NiO-thickness dependent magnetic anisotropies in Fe/NiO/Au(001) and Fe/NiO/MgO(001) systems

The in-plane magnetic anisotropy of Fe/NiO bilayers was studied quantitatively as a function of NiO thickness using the magneto-optical Kerr effect with a rotating field. For NiO thicker than the ordering transition thickness, the total in-plane fourfold anisotropy of the Fe layer decreases with NiO thickness in Fe/NiO/Au(001), but increases in Fe/NiO/MgO(001). Our result indicates that the exchange coupling in an Fe/NiO bilayer might induce an additional in-plane fourfold anisotropy, and the opposite thickness dependent behaviors may be attributed to the different Ni²⁺ antiferromagnetic spin orientations for NiO films grown on Au(001) and MgO(001) surfaces.     

Perpendicular antiferromagnetic ordering of Mn and exchange anisotropy in Fe/Mn multilayers

Fe/Mn is a model system in which to study exchange bias, since the antiferromagnetic (AF) Mn layers are believed to have uncompensated moments with all spins aligned in the plane and parallel to those of the Fe. We have determined the microscopic AF ordering at the interfaces using single-crystal neutron diffraction. An unexpected magnetic structure is obtained, with out-of-plane Mn moments perpendicular to those of Fe. This explains the low bias field and shows that the simple AF ordering assumed in a variety of exchange-biased systems may well have to be revised.     

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.     

First-principle study on the electronic structure and magnetic properties in Fe/MgO/Fe magnetic tunnel junction with Mg insertion layer

The effect of a Mg insertion layer between the Fe electrode and the MgO barrier layer on the electronic structure and magnetic properties of Fe/MgO/Fe magnetic tunnel junction has been studied by first-principle method. Two models of (a) Fe(1 0 0)/MgO(1 0 0)/Fe(1 0 0) and (b) Fe(1 0 0)/Mg/MgO(1 0 0)/Mg/Fe(1 0 0) were established. Our calculation results show that the Mg insertion layer has enhanced both the spin polarization and the magnetic moment of its adjacent Fe layer. The results have been discussed in terms of the variation in the DOS features and charge transfer with the Mg insertion layer.     

Magnetic state and properties of the Fe0.5TiSe2 intercalation compound

The Fe_0.5TiSe₂ compound with a monoclinic crystal structure has been prepared by intercalation of Fe atoms between Se-Ti-Se sandwiches in the layered structure of TiSe₂. The crystal and magnetic structures, electrical resistivity, and magnetization of the Fe_0.5TiSe₂ compound have been investigated. According to the neutron diffraction data, the Fe_0.5TiSe₂ compound has a tilted antiferromagnetic structure at temperatures below the Néel temperature of 135 K, in which the magnetic moments of Fe atoms are antiferromagnetically ordered inside layers and located at an angle of approximately 74.4° with respect to the layer plane. The magnetic moment of Fe atoms is equal to (2.98 ± 0.05)μ_B. The antiferromagnetic ordering is accompanied by anisotropic spontaneous magnetostrictive distortions of the crystal lattice, which is associated with the spin-orbit interaction and the effect of the crystal field.     

First-order phase transitions in magnetization of Fe/Cr/Fe three-layer films

The possibility of appearing jumps in the magnetization curves of Fe/Cr multilayers has been predicted under the assumption that magnetic ordering in chromium interlayers has the form of a linearly polarized spin-density wave. This possibility has been analyz ed for an Fe/Cr/Fe three4ayer film with a conventional quality of Fe/Cr interfaces, which does not ensure a change in the ferromagnetic and antiferromagnetic orientations of the magnetizations of neighboring iron layers with a variation in the thickness of the chromium interlayer by one atomic layer (short oscillation period). The model used suggests that the wave vector of the spin-density wave is responsible for the experime ntally observed long period of these oscillations. Relationships have been derived for the range of thicknesses of chromium interlayers in which the appearance of the predicted effect can be expected, and the magnitude of the effect has been evaluated.     

