Time resolved AC-polarized neutron reflectometry

Diese Dissertation beschreibt neue Methoden der Neutronenstreuung zur Untersuchung von Magnetisierungsdynamik und Domänenwandkinetik in dünnen Schichten, Heterostrukturen und spintronischen Materialen. Diese Methoden liefern Information über die räumliche und zeitliche Entwicklung der Magnetisierung wenn sie einem magnetischen Wechselfeld ausgesetzt werden. Dabei wird mit Hilfe von polarisierter Neutronenreflektivität die zeitliche gemittelte reflektierte Intensität gemessen. In einer zweiten Methode wird das Messsignal im relevanten Frequenzbereich mit Hilfe von schnellen Neutronendetektoren zeitlich aufgelöst. Da diese Methode schnell an ihre Grenzen stößt, wurde eine dritte Methode entwickelt, bei der die Polarisation des einfallenden Strahls zeitlich moduliert wird und die Analyse über Lock-in Technik erfolgt. Dies erlaubt eine deutliche Erweiterung des relevanten Frequenzraums. Diese Methoden wurden erfolgreich an verschiedenen Schichtsystemen erprobt

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

International audienceThe 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 biomembrane

Functional lipid pairs as building blocks of phase-separated membranes

Biological membranes exhibit a great deal of compositional and phase heterogeneity due to hundreds of chemically distinct components. As a result, phase separation processes in cell membranes are extremely difficult to study, especially at the molecular level. It is currently believed that the lateral membrane heterogeneity and the formation of domains, or rafts, are driven by lipid–lipid and lipid–protein interactions. Nevertheless, the underlying mechanisms regulating membrane heterogeneity remain poorly understood. In the present work, we combine inelastic X-ray scattering with molecular dynamics simulations to provide direct evidence for the existence of strongly coupled transient lipid pairs. These lipid pairs manifest themselves experimentally through optical vibrational (a.k.a. phononic) modes observed in binary (1,2-dipalmitoyl-sn-glycero-3-phosphocholine [DPPC]–cholesterol) and ternary (DPPC–1,2-dioleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-glycero-3-phosphocholine [DOPC/POPC]–cholesterol) systems. The existence of a phononic gap in these vibrational modes is a direct result of the finite size of patches formed by these lipid pairs. The observation of lipid pairs provides a spatial (subnanometer) and temporal (subnanosecond) window into the lipid–lipid interactions in complex mixtures of saturated/unsaturated lipids and cholesterol. Our findings represent a step toward understanding the lateral organization and dynamics of membrane domains using a well-validated probe with a high spatial and temporal resolution

Topotactic transition mechanisms in SrCoO2.5+x_{2.5+x} films

Transition metal oxides are a big research topic, because they offer a wide range of possible applications, particularly in information and energy technology. One such system is strontium cobaltite (SrCoO2.5+x), which exists in two distinct topotactic phases, depending on the oxygen content. SrCoO3 is a ferromagnetically ordered metal with a Curie temperature of 305 K, but the system becomes an antiferromagnetic insulator with a Néel temperature of 570 K, when the oxygen content is decreased to SrCoO2.5. Along with this magnetic transition, the structure changes from perovskite to the orthorhombic brownmillerite, with the missing oxygen atoms forming vacancy channels [1]. Because of the multivalent Co states and high oxygen mobility it is a promising material for device applications [2]. To control the oxygen content, several possibilities exist. We focus on annealing in oxidising conditions and applying variable strain with a piezoelectric substrate to the film.We grow thin films of SrCoO2.5 by molecular beam epitaxy on SrTiO3 and LSAT substrates for investigations of oxygen annealing induced transitions and 0.7(Pb(Mg1/3Nb2/3)O3)-0.3(PbTiO3) (PMN-PT), a piezoelectric substrate, to study the possibility of a strain dependent oxidation state.To be able to successfully control the oxidation state and transfer strain from the substrate to the film, a high sample quality and epitaxy is mandatory. Thus, we present the results of the film growth and quality, as well as first results of the magnetic characterisation by SQUID and neutron reflectometry for annealed and strained samples.[1] C.K. Xie et al., Appl. Phys. Lett. 99, 2011 [2] H. Jeen et al., Nature Materials 12, 201

