W. J. James

Crystal structure, magnetic properties and electronic structure of the MnBi intermetallic compound

The low-temperature phase of the MnBi alloy has a coercivity μ0Hc of 2.0 T at 400 K and exhibits a positive temperature coefficient from 0 to 400 K. In the higher temperature range it shows a much higher coercivity than that of the NdFeB magnets, which suggests that it has considerable potential as a permanent magnet for use at high temperatures. In the temperature range from 30 to 150 K, the Mn atom is found to change its spin direction from a perpendicular to a parallel orientation with respect to the c axis. The anisotropy field increases with increasing temperature which gives rise to a higher coercivity at the higher temperatures. The maximum energy product (BH)max of the magnet is 7.7 and 4.6 MG Oe at room temperature and 400 K, respectively. The electronic structure of MnBi indicates that the Mn atom possesses a magnetic moment of 3.6μB, and that the Bi atom has a magnetic moment of −0.15μB which is due to the s–d and p–d hybridization between Bi and Mn atoms. We have also investigated the volume dependence of the magnetic moments of Mn and Bi. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume expansion leads to a large magnetic moment for the Mn atom. The Mn magnetic moment attains a value of 4.6μB at a volume expansion rate of ΔV/V ≈ 100%.

Structural evolution of ammonia borane for hydrogen storage

We have studied the crystal structure of fully deuterated BH3NH3 using powder neutron diffraction at different temperatures. It is evident that an order-disorder phase transition occurs around 225K. At low temperature, the compound crystallizes in the orthorhombic structure with space group Pnm21 and gradually transforms to a high temperature tetragonal structure with space group I4mm above 225K. At 16K, the BD3–ND3 unit stacks along the c axis with a tilt angle of about 16° between the N–B bond and the c axis. As the temperature is increased, the BD3–ND3 groups start to reorient along the c axis and the deuterium atoms become disordered, leading to the tetragonal phase transition.

Possible ordering of Ru and Cu in the charge-reservoir of magneto-superconductor RuSr2GdCu2O8 (Ru-1212): Magnetic, transport, and TEM microstructural studies

Magnetization vs temperature behavior of RuSr2GdCu2O8−δ (Ru-1212) measured in an field of 5 Oe, shows a clear branching of zero-field-cooled (ZFC) and field-cooled (FC) curves around 140 K, a cusp at 135 K, and a diamagnetic transition around 20 K (in the ZFC branch). The cusp at 135 K is due to the antiferromagnetic ordering of the Ru moments. The magnetization-field isotherms, below 50 K, show a nonlinear contribution from a ferromagnetic component. The resistance vs temperature behavior of the compound, in applied fields of 0, 3, and 7 T, confirms that the sample is superconducting at around 20 K. The superconducting transition exhibits field broadening of a type different than that known for conventional high Tc superconductors. The magnetoresistance (MR) is negative above the Ru magnetic ordering temperature of 135 K, while below this temperature, MR displays a positive peak in low fields and becomes negative in higher fields. A maximum of 2% is observed for the negative MR value at the Ru magnetic ordering temperature. An electron diffraction pattern obtained for this Ru-1212 sample shows two types of superstructure; one with a weak spot at the center of the a–b rectangle and the other only along the b direction. It is possible that either Ru/Cu or Ru4+/Ru5+ ordering of 2b periodicity takes place along the b direction.Magnetization vs temperature behavior of RuSr2GdCu2O8−δ (Ru-1212) measured in an field of 5 Oe, shows a clear branching of zero-field-cooled (ZFC) and field-cooled (FC) curves around 140 K, a cusp at 135 K, and a diamagnetic transition around 20 K (in the ZFC branch). The cusp at 135 K is due to the antiferromagnetic ordering of the Ru moments. The magnetization-field isotherms, below 50 K, show a nonlinear contribution from a ferromagnetic component. The resistance vs temperature behavior of the compound, in applied fields of 0, 3, and 7 T, confirms that the sample is superconducting at around 20 K. The superconducting transition exhibits field broadening of a type different than that known for conventional high Tc superconductors. The magnetoresistance (MR) is negative above the Ru magnetic ordering temperature of 135 K, while below this temperature, MR displays a positive peak in low fields and becomes negative in higher fields. A maximum of 2% is observed for the negative MR value at the Ru magnetic o...

