Magnetovolume Coupling Research Articles

Anomalous properties and magnetic heterogeneity of Fe-Ni invar alloys

The temperature dependence of the enhancement factor due to the magnetovolume coupling in the high-field susceptibility, compressibility and forced volume magnetostriction are obtained for 29 at% Ni-Fe alloy and its relation to the martensitic transformation is discussed. A large enhancement in Zr(Fe 1-xCo x ) 2 compounds is also pointed out.

Origin of Anomalies and Thermodynamic Aspects in Invar Alloys II. Effect of Magnetic Phase Mixing

The origin of the magnetic inhomogeneity in Fe–Ni Invar alloys is attributed to the first-order magnetic transition, which was predicted before in the itinerant electron model. The large effect of the magnetovolume coupling on various thermodynamic properties is examined in the model of magnetic mixing of the ferromagnetic phase with larger atomic volume and the paramagnetic phase with smaller atomic volume. At low temperatures the mixing is assumed to be frozen in. The mutual transformation between the ferromagnetic and paramagnetic phases due to the changes of temperature, magnetic field and pressure give additional contributions to the respective thermodynamic properties. The difference between the observed values of the field derivative of the volume and the pressure derivative of the magnetization is discussed. The experimental results of various thermodynamic properties in Fe–Ni Invar alloys are analyzed in connection with the present theoretical results and are satisfactorily explained.

Magnetovolume effects in Zr-(Fe1-xCox)2

The thermal expansion and Zr(Fe1-xCox)2 have been measured. The results indicate that the magnetovolume coupling between spontaneous volume magnetostriction and spontaneous magnetisation is very large and is almost independent of the concentration of Co in the ferromagnetic region (i.e. for x=0.0-0.4). Furthermore, an extraordinarily large negative thermal expansion at low temperatures and a large forced volume magnetostriction at 4.2K have been observed in the ferromagnetic compound with x=0.4, both of which are comparable in magnitude to those observed in Fe-Ni Invar alloys. Fairly large forced volume magnetostriction, which does not vary linearly with the applied fields, has been observed in mictomagnetic compounds with x=0.5 and 0.6.

Magnetovolume effects in ferromagnetic and mictomagnetic Zr(Fe-Co) 2

Thermal expansion and forced volume magnetostriction have been measured on the Zr(Fe 1- x Co x ) 2 system. The ferromagnetic compounds with x = 0-0.4 indicate a strong magnetovolume coupling; particularly anomalously large magnetovolume effects were found for x = 0.4. A large forced volume magnetostriction, ω, has been observed in mictomagnetic compounds with x = 0.5 and 0.6, which is well described by a simple relation, ω ⪖ MH.

Thermodynamic aspects of magnetoelastic properties in itinerant ferromagnetism

The enhancement factor due to the magnetovolume coupling and the coupling constant in a simple model are estimated for various itinerant ferromagnets. The spontaneous volume magnetostriction and dTc/dP are calculated and compared with the experimental results. Anomalies of the magnetovolume effects in Fe-Ni Invar alloys are discussed.

Relation between the Thermal Expansion Coefficient and the Magnetization in Fe-Ni Invar Alloys

Measurements of magnetization curves for Fe-Ni Invar alloys have been made at various temperatures between room temperatures and about 450°C. Specimens used were in a sphere shape, which were obtained from the same ingots that the specimens used for the thermal expansion measurements had been taken from. By comparing the results of the magnetization measurements with the observed temperature dependence of the thermal expansion coefficients, it was found that the anomalous part of the thermal expansion vs temperature curve can be well expressed with the quantity, \(\frac{2}{3}CM_{s}\text{d}M_{s}/\text{d}T\), where C is a magneto-volume coupling constant and M s is the spontaneous magnetization.

Origin of the anomalies and thermodynamic aspects in iron-nickel invar alloys

The thermodynamic properties of Fe-Ni Invar alloys are analyzed in terms of the theoretical results obtained in the itinerant electron model accompanying in the mixing of the ferromagnetic and paramagnetic phases with different atomic volumes. All the anomalies in Fe-Ni Invar alloys are explained by the effect of a large magnetovolume coupling and this large coupling is attributed to the magnetic transformation due to the changes in temperature, magnetic field and pressure. There is also an enhancement due to the magnetovoume coupling to the high-field susceptibility, compressibility and forced magnetostriction. The pressure dependence of T c is also discussed.

Origin of Anomalies and Thermodynamic Aspects in Invar Alloys

The effect of the forced magnetostriction on various thermodynamic properties of Invar alloys is examined. The temperature variations of the thermal expansion coefficient and bulk modulus defined at constant magnetization and the concentration dependences of this bulk modulus and the high-field susceptibility at constant volume are shown to be normal. There is an enhancement due to the magnetovolume coupling in the high-field susceptibility, compressibility and forced magnetostriction. The various anomalies are attributed to the large magnetovolume coupling in Fe–Ni Invar alloys. It is seen that Fe–Pt alloys are rather normal at low temperature. The magnetovolume coupling term is calculated in a simple itinerant electron model for fcc Fe–Ni alloys. To explain the large magnetovolume coupling in Fe–Ni alloys, the homogeneous itinerant electron model is not sufficient and a magnetic inhomogeneity with different atomic volumes will explain this discrepancy.

Spontaneous Volume Magnetostriction of RCo2 Compounds

The thermal expansion of RCo 2 compounds (R=Gd, Tb, Dy, Ho and Er) has been measured from 4.2 K up to 700 K. All the RCo 2 compounds show an Invar-like thermal expansion anomaly below their Curie temperatures: a large positive spontaneous volume magnetostriction was found. The large magneto-volume coupling is presumably of help towards a first order phase transition observed in DyCo 2 , HoCo 2 and ErCo 2 .