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The neutron yield of a compact accelerator driven neutron source depends strongly on the target performance. This performance is influenced by the target composition and geometry, the cooling system design and which primary particles are used. We show that the temperature difference inside the target depends directly on the target thickness determined by the ion stopping range and therefore on the type and energy of the primary particle. Deuterons with a larger stopping power show a smaller temperature difference inside the target than protons allowing thus for a better target cooling.

Journal of physics / Condensed matter Condensed matter 32(16), 165801 - (2020). doi:10.1088/1361-648X/ab678c, Published by IOP Publ., Bristol

By performing high-pressure single-crystal neutron-diffraction measurements, the evolution of structure and magnetic ordering in ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ under hydrostatic pressure were investigated. Both the tetragonal-to-orthorhombic structural transition and the Fe spin-density-wave transition are gradually suppressed and become decoupled with increasing pressure. The antiferromagnetic order of the Eu sublattice is, however, robust against the applied pressure up to 24.7 kbar, without showing any change of the ordering temperature. Under the pressure of 24.7 kbar, the lattice parameters of ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ display clear anomalies at 27(3) K, well consistent with the superconducting transition observed in previous high-pressure resistivity measurements. Such an anomalous thermal expansion around ${T}_{c}$ strongly suggests the appearance of bulk superconductivity and strong electron-lattice coupling in ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ induced by the hydrostatic pressure. The coexistence of long-range ordered Eu antiferromagnetism and pressure-induced superconductivity is quite rare in the ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$-based iron pnictides.

The manipulation of the magnetism of self-assembled iron oxide nanoparticle (NP) monolayers on top of BaTiO3 (BTO) single crystals is reported. We observe strain induced magnetoelectric coupling (MEC) as shown by measurements of both the magnetization and magneto-electric AC susceptibility (MEACS). The magnetization, coercivity, remanent magnetization and MEACS signal as a function of temperature show abrupt jumps at the BTO phase transition temperatures. Hereby the jump values are opposite for in-plane and out-of-plane measurements. Grazing incidence small angle X-ray scattering (GISAXS) and scanning electron microscopy (SEM) confirm a hexagonal close-packed supercrystalline order of the NP monolayers. Cross-sectional scanning transmission electron microscopy (STEM) experiments provide information about the layer structure of the sample. This work opens up viable possibilities for fabricating energy-efficient electronic devices by self-assembly techniques.

We report about the magnetic properties of antiferromagnetic (AF) MnO nanoparticles (NPs) with different sizes (6--19 nm). Using a combination of polarized neutron scattering and magnetometry, we were able to resolve previously observed peculiarities. Magnetometry, on the one hand, reveals a peak in the zero-field-cooled (ZFC) magnetization curves at low temperatures $(\ensuremath{\sim}25$ K) but $no$ feature around the N\'eel temperature at 118 K. On the other hand, polarized neutron scattering shows the expected behavior of the AF order parameter vanishing around 118 K. Moreover, hysteresis curves measured at various temperatures reveal an exchange-bias effect, indicating a coupling of an AF core to a ferromagnetic (FM)-like shell. ZFC data measured at various fields exclude a purely superparamagnetic (SPM) scenario. We conclude that the magnetic behavior of MnO particles can be explained by a superposition of SPM-like thermal fluctuations of the AF-N\'eel vector inside the AF core and a magnetic coupling to a ferrimagnetic ${\mathrm{Mn}}_{2}{\mathrm{O}}_{3}$ or ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ shell. In addition, we have studied heterodimer (``Janus'') particles, where a FM FePt particle is attached to the AF MnO particle. Via the exchange-bias effect, the magnetic moment of the FePt subunit is stabilized by the MnO.

