Spin-reorientation phase transition on the surface and in the bulk of $\alpha $-Fe$_2$O$_2$ single crystals

Direct comparison of the properties of a thin surface layer and the bulk of macroscopic hematite ($\alpha $-Fe$_2$O$_2$) crystals was used to study the magnetic structure of the surface layer and the bulk and the processes attendant on spin-reorientation phase transition (SRT). The investigation tool was simultaneous $\gamma$-ray, X-ray, and electronic Mössbauer spectroscopy, which enabled us to study the bulk and surface properties of macroscopic samples simultaneously and to compare them directly. Direct evidence of the existence of a surface «transition layer» on hematite crystals is obtained. The existence of this layer was suggested and described by Krinchik and Zubov [JETP 69, 707 (1975)]. The study in the SRT region showed that (1) the Morin SRT in the crystal bulk occurs in a jump (as a first-order phase transition), whereas in the surface layer of about 200 nm thick, some smoothness appears in the mechanism of magnetic-moment reorientation; (2) SRT in the surface layer, as in the bulk, involves an intermediate state in which low-and high-temperature phases coexist; and (3) SRT in the surface layer occurs at a temperature several degrees higher than in the bulk. Our experimental evidence on the SRT mechanism in the surface layer correlates with the inferences from phenomenological theory developed by Kaganov [JETP 79, 1544 (1980)].