IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 9, SEPTEMBER 2014 4800204 Study of Exchange Bias in All Ferromagnetic Fe/Co Soft/Hard Bilayer Harkirat Singh 1 , Ratnesh Gupta 2 , Tanmoy Chakraborty 1 , Ajay Gupta 3 , and Chiranjib Mitra 1 1 Indian Institute of Science Education and Research Kolkata, Mohanpur 741252, India 2 School of Instrumentation, Devi Ahilya Vishwavidyalaya, Indore 452001, India 3 UGC DAE CSR, Indore Centre, Indore 452001, India We report the unusual and exotic phenomenon of positive exchange bias observed in all ferromagnetic layered soft/hard Fe/Co spring magnet systems. Interestingly, the results are well reproducible both at low (5 K) and room temperatures (300 K). The effect of applied magnetic pinning field of 7 and -5 T reveals a noticeable shift of magnetic hysteresis loops along the field direction. We argue that the reason behind observed positive exchange bias is the establishment of antiferromagnetic (AFM) exchange coupling at the interface as a result of intermixing of Fe into the Co layer. The collective coupling of this intermix layers is AFM which facilitates positive shift of magnetic hysteresis loop. Index Terms— Antiferromagnetic (AFM) materials, magnetic films, magnetization, sputtering. I. I NTRODUCTION E XCHANGE bias (EB) is an interesting phenomenon pertaining to its immense technological applications in data storage industries [1]. It has been studied extensively in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers, when one field cools the system through the Neèl temperature of AFM phase [2]. Owing to FM/AFM interfacial exchange interaction, a shift in magnetization loop is usually observed. Immense efforts have been put together to understand the underlying physics of EB, yet an appropriate theoretical model is still lacking. It has been predicted that the uncompensated AFM spins at the interface of AFM–FM system might be contributing to EB through short-range exchange coupling between the AFM–FM bilayer [3]. The EB has been studied thoroughly in various systems (in-homogeneous materials [4], small systems, and thin films [5]) considering FM and AFM as two magnetic materials. However, recent reports have explored EB in other magnetic systems, namely phase separated man- ganites (bulk) [6], ferrimagnet (FI)–FM, FI–FI [7], nanostruc- tures [8], and AFM–dilute AFM [9]. These experimental inves- tigations have reignited the interest of researchers to investi- gate further such an old and interesting scientific phenomenon. Depending on the nature of exchange coupling at the inter- face (FM or AFM), it was proposed that the shift in magnetic hysteresis loop can be negative or positive [3], [10]. The EB shift is usually defined as H E = ( H C 1 + H C 2 )/2, where H C 1 and H C 2 are the coercive fields on the left and right side of the magnetic hysteresis loop, respectively. In comparison with usual negative EB, positive EB (PEB) is quite rare and treated as an exotic phenomenon. The reason behind the more Manuscript received October 18, 2013; revised March 12, 2014 and March 19, 2014; accepted March 21, 2014. Date of publication March 27, 2014; date of current version September 9, 2014. Corresponding author: C. Mitra (e-mail: chiranjib@iiserkol.ac.in). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2014.2313590 interesting PEB is believed to be the establishment of AFM interfacial coupling. Hence, observation of PEB in an all FM bilayer system has been considered to be unique and intriguing [11]. Ke et al. [11] have demonstrated that AFM exchange coupling that give rise to PEB, can exist between two FM layers. In contrast to conventionally used AFM layers to observe EB, an FM pinning layer provides a unique advan- tage of experimental access to change in their magnetization states [12]. The prototypical properties possessed by these FM bilayer systems are interesting for the fundamental understand- ing of EB. Experimentally, these systems are far more ideal to study EB than the conventional FM–AFM systems owing to the FM nature of the pinning layer [13]. Two FM layers, one soft magnetic (SM) and another hard magnetic (HM) material forms an exchange-spring magnet, where SM grains provide enhanced magnetic moment and HM gives high anisotropy and coercive field [14]–[16]. Exchange-spring magnets have attracted considerable attention because of their application as permanent magnets [17]. Presence of direct exchange coupling between soft and hard FM phases causes single phase-like magnetization reversal [18]. Goto et al. [14] have studied the switching behavior of composite thin films where SM and HM layers are ferromagnetically coupled. They assumed that HM layer is rigid while SM layer has no anisotropy. Study of EB in exchange-spring magnet systems could be of great importance from the perspective of exploring interesting aspects of interfacial physics [18], [19]. In this paper, we have demonstrated PEB in exchange- coupled soft/hard Fe/Co bilayer. The bilayer of Fe and Co form a suitable exchange coupled spring magnet, where Co, being highly coercive, is chosen as HM and Fe, with less anisotropy, play the role of SM. Magnetization curves were obtained with the applied field direction along the plane of thin film, which is the same as that of the deposition field axis. A positive exchange bias was observed at all measured temperatures. The effect of large applied magnetic pinning field (+7 and -5 T) results in complete shift of magnetic 0018-9464 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.