Magnetic cluster expansion simulation and experimental study of high temperature magnetic properties of Fe-Cr alloys

We present a combined experimental and computational study of high temperature magnetic properties of Fe-Cr alloys with chromium content up to about 20?at.%. The magnetic cluster expansion method is applied to model the magnetic properties of random Fe-Cr alloys, and in particular the Curie transition temperature, as a function of alloy composition. We find that at low (3-6?at.%) Cr content the Curie temperature increases with the increase of Cr concentration. It is maximum at approximately 6?at.% Cr and then decreases for higher Cr content. The same feature is found in thermo-magnetic measurements performed on model Fe-Cr alloys, where a 5?at.% Cr alloy has a higher Curie temperature than pure Fe. The Curie temperatures of 10 and 15?at.% Cr alloys are found to be lower than the Curie temperature of pure Fe.     

A study of the magnetic moments and Fe site hyperfine fields in iron-chromium alloys

The magnetization of Fe-Cr alloys ranging from 1 to 15 atomic % of Cr has been measured at room temperature in order to study the relationship between the Fe site hyperfine fields and the magnetic moment. The average moment decreases linearly, at a rate of -2.36 μ_B per Cr atom, up to 10% Cr concentration. The Fe site hyperfine fields were measured in a previous study¹ using the same samples. It is found that the hyperfine fields measured are not proportional to the corresponding magnetic moments. The results are interpreted using a model previously developed for other binary alloys of iron².     

Vector magnetization of a Cr monolayer on an Fe(110) film

The non-collinear magnetic moment distribution at the Cr/Fe(110) interface of an Fe(110) thin film covered by a Cr monolayer is revealed by means of periodic Anderson model calculations. It is shown that the Cr surface has two magnetically non-equivalent sites with magnetic moments canted to each other. The Fe atoms in the subsurface monolayer also acquire two magnetically non-equivalent sites with magnetic moments canted to each other. Non-collinearity is shown to be a ground state for the Cr/Fe(110) multilayer.     

Magnetic and structural properties of FeCr alloys

Synergistic synchrotron x-ray absorption experiments using imaging magnetic microspectroscopy, x-ray magnetic circular dichroism, and ab initio calculations on FeCr alloys reveal that the Cr content strongly influences the ferromagnetic microstructure and the Fe magnetic moments. The Cr local structure resolved by extended x-ray absorption fine structure (EXAFS) is also found to be affected by the alloy's composition. Both EXAFS and ab initio calculations show a change in the Cr local atomic structure above 10 at.% Cr content from the distance contraction of the first two coordination shells around the Cr absorbing atom. These results indicate the strong dependence of magnetic and structural properties of these alloys on Cr concentration.     

Anisotropy of Linear Thermal Expansion and Compressibility of Y 2 Fe 17 Under Pressure and its Correlation to Magnetic Structure

A complex temperature dependence of a.c. susceptibility of Y 2 Fe 17 under high pressures together with recent neutron diffraction studies under pressure proved the instability of the collinear ferromagnetic structure and the development of a non-collinear magnetic arrangement of Fe moments in Y 2 Fe 17 . To study the correlation between magnetic structure and volume in more detail we performed compressibility and linear thermal expansion studies under high pressures up to 100 kbar and 10 kbar, respectively. The compressibility in the paramagnetic state s P (above 10 kbar at room temperature) was determined from the Murnaghan equation of state using the X-ray data, \kappa_{P}=0.80\ {\rm Mbar}^{-1} . The linear thermal expansion and compressibility in the ferromagnetic state at low temperatures are highly anisotropic. As a consequence, the c/a ratio decreases with increasing pressure. The magnetic phase diagram of Y 2 Fe 17 compound was compiled up to 20 kbar.     

Mechanical properties of bcc Fe-Cr alloys by first-principles simulations

The effect of chromium content on the fundamental mechanical properties of Fe-Cr alloys has been studied by first-principles calculations. Within a random solid solution model, the lattice constants and the elastic constants of ferromagnetic bcc Fe_1?x Cr _x (0? · ?0.156) alloys were calculated for different compositions. With addition of Cr content, the lattice parameters of Fe-Cr alloys are larger than that of pure Fe solid, and the corresponding Young??s modulus and shear modulus rise nonmonotonically with the increasing Cr content. All alloys (except 9.4 at% Cr) exhibit less ductile behavior compared with pure bcc Fe. For the Fe_1?x Cr _x (0? · ?0.156) alloys, the average magnetic moment per atom decreases linearly with the increasing Cr concentration.     

