Magnetization depth profile of (Fe/Dy) multilayers

An experimental and numerical study of the magnetization in (Fe 3 nm/Dy 2 nm) multilayers is presented. The samples were thermally evaporated under ultra-high vacuum at two different substrate temperatures, 320 and 570 K. In order to get the magnetization depth profile of these transition metal/rare earth (TM/RE) multilayers, a fine investigation of the structural, chemical, and magnetic properties was carried out. The samples were studied by X-ray reflectivity (XRR), high resolution transmission electron microscopy (HRTEM), conversion electron Mössbauer spectrometry (CEMS), SQUID magnetometry and polarized neutron reflectivity (PNR). Magnetization profiles were obtained by Monte Carlo simulations to support the PNR fits. The key role of the crystalline structure is emphasized by magnetic depth profile measurements performed using polarized neutron reflectometry. The antiparallel configuration of Fe and Dy layers’ magnetizations was evidenced, as well as the perpendicular magnetic anisotropy (PMA), especially in the case of the sample prepared at 570 K.     

Induced moment due to perpendicular field cycling in trained exchange bias system

Depth-sensitive polarized neutron scattering in specular and off-specular mode has recently revealed that perpendicular field cycling brings about a modification in the interfacial magnetization of a trained exchange coupled interface. We show here by various model fits to our neutron reflectivity data that a restoration of the untrained state is not possible in the case of our polycrystalline multilayer specimen. This is due a magnetic moment at the interface induced only after perpendicular field cycling, changing the initial field-cooled state.     

Transverse and lateral structure of the spin-flop phase in fe/cr antiferromagnetic superlattices

Direct evidence of the nonuniformly canted state of the spin-flop phase induced by a magnetic field applied to Fe/Cr(100) superlattices is obtained by polarized neutron reflectometry. It is unambiguously demonstrated that the magnetization of the alternating Fe layers is twisted through the multilayer stack proving a stable noncollinear configuration. The maximal tilt at the end layers progressively reduces towards the center of the multilayer. The set of tilt angles is deduced from a model-free data evaluation employing the supermatrix routine. Spin-flip off-specular scattering is determined by the in-plane magnetization fluctuations and is fitted by a theoretical model of domains.     

Ultrathin multilayer of Fe and Ge: structure and magnetic properties

Metal-semiconductor multilayers are interesting, artificial structures as prospective candidates for spin injection devices. A Fe–Ge multilayer sample with very thin individual layers (few crystallographic planes) has been deposited by sputtering on Si[1 0 0] substrate. We have characterized the structure of this multilayer sample using X-ray diffraction, X-ray reflectometry and neutron reflectometry. The magnetic moment density in the ferromagnetic Fe layer has been obtained by polarized neutron reflectometry and the bulk magnetic behavior of the thin film by SQUID magnetometer measurements. We found that the film is a soft ferromagnet at room temperature with a substantially reduced magnetic moment of the Fe atoms.     

Polarized neutron reflectometry at Dhruva reactor

Polarized neutron reflectometry (PNR) is an ideal non-destructive tool for chemical and magnetic characterization of thin films and multilayers. We have installed a position sensitive detector-based polarized neutron reflectometer at Dhruva reactor, Trombay. In this paper we will discuss the results obtained from this instrument for two multilayer samples. The first sample is a (Ni-Mo alloy)/Ti multilayer sample. We have determined the chemical structure of this multilayer by unpolarized neutron reflectometry (NR). The other sample is a Fe/Ge multilayer sample for which we obtained the chemical structure by NR and magnetic moment per Fe atom by PNR.     

Polarized neutron reflection study of an Er/Fe multilayer

Low angle polarized neutron scattering experiments have been performed on an Er91 Å/Fe39 Å multilayer. The evolution of the intensity of the first peak with the temperature and the external magnetic field shows that the average magnetizations of Fe and Er layers are in the applied field direction. The study of the intensities of the second and the third peaks indicates that there is an interface effect: some erbium atoms remain antiferromagnetically coupled to iron at the interface.     

Highlights from the magnetism reflectometer at the SNS

The magnetism reflectometer at the SNS has passed the phase of commissioning and is in operation for users. The high power of the neutron source demands that special attention be paid to the optimization of the background in order to be able to measure the two-dimensional maps of reflected and scattered intensities with polarized neutrons in a broad range of momentum transfer. It implies an effective separation of the magnetic and non-magnetic reflectivity, off-specular scattering and the grazing incidence SANS in a high range of momentum transfer. Therefore the polarizing and the analyzing efficiencies are of particular importance on this time-of-flight instrument. At the beginning of July 2008 the world's first ³He neutron analyzer with on-line pump-up polarization was successfully installed and the tests with a magnetic multilayer film showing a strong off-specular spin-flip scattering were made. The performance of the instrument is under constant improvement in order to make it an effective and optimal instrument for the applications in nanosciences.     

