Spin waves in exchange-coupled NiFe/IrMn/NiFe trilayers

Magnetic films consisting of an antiferromagnetic (AFM) layer sandwiched between two non-equivalent ferromagnetic (FM) layers have recently attracted a great interest. In fact, in this configuration, each interface of the AFM layer is exchange-coupled to a different FM layer and information on the AFM magnetic structure may be obtained by the analysis of the exchange bias effect [1]. In this work, we focus on the spin wave properties dynamics of samples (deposited by DC magnetron sputtering in a magnetic field of 400 Oe) with layer-stacking sequence Si/Cu[5 nm]/Py[5 nm]/IrMn[10 nm]/Cu[tCu]/Py[10 nm], where Py is Ni80Fe20, IrMn is Ir25Mn75, and tCu, the nominal thickness of a Cu spacer, is varied in the 0 - 0.2 nm range. Since tCu is lower than the Cu lattice parameter, Cu is expected to grow in form of islands at the top AFM/FM interface, thus providing another tool, a part from the FM layer thickness, to diversify the top from the bottom interface and to tune the strength of the exchange coupling. As a consequence, the magnetization loops of the samples, measured by magneto-optic Kerr effect (MOKE) magnetometry, actually feature two distinct loops, corresponding to the magnetization process of the two Py layers. The relative orientation of the magnetization vectors of the Py layers (parallel P or antiparallel AP) can be thus controlled by an external magnetic field H. The spin-wave properties were studied by Brillouin Light Scattering (BLS): spectra were acquired at room temperature at an angle of incidence of 40° by sweeping H over the upper branch of the hysteresis loop (from negative to positive saturation) and encompassing both the P and AP alignment of the Py layers. The BLS results were satisfactorily reproduced by a theoretical model we developed for exchange-coupled AFM/FM bilayers [2]. To exemplify the information obtainable through this approach, in Fig. 1 the results for the sample with tCu = 0 Å are shown. In the P states the Stokes (S) and the anti-Stokes (AS) modes are degenerate, so only the difference between optical and acoustic branches is observed. The calculated frequencies are in good agreement with the BLS ones apart from slight discrepancies, especially observed for the high-frequency mode. The S-AS degeneracy is removed in the AP state, between -0.05 kOe and -0.15 kOe, where four modes are found, well reproduced by the analytical model: the agreement is good for the high frequency modes, but only satisfactory for the low frequency ones. Similar results were detected for tCu = 0.1 nm and 0.2 nm. The Cu insertion induced just a change of the extension of the field regions corresponding to the P and AP configurations, whilst the frequency values were nearly unchanged, so the frequency values in the AP configuration are still not well approximated. This discrepancy can be tentatively ascribed to the fact that in the AP state the IrMn spins must comply the exchange coupling with the spins of the two Py layers, which have opposite orientations. This last process is expected to directly involve the interfacial IrMn spins, but reasonably also the ‘bulk’ ones will be implicated. Hence, compared to the case of AFM/FM bilayers, a more accurate analytical description of the IrMn magnetic configuration, that is the aim of this investigation, is necessary. This work was partially supported by MIUR-PRIN 2010–11 Project No. 2010ECA8P3 “DyNanoMag” and by MIUR-FIRB2010 Project No. RBFR10E61T “NANOREST” [1] A. N. Dobrynin, D. Givord, Phys. Rev. B 85 (2012) 014413 [2] G. Gubbiotti, S. Tacchi, L. Del Bianco, E. Bonfiglioli, L. Giovannini, M. Tamisari, F. Spizzo, R. Zivieri, J. Appl. Phys. 117 (2015) 17D150.