Polarized Neutron Diffraction Experiments Research Articles

Determining the magnetic ground state of TbNi5 single crystal using polarized neutron scattering technique

The TbNi5 compound shows an interesting magnetic phase transition with an incommensurate structure below 23K, whose true nature remains unresolved. In order to solve this question, we have carried out polarized neutron diffraction experiments by measuring temperature and field dependence of the intensities of satellites and Bragg reflections. From the temperature dependence of both satellite peaks and Bragg reflection, we demonstrated that it has only one magnetic structure at a given temperature. Furthermore, unlike previous reports, we found that both ferromagnetic and modulated components of the Tb ion magnetic moments should be collinear to each other. Our data also show strong depolarisation effects that are most likely to arise from domain structure of ferromagnetic component. A critical field, which destroyers a modulated magnetic structure is found to be lower than a field value to saturate the ferromagnetic component, in which the intensity of Bragg ferromagnetic reflections reaches saturation.

Magnetic moment distribution in non-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys

In this work we present a model for the net magnetic moment per formula unit measured in Ni-Mn-Ga Ferromagnetic Shape Memory Alloys, analyzing the different types of magnetic interactions appearing in such compounds. We have studied four compositions ranging from a 44% to a 52% Ni content. We found that excess Mn atoms occupy empty positions at Ni and Ga sites. We consider that the Mn atoms at Ni sites (Mn/Ni) can determine that Mn/Ga atoms couple either ferromagnetic (FM) or antiferromagnetically (AF) to Mn/Mn atoms. If Mn/Ga are nearest neighbors to properly-sited Mn/Mn atoms they present AF coupling. However, when Mn/Ni atoms are at closer distances than Mn/Ga ones, so that they become nearest neighbors to Mn/Mn, the AF coupling between Mn/Ni and Mn/Mn atoms is stronger and overcomes that between Mn/Ga and Mn/Mn ones. Therefore Mn/Ga and Mn/Mn will couple ferromagnetically. Polarized neutron diffraction experiments will help to elucidate the validity of our model.

Antiferromagnetic order in MnO spherical nanoparticles

We have performed unpolarized and polarized neutron diffraction experiments on monodisperse 8 nm and 13 nm antiferromagnetic MnO nanoparticles. For the 8 nm sample, the antiferromagnetic transition temperature $T_N$ (114 K) is suppressed compared to the bulk material (119 K) while for the 13 nm sample $T_N$ (120 K) is comparable to the bulk. The neutron diffraction data of the nanoparticles is well described using the bulk MnO magnetic structure but with a substantially reduced average magnetic moment of 4.2$\pm$0.3 $\mu_B$/Mn for the 8 nm sample and 3.9$\pm$0.2 $\mu_B$/Mn for the 13 nm sample. An analysis of the polarized neutron data on both samples shows that in an individual MnO nanoparticle about 80$%$ of Mn ions order. These results can be explained by a structure in which the monodisperse nanoparticles studied here have a core that behaves similar to the bulk with a surface layer which does not contribute significantly to the magnetic order.

Neutron diffraction and X-ray magnetic circular dichroism studies of the Co2SiO4 magnetic properties

Synthetic Co2SiO4 crystallizes in the olivine structure (space group Pnma) with two crystallographically non-equivalent Co positions and shows antiferromagnetic ordering below 50K. We have studied the magnetic properties of Co2SiO4 by means of both nonpolarized and polarized neutron diffraction as well as X-ray magnetic circular dichroism. Measurements with non-polarized neutrons were made at 2.5 K whereas polarized neutron diffraction experiments were done at 70 K and 150 K in an external magnetic field of 7 T parallel to the b axis. Symmetry analysis of the Co2SiO4 magnetic structure proves that it corresponds to the magnetic (Shubnikov) group Pnma which allows the antiferromagnetic configuration (Gx,Cy,Az) for the 4a site with inversion symmetry 1̄ (Co1 position) and (0,Cy,0) for the 4c site with mirror symmetry m (Co2 position). The polarized neutron study of the magnetization induced by an applied field indicates a delocalization of the magnetic moment from Co towards neighbouring O due to the superexchange coupling. This is also in agreement with X-ray magnetic circular dichroism measurements performed at the Co K-edge in the temperature range 7K–150K in an external magnetic field of 6T applied close to the b axis of Co2SiO4. An important orbital contribution to the total magnetic moment of Co2+ in Co2SiO4 predicted by non-polarized neutron diffraction was independently proven by means of the X-ray magnetic circular dichroism and polarized neutron diffraction studies which indicate an orbital to spin magnetic moment ratio μL/μs ≈ 0.3.

