Neutron Polarization Research Articles

On the Possibility of Measuring the Energy Gap in the Spectrum of Spin Waves of Ferromagnets by the Small-Angle Scattering of Polarized Neutrons

On the Possibility of Measuring the Energy Gap in the Spectrum of Spin Waves of Ferromagnets by the Small-Angle Scattering of Polarized Neutrons

Method of Phase–Amplitude Functions in the Polarized Neutron Reflectometry

Method of Phase–Amplitude Functions in the Polarized Neutron Reflectometry

Polarized neutron reflectometry study on the modulation of resistance and magnetism in resistive switching cobalt ferrite thin films

In this work, the resistive switching and electrical-control of magnetization in Pt/CoFe2O4/Nb:SrTiO3 heterostructures have been investigated. The films exhibit a classic bipolar resistive switching effect with a maximum switch ratio of about 5 × 103 and good anti-fatigue performance. Associated with resistive switching, the saturated magnetization of the thin film at high resistance state is found to be larger than that at low resistance state. Meanwhile, polarized neutron reflectivity of the thin film under different resistance states was in situ measured. The results reveal that the interfacial migration of oxygen vacancies driven by an applied electric field plays an important role in the modulation of resistive and magnetism of CoFe2O4 resistive switching devices.

Vertical inhomogeneous magnetic order in FeRh film

FeRh films, known as antiferromagnetic materials at low temperatures, exhibit unexpected ferromagnetic (FM) characteristics, unlike the bulk. The detailed temperature-dependent physical properties of FeRh films were examined to elucidate the causes. Temperature-dependent magnetic and electrical measurements showed that the coexistence of antiferromagnetic and residual FM states induces thermomagnetic irreversibility (e.g., spin-glass-like behavior) and negative magnetoresistance. Polarized neutron reflectometry (PNR) revealed the presence of non-uniform FM states at the interfaces. Additional temperature-dependent PNR profile and room temperature structural and spectroscopic analyses confirmed that the bottom FM region is correlated with structural distortion. In contrast, the top FM region is temperature-dependent and originates from non-stoichiometry.

Magnetized FeSi film with micrometer thickness inserted with Cr layers as [formula omitted]-flippers for neutron spin-echo spectrometry

Magnetized soft ferromagnetic films with micrometer thickness can be used as π-flippers for a spin echo small-angle neutron scattering (SESANS) instrument. Two FeSi monolayers and two FeSi/Cr multilayers were fabricated using a direct-current magnetron sputtering technique. The thicknesses of the two FeSi monolayers were 114 nm and 1140 nm, respectively. The two FeSi/Cr multilayers with 2 and 20 periods, respectively, were composed of FeSi layers having thicknesses of 1000 nm and 100 nm, respectively, and 10 nm-thick Cr layers. Hysteresis loop measurements and X-ray diffraction measurements were used to characterize their magnetic properties and texture, respectively. Polarized neutron flipping efficiency measurements were conducted using the TPNR (Time-of-flight and Polarized Neutron Reflectometer) to determine the flipping efficiency of the FeSi/Cr multilayer with [100 nm FeSi/10 nm Cr]20. The experimental results show that the saturation magnetization of the FeSi-monolayers decreased and the coercivity increased as their thickness increased. The FeSi/Cr multilayer with [100 nm FeSi/10 nm Cr]20 had higher saturation magnetization (1300 emu/cm 3) and lower coercivity than those of [1000 nm FeSi/10 nm Cr]2, although the total thickness of the FeSi layers was almost the same, and its maximum flipping efficiency was up to 98% for polarized neutrons.

Neutron Polarization Observables in d(vec {gamma },vec {n})p at Low Energies of Interest to Astrophysics

A model-independent theoretical analysis of neutron polarization observables in d(vec {gamma },vec {n})p using circularly polarized photons at the range of energies of interest to Big Bang Nucleosynthesis is presented. An investigation of various spin dependent observables is carried out including the isoscalar multipole amplitudes M{{1}_{s}} and E{{2}_{s}}. It is suggested that the measurement of neutron polarization in the final state at near threshold energies will be very useful to assess the contribution of isoscalar amplitudes at range of energies of interest to Big Bang Nucleosynthesis.

