Polarized Neutron Diffraction Research Articles

Directin situmeasurement of coupled magnetostructural evolution in a ferromagnetic shape memory alloy and its theoretical modeling

In this study, ferromagnetic shape memory alloys (FSMAs) have shown great potential as active components in next generation smart devices due to their exceptionally large magnetic-field-induced strains and fast response times. During application of magnetic fields in FSMAs, as is common in several magnetoelastic smart materials, there occurs simultaneous rotation of magnetic moments and reorientation of twin variants, resolving which, although critical for design of new materials and devices, has been difficult to achieve quantitatively with current characterization methods. At the same time, theoretical modeling of these phenomena also faced limitations due to uncertainties in values of physical properties such as magnetocrystalline anisotropy energy (MCA), especially for off-stoichiometric FSMA compositions. Here, in situ polarized neutron diffraction is used to measure directly the extents of both magnetic moments rotation and crystallographic twin-reorientation in an FSMA single crystal during the application of magnetic fields. Additionally, high-resolution neutron scattering measurements and first-principles calculations based on fully relativistic density functional theory are used to determine accurately the MCA for the compositionally disordered alloy of Ni2Mn1.14Ga0.86. The results from these state-of-the-art experiments and calculations are self-consistently described within a phenomenological framework, which provides quantitative insights into the energetics of magnetostructural coupling in FSMAs. Based onmore » the current model, the energy for magnetoelastic twin boundaries propagation for the studied alloy is estimated to be ~150kJ/m3.« less

Heterometallic Rings: Their Physics and use as Supramolecular Building Blocks.

An enormous family of heterometallic rings has been made. The first were Cr7 M rings where M = Ni(II), Zn(II), Mn(II), and rings have been made with as many as fourteen metal centers in the cyclic structure. They are bridged externally by carboxylates, and internally by fluorides or a penta-deprotonated polyol. The size of the rings is controlled through templates which have included a range of ammonium or imidazolium ions, alkali metals and coordination compounds. The rings can be functionalized to act as ligands, and incorporated into hybrid organic-inorganic rotaxanes and into molecules containing up to 200 metal centers. Physical studies reported include: magnetic measurements, inelastic neutron scattering (including single crystal measurements), electron paramagnetic resonance spectroscopy (including measurements of phase memory times), NMR spectroscopy (both solution and solid state), and polarized neutron diffraction. The rings are hence ideal for understanding magnetism in elegant exchange-coupled systems.

Control of multiferroic domains by external electric fields in TbMnO3

The control of multiferroic domains through external electric fields has been studied by dielectric measurements and by polarized neutron diffraction on single-crystalline TbMnO3. Full hysteresis cycles were recorded by varying an external field of the order of several kV mm−1 and by recording the chiral magnetic scattering as well as the charge in a sample capacitor. Both methods yield comparable coercive fields that increase upon cooling.

Gradual Localization of 5fStates in Orthorhombic UTX Ferromagnets:Polarized Neutron Diffraction Study of Ru Substituted UCoGe

We report on a microscopic study of the evolution of ferromagnetism in the Ru substituted ferromagnetic superconductor (FM SC) UCoGe crystallizing in the orthorhombic TiNiSi-type structure. For that purpose, two single crystals with composition UCo0.97Ru0.03Ge and UCo0.88Ru0.12Ge have been prepared and characterized by magnetization, AC susceptibility, specific heat and electrical resistivity measurements. Both compounds have been found to order ferromagnetically below TC = 6.5 and 7.5 K, respectively, which is considerably higher than the TC = 3 K of the parent compound UCoGe. The higher values of TC are accompanied by enhanced values of the spontaneous moment μspont = 0.11 μB/f.u. and μspont = 0.21 μB/f.u., respectively in comparison to the tiny spontaneous moment of UCoGe (about 0.07 μB/f.u.). No sign of superconductivity was detected in either compound. The magnetic moments of the samples were investigated on the microscopic scale using polarized neutron diffraction (PND) and for UCo0.88Ru0.12Ge also ...

