Yasuaki Nakagawa

Dense radial growth of silver metal leaves in a high magnetic field

Growth patterns of silver metal leaves from AgNO 3 solutions were investigated in a high magnetic field of 8 T as experimental research of diffusion-limited aggregation (DLA) with magnetohydrodynamic drifts of ions. Silver metal leaves grew into a typical DLA form with the fractal dimension of 1.62 in the absence of magnetic fields, and they grew into a dense radial form in the field of 8 T, where the crossover in the fractal dimension from 1.69 to 2 was observed with increasing length scale.

Lattice Distortion and Elastic Properties of Antiferromagnetic γ Mn-Ni Alloys

Lattice parameters and elastic moduli of polycrystalline γ Mn alloys containing 12–40 at% Ni have been measured in a temperature range between –180° and 250°C. The face-centred cubic lattice undergoes a tetragonal distortion, either c / a 1, or an orthorhombic distortion at low temperatures. Both Youngs modulus and shear modulus exhibit a step-type change at the Neel point and a broad minimum near the transition temperature of lattice distortion. The elastic behaviour is discussed on the basis of the so-called Δ E effect due to the displacement of antiferromagnetic domain walls. This effect may possibly be enhanced by the lattice softening in the γ Mn–Ni alloys.

Magnetization and Spin Correlation of Two-Dimensional Triangular Antiferromagnet LuFe2O4

Magnetization, neutron diffraction and Mossbauer measurements have been performed on a two dimensional (2D) double-layered triangular antiferromagnet LuFe 3+ Fe 2+ O 4 single crystal. Two magnetic rods (0, 1, l ) and (1/3, 1/3, l ) with broad widths across them reveal that 2D magnetic ordering is not of long range and no 3D one occurs down to 4.2 K in contrast to an ordinary magnetic system. Thermoremanent magnetization measurements suggest that the system consists of various size ferrimagnetic clusters. The characteristic profile of l -scan in (0, 1, l ) can be realized by taking account of correlation among clusters for intra double-layers and inter double-layers. The temperature variations of intensity in magnetic scattering can be explained qualitatively by assuming unequal exchange interactions among three sublattice.

Change of Magnetic Susceptibility of Transition Metals and Alloys at their Melting Points

The magnetic susceptibility of Mn, Fe, Co, Ni and their alloys has been measured through their melting points up to 1600°C. In general, the susceptibility of transition metals shows a decrease or an increase on melting, depending on whether the interatomic interactions favour parallel alignment of atomic magnetic moments or not, indicating that the interactions are always weaker in liquid than in solid. Although some Fe alloys of face-centred cubic lattice form exceptions to the above simple rule, they are also explained on the basis of the Bethe-Slater exchange interaction curve together with the volume increase on melting. It is to be noted that a remarkable supercooling of liquid, often exceeding one hundred degrees, has been observed during the course of these susceptibility measurements.

Low-Temperature Phase Transitions and Magnetic Properties of YFe2O4

Two-step phase transitions are found in stoichiometric YFe 2 O 4 at low temperatures, about 230 K and 190 K during cooling. A hexagonal lattice at room temperature is distorted to be monoclinic and subsequently triclinic. No parasitic ferrimagnetism is found in YFe 2 O 4.00 , in contrast to oxygen-deficient specimens such as YFe 2 O 3.94 .

Magnetic Properties of Amorphous Fe-Nd Alloys*

Melt-spun amorphous Fe 1- x Nd x alloys (0.11≤ x ≤0.60) were investigated by the magnetization measurement up to 143 kOe and the Mossbauer spectroscopy. The coercive field at 4.2 K increases abruptly at around x =0.30 with increasing x , amounting to 53 kOe at x =0.40. The hyperfine field of 57 Fe at 77 K is independent of x and about 300 kOe on an average, suggesting that the magnetic moment of Fe is about 2.0 µ B /atom throughout this system. On this assumption and the measured magnetization at 4.2 K, it is estimated that the average Nd moment is about 3.2 µ B /atom at the low x limit. This value corresponds to the free ion value of Nd 3+ . The Nd moment decreases with increasing x to one half, 1.6 µ B /atom. A ferromagnetic cluster model with large random anisotropy is proposed for the high coercivity alloys of this system. Magnetization curves at 4.2 K are well explained by the model, by an adjustment of the parameters for the anisotropy energy of the cluster and the inter-cluster coupling. The concen...

High‐resolution neutron powder diffraction study on nitrogenated Nd2Fe17

High‐resolution neutron powder diffraction measurements with λ=1.8232 A and collimation of 6’‐20’‐6’ on Nd2Fe17Nx with x=0, 2.85, and 2.91 were carried out at room temperature. Structural parameters and magnetic moments were determined by the Rietveld profile‐fitting calculation. The magnetic moments of iron atoms increased with increasing nitrogen concentration; e.g., the magnetic moments of iron located at 6c, 9d, 18f, and 18h sites increase from less than 0.7μB in the x=0 sample to about 2.1μB in the x=2.91 sample. It is noticed that a local atomic group composed of two Fe(1) atoms at the 6c site plus six Fe(3) atoms at the 18f site keeps its shape against the nitrogen uptake.

Magnetization of YbFe2O4+x

Magnetizations of hexagonal compounds YbFe 2 O 4+ x (polycrystals with x =-0.027, -0.003 and +0.015 and a single crystal with x =+0.03) have been measured in a temperature range between 4.2 K and room temperature. Remanent magnetizations are induced by magnetic fields applied during cooling below about 250 K. Dependences of lattice parameters on x and temperature have also been determined. No lattice distortion occurs at low temperatures in contrast to the case of YFe 2 O 4 .

Magnetizations and Mössbauer Spectra of YFe2O4-x

Effects of oxygen deficiency on magnetic properties of YFe 2 O 4- x ( x =0.00, 0.031, 0.040, 0.055 and 0.095) have been investigated by magnetization and Mossbauer effect measurements. When x ≧0.055, the compounds show so-called parasitic ferrimagnetism below 210 ∼220 K. When x ≦0.031, on the other hand, they undergo two successive first-order phase transitions at around 230 K and 190 K, without any thermoremanence at any temperature. The compound with x =0.040 exhibits intermediate characteristics. Lattice defects introduced by the oxygen deficiency are considered to play an important role in the disappearance of the first-order transitions and the appearance of the parasitic ferrimagnetism.

Magnetic Susceptibility of Iron-Nickel Alloys in Liquid and Solid States

The magnetic susceptibilities of various Fe-Ni alloys have been measured through their melting points up to 1600°C. The susceptibilities of the alloys with less than 60 % Ni increase discontinuously on melting, while those of the alloys with more than 60 % Ni decrease. The temperature variation of the susceptibilities is approximately expressed by the Curie-Weiss law in the liquid state as well as in the solid state, but the Curie constants and the paramagnetic Curie temperatures of liquids are in general different from those of solids. These results can be explained by assuming that the atomic magnetic moments remain unaltered and that the magnetic interaction varies with the interatomic distance as indicated by the Bethe-Slater curve. The change of susceptibility at the melting point is solely due to the difference in interatomic distance between the liquid and the solid. It is expected, in some cases, that the especially strong dependence of the magnetic interaction on the interatomic distance results ...