A neutron powder diffraction (NPD) study on the crystal and magnetic structure of a crushed La1-xSrxMnO3 (x ≈ 1/8) single crystal has been performed. The sample belongs to orthorhombic (Pnma, O) above the Jahn-Teller (JT) transition temperature (TJT) and monoclinic (P121/c1, M′) in the JT regime. We have also refined the NPD data below the charge/orbital ordering (CO/OO) temperature (TCO/OO) with a monoclinic (P121/c1, M′′) model because the experimental resolution was insufficient to clearly identify a triclinic $(P\bar {1})$ structure. The refined lattice parameters show an obvious breathing-mode distortion between TCO/OO and TJT, accompanied by a large deviation of the monoclinic angle β from 90°, signifying a very strong cooperative JT distortion. A ferromagnetic (FM) moment of 3.43(5)μ B/Mn besides an A-type antiferromagnetic (A-AFM) moment of 0.54(2) μ B/Mn is directed mainly along the b axis in P121/c1 symmetry at 5 K. With increasing temperature, the A-AFM domains transform into FM ones above ~100 K and the FM spin orientation turns from the b to the c axis in crystallographic b-c plane below Tc = 187(1) K. The magnetization measurements show typical anomalies around TCO/OO and TJT. The measured saturation moment of 3.9(1)μ B/Mn at 70 kOe and 5 K is well consistent with the sum 3.97(5)μ B/Mn of the refined FM and A-AFM moments at 5 K, implying the A-AFM spins are aligned in field direction at 70 kOe. The applied magnetic field can affect the paramagnetic insulating (PMI) state in the range of magnetic polarons. Based on the size of JT distortion and the bond-valence sums (BVS’s), the CO/OO phenomenon is being discussed.

A high intensity triple-axis spectrometer is described, which has been commissioned only a year before the final shut-down of the FRJ-2 reactor in May 2006. It was designed both as an instrument for quasi-elastic scattering with polarisation analysis on an area detector, and as a triple-axis spectrometer with polarization analysis for measurements of magnetic and nuclear inelastic scattering. The instrument features two exchangeable monochromators (Cu [200] and PG [002]), which enable both horizontal and vertical focusing. High monochromatic neutron current densities at the sample position of up to 4 × 107cm− 2 s− 1 (for pyrolytic graphite at 34 meV) and energies of up to 117 meV (with copper) have been achieved at the FRJ-2 (thermal flux density: 3 × 1014cm− 2 s− 1). The utilization of 3He filters both as polarizer and analyzer will enable polarization analysis for high-energy neutrons. The spectrometer is presently being prepared for installation at the new Chinese research reactor CARR at the China Ins...

We report a comprehensive specific heat and inelastic neutron scattering study to explore the possible origin of multiferroicity in HoCrO$_3$. We have performed specific heat measurements in the temperature range 100 mK - 290 K and inelastic neutron scattering measurements were performed in the temperature range 1.5 - 200 K. From the specific heat data we determined hyperfine splitting at 22.5(2) $\mu$eV and crystal field transitions at 1.379(5) meV, 10.37(4) meV, 15.49(9) meV and 23.44(9) meV, indicating the existence of strong hyperfine and crystal field interactions in HoCrO$_3$. Further, an effective hyperfine field is determined to be 600(3) T. The quasielastic scattering observed in the inelastic scattering data and a large linear term $\gamma=6.3(8)$ mJmol$^{-1}$K$^{-2}$ in the specific heat is attributed to the presence of short range exchange interactions, which is understood to be contributing to the observed ferroelectricity. Further the nuclear and magnetic entropies were computed to be, $\sim$$17.2$ Jmol$^{-1}$K$^{-1}$ and $\sim$34 Jmol$^{-1}$K$^{-1}$, respectively. The entropy values are in excellent agreement with the limiting theoretical values. An anomaly is observed in peak position of the temperature dependent crystal field spectra around 60 K, at the same temperature an anomaly in the pyroelectric current is reported. From this we could elucidate a direct correlation between the crystal electric field excitations of Ho$^{3+}$ and ferroelectricity in HoCrO$_3$. Our present study along with recent reports confirm that HoCrO$_3$, and $R$CrO$_3$ ($R=$ Rare earth) in general, possess more than one driving force for the ferroelectricity and multiferroicity., Comment: 17 pages, 6 figures, published in J. Phys.: Condens. Matter

While structural and magnetic lateral correlations in thin film materials can be investigated at the μm length scale by neutron off-specular scattering (OSS) with polarization analysis, they can also be investigated at the nm length scale by grazing incidence small-angle scattering of polarized neutrons (polarized GISANS). We exemplify this issue showing a combined OSS and GISANS study of the lateral correlations in a remanent polarizing supermirror.