Neutron diffraction of hydrogenous materials: Measuring incoherent and coherent intensities separately

(short version) Accurate determination of the coherent static structure factor of disordered materials containing proton nuclei is prohibitively difficult by neutron diffraction, due to the large incoherent cross section of $^1$H. This notorious problem has set severe obstacles to the structure determination of hydrogenous materials up to now, via introducing large uncertainties into neutron diffraction data processing. Here we present the first accurate separate measurements, using polarized neutron diffraction, of the coherent and incoherent contributions to the total static structure factor of 5 mixtures of light and heavy water, over an unprecedentedly wide momentum transfer range. The structure factors of H$_2$O and D$_2$O mixtures derived in this work may signify the beginning of a new era in the structure determination of hydrogenous materials, using neutron diffraction.

Direct observation of finite size effects in chains of antiferromagnetically coupled spins.

Finite spin chains made of few magnetic ions are the ultimate-size structures that can be engineered to perform spin manipulations for quantum information devices. Their spin structure is expected to show finite size effects and its knowledge is of great importance both for fundamental physics and applications. Until now a direct and quantitative measurement of the spatial distribution of the magnetization of such small structures has not been achieved even with the most advanced microscopic techniques. Here we present measurements of the spin density distribution of a finite chain of eight spin-3/2 ions using polarized neutron diffraction. The data reveal edge effects that are a consequence of the finite size and of the parity of the chain and indicate a noncollinear spin arrangement. This is in contrast with the uniform spin distribution observed in the parent closed chain and the collinear arrangement in odd-open chains.

Investigation of the evolution of magnetic structure of multiferroics Tb1−x Ce x Mn2O5 (x = 0, 0.2) by polarized neutron diffraction

The evolution of multiferroics TbMn2O5 and Tb0.8Ce0.2Mn2O5 magnetic structure was studied by means of polarized neutron diffraction. It is shown that in all magnetically ordered phases there exists nonequilibrium population of domains with “right” and “left” spirals. It is revealed that in Tb0.8Ce0.2Mn2O5 in all magnetic phases and in TbMn2O5—in low-temperature magnetic phases incommensurate as well as in commensurate — the orientation of the rotation plane of magnetic moments remains the same and makes an angle of ~10° with the ab-plane of crystal. At the transition to the high temperature incommensurate phase in TbMn2O5 the orientation transition takes place.

Spin reorientation inBa0.65Na0.35Fe2As2studied by single-crystal neutron diffraction

We have studied the magnetic ordering in Na doped BaFe$_2$As$_2$ by unpolarized and polarized neutron diffraction using single crystals. Unlike previously studied FeAs-based compounds that magnetically order, Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ exhibits two successive magnetic transitions: For x=0.35 upon cooling magnetic order occurs at $\sim$70\ K with in-plane magnetic moments being arranged as in pure or Ni, Co and K-doped BaFe$_2$As$_2$ samples. At a temperature of $\sim$46\ K a second phase transition occurs, which the single-crystal neutron diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ point along the $c$ direction. Magnetic correlations in these materials cannot be considered as Ising like, and spin-orbit coupling must be included in a quantitative theory.

Polarized neutron powder diffraction studies of antiferromagnetic order in bulk and nanoparticle NiO

In many materials it remains a challenge to reveal the nature of magnetic correlations, including antiferromagnetism and spin disorder. Revealing the spin structure in magnetic nanoparticles is further complicated by the large incoherent neutron scattering cross section from water adsorbed at the particle surfaces and by the broadening of diffraction peaks due to the finite crystallite size. Moreover, the spin structure in magnetic nanoparticles may deviate significantly from that of the corresponding bulk material because of the low-symmetry surroundings of surface atoms and the large relative surface contribution to the magnetic anisotropy. Here we explore the potential use of polarized neutron diffraction to reveal the magnetic structure in NiO bulk and nanoparticle powders by applying the $XYZ$-polarization analysis method. Our investigations address in particular the spin orientation in bulk NiO and platelet-shaped NiO nanoparticles with thickness from greater than 200 nm down to 2.0 nm. The advantage of the applied method is that it is able to clearly separate the structural, the magnetic, and the spin-incoherent scattering signals for all particle sizes. For platelet-shaped particles with thickness from greater than 200 nm down to 2.2 nm we find that the spin orientation deviates about ${16}^{\ensuremath{\circ}}$ from the primary (111) plane of the platelet-shaped particles. In the smallest particles (2.0 nm thick) we find the spins are oriented with a ${30}^{\ensuremath{\circ}}$ average angle to the primary (111) plane of the particles. The results show that polarization analyzed neutron powder diffraction is a viable method to investigate magnetic order in powders of antiferromagnetic nanoparticles.