Asymmetric magnetization reversal on exchange biased CoO/Co bilayers

We study magnetic hysteresis loops after field cooling of a CoO/Co bilayer by MOKE and polarized neutron reflectivity. The neutron scattering reveals that the first magnetization reversal after field cooling is dominated by domain wall movement, whereas all subsequent reversals proceed essentially by rotation of the magnetization. In addition, off-specular diffuse scattering indicates that the first magnetization reversal induces an irreversible change of the domain state in the antiferromagnet.     

X-ray and neutron reflectivity analysis of thin films and superlattices

X-ray and neutron reflectivity measurements provide a wealth of information on thickness and interfacial properties on the nanometer scale. This method is therefore well suited for the study of nano-structured thin films and superlattices. Neutrons provide a different contrast between the elements than X-rays and, in addition, are sensitive to the magnetization in the sample. Using polarized neutrons, magnetic as well as chemical profiles can be probed. In this review a basic introduction into the theory of X-ray and neutron reflectivity is provided along with some recent examples including the oxidation of Fe films and the structural and magnetic properties of Co/Cu superlattices.     

Nuclear resonance reflectivity from a [57Fe/Cr]30 multilayer with the Synchrotron Mössbauer Source

Mössbauer reflectivity spectra and nuclear resonance reflectivity (NRR) curves have been measured using the Synchrotron Mössbauer Source (SMS) for a [⁵⁷Fe/Cr]₃₀ periodic multilayer, characterized by the antiferromagnetic interlayer coupling between adjacent ⁵⁷Fe layers. Specific features of the Mössbauer reflectivity spectra measured with π‐polarized radiation of the SMS near the critical angle and at the `magnetic' maximum on the NRR curve are analyzed. The variation of the ratio of lines in the Mössbauer reflectivity spectra and the change of the intensity of the `magnetic' maximum under an applied external field has been used to reveal the transformation of the magnetic alignment in the investigated multilayer.     

超薄Fe(4(A))膜磁特性极化中子反射研究

利用分子束外延薄膜生长技术,制备了200 (A)V/4 (A) Fe/900 V/MgO(100)薄膜样品,通过X射线反射和极化中子反射两种测量手段获得了薄膜的表面、界面及各层膜厚的相关结构信息.中子反射结果表明,Fe原子磁矩在室温下约为1.0±0.1μB,随着温度的降低,Fe原子磁矩增加,在10 K时达到1.5±0.1μB.利用指数定律拟合磁矩随温度的变化情况,外推得出4(A)铁薄膜样品的居里温度约为310±30 K.

Abstract：

Uhrathin Fe film 200 (A)V/4 (A)Fe/900 (A)V/MgO(100) has been prepared by molecular beam epitaxy (MBE). The structure parameters, such as the surface and interface roughness and the thickness of each layer, were obtained by X-ray and neutron reflectivity measurement. The magnetic properties of the thin Fe layer were investigated by polarized neutron reflectometry at different temperature. The result shows that the magnetic moment of an Fe atom is about 1.0 ± 0.1 μB at room temperature and increases to 1.5 ±0.1 μBat 10 K. The Curie temperature of the thin Fe film is estimated to be 310 ± 30 K.     

Application of polarized neutron reflectometry and X-ray resonant magnetic reflectometry for determining the inhomogeneous magnetic structure in Fe/Gd multilayers

The evolution of the magnetic structure of multilayer [Fe (35 Å)/Gd (50 Å)₅] with variation in temperature and an applied magnetic field was determined using a complementary approach combining polarized neutron and X-ray resonant magnetic reflectometry. Self-consistent simultaneous analysis of X-ray and neutron spectra allowed us to determine the elemental and depth profiles in the multilayer structure with unprecedented accuracy, including the identification of an inhomogeneous intralayer magnetic structure with near-atomic resolution.     

