Exchange Bias in Surface-Crystalline Fe-Nb-B Ribbons

Exchange bias in Fe-Nb-B ribbons is investigated. The as-quenched ribbon exhibits partial crystallinity on both surfaces while the bulk of ribbon is amorphous. The coexistence of amorphous phase and crystalline $\alpha$-Fe on the ribbon surfaces is confirmed by X-ray diffraction, conversion electron MÖssbauer spectroscopy (CEMS), and transmission electron microscopy (TEM). The crystalline phase can be removed by a short etching in a diluted nitric acid. The properties of the two surfaces are different. A high concentration of the crystallites in a very thin layer at the surface is typical for the wheel side. On the shiny side the crystallites are spread, with the volume fraction about 5%, in a layer about 1.5 $\mu$ m thick. Whereas the amorphous phase is magnetically soft ($H_{c} \approx$ 3.5 A/m), the coercive fields of the surface layers, measured by magneto-optic Kerr effect (MOKE), are 0.8 and 3.2 kA/m for the wheel and shiny side, respectively. Bulk hysteresis loops of strained ribbons (straight or toroids) are usually asymmetric and shifted with respect to zero magnetic field. It is shown that mainly the surface layer on the shiny side is responsible for this bias effect. An explanation based on the exchange coupling between the Fe grains and the amorphous phase is proposed.