Strain controlled Morin temperature in epitaxial a-Fe2O3(0001) film

Abstract

Morin transition in α-Fe2O3(0001) films have been one of the puzzling topic. Differently from bulk and nanoparticle, the Morin transition have been not observed in the (0001) orientated thin films and it is very likely because of the tensile strain. The absence of Morin transition in thin films have gave limit on the understanding of spin reorientation transition in antiferromagnetic films and its application utilizing the spin manipulation.In this dissertation, we fabricated the first epitaxial α-Fe2O3 (0001) films un-dergoing Morin transition. X-ray spectroscopy, x-ray diffraction and microscopy techniques makes it suitable for investigating phase-pure and epitaxial films. In particular, x-ray magnetic linear dichroism (XMLD) and grazing incident diffraction (GID) enable us to determinethe Morin transition temperature (TM) and the degree of in-plane strain of films even through some films are even a few monolayers due to their surface-sensitivity.We show that the strain of α-Fe2O3 films grown onto the Cr2O3 buffer layer changes from compressive to unstrained state as the film thickness increases, which indicates they follow the strain relaxation mechanism. Morin temperature TM, which increases up to 360 K for the film thickness 3 nm, gradually decreases down to near the bulk value as the film thickness increases. We found that the correlation between TM and in-plane strain is well explained in terms two competing energy of magnetic dipolar anisotropy and single ion magnetocrystalline anisotropy, and that TM is truly controlled by the in-plane strain. For the ultra-thin films d < 3 nm, TM decreases as film thickness decreases and it is due to the finite size effect of TM as well as N´eel temperature(TN). The measurements of TN and the finite size scaling in ultra-thin region of α-Fe2O3(0001) films show TM/TN is used as a good quantity to explain the the correlation between TM and the strain, which means that the two magnetic ordering temperature, TM and TN in α-Fe2O3 are intimately coupled to each other.In addition, we observed that ferromagnetic Co layer grown onto α-Fe2O3 films severely suppress the TM of α-Fe2O3 films and anneal-induced perpendicular magnetic anisotropy in Pt/Co/α-Fe2O3 films. The observations will encouraged to develop the spin-manipulated device utilizing α-Fe2O3 films.