Crystal structure of high–density Fe56 cluster Nd2Fe14B under high pressure

Nd2Fe14B is a high-density Fe cluster containing 56 Fe atoms in one unit cell. We investigated the crystallographic structure of an isotropic Nd2Fe14B magnet comprising nanocrystals of a size of ~30 nm at pressures up to 2 GPa. The results of X-ray diffraction measurements using Rietveld refinement revealed the displacements of each Fe atomic site in the Fe cluster, Nd, and B atomic sites. The lattice constants, a and c, of tetragonal symmetry decreased proportionally with external pressure, whereas the shrinkage ratio for both a and c changed at approximately 0.5 GPa. However, each atomic position exhibited non-monotonic pressure dependence. The trend of displacement of atomic positions changed at a characteristic pressure of 0.4 ± 0.1 GPa. When exceeded, most atoms shifted to the direction opposite their displacement at lower pressures. Thus, they exhibited restoration tendencies toward the positions at ambient pressure. The bond angles and bond lengths among Nd, Fe, and B atoms also exhibited characteristic pressure dependences. As pressure increased, the basal triangle of the trigonal prism in the Fe cluster layers distorted up to ~0.5 GPa, whereas the strain of the trigonal prism gradually reduced just above 0.5 GPa. The atomic position of the heaviest Nd atoms was a key structural parameter to characterize the change

SIZE EFFECTS ON MAGNETIC PROPERTY AND CRYSTAL STRUCTURE OF MN3O4 NANOPARTICLES IN MESOPOROUS SILICA

Mn3O4 nanoparticles with particle sizes of 7.8, 11.4, and 18.3 nm were synthesized in the pores of mesoporous silica, and their crystal structure and magnetic properties were investigated. The powder X-ray diffractions at room temperature indicated that the crystal structural symmetry was the same as that for bulk crystal, and the lattice constants deviated from those for bulk crystal, which depended on the particle size. In addition, compared with the bulk crystal, the Jahn-Teller distortion for the nanoparticles was suppressed and decreased with decreasing the particle size. The coercive field for 7.8 nm was rather smaller than those for 11.4 and 18.3 nm. The nanoparticles with 11.4 and 18.3 nm exhibited pronounced three kinds of magnetic transition temperatures, whereas the susceptibility for 7.8 nm indicated the existence of two transition temperatures. These experimental results suggested that the Mn3O4 nanoparticles have a strong correlation between crystallographic structure and magnetic property, and the characteristic magnetic size effects are attributed to the reduction of Jahn-Teller distortion.The 21st International Conference on Magnetism (ICM2018), July 15-20, 2018, San Francisco, US

Effect of pressure on single-chain magnets with repeating units of the MnIII-NiII-MnIII trimer

The single-chain magnet (SCM) system [Mn2(saltmen)2Ni(pao)2(L)2](A)2 (L: intrachain attaching ligand of NiII ion; A-1: interchain counteranion) is a ferromagnetic one-dimensional network system with repeating units of the MnIII-NiII-MnIII trimer which itself behaves as a single-molecule magnet with an S=3 spin ground state and negative uniaxial single-ion anisotropy (D) parallel to the bridging direction. The slow relaxation of the magnetic moment in this SCM system originates in an energy barrier for spin reversal (ΔE), which is closely related to the ferromagnetic interaction between the trimers (Jtrimer) as well as to the D of the trimer. We have investigated the effects of pressure on three compounds representative of the above SCM family through ac susceptibility measurements under hydrostatic pressures up to P=13.5 kbar and crystal structural analysis experiments up to P=20.0 kbar, and have observed a pronounced enlargement of ΔE when J was artificially increased. The application of hydrostatic pressure brought about the systematic enhancement of EΔ (a maximum increase of 10% within the pressure region of the experiments). The pressure dependence of EΔ varied according to the kind of attaching ligand L involved and the intrachain structure, and we have experimentally found that isotropic lattice shrinkage is desirable if a continuous increase of ΔE in this system is aimed at

Contactless measurement of electrical conductivity for bulk nanostructured silver prepared by high-pressure torsion: A study of the dissipation process of giant strain

We measured the electrical conductivity of bulk nanostructured silver prepared by high-pressure torsion (HPT) in a contactless manner by observing the AC magnetic susceptibility resulting from the eddy current, so that we could quantitatively analyze the dissipation process of the residual strain with sufficient time resolution as a function of temperature T and initial shear strain γ. The HPT process was performed at room temperature under a pressure of 6 GPa for revolutions N = 0–5, and we targeted a wide range of residual shear strains. The contactless measurement without electrode preparation enabled us to investigate both the fast and slow dissipation processes of the residual strain with sufficient time resolution, so that a systematic study of these processes became possible. The changes in the electrical conductivity as a function of N at room temperature were indeed consistent with changes in the Vickers microhardness; furthermore, they were also related to changes in structural parameters such as the preferred orientation, the interplanar distance, and the crystallite size. The dissipation process at N = 1, corresponding to γ ≈ 30, was the largest and the fastest. For N = 5, corresponding to γ ≈ 140, we considered the effects of grain boundaries, as well as those of dislocations. The strain dissipation was quite slow below T = 290 K. According to the analytical results, it became successful to conduct the quantitative evaluation of the strain dissipation at arbitrary temperatures: For instance, the relaxation times at T = 280 and 260 K were estimated to be 3.6 and 37 days, respectively

Characteristic Size Effects on the Crystallographic Structure and Magnetic Properties of RMnO3 (R = Eu, Gd, Tb, Dy) Nanoparticles

