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.     

Coupled perpendicular magnetization in Fe/Cu/Fe trilayers

Ultrathin epitaxial Fe films on Cu(1 0 0) with perpendicular magnetization have been used as templates for the preparation of FCC Fe/Cu/Fe trilayers. The magnetic anisotropy and the coupling of these films have been studied by in-situ magneto optical Kerr effect measurements and Kerr microscopy. The magnetic coupling of both Fe layers is found to be dominated by magnetostatic interaction. Adsorbate-induced spin reorientation in the top layer also causes spin reorientation in the bottom layer. The governing role of the Fe-vacuum interface for the magnetism of the whole trilayer is demonstrated.     

Microwave switching behaviors of Fe/Ag/Fe/Ag epitaxial films

In this study, Fe/Ag/Fe/Ag ferromagnetic structures were epitaxially grown on a GaAs(1 0 0) substrate. Using the Network Analyzer (Agilent 8510C), FMR (ferromagnetic resonant) signals were observed in samples as the in-plane magnetic field was applied to either hard- or easy-axes. Of interest was that our sample demonstrated a FMR-switching behavior in hard-axis but not in easy-axis. The detecting data showed that the switch magnetic field was much smaller than what has been stated previously in Fe/Cu/Fe systems by other laboratories. In addition to the frequency switch in FMR, we also observed a magnetic reversal behavior in its M–H curve. Data presented that both of the FMR and magnetic reverse took place in the same direction (hard-axis).     

Anisotropic pinned/biased magnetization in SrRuO3/SrMnO3 superlattices

The exchange coupling at the interfaces of magnetic superlattices consisting of ferromagnetic SrRuO3 and antiferromagnetic SrMnO3 grown on (001) oriented SrTiO3 is studied with in-plane and out-of-plane orientations of the cooling magnetic field, with respect to the substrate plane. The magnetization of the in-plane, field cooled hysteresis loop is lower than the corresponding in-plane zero-field-cooled hysteresis loop. The out-of-plane field cooled hysteresis loop is shifted, from the origin, along the graphical magnetization axis. We attribute this irreversible rotation of the moment to the pinning/biasing of spin in the SrRuO3 layer in the vicinity of interfaces by the antiferromagnetic SrMnO3 layer.     

Magnetism and structure of hexagonal Fe in Fe/Ru superlattices

Mössbauer spectroscopy is applied to study the magnetism and the local structure of Fe/Ru superlattices. Fe Layers show hcp structure, isomorphous with Ru. A well-defined symmetry is observed at Fe sites, withV _zz along thec axis. The Fe/Ru interfaces are non-magnetic over the two first layers, but a large moment (2μ_B/Fe) exists beyond. Magnetic coupling between neighbouring Fe Layers is observed at small spacing, i.e. for Ru thickness lower than 13 Å typically.     

Radiofrequency magnetoresistance of Fe/Cr superlattices

The rf magnetoresistance of Fe/Cr superlattices is studied for two orientations of the current: parallel and across the superlattice layers. A mutually single-valued correspondence is established between the relative magnetoresistance measured at dc current and the change in the transmission coefficient of electromagnetic waves in the magnetic field. When rf currents flow across the layers, the relative change in the signal amplitude is proportional to twice the change in the electrical resistance of the superlattice and is of opposite sign. It is shown that the rf losses are determined by the surface resistance which is proportional to the superlattice thickness and inversely proportional to its conductivity. An equation is derived for the rf electric field distribution in the superlattice. It is established that when the thickness of the superlattice is small compared with the skin layer depth, field and current components which penetrate through the entire superlattice exist.     

Superconducting properties of V/Fe superlattices

We report measurements on the superconducting properties of V/Fe superlattices with various layer thicknesses. These samples were prepared with a novel UHV evaporator which can produce up to twenty different samples in the same run. The Fe layer, a strong pair breaker, suppresses the superconducting transition temperature in a systematic way. When the V layer thickness is on the order of the BCS coherence length and the Fe layer is only a few atomic planes thick, a 2D–3D crossover has been observed in the temperature dependence of the parallel upper critical field H_C2∥. This implies the coexistence of superconductivity and ferromagnetismm. We observe three dimensional behavior for thinner Fe layers (～1 atomic plane) and two dimensional behavior for thicker Fe layers (greater than 10 atomic planes).     

