Masayuki Shiga

Magnetism and spin fluctuations of laves phase manganese compounds

Abstract The magnetism of Laves phase compounds RMn2 is reviewed. The conditions for the onset of Mn moments and the type of spin fluctuations of the Mn sublattice are discussed. It is shown that the Mn moments, if they exist, are unstable and easily collapse on raising the temperature or on substituting R or Mn by a third element with a smaller atomic volume. By analyzing the thermal expansion curves, the amplitudes of spin fluctuations in YMn2 and related compounds are estimated.

Magnetic Properties of Fe65(Ni1-xMnx)35 Ternary Alloys

Magnetic properties of Fe 65 (Ni 1- x Mn x ) 35 ternary alloys which have the f.c.c. structure over the whole range of x were investigated. It was found that the alloys are antiferromagnetic in the composition range of 1≥ x >0.3 and are ferromagnetic in the composition range of 0≤ x <0.3. In the ferromagnetic region, the Curie temperature and the spontaneous magnetization decrease rapidly with increasing x or with the decrease of mean electron concentration. Many physical properties of the alloys are discussed on the basis of three postulates, i.e. , 1) electrons at Mn sites have itinerant character, 2) electrons at Ni sites have localized character and 3) at Fe sites, electrons have itinerant character in the antiferromagnetic range and localized character in the ferromagnetic range. The relation between the atomic distance and the magnetic moment is discussed. The origin of the Invar effect is qualitatively explained on these bases.

Magnetovolume Effects in Ferromagnetic Transition Metals

Magnetovolume effects in ferromagnetic transition metals and alloys, such as the spontaneous volume magnetostriction, the forced volume magnetostriction, are described by a phenomenological theory based upon a fluctuating local band picture, in which both itinerant electron and local moment characters are taken into account. The magnetovolume effects of Fe, Ni and the Fe-Ni Invar alloy are analyzed on the basis of the proposed theory. It has been shown that the degree of shrinkage of local moments above the Curie temperature can be estimated from this analysis. It is concluded that local moments remain almost unchanged in bcc Fe and, on the contrary, they shrink markedly in the Invar alloy.

Stability of Mn moments and spin fluctuations in RMn2 (R: Rare earth)

Abstract On the basis of the results of thermal expansion and NMR measurements, the stability of Mn moments and spin fluctuations in RMn2 are discussed. It has been shown that the interatomic distance of RMn2 plays a crucial role in determining the Mn magnetism in RMn2.

Magnetocaloric properties of a first-order magnetic transition system ErCo2

Abstract The magnetocaloric effect was examined for ErCo 2 , which shows a first-order magnetic transition from a ferrimagnetic to paramagnetic state at 32 K. A large magnetic entropy change was caused by a magnetic field. The corresponding entropy change ΔS and the adiabatic temperature change ΔT ad are 12 J/K mol and 13 K, respectively in a change of magnetic field 0–8 T at 32 K. The substitution of 20% Y for Er leads to a reduction of the magnetocaloric properties by 20–30%. The origin of the large magnetocaloric effect is also discussed.

Magnetocaloric effects of Laves phase Er(Co1―xNix)2 compounds

The magnetocaloric effect (MCE) has been studied for ErCo2 single crystals and Er(Co1−xNix)2 polycrystalline samples with x≤0.10 by means of specific heat measurements and magnetization measurements. The magnetocaloric properties estimated from the calorimetric study are in agreement with those from the magnetization measurements for ErCo2 which undergoes a first-order magnetic transition at 32 K. The substitution of 10% Ni for Co lowers the Curie temperature from 32 to 13 K. Although the first-order transition vanishes at around x=0.05, the large MCE is retained for x=0.10. The results suggest a high potential of Er(Co1−xNix)2 as active magnetic regenerative materials.

Mössbauer Study of Invar-Type Iron-Nickel Alloys

By means of the Mossbauer effect intenal magnetic fields at iron atom in invar-type iron-nickel alloys were measured using fine powdered samples in which the γ→α martensitic transformation can be suppressed. The absorption spectra of 30 and 32% iron-nickel alloys have a marked feature that a paramagnetic absorption line coexists with normal ferromagnetic ones even at liquid nitrogen temperature which is well below the Curie temperature. The intenal magnetic field was found to be nearly constant at 330 kOe over the composition range and the sharp decrease of saturation magnetization of alloys around the composition can semiquantitatively be explained when both of intensities of ferromagnetic and paramagnetic lines are taken into consideration.

Spontaneous Volume Magnetostriction and Lattice Constant of Face-Centered Cubic Fe-Ni and Ni-Cu Alloys

Thermal expansion measurements were made on f.c.c. Fe-Ni and Ni-Cu alloys from liquid helium temperature up to 1000 K. The spontaneous volume magnetostriction of these alloys at 0 K evaluated from their thermal expansion curves can be quantitatively explained on the basis of the collective electron model, using the density of states curve for nickel. The lattice constants of these alloys at various temperatures evaluated from their thermal expansion curves deviate from Vegards law in the Invar region. This deviation is discussed in connection with the magnetic state of the Invar alloy and with the origin of the Invar effects.

Moment-Induced Magnetic Scattering and Magnetovolume Effect in Fe65(Ni1-xMnx)35 Alloys

The temperature dependences of electrical resistivity and thermal expansion were measured for face-centered cubic Fe 65 (Ni 1- x Mn x ) 35 ternary alloys. The electrical resistivity of ferromagnetic alloys ( x 0.3) increases below the Neel temperature and shows a resistance minimum. These behaviors are explained in terms of the magnetic scattering by induced moments below the magnetic ordering temperature. The expansion of volume due to the magnetic ordering was observed in both ferro- and antiferromagnetic alloys. To explain the volume expansion by the magnetic ordering, a new mechanism of magnetovolume effect based on the change of cohesive energy due to the spin polarization in 3 d band is proposed.

Characteristic magnetovolume effects in Invar type Fe-Pt alloys

The thermal expansion and the forced volume magnetostriction have been measured for the Invar type Fe-Pt alloys. Derived values of the spontaneous volume magnetostriction are large and the same order of magnitude as those for the Fe-Ni Invar alloys. Their dependence on concentration and chemical ordering can be explained by the change of Fe-Fe atom pairs and their temperature dependence satisfies the empirical relation, omega s(T)=CM2(T). The forced volume magnetostriction observed at 4.2K is about one order smaller than that for Fe-Ni Invar alloys. This suggests the presence of strong ferromagnetism in Fe-Pt Invar alloys, although this contradicts the observation of large values for the high-field susceptibility in these alloys.