Rotatable anisotropy of Ni 81 Fe 19 =Ir 20 Mn 80 films: A study using broadband ferromagnetic resonance R. Dutra a , D.E. Gonzalez-Chavez a , T.L. Marcondes a , A.M.H. de Andrade b , J. Geshev b , R.L. Sommer a,n a Centro Brasileiro de Pesquisas Físicas, 22290-180 Rio de Janeiro, RJ, Brazil b Instituto de Física, UFRGS, 91501-970 Porto Alegre, RS, Brazil article info Article history: Received 27 April 2013 Received in revised form 10 June 2013 Available online 10 July 2013 Keywords: Exchange bias Rotatable anisotropy Broadband FMR abstract We investigate the broadband ferromagnetic resonance dispersion relation of NiFe/IrMn multilayers using the vector network analyzer ferromagnetic resonance (VNA-FMR) method. Multilayered films with structure [NiFe(20 nm)/IrMn (t IrMn )/Ta(3 nm)] Â 10 (with t IrMn ¼4 and 15 nm) were produced onto Si (100) substrates using magnetron sputtering. The dispersion relations of the samples were extracted from the resonance spectra in the range of 0.1–7 GHz under magnetic fields of up to 7300 Oe. Static magnetization curves were also obtained in the same field range. The experimental data were compared with numerical calculations performed using the granular exchange-bias model which takes into account both, the unidirectional and rotatable anisotropy present in the samples. A good agreement between numerical and experimental results was achieved without the need of any frequency or field-dependent parameter. & 2013 Elsevier B.V. All rights reserved. 1. Introduction The exchange bias (EB) phenomenon comes from the interfacial interaction between ferromagnetic (FM) and antiferromagnetic (AFM) [1] films. Its investigation has attracted much attention in the past decades because exchange-coupled FM/AFM systems have wide application in spin valves and magnetic tunnel junctions. As a matter of fact, EB is currently used in hard disk read/head sensors and magnetic random access memory cells as well as in spin torque oscillators. The fingerprint of the EB phenomenon is the characteristic shift of the magnetization curve along the magnetic field axis. The EB field is estimated by taking the mean value of the reversal fields of the magnetic hysteresis curve, descending and ascending branches, referred to as coercive fields in EB systems. This DC EB field was observed to be different from the values obtained from usual ferromagnetic resonance (FMR) measurements [2]. Moreover, FMR experiments on systems exhi- biting EB commonly present an isotropic shift of the angular variation of the resonance field H res attributed to the so-called rotatable anisotropy (RA), as discussed in Refs. [3–5] and the references cited therein. It is accepted that the RA comes from uncompensated spins (UCSs) at the FM/AFM interface whose magnetizations follow the rotation of the FM layer magnetization and that the RA is responsible for the increased coercivity (H C ) in bilayers with polycrystalline AFM. In FMR experiments, the RA can strongly depend on the excitation frequency [6]; it has also been found [7] that in IrMn/Cr/Co thin films the RA is due to the Cr interface coupled antiferromagnetically with the Co atoms and that, near saturation, the RA field is antiparallel to the external magnetic field H. It will be shown in this paper that the usual hysteresis and FMR measurements are not sufficient to get a wide panorama of the EB phenomenon due to the strong dependence on frequency, even by taking into account the angular dependencies. The only way to proceed is to perform broadband FMR measurements using the Vectorial Network Analyser (VNA) or similar techniques. A few indications of this situation have been discussed in the paper by Geshev et al. [8] , Bilzer et al. [9] and Yang et al. [10], the first with usual FMR measurements and the others with preliminary broad- band measurements. Although the description of these EB features has received a great deal of attention in the past decades [11,12], only recently a realistic model able to describe experimental data for polycrystal- line EB systems has been proposed [13]. In this model, the authors consider FM domains interacting with two types of interfacial grains with UCSs, namely stable (or biasing) and rotatable grains. In the corresponding energy expression, there are two terms corresponding to the effective RA, i.e., that of the magnetic anisotropy of the rotatable grains and a term representing the FM/UCS (rotatable) exchange coupling. The RA changes when H is Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.06.040 n Corresponding author. Tel.: +55 21 2141 7279; fax: +55 21 2141 7400. E-mail addresses: rlsommer@gmail.com, sommer@cbpf.br (R.L. Sommer). Journal of Magnetism and Magnetic Materials 346 (2013) 1–4