Coexistence of different magnetic moments in CeRuSn probed by polarized neutrons

We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure, commensurate with the CeCoAl type, contains two different crystallographic Ce sites, we observe that one Ce site is clearly more susceptible to the applied magnetic field whereas the other is hardly polarizable. This finding clearly documents that distnictly different local environment of the two Ce sites causes the Ce ions to split into magnetic Ce3+ and non-magnetic Ce(4-delta)+ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the c axis. This leads to new inequivalent crystallographic Ce sites resulting in a re-distribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions co-exist in different valence states. Localized 4f states that fulfill the third Hund's rule are found to be close to the ideal Ce3+ state (at sites with the largest Ce-Ru interatomic distances) whereas Ce(4-delta)+ valence states are found to be itinerant and situated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous alpha-gamma transition in elemental cerium is discussed.

Updating the phase diagram of the archetypal frustrated magnetGd3Ga5O12

The applied magnetic field and temperature phase diagram of the archetypal frustrated magnet, ${\mathrm{Gd}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$, has been reinvestigated using single-crystal magnetometry and polarized neutron diffraction. The updated phase diagram is substantially more complicated than previously reported and can be understood in terms of competing interactions with loops of spins, trimers, and decagons, in addition to competition and interplay between antiferromagnetic, incommensurate, and ferromagnetic order. Several additional distinct phase boundaries are presented. The phase diagram centers around a multiphase convergence to a single point at 0.9 T and $\ensuremath{\sim}0.35$ K, below which, in temperature, a very narrow magnetically disordered region exists. These data illustrate the richness and diversity that arise from frustrated exchange on the three-dimensional hyperkagome lattice.

Local magnetic order in La2CuO4 seen via μ+SR spectroscopy

The origin of a pseudogap in the underdoped phase of cuprates has become one of the leading issues in understanding the mechanism(s) of high-Tc superconductivity. Several experiments (i.e. polarized neutron diffraction studies) support theoretical models based on the circulating current (CC) picture. These CC models suggest a novel ordered phase formed by orbital magnetic moments caused by spontaneous orbital currents. However to date, μSR experiments in cuprate superconductors have not revealed any magnetic field higher than 0.2 G associated with orbital moments. Here we present high magnetic field μSR spectroscopy of a stoichiometric La2CuO4 crystal which shows antiferromagnetic splittings and additional 45 G line splittings, consistent with orbital currents.

Polarized-neutron-diffraction study of the microscopic magnetic structure inα′′−Fe16N2nanoparticles

Polarized-neutron-diffraction (PND) measurements were carried out using a pseudo-single-phase powder sample of ferromagnetic ${\ensuremath{\alpha}}^{\ensuremath{'}\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$ nanoparticles. For the well-identified ${\ensuremath{\alpha}}^{\ensuremath{'}\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$ phase, sizes of the magnetic moments at the three crystallographic Fe sites were determined in the absolute scale. The agreement between the magnetization value deduced from the present PND and that measured by a magnetometer $({M}_{\mathrm{VSM}})$ supports the hypothesis that ${M}_{\mathrm{VSM}}$ is primarily caused by the magnetization value in the target ${\ensuremath{\alpha}}^{\ensuremath{'}\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$; thus there is no evidence for macroscopic giant saturation magnetization, at least for ${\ensuremath{\alpha}}^{\ensuremath{'}\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$ nanoparticles. On the basis of the large magnetic moment size at one of the Fe sites, a possible coexisting state of localized spins and itinerant electron spins is inferred. Drawing a distinction between thin films and nanoparticles is currently necessary because of their divergent magnetic evolutions.

Effect of double nuclear scattering on nuclear-magnetic interference in experiment with small-angle diffraction of polarized neutrons

An experiment on small-angle polarized-neutron diffraction by a two-dimensional spatially ordered array of nickel nanowires embedded in a porous anodic alumina matrix is discussed. The contributions of nonmagnetic (nuclear) structures and nuclear magnetic interference indicating the correlation between magnetic and nuclear structures are discussed. Magnetic scattering is two orders of magnitude smaller than nuclear scattering and, hence, turns out to be weakly distinguishable. The ordered magnetic composite nanostructure of a sample leads to strong interaction between the neutron wave and the structure itself, which, in turn, implies a twofold (miltiple scattering) nuclear scattering process. Nuclear magnetic interference scattering must be analyzed allowing for twofold scattering conditions, which substantially distorts the intensity distribution of the interference contribution of first-order diffraction peaks.