Magnetic and structural properties of Nd2Fe17−xMnx solid solutions

A series of Nd2Fe17−xMnx solid solutions with x values between 0 and 6 were prepared and analyzed using magnetic measurements, neutron diffraction, and Mossbauer spectroscopy. All of the Nd2Fe17−xMnx samples crystallized in the Th2Zn17−x-type rhombohedral structure. The lattice parameters and unit cell volumes decrease with increasing manganese content up to ∼x equal to 2, and then increase for higher manganese content. The magnetizations of Nd2Fe17−xMnx decrease with increasing manganese content and Nd2Fe17−xMnx is paramagnetic at room temperature for x greater than 3. The Curie temperature in Nd2Fe17−xMnx solid solutions is maximum for x equal to 0.5 and decreases at a rate of ∼10° per substituted manganese up to x equal to 3, after which it drops sharply. These results are discussed in terms of the manganese site occupancies in Nd2Fe17−xMnx.

The effect of Cu-doping on the magnetic and transport properties of La0.7Sr0.3MnO3

The effects of Cu-doping on the structural, magnetic, and transport properties of La0.7Sr0.3Mn1−xCuxO3(0⩽x⩽0.20) have been studied using neutron diffraction, magnetization, and magnetoresistance (MR) measurements. All samples show the rhombohedral structure with the R3¯c space-group from 10 K to room temperature (RT). Neutron diffraction data suggest that some of the Cu ions have a Cu3+ state in these compounds. The substitution of Mn by Cu affects the Mn–O bond length and Mn–O–Mn bond angle resulting from the minimization of the distortion of the MnO6 octahedron. Resistivity measurements show that a metal to insulator transition occurs for the x⩾0.15 samples. The x=0.15 sample shows the highest MR(≈80%), which might result from the co-existence of Cu3+–Cu2+ and the dilution effect of Cu-doping on the double exchange interaction.

Neutron‐diffraction and Mössbauer effect study of the preferential silicon site occupation and magnetic structure of Nd2Fe14−xSixB

A neutron‐diffraction study of Nd2Fe14−xSixB has shown that silicon preferentially occupies the 4c site in the transition‐metal sublattice in Nd2Fe14B. Silicon also exhibits a moderate preference for the 8j1 site, is almost excluded from the 16k2 site, and avoids the 16k1, 8j2, and 4e sites. The silicon site occupancy is correlated with a preference for a silicon atom to have rare‐earth atoms in its coordination environment. The Mossbauer spectra of Nd2Fe14−xSixB have been fit with a model which takes into account the distribution of near‐neighbor environments of an iron atom due to the presence of silicon. These fits show that the substitution of silicon in the near‐neighbor environment of an iron atom primarily influences the long‐range contributions to the hyperfine field experienced by the iron. The mechanism for the increase in the Curie temperature when silicon is added to Nd2Fe14B‐type magnets is more subtle than previously believed, but can be explained by the relative decrease in the proportion o...

Structure, magnetic, and transport properties of Ti-substituted La 0.7 Sr 0.3 MnO 3

Ti-substituted perovskites

A magnetic and crystallographic study of (Sm/Gd)2(Fe/Si)17Cz solid solutions

{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{3}

Magnetic and transport properties of nanocomposite Fe/Fe3−δO4 and Fe3−δO4 films prepared by plasma-enhanced chemical vapour deposition

with

Magnetic properties of iron-rich Nd2−yDyyFe17−xSix mixed rare-earth system

0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.20