bigger particles that have allowed their indexation as Co 3 B. Other diffraction spots which are observed superimposed to those of Co 3 B give hints to 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.05.059 * Corresponding author. E-mail address: rivas@uniovi.es (M. Rivas). Journal of Non-Crystalline Solids 354 (2008) 5123–5125 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol