Magnetic nanostructures in liquid and amorphous materials as probed by small angle neutron scattering

Small angle neutron scattering using polarized neutrons is introduced as a special type of contrast variation for magnetic systems. Examples of diluted magnetic systems are reviewed where low magnetic contrasts had to be analyzed beside strong nuclear contributions or vice versa. In Ferrofluids magnetic core–shell composite particles and magnetic aggregates could be precisely evaluated beside non-magnetic micelles and free surfactants of similar sizes. In more concentrated Co-Ferrofluids an external magnetic field induces a pseudo-crystalline hexagonal ordering which coexists with chain like arrangements of particles. In magnetic glasses magnetization density profiles have been monitored. Magnetically inactive deadlayers or weakly magnetic interfaces separate nanocrystalline ferromagnetic particles from amorphous (para- or ferromagnetic) matrices in soft magnetic materials based on Fe₃O₄-glass ceramics, Fe–Si–B and Fe–Nb–B alloys. Nanosized inclusions have been detected and identified to be at the origin of the hard magnetic properties of Nd based alloys.     

Atomic structure of Al55(Cr1−xMnx)15Si30 metallic glasses observed by neutron diffraction

An atomic structure of Al₅₅(Cr_1−xMn_x)₁₅Si₃₀ (x = 0, 0.49,1) metallic glasses was studied by neutron diffraction. An advantage of the neutron diffraction technique was fully exploited by utilizing the negative scattering length of Mn to form a neutron zero scattering ‘alloy’ for the component Cr_0.51Mn_0.49 in this quaternary Al---(Cr, Mn)---Si alloy. This allows the atomic distribution of the resulting quasibinary Al---Si metallic glass to be derived directly. Moreover, the (Al, Si)---TM (TM = Mn, Cr) and TM---TM pair correlations were also extracted by taking appropriate linear combinations of the atomic structures for the Al₅₅(Cr_1−xMn_x)₁₅Si₃₀ (x = 0, 0.49, 1) metallic glasses. A sharp first peak in the (Al,Si) ---TM pair correlations thus obtained led to the conclusion that a strong attractive interaction exists between (Al, Si) and TM atoms and, hence, that the presence of the TM atoms is responsible for the formation of an amorphous phase.     

Growth of large doped Bi12SiO20 crystals

The conditions for growing Cr-, Mn-, Fe-, Co-, Ni-, and Cr + Al-doped BSO crystals from a stoichiometric melt by the Czochralski method have been the subject of research. A decrease of the absorption coefficient along the growth axis was observed in undoped crystals of 50 mm diameter and 150 mm long, which is indicative of melt stoichiometry change in the course of the growth run. The transmission and reflection spectra of different crystals were measured and the absorption coefficients was determined in the range 0.38–0.70 μm. When Fe dopant concentration exceeds a limit value, the absorption spectrum is blue-shifted relative to the undoped case and the absorption coefficient decreases. Co, Cr, Mn, and Cr + Al dopants produce a red shift and increase the absorption coefficient. Combined Cr + Al doping shift the absorption spectrum to the blue region as compared to Cr-only doping with the same Cr concentration.     

Feasibility of study magnetic proximity effects in bilayer “superconductor/ferromagnet” using waveguide-enhanced polarized neutron reflectometry

A resonant enhancement of the neutron standing waves is proposed to use in order to increase the magnetic neutron scattering from a “superconductor/ferromagnet” (S/F) bilayer. The model calculations show that usage of this effect allows to increase the magnetic scattering intensity by factor of hundreds. Aspects related to the growth procedure (order of deposition, roughness of the layers etc.) as well as experimental conditions (resolution, polarization of the neutron beam, background etc) are also discussed. Collected experimental data for the S/F heterostructure Cu(32 nm)/V(40 nm)/Fe(1 nm)/MgO confirmed the presence of a resonant 60-fold amplification of the magnetic scattering.     

Crystal Structure of Synthetic High-Chromium Analogue of Yimengite

Crystallography Reports - The structure of a potassium compound with the chemical formula K0.90Ti5.16Cr2.94Fe2.54Mg0.87Al0.22Mn0.30O19 has been investigated by single-crystal X-ray diffraction...     

Neutron nano-spin-echo spectrometer based on magnetic nanostructures

A neutron spin-echo spectrometer based on spin precessors in the form of magnetic layered nanostructures is described. A model of a spin-echo spectrometer is developed on beam no. 9 in the IBR-2 reactor. In this model, spin precession occurs during motion of neutrons in a magnetic field and their double reflection from Al(30 nm)/Fe(15 nm)/Al(120 nm)/Cu(150 nm) magnetic layered structures. The obtained spectrometer parameters make it possible to investigate excitations in films with a wave vector oriented along the neutron beam direction in the range from 10^?3 to 10^?1 Å^?1 and perpendicularly to the beam in the range from 10^?4 to 10^?5 Å^?1.     