Layer magnetization canting in 57Fe/FeSi multilayer observed by synchrotron Mössbauer reflectometry

Synchrotron Mössbauer reflectometry and CEMS results on a [⁵⁷Fe(2.55 nm)/FeSi\break(1.57 nm)]₁₀ multilayer (ML) on a Zerodur substrate are reported. CEMS spectra are satisfactorily fitted by α‐Fe and an interface layer of random α‐(Fe, Si) alloy of 20% of the ⁵⁷Fe layer thickness on both sides of the individual Fe layers. Kerr loops show a fully compensated AF magnetic layer structure. Prompt X‐ray reflectivity curves show the structural ML Bragg peak and Kiessig oscillations corresponding to a bilayer period and total film thickness of 4.12 and 41.2 nm, respectively. Grazing incidence nuclear resonant Θ–2Θ scans and time spectra (E = 14.413 keV, λ = 0.0860 nm) were recorded in different external magnetic fields (0 < B^ext < 0.95 T) perpendicular to the scattering plane. The time integral delayed nuclear Θ–2Θ scans reveal the magnetic ML period doubling. With increasing transversal external magnetic field, the antiferromagnetic ML Bragg peak disappears due to Fe layer magnetization canting, the extent of which is calculated from the fit of the time spectra and the Θ–2Θ scans using an optical approach. In a weak external field the Fe layer magnetization directions are neither parallel with nor perpendicular to the external field. We suggest that the interlayer coupling in [Fe/FeSi]₁₀ varies with the distance from the substrate and the ML consists of two magnetically distinct regions, being of ferromagnetic character near substrate and antiferromagnetic closer to the surface.     

Effect of ion irradiation on magnetic property of exchange coupled interfacial structures of Fe/NiO and NiO/Fe on Si substrates

The role of defects on the magnetic behaviour of exchange coupled interfacial structures of Fe/NiO and NiO/Fe on Si substrates has been studied. For introduction of defects in the structures, swift (～ 100 MeV) heavy ion irradiation has been used, which is known to cause structural and microstructural modifications. In our earlier study [Srivastava, N; Srivastava, P.C. J. Appl. Phys. 2012, 111, 123909] on similar structures, the significant magnetic behaviour (of exchange bias (EB) and coercivity) for Fe/NiO/nSi interfacial structure was observed and discussed in the realm of interfacial structural modification in the antiferromagnetic layer of the structure. The irradiated interfacial structures have been characterized from X-ray diffraction and M–H characteristics. Structural investigation has shown the formation of various silicide and oxide phases due to the irradiation-induced interfacial intermixing. A significant enhancement in EB field and coercivity has been observed for Fe/NiO/nSi interfacial structure on the irradiation (as compared to unirradiated ones). The observed enhanced EB and coercivity on the irradiation has been understood due to the creation of domain wall pinning centres across the interface as a result of ion irradiation. Moreover, the present study confirms the role of defects in the antiferromagnetic layer to cause the significant change in EB and coercivity. The observation supports the domain state model of EB in the exchange-coupled structures.     

Magnetic Properties of Fe/Ni and Fe/Co Multilayer Thin Films

In this work, the magnetic and transport properties of Fe/SiO₂/Ni and Fe/SiO₂/Co multilayers grown on Si/SiO₂ substrates have been studied. The samples have been prepared by two-stage deposition process. In the first stage, Fe layer and SiO₂ interlayer of both samples are grown by ion beam deposition technique at room temperature. Then the samples are taken out to ambient atmosphere and loaded into a pulse laser deposition (PLD) chamber. Prior to the deposition of top layer, the samples are cleaned by annealing at 150 °C. In the second stage, Ni (or Co) layer is prepared by PLD technique at room temperature. The thickness of deposited layers has been measured by Rutherford back scattering (RBS). Magnetic properties of ferromagnetic bilayers have been investigated by room-temperature ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques. Standard four-point magneto-transport measurements at various temperatures have been performed. Two-step switching in the in-plane hysteresis loops of Fe/SiO₂/Ni and Fe/SiO₂/Co samples is observed. A crossing in the middle of hysteresis loops of both samples points to a weak antiferromagnetic interaction between the magnetic layers of the stacks. Saturation magnetization values have been obtained from the VSM measurements of samples with DC magnetic field perpendicular to the films surface. Magneto-transport measurements have shown the predominant contribution of anisotropic magnetic resistance both at room and low temperatures. FMR studies of Fe/SiO₂/Ni and Fe/SiO₂/Co samples have revealed additional non-uniform (surface and bulk SWR) modes, which behavior has been explained in the framework of the surface inhomogeneity model. An origin of the antiferromagnetic interaction has been discussed.     

