Spin-wave properties of IrMn/NiFe based spin-valves

Brillouin light scattering (BLS) was exploited to study the effect of the exchange coupling (EC) at the interface between antiferromagnetic (AF) and ferromagnetic (FM) layers on spin-wave properties in spin-valves (SVs). The stacking of the samples was Cu(5 nm)/IrMn(tu)/NiFe(5nm)/Cu(5nm)/NiFe(5nm)/IrMn(to); tu and to are the thicknesses of the AF IrMn (Ir25Mn75) underlayer and overlayer, respectively. Three different samples were studied: SVA, with tu = 10 nm, to = 0 nm, SVB, with tu = 10 nm, to = 6 nm, and a reference sample, SVR, with tu = to = 0 nm, where just unpinned NiFe (Ni80Fe20) films are included. The samples were deposited by DC magnetron sputtering in a magnetic field of 400 Oe. The EC strongly affects the SV magnetization process by inducing a bias field (HB) on the pinned NiFe layer, that results in a shift of the NiFe loop along the measurement axis (fig. 1). BLS spectra were acquired by sweeping the applied field (H) over the upper branch of the hysteresis loop (from positive to negative saturation) and encompassing both the parallel (P) and antiparallel (AP) alignment of the NiFe layers. In the saturated state, in SVA and SVB the frequencies of the two measured modes increase with respect to the ones in SVR, and their frequency difference depends on H orientation with respect to HB direction. At the transition from the P to the AP ground-state, the mode frequencies undergo an abrupt variation and the frequency modes dependence remarkably changes. The experimental frequencies are compared with those calculated by a theoretical model, based on the thin film formalism, that includes in a phenomenological way the effects of the EC at the IrMn/NiFe interface. For SVA, the accordance between experimental and theoretical frequencies is good in the P state, and larger differences are present in the AP state. In SVB, with two IrMn layers, larger discrepancies are found. These results indicate that additional effects have to be included to describe the dynamic effects of the NiFe/IrMn coupling: different hypotheses will be presented and discussed.