Texture investigations of Mn?Al magnets by neutron time-of-flight diffraction

In the present work the connection between magnetic properties and texture of two samples of the tetragonal Mn Al phase is investigated. The specimens have been prepared by different cold deformation methods to get various texture types. For texture analysis neutron time-of-flight technique has been applied. Some aspects of the mathematical treatment of experimental data has been discussed. The main texture components in the inverse pole figures are inclined with respect to the direction of easy magnetization. Therefore, improved magnetic properties of the material can be expected using more optimal preparation techniques.     

Interatomic force interaction in an i-AlCuFe quasicrystal

Partial spectra of thermal vibrations of Al, Cu, and Fe atoms in an icosahedral quasicrystal have been obtained by the isotopic-contrast method in inelastic neutron scattering. Joint analysis of these results and the published data on the atomic and electronic structures of the icosahedral i-AlCuFe quasicrystal has been performed. A physical model of the quasicrystal structure is proposed that is in agreement with the existing experimental data and qualitatively describes the peculiarities of interatomic interaction.     

Magnetic correlations in the CeAl3 heavy-fermion system

The inelastic neutron scattering spectra of the CeAl₃ heavy-fermion system are measured over a wide range of momentum transfers with the aim of determining the type of ion-ion magnetic correlations. The oscillatory dependence of the intensity of quasi-elastic magnetic neutron scattering on the momentum transfer is revealed at temperatures lower than those corresponding to the energy of splitting in the crystal electric field. An analysis of the obtained dependence of the intensity of quasi-elastic magnetic neutron scattering indicates the presence of short-range ferromagnetic correlations in the system.     

Orientational Disorder in the Plastic Phases of SF6 and CBr4

The coherent, elastic neutron scattering from polycrystalline samples of SF₆ and CBr₄ in their plastic phases has been measured. The powder line component of the neutron distributions has been analysed in terms of Debye-Waller factors to give information about the orientational disorder in the solids. An extensive diffuse scattering component is also observed in CBr₄. The scattering calculated for a simple model of the molecular disorder in CBr₄ agrees reasonably well with this observed diffuse scattering, but improvements in the model are clearly necessary.     

Observation of chiral ordering of moments in the magnetic mesostucture of the (Pd0.984Fe0.016)0.95Mn0.05 alloy by means of polarized neutrons

Investigation of the magnetic structure of the diluted (Pd_0.984Fe_0.016)_0.95Mn_0.05 alloy at meso-and nanoscale levels by means of two techniques of small-angle polarized-neutron scattering (within a direct beam and beyond it) has been performed in the temperature range 10<T<60 K. The dependences of the neutron beam depolarization, the polarization rotation angle, and the polarization-dependent magnetic scattering cross section on a weak (0<H<40 A/cm) external magnetic field have been studied. A simple model of neutron beam depolarization by a sample with uniaxial magnetic anisotropy was used to analyze the results obtained. The experiment on polarized-neutron scattering in the so-called direct geometry showed the existence of a polarization-dependent scattering cross section. This scattering has left-right asymmetry and depends on temperature. Comparison of these results with the depolarization data leads to a conclusion about the existence of static chiral fluctuations in large-scale inhomogeneities.     

Neutron measurements of magnetic correlations in metallic glasses

Neutron diffraction experiments have been made on FeB and PdSi transition metal alloy glasses prepared by melt-spinning. The magnetic neutron scattering has been analysed to provide information about the magnetic order in the specimens. An Fe₈₃B₁₇ metallic glass has magnetic correlations at room temperature with a range commensurate with the variations in atomic structure. These correlations appear to have a structure based on fcc gamma-iron, and the magnetic disorder is small - given that the specimen was in a field of 1 kG. Magnetic scattering was observed from Pd₇₀Si₂₀Co₁₀ and Pd₆₇Si₂₀Fe₁₃ at room temperature, which indicated a large magnetic moment value for the transition metal atoms. Cooling the PdSiCo sample to 80 K induced an antiparallel moment correlation, while further cooling to 7 K produced little obvious change in the scattering. These latter results are discussed in the light of the considerable disagreement which exists between different investigations of these alloys.     

Algorithms for solving atomic structures of nanodimensional clusters in single crystals based on X-ray and neutron diffuse scattering data

New algorithms for solving the atomic structure of equivalent nanodimensional clusters of the same orientations randomly distributed over the initial single crystal (crystal matrix) have been suggested. A cluster is a compact group of substitutional, interstitial or other atoms displaced from their positions in the crystal matrix. The structure is solved based on X-ray or neutron diffuse scattering data obtained from such objects. The use of the mathematical apparatus of Fourier transformations of finite functions showed that the appropriate sampling of the intensities of continuous diffuse scattering allows one to synthesize multiperiodic difference Patterson functions that reveal the systems of the interatomic vectors of an individual cluster. The suggested algorithms are tested on a model one-dimensional structure.     