Magnetism in Fe/V superlattices and alloys

The first principles discrete variational X_α method was used to calculate the electronic structures, magnetic moments and the conduction electron contact spin densites at a central V site in a number of 15 atom clusters representing [110]-interfaces in Fe/V superlattices and alloys. It was found that interactions between the spin-up and spin-down 3d electrons behave differently as the number of Fe atoms, P, in the first coordination shell of a central V atom changes. The 3d magnetic moments at the V sites were found to vary strongly with P, and their variation with respect to changes in the number of Fe atoms in the second shell, Q, was found to depend on P. The sign of 4s and 4p moments was found to change from positive to negative as the average 3d magnetic moment per atom at the first shell sites goes from negative to positive. The conduction electron contact spin densities follow the pattern of the 4s moment as P varies. The results of previously calculated Fe-centered and the present V-centered clusters are combined to study the average magnetic moment in iron-vanadium superlattices and alloys.     

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.     

Magnetization temperature dependence in Fe/Cr superlattices

We report the temperature dependence of the saturation magnetization, M_s(T), of Fe(t)/Cr(40 Å) superlattices grown on MgO(1 0 0) and MgO(1 1 0) and its variation with Fe-layer thickness (5 Å<t<80 Å). The structure was characterized by X-ray diffraction. M_s(t) vs. T, as measured by DC magnetization and ferromagnetic resonance, show a large interface effect. The origins of this effect are discussed.     

Magnetic anisotropy and magnetization dynamics of Fe nanoparticles embedded in Cr and Ag matrices

Static and dynamical magnetic properties of Fe nanoparticles (NPs) embedded in non-magnetic (Ag) and antiferromagnetic (Cr) matrices with a volume filling fraction (VFF) of 10% have been investigated. In both Fe@Ag and Fe@Cr nanocomposites, the Fe NPs have a narrow size distribution, with a mean particle diameter around 2 nm. In both samples, the saturation magnetization reaches that of Fe bulk bcc, suggesting the absence of alloying with the matrices. The coercivity at 5 K is much larger in Fe@Cr than in Fe@Ag as a result of the strong interaction between the Fe NPs and the Cr matrix. Temperature-dependent magnetization and ac-susceptibility measurements point out further evidence of the enhanced interparticle interaction in the Fe@Cr system. While the behaviour of Fe@Ag indicates the presence of weakly interacting magnetic monodomain particles with a wide distribution of blocking temperatures, Fe@Cr behaves like a superspin glass produced by the magnetic interactions between NPs.     

Size effect on magnetism of Fe thin films in Fe/Ir superlattices

In ferromagnetic thin films, the Curie temperature variation with the thickness is always considered as continuous when the thickness is varied from n to n+1 atomic planes. We show that it is not the case for Fe in Fe/Ir superlattices. For an integer number of atomic planes, a unique magnetic transition is observed by susceptibility measurements, whereas two magnetic transitions are observed for fractional numbers of planes. This behavior is attributed to successive transitions of areas with n and n+1 atomic planes, for which the T(c)'s are not the same. Indeed, the magnetic correlation length is presumably shorter than the average size of the terraces. Monte Carlo simulations are performed to support this explanation.     

Step-induced frustration of antiferromagnetic order in Mn on Fe(001)

We studied the spin arrangement in ultrathin antiferromagnetic Mn films in contact with a ferromagnetic Fe(001) substrate using spin-polarized scanning tunneling microscopy. Mn shows a layerwise antiferromagnetic order on Fe(001). In regions where Mn overgrows Fe steps, a frustration of the antiferromagnetic order occurs which is similar to a 180 degrees domain wall. This topologically enforced frustration was studied as a function of Mn thickness. A linear increase of the width of the frustration region with the Mn thickness was found.     

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.