Magnetic behaviour of synthetic Co2SiO4

Synthetic Co(2)SiO(4) crystallizes in the olivine structure (space group Pnma) with two crystallographically non-equivalent Co positions and shows antiferromagnetic ordering below 50 K. We have investigated the temperature variation of the Co(2)SiO(4) magnetic structure by means of non-polarized and polarized neutron diffraction for single crystals. Measurements with non-polarized neutrons were made at 2.5 K (below T(N)), whereas polarized neutron diffraction experiments were carried out at 70 and 150 K (above T(N)) in an external magnetic field of 7 T parallel to the b axis. Additional accurate non-polarized powder diffraction studies were performed in a broad temperature range from 5 to 500 K with small temperature increments. Detailed symmetry analysis of the Co(2)SiO(4) magnetic structure shows that it corresponds to the magnetic (Shubnikov) group Pnma, which allows the antiferromagnetic configuration (G(x), C(y), A(z)) for the 4a site with inversion symmetry 1 (Co1 position) and (0,C(y),0) for the 4c site with mirror symmetry m (Co2 position). The temperature dependence of the Co1 and Co2 magnetic moments obtained from neutron diffraction experiments was fitted in a modified molecular-field model. The polarized neutron study of the magnetization induced by an applied field shows a non-negligible amount of magnetic moment on the oxygen positions, indicating a delocalization of the magnetic moment from Co towards neighbouring O owing to superexchange coupling. The relative strength of the exchange interactions is discussed based on the non-polarized and polarized neutron data.

Spin chirality and electric polarization in multiferroic compounds RMn2O5 (R=Ho, Er)

Polarized neutron diffraction experiments have been performed on multiferroic materials $R$Mn$_{2}$O$_{5}$ ($R=$Ho, Er) under electric fields in the ferroelectric commensurate (CM) and the low-temperature incommensurate (LT-ICM) phases, where the former has the highest electric polarization and the latter has reduced polarization. It is found that, after cooling in electric fields down to the CM phase, the magnetic chirality is proportional to the electric polarization. Also we confirmed that the magnetic chirality can be switched by the polarity of the electric polarization in both the CM and LT-ICM phases. These facts suggest an intimate coupling between the magnetic chirality and the electric polarization. However, upon the transition from the CM to LT-ICM phase, the reduction of the electric polarization is not accompanied by any reduction of the magnetic chirality, implying that the CM and LT-ICM phases contain different mechanisms of the magnetoelectric coupling.

Magnetic octupole order in Ce0.7La0.3B6: A polarized neutron diffraction study

Abstract Recently, in phase IV of Ce x La 1 - x B 6 , weak but distinct superlattice reflections from the order parameter of phase IV have been detected by our unpolarized neutron scattering experiment [K. Kuwahara, K. Iwasa, M. Kohgi, N. Aso, M. Sera, F. Iga, J. Phys. Soc. Japan 76 (2007) 093702]. The scattering vector dependence of the intensity of superlattice reflections is quite unusual; the intensity is stronger for high scattering vectors. This result strongly indicates that the order parameter of phase IV is the magnetic octupole. However, the possibility that the observed superlattice reflections are due to lattice distortions could not be completely ruled out only on the basis of the unpolarized neutron scattering experiment. To confirm that the superlattice reflections are magnetic, therefore, we have performed a single crystal polarized neutron diffraction experiment on Ce 0.7 La 0.3 B 6 . The obtained result has clearly shown that the time reversal symmetry is broken by the order parameter of phase IV. This is further evidence for the magnetic octupole order in Ce x La 1 - x B 6 .

