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

Magnetic correlations in the S=5/2 quadratic lattice Heisenberg antiferromagnet Mn(HCOO)2･2(ND2)2CO

The magnetic correlations in the quadratic lattice S=5/2 Heisenberg antiferromagnet Mn(HCOO)2⋅2(ND2)2CO (TN=3.77K) have been studied by means of specific heat and neutron-scattering experiments. With a universal temperature scale, the temperature behavior of both the magnetic heat capacity and spin correlations are quantitatively accounted for by the pure quantum self-consistent harmonic approximation by Cuccoli et al. for S=5/2

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

Pressure-Induced Ferromagnetic to Nonmagnetic Transition and the Enhancement of Ferromagnetic Interaction in the Thiazyl-Based Organic Ferromagnet γ-BBDTA·GaCl4

A thiazyl-based ferromagnet, the γ-phase of BBDTA (i.e., benzo[1,2- d :4,5- d \u27]bis[1,3,2]dithiazole)·GaCl 4 , has a high ferromagnetic ordering temperature of 7.0 K in organic radical ferromagnets. In this system, pressurization generated more compact molecular packing, resulting in that the ferromagnetic state at P = 16.2 kbar is stabilized over a temperature range of more than twice of the initial range. However, the saturation magnetic moment was reduced with increasing pressure, decreasing to about 12% of the initial value even at the low pressure level of P = 1.0 kbar. This suggests that the ferromagnetic molecular packing of the monoclinic γ-phase is easily transformed into that of the diamagnetic phase. Powder X-ray diffraction experiments revealed that the diamagnetic non-monoclinic (α- or β-) phase became stable instead of the monoclinic γ-phase across the pressure of 2.5–5.8 kbar. The increase in the temperature of onset of ferromagnetic state occurs in the surviving ferromagnetic domain surrounded by the diamagnetic domains

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

High-pressure dc magnetic measurements on a bisdiselenazolyl radical ferromagnet using a vibrating-coil SQUID magnetometer

The high-pressure magnetic properties of the iodo-substituted bisdiselenazolyl radical ferromagnet IBPSSEt have been studied by vibrating-coil SQUID magnetometry. The magnetic state at a pressure (P) of approximately 2 GPa has the highest Curie temperature (TC) of 27.5 K, and displays an ideal three-dimensional (3D) ferromagnetic interaction network. The value of TC observed by ac magnetic susceptibility measurements is consistent with that obtained from dc measurements below approximately 4 GPa. Field-cooled dc measurements at more elevated pressures reveal a slow evolution of magnetic ordering, so that atP >6 GPa the structure may be described in terms of a 1D ferromagnetic chain with predominantly antiferromagnetic lateral (interchain) interactions, in accord with the results of density functional theory calculations

Effects of Hydrostatic Pressure and Uniaxial Strain on Spin-Peierls Transition in an Organic Radical Magnet, BBDTA·InCl4

We investigated the effects of hydrostatic pressure and uniaxial strain on the spin-Peierls (SP) transition of an organic radical magnet, benzo[1,2-d:4,5-d\u27]bis[1,3,2]dithiazole(BBDTA)·InCl 4 . It has a one-dimensional coordination polymer structure along its c -axis and its SP transition occurs at 108 K. The SP transition temperature T SP decreased to 99 K at a hydrostatic pressure of 10 kbar, while it increased to 132 K at a uniaxial strain along the c -axis of 8 kbar. The pressure dependences of T SP under these two conditions were discussed by evaluating two parameters, namely, the intrachain interaction 2 J / k B and the effective spin–lattice coupling parameter η, that are related to T SP by the equation T SP =1.6η J / k B . Under ambient pressure, the a - and c -axes of this material shortened monotonically with decreasing temperature, while the b -axis elongated below T SP . In this study, we found the correlation between η and the change in the lattice constant b . 2 J / k B increased with increasing hydrostatic pressure and uniaxial strain, suggesting that the contraction along the c -axis does not depend on the manner of pressurization. From the evaluation of η, the observed variation in T SP is explained by the difference between the changes in b under the two pressurization conditions

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