Magnetism Of Fe Films Research Articles

Fine-tuning of canted magnetization in stepped Fe films through thickness variation, Au capping, and quantum confinement

Author(s): Daobrowski, M; Cinal, M; Schmid, AK; Kirschner, J; Przybylski, M | Abstract: We present a joint experimental and theoretical study that demonstrates how to efficiently control a canted state of magnetization in Fe films grown on Ag(001) vicinal surface and precisely characterize it with the magneto-optical Kerr effect. It is shown that by employing different mechanisms to tune the magnetization tilting angle, any magnetization orientation within the plane perpendicular to the step edges can be achieved. In particular, increasing the Fe film thickness leads to continuous rotation of the magnetization easy axis toward the film surface and the sense of this rotation in uncovered films is opposite to that in films covered with Au. Another tuning mechanism is provided by oscillatory changes of the tilting angle at low temperatures due to formation of quantum well states in Fe films. The observed canting of magnetization is explained within a phenomenological model by an interplay of the shape anisotropy and two magnetocrystalline anisotropy terms, perpendicular and step-induced anisotropies, which results in an effective uniaxial magnetic anisotropy. The fitted thickness dependencies of the anisotropy constants accurately reproduce experimental variations of the tilting angle with both Fe and Au thicknesses as well as transient changes of the magnetization orientation in ultrathin Fe films upon submonolayer Au coverage, observed with spin-polarized low-energy electron microscopy.

Growth, magnetism and ferromagnetic thickness gap in Fe films on the W(111) surface

The growth, structure, and magnetism of Fe films on the W(111) surface were investigated using low energy electron microscopy (LEEM) and diffraction (LEED), spin polarized LEEM (SPLEEM), and work function measurements. In contrast to an earlier report that Fe grows with fcc structure following initial pseudomorpic layer growth, we observe no evidence of the formation of fcc Fe over the entire thickness range studied, up to 18 monolayers (ML). Observations are instead consistent with the formation of a well-ordered, laterally (tensile) strained bcc Fe layer that gradually relaxes vertically and develops increasing disorder with increasing thickness. Ferromagnetic order appears at 6 ML, but surprisingly vanishes at 8 ML, and reappears just as suddenly at 9 ML during Fe deposition at room temperature. Ferromagnetism between 6 and 8 ML also vanishes at only 5 deg above room temperature. The magnetization direction of a monodomain structure remains constant before and after the ferromagnetic thickness gap at 8--9 ML until the formation of a multidomain structure at about 12 ML. Variations of exchange asymmetry in spin-polarized elastic electron scattering are also observed with increasing film thickness, particularly above 12 ML, that indicate changes in the spin-polarized electron band structure above the vacuum level. The evolution of magnetism and exchange asymmetry with increasing thickness and the appearance of the ferromagnetic gap are attributed to structural and morphological changes in the strained Fe layer, which eventually lead to a relaxed although highly disordered bcc Fe layer.

Kinetic Stabilization and Magnetism of Fe films on NiO(001)

Kinetic Stabilization and Magnetism of Fe films on NiO(001)

The role of structural variations in the magnetism of Fe/Cu(1 1 1): First-principles calculations

The effects of three crucial structural variations, i.e., sub-monolayer coverage, intermixing, and stacking fault, on the magnetism of Fe/Cu(1 1 1) were quantitatively investigated using density functional theory calculations. A single Fe atom (0.25 ML) on the fcc (2 × 2) Cu(1 1 1) surface has a higher magnetic moment (3.18 μ B) than bulk Fe (2.15 μ B). The calculated magnetic moments of 0.5, 0.75 and 1 ML coverage Fe on the Cu(1 1 1) surface, were 3.01 μ B, 2.87 μ B and 2.72 μ B, respectively. For intermixing between 1 ML of Fe and Cu(1 1 1), the magnetic moment of the Fe adatoms monotonically decreased with the intermixing amount. The electron density of states showed that the decreasing magnetic moment of Fe adatoms resulted from the filling of down-spin 3 d-electrons below Fermi level by the hybridization of 3 d-electron among Fe atoms. The influence of Cu(1 1 1) atoms on the electronic structure and magnetic moment was observed to be negligible. In the artificially prepared 2 and 3 ML thick Fe films on Cu(1 1 1), the magnetic moments of Fe atoms were not dependent on the stacking sequences of Fe layers. However, the demagnetization energy calculation revealed a more stable perpendicular magnetization for fcc Fe films compared to hcp Fe films on Cu(1 1 1). In brief, the structure dependence of magnetism of Fe films on Cu(1 1 1) mainly originates from the bonding among Fe atoms, while the effect of bonding with Cu atoms was negligible.