A neutron diffraction study of RMn2O5 multiferroics

The magnetic properties of RMn2O5 multiferroics as obtained by unpolarized and polarized neutron diffraction experiments arereviewed. We discuss the qualitative features of the magnetic phase diagram inboth zero magnetic field and in field and analyze the commensurate magneticstructure and its coupling to an applied electric field. The origin of ferroelectricity isdiscussed based on calculations of the ferroelectric polarization predicted by differentmicroscopic coupling mechanisms (exchange-striction and cycloidal spin–orbitmodels). A minimal model containing a small set of parameters is also presentedin order to understand the propagation of the magnetic structure along thec-direction.

Spin-density distribution in the partially magnetized organic quantum magnetF2PNNNO

Polarized neutron diffraction experiments on an organic magnetic material reveal a highly skewed distribution of spin density within the magnetic molecular unit. The very large magnitude of the observed effect is due to quantum spin fluctuations. The data are in quantitative agreement with direct diagonalization results for a model spin Hamiltonian, and provide insight on the actual microscopic origin of the relevant exchange interactions.

P–f hybridization effect on the metal–nonmetal phase transition in [formula omitted

Variation of electronic state in the metal-nonmetallic phase transition of PrRu 4 P 12 has been investigated by a polarized neutron diffraction experiment. Two Pr ions placed at the center and the corner sites in the bcc unit cell surrounded by smaller and larger Ru-ion cubic sublattices have Γ 1 and Γ 4 ( 2 ) crystal field (CF) ground states, respectively. This modulated electronic state as well as the drastic evolution of the CF schemes of Pr ions in the nonmetal phase can be explained by p–f hybridization combined with the nesting behavior of Fermi surface with the wave vector q = ( 1 , 0 , 0 ) .

Role of p– f Hybridization in the Metal–Nonmetal Transition of PrRu4P12

Electronic state evolution in the metal-non-metal transition of PrRu4P12 has been studied by X-ray and polarized neutron diffraction experiments. It has been revealed that, in the low-temperature non-metallic phase, two inequivalent crystal-field (CF) schemes of Pr3+ 4f^2 electrons with Gamma_1 and Gamma_4^(2) ground states are located at Pr1 and Pr2 sites forming the bcc unit cell surrounded by the smaller and larger cubic Ru-ion sublattices, respectively. This modulated electronic state can be explained by the p-f hybridization mechanism taking two intermediate states of 4f^1 and 4f^3. The p-f hybridization effect plays an important role for the electronic energy gain in the metal-non-metal transition originated from the Fermi surface nesting.

Combined Charge and Spin Density Experimental Study of the Yttrium(III) Semiquinonato Complex Y(HBPz3)2(DTBSQ) and DFT Calculations

High-resolution X-ray diffraction and polarized neutron diffraction experiments have been performed on the Y-semiquinonate complex, Y(HBPz3)2(DTBSQ), in order to determine the charge and spin densities in the paramagnetic ground state, S = (1/2). The aim of these combined studies is to bring new insights to the antiferromagnetic coupling mechanism between the semiquinonate radical and the rare earth ion in the isomorphous Gd(HBPz3)2(DTBSQ) complex. The experimental charge density at 106 K yields detailed information about the bonding between the Y3+ ion and the semiquinonate ligand; the topological charge of the yttrium atom indicates a transfer of about 1.5 electrons from the radical toward the Y3+ ion in the complex, in agreement with DFT calculations. The electron density deformation map reveals well-resolved oxygen lone pairs with one lobe polarized toward the yttrium atom. The determination of the induced spin density at 1.9 K under an applied magnetic field of 9.5 T permits the visualization of the delocalized magnetic orbital of the radical throughout the entire molecule. The spin is mainly distributed on the oxygen atoms [O1 (0.12(1) mu B), O2(0.11(1) mu B)] and the carbon atoms [C21 (0.24(1) mu B), C22(0.20(1) mu B), C24(0.16(1) mu B), C25(0.12(1) mu B)] of the carbonyl ring. A significant spin delocalization on the yttrium site of 0.08(2) mu B is observed, proving that a direct overlap with the radical magnetic orbital can occur at the rare earth site and lead to antiferromagnetic coupling. The DFT calculations are in good quantitative agreement with the experimental charge density results, but they underestimate the spin delocalization of the oxygen toward the yttrium and the carbon atoms of the carbonyl ring.

