Epitaxial growth and magnetic exchange anisotropy in Fe3O4<Emphasis Type="Bold">/NiO bilayers grown on MgO(001) and Al</Emphasis>2O3(0001)

Epitaxial Fe3O4/NiO bilayers were epitaxially grown on MgO(001) and Al2O3(0001) substrates to investigate the influence of the fully spin compensated (001) and the non-compensated (111) NiO interface planes between the ferromagnetic (F) and antiferromagnetic (AF) layers on the AF/F exchange coupling. Bilayers of different magnetite thicknesses and constant NiO thickness were investigated. The structural characterizations indicate a perfect epitaxy of the two layers for the both growth directions in the two Fe3O4/NiO/MgO(001) and NiO/Fe3O4/Al2O3(0001) systems. An epitaxial ferrimagnetic (Ni,Fe)Fe2O4 phase is observed at the AF/F interface when the NiO oxide is grown on the top of the Fe3O4 layer while a perfectly flat AF/F interface is observed in the Fe3O4/NiO/MgO(001) system exhibiting only a very slight interdiffusion. Magnetic measurements indicate a relative strong bias at 300 K for the bilayers grown on Al2O3(0001), which decreases with the inverse of the ferrimagnetic layer thickness as theoretically expected. On the contrary, a zero exchange biasing is observed at 300 K for the bilayers grown on MgO(001).     

Development of magnetic anisotropies in ultrathin epitaxial films of Fe(001) and Ni(001)

Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asK _us = 0.96, 0.63, and 0.3 ergs/cm² respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K _1S=0.012 ergs/cm². Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.     

Perpendicular interlayer coupling in Ni80Fe20/NiO/Co trilayers

An in-plane perpendicular magnetic coupling between Ni80Fe20 and Co has been found in NiFe/NiO/Co trilayers for a NiO thickness ranging from 4 to 25 nm by magneto-optical Kerr effect and x-ray magnetic circular dichroism measurements. In the easy magnetization direction of the Co layer, the Co coercive field H(C) increases when the thickness of the NiO layer t(NiO) increases. Because of the coupling, H(C) is always larger than for NiO/Co bilayers with the same thicknesses. The saturation field of the NiFe layer H(S) decreases when t(NiO) increases, indicating a weakening of the coupling. Numerical simulations show that the presence of interface roughness combined with a small value of the NiO anisotropy can explain the observed 90 degrees coupling.     

Chemisorption of Fe on Au-passivated Si（001） surface

The chemisorption of one monolayer of Fe atoms on a Au-passivated Si(001) surface is studied by using the self-consistent tight-binding linear muffin-tin orbital method. The Fe adatom chemisorption on an ideal Si(001) surface is also considered for comparison. The chemisorption energy and layer projected density of states for a monolayer of Fe atoms on Au-passivated Si(001) surface are calculated and compared with that of the Fe atoms on an ideal Si(001) surface. The charge transfer is investigated. It is found that the most stable position is at the fourfold hollow site for the adsorbed Fe atoms, which might sit below the Au surface. Therefore there will be a Au－Fe mixed layer at the Fe/Au－Si(100) interface. It is found that the adsorbed Fe atoms cannot sit below the Si surface, indicating that a buffer layer of Au atoms may hinder the intermixing of Fe atoms and Si atoms at the Fe/Au－Si(001) interface effectively, which is in agreement with the experimental results.     

Complex angular dependence of exchange bias on (001) epitaxial NiO-Co bilayers

We have sputter-deposited NiO-Co bilayers on MgO(001) substrates. NiO and Co grow epitaxially on MgO and reproduce its fcc structure. The high quality of our samples, in terms of flatness and crystallographic coherence of the interface, allows the observation of an additional fourfold magnetic anisotropy term by standard magnetometry. This term is induced by interfacial interaction assigned to the same origin as exchange bias. Additional measurements of exchange bias azimuthal dependence versus the crystallographic axes of the film plane reveal unusual behaviors with several sign changes related to this fourfold anisotropy.     

