Research articles Magnetic anisotropies and rotational hysteresis in Ni 81 Fe 19 /Fe 50 Mn 50 films: A study by torque magnetometry and anisotropic magnetoresistance O.E. da Silva a , J.V. de Siqueira a , P.R. Kern a , W.J.S. Garcia a , F. Beck b , J.N. Rigue b , M. Carara a,⇑ a Departamento de Física, Universidade Federal de Santa Maria, Santa Maria 97105-900, RS, Brazil b Coordenadoria Acadêmica, Universidade Federal de Santa Maria – Campus Cachoeira do Sul, Cachoeira do Sul 96506-302, RS, Brazil article info Article history: Received 10 July 2017 Received in revised form 21 September 2017 Accepted 10 October 2017 Available online 21 November 2017 Keywords: Exchange bias Rotational hysteresis Torque measurements Anisotropic magnetoresistance Magnetic anisotropies abstract Exchange bias properties of NiFe/FeMn thin films have been investigated through X-ray diffraction, hys- teresis loops, angular measurements of anisotropic magnetoresistance (AMR) and magnetic torque. As first predicted by Meiklejohn and Bean we found a decrease on the bias field as the NiFe layer thickness increases. However such reduction is not as strong as expected and it was attributed to the increase on the number of uncompensed antiferromagnetic spins resulting from the increase on the number of FeMn grains at the interface as the thickness of the NiFe layer is increased. The angular evolution of AMR and the magnetic torque were calculated and compared to the experimental ones using the minimization of the free magnetic energy and finding the magnetization equilibrium angle. The free energy, for each grain of the polycrystalline sample, is composed by the following terms: Zeeman, uniaxial, unidirectional and the rotatable energies. While from the AMR curves we obtain stable anisotropy fields independently on the measuring fields, from the torque curves we obtain increasing values of the uniaxial and rotatable fields, as the measuring field is increased. These results were attributed to the physical origin and sensi- tivity of the two different techniques. Magnetoresistance is mainly sensitive to the inner portion of the ferromagnetic layer, and the torque brings out information of the whole ferromagnetic layer including the interface of the layers. In this way, we believe that the increase in the uniaxial and rotatable values were due to an increase on the volume of the ferromagnetic layer, near the interfaces, which is made to rotate with the measuring field. Studying the rotational hysteresis by both techniques allows to sepa- rately obtain the contributions coming from the inner portion of ferromagnetic layer and from the interface. Ó 2017 Published by Elsevier B.V. 1. Introduction The exchange bias phenomenon (EB) occurs due to the interfa- cial coupling of a ferromagnet (F) to an antiferromagnet (AF) [1,2]. The main features observed and studied in such systems are the shift of the magnetization curve by an exchange bias field (H B ) [1,9], the increase in the coercive field (H C ), the existence of the training effect, which is especially important for polycrystalline films [3,4] and the rotational hysteresis in the angular magnetic torque measurements [5,6]. The EB has been studied for 60 years due to its important technological drive [see for ex. [7] and references therein]. The phenomenon has already been verified in different systems within a plentiful diversity of materials, adding difficulties to achieve its total comprehension [2,8–10]. Different models have been developed, aiming to including specific characteristics of the EB in different systems [11,12]. Important hypotheses have been made in different models, for example: the splitting of the interfacial structures in stable and unstable [6,8]; the existence of a rotatable anisotropy associated to the unstable grains [13,14]; domain wall formation at the interfaces [15–17]; angular distribution on the anisotropy of inter- facial grains [18]; misalignment between the unidirectional (AF – F coupling) and uniaxial anisotropies (F layer) [19–21]. The main techniques used to the study of EB systems are magnetometry (VSM, vector VSM, MOKE) and ferromagnetic reso- nance. Other techniques capable to provide additional information on the subject, but used less, are the torque magnetometry and anisotropic magnetoresistance (AMR). https://doi.org/10.1016/j.jmmm.2017.10.043 0304-8853/Ó 2017 Published by Elsevier B.V. ⇑ Corresponding author. E-mail address: carara@smail.ufsm.br (M. Carara). Journal of Magnetism and Magnetic Materials 451 (2018) 507–514 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm