Intermetallic compounds of the type RFe10Si2 and RCo10Si2 crystallize in the ThMn12 structure (space group I4/mmm) whilst the heavy rare earth series RNi10Si2 crystallize in a maximal subgroup of I4/mmm, P4/nmm. Reported here are neutron powder diffraction investigations for TbNi10Si2 and ErNi10Si2 which show that the P4/nmm structure undergoes a high temperature order-disorder phase transition at approximately 930 degrees C above which the ordered Ni and Si fractions revert to a random distribution on 4d and 4e sites. The volume expansion has been tracked in detail via the temperature dependence of the lattice parameters, whilst the temperature dependence of the thermal expansion coefficients alpha(11), alpha(33) and alpha(volume) has been determined from the lattice parameters. Associated with the order-disorder transition is a transition associated with a displacement of the R ion along the c-axis. Both transitions are of second order and the critical exponent associated with the order- disorder and displacive transitions, beta = 0.31, is in excellent agreement with the exponent determined for the three-dimensional Ising model.

The molecular magnet Hpyr[Fe 17O16OH12py12Br4Br4] “Fe17” has a well-defined cluster spin ground state of S=35/2 at low temperatures and an axial molecular anisotropy of only D ≈−0.02 K. Dipolar interactions between the molecular spins induce long-range magnetic order below 1.1 K. We report here the magnetic structure of Fe17, as determined by unpolarized neutron diffraction experiments performed on a polycrystalline sample of deuterated Fe17 in zero applied magnetic field. In addition, we report bulk susceptibility, magnetization, and specific heat data. The temperature dependence of the long-range magnetic order has been tracked and is well accounted for within mean-field theory. Ferromagnetic order along the crystallographic c axis of the molecular spins, as determined by the neutron diffraction experiments, is in agreement with ground-state dipolar energy calculations.

The molecular magnet Hpyr[Fe17O16(OH)(12)(py)(12)Br-4]Br-4 ("Fe-17") has a well-defined cluster spin ground state of S=35/2 at low temperatures and an axial molecular anisotropy of only D similar or equal to -0.02 K. Dipolar interactions between the molecular spins induce long-range magnetic order below 1.1 K. We report here the magnetic structure of Fe-17, as determined by unpolarized neutron diffraction experiments performed on a polycrystalline sample of deuterated Fe-17 in zero applied magnetic field. In addition, we report bulk susceptibility, magnetization, and specific heat data. The temperature dependence of the long-range magnetic order has been tracked and is well accounted for within mean-field theory. Ferromagnetic order along the crystallographic c axis of the molecular spins, as determined by the neutron diffraction experiments, is in agreement with ground-state dipolar energy calculations.

We present small-angle neutron scattering (SANS) data for the temperature variation of the recently observed dipole-field-induced spin-misalignment scattering in the soft magnetic nanocomposite Nanoperm (Fe89Zr7B3Cu1). The associated clover-leaf-shaped angular anisotropy of the SANS pattern, which is due to spin disorder arising from dipolar stray fields of the iron nanoparticles, persists up to several hundred Kelvin above the decoupling point of the intergranular amorphous matrix phase (TCam≅345K). This observation, in conjunction with the q-dependence of the scattering, suggests the existence of long-range magnetic correlations between the iron particles through the paramagnetic matrix, in agreement with previous investigations. The characteristic wavelength of the dipole-field-induced spin disorder appears to be temperature independent.

J. L. Zarestky,1 O. Moze,2 J. W. Lynn,3 Y. Chen,3,4 T. A. Lograsso,5 and D. L. Schlagel5 1Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA 2Dipartimento di Fisica, Universita di Modena e Reggio Emilia, Modena 41100, Italy 3NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, USA 4Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA 5Ames Laboratory, Materials and Engineering Physics Program, Iowa State University, Ames, Iowa 50011, USA Received 16 November 2006; published 12 February 2007

The magnetic structure and magnetic excitations in nanocrystalline Tb have been investigated by neutron diffraction and neutron spectroscopy. This is a report on the long-range magnetic order and the magnetic excitations in a nanocrystalline elemental rare earth. Refinement of the neutron-diffraction data reveals an “average” magnetic structure of each crystallite which contains a significant out-of-plane component to the magnetic moment as well as a suppression of the high-temperature antiferromagnetic phase observed for coarse-grained Tb. The inelastic-neutron-scattering measurements reveal the presence of a magnetic excitation of approximately 10meV at 2.5K. The excitation energy decreases with increasing temperature. The origins of this excitation are discussed with particular reference to the magnetic modes at the zone center observed for single-crystal Tb.

We present results for the magnetic-field, temperature, and neutron-polarization dependence of the small-angle neutron scattering intensity in the soft magnetic iron-based nanocomposite Nanoperm (Fe89Zr7B3Cu). An unusual "clover-leaf-shaped" intensity distribution on the detector is attributed to the dipolar stray fields around the nanosized iron particles, which are embedded in an amorphous magnetic matrix of lesser saturation magnetization. The dipole field induces spin disorder, correlating the spin misalignment of neighboring particles and matrix over several particle spacings. The clover-leaf-shaped anisotropy is observed over a wide range of applied magnetic field and momentum transfer. It persists up to several hundred degrees Kelvin above the Curie temperature of the matrix phase, indicating that some degree of magnetic coupling persists even when the matrix is paramagnetic.

Body centered cubic Fe1−xBex alloys are known to display an enhanced tetragonal magnetostriction compared to Fe. In order to characterize the enhanced magnetoelasticity observed in this alloy system, we present detailed inelastic neutron scattering measurements of the phonon dispersion relations of a single crystal of bcc Fe, 10at.% Be along the high symmetry directions [100], [110], and [111] at room temperature. We observe in particular that the frequency of transverse phonons propagating along the [110] direction with a [11¯0] polarization at the zone boundary is reduced by 10% with respect to bcc Fe (the corresponding elastic shear constant c′=12(c11−c12) associated with this mode is approximately 70% that of pure Fe). The dispersion of spin waves has also been determined for energy transfers up to 40meV and is found to follow the isotropic dispersion relation E(q)=Dq2, with D∼200meV∕Å2 [for Fe, E(q)=Dq2, with D=280meV∕Å2].

The technique of SANS (small angle neutron scattering) furnishes unique information on the characteristic magnetic length scales and local magnetic anisotropies at the nanoscale in nanocomposite ferromagnets. Such information is not presently available using any other microscopic technique. The basic principles and results of the technique will be presented with regard to a unique and unexpected observation of a dipole field controlled spin disorder in a prototypical soft nanocomposite ferromagnet of the Nanoperm type.

We have determined the magnetic structure of the orthorhombic (Cmcm) intermetallic compound DyFe6Sn6 by neutron powder diffraction. The Fe sublattice orders antiferromagnetically at 559(5)K with moments along [100] and a propagation vector k1=[010]. At 3.6K, the Fe magnetic moment is 2.6(4)μB. The Dy sublattice orders at 19(2)K, quite independently of the Fe sublattice. The Dy order is canted and comprises ferromagnetic order along [001] and antiferromagnetic order along [010]; the respective propagation vectors are [000] and [010]. At 3.6K, the Dy magnetic moment components are FM 5.0(3)μB and AF 1.6(3)μB, leading to a net Dy moment of 5.2(4)μB and a canting angle of 17(4)∘ away from the crystal c-axis towards the b-axis.

We report on a study of a magnetic nanocomposite of the Nanoperm type (Fe89Zr7B3Cu1) by magnetic small-angle neutron scattering (SANS). The understanding of the magnetic microstructure of these materials leaves much to be desired since we lack techniques capable of resolving the spin structure in the bulk with nanoscale resolution. Here, we present an analysis of the SANS signal by which one cannot only characterise the nanoscale structure of the spin system, but which allows to identify origin and structure of the perturbing field. In Nanoperm, an unusual angular anisotropy of the scattering suggests that the local spin misalignment decorates, as the most important perturbing field, dipole stray fields around the crystalline phase of the composite.

We present magnetic-field-dependent small-angle neutron scattering data for the ferromagnetic nanocomposite Nanoperm (Fe89Zr7B3Cu1). The spin-misalignment scattering in the approach-to-saturation regime unexpectedly reveals pronounced lobes of high intensity at angles +/- 30-40 degrees relative to the magnetic-field axis. Based on numerical calculations, the four-fold angular symmetry of the scattering pattern can be explained in terms of local spin misalignment, which originates from dipolar stray fields due to the mismatch of the saturation-magnetization values between the bcc Fe particles and the amorphous magnetic matrix. (C) 2005 American Institute of Physics.

