Magnetism Of Ultrathin Fe Films Research Articles

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

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.

Magnetism of ultrathin Fe films on MgO(001)

We grow ultrathin (<2-monolayer (ML)) Fe films on MgO(001) at 80 K, limiting oxide formation to the very early stages of Fe growth, and then investigate in situ the magnetism of the films using x-ray magnetic circular dichroism (XMCD). The 0.2-ML-thick Fe film exhibits superparamagnetism, disproving the existence of a magnetically dead layer. For all the films, the spin moments per Fe atom are far less than that of bulk Fe, while anomalously large orbital moments are observed. Those are attributed, respectively, to both the oxidation Fe film at the interface and the existence of some Fe nanoclusters in fcc phase and to the high population of low coordinated atoms, such as Fe at the edges of the nanoclusters.

Magnetism of Ultrathin Fe Films in the Vicinity of Transition from Ferromagnetism to Superparamagnetism

We have investigated the magnetism of ultrathin Fe films grown on the inclined Al2O3(0001), especially, in the vicinity of ferromagnetic-superparamagnetic transition. In the transition region, the magnetic state of Fe films is the coexistence of ferromagnetism and superparamagnetism. The ratio of the components in ferromagnetic and superparamagnetic state is dependent on the growth temperature. For Fe grown on the flat substrate without the inclination, as the growth temperature increases, the ratio of the component in ferromagnetic state increases, reaches to the maximum and decreases again. On the presence of substrate inclination, the ratio of the component in ferromagnetic state increases, reaches to the maximum and decreases again. decreases monotonically with increasing growth temperature. We investigated the dominant factor of the complex magnetic state, and clarified that the complex magnetic state is dominated by the change of the film structure and the effective magnetic anisotropy energy.

Growth and magnetism of ultrathin Fe films on Pt(100)

The scanning tunneling microscopy, low energy electron diffraction, and surface magneto-optic Kerr effect are used to investigate the morphology, structure, and magnetism of ultrathin Fe films on a Pt(100) surface. At room temperature, the deposited Fe atoms exchange sites with the Pt atoms of the substrate, and then grow in a quasi layer-by-layer mode. The Fe films show in-plane anisotropy for all coverages (up to 7 ML). Annealing at $600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ switches the easy magnetization axis from in-plane to perpendicular when the thickness is smaller than 5.2 ML. The coercivity increases gradually to its maximum at 3.3 ML and then decreases abruptly, accompanied with a surface structure transition from $(1\ifmmode\times\else\texttimes\fi{}1)$ to $\mathrm{c}(2\ifmmode\times\else\texttimes\fi{}2)$. These results demonstrate the formation of $\mathrm{Fe}\penalty1000-\hskip0pt\mathrm{Pt}$ $\mathrm{L}{1}_{0}$ and $\mathrm{L}{1}_{2}$ chemically ordered alloys and a phase transition between them. The atom exchange process is found to play a key role in the alloy formation at significantly reduced growth temperature.

Surface Structure of Ultrathin Fe Films on Cu(001) Revisited

The structure and magnetism of ultrathin Fe films epitaxially grown on a Cu(001) surface are investigated by grazing scattering of fast H and He atoms or ions. By making use of a new variant of ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of the film surface. We observe for room temperature growth a dominant and defined fcc-like structure. Complex surface reconstructions as reported in recent STM and LEED studies are observed only for cooling and H2 dosing.

Magnetism of ultrathin Fe films on GaAs(1 0 0) investigated by photoelectron spectroscopy

Ultrathin Fe films were epitaxially grown on Ga- and As-terminated GaAs(1 0 0) at room temperature by molecular beam epitaxy. The spin polarisation was studied by synchrotron-based photoelectron spectroscopy (PES). Magnetically dead layers of a few Å thickness were observed and the behaviour is attributed to the superparamagnetic phase rather than the formation of non-ferromagnetic compounds. For Fe on the Ga-terminated substrate, the spin polarisation increased abruptly and then remained at a constant value when the film was deposited in thickness. In contrast, Fe grown on the As-terminated GaAs(1 0 0) substrate reveals a variable behaviour of spin polarisation with two spin polarisation maximal, along with increasing the thickness of Fe coverlayers. The underlying decreases in the spin polarisation may be attributed to structural changes of Fe coverlayers, related to the different ternary metastable phases.

