Journal of Magnetism and Magnetic Materials 321 (2009) 963–965 Exchange bias in bulk Ni–Mn–In-based Heusler alloys Arjun Kumar Pathak, Mahmud Khan, Bhoj Raj Gautam, Shane Stadler, Igor Dubenko à , Naushad Ali Department of Physics, Southern Illinois University, 1245 Lincoln Drive, Carbondale, IL 62901, USA Available online 7 March 2008 Abstract The crystal structure and exchange bias of the bulk Heusler alloys Ni 50 Mn 50x In x with 14.5pxp15.2 have been investigated using X- ray diffraction and magnetization measurements, respectively. Magnetic measurements were performed with SQUID magnetometry after samples were zero-field cooled and field cooled (FC) in positive magnetic fields up to H ¼ 50 kOe, from a temperature T ¼ 380 K. Three temperatures of the phase transitions, T 1 oT M oT C , and a shift of the FC (50 kOe) magnetic hysteresis loops up to 120 Oe at 5 K have been detected for all samples. The exchange bias field (H E ) was almost constant for intermediate In concentrations 14.8oxo15.2, and sharply decreased to about 20 Oe on the borders of this concentration interval (xp14.5; 15.2px). The changes of H E have been related to changes in the ratio of T 1 to T M : the overlapping of transitions at T 1 and T M (for x ¼ 14.8, and 15.2) results in a decrease in H E . r 2008 Elsevier B.V. All rights reserved. Keywords: Heusler alloys; First-order phase transition; Magnetic properties; Exchange bias 1. Introduction The importance of magnetic materials exhibiting uni- directional anisotropy (exchange bias phenomena—EB) is revealed in many spintronic devices such as spin valves, read heads, and nonvolatile memory [1]. The EB is attributed to exchange interactions between antiferromag- netic (AFM) and ferromagnetic (FM) phases in magneti- cally heterogeneous systems that, in some cases, can result in a shift, H E , of the magnetization hysteresis loop M(H) from zero applied magnetic field. These systems show asymmetric M(H) when cooling in the presence of magnetic field from above magnetic ordering temperature of the AFM phase, T N [2]. The value of H E depends on the relative orientation of the FM moment (M F ) and the magnetic moment of one of the magnetic sublattices of the AF phase (M AF ). The coupling between AF and FM moments is maximal when M F and M sub are parallel. Hence, the exchange bias field, H E , is maximal when such a system is cooled down below T N in the presence of a magnetic field that is larger than the saturation field of FM phase. In this case, the external magnetic field (H) can ‘‘freese’’ M AF along the M F direction, resulting in the shift of hysteresis loop. EB is observed in materials and composites with developed AFM–FM interfaces such as AF–FM bilayers, magnetic nanoparticles [2,3], spin glass systems [4], polycrystalline materials such as Cu 2 MnAl, Ni 50 Mn 35 Sn 14 and Ni–Mn–Sb system [5–7]. One of the possible class of promising EB materials are those that show the temperature-induced first-order phase transitions (FOT) below T C [6,7]. Indeed, such systems are characterized by the coexistence of supercooling/super- heating phases in the vicinity of the first-order transition temperature—T M (thermal hysteresis), i.e. by magnetic/ structure heterogeneity. The fraction of sample that is in the high temperature (HT) phase is normally sharply reduced when cooled. However, in some systems, a significant amount of HT phase can be detected even at 5K [8,9]. Therefore, EB behavior can be expected in FOT systems if the HT phase is FM, while the low temperature (LT) phase undergoes a transition to AFM state. Recently studied Ni 50 Mn 50x In x ferromagnetic Heusler alloys possess at least three temperature-induced phase transitions: the ferromagnetic–paramagnetic transition of the HT austenetic phase at T C ; the first-order martensitic ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.03.008 à Corresponding author. Tel.: +1 618 453 7126; fax: +1 618 453 1056. E-mail address: igor_doubenko@yahoo.com (I. Dubenko).