Study of obliquely deposited $^{57}$Fe layer on organic semiconductor (Alq$_3$); interface resolved magnetism under x-ray standing wave

Ultra-thin Fe film is deposited obliquely on the Tris(8-hydroxyquinolinato) aluminum (Alq$_3$) layer to study the interface magnetism and compared with the normally deposited film of the same thickness. The overall magnetic property of the film, as observed using magneto-optic Kerr effect measurements, is understood precisely through interface resolved grazing incident nuclear resonance scattering (GINRS) measurements under x-ray standing wave (XSW) conditions. To excite XSW modes, the bilayer is deposited between high dense Ag and Pt layers to make a waveguide structure, where the position of the antinode is varied by changing incident angles. The Fe layer is enriched with $^{57}$Fe isotope during deposition to make it active for GINRS measurements. Compared with obliquely deposited $^{57}$Fe film, reduced hyperfine fields at the interface of normally deposited $^{57}$Fe/Alq$_3$ film, providing evidence for the deep penetration of $^{57}$Fe atoms into Alq$_3$ film. Due to the large shadowing effect at oblique angle deposition, Fe atoms try to organize into isolated superparamagnetic clusters to minimize their surface/interface energy. On the other hand, besides the relatively deep penetration in the normally deposited thin film, ferromagnetism is observed due to the connecting bigger $^{57}$Fe clusters. Our results demonstrate that deposition geometry has a direct and crucial bearing on the interfacial microstructure, which can control diffusion at the Alq$_3$ interfaces and tune magnetic anisotropy in the ferromagnetic layer