We synthesized lanthanoid manganese oxide RMnO3 (R = Eu, Gd, Tb, and Dy) nanoparticles with particle sizes ranging from approximately 6.5 to 23 nm and investigated both their crystal structure and magnetic properties. The RMnO3 nanoparticles showed a strong correlation between crystal structure and magnetic properties, and particle size effects on these properties vary owing to the different atomic radii of the lanthanoid ions. The magnetic properties of all of the nanoparticles exhibited significant changes as the lattice constants changed at characteristic sizes that depend on the lanthanoid ionic radius; however, the characteristic size for magnetic properties corresponded to the magnitude of the orthorhombic distortion b/a = 1.10, regardless of the lanthanoid ionic radius. With decreasing particle size, EuMnO3, GdMnO3, and TbMnO3 nanoparticles induced tensile strain of MnO6 octahedra, whereas compressive strain occurred in DyMnO3 nanoparticles. The deformation of MnO6 octahedra changed the magnetic interactions, resulting in changes in the magnetic properties. As the particle size decreased, for R = Eu, Gd, and Tb, the magnetic properties, such as transition temperature, coercive field, and blocking temperature, decreased; conversely, these values increased in DyMnO3. The distortion of the unit cell induced changes in the magnetic ordering state due to decreasing particle size

Spin correlation and relaxational dynamics in molecular-based single-chain magnets

We report the combined measurements of the dc susceptibility X0, the ac susceptibility X, and the NMR relaxation rate T for the molecular-based heterometallic single-chain magnet [Mn(saltmen)]2[Ni(pao)2(py)2](PF6)2. At low temperatures, this system is well described by a one-dimensional array of effective spin S=3 chains comprising the MnIII-NiII-MnIII trimers and treated as the S=3 Ising chain with the single-ion term (Blume-Capel model). Using the exact solution of the model and based on the picture that the random motion of the local domain walls dominates the low-temperature spin dynamics, we succeeded in reproducing the experimental results of the dc susceptibility X0, the ac susceptibility X, and the 19F-NMR relaxation rate T in a consistent manner

Spin correlation and relaxational dynamics in molecular-based single-chain magnets

We report the combined measurements of the dc susceptibility 0, the ac susceptibility , and the NMR relaxation rate T for the molecular-based heterometallic single-chain magnet [Mn(saltmen)]2[Ni(pao)2(py)2](PF6)2. At low temperatures, this system is well described by a one-dimensional array of effective spin S=3 chains comprising the MnIII-NiII-MnIII trimers and treated as the S=3 Ising chain with the single-ion term (Blume-Capel model). Using the exact solution of the model and based on the picture that the random motion of the local domain walls dominates the low-temperature spin dynamics, we succeeded in reproducing the experimental results of the dc susceptibility 0, the ac susceptibility , and the 19F-NMR relaxation rate T in a consistent manner

Hydrostatic contraction and anisotropic contraction effects on oxygen molecule nanorods

We study the effects of both hydrostatic and anisotropic contractions on the molecular condensation of oxygen molecules (O2) physisorbed to nanosized pores, termed “O2 nanorods”, through the magnetization measurements. Multisteps of O2 solidification accompany the reduction in structural symmetry with decreasing temperature, such that the structural change by external stress varies the stability of O2 solidification. For initial pore diameters of D = 8.5, 14.5, and 24.0 nm, anisotropic compression for nanorods (preferential compression along radial direction of the pores) occurred, and molecule solidification is suppressed at the lower temperature side compared with that under hydrostatic compression. For the smallest D = 6.5 nm, a hydrostatic contraction almost occurred, and the high adsorption capability enabled the detection of both the melting transition and change in crystal structure within the β phase, in addition to α–β and β–γ transitions

Synthesis and Magnetic Property of Multiferroic BiMnO3 Nanoparticles in the Pores of Mesoporous Silica

The mesoporous silica material SBA-15 was prepared with control pore size for use as a nanoparticle synthesis template. The X-ray diffraction results show that the nanopores were ordered in a hexagonal structure. The pore size was controlled by the synthesis conditions. Nanoparticles of the multiferroic compound BiMnO3 with an average size of about 14 nm were successfully synthesized in the pores of SBA-15, and their magnetic properties were investigated. The prepared BiMnO3 nanoparticles showed both antiferromagnetic and ferromagnetic behavior, whereas BiMnO3 bulk crystals consistently showed ferromagnetic behavior. The prepared BiMnO3 nanoparticles exhibited unique magnetic size effects: the appearance of antiferromagnetic behavior and the coexistence of antiferromagnetic and ferromagnetic components

Anisotropic compression effects on nanocrystalline crystals of nickel oxide

Nickel oxide (NiO) with bulk crystalline size is an antiferromagnetic insulator correlated strongly with crystal structure. A reduction in crystalline size causes a change in the magnetic sublattice, resulting in the appearance of ferromagnetic moments. Furthermore, in the nano crystals fabricated in mesoporous silica, there is a lattice distortion at the unit cell level that brings about the change in the magnetic property, suggesting a prominent magneto-structural correlation. We investigate anisotropic compression effects on nanocrystalline NiO with a crystalline size (D) of 11.4 nm to observe this remarkable magneto-structural correlation. Magneto-crystalline anisotropy is at its maximum when negative contraction occurs. The negative contraction appears even in a bulk crystal under non-ideal hydrostatic compression, and its anomalous structural change is suppressed with decreasing D