Neutron diffraction measurements of a high quality single crystal of CaFe 2 As 2 are reported. A sharp transition was observed between the high temperature tetragonal and low temperature orthorhombic structures at T S = 172.5K (on cooling) and 173.5K (on warming). Coincident with the structural transition we observe a rapid, but apparently continuous, ordering of the Fe moments, in a commensurate antiferromagnetic structure is observed, with a saturated moment of 0.80(5)μ B /Fe directed along the orthorhombic aaxis. The hysteresis of the structural transition is 1K between cooling and warming and is consistent with previous thermodynamic, transport and single crystal x-ray studies. The temperature onset of magnetic ordering shifts rigidly with the structural transition providing the clearest evidence to date of the coupling between the structural and magnetic transitions in this material and the broader class of iron arsenides.

Applying a magnetic field to a ferromagnetic Ni 50 Mn 34 In 16 alloy in the martensitic state induces a structural phase transition to the austenitic state. This is accompanied by a strain which recovers on removing the magnetic field, giving the system a magnetically superelastic character. A further property of this alloy is that it also shows the inverse magnetocaloric effect. The magnetic superelasticity and the inverse magnetocaloric effect in Ni-Mn-In and their association with the first-order structural transition are studied by magnetization, strain, and neutron-diffraction studies under magnetic field.

High resolution neutron powder diffraction and single crystal measurements on the ferromagnetic shape memory compound Ni 2 MnGa have been carried out. They enabled the sequence of transformations which take place when the unstressed, stoichiometric compound is cooled from 400 to 20 K to be established. For the first time the crystallographic structure of each of the phases which occur has been determined. At 400 K the compound has the cubic L2 1 structure, and orders ferromagnetically at T C ≈ 365 K. On cooling below ∼260 K a super-structure, characterized by tripling of the repeat in one of the 110 cubic directions, forms. This phase, known as the pre-martensitic phase, persists down to the structural phase transition at T M ≈ 200 K and can be described by an orthorhombic unit cell with lattice parameters a ortho = 1 √ 2 a cubic , b ortho = 3 √ 2 a cubic , c ortho = a cubic and space group Pnnm. Below T M the compound has a related orthorhombic super-cell with b ortho ≈ 7 √ 2 a cubic , which can be described within the same space group. The new modulation appears abruptly at T M and remains stable down to at least 20 K.

Magnetization and high resolution neutron powder diffraction measurements on the magnetic shape memory compound Ni2Mn1.44Sn0.56 have confirmed that it is ferromagnetic below 319 K and undergoes a structural phase transition which takes place at TM = 221 K on cooling and 239 K on warming. The high temperature phase has the cubic L 21 structure, a = 5.973 Å, with the excess manganese atoms occupying the 4(b) tin sites. In the cubic phase at 245 K the manganese moments at both sites were found to be ferromagnetically aligned. The magnetic moment at the 4(a) sites was 1.88(10) μB but it was only 0.53(18) μB/Mn at the 4(b) sites. The low temperature phase stable below TM has an orthorhombic structure with space group Pmma related to the cubic phase through a Bain transformation aortho = (acub+bcub)/2; bortho = ccub; cortho = (acub− bcub). The change in cell volume in the transition is only ≈0.5%, suggesting that the atomic moments are unchanged although the spontaneous magnetization drops significantly.

Neutron scattering from single crystals has been used to determine the magnetic structure and magnon dynamics of FePS 3 , an S = 2 Ising-like quasi-two-dimensional antiferromagnet with a honeycomb lattice. The magnetic structure has been confirmed to have a magnetic propagation vector of k M = [ 01 1 2 ] and the moments are collinear with the normal to the ab planes. The magnon data could be modeled using a Heisenberg Hamiltonian with a single-ion anisotropy. Magnetic interactions up to the third in-plane nearest neighbor needed to be included for a suitable fit. The best fit parameters for the in-plane exchange interactions were J 1 = 1.46, J 2 = −0.04, and J 3 = −0.96 meV. The single-ion anisotropy is large, = 2.66 meV, explaining the Ising-like behavior of the magnetism in the compound. The interlayer exchange is very small, J = −0.0073 meV, proving that FePS 3 is a very good approximation to a two-dimensional magnet.