We have investigated the structural and magnetic properties of (Er|Tb) multilayers by different scattering methods. Diffuse X-ray scattering under grazing incidence reveals the interface structure in (Er|Tb) bilayers and trilayers, indicating vertically correlated roughness between the Er and Tb interfaces. The magnetic properties of (ErnEr |TbnTb ) superlattices have been studied as a function of the superlattice composition (indices denote the number of atomic layers). Coupled ferromagnetic structures exist in all investigated samples. The phase transition temperature varies with the Tb layer thickness. Modulated magnetic order is short range for all samples beside the (Er20|Tb5) superlattice, the sample with the smallest Tb layer thickness. We observe dipolar antiferromagnetic coupling between single ferromagnetic Tb layers in all samples, with the onset of this ordering depending on the Tb layer thickness. Due to competing interactions, exchange coupling is limited to the interface near region. Therefore long range modulated magnetic order is observed in the (Er20|Tb5) superlattice only, where the interface regions overlap. The distinct differences to the magnetic structure of an Er0.8Tb0.2 alloy film are explained by a highly anisotropic arrangement of neighbouring atoms due to the correlated roughness. PACS. 75.75.+a Magnetic properties of nanostructures - 75.25.+z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.) - 61.18.Fs Magnetic resonance techniques; Mossbauer spectroscopy

While magnetic multilayers can be regarded as systems that are artificially structured along one dimension, laterally patterned magnetic structures are systems that are structured, in addition, along a second (nanostripes) or third (nanodots) dimension. Fundamentally, novel properties analogous to the interlayer exchange coupling and the giant magnetoresistance effect in magnetic multilayers can be expected if the size of the structures become comparable to or smaller than certain characteristic length scales, such as the spin diffusion length, carrier mean free path, magnetic domain wall width, etc. [1]. Typically, those length scales lie in the several-to-hundreds nm range.

Nuclear spin-polarized 3 He efficiently polarizes neutrons over a wide energy range including cold, thermal and hot neutron spectra by means of its strongly spin-dependent absorption of neutrons. At Forschungszentrum Julich we are studying the optical polarization of 3 He by two methods, metastable exchange and spin-exchange, in order to optimize the neutron polarization at various spectrometers of the FRJ-2 reactor. For the spin-exchange method we designed an open, refillable cell system. In contrast to the conventional sealed cells adopted at many other laboratories, we will be able to vary many crucial parameters in the 3 He cell, such as gas ratios of 3 He and N 2 , total gas pressure, and temperature while always working in the same experimental environment of a single cell. Laser polarized Rb vapor is used to polarize 3 He through the spin-exchange interaction, and NMR is employed to determine the polarization of the 3 He after calibration with a proton (water) NMR signal. In this paper we will report the 3 He polarization and relaxation rate, T 1, observed for various gas conditions.

Proximity effects in an [Er20|Tb5] superlattice lead to the formation of new magnetic phases. Modulated magnetic order, expected for pure Er, and ferromagnetic order, expected for pure Tb, coexist at low temperatures. Employing X-ray resonance exchange scattering, we could probe directly the respective spin polarisations of the conduction band electrons, providing a mechanism for the interlayer coupling. The different anisotropies of Er and Tb compete with this tendency to long-range magnetic order, leading to substantial thermal-hysteresis effects.

The surface layer of an epitaxial Fe/Cr multilayer system has been structured lithographically into stripes with 1 μm period. Polarized neutron reflectometry and diffuse scattering under grazing incidence measurements show strongly enhanced domain formation compared to the unstructured sample. The domains are correlated throughout the layers. The lateral domain size is partly the period of the stripe pattern of the surface layer, and partly the double.

A special Microsymposium on polarized neutron methods and technologies on existing and future pulsed neutron sources was held at PNCMI-2002. We present the recommendations formulated by three working groups on “Diffraction,” “Inelastic Scattering” and “Reflectometry.” Needs for method development as well as opportunities for novel polarized neutron instrumentation are identified.

${\mathrm{La}}_{0.4}{\mathrm{Na}}_{0.6}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ single crystals have been grown out of an NaAs flux in an alumina crucible and characterized by measuring magnetic susceptibility, electrical resistivity, specific heat, as well as single-crystal x-ray and neutron diffraction. ${\mathrm{La}}_{0.4}{\mathrm{Na}}_{0.6}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ single crystals show a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase at ${T}_{s}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}125$ K. This structural transition is accompanied by an anomaly in the temperature dependence of electrical resistivity, anisotropic magnetic susceptibility, and specific heat. Concomitant with the structural phase transition, the Fe moments order along the $a$ direction with an ordered moment of 0.7(1) ${\ensuremath{\mu}}_{\mathrm{B}}$ at $T=5$ K. The low-temperature stripe antiferromagnetic structure is the same as that in other $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($A$ = Ca, Sr, Ba) compounds. ${\mathrm{La}}_{0.5\ensuremath{-}x}{\mathrm{Na}}_{0.5+x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ provides a material platform for the study of iron-based superconductors where the electron-hole asymmetry could be studied by simply varying the La/Na ratio.