Polarized Neutron Diffraction study of the molecular magnetic anisotropy

The magnetic anisotropy is a prerequisite for a metal complex to behave as a single-molecule magnet (SMM). Unfortunately, today we do not fully understand the relationships between the local structural parameters and the magnetic anisotropy that results at the molecular level. This is an issue that has become recursive in this area. Out of the synthesis work which is still important, but generates a multiplication of SMMs with frustrating properties, there are various studies to understand these relationships among which most are theoretical studies. In this context, we believe that polarized neutron diffraction (PND) can provide an experimental and complementary point of view to these theoretical studies. PND is indeed well known to allow an accurate determination of the spin density in magnetic compounds and in the field of molecular magnetism it has provided unique information on the pathways and the nature of intra- or intermolecular magnetic coupling [1]. In the case of highly anisotropic paramagnetic materials, where local magnetic moments cannot be aligned by an external magnetic field, that is more tricky, but a method based on local magnetic susceptibility tensor, has been recently developed that allows now analysing the data in this case and obtaining the magnetization distribution [2]. This approach was first used for inorganic compounds. Our idea has been to use this approach to go beyond the reconstruction of spin density to study the magnetic anisotropy in molecular systems. In this paper, we present the results of such an approach applied for the first time to metal complexes that are simple mono and dinuclear cobalt(II) complexes.

Polarized neutron diffraction and anisotropic exchange in pyrochlores

Rare earth frustrated pyrochlore magnets are among the materials that exhibit no long range order down to the lowest explored temperatures. To better understand the many complex phenomena observed in the pyrochlores, knowledge of the local interactions present in these materials is necessary. In the pyrochlore lattice, selection between Ising, Heisenberg, or XY types of the anisotropy cannot be based on the analysis of the macroscopic properties because of the presence of four equivalent local anisotropy (<111> -type) axes. Then only an average over the four axes can be measured by classical methods. Polarized neutron diffraction based on the `site susceptibility approach' (A Gukasov and P. J. Brown, J. Phys.: Cond. Matt., 14, 8831, 2002) allows to determine the local susceptibility tensor on the magnetic site. Its temperature dependence in the paramagnetic phase can be accounted for by the crystal field anisotropy and a molecular field tensor that encompasses exchange and dipolar interactions. It was found that for rare earth pyrochlores an isotropic exchange model is not suitable to explain their magnetic behavior and that, instead, the exchange interaction appear anisotropic to large extent (H. Cao et al., PRL 103, 056402, 2009). The experimental evaluation of the anisotropic exchange in pyrochlores can provide a clue to the richness of the phase diagram observed in rare earth frustrated magnets.

An unique model for joint refinement of (polarised) neutron and X-ray data

A new charge and spin density model and the corresponding refinement software were recently developed to combine X-ray and polarised neutron diffraction experiments [1,2]. This joint refinement procedure allows for an access to both the charge and spin densities but also to spin up ( ) and spin down ( ) electron distributions. These two quantities ( and ) were thus separately modelled and for the first time it was possible to compare them with theoretical results. The first part of the presentation will introduce the refinement procedure and describe its application to the case of an end-to-end azido double bridged copper(II) complex[3]. The results of this joint refinement of X-ray and polarized neutron diffraction data will be compared to theoretical calculations. The second part will be devoted to recent applications to other materials including a purely organic radical.