Polarized neutron reflectometry of Fe/Cr/Gd superlattices

The magnetic structure of Fe/Cr/Gd superlattices is investigated using complementary methods of SQUID magnetometry and polarized neutron reflectometry. The complex magnetic behavior of the given system is caused by exchange interaction between the 3d (Fe) and 4f (Gd) layers of the ferromagnetic metals through the Cr antiferromagnetic spacer layer. It is found that a nonuniform profile of magnetization forms within the Gd layers under the influence of this interlayer interaction.     

Structure and magnetic properties of magnetron-sputtered [(Fe/Pt/Fe)/Au]n multilayer films

Using dc magnetron sputtering, Fe/Pt/Au multilayer films were prepared, and the effects of Au layer thickness and annealing temperature on structure and magnetic properties of the Fe/Pt/Au multilayer films were investigated. The as-deposited Fe/Pt/Au multilayer films have good periodic structure with composition modulation along the growth direction. The stress stored in the as-deposited films promoted the ordering of the films annealed at 400 °C. When the films were annealed at 500 °C, the thicker Au layer could restrain the order-disorder transformation region volume and lead to the decrease of the ordered volume fraction with Au layer thickness increasing.     

Magnetization process of Fe layers in Fe/Nd multilayered films investigated by57Fe Mössbauer spectroscopy

The magnetization process of Fe and Nd layers at 5K in Fe/Nd multilayered films with strong perpendicular magnetic anisotropy is elucidated from a comparison of⁵⁷Fe Mössbauer spectra in the presence of the external field applied parallel to the film plane with total magnetization. At zero external field, the film has a magnetic multi-domain structure. The Nd layer moment is perpendicular to the film plane and the Fe layer moment points in the out-of-plane direction. The Fe layer moment monotonically rotates to the in-plane direction with increasing external field parallel to the film plane, while the Nd layer moment is oriented to the film normal direction up to the external field of 10kOe, above which the Nd layer moment gradually turns to the direction of the external field.     

Structure and magnetic properties of Fe/Fe oxide clusters

Fe clusters have been synthesised in ultra-high-vacuum chamber using a gas-stabilized cluster aggregation method that ensures good control of the cluster size and naturally oxidized in order to obtain Fe/Fe oxide core-shell nanoparticles. The morphology of an individual nanoparticle, as revealed by transmission electron microscopy, consists of a Fe core of an average diameter of 4.4 nm surrounded by an oxide shell of uniform thickness of about 1.2 nm in average. The nanoparticles may be assimilated with a ferro-/antiferromagnetic (FM/AF) system. The morpho-structural features have been correlated with magnetic measurements on the core-shell nanoparticles. A significant exchange bias effect has been measured, when the sample was field-cooled under an applied field of 3 T. As the morphology of core-shell nanoclusters is much more complicated than in FM/AF bilayers of regular thickness due to the particular geometry of the coronal AF layer, the shape and surface anisotropy have to be taken into account for a correct interpretation of the magnetic data.     

Interface-induced states with an incommensurate spin-density wave in Fe/Cr-type multilayers

A model is proposed for magnetic ordering in Fe/Cr-type multilayers substantially above the Néel temperature of bulk chromium. Redistribution of the charge (and, hence, spin) density near the Fe/Cr interfaces gives rise to the formation of an essentially inhomogeneous spin-density-wave (SDW) state in the chromium spacer. The spatial structure of the antiferromagnetic order parameter in thick spacers is described. The SDW contribution to the effective exchange coupling between the moments in adjacent iron layers is calculated. The data obtained are used in the interpretation of experimental data on the tunneling spectroscopy of trilayers and neutron diffraction from Fe/Cr superlattices.     

