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).     

Monte Carlo simulations of high-moment - low-moment transitions in Invar alloys

We have reproduced magneto-volume effects typical for Invar alloys by examining a spin-analogous model which describes coupled spatial and magnetic degrees of freedom and, additionally, chemical disorder. Constant pressure Monte Carlo simulations of this model show an almost vanishing thermal expansion over a broad range of temperatures below ,a softening of the bulk modulus and the absence of a sharp peak in the specific heat at the magnetic phase transition as observed in Fe₆₅Ni₃₅ Invar. Received: 9 September 1996 / Revised: 23 May and 23 September 1997 / accepted: 3 September 1997     

Modeling Structural and Magnetic Phase Transitions in Iron-Nickel Nanoparticles

Martensitic phase transformations and magnetovolume effects in iron-nickel alloys are intimately related. The term Invar is widely used to characterize the unusual physical properties accompanying structural and magnetic instabilities such as those observed in the vicinity of the critical composition Fe 65 Ni 35 . We discuss the crossover from bulk iron-nickel alloys to nanoparticles with respect to structural and magnetic behavior. By employing molecular-dynamics and Monte Carlo methods, we find the absence of structural instabilities in defect-free particles, a linear scaling of the austenitic transformation temperature with the reciprocal cluster radius, as well as a decrease of the magnetic transition temperature with decreasing particle size.     

Metallic magnetism in amorphous TM-Y (TM = Mn,Fe, Co,Ni) alloys

Systematic variations of magnetic properties in amorphous TM-Y (TM = Mn, Fe, Co, Ni) alloys are investigated on the basis of a finite temperature theory of amorphous metallic magnetism which takes into account both thermal spin fluctuations and the fluctuations due to structural and configurations disorder. It is shown that the magnetic phase diagrams calculated in the most random atomic configuration explain qualitatively the spinglass (SG) in Mn-Y, the SG ferromagnetism (F) transition in Fe-Y, and the F-paramagnetism transition in Co-Y and Ni-Y alloys. Magnetization vs concentration curves and susceptibility vs concentration curves as well as the effective Bohr magneton numbers are also shown to be explained qualitatively or semi-quantitatively by the theory. Their miscroscopic mechanisms are elucidated by means of their electronic structures, magnetic couplings, and atomic short range order. It is found that the magnetism in Fe-Y and Mn-Y amorphous alloys is strongly influenced by the atomic short range order. The result explains different magnetic phase diagrams in amorphous Fe-Y alloys and experimental SG transition temperatures in amorphous Mn-Y alloys.     

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.     

Quantitative calculations of thermal-expansion coefficient in Fe–Ni alloys: First-principles approach

The thermal expansion coefficients (α) of Fe_1-xNi_x alloys are calculated by means of the Debye-Grüneisen model, which uses the input parameters calculated from density functional theory (DFT) with collinear spin alignments. The various atomic configurations of fcc and bcc supercells with x being 0–0.5 are calculated to conduct the composition-specific analysis. Thermodynamic analysis is employed to facilitate the evaluation of α in bulk alloys, where the calculated supercells are used as the canonical ensemble. Such calculated α exhibits very similar composition-dependency in comparison with the experimental data from literature, particularly providing the well-known Invar effect at 65 wt% of Fe. The calculations also demonstrated that the pressure-derived magnetic frustration, i.e. the magneto-volume effect, is strongly correlated with the Invar effect. The present approach combining the Debye-Grüneisen formalism and the collinear DFT calculation is shown to be a comprehensive theoretical framework for analysis on the thermal expansion properties in metal alloys.     

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.     

Virtual miscibility gap and interdiffusion coefficient in iron-nickel invar alloys

Fe-Ni Invar alloys exhibit anomalies in thermodynamic and physical properties at high temperatures. This is considered to result from the tendency to two-phase separation in the fcc phase. This idea is supported also by interdiffusion experiments in the Fe-Ni system, where the interdiffusion coefficient is anomalously small in the Invar region at lower temperatures. A simple theory of regular solutions based on a pair interaction model is applied to the Fe-Ni system; with the assumption of two gamma states this gives a miscibility gap in the Invar region.     

Correlation between mechanical,thermal and electronic properties in Zr–Ni,Cu amorphous alloys

We show that mechanical properties (stiffness and hardness) of Zr–Ni, Cu amorphous alloys increase linearly with Ni, Cu content over a wide composition range (22?≤?x _Ni,Cu?≤?65 at%). This correlates with the observed increase in the Debye temperatures and densities with x and shows that the strength of interatomic bonding increases with x in these alloys. Accordingly, the thermal stability (e.g. the crystallization and glass transition temperatures) of these alloys also increases with x. Since the electronic density of states at the Fermi level decreases linearly with x within the same x-range, a very simple relationship exists between the electronic structure and mechanical and thermal properties. We also deduce the mechanical properties of hypothetic amorphous Zr and briefly discuss the possibility of its preparation.     

Lattice dynamics in ferromagnetic invar alloys

The lattice dynamical properties of the Invar alloys Fe₆₅Ni₃₅ and Fe₇₂Pt₂₈ are discussed. The experiments on the lattice vibration by inelastic neutron scattering have shown an apparent correlation of phonon anomalies with the ferromagnetic long range order. The elastic softening below the Curie temperature is attributed to the dynamical aspects, at least for the [ςς0]TA₁ mode. The lattice dynamical features are derived from the electronic structure of the Invar alloys. They are consistent with the theoretical model that Invar characters are closely related to the detailed band structure of the d electrons near the Fermi level. The present studies indicate significant contributions of phonon anomalies to the Invar problem.     

