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

Abstract

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.

In situsmall-angle x-ray and nuclear resonant scattering study of the evolution of structural and magnetic properties of an Fe thin film on MgO (001)

“…In the case that the fluctuation rate of the magnetization direction due to superparamgnetic relaxation is higher than the Larmor precession rate of Fe nuclei, the hyperfine field averages out to zero [41]. The presence of superparamagnetic relaxation in thin films of Fe on MgO has also been seen in some earlier NRS and CEMS studies [10,25]. At a thickness of 2.0 nm, the film exhibits a finite hyperfine field.…”

“…Systematic measurements suggest that PMA has an interfacial origin, as predicted by theory [19,26]. Balogh et al attributed the perpendicular anisotropy to mechanical stresses in the film [25], arising from the epitaxial relation and different temperature dilation of various layers; however, no direct evidence in support of this claim was provided.…”

“…Therefore, from the point of view of basic understanding, most of the studies, both theoretical as well as experimental, have been done on simpler systems such as bcc-Fe or FeCo [8,11,19,23,24]. Systematic study of PMA as a function of Fe film thickness in MgO/Fe/MgO trilayer system has been done using nuclear resonant scattering (NRS) and conversion electron Mössbauer spectroscopy (CEMS) [10,25]. In both of the studies, existence of PMA at low temperatures has been confirmed.…”

Interface-related magnetic and vibrational properties in Fe/MgO heterostructures from nuclear resonant spectroscopy and first-principles calculations

“…Here, TMR is controlled by the electronic interface states in the minority spin channel 9,11 . Complementary work was concerned with the electronic structure, magnetic moments of Fe/MgO(001) stackings and also magnetocrystalline anisotropy [18][19][20][21][22][23][24][25][26] . The early density functional theory work of Li and Freeman investigated the properties of single and double layers of Fe on a MgO(001) surface 27 .…”

Effects of interfacial Fe electronic structures on magnetic and electronic transport properties in oxide/NiFe/oxide heterostructures

“…When a ferromagnetic metal (FM) layer is in direct contact with an oxide layer, rich physical and chemical behaviors such as charge transfer, stress, diffusion and hybridization may occur at FM/oxide interface [1][2][3][4], which play important roles in magnetic and transport properties of the FM/oxide heterostructure [5][6][7]. Previous researches on FM/oxide interfaces mainly focused on the effects of interfacial diffusion, roughness, electron scattering and lattice matching on the properties of FM films [8][9][10][11].…”