We have employed a new experimental configuration to measure ferromagnetic X-ray resonance exchange scattering (XRES) from a EuS single crystal. Using polarisation analysis we could achieve a magnetic-scattering intensity stronger than charge scattering resulting in an asymmetry ratio as large as Ra = 0.67 at the EuLII edge. By a combined refinement of the dependencies of the scattered intensity on the energy and the applied magnetic field we could uniquely determine spectroscopic information such as the 5d conduction band exchange splitting = 0.27(1) eV.

We report on an investigation of the magnetic properties of Gd x Eu 1 - x S mixed crystals with compositions in the range of 0.6 < x < 1. For the two samples Gd 0.8 Eu 0.2 S and Gd 0.73 Eu 0.27 S a long-range antiferromagnetic order was observed at low temperatures. Element-specific measurements exhibited a different temperature dependence of the reduced sublattice magnetisation of the two magnetic species. A model calculation and Monte Carlo simulations revealed that the different temperature dependence is due to frustration effects. These frustration effects lead to a breakdown of the long-range order for higher europium contents. For the Gd 0.67 Eu 0.33 S-sample we were able to observe a short-range antiferromagnetic order with correlation lengths of a few 10A with X-ray resonance exchange scattering.

We report on the first results of a structural and magnetic investigation of Fe 1-x Co x /Mn/Fe 1-x Co x trilayers (x = 0-0.75). In situ low energy electron diffraction shows that Mn grows epitaxially on bcc Fe 1-x Co x (x ≤ 0.75) with a good crystallinity up to 30 A. Reflectivity and small angle diffuse scattering of synchrotron radiation reveal sharp interfaces, each of them having two lateral characteristic lengths in the height-height correlation function that differ by one order of magnitude. The long-range height-height correlation function is vertically correlated and the short-range one is not. From magneto-optical Kerr effect measurements, evidence for non-collinear coupling between Fe 1-x Co x layers (x = 0 and 0.5) was found for Mn spacer thicknesses below 12 A. Polarized neutron reflectometry performed at 300 K reveal a net magnetization of 0.8 ± 0.3 μ B /at in the Mn layer in Fe 0.5 Co 0.5 /Mn/Fe 0.5 Co 0.5 and no net magnetization in the Mn layer in Fe/Mn/Fe.

We report on the determination of element-specific magnetic order for a Gd0.8Eu0.2S mixed crystal by means of X-ray resonance exchange scattering. For the first time, a different temperature dependence of the sublattice magnetization of two lanthanide ions was observed. With model calculations we were able to reproduce the measured temperature dependence of the reduced sublattice magnetization. We can show that competing magnetic interactions lead to the observed effects and can reveal the frustration mechanism.

Polycrystalline TbMn2O5 was prepared by the standard solid-state reaction method and characterized by powder X-ray diffraction and magnetization to assure it is of single phase. Heat capacity measurements on the compound reveal an antiferromagnetic phase transition at � 45 K. A broad peak below � 6 K in the heat capacity measurements corresponds to the crossover transition of Tb 3+ ordering. To confirm these magnetic orderings, neutron powder diffractions on TbMn2O5 with XYZ neutron polarization analysis were performed at the diffuse neutron scattering (DNS) spectrometer, FRJ-II, by

The magnetic correlation length of a mixed nickel ferrite powder was studied by a newly commissioned depolarized neutron beamline at the W3 port of Tsing Hua Open Pool Reactor (THOR). In this work, Ni ferrite powder samples with different packing densities were studied. The magnetic correlation lengths of the sample were observed to be 2 μm at virgin state and about 3.1 μm at remanent state from the packing density of 20–60%. This magnetic domain size is smaller than particle size. No significant change of domain size at this packing density implies the domain wall motion is hindered by the porosity effectively up to at least 60% of packing density.

Polarized neutron reflectivity (PNR) studies have been performed on granular multilayers [Co 80 Fe 20 ( t n )/Al 2 O 3 (3 nm)] m . For a non-percolating superferromagnetic (SFM) sample ( t n =1.3 nm) no spin-flip (SF) scattering was observed, measured at various points of the hysteresis, confirming that the magnetization reversal is achieved merely by domain nucleation and growth. For a percolated sample ( t n =1.6 nm) an oscillating magnetization depth profile from FeCo layer to FeCo layer along the multilayer stack was observed around the coercive field. The competition between long-range dipolar, short-range Neel and external field interactions apparently give rise to such a modulated magnetization depth profile in these magnetic multilayers.

