Boris P. Toperverg

Magnetization reversal and exchange bias effects in hard/soft ferromagnetic bilayers with orthogonal anisotropies

The magnetization reversal processes are discussed for exchange- coupled ferromagnetic hard/soft bilayers made from Co0.66Cr0.22Pt0.12 (10 and 20nm)/Ni (from 0 to 40nm) films with out-of-plane and in-plane magnetic easy axes respectively, based on room temperature hysteresis loops and first-order reversal curve analysis. On increasing the Ni layer thicknesses, the easy axis of the bilayer reorients from out-of-plane to in-plane. An exchange bias effect,

Revealing the mechanism of passive transport in lipid bilayers via phonon-mediated nanometre-scale density fluctuations

The passive transport of molecules through a cell membrane relies on thermal motions of the lipids. However, the nature of transmembrane transport and the precise mechanism remain elusive and call for a comprehensive study of phonon excitations. Here we report a high resolution inelastic X-ray scattering study of the in-plane phonon excitations in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine above and below the main transition temperature. In the gel phase, for the first time, we observe low-frequency transverse modes, which exhibit a phonon gap when the lipid transitions into the fluid phase. We argue that the phonon gap signifies the formation of short-lived nanometre-scale lipid clusters and transient pores, which facilitate the passive molecular transport across the bilayer plane. Our findings suggest that the phononic motion of the hydrocarbon tails provides an effective mechanism of passive transport, and illustrate the importance of the collective dynamics of biomembranes.

SuperADAM: Upgraded polarized neutron reflectometer at the Institut Laue-Langevin

A new neutron reflectometer SuperADAM has recently been built and commissioned at the Institut Laue-Langevin, Grenoble, France. It replaces the previous neutron reflectometer ADAM. The new instrument uses a solid state polarizer/wavelength filter providing a highly polarized (up to 98.6%) monochromatic neutron flux of 8 × 10(4) n cm(-2) s(-1) with monochromatization Δλ∕λ = 0.7% and angular divergence Δα = 0.2 mrad. The instrument includes both single and position sensitive detectors. The position sensitive detector allows simultaneous measurement of specular reflection and off-specular scattering. Polarization analysis for both specular reflection and off-specular scattering is achieved using either mirror analyzers or a (3)He spin filter cell. High efficiency detectors, low background, and high flux provides a dynamic range of up to seven decades in reflectivity. Detailed specifications and the instrument capabilities are illustrated with examples of recently collected data in the fields of thin film magnetism and thin polymer films.

Hyper-domains in exchange bias micro-stripe pattern

A combination of experimental techniques, e.g. vector-MOKE magnetometry, Kerr microscopy and polarized neutron reflectometry, was applied to study the field induced evolution of the magnetization distribution over a periodic pattern of alternating exchange bias (EB) stripes. The lateral structure is imprinted into a continuous ferromagnetic/antiferromagnetic EB bilayer via laterally selective exposure to He-ion irradiation in an applied field. This creates an alternating frozen-in interfacial EB field competing with the external field in the course of the re-magnetization. It was found that in a magnetic field applied at an angle with respect to the EB axis parallel to the stripes the re-magnetization process proceeds via a variety of different stages. They include coherent rotation of magnetization towards the EB axis, precipitation of small random (ripple) domains, formation of a stripe-like alternation of the magnetization, and development of a state in which the magnetization forms large hyper-domains comprising a number of stripes. Each of those magnetic states is quantitatively characterized via the comprehensive analysis of data on specular and off-specular polarized neutron reflectivity. The results are discussed within a phenomenological model containing a few parameters, which can readily be controlled by designing systems with a desired configuration of magnetic moments of micro- and nano-elements.

Competing magnetic interactions in exchange-bias-modulated films

The magnetization reversal in stripelike exchange-bias-patterned

Structural characterization and magnetic profile of annealed CoFeB/MgO multilayers

{\text{Ni}}_{81}{\text{Fe}}_{19}/\text{IrMn}

The magnetic structure of coupled Fe/FeO multilayers revealed by nuclear resonant and neutron scattering methods

thin films was investigated by complementary inductive and high-resolution magneto-optical magnetometry, magneto-optical Kerr microscopy, and polarized neutron reflectometry to clarify the effects of competing interfacial exchange-bias and lateral interface contributions. Structures of varying ferromagnetic layer thickness and stripe period were analyzed systematically at the frozen-in domain state of oppositely aligned stripe magnetization. For all samples the mean magnetization of the magnetic hybrid structures was found to be aligned nearly orthogonally with respect to the stripe axis and the set exchange-bias direction. Due to the interaction of interfacial coupling, exchange, and magnetostatic energy contributions, the opening angle of neighboring stripe magnetizations increases with decreasing ferromagnetic layer thickness and increasing stripe period. The experimental observations are in agreement with an earlier proposed model for designing micropatterned exchange-bias films.

AC-polarized neutron reflectometry: Application to domain dynamics in thin Fe film

Magnetic tunnel junctions (MTJ) have become of strategic importance due to the large tunneling magnetoresistance ratio (TMR) that they can achieve at room temperature. The largest TMR values observed until now were recorded in MTJs with MgO barriers and CoFeB electrodes after annealing of the junction above the recrystallization temperature of the amorphous CoFeB layers. We have used x-ray reflectivity combined with polarized neutron reflectivity to characterize the structure and the magnetism of [Co60Fe20B20∕MgO]14 multilayers, where the MgO layers were prepared by different methods and annealed at different temperatures. We have found that the MgO preparation method as well as the annealing temperature play a significant role in the systems. A gradient in thickness together with a variation of the scattering length density along the multilayer stacks induce a process of underoxidation or overoxidation, strictly dependent on the MgO production method.

Self organization of magnetic nanoparticles: A polarized grazing incidence small angle neutron scattering and grazing incidence small angle x-ray scattering study

We have studied the magnetic structure that forms in a Fe/native Fe oxide multilayer by nuclear resonant scattering of synchrotron radiation and polarized neutron reflectometry. Magnetic field-dependent experiments revealed a non-collinear magnetic arrangement of the adjacent metallic layers which is mediated by an antiferromagnetically ordered oxide layer. Despite its antiferromagnetic (AFM) order, the oxide exhibits a small net magnetization attributed to the presence of metallic Fe within the AFM matrix that aligns parallel to the external field. The presence of a strong uniaxial anisotropy prevents the system from forming small magnetic domains in remanence. The canting angle between the two magnetic sublattices remains close to 90° throughout the magnetization reversal on the hard axis. The results and the influence of the uniaxial anisotropy are discussed in the framework of the proximity magnetism model.

Development and application of setup for ac magnetic field in neutron scattering experiments

We report on the first application of polarized neutron reflectometry (PNR) to probe the re-magnetization kinetics in thin films subjected to periodic magnetic fields. Specular PNR and off- specular scattering were recorded from a 100 nm thick MBE grown Fe film under the application of AC magnetic fields with amplitudes up to 60 Oe and frequencies in the range of 0.3 − 1.8 MHz. It was found that up to 0.4 MHz a complete magnetization reversal of the Fe film takes place during each AC field cycle. Higher frequencies cause only a partial re-magnetization and above 0.8 MHz the AC field no longer alters the sample magnetization. High frequencies suppress magnetization fluctuations on a micrometer scale which are otherwise seen via off-specular scattering.