Structural and magnetic properties of La/Fe multilayers

The structural and magnetic properties of La/Fe multilayers were investigated by X-ray diffraction, RHEED, magnetometry and⁵⁷Fe Mössbauer spectroscopy. Comparison is made with previous results obtained for Ce/Fe multilayers. Remarkably sharp interfaces are found, with roughness between 2 and 2.5 Å. The magnetic interface in the Fe sublayers resulting from the distribution of magnetic hyperfine fields distinctly exceeds the extension of the structural interface and points to a magnetic proximity effect. This is discussed in relation to a strong 3d-5d hybridization recently found in measurements of magnetic circular X-ray dichroism. Both the structural and magnetic La/Fe interface is less extended than the interface in Ce/Fe multilayers. Below a thickness of about 25 Å, the individual Fe layers grow in an amorphous structure on the La layers. In this case, Curie temperatures are below 200 K and the Fe-layer saturation magnetization is reduced up to 50%, and there is evidence of a non-collinear spin structure. It is argued that this mainly reflects the properties of pure amorphous Fe.     

Magnetic and structural properties of Fe/Cu multilayers

Fe/Cu multilayers with Fe and Cu layers of equal thicknesses were grown by high-vacuum evaporation on Si(1 1 1) substrates at room temperature. The crystal orientation, the thickness of the elemental layers and the interplanar distances were analysed by both low- and high-angle X-ray diffraction in the θ–2θ configuration. The magnetic properties of Fe/Cu multilayers were studied by both a static and a dynamic technique, namely surface magneto-optical Kerr effect (SMOKE) and Brillouin light scattering (BLS). Longitudinal SMOKE cycles permitted us to determine the orientation of the easy axis of the magnetization and to put in evidence an appreciable in-plane magnetic anisotropy in multilayers with low periodicity and highly coherent structure. Polar loops were then used to determine the out-of-plane anisotropy fields, showing that both first- and second-order contributions are to be considered in order to reproduce the hysteresis cycle. BLS was then exploited to detect thermaly excited spin waves through inelastic scattering of light. The out-of-plane anisotropy fields evaluated by this high-frequency dynamic technique compare fairly well with the first-order values obtained by analysis of polar SMOKE hysteresis cycles.     

Ce 5d and Fe 3d magnetic profiles in CeH2/Fe multilayers probed by XRMS

The element and electronic shell specificities of X-ray resonant magnetic scattering have been used to investigate the magnetization of Ce 5d and Fe 3d states in [CeH₂(19.6 ?)/Fe(25.4 ?)]*38 a multilayer. We show that the measurement of the magnetic contribution to the intensities reflected at low angles at the Ce L₂ and Fe L_2,3 edges allows us to investigate the profile of the Ce 5d and Fe 3d magnetic polarization. The Fe 3d polarization is found to be uniform across the Fe layer and the Ce 5d polarization appears to be restricted close to the interface with Fe. Received: 22 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Hydrogen-controlled interlayer exchange coupling in Fe/LaHx multilayers

Magneto-optic Kerr magnetometry and neutron reflectometry reveal that Fe layers exhibit magnetic exchange coupling through LaH_x spacer layers. Ferromagnetic and antiferromagnetic coupling is observed on multilayers of these materials depending on the thickness of the hydride layers, but without oscillatory behavior. Starting from metallic La dihydride spacer layers the effect of dissolving increasingly more hydrogen was examined. Sign and value of the coupling depend crucially on the hydrogen content x. The coupling can be inverted from antiferromagnetic to ferromagnetic and vice versa. These alterations are due to modifications of the electronic structure of the hydride. When the hydrogen absorption saturates the hydride layers become insulating and the exchange coupling is likely to disappear. In this final state the multilayers are always characterized by a very soft ferromagnetic rectangular hysteresis curve. Upon removal of the hydrogen to the initial concentration the original magnetic structure is restored.     

Magnetic properties of Co nanoparticle granular films capped with Pt

Magnetization and susceptibility measurements have been performed on Co granular (Al₂O₃/Co/Pt)₂₅ multilayers. A cusp-like anomaly is present in the susceptibility, which together with magnetization measurements suggests that the samples behave as an amorphous Correlated-Super-Spin system. The presence of CoPt partial alloying is proven by the XANES spectra. The XMCD spectra show that the interfacial Pt atoms become magnetically polarized by hybridization of the Pt 5d and Co 3d electrons, and are ferromagnetically coupled to the Co particle moments. The Pt layer is effective in transmitting interparticle exchange coupling.     

Application of magnetic Compton scattering for spin‐specific magnetic hysteresis measurement

An application of magnetic Compton scattering as a new tool to measure a spin‐specific magnetic hysteresis (SSMH) loop is introduced and its validity demonstrated. The applied magnetic field dependence of the integrated intensity of magnetic Compton scattering spectra, which reflect only the spin‐dependent magnetic properties of magnetically active electrons, was interpreted as the spin‐specific hysteresis. The spin magnetization of amorphous Tb₃₃Co₆₇ film was observed and its SSMH loop exhibited qualitative agreement with the ordinal magnetic hysteresis loop measured using a conventional vibrating sample magnetometer.     