Direct Probe of the Ferromagnetic Proximity Effect at the Interface of SnTe/Fe Heterostructure by Polarized Neutron Reflectometry.

Introducing magnetic order into a topological insulator (TI) system has attracted much attention with an expectation of realizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) and axion insulator states. The magnetic proximity effect (MPE) is one of the promising schemes to induce the magnetic order on the surface of a TI without introducing disorder accompanied by doping magnetic impurities in the TI. In this study, we investigate the MPE at the interface of a heterostructure consisting of the topological crystalline insulator (TCI) SnTe and Fe by employing polarized neutron reflectometry. The ferromagnetic order penetrates ∼2.2 nm deep into the SnTe layer from the interface with Fe, which persists up to room temperature. This is induced by the MPE on the surface of the TCI preserving the coherent topological states without introducing the disorder by doping magnetic impurities. This would open up a way for realizing next-generation spintronics and quantum computational devices.

Polar magnetization unveiled by polarized neutron diffraction

Polar magnetism is present when ions occupy sites that are not centres of inversion symmetry. Fortunately, such magnetization contributes to neutron scattering that is the bedrock of magnetic structure determinations. Experiments in which the scattered neutron polarization is analysed are not a novelty. Simulations of polarized neutron scattering amplitudes for room temperature haematite (alpha-Fe2O3) demonstrate the wealth of information on offer. Two magnetic motifs distinguished by the orientation of their bulk ferromagnetism are considered. Additionally, the symmetry-inspired simulations challenge a recent claim to have determined the absolute direction of the Dzyaloshinskii-Moriya (D-M) interaction.

High resolution ferromagnetic and antiferromagnetic correlations in La0.7Ca0.3MnO3

La0.7Ca0.3MnO3 (LCMO30) is a perovskite manganite with a phase transition (TC = 238 K) between a paramagnetic insulator and a ferromagnetic metal. We use single-crystal, inelastic neutron scattering to refine previous work on the low energy (ℏω < 10 meV) magnetic excitations. At 300 K the spectrum consists of a diffusive ferromagnetic component at the pseudo-cubic Brillouin zone center coexisting with a quasielastic excitation located at the pseudo-cubic Brillouin zone edge. Both components are anti-correlated with ferromagnetic long-range order, showing weakened intensity at 215 K in the ferromagnetic phase. Polarized neutron scattering shows both features to be purely magnetic in origin. This indicates the simultaneous presence of ferromagnetic and antiferromagnetic correlations (located, respectively, at the zone center and zone edge) in a three-dimensional perovskite, while this coexistence had been previously observed only in a bilayer compound.

Development of Time-of-Flight Polarized Neutron Imaging at the China Spallation Neutron Source

A time-of-flight polarized neutron imaging setup was realized by integrating an in situ pumped polarized 3He spin filter and energy dispersive neutron camera on the neutron technique development beamline (BL-20) of the China Spallation Neutron Source (CSNS). Test experiments were performed with a solenoid with aluminum wire as a sample. These demonstrated that polarized radiography with a field of view in diameter 2.0 cm at different wavelengths can be obtained. The wavelength-dependent polarization was used to distinguish the neutron polarization behavior for different positions inside and outside the solenoid. The results of this work show the possibility of applying the technique at CSNS and marks a milestone for future polarized neutron imaging developments.

Loop currents in quantum matter

In many quantum materials, strong electron correlations lead to the emergence of new states of matter. In particular, the study in the last decades of the complex phase diagram of high temperature superconducting cuprates highlighted intra-unit-cell electronic instabilities breaking discrete Ising-like symmetries, while preserving the lattice translation invariance. Polarized neutron diffraction experiments have provided compelling evidences supporting a new form of intra-unit-cell magnetism, emerging concomitantly with the so-called pseudogap state of these materials. This observation is currently interpreted as the magnetic hallmark of an intra-unit-cell loop current order, breaking both parity and time-reversal symmetries. More generally, this magneto-electric state is likely to exist in a wider class of quantum materials beyond superconducting cuprates. For instance, it has been already observed in hole-doped Mott insulating iridates or in the spin liquid state of hole-doped 2-leg ladder cuprates.