The cerium magnetic form factor and diffuse polarization inCeRh3B2 as functions of temperature

In the compound CeRh3B2, a rather special polarization of the conduction electrons along thec-chains of cerium atoms had been previously reported at low temperatures(Alonso et al 1998 J. Magn. Magn. Mater. 177–181 1048). The distribution of theCeRh3B2 magnetization has now been studied as a function of temperature up to 150 K—that is,above the Curie temperature of 115 K. The magnetization density maps have beenobtained from polarized neutron diffraction experiments by using the maximumentropy method. The cerium form factor has also been analysed. Calculations ofthe form factor including several multiplets are developed and it is shown thatit is necessary to take into account the influence of the higher multiplet of theCe3+ ion. This result is coherent with the observation of a peak at high energy in theinelastic neutron spectra, indicating a very large crystal electric field splitting.Both analyses lead to the same conclusion that, on heating, the diffuse negativemagnetization observed at low temperature along the cerium chains disappears atthe magnetic ordering temperature. The influence of the second multiplet of theCe3+ ioncould be part of the explanation for the low value of the 4f moment and the large Curie temperaturein CeRh3B2.

Magnetic properties of UNi2/3Rh1/3Al single crystal probed by polarized neutron diffraction

A delicate balance between ferromagnetic and antiferromagnetic interactions in URh1/3Ni2/3Al leads to the absence of a long-range magnetic order and anomalous bulk properties of this material in zero magnetic field. The reported polarized neutron diffraction experiment in fields up to 6T indicates that the positional dependence of Ni/Rh atomic distribution is responsible for the abnormal zero field profiles (most reflections have a significant Lorentzian contribution), while the field-induced U moments of 0.2μB (at 2K) cause a magnetic contribution to be of Gaussian type.

X-ray magnetic circular dichroism investigation of magnetic contributions from Mn(III) and Mn(IV) ions in Mn12-ac

We present the results of an x-ray magnetic circular dichroism (XMCD) investigation of the molecular superparamagnet Mn12-ac. Thanks to an approach based on chemical synthesis of model compounds, the contributions of Mn(III) and Mn(IV) ions to the absorption spectrum and the dichroic signal of Mn12-ac have been separated. The two contributions have been analyzed by simulating the XMCD spectra of Mn12-ac by means of crystal-field multiplet calculations. The ferrimagnetic structure of Mn12-ac has been confirmed, and the individual magnetic moments of Mn(III) and Mn(IV) ions are in good agreement with the values obtained by means of first-principles calculations and polarized neutron diffraction experiments. The orbital and spin contributions to the magnetic moment have been evaluated for each manganese ion, providing the evidence of negligible orbital magnetic moments for both Mn(III) and Mn(IV) ions.

Superexchange interaction enhanced through spin delocalisation in Rb2FeBr5·H2O as studied by polarised neutron diffraction

Polarised neutron diffraction experiments on crystals of Rb2FeBr5·H2O are reported. Experimental magnetic structure factors were analysed using a multipolar model for the magnetisation density on the iron and ligand atoms. Our results indicate that there exists an important spin transfer of 20% from the iron to the ligands. Also, a small asphericity is observed on Fe(III) ion due to the presence of the ligands. The hydrogen bridge, in the superexchange pathway FeBr⋯HOFe, seems to favour the spin-density transfer from the iron through bromine, thus enhancing the magnetic interaction.