Spin disorder in Fe-doped manganites

Ln_0.7M_0.3MnO₃ compounds are well-known ferromagnets mediated by a double exchange mechanism. As Mn atoms are substituted by Fe in the ratio Mn_1−xFe_x the magnetic structure dramatically changes, because the ferromagnetic double exchange chain is broken. At low Fe concentrations all compounds are magnetically ordered. For intermediate values ferro (FM), antiferro (AF) and paramagnetic (PM) phases co-exist in a large temperature range, and at x ≈ 0.2 spin or cluster-glass behavior is found. Magnetization, Mössbauer, polarized and low angle neutron scattering as well as muon spin relaxation experiments have been performed on 0 ⩽ x ⩽ 0.30 compounds showing the transit from long range ferromagnetism to spin glass. Co-existence of FM and AF clusters of different size has been found for all doped compounds.     

Investigation of the ultrasonic wave influence on magnetic ordering in a 20 × [Fe(20 Å)/Cr(12 Å)]/MgO layered structure

A 20 × [Fe(20 Å)/Cr (12 Å)]/MgO structure excited by an ultrasonic wave was investigated by polarized neutron reflectometry. It was found that magnetic domains oscillate and their effective size decreases in weak magnetic fields. In close-to-saturation fields, a magnetic lattice is formed in the layered structure; the interplanar spacings in this lattice change with an increase in the field strength and as a result of the excitation of the structure by an ultrasonic wave.     

Hexagonal frustrated RMnO3 manganites (R = Y, Lu) under high pressure

The crystalline and magnetic structures of YMnO₃ and LuMnO₃ hexagonal manganites under pressures of 0–6 GPa and in the temperature range 10–295 K have been investigated by neutron diffraction. Application of pressure leads to a significant decrease in the ordered magnetic moment of Mn ions (at T = 10 K) from 3.27 (0 GPa) to 1.52 μ_B (5 GPa) for YMnO₃ and from 2.48 (0 GPa) to 1.98 μ_B (6 GPa) for LuMnO₃. Under high pressures, spin reorientation of Mn magnetic moments and a change in the symmetry of the antiferromagnetic structure are observed in YMnO₃. The relationship between the triangular lattice distortion parameter and the symmetry of the triangular antiferromagnetic state of RMnO₃ hexagonal manganites is discussed.     

Anwendungsmöglichkeiten der Analyse der diffusen Streuung von Röntgenstrahlen zur Untersuchung von einkristallinen Halbleiter-Mischkristallen

In order to determine the interaction parameters of the atoms in semiconducting solid solutions of the systems Ge Si, GaAs GaP und GaAs AlAs the diffuse scattering of X-rays by single crystals was analyzed on the base of the theory of KRIVOGLAZ . The results enable one to calculate the phase diagrams of the system taken into consideration.     

Evolution of cellular spacing during directional solid-state ferrite–austenite transformation of Fe–Mn–Al alloy

A series of isovelocity experiments was performed to investigate the interface morphology evolution during the solid-state ferrite–austenite transformation of the Fe–Mn–Al alloy, and a quantitative relationship between cellular spacing and growth velocity had been measured. The corresponding cellular spacing of the product phase decreases with the growth velocity increasing. Theoretical model modified from directional solidification process was also applied to study the relationship between cellular spacing and growth velocity, and the theoretical prediction corresponds well with the measured results.     

Effect of Al substitution on the magnetic properties of amorphous Fe73.5Cu1Mo3Si13.5−xAlxB9 alloy

We have synthesized FINEMET type amorphous Fe_73.5Cu₁Mo₃Si_13.5−xAl_xB₉ alloy by the single wheel melt spinning technique. The effect of Al substitution on the magnetic properties has been studied using a vibrating sample magnetometer, SQUID and Mössbauer spectroscopy. Magnetization and Curie temperature of the amorphous phase of the alloys were found to decrease with Al concentration. The results are attributed to the dilution effect of Al on the magnetic moment of Fe and to the increase in Fe–Fe interaction distance resulting in the weakening of exchange interaction.     

Features of the magnetic state of the layered Fe-V nanostructure of the superconductor-ferromagnet type

The magnetic state of the V(39 nm)/20 [V(3 nm)/Fe(3 nm)] nanostructure has been investigated by polarized neutron reflectometry in the temperature range from 1.6 to 30 K in magnetic fields from 0.2 to 15 kOe. The data obtained indicate that the superconductivity of vanadium layers may affect magnetic ordering both over the depth of the structure and in its plane.