Exploring triplet superconductivity by controlling the magnetic non-collinearity

Recent theories predict spin-triplet superconductivity at the interface between a singlet superconductor (SC) and a ferromagnet (FM) with \textit{inhomogeneous} magnetization [1]. Magnetic non-collinearity is a crucial but not quantitatively controlled parameter in most experiments inferring triplet superconductivity [2]. In this work, we use Nb as the SC, and an epitaxial exchange spring Py/Sm-Co bilayer with in-plane uniaxial anisotropy in Sm-Co layer as the FM. Due to the interfacial exchange coupling, a \textit{tunable} noncollinear spin spiral can be achieved by controlling the external field. At a fixed temperature within the superconducting transition, starting from a collinear magnetic configuration, as the spin spiral winding angle \textit{$\phi $} increases, the superconducting critical current ($I_{c})$ first increases. There is an optimized winding angle \textit{$\phi $}$_{o}$, which maximizes $I_{c}$, after which $I_{c}$ decreases with increasing\textit{ $\phi $}. This \textit{non-monotonic} $I_{c}$(\textit{$\phi $}) dependence cannot be explained by the short range proximity effect alone and suggests triplet pairing. More importantly, combining micromagnetic simulations with magnetoresistance measurements, we have determined the~magnetic non-collinearity and correlated it \textit{quantitatively} with the superconducting transport results. Our findings demonstrate the superconducting proximity effect can be tuned by manipulating the magnetic non-collinearity in a \textit{single} sample.\\[0pt] [1] F. S. Bergeret et al., PRL \textbf{86}, 4096. [2] M. Eschrig, Physics Today \textbf{64}, 43.