The longitudinal spin Seebeck effect (SSE) has been investigated using Pt/ferrite bilayers employing two Y-hexagonal ferrites ${\mathrm{Ba}}_2{\mathrm{Zn}}_2{\mathrm{Fe}}_{12}{\mathrm{O}}_{22}$ (Zn2Y) and ${\mathrm{Ba}}_2{\mathrm{Co}}_2{\mathrm{Fe}}_{12}{\mathrm{O}}_{22}$ (Co2Y) deposited by a spin-coating method on ${\mathrm{SrTiO}}_3\left(111\right)$ substrates. The prepared hexagonal ferrites are highly oriented with $c$ axes perpendicular to the substrate plane. The room-temperature magnetic moments of both ferrimagnetic ferrites amount to similar values and, most importantly, both have easy magnetization normal to the $c$ axis. Despite their similar magnetic response the notable SSE signal is only observed for Zn2Y whereas the SSE signal of Co2Y is below the experimental noise level. A plausible explanation for this surprising discrepancy is magnetic disorder induced by cobalt cations, the random distribution of which in the Co2Y ferrite structure might critically limit the spin-wave propagation. This results in suppression of the SSE signal in Co2Y, while the Zn2Y with nonmagnetic substituent exhibits significant SSE signal. The temperature dependence of SSE for Zn2Y was measured over the $30\text{–}300\text{−}\mathrm{K}$ range and quantitatively analyzed considering the heat flow through the Pt/Zn2Y bilayer and thermal gradient across the Zn2Y thin layer as the most relevant parameters. Using this approach the normalized SSE smoothly increases with lowering temperature, which correlates to increasing magnon propagation length and magnetization with decreasing temperature.

• Received 6 May 2017
• Revised 19 July 2017

DOI:https://doi.org/10.1103/PhysRevB.96.064428