J-PARCにおける高性能&lt;sup&gt;3&lt;/sup&gt;He中性子スピンフィルタの開発とその先導研究

3He spin filter is a useful neutron polarization device as a neutron polarizer and an analyzer in neutron scattering experiments because of its capabilities to polarize neutrons in broad energy range and cover wide solid angle. We are developing a 3He spin filter, based on the Spin Exchange Optical Pumping (SEOP) for polarized neutron scattering experiments at Materials and Life Science Experimental Facility (MLF) of J-PARC. A 3He gas-filling station was constructed at J-PARC, and several 3He cells with long spin relaxation times have been fabricated. A compact pumping systems with laser powers of 110 W was developed, and a 3He polarization of 85 % was achieved under real experimental conditions. Several user experiments utilizing the 3He spin filter are now being conducted at several beamlines at MLF. The development and utilization of 3He spin filters at MLF of J-PARC are reported.

Anisotropic spin-wave excitations in multiferroic BiFeO3

Polarized inelastic neutron-scattering experiments have been performed to elucidate the anisotropic behavior of the low-energy spin-wave excitations in a multiferroic ${\mathrm{BiFeO}}_{3}$, which shows a cycloidal spin structure below 640 K. Using neutron polarization analysis for single magnetic domain crystals, magnetic excitation modes in and out of the cycloidal plane below 6 meV were separated successfully. The magnetic excitation spectra were analyzed using linear spin-wave theory. The low-energy magnon density of states consist of several magnon modes, including the two anisotropic modes, $\mathrm{\ensuremath{\Phi}}$ and $\mathrm{\ensuremath{\Psi}}$ modes, distributed in and out of the cycloidal plane, respectively, which were previously observed using optical spectroscopies. Furthermore, there are other magnon modes that are not active in optical measurements. A model spin Hamiltonian, which reproduces the spin-wave frequencies observed using optical spectroscopies, explains the overall spectra reasonably well.

Cosmic-ray atmospheric cutoff energies of polar neutron monitors

The atmospheric cutoff, similarly to the geomagnetic cutoff, is the lower energy limit for cosmic ray particles that can reach a given location on the ground and be registered by a detector there, e.g., by a neutron monitor. It is caused by the decreasing intensity of a cosmic-ray cascade in the lower atmosphere. Although the geomagnetic cutoff is higher than atmospheric over the most of the Earth’s surface, the latter is dominant and therefore defines the neutron monitor count rate in the polar regions. The atmospheric cutoff decreases with the altitude, and this provides additional sensitivity of high-altitude polar neutron monitors to low-energy particles, mainly solar energetic protons during the so-called ground-level enhancement events.In this work, we quantitatively estimated the atmospheric cutoff energies for 21 polar neutron monitor stations in conditions without a significant solar energetic particle event. The cutoff value can be as low as about 300 MeV for protons (VOSTOK and DOMC/DOMB stations), which is notably lower than about 430 MeV at sea level. In addition to that, we estimated the worst case scenario of the strongest-ever observed ground-level enhancement event GLE#05, occurred on the 23rd of February, 1956, and showed that the atmospheric cutoff becomes as low as about 100 MeV. In other words, some neutron monitor stations can register particles with energies of even about 100 MeV during an exceptionally strong solar particle event. It is explained by the highly intensive and soft spectrum of the event in its early delayed phase.

Magnetic structure of the topological semimetal Co3Sn2S2

${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ has recently been predicted to be a Weyl semimetal in which magnetic order is key to its behavior as a topological material. Here, we report unpolarized neutron diffraction and spherical neutron polarimetry measurements, supported by magnetization and transport data, which probe the magnetic order in ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ below ${T}_{\text{C}}=177$ K. The results are fully consistent with ferromagnetic order in which the spins on the Co atoms point along the crystal $c$ axis, although we cannot rule out some canting of the spins. We find no evidence for a type of long-ranged $(\mathbf{k}=\mathbf{0})$ in-plane ${120}^{\ensuremath{\circ}}$ antiferromagnetic order which had previously been considered as a secondary phase present at temperatures between $\ensuremath{\sim}$90 K and ${T}_{\text{C}}$. A discontinuous change in bulk properties and neutron polarization observed at $T=125$ K when samples are cooled in a field and measured on warming is found to be due to a sudden reduction in ferromagnetic domain size. Our results lend support to the theoretical predictions that ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ is a magnetic Weyl semimetal.