Physical review / B 89(6), 064402 (2014). doi:10.1103/PhysRevB.89.064402, We have determined the field dependence of collective magnetic excitations in iron oxide nanoparticles of cubic shape with 8.42(2) nm edge length and a narrow log normal size distribution of 8.2(2)% using time-of-flight neutron spectroscopy. The energy dependence of the uniform precession modes was investigated up to 5 T applied field and yields a Landé factor g = 2.05(2) as expected for maghemite (γ -Fe2O3) nanoparticles. A large effective anisotropy field of BA,eff = 0.45(16) T was determined, in excellent agreement with macroscopic measurements. This anisotropy is attributed to enhanced shape anisotropy in these monodisperse cubic nanoparticles. The combination of our results with macroscopic magnetization information provides a consistent view of the energy scales of superparamagnetic relaxation and collective magnetic excitations in magnetic nanoparticles., Published by APS, College Park, Md.

Antisite disorder is observed to have significant impact on the magnetic properties of the double perovskite Y2CoMnO6 which has been recently identified as a multiferroic. A paramagnetic-ferromagnetic phase transition occurs in this material at T-c approximate to 75 K. At 2K, it displays a strong ferromagnetic hysteresis with a significant coercive field of H-c approximate to 15 kOe. Sharp steps are observed in the hysteresis curves recorded below 8K. In the temperature range 2K

X-ray, neutron, magnetization and Mossbauer studies in the temperature range 10–300 K are reported for Fe3−xCrxAl system with x < 0.6. The experiments indicate that a single phase having the DO3-type structure is maintained in the range of Cr concentration studied. It was found that Cr atoms occupy preferentially B-sites and the lattice constant decreases with increasing concentration of chromium. The magnetic moment of chromium is small and diminishes the value of neighbouring iron atoms by about 0.1 μB per Cr atom. The magnetic moments of iron at (A, C) and B sites were estimated to be 1.54 (22), 2.58 (22), 1.51 (27) and 2.32 (14) μB for T = 10 and 300 K, respectively. The magnetic moment of iron decreases by 0.37 (10) and 0.26 (6) μB at 10 K and T = 300 K, respectively, if one aluminium atom is found as a nearest neighbour of iron.

Since the advent of high brilliance synchrotron radiation sources, magnetic X-ray diffraction has become a standard microscopic probe for the investigation of magnetic structures, magnetic phase transitions and magnetic disorder phenomena. Modern experiments highlight the complementarity of this new probe to the standard neutron diffraction technique by taking advantage of the high momentum space resolution, the element sensitivity, the possibility to separate spinand orbital contributions etc. In this paper we present selected experiments performed at HASYLAB which exemplify the present possibilities of resonant and non-resonant magnetic X-ray diffraction.

Inelastic neutron scattering experiments were performed to investigate the crystalline electric field (CEF) excitations of Nd3+ (J = 9/2) in the iron pnictide NdFeAsO. The crystal field level structures for both the high-temperature paramagnetic phase and the low-temperature antiferromagnetic phase of NdFeAsO are constructed. The variation of CEF excitations of Nd3+ reflects not only the change of local symmetry but also the change of magnetic ordered state of the Fe sublattice. By analyzing the crystal field interaction with a crystal field Hamiltonian, the crystal field parameters are obtained. It was found that the sign of the fourth and sixth-order crystal field parameters change upon the magnetic phase transition at 140 K, which may be due to the variation of exchange interactions between the 4f and conduction electrons.

We present an exchange bias (EB) effect of variable size in La${}_{0.67}$Sr${}_{0.33}$MnO${}_{3\ensuremath{-}\ensuremath{\delta}}$ single and La${}_{0.67}$Sr${}_{0.33}$MnO${}_{3\ensuremath{-}\ensuremath{\delta}}$/SrTiO${}_{3}$ (LSMO/STO) bilayers grown on STO substrates. Samples have been prepared by pulsed laser deposition (PLD) and high-pressure sputter deposition (HSD) in oxygen atmosphere at different oxygen pressures. Increased out-of-plane lattice parameters of the LSMO layers and reduced Curie temperatures indicate oxygen deficiencies in the samples grown at lower oxygen pressures. We found an increase of the EB and the coercive fields with decreasing oxygen pressure, whereas the growth at high pressures does not lead to an EB effect. The depth-sensitive method of polarized neutron reflectometry (PNR) reveals a region with drastically reduced, maybe even completely vanishing net magnetization in the LSMO layer at the interface to STO in the exchange-biased samples, but not in the non-exchanged-biased one. It is suggested that an antiferromagnetic structure is formed in this part of the LSMO thin film, which causes the EB effect in this system. An explanation based on the strain-doping phase diagram of LSMO is provided. Thus our findings open a route for tuning the EB effect by changing the oxygen vacancy concentrations.