超薄Fe(4?)膜磁特性极化中子反射研究

Ultrathin Fe film 200  V/4  Fe/900  V/MgO（100） has been prepared by molecular beam epitaxy （MBE）. The structure parameters, such as the surface and interface roughness and the thickness of each layer, were obtained by X-ray and neutron reflectivity mea 关键词： 超薄Fe膜 磁特性 极化中子反射 分子束外延     

Polarized neutron reflectometry of the influence of Fe/Si,Fe/FeSi2 and Au/Fe interfaces on the magnetic properties of epitaxial Fe films

The magnetic and structural properties of epitaxial Fe films grown on Si(1 1 1) are investigated by polarized neutron reflectometry (PNR) at room temperature. The influence of different types of interfaces, Fe/Si, Fe/FeSi₂ and Au/Fe on the magnetic properties of Fe films deposited by molecular beam epitaxy onto Si(1 1 1) are characterized. We observe a drastic reduction of the magnetic moment in the entire Fe film deposited directly on the silicon substrate essentially due to strong Si interdiffusion throughout the whole Fe layer thickness. The use of a silicide FeSi₂ template layer stops the interdiffusion and the value of the magnetic moment of the deposited Fe layer is close to its bulk value. We also evidence the asymmetric nature of the interfaces, Si/Fe and Fe/Si interfaces are magnetically very different. Finally, we show that the use of Au leads to an enhancement of the magnetization at the interface.     

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.     

Anisotropy of the optical and magneto-optical response of Au/Co/Au/Cu multilayers grown on vicinal Si (111) surfaces

The optical and magneto-optical second harmonic reflectivity response of Au/Co/Au/Cu multilayers grown on vicinal Si (111) substrates has been studied. These azimuthal optical non-linear experiments check the uniaxial character of the crystallinity of the Au buffer layer and the magnetic behavior of the ultrathin Co films in the metallic multilayer. They clearly show the strong dependence of the growth parameters and the misorientation of the vicinal surface on the SHG reflectivity signals. This uniaxial behavior is also correlated to linear MOKE experiments on the magnetic anisotropy with an easy magnetization axis parallel to the step edges. Received: 16 October 2001 / Published online: 29 May 2002     

Suppression of incommensurate spin-density waves in thin epitaxial Cr(110) layers of a V/Cr multilayer

We observed a complete suppression of the incommensurate spin-density wave in thin Cr layers of a V/Cr multilayer in a temperature range from 550 K down to 2 K. The (110)-oriented V/Cr multilayer consisting of 30 nm thick Cr layers and 5 nm thick V layers was investigated by neutron and X-ray diffraction (XRD). From the XRD experiments we were able to determine that the epitaxial strain of the Cr layers in the V/Cr multilayer is about 90% larger than in earlier studied Fe/Cr(110) multilayers. That leads to a completely different magnetic phase diagram as revealed by neutron diffraction experiments. The existence of the commensurate antiferromagnetic structure in the Cr layers can be observed in the whole temperature range without a phase transition to an incommensurate spin-density wave at lower temperatures. In order to elucidate the proximity effects further we also performed experiments in an external magnetic field. Up to a field of 4 T we found no change in the magnetic structure of the Cr films whereas in earlier experiments on Fe/Cr(110) multilayers we could observe a strong perpendicular pinning of the Cr polarization to the Fe magnetization.Received: 28 August 2003, Published online: 8 December 2003PACS: 75.30.Fv spin-density waves - 75.70.-i magnetic properties of thin films, surfaces, and interfaces - 61.12.-q Neutron diffraction and scattering     

Quantification of magnetic domain disorder and correlations in antiferromagnetically coupled multilayers by neutron reflectometry

The in-plane correlation lengths and angular dispersion of magnetic domains in a transition metal multilayer have been studied using off-specular neutron reflectometry techniques. A theoretical framework considering both structural and magnetic disorder has been developed, quantitatively connecting the observed scattering to the in-plane correlation length and the dispersion of the local magnetization vector about the mean macroscopic direction. The antiferromagnetic domain structure is highly vertically correlated throughout the multilayer. We are easily able to relate the neutron determined magnetic domain dispersion to magnetization and magnetoresistance experiments.     

Stress relaxation in Fe/Si neutron supermirrors by He irradiation

Magnetic supermirrors are widely used in polarized neutron applications. A key issue in producing these multilayer structures is to avoid residual stress in the layers, which may cause the layers to peel off. In this work He⁺ ion irradiation of 500 keV energy was applied at fluences of 1, 24, 50, 100×10¹⁴ ions/cm² with the aim of reducing residual stress. Structural, magnetic and neutron-optical properties were investigated by grazing incidence high-angle X-ray diffraction, magneto-optical Kerr effect, conversion electron Mössbauer spectroscopy and polarized neutron reflectometry. We find a monotonous decrease of the tensile stress upon irradiation from 1.76 to 0.37 GPa with a coupled decrease of neutron reflectivity at the critical angle and decrease of the maximum polarizing efficiency from 23% to 4% at the highest fluence compared to the as-prepared reference supermirror. The supermirrors exhibit a superposition of uniaxial and fourfold in-plane magnetic anisotropy.