Magnetic anisotropy in Fe/rare‐earth multilayers

Magnetic anisotropy of Fe/RE multilayers (RE=Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) was studied using ⁵⁷Fe Mössbauer spectroscopy. Perpendicular magnetic anisotropy was observed in Fe/Pr, Fe/Nd, Fe/Tb, and Fe/Dy multilayers. The external field dependence of the direction of magnetic moments was also examined for Fe/Tb multilayers. The results imply that the perpendicular magnetic anisotropy originates from the single ionic anisotropy of RE at the interfaces.     

Effect of diffusion and stresses on the magnetic properties of multilayer Fe/Pd and Fe/Ge films

Multilayer [Fe/Pd]₁₀ and [Fe/Ge]₁₀ films were studied using ferromagnetic resonance, magnetic-force and atomic-force microscopies, magneto-optical Kerr effect, and X-ray diffraction. It was shown that an increase in the interlayer thickness causes changes in magnetic parameters, such as the induced anisotropy, effective magnetization, coercive force, and the parameters of the fine magnetic structure. Reasons of the changes in the magnetic parameters were established.     

Hydrogen-induced changes of the structural and magnetic properties of cerium/iron multilayers

Multilayers of Ce and Fe show a low Curie temperature and saturation magnetization below a critical thickness of the individual Fe layers where amorphous growth occurs. We have studied on a series of such multilayers with different modulation lengths prepared by ion-beam sputtering the impact of hydrogen absorption on their magnetic properties. Hydrogenation was performed during film growth either reactively by introducing hydrogen gas into the UHV chamber or by irradiation with a beam of low-energy hydrogen ions. Hydrogen is absorbed only in the Ce layers, with a concentration near CeH₂. For suitable modulation lengths, the Curie temperature and saturation magnetization are considerably enhanced with respect to the non-hydrided multilayers. This is correlated with the changes in structure and the quality of the interfaces induced by hydrogenation: the irradiation process itself reduces the critical thickness for amorphous growth of Fe, and the chemical interaction of hydrogen causes a considerable sharpening of the interfaces.     

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.     

Enhanced multiferroic properties in La and Ce co-doped BiFeO3 nanoparticles

Room temperature multiferroic properties of BiFeO₃ (BFO), Bi_0.9La_0.1FeO₃ ((La)BFO) and Bi_0.9La_0.075Ce_0.025FeO₃ ((La,Ce)BFO) nanoparticles have been reported in this paper. XRD (X-ray diffraction) analyses of the nanoparticles show a decrease in the lattice constants and cell volume with the substitution of La and Ce. It is evident from the SEM (scanning electron microscope) micrographs that the (La,Ce) co-doped sample possesses dense microstructure made of smaller particles. Raman study accounts for the weakening of the strong hybridization between Bi-O by the substitution of La and Ce ions. This is also accompanied by an increase in the remanent magnetization, dielectric constant, and ferroelectric polarization. BFO nanoparticles show exchange bias effect under an applied magnetic field while the (La)BFO and (La,Ce)BFO samples show no trace of such effect. Ac-conductivity of (La,Ce) co-doped sample is observed to be several orders lesser in magnitude than bulk BFO ceramics. These results are interpreted by means of the subtle change in the structure, suppression of the spin cycloid and reduction of oxygen vacancies in the doped samples.     

Spin reorientation transition in Fe/CeH2 multilayers probed by soft X-ray resonant magnetic scattering

The magnetic domain configurations of Fe 3d spins in Fe/CeH₂ multilayers were measured by soft X-ray resonant magnetic scattering. The interface region could be probed by setting up X-ray standing waves due to the multilayer periodicity. By resolving first- and second-order magnetic scattering contributions, we show that the latter probe directly the magneto-crystalline anisotropy which is dominated by the Fe interface layers causing a spin reorientation transition when the temperature is lowered. Received: 30 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Novel magnetization processes in ferrimagnetic multilayers with random anisotropy

We carried out a numerical study of magnetization processes in ferrimagnetic multilayers in the presence of pinning forces acting on one of the magnetic components. The multilayers consist of an alternation of two ferromagnetic materials (M and T) which are assumed antiferromagnetically coupled through the interfaces (as in Fe/Tb multilayers for instance). A random anisotropy field acts on the magnetization of material T. The reversal of the magnetization of the multilayers when the applied field is swept from positive saturation to negative saturation is characterized by a competition between coherent rotation and nucleation-propagation mechanisms. In this competition the rotational hysteresis induced by the random anisotropy on the magnetization of material T plays a crucial role. Very original features have been observed in the magnetization processes of these systems such as crossed hysteresis loops with a negative remanent magnetization. An overview of these magnetization processes below and above the compensation temperature is given in this paper.     