We report on neutron quantum well states in Fe/Co/Fe trilayers, in which the reflection potential exhibits a quantum well for both spin states of the neutron. It is shown, that the resonance state in such a system exists and manifests itself as a dip in the reflectivity plateau below the critical angle of total reflection. The range of trilayer parameters was numerically analyzed to find optimal experimental conditions for the enhancement of the neutron wave field in the Co spacer. Enhancement of the wave field by a resonance is suggested as a way to substantially increase the off-specular scattering signal from lateral domains in buried layers of Co.

We have investigated low energy nuclear spin excitations in the strongly correlated electron compound HoCrO3. We observe clear inelastic peaks at E = 22.18 ± 0.04 μeV in both energy loss and gain sides. The energy of the inelastic peaks remains constant in the temperature range 1.5–40 K at which they are observed. The intensity of the inelastic peak increases at first with increasing temperature and then decreases at higher temperatures. The temperature dependence of the energy and intensity of the inelastic peaks is very unusual compared to that observed in other Nd, Co, V and also simple Ho compounds. Huge quasielastic scattering appears at higher temperatures presumably due to the fluctuating electronic moments of the Ho ions that get increasingly disordered at higher temperatures. The strong quasielastic scattering may also originate in the first Ho crystal-field excitations at about 1.5 meV.

We report on interlayer coupling in [Ern(Er)|Tbn(Tb)] superlattices. By means of neutron diffraction, helix-like and conical magnetic structures are observed in an [Er20|Tb5] sample. These structures are correlated across terbium layers at low temperatures, where bulk terbium is ferromagnetic. Moreover, resonant X-ray magnetic scattering gives clear evidence that the 5d conduction band of terbium is polarized according to the erbium magnetic structure.

The magnetic structures adopted by the Fe and Sm sublattices in SmFeAsO have been investigated using element specific x-ray resonant and non-resonant magnetic scattering techniques. Between 110 and 5 K, the Sm and Fe moments are aligned along the c and a directions, respectively according to the same magnetic representation $\Gamma_{5}$ and the same propagation vector (1, 0, 0.5). Below 5 K, magnetic order of both sublattices change to a different magnetic structure and the Sm moments reorder in a magnetic unit cell equal to the chemical unit cell. Modeling of the temperature dependence for the Sm sublattice as well as a change in the magnetic structure below 5 K provide a clear evidence of a surprisingly strong coupling between the two sublattices, and indicate the need to include anisotropic exchange interactions in models of SmFeAsO and related compounds., Comment: 8 pages, 7 figures, accepted for publication in Phys. Rev. B

We investigate the structural, magnetic, and specific heat behavior of the hexagonal manganite Dy0.5Y0.5MnO3 in order to understand the effect of dilution of Dy magnetism with nonmagnetic yttrium. In this compound, the triangular Mn lattice orders antiferromagnetic at T-N(Mn) approximate to 68 K observed experimentally in the derivative of magnetic susceptibility as well as in specific heat. In addition, a low-temperature peak at T-N(Dy) similar to 3 K is observed in specific heat which is attributed to rare earth order. The T-N(Mn) increases by 9 K compared to that of hexagonal (h) DyMnO3 while T-N(Dy) is unchanged. A change in slope of thermal evolution of lattice parameters is observed to occur at temperature close to T-N(Mn). This hints at strong magnetoelastic coupling in this geometric multiferroic. In magnetization measurements, steplike features are observed when the magnetic field is applied along the c axis which shift to higher fields with temperature and vanish completely above 40 K. The presence of different magnetic phases at low temperature and strong magnetoelastic effects can lead to such field-induced transitions which resemble metamagnetic transitions. This indicates the possibility of strong field-induced effects in dielectric properties of this material, which is unexplored to date.

