Low-temperature structure of Fe-Ni alloys

The Debye temperature (θ_M) of Fe-Ni alloys was obtained from measurements of the X-ray integrated intensity and the electrical resistivity at low temperatures from 4 to 300 K. A decrease of θ_M, implying a lattice softening effect, was found by lowering the temperature or increasing the iron concentration in the Invar region.     

High-field susceptibility and magnetization in Fe-Ni invar alloys

The high-field susceptibility of Fe-Ni Invar alloys does not give rise to a sudden increase at the Curie temperature. The decrease in magnetization with increasing temperature is attributed to the T² term as well as to the spin wave term for Fe-Ni Invar alloys.     

High-field induced twist deformation in Fe-Ni invar

It has been found that at temperatures below ～ 12 K Fe-Ni Invar rods undergo a twist deformation in a high magnetic field which is nominally parallel to the rod. The effect is consistent with the formation of a net azimuthal component of magnetization which is of the order of 1% of the saturation magnetization. This possibly arises from antiferromagnetic interactions believed to be present in this material.     

Mössbauer spectroscopy of Fe-Ni and Fe-Pt alloys

Mössbauer measurement on Fe-Ni and on Fe-Pt in the ordered and disordered states at various temperatures and in a longitudinal external field (H_ext = 50 kOe) have been analyzed. Hyperfine field distributions of Fe-Ni Invar alloys indicate that in addition to the ferromagnetic phase a second phase or state exists whose magnetic properties seem to be closely related to those of γ-Fe. No second phase or state was detected in the ordered Fe-Pt, while the situation in disordered Fe-Pt is uncertain.     

Invar Behavior in Fe-Ni Alloys is Predominantly a Local Moment Effect Arising from the Magnetic Exchange Interactions Between High Moments

I argue that the main models that have been advanced to explain Invar behavior in Fe-Ni alloys (the original, classical, Invar system) can all be shown to be critically deficient, except one: The local moment frustration model of Rancourt and Dang ( Phys. Rev. B , 54 , 12225, 1996). The latter model explains all the measured structural, magnetic, and magnetovolume features of the Fe-Ni alloys with 0-65 apc (atomic percent) Fe, based on the assumptions that these systems are predominantly high-moment in character at the temperatures of interest and that the Fe-Fe pairs have large inter-atomic separation dependencies of their magnetic exchange parameters. The large magnetovolume Fe-Fe couplings are understood (based on ab initio electronic structure calculations) as a precursor effect of the low-moment/high-moment (LM/HM) transition that has recently been observed to occur at larger Fe concentrations, as a continuous transition occurring in the range , 65-75 apc Fe (Lagarec, Ph.D. thesis, 2001).     

First Observation of a Composition-Controlled Low-Moment/High-Moment Transition in the Fcc Fe-Ni System: Implications Regarding Invar and Anti-Invar Behaviors

We report the first direct observation of a high-moment (HM)/low-moment (LM) transition occurring in face centered cubic (FCC) Fe-Ni alloys. 57 Fe Mössbauer isomer shifts (ISs) give local electronic densities that exhibit a large discontinuity of , 0.4 el./ $ a_{0}^3 $ at the transition that spans the concentration range ～ 65-75 apc (atomic percent) Fe, in agreement with ab initio predictions. In the most Fe-rich alloys that have LM ground states (including n -Fe), we show that thermal stabilization of the HM state occurs at high temperatures, thereby providing an experimental proof that anti-Invar behavior is due to such HM stabilization. In Invar (Fe 65 Ni 35 ) and at near-Invar compositions, we observe temperature-induced changes in electronic density that follow the spontaneous magnetization curves and find that Invar is predominantly a HM phase at all temperatures where an Invar effect occurs. We show that LM phase thermal excitation cannot cause the Invar effect and that such excitation would cause a contraction instead of the required expansion, relative to normal behavior.     

Thickness dependence of magnetization in FeNi invar alloy film

The thickness dependence of magnetization of FeNi Invar alloy films was observed by means of small angle Lorentz electron diffraction. A remarkable reduction of magnetization at room temperature was observed for films with thickness below 400 Å. This may be ascribable to the instability of the ferromagnetic state in Fe-Ni Invar alloys.     

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.     

Invar effects of laves phase untermetallic compounds

The magnetovolume effects in Laves phase compounds such as AFe₂ and ACo₂ are received. In particular, Invar like behaviour which is found in (Zr_1-xNb_x)Fe₂ and Zr(Fe_1-xCo_x)₂ systems is reported. Comparing the present results with the classical Invar such as Fe-Ni and Fe-Pt alloys, it is proposed that so-called Invar anomalies should be classified into two type, i.e. the giant spontaneous volume magnetostriction, which is the essential characteristic of the Invar effect and the other anomalies which are observed in Fe-Ni and Zr(Fe_1-xCo_x)₂ systems but not in Fe-Pt and (Zr_1-xNb_x)Fe₂ systems.     

Electron irradiation effects on iron-nickel invar alloys

Electron irradiation was used as a means of accelerating the diffusion in Fe-Ni alloys. Mössbauer effect, X-ray and electron microscopy experiments on samples with 28 to 50% Ni show that the Invar character disappears after irradiation up to 250°C, in particular the variation of the lattice parameter versus temperature becomes linear around room temperature. At the same time, two phases appear in the alloy, one rich in nickel and ordered with FeNi (AuCu) superstructure, the other rich in iron and probably ordered (Fe₃Ni). The Invar state is therefore shown to be a metastable state. A diagram of the Fe-Ni alloy is proposed.     