Structural properties of high-quality sputtered Fe films on Al2O3(1120) and MgO(001) substrates

High-quality thin Fe films were deposited on MgO(001) and Al₂O₃(1120) substrates in the thickness range from 7 to 50 nm. The structural properties have been studied by out-of-plane and in-plane X-ray scattering experiments. From the out-of-plane measurements the electron density profile was determined together with interface and surface roughness parameters. Fe on Al₂O₃ grows along the [110]-direction with a structural coherence length comprising about the total film thick ness and a very small mosaicity. From in-plane scattering experiments a three-domain structure was observed. On MgO(001) substrates Fe grows in the [001]-direction, with the Fe [100]-axis parallel to the MgO [110]-axis. On both substrates, the Fe films exhibit a very small surface and interface roughness, indicative for a high quality of the sputtered samples.     

Magnetic properties and domain structure of epitaxial (001) Fe/Pd superlattices

The magnetic properties of well-characterized (001)Fe/Pd superlattices prepared by molecular beam epitaxy are presented. The saturation magnetization is enhanced due to the polarization of the Pd interface, and analysis of hysteresis loops indicate low coercive fields, abrupt magnetic reversals, and ferromagnetic coupling between the Fe layers for all Pd thickness investigated (10–50 Å). It is also found that deposition on a stationary substrate can create a weak uniaxial in-plane anisotropy, which, for one of the two easy directions of Fe, causes a spontaneous rotation of the magnetization by 90° at low fields. This effect is clearly demonstrated by optical Kerr-effect imaging of the magnetic domain structure, and can be mistaken for antiferromagnetic coupling with very weak coupling fields. The strength of this uniaxial anisotropy is found to oscillate rapidly with Pd thickness, suggesting that it is very sensitive to the microstructure.     

Magnetic anisotropy of Fe films on GaAs(113)A substrates

We employ superconducting quantum interference device magnetometry to study the thickness dependence of in-plane and out-of-plane magnetic anisotropic properties of Fe films grown on high-index GaAs(113)A substrates by molecular beam epitaxy. The evolution of the in-plane magnetic anisotropy with film thickness is distinguished into two regions. First, for Fe film thicknesses ≤50 MLs, we observe an in-plane uniaxial magnetic anisotropy with the easy axis along the in-plane 〈332̄〉 axes. Second, for Fe film thicknesses ≥70 MLs, we observe a four-fold magnetic anisotropy with the easy axis along the in-plane 〈031̄〉 axes. The existence of an out-of-plane perpendicular magnetic anisotropy is also detected in ultrathin Fe films. Similar to Fe on GaAs(001), our results provide evidence for the interfacial origin of the in-plane uniaxial and out-of-plane perpendicular magnetic anisotropy. Both the uniaxial and the perpendicular interface anisotropy are found to be independent of the epitaxial orientation and are hence an intrinsic property of the Fe/GaAs interface. PACS 75.70.-i; 75.50.Bb; 81.15.Hi     

Field cooling-induced magnetic anisotropy in exchange biased CoO/Fe bilayer studied by ferromagneticresonance

Exchange-biased CoO/Fe bilayer grown on MgO (0 0 1) substrate by sputtering, studied by variable angle and temperature ferromagnetic resonance. Room temperature in-plane measurements reveal that the Fe layer was epitaxially grown on MgO substrate with a fourfold cubic symmetry. The data also show that the easy axis of magnetization is in the film plane and makes an angle of 45° with the [1 0 0] crystallographic direction of MgO substrate. The low temperature data exhibit a sudden onset of a field cooling-induced and shifted cubic anisotropy below the Néel temperature of CoO. This results in a twofold uniaxial or fourfold cubic symmetry for in-plane magnetic anisotropy depending on a field cooling direction. Low temperature measurements also present a reduction in the resonance fields due to the antiferromagnetic/ferromagnetic coupling. The developed theoretical model perfectly simulates the experimental data of coupled CoO/Fe bilayer.     

