Influence of the magnetic state on the chemical order-disorder transition temperature in Fe-Ni permalloy

In magnetic alloys, the effect of finite temperature magnetic excitations on phase stability below the Curie temperature is poorly investigated, although many systems undergo phase transitions in this temperature range. We consider random Ni-rich Fe-Ni alloys, which undergo chemical order-disorder transition approximately 100 K below their Curie temperature, to demonstrate from ab initio calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme where the magnetic state is described by partially disordered local magnetic moments, which in combination with Heisenberg Monte Carlo simulations of the magnetization allows us to reproduce the transition temperature in good agreement with experimental data.     

Interplay between disorder and quantum and thermal fluctuations in ferromagnetic alloys: the case of UCu2Si2-xGex

We consider, theoretically and experimentally, the effects of structural disorder, quantum fluctuations, and thermal fluctuations in the magnetic and transport properties of certain ferromagnetic alloys. We study the particular case of UCu2Si2-xGex. The low temperature resistivity, rho(T,x), exhibits Fermi liquid behavior as a function of temperature T for all values of x, which can be interpreted as a result of the magnetic scattering of the conduction electrons from the localized U spins. The residual resistivity, rho(0,x), follows the behavior of a disordered binary alloy. The observed nonmonotonic dependence of the Curie temperature, T(c)(x), with x can be explained within a model of localized spins interacting with an electronic bath. Our results clearly show that the Curie temperature of certain alloys can be enhanced due to the interplay between quantum and thermal fluctuations with disorder.     

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.     

Temperature magnetism of disordered iron–aluminum alloys in a model of two intra-atomic interactions

A system of band electrons with the two constants of intra-atomic exchange interaction simulates the different local environment of magnetic atoms in alloys. Temperature behavior of the magnetic characteristics of disordered Fe-Al alloys is investigated on the basis of an improved theory of dynamic fluctuations in electron spin density. The nature of the alloys’ different properties and the unusual temperature dependence of the magnetization and magnetic susceptibility is discussed.     

Temperature-dependent Heisenberg exchange coupling constants from linking electronic-structure calculations and Monte Carlo simulations

We propose a method to calculate the temperature dependence of Heisenberg exchange coupling constants J_ij. Within the formalism of disordered local moments (DLM), the magnetization and the J_ij are computed from first principles for any concentration c of the magnetic constituents. The exchange coupling constants are then used in Monte Carlo (MC) simulations to compute the temperature dependence of the magnetization for the given c. By comparing the magnetization from DLM calculations and from MC simulations we obtain a mapping of temperature versus concentration and eventually temperature-dependent J_ij. The approach which is applied to bulk Fe and Co can for example improve critical exponents.     

Study of the influence of hydrogen absorption on the magnetic properties of V−Fe−H alloys by low-temperature Mössbauer spectroscopy

The change of the magnetic properties of disordered bcc V?Fe alloys induced by hydrogen absorption has been studied by the Mössbauer spectra measurements at 4.2 K. The magnetic ordering reflecting the para-/ferromagnetic transition was observed in these alloys as a result of hydrogenation, at the same electron concentration at which the local magnetic moments were observed by room-temperature measurements.     

Local magnetic characteristics of iron atoms in disordered alloys of Fe(Pd1-xAux)3

We investigate the local magnetic characteristics of atomically disordered alloys stoichiometrically close to FePd₂Au using Mössbauer spectroscopy at 77 and 300 K. With this information, we establish the distribution functions of the magnetic fields at the nuclei of the resonant iron atoms and their local magnetic moments. We demonstrate the presence of a dependence of the iron atom magnetic moments on the local atomic neighborhood and estimate parameters of its linear approximation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 44–49, August, 1989.     

Anomalous low temperature magnetic and thermal properties of an amorphous carbon

Very low temperature measurements of magnetic and thermal properties of a disordered carbon are reported. An interaction coupling between the localized spins and the free charge carriers analogous to the existing in dilute alloys is proposed to explain the results.     

Effect of short-range order on electronic and magnetic properties of disordered Co-based alloys

We here study electronic structure and magnetic properties of disordered CoPd and CoPt alloys using augmented space recursion technique coupled with the tight-binding linearized muffin tin orbital (TB-LMTO) method. Effect of short-range ordering present in disordered phase of alloys on electronic and magnetic properties has been discussed. We present results for magnetic moments, Curie temperatures and electronic band energies with varying degrees of short-range order for different concentrations of Co and try to understand and compare the magnetic properties and ordering phenomena in these systems.     

About the local Curie temperature distribution of Invar alloys

Strongly disordered ferromagnetic alloys which exhibit magnetic inhomogeneities are described in the frame of a generalized Ginzburg-Landau theory by averaging the Ginzburg-Landau energy of a subsystem with a definite Curie temperature over the Curie temperature of all subsystems of the inhomogeneous magnetic system. It is shown that this leads to the rough scaling of a smooth phase transition. Furthermore the distribution function of the Curie temperature is derived from the experimental data of the specific heat, the volume thermal expansion coefficient and the compressibility.     

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.     

Ab-initio calculations of the electronic structure of impurities and alloys of ferromagnetic transition metals

We review recent theoretical work on the electronic structure and the magnetic properties of ferromagnetic transition-metal alloys. All calculations are based on density-functional theory in the local-spin-density approximation. We report about calculations for dilute alloys using the KKR-Green's function method and for concentrated disordered alloys using the charge-self-consistent KKR-CPA method.     

The influence of the metalloid species on the electronic structure of disordered transition-metal-metalloid alloys

The results of x-ray emission investigations of the electronic structure of Fe-Si and Fe-P disordered alloys are presented. Relying on the analysis of the spectrum parameters and data available in the literature, a qualitative model of the electronic-structure formation in disordered Fe-P alloys is proposed. The model allows one to account for the concentration dependence of the average magnetic moment per iron atom in the disordered systems investigated.     

Computer simulation of the structure of MZr2 liquid and amorphous alloys

In the present study, we compare the structure of NiZr₂ and CuZr₂ disordered (liquid and amorphous) alloys. While Cu and Ni have similar atomic radii, the formation heats of these alloys differ by more than a factor of two. Moreover, the most stable crystal phase in the CuZr₂ alloy has the C11b lattice while the most stable phase in the NiZr2 alloy has the C16 lattice. Comparing these two alloys allows us to directly explore how the chemistry and atomic size affect the disordered phase structure. We find that all differences in the structures of the disordered alloys are readily explained by the smaller Ni–Zr separation driven by the higher heat of mixing and the structure of the disordered alloys does not necessarily resemble the local packing structure of their crystalline counterparts. Comparison of the disordered alloys and their most stable crystal phase structures explains the better glass formability of the CuZr₂ alloy.     

Magnetic phase diagram of the FexMn0.7-xAl0.3 alloys series obtained by Mössbauer spectroscopy

By Mössbauer spectroscopy the magnetic phase diagram of the Fe_xMn_0.7-xAl_0.3 spin glass alloy has been obtained. X-ray diffraction (XRD) has shown that all the alloys are in the BCC phase. The broad spectral lines observed in the Mössbauer spectra are indicative of the disordered character of these alloys. Depending on the composition and the temperature the alloys behave as paramagnetic (P), ferromagnetic (F) and reentrant spin glass (RSG). For the alloys rich in iron the only detected magnetic transition is from F to P. For medium iron content two transitions were observed, namely from RSG to F and from F to P. The RSG phase is obtained as a consequence of the disordered character of the alloys and the competitive exchanges due to iron and manganese atoms.     

Dependence of the magnetization on the interface morphology in ultra-thin magnetic/non-magnetic films: Monte Carlo approach

Using Monte Carlo simulations, we have studied the dependence of magnetic properties on interface morphology in magnetic/non-magnetic (M/NM) multilayers. Our aim is to relate macroscopic magnetic properties of the multilayers to their concentration profile at the interface. Our model consists of an alternate staking of magnetic and non-magnetic layers with disordered interfaces. We have considered different concentration and the existence of local magnetic domains at the interface. The results indicate the crucial dependence of magnetization amplitude with interface multilayers atomic composition and the spatial arrangement of magnetic atoms. In particular, we show that isolated islands at the interface leads to the apparition of super-paramagnetic behavior.     

Role of disorder and anharmonicity in the thermal conductivity of silicon-germanium alloys: a first-principles study

The thermal conductivity of disordered silicon-germanium alloys is computed from density-functional perturbation theory and with relaxation times that include both harmonic and anharmonic scattering terms. We show that this approach yields an excellent agreement at all compositions with experimental results and provides clear design rules for the engineering of nanostructured thermoelectrics. For Si(x)Ge(1-x), more than 50% of the heat is carried at room temperature by phonons of mean free path greater than 1 μm, and an addition of as little as 12% Ge is sufficient to reduce the thermal conductivity to the minimum value achievable through alloying. Intriguingly, mass disorder is found to increase the anharmonic scattering of phonons through a modification of their vibration eigenmodes, resulting in an increase of 15% in thermal resistivity.     

Role of the chemical ordering on the magnetic properties of Fe–Ni cluster alloys

The spin and orbital moments of fcc Fe-Ni cluster alloys are determined within the framework of a d-band Hamiltonian including the spin-orbit coupling non perturbatively. Different sizes (up to 321 atoms), compositions, and chemical configurations (random alloys as well as core-shell arrays of iron and nickel atoms) are considered in order to reveal the crucial role played by local order and stoichiometry on the magnetic moments of the clusters. Interestingly, we have found considerably reduced average magnetizations for Fe-Ni clusters with Fe cores compared to that of the bulk alloy with the same composition. Indeed, in these configurations not only antiparallel arrangements between the local moments of some Fe atoms within the iron core are found, but also the total magnetization of the surface Ni atoms is significantly quenched. On the opposite, the disordered and Ni-core cluster alloys are characterized by high magnetizations resulting from saturated-like contributions from both Ni and Fe atoms, in agreement with recent ab-initio calculations. In general, the local orbital magnetic moments are strongly enhanced with respect to their bulk values. Finally, the variation of the orbital-to-spin moment ratio with the chemical order is discussed.     

Study of local magnetic non-homogeneities in disordered magnetics by means of Mössbauer spectroscopy

The analysis of the results of experimental studies of hyperfine interactions in disordered magnetics Fe_1?y Pd_y and FePd₂Au presented in the given work allowed to establish the interrelation between their atomic and magnetic structures. The magnetic moments of iron atoms in these alloys were defined and the character of their dependences on local atomic environment was clarified. The prevailing effect of local atomic environment on magnetic states realizing in FePd₂Au alloy was approved.