Bimetallic, oxalate-bridged compounds with bi- and trivalent transition metals comprise a class of layered materials which express a large variety in their molecular-based magnetic behavior. Because of this, the availability of the corresponding single-crystal structural data is essential to the successful interpretation of the experimental magnetic results. We report in this paper the crystal structure and magnetic properties of the ferromagnetic compound {[N(n-C(3)H(7))(4)][Mn(II)Cr(III)(C(2)O(4))(3)]}(n)() (1), the crystal structure of the antiferromagnetic compound {[N(n-C(4)H(9))(4)][Mn(II)Fe(III)(C(2)O(4))(3)]}(n)() (2), and the results of a neutron diffraction study of a polycrystalline sample of the ferromagnetic compound {[P(C(6)D(5))(4)][Mn(II)Cr(III)(C(2)O(4))(3)]}(n)() (3). Crystal data: 1, rhombohedral, R3c, a = 9.363(3) Å, c = 49.207(27) Å, Z = 6; 2, hexagonal, P6(3), a = 9.482(2) Å, c = 17.827(8) Å, Z = 2. The structures consist of anionic, two-dimensional, honeycomb networks formed by the oxalate-bridged metal ions, interleaved by the templating cations. Single-crystal field dependent magnetization measurements as well as elastic neutron scattering experiments on the manganese(II)-chromium(III) samples show the existence of long-range ferromagnetic ordering behavior below T(c) = 6 K. The magnetic structure corresponds to an alignment of the spins perpendicular to the network layers. In contrast, the manganese(II)-iron(III) compound expresses a two-dimensional antiferromagnetic ordering.

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The archetype of geometrically frustrated compounds SrCr 9 p Ga 12Ϫ9 p O 19 is a kagomé bilayer of Heisenberg Cr 3ϩ ions (Sϭ3/2) with antiferromagnetic interactions. We present an extensive gallium nuclear magnetic resonance ͑NMR͒ study over a broad Cr-concentration range (0.72рpр0.95). This allows us to probe locally the susceptibility of the kagomé bilayer and separate the intrinsic properties due to geometric frustration from those related to site dilution. Compared to the partial study on one sample, pϭ0.90, presented in Phys. Rev. Lett. 85, 3496 ͑2000͒, we perform here a refined study of the evolution of all the magnetic properties with dilution, with a great emphasis on the lowest diluted pϭ0.95 sample synthesized for this study. Our major findings are the following ͑1͒ The intrinsic kagomé bilayer susceptibility reaches a maximum at a temperature of Ϸ40Ϫ50 K, which we show here to be robust up to a dilution as high as Ϸ20%; this maximum is the signature of the development of short-range antiferromagnetic correlations in the kagomé bilayer. ͑2͒ At low T, a highly dynamical state induces a strong wipeout of the NMR intensity, regardless of dilution. ͑3͒ The low-T upturn of the macroscopic susceptibility is associated with paramagnetic defects, which stem from the dilution of the kagomé bilayer. The low-T analysis of the pϭ0.95 NMR line shape, coupled with a more accurate determination of the nuclear Hamiltonian at high T, allows us to discuss in detail the nature of the defect. Our analysis suggests that the defect can be associated with a staggered spin response to the vacancies of the kagomé bilayer. This, altogether with the maximum in the kagomé bilayer susceptibility, is very similar to what is observed in most low-dimensional antiferromagnetic correlated systems, even those with a short spin-spin correlation length. ͑4͒ The spin-glass-like freezing observed at T g Ϸ2 -4 K is not driven by the dilutioninduced defects.