Fourfold in-plane magnetic anisotropy in epitaxial Fe(1 1 0)/Cu(0 0 1) and Fe(1 1 0)/Ni(0 0 1) bilayers

Epitaxial Fe(1 1 0) films with thicknesses of 100-800 nm on Cu(0 0 1) and Ni(0 0 1) buffer layers grown on MgO(0 0 1) substrates have been fabricated. These films contain Fe(1 1 0) crystallites which are in the Pitsch orientation relationship. Magnetization and the fourfold in-plane magnetic anisotropy constants of these films have been determined by torque measurements. All the samples under study are characterized by a fourfold magnetic anisotropy with easy axes parallel to the [1 0 0] and [0 1 0] directions of Cu(0 0 1) and Ni(0 0 1) layers. The measured values of the constant for Fe(1 1 0)/Cu(0 0 1) are found to depend on deposition temperature; a maximum value of (2.5±0.1)×10⁵ erg/cm³ is reached after annealing at 600 °С. The in-plane torque measurements on Fe(1 1 0)/Ni(0 0 1) bilayers obtained at 300 °С, on the other hand, exhibit a constant value of (2.7±0.1)×10⁵ erg/cm³. Assuming an exchange interaction between the Fe(1 1 0) crystallites, which are in the Pitsch orientation relationship, the fourfold in-plane magnetic anisotropy has been calculated as 2.8×10⁵ erg/cm³. The deviations of the experimental values from the predicted one may be explained by the formation of a polycrystalline phase within the Fe(1 1 0) layer and a partial disorientation of the epitaxial crystallites.     

超薄Fe层在反铁磁NiO(001)面上沉积的研究

通过分子束外延(MBE)和脉冲激光沉积(PLD)方法，将1—10个Fe原子层(ML)以楔形方式沉积到反铁磁单晶NiO(001)基片上.表面磁光克尔效应的原位测试结果表明：通过MBE沉积的Fe原子层在Fe/NiO界面处产生了约2ML的磁死层；而通过PLD沉积的Fe原子层在Fe/NiO界面处产生了约3ML的磁死层.X射线光电子能谱对Fe/NiO界面进行研究的结果表明，在Fe原子与单晶NiO间发生了界面化学反应. 关键词： 磁性薄膜 表面磁性 X射线光电子能谱     

Ultrafast transport of laser-excited spin-polarized carriers in Au/Fe/MgO(001)

Hot carrier-induced spin dynamics is analyzed in epitaxial Au/Fe/MgO(001) by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au.     

Uniaxial magnetocrystalline anisotropy of metal/semiconductor interfaces: Fe/ZnSe(001)

A theoretical study of the magnetic moments and the in-plane magnetic anisotropy of an interface between a cubic ferromagnet and a cubic semiconductor, Fe/ZnSe(001), is presented. Theory confirms the observed, much debated, uniaxial anisotropy of the iron film. This result is important since the calculations are for perfect interfaces with squarelike environments, proving that the fourfolded symmetry of the interface Fe atoms is broken beyond the nearest neighboring semiconducting layer, effects that are usually assumed small. It is demonstrated how the uniaxial anisotropy is produced by the directional covalent bonds at the interface, even without atomic relaxations.     

Interplay between anisotropic strain relaxation and uniaxial interface magnetic anisotropy in epitaxial Fe films on (001) GaAs

Grazing incidence x-ray diffraction study of Fe epitaxial ultrathin films (1.5-13 nm) on GaAs (001) reveals an anisotropy of both domain shape and strain, with [110] and [1-10] as the principal directions. It is shown that the observed thickness-dependent strain anisotropy, together with a uniaxial interface term, can provide an unambiguous explanation to the usual in-plane magnetic anisotropy and its thickness dependence observed in this magnetic thin-film system.     

Magnetic properties of Fe/ZnSe/Fe trilayers

The magnetic properties of Fe/Zn/Fe trilayers have been studied by ferromagnetic resonance and magnetization measurements. These measurements have been used to investigate the magnetic anisotropy of the iron layers and the magnetic coupling across the semiconductor spacer. The angular dependence of the resonance spectra has been measured in-plane and out-of-plane in order to deduce magnetic anisotropy constants of the samples. Experimental data were fitted by using an energy-density expression that includes bulk cubic anisotropy, growth-induced uniaxial in-plane anisotropy and perpendicular-surface anisotropy terms. A small ferromagnetic coupling is observed in the trilayers with spacer thickness up to .     