Magnetic moment of iron atoms in Fe-Al body-centered cubic alloys as a function of the nearest environment

The Fe-Al systems in the concentration range from 29 to 44 at. % Al are investigated in terms of the density functional theory. It is shown that, in the system under consideration, there can exist three magnetic states with close energies. Two of these three magnetic states have collinear magnetic moments (the ferromagnetic and antiferromagnetic states), and the third is a spin-spiral state. In collinear magnetic structures, the local magnetic moments are determined by the nearest chemical environment and, in the antiferromagnetic state, the iron atoms surrounded by a large number of aluminum atoms in their environment have a negative magnetic moment. The results obtained substantiate the applicability of modified models of the Jaccarino-Walker type for the interpretation of the experimental data obtained for Fe-Al alloys. The results of the calculations also indicate a significant role of Stoner excitations in the formation of magnetic order in these alloys.     

The Role of Intermixing in the Phase Determination of the Interlayer Exchange Coupling in Multilayers: Application to Fe-Cr Superlattices

The phase of the short-range interlayer exchange coupling oscillations in Fe n Cr m superlattices is expressed in terms of the intermixing at the Fe-Cr as well as at the Cr-Fe interfaces. The interdiffusion is modeled through stochastic algorithms, which presupposes floating of the atoms on the surface of the sample during the epitaxial growth. It automatically leads to the different chemical and magnetic structure of Fe on Cr and Cr on Fe interfaces. Self-consistent calculations of the magnetic moments are performed on the basis of the periodic Anderson model. Although short-range (2 monolayers (ML)) oscillations of exchange coupling were detected for all considered structures, its amplitude and phase strongly depend on the interface alloying. Introduction of the same intermixing at both interfaces does not change the phase as compared to the ideal superlattices with sharp interfaces. However different interdiffusion leads to the ~ -phase shift in accordance with experimental results for the Fe-Cr-Fe trilayers grown on an Fe whiskers. Distribution of magnetic moments on Fe atoms contains several distinct peaks but their position and relative area weakly depend on the alloying and interlayer exchange coupling in the superlattice. On the contrary, magnetic moments on Cr atoms are very sensitive to the intermixing and their behavior determines the strength and the phase of exchange coupling oscillations.     

Formation and ordering of local magnetic moments in Fe–Al alloys

The density functional theory is used to study the local magnetic moments in Fe–Al alloys depending on concentration (from 29 to 44 at% Al) and the Fe nearest environment. We have found three different solutions for the system: a spin-spiral wave (SSW) which has a minimum energy and two collinear states, a ferromagnetic one and a state with both positive and negative Fe magnetic moments (the Fe atoms with many neighboring Al atoms around them have negative magnetic moments, while the other Fe atoms—positive). Both the SSW and the negative Fe moments agree with the experiments. Magnetization curves taken from the literature are analyzed. The assumption of percolation character of the size distribution of magnetic clusters describes well the experimental superparamagnetic behavior above 150 K.     

Local spin configurations of Fe atoms in the Rh1−x Fex (x=0.1, 0.2, and 0.3) system with competing exchange interactions

The local spin configurations of Fe atoms in the magnetically ordered alloys Rh_1?x Fe_x (x=0.1, 0.2, and 0.3) have been investigated by Mössbauer spectroscopy. The Mössbauer absorption spectra are measured in the range from 5 K to temperatures of the transition to the paramagnetic state. The measurements in magnetic fields with a strength up to 5 T are carried out at a temperature of 4.2 K. Analysis of the magnetic-hyperfinefield distribution functions demonstrates that Fe atoms form discrete sets of collinear spin configurations corresponding to different net moments of the nearest coordination sphere. The spin structure of the alloys is governed by a random distribution of Fe atoms over the lattice sites and the competition between the Fe-Rh ferromagnetic exchange interaction and the antiferromagnetic interaction of the neighboring Fe atoms. No spin frustration and spin “melting” effects characteristic of spin glasses are revealed in the Rh-Fe alloys.     

Magnetic contribution to the atomic volume of Co,Fe, Mn and Fe-3d metal alloys

The atomic volumes of the polymorphic 3d elements Mn, Fe and Co, and the average atomic volumes of alloys of Fe with Ni, Co, Mn, Cr and V are examined using a simple magnetovolume relation presented previously. The variation of the atomic volumes of the elements and of the alloys is shown to follow the variations in the magnetic moments associated with the atom through this relation.     