Temperature dependence of forced volume magnetostriction in fcc Fe-Ni alloys

The forced volume magnetostriction of the fcc Fe-Ni Invar alloys has been measured in the temperature range from 77 to 900 K to elucidate the magnetovolume effect at finite temperatures. The temperature dependence of (θ ω/θ(H/M) is evaluated as well as (θ ω/θH), where ω is the volume expansion. These results are discussed using a phenomenological model for the magnetovolume effect which takes into account the contribution of the correlation between local magnetic moments.     

The ΔE effect in iron-boron and iron-chromium-boron amorphous invar aloys

Temperature- and field-dependence of elastic properties of amorphous Fe-B and Fe-Cr-B Invar alloys have been investigated and Elinvar characteristics have been found together with a large ΔE effect. These results have been revealed that the elastic properties of these amorphous alloys are very different those of the crystalline Fe-Ni Invar alloy.     

Fe-Pt invar alloys—homogeneous strong ferromagnets

The thermal expansion and magnetic properties of Fe-Pt Invar alloys in both ordered and disordered states indicate that inhomogeneities play no essential role in determining large magnetovolume effects in Fe-Pt alloys. On the other hand, the concentration dependence of the magnetization, the hyperfine field at Fe nuclei and other related properties clearly show the strong ferromagnetism in Fe-Pt Invar alloys. Comparing the Invar behaviour found in Fe-Pt alloys with that in Fe-Ni alloys, it has been concluded that the so-called Invar effect generally consists of two types of anomalies. One is the essential effect, i.e., the large magnetovolume effect arising from the 3d band polarization and the other is the secondary or additional effect manifested as various magnetic anomalies associated with heterogeneities or weak ferromagnetism.     

The effect of hydrogen on the properties of invar alloys

The characteristic weak internal field contribution of a Mössbauer spectrum of Fe-Ni Invar alloys was diminished by an intense hydrogenation, and instead, the strong ferromagnetic part increased. This effect of hydrogen was interpreted in terms of 3d hole filling with electrons from solute hydrogen in Stoner's band model.     

A model of magnetic inhomogeneous structure for Fe-Ni invar alloys

The magnetic inhomogeneous structure revealed by neutron small angle scattering for Fe-Ni Invar alloys can be interpreted in terms of a model in which an effective local concentration of Fe for each atom within a sphere is taken into account in order to determine the magnetic moment of the atom.     

A vibronic state model of the invar problem

A new model is proposed to explain the Invar effect of Fe-Ni alloys and related problems, which an admixing of low and high spin states in vibrating Fe atoms and an itinerant electron ferromagnetism for the admixed state are taken into account.     

Invar effects on some iron-base amorphous alloys

The thermal expansion and magnetic properties of Fe-B and Fe-P amorphous alloys prepared from melts have been investigated. These amorphous alloys show distinct Invar characteristics. heir magnetic properties are also very similar to those of Fe-Ni crystalline Invar alloys; that is, the high-field susceptability and forced-volume magnetostriction are remarkably large, the magnetic moment per Fe atom does not increase linearly, the Curie temperature decreases with a decrease in concentration of B or P, and their reduced their magnetization curves are much flatter than those of crystalline pure Fe.     

Pressure-induced Invar effect in Fe-Ni alloys

We have measured the pressure-volume (P-V) relations for cubic iron-nickel alloys for three different compositions: Fe 0.64Ni (0.36), Fe 0.55Ni (0.45), and Fe 0.20Ni (0.80). It is observed that for a certain pressure range the bulk modulus does not change or can even decrease to some minimum value, after which it begins to increase under still higher pressure. In our experiment, we observe for the first time a new effect, namely, that the Fe-Ni alloys with high Ni concentrations, which show positive thermal expansion at ambient pressure, become Invar system upon compression over a certain pressure range.     

Local moment model for the Invar effect

It has been shown by small-angle neutron scattering that the magnetic structure of Invar alloys in the ground state is characterized by the occurrence of static magnetic fluctuations on the scale of 10–12 Å. Centres of such fluctuations are iron atoms surrounded by the same sort of atoms in the nearest coordination sphere. These fluctuations are due to antiferromagnetic iron pair interactions. The magnetic part of thermal expansion coefficient of Fe-Ni alloys is evaluated within the framework of the molecular field approximation. The Invar effect is brought about by a mixed exchange interaction and by a strong dependence of the exchange integral J_FeFe on the interatomic distance.     

Mössbauer effect study of the magnetic state of iron atoms in Fe-Ni 36 wt.% Invar alloys

The⁵⁷Fe Mössbauer spectra of the Fe-Ni 36 wt.% Invar alloy at different temperatures (300–530 K) have been measured. The experimental results indicate that the hyperfine field distributions are characterized by dual peaks. The temperature dependence of the hyperfine field indicates that some iron atoms may transfer from the ferromagnetic state to the antiferromagnetic one with increasing temperature, and that the variation of the ratio of numberN ₀ of the iron atoms in the antiferromagnetic state to the numberN _h in the ferromagnetic state with temperature will obey the thermodynamic relationship.