Polarized Neutron Reflectometry Experiments Research Articles

Magnetic contrast layers with functional SiO2 coatings for soft-matter studies with polarized neutron reflectometry

This study introduces silicon substrates with a switchable magnetic contrast layer (MCL) for polarized neutron reflectometry (PNR) experiments at the solid–liquid interface to study soft-matter surface layers. During standard neutron reflectometry (NR) experiments on soft-matter samples, structural and compositional information is obtained by collecting experimental data with different isotopic contrasts on the same sample. This approach is normally referred to as contrast matching, and it can be achieved by using solvents with different isotopic contrast, e.g. different H2O/D2O ratios, and/or by selective deuteration of the molecules. However, some soft-matter systems might be perturbed by this approach, or it might be difficult to implement, particularly in the case of biological samples. In these scenarios, solid substrates with an MCL are an appealing alternative, as the magnetic contrast with the substrate can be used for partial recovery of information on the sample structure. More specifically, in this study, a magnetically soft Fe layer coated with SiO2 was produced by ion-beam sputter deposition on silicon substrates of different sizes. The structure was evaluated using X-ray reflectometry, atomic force microscopy, vibrating sample magnetometry and PNR. The collected data showed the high quality and repeatability of the MCL parameters, regardless of the substrate size or the thickness of the capping SiO2 layer. Previously proposed substrates with an iron MCL used an Au capping layer. The SiO2 capping layer proposed here allows reproduction of the typical surface of a standard silicon substrate used for NR experiments and is compatible with a large variety of soft-matter samples. This application is demonstrated with ready-to-use 50 × 50 × 10 mm substrates in PNR experiments for the characterization of a lipid bilayer in a single solvent contrast. Overall, the article highlights the potential of PNR with an MCL for the investigation of soft-matter samples.

Design and realization of a sputter deposition system for the in situ and in operando use in polarized neutron reflectometry experiments: Novel capabilities

Recently, the design and realization of a sputter-deposition system for in situ and in operando polarized neutron reflectometry (PNR) was reported. The device allows magnetic thin films and heterostructures to be grown, while the sample remains aligned in the neutron beam for PNR. By now, it has been applied in experiments that investigated the magnetic and structural properties of thin Fe and Pd/Fe/Pd heterostructures as a function of the layer thickness. Here, we report on significant upgrades of the deposition system with advanced thin film growth and measurement capabilities. These include improvements to the generation of the vacuum to realize ultra-high-vacuum (UHV) conditions. A base pressure below 5 × 10−9mbar can now be obtained within less than one day of pumping time. To allow experiments over a wide range of temperatures, the system was upgraded to include a cryo-furnace. It allows the sample to be cooled or heated in the range of 10K to 1000K for both applications, sample growth and measurement. Further, a new magnetic coil setup with soft iron yoke was designed which can realize a homogeneous in-plane magnetic field of up to 300mT and electric fields at the sample position.

Exploiting Symmetry Mismatch to Control Magnetism in a Ferroelastic Heterostructure.

In the bulk, LaCoO_{3} (LCO) is a paramagnet, yet the low-temperature ferromagnetism (FM) is observed in tensile strained thin films, and its origin remains unresolved. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by polarized neutron reflectometry (PNR) and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced atomic density, whereas the film's interior (i.e., its film bulk) shows the opposite trend. We attribute the nonuniformity to the symmetry mismatch at the interface, which induces a structural distortion related to the ferroelasticity of LCO. This assertion is tested by systematic application of hydrostatic pressure during the PNR experiments. The magnetization can be controlled at a rate of -20.4% per GPa. These results provide unique insights into mechanisms driving FM in strained LCO films while offering a tantalizing observation that tunable deformation of the CoO_{6} octahedra in combination with the ferroelastic order parameter.

Design and realization of a sputter deposition system for the in situ- and in operando-use in polarized neutron reflectometry experiments

We report on the realization of a sputter deposition system for the in situ- and in operando-use in polarized neutron reflectometry experiments. Starting with the scientific requirements, which define the general design considerations, the external limitations and boundaries imposed by the available space at a neutron beamline and by the neutron and vacuum compatibility of the used materials, are assessed. The relevant aspects are then accounted for in the realization of our highly mobile deposition system, which was designed with a focus on a quick and simple installation and removability at the beamline. Apart from the general design, the in-vacuum components, the auxiliary equipment and the remote control via a computer, as well as relevant safety aspects are presented in detail.

The study of perpendicular magnetic anisotropy in the magnetic sensors with linear sensitivity using polarized neutron reflectometry

The CoFeB sandwiched by Ta and MgO layers enables a perpendicular magnetic anisotropy (PMA) and provides a pathway for such application. In this paper, we reported the origin of PMA in CoFeB using the anomalous Hall effect (AHE) and polarized neutron reflectometry (PNR). From PNR experiments, we obtained the details of the magnetic and structural depth profiles inside the film. It is found that the PMA properties of CoFeB layers deposited above and under MgO layer are different and PNR measurements confirmed that a large PMA in the CoFeB above MgO layer is related to its low magnetization. Based on this PMA mechanism, we obtain a high sensitivity of AHE in the perpendicular CoFeB, which opens a new avenue to detect ultralow magnetic field.