Purely organic magnets: How to get ferromagnetic interactions

Purely organic magnets, materials with spontaneous magnetization despite no containing magnetic ions, are very promising for technological applications due to their peculiar properties as flexibility, lightness or even biocompatibility. Most of the published purely organic magnets are free radical compounds. One of the most important difficulties in order to get spontaneous magnetization in such materials is that magnetic interactions among free radicals are usually antiferromagnetic and, in addition, ferromagnetic interactions are weaker than the antiferromagnetic ones. A possible strategy to overcome this issue is the use of triradical molecules with total spin S=1/2. In this case, an adequate packing of the triradical molecules can give place to antiferromagnetic interactions between regions with positive spin density and regions with negative spin density of two close molecules. This antiferromagnetic interaction between regions with opposite spin density would result in an overall ferromagnetic interaction between the two close triradicals. With this idea in mind we have performed an study of the spin density distribution and of the intramolecular and intermolecular magnetic interactions of the triradical compound 2-[3',5'-bis(Ntert-butylaminoxyl)phenyl]-4,4,5,5-tetramethyl-4,5-dihydro-1-Himidazol-1-oxyl-3-oxide, containing two N-tert-butyl aminoxyls and a nitronyl nitroxide groups. Combination of experimental data from a polarized neutron diffraction experiment and ab initio calculations (DFT) has allowed us to obtain the spin density distribution. In addition, the intramolecular and intermolecular magnetic interactions have been computed by ab initio quantum chemistry methods. The values for the intramolecular interactions confirm the S=1/2 ground state of the triradical. As for the intermolecular interactions, the two strongest ones are ferromagnetic, what is in agreement with the overlapping of regions with opposite spin density of the two interacting triradicals. These results support the strategy of using triradical molecules for obtaining purely organic magnets with higher magnetic transition ordering temperatures since is easier to obtain ferromagnetic interactions between the radicals and these interactions, having an antiferromagnetic origin, can be stronger than typical ferromagnetic interactions between radicals.

Spin Density Distribution in a MOF presenting electric and magnetic order

One of the main features of molecular compounds is the possibility of combining different properties in a synergic way giving a multifunctional material. Here we will discuss the case of a compound combining electric and magnetic order, thus giving a "multiferroic" molecular material. The disorder-order of the dimethylammnoium molecule in the Iron(II)-Iron(III) system [NH2(CH3)2]n[FeIIIFeII(HCOO)6]n is in the origin of the observed electric transition from paraelectric to antiferroelectric. In combination with the mentioned electric properties, this compound shows also magnetic order in the form of Nel N-Type ferrimagnetism [1]. The structure of [NH2(CH3)2]n[FeIIIFeII(HCOO)6]n has been characterized by means of neutron diffraction at VIVALDI and D19 instruments at Institut Laue-Langevin (ILL, Grenoble, France), where a crystallographic phase transition was observed from the high temperature structure [P-31c; a =b=8.2550(12) and c=13.891(3) at room temperature] to a lower symmetry one [R-3c; a = b=14.2600(17) and c=41.443(8) at low temperature]. On the other hand, the magnetic behaviour of this compound can be described as a result of two sublattices anti-ferromagnetically coupled, containing different spin carriers FeIII and FeII, respectively, with an ordering temperature of 37 K, and which are responsible of the different magnetic behaviours at low temperature.[2] The different spins of the neighbouring ions [S=5/2 and S=2] result in a ferrimagnetic state. Polarized neutron diffraction measurements aimed at clarifying the spin density map in order to understand the influence of the counter ion in the magnetic properties were carried out at D3 instrument (ILL). Measurements of the flipping ratios were performed with magnetic field of 9 Tesla at 45K, over the magnetic ordering temperature. The results point to an unusual weak spin density located around the counterions which suggests a non-negligible role in the magnetic behavior for the amine group.

Spin correlations inCa3Co2O6: Polarized-neutron diffraction and Monte Carlo study

We present polarised-neutron diffraction measurements of the Ising-like spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature TN. Below TN, a clear evolution from a single-phase spin-density wave (SDW) structure to a mixture of SDW and commensurate antiferromagnet (CAFM) structures is observed on cooling. For a rapidly-cooled sample, the majority phase at low temperature is the SDW, while if the cooling is performed sufficiently slowly, then the SDW and the CAFM structure coexist between 1.5 and 10 K. Above TN, we use Monte Carlo methods to analyse the magnetic diffuse scattering data. We show that both intra- and inter-chain correlations persist above TN, but are essentially decoupled. Intra-chain correlations resemble the ferromagnetic Ising model, while inter-chain correlations resemble the frustrated triangular-lattice antiferromagnet. Using previously-published bulk property measurements and our neutron diffraction data, we obtain values of the ferromagnetic and antiferromagnetic exchange interactions and the single-ion anisotropy.