We present the results of a combined neutron scattering and magnetization study of the pseudo-one-dimensional magnetic compounds [(CH 3 ) 3 NH] Co 1-x Ni x Cl 3 .2 H 2 O, abbreviated as CoNiTAC. For Co rich samples, we determined the 3d magnetic structure to be canted antiferromagnetic having the magnetic space group Pnm«a». We performed Monte-Carlo simulations and could reproduce well the measured temperature dependence of the sublattice magnetization assuming an Ising type, spatially strongly anisotropic interaction. The exchange integral along the chains (b-direction) is at least 100 times larger than the average exchange in the a-c-plane. Despite this strong one dimensional character of the exchange interactions the critical exponent β was found to be very close to the theoretical value for a 3d Ising system (0.31). We did not discover any anomaly of the long range order component for the mixed crystals, for which a spin glass phase had been proposed by other authors on the basis of a decay of the TRM on macroscopic time scales. Instead, our polarized neutron diffraction and magnetization measurements reveal that the low temperature dynamics is connected to domain relaxation processes and not to a spin glass transition

The magnetization, neutron diffraction and Mössbauer spectroscopy studies have been carried out on Fe3−xCrxSi alloys with x

We report on measurements of the energy integrated neutron scattering due to magnetic short range order on an amorphous dilution series MnxCa3-xAl2(SiO4)3. We found experimental evidence for a transition from a frustrated magnetic system, present in samples with high Mn2+ concentrations, into one with magnetic short range order only (x=0.6).

We present results of a structural investigation of oxygen ordering in a 0.1-cm{sup 3} YBa{sub 2}Cu{sub 3}O{sub 6.35} single crystal. For the first time nonmagnetic superstructure reflections have been observed by neutron diffraction. From the intensities a 2 {radical}2 {ital a}{times}2 {radical}2 a{times}{ital c} oxygen ordering model is derived. In this structure half-filled'' CuO chains alternate with quarter-filled'' chains. Only 15% of the basal-plane oxygen atoms contribute to the superstructure.

We have investigated the incommensurate-commensurate lock-in phase transition in EuAs3 by the relatively new resonant X-ray magnetic scattering technique. Strong resonance enhancements of the magnetic scattering have been observed at the LII and LIII absorption edges of Eu. The intensity of the 1, 0, 5 2 magnetic reflection of the commensurate phase decreases continuously with increasing temperature and at TL = 10.3 K drops abruptly to zero. Satellite reflections characteristic of the incommensurate phase appear at T ⩾ TL. By using the excellent Q-resolution of the X-ray magnetic scattering experiment we have determined the temperature variation of both x and z components of the propagation vector of the incommensurate sine-wave phase stable in the temperature range 10.3 K ⩽ T ⩽ 11 K.

Polarized neutron reflectivity (PNR) and magnetometry studies have been performed on the metal-insulator multilayer ${[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}(1.6\phantom{\rule{0.3em}{0ex}}\mathrm{nm})∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(3\phantom{\rule{0.3em}{0ex}}\mathrm{nm})]}_{9}$ which exhibits dominant dipolar coupling between the ferromagnetic layers. Our PNR measurements at the coercive field reveal a novel and unexpected magnetization state of the sample, exhibiting an oscillating magnetization depth profile from CoFe layer to CoFe layer with a period of five bilayers along the multilayer stack. With the help of micromagnetic simulations we demonstrate that competition between long- and short-ranged dipolar interactions apparently gives rise to this unprecedented phenomenon.

Polarized neutron reflectivity (PNR) and magnetometry studies have been performed on the granular multilayer [Co80Fe20(1.3 nm)/Al2O3(3 nm)](10). Due to strong interparticle interactions, a collective superferromagnetic state is encountered. Cole-Cole plots drawn from the complex ac susceptibility are measured as functions of frequency, temperature, and field amplitudes that hint at the relaxation, creep, sliding, and switching regimes of pinned domain walls that are in close agreement with results obtained from simulations. Very slow switching with exponential relaxation under near-coercive fields is confirmed by PNR measurements. The complete absence of spin-flip scattering confirms that the magnetization reversal is achieved merely by domain nucleation and growth.

We present results of a neutron scattering investigation of the pseudo-one-dimensional magnetic compounds [(CH 3 ) 3 NH]Co 1− x Ni 3 Cl 3 ·2H 2 O, abbreviated as CoNiTAC. For three Co rich samples, we determined the magnetic structure to be canted antiferromagnetic with spins in the a - c plane. Furthermore we have investigated the temperature dependence of the long range order component through the proposed re-entrant spin glass transition.