Magneto-volume effects in Fe–Cu solid solutions

The magnetic properties of Fe–Cu metastable solid solutions have been investigated by means of neutron diffraction and magnetisation measurements. These compounds exhibit ferromagnetic order with Curie temperatures above room temperature for concentrations beyond 40 at% in Fe. The magnetic moment at 5 K can reach values over 2 μ_B, while the high field susceptibility is similar to that found in FCC–FeNi Invar alloys. These features together with the low values for the linear coefficient for thermal expansion in the ferromagnetic region suggest that magneto-volume anomalies, including Invar behaviour, play a major role in the magnetic properties of this system when the crystal structure is face centred cubic. Such behaviour could be explained using theoretical total-band energy calculations.     

Theory of volume anomalies in magnetic transition metal alloys

The electronic contribution to the pressures which determines the equilibrium volume of transition metals is parametrized by use of the interacting Anderson model. It is discussed that there are two mechanisms which can explain the volume anomalies in the Invar alloys. The Fe-Fe atomic interaction which seems to be important in the Invar effect is also discussed.     

Structural, thermodynamic and electronic properties of zinc-blende AlN from first-principles calculations

Structural, thermodynamic and electronic properties of zinc-blende AlN under pressure are investigated by first-principles calculations based on the plane-wave basis set. Through the analysis of enthalpy variation of AlN in the zinc-blende (ZB) and the rock-salt (RS) structures with pressure, we find the phase transition of AlN from ZB to RS structure occurs at 6.7 GPa. By using the quasi-harmonic Debye model, we obtain the heat capacity C_V, Debye temperature Θ_D, Grüneisen parameter γ and thermal expansion coefficient α. The electronic properties including fundamental energy gaps and hydrostatic deformation potentials are investigated and the dependence of energy gaps on pressure is analysed.     

Magnetic Transitions in Fe 3 Pt Invar Alloy Under High Pressure and Temperature Studied by Inelastic X-ray Scattering

The magnetic properties of the Invar alloy Fe 72 Pt 28 have been investigated by X-ray emission spectroscopy as a function of temperature up to 900 K and pressure up to 25 GPa. With increasing temperature and pressure, the amplitude of the Fe local moment, deduced from the Fe K g line satellite intensity, is reduced. Both the temperature and pressure dependence are interpreted in terms of transitions from a high-spin state to a low-spin state. This behavior provides a microscopic picture of the Fe magnetism in Invar alloys in agreement with the 2 n state model.     

Investigation of spin structure in fcc Fe-Ni-Mn

⁵⁷Fe Mössbauer spectra of Invar type (Fe_.65Ni_.35)_1–xMn_x alloys (O相似文献   

Effects of Co on the Néel temperature and thermal expansion in dilute CrFe alloys

The Néel temperature and thermal expansion characteristics of dilute CrFe alloys containing Co have been investigated. It has been found that the Néel temperature of Cr_99-xFe₁Co_x alloys (0.5?x?1) increases with increasing Co content and that these ternary alloys exhibit a large spontaneous volume magnetostriction, indicating Invar characteristics around room temperature. These behaviors are extremely different from those of CrFe and CrCo binary alloys.     

Ce-La-Th合金高压相变的第一性原理计算

采用第一性原理计算对Ce_(0.8)La_(0.1)Th_(0.1)在高压下fcc-bct的结构相变、弹性性质及热力学性质进行了研究讨论.通过对计算结果的分析,发现了合金在压力下的相变规律,压强升高到31.6 GPa附近时fcc相开始向bct相转变,到34.9 GPa时bct相趋于稳定.对弹性模量的计算结果从另一角度反映了结构相变的信息.最后,利用准谐德拜模型对两种结构的高温高压热力学性质进行了理论预测.     

Temperature dependences of thermal expansion and saturation magnetization in Fe(67.0%)-Ni(32.5%)-Co(0.5%) Invar alloy with nanocrystalline structure

The effect of the nanocrystalline structure of Fe(67.0%)-Ni(32.5%)-Co(0.5%) Invar alloy on its thermal expansion is considered. It is found that the structure with grain mean sizes of about 100 nm increases its temperature coefficient of thermal expansion in the range of “invarness,” i.e., in the temperature range where the alloy offers the Invar properties. Reasons for this behavior are analyzed by taking the temperature dependence of the saturation magnetization.     

X-ray photoelectron studies of changes in the electronic structure upon phase transitions in Co–Ni and Co–Fe alloys

The changes in the electronic structure of Co–Ni and Co–Fe systems upon phase transitions are studied. X-ray photoelectron study of the valence-band spectra and the parameters of the multiplet splitting of Co, Ni and Fe 3s spectra is carried out at different temperatures. It is established that the ordering–separation phase transition in Co–Ni alloys takes place in the temperature range of 600–700°C. As opposed to Co–Ni alloys, in the Fe–Co alloy, ordering–separation–ordering phase transitions are observed. High-temperature ordering of the Fe₅₀Co₅₀ alloy is observed above 1200°C. The transition from ordering to separation is shown to lead to changes in the d electron spectra of the valence band and in the parameters of the multiplet splitting of the 3s spectra.