Neutron diffraction, ac susceptibility and low field dc magnetic measurements were performed on the ternary intermetallic phase Y3Co8Sn4. This compound displays a ferromagnetic transition at 53 K and then undergoes a further transition below 10 K into an incommensurate anti ferromagnetic modulated structure. The low temperature magnetic structure is a long period sine or helical modulation with a propagation vector of [0.095, 0, 0] corresponding to a modulation length of about 80 Angstrom at 1.6 K. A possible mechanism for such a peculiar behaviour is a change from a high temperature itinerant character of the magnetic interactions to a low temperature localized magnetic state in a multi-sublattice system.

In the MnFe(P,As) compounds which are promising magnetorefrigerant materials, we have studied the effect of Si substitution and successfully replaced As by Si. Surprisingly besides all the other changes, a peculiar history dependence of the magnetic phase transition was disclosed. The as-prepared sample shows a significantly lower transition temperature (namely a virgin TC) than the sample that has experienced thermal cycling. The neutron diffraction patterns recorded during the first cooling manifest the first-order and magnetic-field-induced characters of the virgin phase transition. However, the refinement of the diffraction patterns does not provide evidence for atomic-position swapping, which might account for this history dependence.

The magnetic ordering of the compound CeCoAl4 (orthorhombic; Pmma space group) has been investigated by means of neutron diffraction from powder and single-crystal samples. Only the Ce moments order antiferromagnetically below TN = 12.8(1) K, with a propagation vector . Their collinear magnetic moments point along the b axis of the magnetic unit cell, which is doubled along both the nuclear b and c axes. At 1.5 K the refined magnetic moment value is 1.29(3) μB/Ce atom.

The magnetic and crystal structures of the series FexMn5−xSi3 (x=4,3.5,3,2,1) have been reinvestigated by neutron powder diffraction between 315 and 8 K. Fe atoms are found to preferentially occupy the 4d site of the hexagonal D88 structure (P63/mcm). The compound Fe4MnSi3 orders ferromagnetically below 300 K. Refinement of the neutron data for this compound indicates that the moments are aligned at ∼40° off the c axis. A high moment at the 6g site is due to a large Mn moment which is forced into a ferromagnetic alignment with the Fe moments. Associated with this moment is a moderate magnetic entropy change, with a maximum value of 4 J kg−1 K−1 for a magnetic field change of 5 T. The magnetic structure of Fe3.5Mn1.5Si3 is similar to that of Fe4MnSi3, but the Curie point is lower, below 240 K. A feature for this composition is the appearance of a spiral structure below 60 K. The compounds with x=1, 2, and 3 are all antiferromagnets with a spiral structure observed for Fe3Mn2Si3 at all temperatures up to the Néel point of 150 K. Two separate antiferromagnetic structures exist for Fe2Mn3Si3, one between 120 and 45 K and another structure for temperatures below 45 K. Similar features in the magnetic ordering are exhibited by FeMn4Si3. At temperatures between 70 and 95 K, the magnetic structure is similar to that of the low temperature phase of Fe2Mn3Si3. Accompanying the magnetic transition at 70 K is a structural distortion to space group Cmcm, a feature also observed for Mn5Si3.

The effects of diluting Gd with Y on the magnetic properties of the giant magnetocaloric effect material Gd5Si2Ge2 have been investigated by high incident energy inelastic neutron scattering, magnetization, and x-ray powder diffraction measurements. The dynamic response for Gd5Si2Ge2 at 15 and 150 K consists of two magnetic excitations extending up to 40 meV. From the x-ray powder diffraction data, Gd4YSi1.9Ge2.1 is monoclinic at room temperature, space group P1121/a with the 4e site at x=−0.0092(3), y=0.1005(2), z=0.1808(4) almost exclusively occupied by Gd (97%), while the remaining four Gd/Y sites are randomly occupied by Gd and Y. The magnetization measurements show that the ordering temperature of Gd4YSi1.9Ge2.1 is about half that of Gd5Si2Ge2, as the former orders ferromagnetically at 149 K with an effective paramagnetic moment of 7.87μB. The observed magnetic dynamic response for the Gd4YSi1.9Ge2.1 at 7 K is also different from the nondiluted compound, whereby the positions and intensities of the magnetic excitations have decreased markedly with respect to Gd5Si2Ge2.