Magnetism of ultrathin Fe films on Mo substrates with low and high Miller indices

The magnetism of iron (Fe) monolayers (ML) adsorbed on low- and high-index orientations of molybdenum (Mo) surfaces is investigated using the real-space self-consistent tight-binding recursion method in the Hartree–Fock approximation with Hubbard Hamiltonian. The (0 1 1) orientation is found to be the most favored to exhibit magnetism due to Fe neighboring atoms. The least magnetic moment is found in (0 0 1) orientation due to the larger number of Mo neighboring atoms. In the vicinal systems, the net magnetization decreases as the step length increases.

Modification of initial growth and magnetism in Fe/Cu(100)

The influence of the deposition rate upon the initial growth morphology and magnetism of ultrathin Fe films on the Cu(100) surface has been studied with low-energy electron diffraction, reflection high-energy electron diffraction, surface magneto-optical Kerr effect (SMOKE), and spin-polarized low-energy electron microscopy (SPLEEM). Dramatic changes in the first diffraction intensity oscillation during growth at room temperature indicate that simultaneous growth of the first and second atomic layers is converted to more perfect layer-by-layer growth when the deposition rate is reduced below 0.5 ML/min. SPLEEM and SMOKE demonstrate the sensitivity of magnetism in Fe/Cu(100) to details of interface formation during the initial growth in this range of deposition rates. Kinetic and thermodynamic mechanisms that relate to place exchange at the interface are discussed as the source of the observed growth and magnetic behavior.

Surface alloying and magnetism of ultrathin Fe films on Pd(001)

The atomic structure and the chemical composition of ultrathin $(<3$ monolayer) Fe films grown on a clean Pd(001) at room temperature are studied by low energy electron diffraction (LEED) $I/V$ (spot intensity versus beam energy) analysis with average t-matrix approximation. We find sizable amount of alloying between Fe film and Pd substrate near the interface, and the alloyed Fe films show large tetragonal distortion. Fully relativistic ab initio band structure calculation is performed to elucidate the stability and the correlation between the surface alloying and the magnetic properties of the alloyed Fe film. We find that the sublayer Fe film below 1 ML Pd capping layer is stable compared with the Fe overlayer, supporting efficient inter-diffusion between Fe and Pd, and that lattice spacing in the alloyed Fe films contract as observed by the LEED $I/V$ analysis. The magnetic moments of Fe are found strongly enhanced, and the induced magnetic moments of Pd are also substantial.

Morphology and magnetism of ultrathin Fe films on Pd(100)

Growth and magnetism of ultrathin Fe films on Pd(100) have been studied in situ by reflection high-energy electron diffraction, scanning tunneling microscope, and magneto-optical Kerr effect technique. It is found that the growth at the initial stage is in the layer-by-layer mode and the magnetic hysteresis loops are detectable only when the Fe films are thicker than 1.3 monolayer (at 300 K) and 0.5 monolayer (at 55 K).

Magnetization of ultrathin Fe films deposited on Gd (0001)

The magnetization of ultrathin (1.5–6 ML) Fe films deposited on Gd (0001) was studied using spin polarized secondary electron emission spectroscopy. An anomalously high Curie temperature (>320 K) was observed for very low coverages of Fe (1.5 ML). The anomalously high Curie temperature results in part from the strong antiferromagnetic exchange interaction between the Fe and Gd at the interface. In addition, a spin reorientation transition of the Fe magnetization from in-plane to perpendicular was observed as the temperature is increased for Fe thickness between 1.5–4 ML. The spin reorientation is attributed to the competition between the perpendicular anisotropy of the Fe overlayer and the exchange interaction with the in-plane Gd bulk magnetization. Therefore the effects of varying the thickness of the Gd and hence the bulk Curie temperature on the spin reorientation transition can be studied.