Magnetic properties of the Fe-MgO interface studied by Mössbauer spectroscopy

Thin ${}^{57}$Fe layers evaporated onto an MgO(100) single-crystal substrate and covered by an evaporated MgO layer were studied by low-temperature conversion electron Mössbauer spectroscopy. The temperature dependence of the spectra indicates superparamagnetic behavior below 8 ML nominal thickness of the Fe layer signaling a cluster-type growth mode. The low-temperature hyperfine fields are consistent with a model that defines two types of metallic Fe atoms: bulklike and interfacial ones. Formation of FeO or (Fe,Mg)O at the interface layer is not observed. The sample with a 4-ML Fe layer when grown over a cleaved MgO substrate shows almost perfect perpendicular magnetization, as locally probed at 15 K by the hyperfine magnetic field, while random magnetization orientation and lower blocking temperature is observed in the case of a polished substrate. The perpendicular anisotropy observed at low temperature is attributed to mechanical stresses arising from the epitaxial relation and the different temperature dilatation of the subsequent layers.

Figure 1

Room-temperature CEMS of samples over cleaved MgO(100) substrate with ${}^{57}$Fe layer thickness as indicated in the figure. The fitted subspectra (see text for details) are also indicated. The scale bars indicate the 1$\%$ relative transmission.

Figure 2

Conversion electron Mössbauer spectra of the samples over cleaved MgO(100) substrates with different ${}^{57}$Fe layer thickness as measured at 15 K. The calculated spectra result from fitting by sextets—indicated by the subspectra for each spectrum—which have lines with Voight line shape. In case of the 4-ML sample, a small quadruple doublet is also present among the components. The scale bars indicate a 1$\%$ relative transmission.

Figure 3

Hyperfine field distributions [connected (blue) dots] and the isomer shift values [connected (black) crosses] linearly correlated to the hyperfine fields. The spectral intensity of the components is shown on the left scale and the isomer shifts on the right scale.

Figure 4

Temperature dependence of the intensities of the lines belonging to the $\Delta m=0$ transitions in case of the 8-ML sample.

Figure 5

CEMS of a 4-ML thick ${}^{57}$Fe layer grown over a polished MgO(100) substrate and covered by an evaporated MgO layer as measured at the indicated temperatures. For the indicated components see text. The scale bars indicate the 0.5$\%$ relative transmission.