Temperature dependence of biased hysteresis loops in hard-soft nanocrystalline Co-based ribbons M. Rivas ⁎, J.A. García, M.A. Cerdeira, J.C. Martínez-García Departamento de Física, Campus de Viesques, Universidad de Oviedo, 33204 Gijón, Spain abstract article info Article history: Received 22 June 2011 Received in revised form 23 September 2011 Available online 17 October 2011 Keywords: Nanocrystalline ribbons; Biased magnetic hysteresis; Temperature dependence The low field hysteresis loops of hard-soft magnetic materials can be biased as a consequence of the magnetic coupling of both phases. In hard-soft nanocrystalline materials, magnetostatic and exchange interactions be- tween the hard crystalline phase and the much softer embedding amorphous matrix coexist and the rele- vance of their effect on the hysteresis is difficult to distinguish. In this work, the thermal evolution of the biased magnetic hysteresis loops of hardsoft devitrified Co 66 Fe 4 Mo 2 Si 16 B 12 ribbons has been studied between room and Curie temperature. The nature of the coupling between the two magnetic phases is discussed in terms of the influence of the temperature on the magnetic properties of the soft phase, mainly on its satura- tion magnetization and exchange constant, and on the switching field distribution of the hard phase. The re- sults allow to conclude that the effect of the exchange interaction predominates over the magnetostatic one. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Multiphase magnetic systems have been reported which present biased hysteresis loops (HL) caused by the coupling of magnetically different adjacent phases [1,2,3]. The detailed features of such hyster- esis depend on the morphology as much as on the intrinsic properties of the involved phases [4]. In the case of nanocrystalline materials the formation of individual nanograins, completely surrounded by the amorphous matrix is a sine qua non [5]. The pinning of the soft amor- phous phase by the harder crystalline one, via the exchange and mag- netostatic interactions, is responsible for the shift and widening of the HL. The elucidation of the relative importance of both ways of coupling is still an open question because theoretically both produce similar consequences on the hysteretical features, as reported elsewhere [5]. In this situation, the analysis of the temperature dependence of the hysteresis can throw light because the thermal evolution of both effects is expected to be radically different. This has been the motiva- tion for the work presented in this paper which deals with the varia- tion of the biased HL of nanocrystalline ribbons between room and Curie temperatures. For this study, the chosen amorphous precursor has been Co 66 Fe 4 Mo 2 Si 16 B 12 because for this composition HL shifts as large as five times the coercive field were obtained [6], besides other interesting implications of the hard–soft coupling as auto-biased magnetoimpe- dance curves [7]. We will show that the consequences of heating above room temperature (RT) are consistent with the preponderance of the effect of the exchange interaction versus the magnetostatic one. 2. Experimental procedure In a previous work it was already reported that annealing Co 66 Fe 4 Mo 2 Si 16 B 12 amorphous ribbons at temperatures slightly below the crystallization temperature, T cr =558 °C (see the differential scan- ning calorimetry diagrams in [8]), give rise to samples with biased mag- netic HL [6]. For the present work samples of this amorphous precursor, 40 mm long, 5 mm wide, and 25 μm thick, were annealed in Argon at- mosphere in a preheated furnace at temperatures ranging from 500 °C and 530 °C and for times between 10 min and 360 min. To check the multiphase character of the samples Transmission Electron Microscopy (TEM) and X-Ray Difraction (XRD) investigations were performed. For each sample, the grain size distribution and the crystallized volume fraction were determined from the visible area of four TEM micrographs taken with magnifications of 100× and 120×. The micrographs were taken with a Jeol-2000 microscope using an acceleration voltage of 160 kV after thinning by dual ion milling. Two ion beams were incident on both sides of the samples, so the results presented in this paper are characteristic of regions far away from the surfaces. The HL were obtained from RT up to Curie temperature with an in- ductive loop tracer in which the coercive field can be determined with a precision of 4 A/m. Heating was achieved by the incident light of two halogen lamps symmetrically placed at both sides of the sensing coil in which the sample was located. The temperature was continuously measured with a T-thermocouple in contact with the sample. The measurements were performed proceeding systematical- ly with the following sequence: 30 min after submitting the sample to Journal of Non-Crystalline Solids 358 (2012) 310–315 ⁎ Corresponding author. Tel.: + 34 985 182389; fax: + 34 985 182390. E-mail addresses: rivas@uniovi.es (M. Rivas), joseagd@uniovi.es (J.A. García), ance@uniovi.es (M.A. Cerdeira), jcmg@uniovi.es (J.C. Martínez-García). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.09.035 Contents lists available at SciVerse ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol