Theoretical aspects of the FecNi1-c Invar alloy

Invar Effect of the Fe13 Cluster

The Invar effect, which refers to magnetic alloys having an anomalously small thermal expansion, has been under extensive investigations over the past century. However, study on the thermal expansion of clusters and their Invar effect is still lacking. In this letter, we calculate the thermal expansion properties of Fe_13-nNi_n (n = 0, 3, 6, 9, 12, 13) clusters and find that the Fe₁₃ cluster has an abnormally small thermal expansion, which is due to its remarkable magneto-volume effect. It is also revealed that the strong magneto-volume effect of the Fe₁₃ cluster is highly related with the two magnetic states, the same as the interpretation for the Invar effect of a bulk alloy. The anomalously small thermal expansion of the Fe₁₃ cluster is the manifestation of the Invar effect at the cluster scale.

Growth of invar nanoparticles on a graphene oxide support

Alloyed binary invar-like FeNi nanocrystals are synthesized on the surface of graphene oxide in a two-step impregnation–annealing procedure.

Systematic Quantum Cluster Typical Medium Method for the Study of Localization in Strongly Disordered Electronic Systems

Great progress has been made in recent years towards understanding the properties of disordered electronic systems. In part, this is made possible by recent advances in quantum effective medium methods which enable the study of disorder and electron-electronic interactions on equal footing. They include dynamical mean-field theory and the Coherent Potential Approximation, and their cluster extension, the dynamical cluster approximation. Despite their successes, these methods do not enable the first-principles study of the strongly disordered regime, including the effects of electronic localization. The main focus of this review is the recently developed typical medium dynamical cluster approximation for disordered electronic systems. This method has been constructed to capture disorder-induced localization and is based on a mapping of a lattice onto a quantum cluster embedded in an effective typical medium, which is determined self-consistently. Unlike the average effective medium-based methods mentioned above, typical medium-based methods properly capture the states localized by disorder. The typical medium dynamical cluster approximation not only provides the proper order parameter for Anderson localized states, but it can also incorporate the full complexity of Density-Functional Theory (DFT)-derived potentials into the analysis, including the effect of multiple bands, non-local disorder, and electron-electron interactions. After a brief historical review of other numerical methods for disordered systems, we discuss coarse-graining as a unifying principle for the development of translationally invariant quantum cluster methods. Together, the Coherent Potential Approximation, the Dynamical Mean-Field Theory and the Dynamical Cluster Approximation may be viewed as a single class of approximations with a much-needed small parameter of the inverse cluster size which may be used to control the approximation. We then present an overview of various recent applications of the typical medium dynamical cluster approximation to a variety of models and systems, including single and multiband Anderson model, and models with local and off-diagonal disorder. We then present the application of the method to realistic systems in the framework of the DFT and demonstrate that the resulting method can provide a systematic first-principles method validated by experiment and capable of making experimentally relevant predictions. We also discuss the application of the typical medium dynamical cluster approximation to systems with disorder and electron-electron interactions. Most significantly, we show that in the limits of strong disorder and weak interactions treated perturbatively, that the phenomena of 3D localization, including a mobility edge, remains intact. However, the metal-insulator transition is pushed to larger disorder values by the local interactions. We also study the limits of strong disorder and strong interactions capable of producing moment formation and screening, with a non-perturbative local approximation. Here, we find that the Anderson localization quantum phase transition is accompanied by a quantum-critical fan in the energy-disorder phase diagram.

Large negative magnetic contribution to the thermal expansion in iron-platinum alloys: quantitative theory of the Invar effect

We show that the large negative magnetic contribution to the thermal expansion in disordered Fe-Pt alloys can be understood within the disordered local moment (DLM) approach. On the basis of first principles calculations we quantitatively describe the spontaneous volume magnetostriction for various Pt concentrations. It is found that the Invar effect in these alloys is entirely related to the state of thermal magnetic disorder modeled by the DLM states. We also show that the experimentally observed anomaly in the temperature dependence of the magnetization is due to a spontaneous reduction of the local magnetic moments rather than to "hidden excitations."

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.