Revealing magnetic properties of thin films utilizing polarized neutrons

Polarized neutron reflectometry (PNR) is a versatile probe for the study of the magnetic moment with depth resolution. As self-calibrating technique, it provides independently values of the magnetic moment, its direction in the film plane, and film thickness together with its scattering length density values. It is layer selective and buried layers in multilayer systems can be analyzed [1].In this contribution we will provide an overview of the possibilities of PNR as well as present the reflectometer MARIA [2]. It is a state of the art reflectometer at the constant neutron flux reactor in Garching, Germany. MARIA exhibits a high dynamic range of up to 7-8 orders of magnitude and a maximum Q (momentum transfer vector) higher than 0.25 Å$^{-1}$. With the combination of a 400 x 400 mm$^2$ position sensitive detector and a time-stable 3He polarization spin filter based on Spin-Exchange Optical Pumping (SEOP), the instrument is well equipped for investigating specular reflectivity and off-specular scattering from magnetic thin films and artificially fabricated structures like nano-dots, gratings, etc. down to the monolayer regime in full spin polarization. Furthermore, the GISANS option can be used to investigate lateral correlations in the nm range. Due to the large detector and pinhole collimation of the incident neutron beam even grazing incidence diffraction measurements are possible. All the options, like GISANS, neutron polarization and 3He polarization spin filter can be moved in and out of the neutron beam within seconds by remote controlled push button operation and do not require any realignment.Magnetic fields can be applied up to 5 T and a low temperature sample environment (down to 3 K) is offered. Thin film samples may be fabricated in a MBE system nearby (deposition materials according to the requirements of the user). For investigation of samples which are sensitive to ambient conditions a UHV transport and measurement chamber with base pressure in 10−10 mbar range is provided (transfer forth and back) [3]. Typical substrate size for investigation is 10x10 mm$^2$.Examples for PNR investigation of thin films like e.g. NiO/Fe/L10-FePt, SrCoO$_x$, Co/W(110), Fe$_4$N/LaAlO$_3$(001) are discussed. However, the MARIA reflectometer and the MBE system are user instruments. Hence we offer measurement and sample preparation time to interested users [4]. Let’s discuss your ideas![1] J. A. C. Bland and C. A. F. Vaz, Chapter 7 in J. A. C. Bland and B. Heinrich, Eds., Ultrathin Magnetic Structures III, Springer-Verlag Berlin (2005)[2] Heinz Maier-Leibnitz Zentrum. (2015). J. large-scale research facilities, 1, A8. http://dx.doi.org/10.17815/jlsrf-1-29; S. Mattauch, A. Koutsioumpas, et al., J. appl. Crystallography, 51, 646 (2018)[3] A.Syed Mohd, S.Pütter, et al., Rev. Sci. Instrum. 87, 123909(2016)[4] www.mlz-garching.de/maria;www.mlz-garching.de/mb

Direct observation of spin correlations in an artificial triangular lattice Ising spin system with grazing-incidence small-angle neutron scattering

The triangular lattice with Ising magnetic moments is an archetypical example of geometric frustration. In the case of dipolar-coupled out-of-plane moments, the geometric frustration results in a disordered classical spin-liquid state at higher temperatures while the system is predicted to transition to an anti-ferromagnetic stripe ground state at low temperatures. In this work we fabricate artificial triangular Ising spin systems without and with uniaxial in-plane compression to tune the nature and temperature of the correlations. We probe the energy scale and nature of magnetic correlations by grazing-incidence small-angle neutron scattering. In particular, we apply a newly-developed empirical structure-factor model to describe the measured short-range correlated spin-liquid state, and find good agreement with theoretical predictions. We demonstrate that grazing-incidence neutron scattering on our high-quality samples, in conjunction with detailed modeling of the scattering using the Distorted Wave Born Approximation, can be used to experimentally quantify the spin-liquid-like correlations in highly-frustrated artificial spin systems.ISSN:2055-6756ISSN:2055-676

Fluorination of Diamond Nanoparticles in Slow Neutron Reflectors Does Not Destroy Their Crystalline Cores and Clustering While Decreasing Neutron Losses