Magnetic properties and magnetic entropy changes of MRE_2Co_7 compounds

(La0.5Ce0.5)2Co7 and(Ce0.65Pr0.35)2Co7 compounds for magnetic refrigeration were studied by X-ray diffraction, ac susceptibility and isothermal magnetization measurements. X-ray powder diffraction shows that all the compounds have hexagonal Ce2Ni7-type structure. The Curie temperatures(TC) are 258 K and 222 K for(La0.5Ce0.5)2Co7 and(Ce0.65Pr0.35)2Co7 compounds, respectively. High coercivities(HC) of about 1.74 and 6.61 k Oe at 5 K with a smooth demagnetization curves were obtained for the(La0.5Ce0.5)2Co7 and(Ce0.65Pr0.35)2Co7 compounds, respectively. For an applied field change from 0 to 50 k Oe, the maximum(??SM) for(La0.5Ce0.5)2Co7 and(Ce0.65Pr0.35)2Co7 compounds are 0.52 and 0.67 J/(kg K), respectively.     

垂直取向FePt/Ag纳米颗粒薄膜的结构和磁性

用磁控溅射法在单晶MgO(100)基片上制备了［FePt 2 nm/Ag dnm］₁₀多层膜， 经真空热处理后，得到具有高矫顽力的垂直取向L1₀-FePt/Ag颗粒膜.x射线衍射结 果表明，在250 ℃的热基片上溅射，当Ag层厚度d=3—11 nm时，FePt颗粒具有很好的［001］取向，随着Ag层厚度的增加，FePt颗粒尺寸减小.［FePt 2 nm/Ag 9 nm］₁₀经过6 00 ℃真空热处理15 min后，颗粒大小仅约8 nm，垂直矫顽力达到692 kA/m.这种无磁耦合作用的颗粒膜，适合用作超高密度的垂直磁记录介质. 关键词： 磁控溅射 垂直磁记录 纳米颗粒膜 0-FePt/Ag')" href="#">L1₀-FePt/Ag     

Magnetic properties of ternary rare-earth compounds of the type R2Fe14B

Ternary tetragonal compounds of the composition R₂Fe₁₄B were observed for R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. The lattice constants and the X-ray density of these compounds were determined. Also determined were the magnetic properties, comprising the temperature dependence of the magnetization in the range 4.2–700 K and the field dependence of the magnetization at 4.2 K in fields up to 20 T. These latter measurements were made in two mutually perpendicular directions, making it possible to determine the anisotropy fields. The magnetocrystalline anisotropy was found to consist of contributions due to the Fe and rare-earth sublattice, respectively.     

非晶TbCo/Si多层膜的磁性和磁光特性

利用射频磁控溅射方法制备了非晶TbCo/Si多层膜,并对多层膜的磁性和磁光特性进行了测量.实验发现,随着Si层厚度的增加,非晶TbCo/Si多层膜的饱和磁化强度M_s、垂直各向异性常数K_u、磁光克尔角θ_K都显著下降.分析认为这是由于在TbCo层与Si层之间的层间互扩散形成了非磁性的Co₂Si所致. 关键词：     

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.     

Dynamical properties of magnetization reversal in an ultrathin AuCo film

We present a dynamical study of hysteresis loops of a MoS₂/[Au/Co/Au] sandwich performed by surface magneto-optical Kerr effect with a field variation rate up to 1.2 MOe/s. An interpretation of dynamical effects at room temperature is proposed, using a modelization of the magnetization reversal. We discuss simulations which describe two different processes of the magnetization reversal to interpret the evolution of the hysteresis loops for several rates of variation of the magnetic field. For a first range of field variation rates lower than 180 kOe/s, the predominant mechanism seems to be wall motion and beyond 180 kOe/s, an expression for the magnetization is given, which supposes micro-domains reversal as a prevailing process. Finally, the general behaviour of the relaxation time, depending on the magnetic field, is investigated.     

Study of magnetization reversal and anisotropy of single crystalline ultrathin Fe/MgO(001) film by magneto-optic Kerr effect

The magnetization reversal process of Fe/MgO(001) thin film is investigated by combining transverse and longitudinal hysteresis loops. Owing to the competition between domain wall pinning energy and weak uniaxial magnetic anisotropy,the typical magnetization reversal process of Fe ultrathin film can take place via either an "l-jump" process near the easy axis, or a "2-jump" process near the hard axis, depending on the applied field orientation. Besides, the hysteresis loop presents strong asymmetry resulting from the variation of the detected light intensity due to the quadratic magneto-optic effect. Furthermore, we modify the detectable light intensity formula and simulate the hysteresis loops of the Kerr signal. The results show that they are in good agreement with the experimental data.