Superparamagnetic limit to spin-polarized vacuum tunneling from Fe-coated W(001) tips

Superparamagnetic fluctuations of magnetization in thin Fe films that coat a sharp W(001) tip are evident in the spin polarization of field emitted electrons. Fluctuations of polarization between four transverse directions that align with the tip ⟨100⟩ directions are consistent with the in-plane biaxial magnetization of Fe films on macroscopic W(001) surfaces. This superparamagnetic behavior imposes a limit to spin polarized vacuum tunneling applications that proves to be more stringent than the suppression of the Curie temperature in thin films due to finite size effects.

Surfactant-assisted epitaxial growth and magnetism of Fe films on Cu(111)

The magnetic properties of thin epitaxial layers of Fe grown on Cu(111) depend sensitivelyon the films’ structure and morphology. A combination of experiments and numericalsimulations reveals that the use of a surfactant monolayer (ML) of Pb during molecularbeam epitaxy (MBE) growth at room temperature reduces the amount of interdiffusion atthe Cu–Fe interface, retards the fcc-to-bcc transformation by about 2 ML and substantiallyincreases the films’ coercivity. The origin of all these alterations to the magnetic behaviorcan be traced back to the structural modifications provoked by the surfactantduring the early growth stages. These results open the way for the controlledfabrication of custom-designed materials with specific magnetic characteristics.

Effect of oxygen adsorption on the structure and spin-reorientation transition of Fe films on Cu(1 1 25)

The morphology, structure, and magnetism of Fe films on an oxygen-precovered, stepped Cu(1 1 25) surface are investigated by scanning tunneling microscopy, low-energy electron diffraction, and magneto-optical Kerr effect analysis. After exposure of Cu(1 1 25) to about 600 L of oxygen at 493 K, a well-ordered (2√×√)R45° superstructure is formed. The O-induced structure has a zig-zag morphology, showing a high density of facets and is very stable during subsequent Fe deposition. Fcc Fe can be stabilized up to 20 monolayer (ML) thickness, while the magnetization of the films reorients from perpendicular to in-plane at 15 ML. From 24 ML, the linear extrapolation line of the thickness dependence of the Kerr intensity of the transformed bcc Fe films does not go along with the pure fully magnetized bcc Fe films. We conclude that oxygen adsorption strongly affects the structure and spin-reorientation transition of Fe films.

Temperature dependence of the magnetization of Fe films on Cu(1 0 0) studied by SPMDS and SMOKE: effects of relaxation

Temperature dependence of the magnetization of Fe films on Cu(1 0 0) has been investigated by spin-polarized metastable deexcitation spectroscopy (SPMDS), which is extremely surface sensitive, and by surface magneto-optic Kerr effect (SMOKE). It has been observed that the remanent magnetization measured by the two methods exhibits different behaviors at around the critical temperature. This has been found to be caused by the relaxation of the magnetization. The relaxation may be caused by the formation of the multi-domain state or the effect of superparamagnetism.

A Spin-Polarized Metastable-Atom Deexcitation Spectroscopy (SPMDS) Study on Surface Curie Temperature of Fe Films on Cu(100)

The temperature dependence in the surface magnetism of Fe films on Cu(100) was investigated by spin-polarized metastable-atom deexcitation spectroscopy (SPMDS), which is extremely surface sensitive, and the surface magneto-optic Kerr effect (SMOKE). By comparing these two measurements, it has been shown that the surface and the bulk of the Fe films on Cu(100) have the same Curie temperature, and that the magnetic moment of the surface atoms is almost proportional to that of the bulk atoms at 150–320 K.

Au-induced perpendicular magnetization in Fe films grown on Si(001)

We report on Fe films grown on Au-buffered Si(001) showing perpendicular magnetization at ambient temperatures. The in-plane magnetization in ultra-thin Fe films grown at 150 K reorients irreversibly out-of-plane as a result of annealing close to room temperature. A Au cap layer which is expected to segregate to the top of the Fe film may be responsible for this behaviour. This hypothesis is based on the observation of an in-plane to out-of-plane reorientation of magnetization upon the growth of an ultra-thin Au cap layer on a slightly thicker Fe film. The origin of such a strong out-of-plane anisotropy of Fe induced by Au is discussed.

