Spin Seebeck effect through antiferromagnetic NiO

We report temperature-dependent spin Seebeck measurements on Pt/YIG bilayers and Pt/NiO/YIG trilayers, where YIG (yttrium iron garnet, ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$) is an insulating ferrimagnet and NiO is an antiferromagnet at low temperatures. The thickness of the NiO layer is varied from 0 to 10 nm. In the Pt/YIG bilayers, the temperature gradient applied to the YIG stimulates dynamic spin injection into the Pt, which generates an inverse spin Hall voltage in the Pt. The presence of a NiO layer dampens the spin injection exponentially with a decay length of 2 ± 0.6 nm at 180 K. The decay length increases with temperature and shows a maximum of 5.5 ± 0.8 nm at 360 K. The temperature dependence of the amplitude of the spin Seebeck signal without NiO shows a broad maximum of 6.5 ± 0.5 μV/K at 20 K. In the presence of NiO, the maximum shifts sharply to higher temperatures, likely correlated to the increase in decay length. This implies that NiO is most transparent to magnon propagation near the paramagnet-antiferromagnet transition. We do not see the enhancement in spin current driven into Pt reported in other papers when 1–2 nm NiO layers are sandwiched between Pt and YIG.

Figure 1

(a) A raw trace of the spin Seebeck voltage $V_y$ vs applied field at 300 K for an applied heater power of 0.495 W and (b) spin Seebeck coefficient as a function of temperature for a Pt(6 nm)/YIG(250 nm) bilayer deposited on GGG $\langle 111\rangle$. In (b), the results of several different runs in different instruments are superimposed on each other, illustrating the reproducibility of the results, which is of the order of 30%. A maximum spin Seebeck coefficient of ∼6.5 ± 0.5 μV/K is observed on the YIG film within a broad peak around 20 K.

Figure 2

Temperature dependence of the SSE signal on the Pt/YIG bilayer is compared to SSE signals from the YIG/NiO/Pt trilayers, with the NiO film thickness as noted. A decrease in SSE signal is observed with increasing NiO film thickness. A maximum at a temperature $T_M$ is observed for each of the YIG/NiO/Pt trilayers. $T_M$ increases with increasing NiO thickness.

Figure 3

(a) Magnitude of the SSE signal at 420, 360, 300, 240, and 180 K plotted as a function of NiO thickness. The experimental values are the data points. The lines through them are fitted exponential decay functions, with a characteristic attenuation length scale given as a function of temperature in (b).