Proximity effect induced enhanced spin pumping in Py/Gd at room temperature

Investigating the damping processes and the behavior of dynamic magnetic properties in ferromagnetic (FM) thin films has been an important key towards design and fabrication of different microwave and magnetic recording devices. Damping in a magnetic material can be enhanced due to spin pumping, in which magnetization precession in the FM layer produces a spin current that flows into the adjacent non-magnetic (NM) layer[1], [2]. It is an interfacial effect and plays an important role in thin films. A recent theoretical work predicted that magnetic relaxation can be significantly enhanced when spin pumping is performed into a ferromagnet near Curie temperature $(T_{C})$ due to the fluctuation enhancement of the spin conductance across the interface [3]. This was qualitatively confirmed by Khodadadi et al. [4] in Py/ Gd system at low temperature. Here, we discuss the relaxation mechanism in Py/Gd structure by means of broadband ferromagnetic resonance (FMR) technique at room temperature. We show that a portion of the Gd layer at the interface becomes ferromagnetically ordered at room temperature and become antiferromagnetically (AFM) coupled to the Py due to the magnetic proximity effect (MPE). The ordered Gd at the interface of Py/Gd acts as a spin sink and contributes to the increase in spin pumping. In this work, we performed FMR measurement on two series of samples consisting of Py (15nm)/Gd $(t)$ and Py (15 nm)/Al $(t)$, where $t$ is the varying thickness of material from 0 to 16 nm. Py/Gd films were designed to study proximity induced magnetization in Gd due to neighboring Py. A series of Py/ Al reference samples were used to distinguish proximity induced effect in Py/Gd films from other interface-related effects due to long spin diffusion length of Al compared to Gd. FMR measurements were carried out using a co-planner waveguide (CPW) based FMR set-up in the frequency range of 3–17 GHz. The raw FMR spectra were fitted using a derivative of Lorentzian line shape to determine the resonance field $(H_{R})$ and line-width $\left({ \Delta H}\right)$. Fig.1 shows the FMR spectra for Py/Gd samples at a fixed frequency of 10 GHz. The inset shows Gd-caused FMR resonance field shift $(H_{shift})$ as a function of $t_{Gd}$. This change in $H_{shift}$ with $t_{Gd}$ can be attributed to the MPE. The observed negative $H_{shift}$ indicate a reduction of effective magnetization of the Py layer due to AFM coupling between the Py and the ordered interfacial Gd layer. We found that $M_{eff}$ decreases with increase in $t_{Gd}$ due to the AFM coupling between Py and ordered portion of the Gd layer. To further confirm that this is caused due to the MPE, not from other interfacial effects, we performed similar FMR measurements in a series of Py/Al samples. We did not observe any decrease of magnetization in Py/Al samples as shown in the inset of fig. 2(a). In these samples, the behavior of $M_{eff}$ with $t_{Al}$ is almost constant, indicating the absence of AFM coupling. The effective Gilbert damping parameter $(\alpha _{eff})$ increases significantly with increase in $t_{Gd}$ as shown in Fig.2 (b). An increase of 63.5% is observed in $\alpha _{eff}$ for Py/Gd bilayers as compared to Py alone. The inset of Fig. 2(b) shows the behavior of $\alpha _{eff}$ with $t_{Al}$ for which we did not find significant enhancement of $\alpha _{eff}$. We will show that a major contribution of this enhancement is due to the spin pumping effect into the ordered ferromagnet Gd layer which is near it's Curie temperature. This provides a qualitative confirmation of a recent theoretical prediction of spin sinking enhancement in this situation.