Probing Flat Band Physics in Spin Ice Systems via Polarized Neutron Scattering.

In this Letter, we illustrate how polarized neutron scattering can be used to isolate the spin-spin correlations of modes forming flat bands in a frustrated magnetic system hosting a classical spin liquid phase. In particular, we explain why the nearest-neighbor spin ice model, whose interaction matrix has two flat bands, produces a dispersionless (i.e., "flat") response in the non-spin-flip (NSF) polarized neutron scattering channel and demonstrate that NSF scattering is a highly sensitive probe of correlations induced by weak perturbations that lift the flat band degeneracy. We use this to explain the experimentally measured dispersive (i.e., nonflat) NSF channel of the dipolar spin ice compound Ho_{2}Ti_{2}O_{7}.

Design of a neutron polarizing bender for a cold triple-axis spectrometer

We report on a design study for a supermirror based neutron polarizer for a cold neutron triple-axis spectrometer to be installed at the MLZ in Garching. The spectrometer will be located at a midway position of a neutron guide, with other non-polarized instruments following downstream, such that the neutron polarizer has to be installed in the monochromatic beam downstream but close to the monochromator inside the monochromator shielding. The polarizer operates in a wavelength band 2Å –6Å . The design is optimized by ray tracing simulations with the McStas software. We compare the performance of the conventional reflection and transmission type C-benders, V-bender (kink-polarizer), S-bender, and V-cavity. The latter two will only be briefly discussed here. We also propose a new design for a transmission C-bender with additional polarizing collimators.

Polarized neutron reflectometry characterization of interfacial magnetism in an FePt/FeRh exchange spring

We report on the depth-sensitive, temperature-dependent exchange coupling in an FePt/FeRh thin-film exchange-spring structure. The depth-dependent in-plane magnetization is measured as a function of applied magnetic field and sample temperature using polarized neutron reflectometry (PNR). The magnetization profiles are interpreted in terms of the competition between anisotropy, exchange coupling, and dipolar coupling as the FeRh undergoes the magnetic phase transition from antiferromagnetic to ferromagnetic ordering. The PNR data are combined with bulk magnetometry and x-ray characterization, allowing us to determine characteristic length scales over which the exchange-spring mechanism is effective at ambient and elevated temperatures.

Magnetic correlations of iron oxide nanoparticles as probed by polarized SANS in stretched magnetic nanoparticle-elastomer composites.

We have investigated the magnetic correlations among 7 nm iron oxide nanoparticles embedded in stretched silicone elastomers using polarized Small Angle Neutron Scattering (SANS). The magnetic nanoparticle (MNP)-elastomer composite can be stretched during experiments, and macroscopic deformations cause rearrangement of the iron oxide particles on the nanoscale. Polarized neutrons can be used to nondestructively probe the arrangement of magnetic nanoparticles before and after stretching, so that the relationship between applied stress and nanoscale magnetization can be interrogated. We find that stretching the MNP-elastomer composite past a certain threshold dramatically changes the structural and magnetic morphology of the system. The unstretched sample is modeled well by ~40 nm clusters of ~7 nm particles arranged in a hard sphere packing with a "volume fraction" parameter of 0.3. After the sample is stretched 3× its original size, however, the scattering data can be modeled by smaller, 16 nm clusters with a higher volume fraction of 0.4. We suggest that the effect can be understood by considering a stretching transformation on FCC-like crystallites of iron oxide nanoparticles embedded in an elastomeric medium.

Study of Chromium Migration in a Nickel-Based Alloy Using Polarized Neutron Reflectometry and Rutherford Backscattering Spectrometry

Study of Chromium Migration in a Nickel-Based Alloy Using Polarized Neutron Reflectometry and Rutherford Backscattering Spectrometry