If the wavelength of radiation and the size of inhomogeneities in the medium are approximately equal, the radiation might be intensively scattered in the medium and reflected from its surface. Such efficient nanomaterial reflectors are of great scientific and technological interest. In previous works, we demonstrated a significant improvement in the efficiency of reflection of slow neutrons from a powder of diamond nanoparticles by replacing hydrogen located on the surface of nanoparticles with fluorine and removing the residual sp2 amorphous shells of nanoparticles via the fluorination process. In this paper, we study the mechanism of this improvement using a set of complementary experimental techniques. To analyze the data on a small-angle scattering of neutrons and X-rays in powders of diamond nanoparticles, we have developed a model of discrete-size diamond nanospheres. Our results show that fluorination does not destroy either the crystalline cores of nanoparticles or their clustering in the scale range of 0.6–200 nm. This observation implies that it does not significantly affect the neutron scattering properties of the powder. We conclude that the overall increase in reflectivity from the fluorinated nanodiamond powder is primarily due to the large reduction of neutron losses in the powder caused by the removal of hydrogen contaminations

Effect of Nanodiamond Sizes on the Efficiency of the Quasi-Specular Reflection of Cold Neutrons

Nanomaterials can intensively scatter and/or reflect radiation. Such processes and materials are of theoretical and practical interest. Here, we study the quasi-specular reflections (QSRs) of cold neutrons (CNs) and the reflections of very cold neutrons (VCNs) from nanodiamond (ND) powders. The fluorination of ND increased its efficiency by removing/replacing hydrogen, which is otherwise the dominant cause of neutron loss due to incoherent scattering. The probability of the diffuse reflection of VCNs increased for certain neutron wavelengths by using appropriate ND sizes. Based on model concepts of the interaction of CNs with ND, and in reference to our previous work, we assume that the angular distribution of quasi-specularly reflected CNs is narrower, and that the probability of QSRs of longer wavelength neutrons increases if we increase the characteristic sizes of NDs compared to standard detonation nanodiamonds (DNDs). However, the probability of QSRs of CNs with wavelengths below the cutoff of ~4.12 Å decreases due to diffraction scattering on the ND crystal lattice. We experimentally compared the QSRs of CNs from ~4.3 nm and ~15.0 nm ND. Our qualitative conclusions and numerical estimates can help optimize the parameters of ND for specific practical applications based on the QSRs of CNs

Fluorination of Diamond Nanoparticles in Slow Neutron Reflectors Does Not Destroy Their Crystalline Cores and Clustering While Decreasing Neutron Losses

If the wavelength of radiation and the size of inhomogeneities in the medium are approximately equal, the radiation might be intensively scattered in the medium and reflected from its surface. Such efficient nanomaterial reflectors are of great scientific and technological interest. In previous works, we demonstrated a significant improvement in the efficiency of reflection of slow neutrons from a powder of diamond nanoparticles by replacing hydrogen located on the surface of nanoparticles with fluorine and removing the residual sp2 amorphous shells of nanoparticles via the fluorination process. In this paper, we study the mechanism of this improvement using a set of complementary experimental techniques. To analyze the data on a small-angle scattering of neutrons and X-rays in powders of diamond nanoparticles, we have developed a model of discrete-size diamond nanospheres. Our results show that fluorination does not destroy either the crystalline cores of nanoparticles or their clustering in the scale range of 0.6–200 nm. This observation implies that it does not significantly affect the neutron scattering properties of the powder. We conclude that the overall increase in reflectivity from the fluorinated nanodiamond powder is primarily due to the large reduction of neutron losses in the powder caused by the removal of hydrogen contaminations

Effect of Particle Sizes on the Efficiency of Fluorinated Nanodiamond Neutron Reflectors

International audienceOver a decade ago, it was confirmed that detonation nanodiamond (DND) powders reflect very cold neutrons (VCNs) diffusively at any incidence angle and that they reflect cold neutrons quasi-specularly at small incidence angles. In the present publication, we report the results of a study on the effect of particle sizes on the overall efficiency of neutron reflectors made of DNDs. To perform this study, we separated, by centrifugation, the fraction of finer DND nanoparticles (which are referred to as S-DNDs here) from a broad initial size distribution and experimentally and theoretically compared the performance of such a neutron reflector with that from deagglomerated fluorinated DNDs (DF-DNDs). Typical commercially available DNDs with the size of ~4.3 nm are close to the optimum for VCNs with a typical velocity of ~50 m/s, while smaller and larger DNDs are more efficient for faster and slower VCN velocities, respectively. Simulations show that, for a realistic reflector geometry, the replacement of DF-DNDs (a reflector with the best achieved performance) by S-DNDs (with smaller size DNDs) increases the neutron albedo in the velocity range above ~60 m/s. This increase in the albedo results in an increase in the density of faster VCNs in such a reflector cavity of up to ~25% as well as an increase in the upper boundary of the velocities of efficient VCN reflection