Oxygen-induced symmetrization and structural coherency in Fe/MgO/Fe(001) magnetic tunnel junctions

We present x-ray diffraction experiments and multiple-scattering calculations on the structure and transport properties of a Fe/MgO/Fe(001) magnetic tunnel junction (MTJ). Coherent growth of the top Fe electrode on the MgO spacer is observed only for Fe deposition in ambient oxygen atmosphere leading to a coherent and symmetric MTJ structure characterized by FeO layers at both interfaces. This goes in parallel with calculations indicating large positive tunnel magnetoresistance (TMR) values in such symmetric junctions. The results have important implications for achieving giant TMR values.     

Magnetic and magnetooptical properties of Fe/Pt and Fe/Pt/Fe thin-film magnetic structures

Results of a study of magnetic and magnetooptical properties of Fe/Pt double-layer and Fe/Pt/Fe three-layer thin-film magnetic structures are presented. A strong effect of the Pt layer on magnetic properties of the studied samples was revealed. It was established that the saturation field of three-layer magnetic structures has an oscillating magnitude with varying Pt layer thickness, and the oscillation period is a function of the Fe layer thickness. The data obtained are explained by the presence of exchange interaction between the Fe layers via the Pt layer. A strong effect of Pt on spectral dependences of the equatorial Kerr effect in the thin-film structures under study is revealed.     

Fe/Si多层膜的层间耦合与界面扩散

对以本征Si及重掺杂ｐ型和ｎ型Si作为中间层的Fe/Si多层膜的层间耦合进行研究，并通过退火，增大Fe，Si之间的扩散，分析界面扩散对层间耦合的影响. 实验结果表明，层状结构良好的制备态的多层膜，Fe，Si之间也存在一定程度的扩散，它是影响层间耦合的 主要因素，远远超过了半导体意义上的重掺杂，使不同种类的Si作为中间层的层间耦合基本 一致.进一步还发现，在一定范围内增大Fe，Si之间的扩散，即使多层膜的层状结构已经有了相当的退化，Fe/Si多层膜的反铁磁耦合强度基本保持不变. 关键词： Fe/Si多层膜 层间耦合 界面扩散     

Noncollinear magnetic states in iron-chromium-type multilayers

A model of noncollinear magnetic ordering in Fe/Cr-type multistructures was suggested. The model was based on the idea of charge (and, as a consequence, spin) density redistribution near a metal-metal interface. A peculiar state of the whole structure characterized by strong short-range antiferromagnetic ordering in the interlayer and a pronounced dependence of magnetic characteristics on the properties of the boundary between iron and chromium layers was shown to be formed in a certain temperature range. Inhomogeneous antiferromagnetic structures with a vector order parameter were found, and the effective exchange coupling between neighboring iron layer moments was calculated using the Ginzburg-Landau expansion of the thermodynamic potential. The results were used to discuss the experimental data on Fe/Cr superlattices obtained in neutron scattering and magnetization measurements.     

The fragile magnetic structures of Fe/CeH2−δ multilayers

Fe/CeH_2−δ multilayers exhibit at room temperature evidence of interlayer exchange coupling. Subsequent Fe layers are either parallel or antiparallel to each other, depending on the Fe and CeH_2−δ layer thickness. However, when both layers have thickness larger than 15 Å, the antiferromagnetic structure becomes fragmented into domains laterally limited to a few microns, and the magnetic structures become very fragile. Small magnetic fields of a few Oersteds acting on the samples during growth induce helimagnetic configurations which coexist with antiferromagnetic coupling. The magnetic structures can be permanently destroyed by applying magnetic fields larger than 150 Oe.     

Ab initio calculations of interfacial magnetism in Fe/Mo superlattices

Ab initio calculations have been performed on Fe/Mo(1 0 0) superlattices in order to study the interfacial magnetic properties and layer thickness effect on the magnetic moments. In most cases, the magnetic moments of interfacial Fe monolayers are always smaller than those of the inner layers, and the induced magnetic moments of interfacial Mo monolayers oriented in the opposite direction. Calculation results show that the Fe layers are ferromagnetic when n = 3. As the thickness of the Mo layers increases, the influence of the Mo layer increases and the magnetic state of the Fe layer gradually changes into an antiferromagnetic or non-magnetic state. The change of magnetic moments of Fe/Mo superlattices is in agreement with the experimentally observed oscillation periods.