Examining the role of antiferromagnetism in spin-charge conversion in chromium thin films via temperature-dependent measurements of the local spin Seebeck effect

The potential contribution to spin-charge conversion from antiferromagnetism in chromium thin films remains a topic of debate. Here we experimentally demonstrate significant spin-to-charge conversion in thermally evaporated chromium thin films via the longitudinal spin Seebeck effect (LSSE) measured with lithographically patterned, self heated metal structures on bulk polycrystalline yttrium-iron-garnet (YIG) substrates, with specific focus on the temperature dependence. We present LSSE between 78 K and 350 K for 10 nm evaporated and sputtered Cr thin films, along with comparison to Pt and Cu films, all deposited at room temperature on a bulk polycrystalline yttrium-iron-garnet (YIG) substrate. Both Cr films show a significant spin-charge conversion via the LSSE, with roughly similar magnitude but quantifiably different temperature dependence. Dimensionless sensitivity measurements, $(T/R)\ifmmode\cdot\else\textperiodcentered\fi{}dR/dT$, of our Cr structures show two distinct peaks centered at roughly 350 K and 200 K for the evaporated Cr film. These two peaks likely indicate the separate N\'eel temperatures of the incommensurate and commensurate spin density wave antiferromagnetic states coexisting in a strain-driven mixed state for this film. $(T/R)\ifmmode\cdot\else\textperiodcentered\fi{}dR/dT$ for the sputtered film shows clear evidence for only one peak, near the bulk N\'eel temperature of $\ensuremath{\sim}300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Comparison of the normalized spin conversion efficiency for evaporated Cr, sputtered Cr, and literature values for sputtered Cr highlights the potential role of this strained mixed spin density wave state for spin-charge conversion. The temperature evolution of the two sputtered films agrees at all measured temperatures, and with the evaporated film above 200 K. Below 200 K there is a significant enhancement to the LSSE voltage for the evaporated film as compared to the sputtered films. This suggests that spin-charge conversion in our evaporated Cr thin film is affected by the modified spin-density wave antiferromagnetic state, which is driven by a combination of finite-size effects and strain.