Project

Discipline

Polarization Analysis; Photo-excited triplet states; Dynamic Nuclear Polarization; Magnetic Small Angle Neutron Scattering; Neutron Physics; Spin Filters

Lead
Die magnetische Neutronen Kleinwinkelstreung (SANS) ist gegenwärtig die einzige Technik welche erlaubt die Spin Mikrostruktur in einem Magneten auf der Längenskala von Nanometern (1-200 nm) zu untersuchen. Durch die Verwendung von Spin polarisierten Neutronen und einer Spin-Analyse der Streuung können Schlüsselfragen von fundamentaler sowie technologischer Relevanz angegangen werden. Diese Art von Experimenten sind erst kürzlich durch die Entwicklung von 3He Spin Filter möglich geworden, welche erlauben die Polarisation von divergent gestreuten Neutronen zu analysieren. Gegenwärtig treiben wir die Entwicklung einer alternativen Methode zur Neutronen Spin Filterung voran, welche auf der stark spin-abhängigen Neutron-Proton Wechselwirkung basiert. Die benötigte hohe Proton Polarisation im Filtermaterial erzeugen wir mittels einer neuartigen Methode der dynamischen Kernpolarisation (DNP), welche optisch angeregte Elektron Triplett Zustände verwendet.
Lay summary
Ausgangspunkt des Projektes ist ein kürzlich erfolgreich durchgeführtes „proof of principle“ Experiment indem wir erstmalig ein Triplet DNP Spin Filter zur Analyse der magnetischen Streuung verwendet haben. Darauf basierend möchten wir ein Instrument entwickeln, das spezifisch zur longitudinalen Polarisationsanalyse in der Neutronen Kleinwinkelstreuung ausgelegt ist und auch in inhomogenen magnetischen Feldern funktioniert. Das anvisierte Design integriert den Analysator Kristall mit der zu untersuchenden Probe was eine grosse Winkelakzeptanz mit einem Filter von moderater Grösse ermöglicht. Dies erlaubt einen kompakten Setup zu realisieren welcher kontinuierlich auf einer Strahllinie betrieben werden kann. Das in den letzten Jahren erarbeitete theoretische Verständnis wird uns helfen die Polarisationsprozesse für grössere Filter Kristalle bei tieferen Magnetfeldern zu optimieren und möglicherweise die heute maximale Polarisation zu erhöhen und damit die Filter Effizienz zu steigern. Das neue Instrument soll das Spektrum der Möglichkeiten der SANS Instrumente an der SINQ am PSI signifikant erweitern und bislang nicht mögliche Experimente auf dem Gebiet des Magnetismus erlauben. Im Speziellen möchten wir den neuen Analyzer benutzen um den chiralen Magnetismus in Skyrmion Materialien zu untersuchen. Eine longitudinale Polarisationsanalyse ergibt eine direkte und empfindliche Messung der Drehrichtung der magnetischen Ordnung im material. Zusätzlich planen wir die Spin Mikrostruktur in metallischen Gläsern zu studieren welche aus Deformation resultiert. Diese Untersuchungen tragen bei zum Verständnis der fundamentalen Instabilität und zur Verbesserung der mechanischen Eigenschaften dieser wichtigen Klasse von Materialien.

Direct link to Lay Summary

Last update: 29.06.2016

Active participation

Number	Title	Start	Funding scheme

The recent advances in the field of nanomagnetism have resulted in a growing interest to use the magnetic small-angle neutron scattering (SANS) method as the main characterization tool. In fact, magnetic SANS is presently the only technique which allows one to resolve the spin microstructure on the nanometer length scale (1 - 200 nm) and in the bulk of the magnet. It measures the diffuse scattering along the forward direction, which arises from nanoscale variations in both, the magnitude and orientation of the magnetization vector field. Indeed, SANS with polarized neutrons and (uniaxial) polarization analysis could address key outstanding questions in studies with both fundamental and technological relevance; examples include the study of interfacial magnetic effects in nanoscopic heterostructures and the manipulation of magnetism with strain and electrical current [1], electric-field-induced magnetization in multiferroics [2], magnetostriction in Fe-Ga alloys [3], vortex structures in Fe-based superconductors [4], or studies on the intraparticle spin misalignment in arrays of nanostructured materials [5]. These kinds of experiments became only recently possible with the development of 3He spin filters [6] that allow the neutron spins from a divergently scattered beam to be analyzed. However, the size and sensitivity for magnetic field inhomogeneities of 3He spin filters puts severe restriction on their application in these fields.At present we are pushing the development of a neutron spin filter based on the strong spin dependence of the neutron scattering on protons, which is small and works in inhomogeneous fields. We create the necessary large proton polarization in a solid with a recent method of dynamic nuclear polarization (DNP) [7] that uses photo-excited triplet states [8] and requires only moderate experimental means. In our team the development of the theory and experiments were going in parallel. In an initial step we demonstrated that the method can be used to build a reliably working spin filter for neutrons operating at 0.3 T at a temperature of 100 K [9], and next showed that a sizable proton spin polarization can be achieved in single crystals of naphthalene doped with deuterated pentacene employing a well suited laser source for excitation [10]. Further optimization of several crucial parameters for, e.g. the optical excitation and the polarization transfer, very recently resulted in a record proton polarization of above 70% at a field of 0.36 T using a simple flow cryostat for cooling [11]. Decisive for this step was the theoretical understanding of the polarization process [12,13] and the photo-excitation of triplet states in pentacene [14]. With this recent breakthrough the triplet DNP method becomes a serious alternative to the well-established methods of polarizing neutrons, super mirror polarizers and polarized gaseous 3He, with the further advantage of its small size and insensitivity for magnetic field inhomogeneity. Very recently we demonstrated that our spin filter is well suited to perform polarization analysis in SANS in a magnetic field environment where a 3He filter can only be operated with a proper magnetic shielding. Based on this successful proof of principle, we propose to build a device specifically designed for longitudinal neutron polarization analysis in small angle neutron scattering. The design integrates a moderate sized analyzer and the sample under investigation and yields a large angular acceptance, covering a q-range of more than 1 nm-1 with a moderate sized analyzer. This allows a compact setup that can be conveniently operated on a beamline in a continuous mode. To realize this novel device we can profit from the unique and versatile apparatus we have already developed and our present theoretical understanding of the fundamental processes. The latter will guide the planned basic investigations on how to optimize the polarization properties for larger samples at lower magnetic fields and possibly even increase the maximum polarization above the present level.This instrument will significantly expand the possibilities of the SANS instruments at SINQ at PSI enabling experiments in the field of magnetism hitherto not possible. Specifically, we propose to use the novel analyzer to study the chiral magnetism displayed by Skyrmion lattice materials. Here, the longitudinal polarization analysis yields unambiguous, direct and sensitive measurements of the chiral sense of the magnetic order in the material. Additionally we will investigate the spin microstructure related to a displacement field of a bulk metallic glass (BMG). Spin resolved measurements provide insights into the existence, structure and size of displacement fields in a BMG and contribute to the understanding of the fundamental instability and to the improvement of the mechanical properties of this important class of materials.