Magnetism of ultrathin Fe films in MgO/Fe/MgO in epitaxial structures probed by nuclear resonant scattering of synchrotron radiation

Abstract

Nuclear resonant scattering (NRS) of synchrotron radiation was used to investigate the magnetic anisotropy of iron films in MgO/Fe(t)/MgO(001) structures for t = (4–10) Å. The low-temperature NRS spectra were analyzed using a static magnetization model involving two interface-like components and a bulk-like component. We confirmed the existence of perpendicular magnetic anisotropy in MgO/Fe/MgO structures at 10 K with an increasing in-plane component of the magnetization for t > 8 Å over the entire thickness of the Fe film. The evolution of the magnetic structure with increasing temperature was studied for an Fe film thickness of 8.8 Å, and the temperature dependence of superparamagnetic fluctuations with characteristic frequencies ranging over tens of MHz was interpreted in terms of a spin reorientation transition. We showed that interfacial magnetic moments are less sensitive to thermal excitations than the magnetic moments in the film center, which was attributed to the spin pinning at the interface.

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.…”

“…Interfacial imperfections in terms of possible oxidation of transition metal in the interfacial region or oxygen vacancies, etc., are suggested to be possible reasons for deterioration in TMR value [3][4][5]. This has led to extensive study in the literature on characterization of interfaces in MgO based MTJs [6][7][8][9][10][11]. However, understanding of various interfacial imperfections and their correlation with magnetic and magneto-transport properties of magnetic electrodes is still far from satisfactory.…”

“…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.…”

Tunable magnetic properties of monoatomic metal-oxide Fe/MgO multilayers

“…Ultrathin Fe(001) films embedded between the MgO(001), with thicknesses below the critical value necessary to establish perpendicular magnetization [15,16], are superparamagnetic at RT, as indicated in Fig. 2 is characterized by a typical broadening that reveals that the ferromagnetic order is averaged out because of random jumps in the magnetization of the superparamagnetic particles.…”

“…Recently, we have demonstrated that iron in epitaxial MgO/Fe/MgO trilayers grown on MgO(001) has perpendicular anisotropy below a certain critical Fe thickness [15,16]; however, because of the cluster growth, superparamagnetism significantly influences the magnetic properties of MgO/Fe/MgO at room temperature (RT). Simultaneously, granular Fe/MgO multilayers fabricated on amorphous [13,17] and single-crystal substrates [18] have been optimized for maximum tunneling magnetoresistance (TMR) and field sensitivity at RT.…”