Polarized neutron reflectometry in high magnetic fields

A simple method is described to maintain the polarization of a neutron beam on its way through the large magnetic stray fields produced by a vertical field of a cryomagnet with a split-coil geometry. The two key issues are the proper shielding of the neutron spin flippers and an additional radial field component in order to guide the neutron spin through the region of the null point (i.e., point of reversal for the vertical field component). Calculations of the neutron’s spin rotation as well as polarized neutron reflectometry experiments on an ErFe2∕DyFe2 multilayer show the perfect performance of the used setup. The recently commissioned cryomagnet M5 with a maximum vertical field of up to 7.2T in asymmetric mode for polarized neutrons and 9T in symmetric mode for unpolarized neutrons was used on the C5 spectrometer in reflectometry mode, at the NRU reactor in Chalk River, Canada.

Magnetization of uncovered and V-covered ultrathin Fe(100) films on V(100)

We used polarized neutron reflectometry (PNR) to determine the absolute magnetic moment of uncovered and V-covered Fe films in the thickness range from $0.3\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The films were prepared by molecular beam epitaxy on a V(100) buffer layer grown on a MgO(100) crystal. The magnetic moment shows a linear dependence on the Fe film thickness with a reduction (compared to the Fe bulk value) of the magnetic moment equivalent to $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ bulk Fe for the V-covered films and a reduction equivalent to $0.03\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ bulk Fe for the uncovered Fe films. For the case of the $\mathrm{V}∕\mathrm{Fe}∕\mathrm{V}$ samples we observe a much smaller reduction of the magnetic moment than reported for experiments on $\mathrm{Fe}∕\mathrm{V}$ multilayers. As theoretical calculations show a strong decrease of the magnetic moment for an interface alloy we conclude that the larger reduction of the magnetization in $\mathrm{Fe}∕\mathrm{V}$ multilayers is due to an increase in interface roughness with increasing film thickness. For the uncovered Fe(100) films we find a much smaller reduction of the magnetic moment than in earlier in situ PNR experiments on $\mathrm{V}(110)∕\mathrm{Fe}(110)$ where we observed a reduction equivalent to $0.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ bulk Fe.

Polarization analysis of off-specular neutron scattering from domains and rough interfaces in a FeCoV/TiZr multilayer

Specular neutron reflectivity is commonly used as a powerful tool in determining the scattering length density (SLD) profile perpendicular to the surface of thin films. Information about the lateral fluctuations in the SLD, such as the presence of magnetic domains and rough interfaces between layers, can be obtained by studying the pattern of off-specular scattering. Advancements in neutron-source brilliance, beam-line optics and detector efficiency, as well as advances in the theoretical description of off-specular scattering, have contributed to the increasing importance of off-specular neutron-reflectometry studies. We present specular and off-specular polarized neutron reflectometry experiments on FeCoV/TiZr multilayers, with a detailed description of the orientations of the magnetic moments inside the FeCoV layers and at the FeCoV/TiZr interfaces.

Polarization analysis of neutron reflectometry on non-collinear magnetic media: polarized neutron reflectometry experiments on a thin cobalt film

In this work we present a full data analysis of polarized neutron reflectometry experiments on a thin magnetic film. A magnetic field was applied perpendicular to the layer magnetization, resulting in non spin-flip and spin-flip reflectivities. For moderate external fields, the gain or loss of Zeeman energy for spin-flipped neutrons results in off-specular reflected spin-flipped beams. The theoretical model used to explain all data simultaneously consists of a bulk cobalt layer, together with interfacial layers with low magnetization values.

A polarized neutron reflectometry study on 60×Fe 31Co 68V 1/Ti 75Zr 25 multilayers

We present a study of the magnetic behavior of 60×Fe 31Co 68V 1/Ti 75Zr 25 multilayers with applied field along the easy and hard axis. We performed polarized neutron reflectometry experiments with polarization analysis at a time-of-flight setup. At applied fields above 16 kA/m the sample was magnetically saturated. The reduction of high-order Bragg superlattice reflections is interpreted as inter-layer roughness of about 2 nm. In the low-field region, the first-order Bragg reflection consists of a broad and a sharp peak, indicating separate contributions from macroscopic magnetically ordered and disordered regions. Spin flipped neutron reflectivities observed at low fields are attributed to domains with local easy axis not parallel to the applied field. The increase in spin flipped neutron reflectivity at high incident angle is ascribed to domains sizes in the order of tens of microns.

Neutron reflectometry on an exchange biased Ni 80Fe 20/Fe 50Mn 50 bilayer

Polarized neutron reflectometry experiments were performed on a thin MBE-grown [111]-oriented exchange biased Ni 80Fe 20/Fe 50Mn 50 bilayer. Analysis of the data shows that the magnetic moment of the ferromagnetic Ni 80Fe 20 layer appears constant throughout the layer. In addition, this experiment restrics the possible spin arrangements or domains that may influence the magnitude of the exchange biasing.