Microscopic imaging of Fe magnetic domains exchange coupled with those in a NiO(001) surface

We revealed the micromagnetic structure of an Fe thin film exchange interacting with the spins of a fully compensated (001) surface of antiferromagnetic NiO. The interface exchange interaction causes the Fe domains to follow the NiO domains. The Fe spin polarization is in plane and the spin polarization in each domain is roughly perpendicular to an easy-spin axis of the NiO. These results agree with numerically calculated spin directions. Our numerical results also show that the NiO spins at the interface cant from the easy-spin axis towards the Fe spin because of exchange coupling.     

Isolating the step contribution to the uniaxial magnetic anisotropy in nanostructured Fe/Ag(001) films

We have investigated the possibility of isolating the step-induced in-plane uniaxial magnetic anisotropy in Fe/Ag(001) films on which nanoscale surface ripples were fabricated by the ion sculpting technique. For rippled Fe films deposited on flat Ag(001), the steps created along the ripple sidewalls are shown to be the only source of uniaxial anisotropy. Ion sculpting of ultrathin magnetic films allows one to selectively study the step-induced anisotropy and to investigate the correlation between local atomic environment and magnetic properties.     

Iron oxidation, interfacial expansion, and buckling at the Fe/NiO(001) interface

In order to provide a structural basis for a physical understanding of exchange bias in metal/magnetic-oxide interfaces, we have determined the structure of the Fe/NiO(001) interface by means of x-ray absorption spectroscopy and ab initio density functional theory calculations. A Fe-Ni alloyed phase on top of an interfacial FeO planar layer is formed. The FeO layer exhibits a 7% expanded interlayer distance and a 0.3 A buckling; its presence is predicted to increase the spin magnetic moment of the interface Fe atoms by 0.6 mu(B), compared to the ideally abrupt interface.     

Interface coupling transition in a thin epitaxial antiferromagnetic film interacting with a ferromagnetic substrate

We report experimental evidence for a transition in the interface coupling between an antiferromagnetic film and a ferromagnetic substrate. The transition is observed in a thin epitaxial NiO film grown on top of Fe(001) as the film thickness is increased. Photoemission electron microscopy excited with linearly polarized x rays shows that the NiO film is antiferromagnetic at room temperature with in-plane uniaxial magnetic anisotropy. The anisotropy axis is perpendicular to the Fe substrate magnetization when the NiO thickness is less than about 15 A, but rapidly becomes parallel to the Fe magnetization for a NiO coverage higher than 25 A.     

Magnetic anisotropy in (, .44, .55, .67, and .78) alloy film grown on and MgO(001) by molecular beam epitaxy

Fe_xPd_1−x (x=.30, .44, .55, .67, and .78) films were directly grown on SrTiO₃(001) and MgO(001) by molecular beam epitaxy at 500 C. The thickness of all films is 50 nm. X-ray diffraction shows epitaxial quality and face-center- tetragonal (00l), (002), and (003) peaks indicating an FePd L1₀ order state for films of x=.30, .44 and .55. X-ray diffraction only shows (002) peaks with a relatively weak intensity for the film of x=.67 and no (00l) peak is observed, but a broad body-center-cubic Fe(002) is identified for the film of x=.78. Magnetic hysteresis curves are carried out by a vibrating sample magnetometer (VSM) with an applied field within 12 kOe. Magnetization of both in-plane and perpendicular-to- the-plane measurements show a linear increase of the magnetization saturation from 560 emu/cm³ to 1250 emu/cm³ as x increasing from .30 to .78. For the results of the in-plane measurements, remanence (M_r), however, shows a minimum while the anisotropy field (H_k) shows a maximum for the film with x=.44 indicating the optimal content ratio of Fe/Pd for perpendicular anisotropy in the present alloy films. Further, negative remanence is observed in the hysteresis curves where the field is perpendicular to the film of x=.78 This may indicate that the L1₀order state still affects the magnetic anisotropy for high Fe content films even though the film has a body-center-cubic structure.