Angular dependence of magnetism of Fe films sputtered with ion beam perpendicular to target

Fe films were deposited by sputtering with ion beam perpendicular to the target. The substrates were arranged to be in the arc shaped position around the center of the ion bombarded area of the target. The sputtering voltage Vs was 1200 V. The angle θs between the direction of the ion beam and the normal line of the substrate plane was varied in the range of 10°–80°. The energy of the sputtered atoms arriving to the substrate was calculated by Monte Carlo simulation, and the average energy EFe of the sputtered Fe atoms varied drastically with θs. EFe increased with increase of θs. The morphology and the magnetic properties of the Fe films significantly depended on θs. The morphology changed from a columnar structure to a round-grain one by propagation of recrystallization with the increase of θs. The saturation magnetization 4πMs was about 21.5 kG. The coercivity Hc took the minimum value of 5 Oe for θs above 50°. This suggests that the angular relation between the target and the substrate should be one of essential factors for controlling the magnetic properties of the Fe films.

Changes of morphology, structure, and magnetism of Fe on stepped Cu(111)

We have studied the morphology, structure and magnetism of Fe films thermally deposited on stepped Cu(111) by scanning tunnelling microscopy (STM), low energy electron diffraction (LEED) and magneto-optical Kerr effect (MOKE). At room temperature, in the submonolayer range Fe films grow as quasi one-dimensional stripes along the upper step edges due to a strong decoration effect. Between 1.4 and 1.8 monolayer coverage the stripes percolate and become two-dimensional films. Between 2.3 and 2.7 monolayer coverage the films undergo a structural transition from fcc(111) to bcc[110] with Kurdjumov-Sachs orientation. In the fcc range the films have a low net magnetic moment, and a perpendicular easy magnetization axis. The magnetization switches to an in-plane high-moment phase correlated with the fcc to bcc structural transformation.

Kerr effect study of the onset of magnetization in Fe films on GaAs(001)-2×4

We have studied the magnetic behavior of epitaxial bcc Fe(001) films grown on molecular beam epitaxy-prepared GaAs(001)-2×4 substrates, with film thicknesses ranging up to ∼16 monolayers (ML) (∼23 Å). We present measurements of the thickness-dependent evolution of the magnetic properties of the Fe films as determined by in situ magneto-optical Kerr effect. Particular attention is given to the substrate-induced in-plane uniaxial magnetic anisotropy. We find that the onset of room-temperature ferromagnetism occurs between 4 and 8 ML, and that the anisotropy is dominated by a uniaxial in-plane component such that the two 〈110〉 axes are inequivalent for all coverages studied. Although the initial nucleation and growth results in 2D islands elongated along [1̄10], the in plane easy axis is perpendicular to this direction along [110], indicating that shape anisotropy does not play a significant role.

Annealing effect on morphology and magnetism of ultrathin films of Fe and Ni on Cu(100)

Scanning tunneling microscopy (STM) and the magneto-optical Kerr effect (MOKE) have been used to investigate the effects of annealing on the morphology and magnetism of Fe and Ni films on Cu(100). Relatively mild annealing (490 K) created pinholes in the Fe/Cu(100) films, opening channels for the surface diffusion of copper to the top of the film surface along the walls of the pinholes. The magnetization of the Fe films is strongly affected by annealing. Annealing smooths the surface of Ni/Cu(100) films significantly, but no interdiffusion was observed upon annealing below 500 K. For Ni films thicker than 7 ML, the magnetization remains perpendicular after annealing, with only a slight reduction in its amplitude.

Magnetic Anisotropy Constants of Epitaxial (110) Fe/GaAs Films from 77k To 293k Studied by Magneto-Resistance

ABSTRACTA first-order phase transition in the magnetization of Fe films, driven by an applied magnetic field, was first reported by Hathaway and Prinz [1]. Further studies were performed on this phase transition using anisotropic magnetoresistance measurements by Riggs and Dahlberg [2]. Here we report the extension of these studies to include temperatures between 77K and 293K. Emphasis is on a determination of the fourth-order and uniaxial anisotropy constants (K1 and Ku). It is shown that the temperature dependence of the anisotropy energies in these films varies with thickness, which may be useful in sorting out the origin and magnitude of different contributions to the total effective anisotropy. The present study suggests that similar studies of (110) iron on other substrates might contribute to achieving a better understanding of in-plane anisotropies in epitaxial films.

New Magnetic Material Having Ultrahigh Magnetic Moment

The change of the saturation magnetization of Fe films with the pressure of nitrogen during deposition ranging from 2 × 10−5 to 7 × 10−3 Torr has been investigated systematically. We found a new magnetic material which has the highest saturation magnetization at room temperature, 2050 G, among those of all the magnetic materials. This was attributed to Fe16N2, which has a bct structure, and the magnetic moment associated with Fe atoms of Fe16N2 was deduced to be 3.0 μB.