Discontinuous metal–insulator multilayers (DMIMs) of [CoFe(t n)/Al2O3] m containing soft ferromagnetic (FM) Co80Fe20 nanoparticles embedded discontinuously in a diamagnetic insulating Al2O3 matrix are ideal systems to study interparticle interaction effects. Here the CoFe nanoparticles are treated as superspins with random size, position and anisotropy. At low particle density, namely nominal layer thickness t n = 0.5 nm, single-particle blocking phenomena are observed due to the absence of large enough interparticle interactions. However at 0.5 nm < t n < 1.1 nm, the particles encounter strong interactions which give rise to a superspin glass (SSG) phase. The SSG phase has been characterized by memory effect, ageing, dynamic scaling, etc. With further increase in particle concentration (1.1 nm < t n < 1.4 nm) and, hence, smaller interparticle distances, strong interactions lead to a FM-like state which is called superferromagnetic (SFM). The SFM state has been characterized by several techniques, e.g. dynamic hysteresis, Cole–Cole plots extracted from ac susceptibility, polarized neutron reflectometry, etc. Moreover, the SFM domains could be imaged by x-ray photoemission electron microscopy and magneto-optic Kerr effect microscopy. At t n > 1.4 nm physical percolation occurs between the particles and the samples are no longer discontinuous and then termed as metal insulating multilayers. Competition between long- and short-ranged dipolar interactions leads to an oscillating magnetization depth profile from CoFe layer to CoFe layer with an incommensurate periodicity.

Pd almost fulfils the Stoner criterion to be ferromagnetic. In order to understand effects of ferromagnetic inclusions in Pd it is of great interest to study in detail the influence of an ultrathin Fe layer in Pd. Here we report on the preparation and characterization of epitaxial Pd containing Fe layers of a few monolayer thickness. The growth procedure on Ag buffered GaAs(001) substrates was optimized by in-situ Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES) to obtain chemical purity and monocrystallinity of the layers with smooth interfaces. By using a variable deposition temperature during the growth of the first Pd layer on the Ag buffer we were able to avoid interdiffusion between Ag and Pd. X-ray- and polarized neutron reflectometry revealed a very well defined interface between the Pd and Fe on top, but an intermixing zone between Fe and Pd on top. With decreasing Fe layer thickness we have observed an increasing magnetization compared to bulk Fe moments.

We study the magnetic correlations in Er|Tb superlattices by means of off-specular scattering of polarized neutrons. We show here the co-existence of inhomogeneous magnetic states: i) ferromagnetic order of moments within the Tb layers below 230 K (FM), correlation length of about 10 bilayer, ii) an incommensurate modulated magnetic order, restricted to single Er layers and iii) antiferromagnetic coupling of ferromagnetic layers below 70K (AFC). Polarised off-specular neutron scattering under grazing incidence reveals that i) magnetic fluctuations appear when the sample is cooled below 70 K, ii) these fluctuations lead to AFC, when the sample is cooled to 10 K, which iii) persists, when the sample is subsequently heated up to 45 K, while the order is not present during the cooling cycle. Also the short range incommensurate order changes accordingly, implying that the magnetic order in the Er layers mediates the interlayer coupling between ferromagnetic Tb layers.

We study the magnetization of antiferromagnetically (AF) coupled Fe/Cr multilayers, with 7 and 19 periods respectively, under the influence of an external magnetic field. Due to the odd number of Fe layers the uncompensated magnetization of the multilayer at remanence is equal to the magnetization of a single Fe layer. Measurements have been performed using the magnetooptical Kerr effect (MOKE) for an integral overview and polarized neutron scattering under grazing incidence for a depth resolved information about the magnetization orientation of the individual Fe layers. We find a dependence of the orientation of the remanent magnetization on the number of AF coupled Fe double layers. For a small number of periods (here: 7). The uncompensated magnetic moment is sufficient to align the magnetization of all layers parallel or antiparallel to the applied magnetic field. On the other hand, in the multilayer with 19 AF-coupled Fe layers all Fe layers are magnetized perpendicular to the field.

We report on a structural and magnetic investigation of Fe/δ-Mn multilayers. In situ LEED shows that Mn grows epitaxially on BCC Fe with a good crystallinity up to 30 A. Reflectivity and small-angle diffuse scattering of synchrotron radiation on Fe/Mn/Fe sandwiches reveal sharp interfaces with correlated roughness. From MOKE measurements, evidence for non-collinear coupling between the Fe layers was found for Mn spacer thicknesses below 13 A. With polarized neutron reflectometry and high-angle neutron diffraction we could not detect any sign of magnetic order inside the Mn layers.