Effect of spin delocalisation in K 2FeCl 5·H 2O on its superexchange pathways

Polarised-neutron diffraction experiments on crystals of K 2FeCl 5·H 2O are reported. Experimental magnetic structure factors were analysed using a multipolar model for the magnetisation density on the iron and ligand atoms. Our results indicate that there exists an important spin transfer of 17% from the iron to the ligands. Also a small asphericity is observed on the Fe(III) ion due to the presence of the ligands. The hydrogen bridge, in the superexchange pathway Fe–Cl⋯(H)–O–Fe, seems to favour the spin-density transfer from the iron through the chlorine, thus enhancing the magnetic interaction.

Influence of density and temperature on the microscopic structure and the segmental relaxation of polybutadiene.

We investigate the influence of temperature and density on the local structure and the dynamics of polybutadiene by controlling both hydrostatic pressure and temperature in polarized neutron diffraction experiments on deuterated polybutadiene and in inelastic incoherent scattering experiments on protonated polybutadiene. We observe that the static structure factor S(Q) does not change along macroscopic isochores. This behavior is contrary to the relaxations observed on the nanosecond and picosecond time scales and viewed by the dynamic incoherent scattering law S(Q,omega), which differ strongly along the same thermodynamic path. We conclude that the static behavior, i.e., S(Q), is dominated by macroscopic density changes, similar to the vibrational excitations in the meV range. However, the relaxation dynamics is more sensitive to thermal energy changes. This is confirmed by the finding that lines of identical relaxation behavior (in time, shape, and Q dependence), isochrones on the 10(-9) sec time scale, clearly cross the constant density lines in the (P,T) plane. Concerning S(Q), we can reasonably relate the variation of the main-peak position to the average neighbor chain distance and deduce crude microscopic thermal expansion and compressibility coefficients. In the low-Q regime, the observed pressure and temperature variation of S(Q) exceeds the compressibility contribution and suggests the existence of additional scattering, which might originate from structural correlations arising at higher temperature and low pressure.

Neutron scattering studies of the one-dimensional quantum spin magnetism in Yb 4 As 3

Inelastic neutron scattering and polarized neutron diffraction experiments on Yb4As3 revealed that it exhibits typical behaviors of a 1D S=1/2 Heisenberg antiferromagnet at low temperatures due to the formation of 1D arrays of Yb3+ ions by the charge-ordering transition. The staggered field effects on the Yb3+ chains induced by applying a magnetic field were also detected by the neutron scattering experiments.

Spin densities in a ferromagnetic bimetallic chain compound: polarized neutron diffraction and DFT calculations.

The spin population distribution in the ferromagnetically coupled hetero-bimetallic chain compound [MnNi(NO(2))(4)(en)(2)] (en = 1,2-ethanediamine) has been investigated by means of polarized neutron diffraction experiments, and the results compared with those from theoretical estimates obtained via calculations based on density functional theory on dinuclear molecular models of the chain. The spin distributions obtained from experiment and from theory are consistent and reflect a larger spin delocalization from the Ni atom due to the more covalent character of the Ni-N bonds compared to the Mn-O ones. Also a nearly isotropic spin distribution is observed for the more ionic d(5) Mn(2+) ion and a clearly anisotropic distribution for the d(8) Ni(2+) ion. The use of dinuclear molecular models for the calculation of the exchange coupling constant between Ni and Mn provide upper and lower limits (+17.6 and -4.2 cm(-)(1)) for the experimentally determined value (+1.3 cm(-)(1)), depending on how the missing part of the chain is simulated, but yield essentially the same spin distribution. The Mn(II)-Ni(II) weak ferromagnetic coupling in the chain is interpreted in a spin delocalization mechanism as resulting from the weakness of the overlap between the magnetic orbitals centered on nickel and those centered on manganese which are only weakly delocalized on the ligands.