Surface dependence of the magnetic configurations of the ordered B2 FeCr alloy

Tight-binding linear muffin tin orbitals calculations with generalized gradient approximation were carried out for the magnetic configurations at the surface of the ferromagnetic ordered B2 FeCr alloys. For both (001) and (111) crystallographic phases, non ferromagnetic configurations are shown to be more stable than the ferromagnetic configuration of the bulk alloy. For (001) surface we display a c ground state for either Cr or Fe at the surface. For Cr top layer the magnetic moments are larger than in the bulk B2 FeCr while they are slightly enhanced for Fe top layer. For (111) surface an antiferromagnetic coupling between surface and subsurface is always obtained i.e. for either Fe or Cr at the surface. This change of coupling between Fe and Cr (from ferromagnetic to antiferromagnetic) is expected to be fundamental to any explanation of the experimental results obtained for the interface alloying at the Fe/Cr interfaces. Received 23 March 1998     

Magnetic behaviour of cobalt,iron and manganese dissolved in palladium

This paper is meant to be a report on the experimental work on dilute Pd-based alloys with Co, Fe and Mn. These alloys exhibit the phenomenon of giant moments. The importance of measurements on paramagnetic alloys is emphasized. From these measurements the conclusion can be drawn that Co and Fe dissolved in Pd does not behave like a normal paramagnet, i.e. according to a Brillouin function. This result makes it possible to explain the existing discrepancy in the interpretations of magnetic measurements on one hand and of specific-heat experiments on the other.

The main conclusions of this paper are:

The giant moment should be accounted for by ‘normal’ values of the magnetic quantum number (3/2 for Co, 2 for Fe and 5/2 for Mn) and a large value of geff.

Paramagnetic alloys of Mn in Pd behave according to Brillouin functions, but alloys of Co or Fe in Pd do not. Hence, a number of interpretations of magnetic measurements should be considered as incorrect.

The localized model for ferromagnetism can well account for the magnetic ordering of dilute Pd-based alloys (certainly if c < 1 at.%). A straightforward generalization of the Weiss molecular-field model may be applied.

The transition temperature of Pd-Mn alloys is not proportional to the concentration, but after scaling the behaviour is similar to what has been found for Pd-Co and Pd-Fe alloys. The concentration dependence can be explained from a calculation of the strength of the interaction between two impurity atoms as a function of the distance.

Comparison between alloys with equal concentrations shows that the magnetic ordering in Pd-Mn is not at all exceptional, but analogous to that in Pd-Co and in Pd-Fe. It should be mentioned, however, that Pd-Mn at c > 3 at.% is a so-called spin glass.

Addition of Ag or Rh to Pd alloys with Co, Fe and Mn has important influences on their properties. Unfortunately these effects are not completely understood.     

Sperimagnetism in Fe78Er5B17 and Fe64Er19B17 metallic glasses: I. Moment values and non-collinear components

Magnetization measurements have been made on a Fe(64)Er(19)B(17) glass, which exhibits ferrimagnetic compensation at T(comp) = 112 K, and polarized beam neutron scattering measurements have been made on Fe(78)Er(5)B(17) and Fe(64)Er(19)B(17) glasses to supplement the measurements made earlier on Fe(64)Er(19)B(17). The magnetization data were analysed with a phenomenological model, to find the magnetic moments and their components needed to interpret the neutron data. Four spin-dependent scattering cross-sections were obtained in absolute units from each neutron experiment, to determine the atomic-scale magnetic structures of the two glasses. The finite spin-flip cross-sections confirmed that these (Fe,Er)(83)B(17) glasses are non-collinear ferrimagnets. The cross-sections were calculated using a model based on random cone arrangements of the magnetic moments. The moment values and the random cone angles were refined in the calculations, which produced good agreement between the calculated curves and the experimental data. The forward limit of the spin-flip cross-sections |?σ(±?)/?Ω|(Q=0) of the Fe(64)Er(19)B(17) glass which peaked at T(comp) and the temperature variation of the total scattering amplitudes (b(?)p(∥)(Q)) suggested that the random cone angles open fully so that the collinear components p(∥)(Q) tend to zero at T(comp). The ferrimagnetic compensation is therefore characterized by an equality of the magnetic sublattices; the reversal of the magnetic structure and a compensated sperimagnetic phase which appears at T(comp).     

Theoretical study of magnetism and electronic structure of Fe3/Crn(1 1 0) superlattices

The electronic structure and magnetism of Fe₃/Cr_n(1 1 0) (n=1, 3, 5) superlattices (SL) with varying layer thickness have been studied using the full-potential linearized augmented plane-wave (FLAPW) method within the first-principle formalism. The results show that the ferromagnetic state is the preferable phase in the ground state. The magnetic moments of the Fe layers are slightly modified by the presence of the Cr layers. The Cr magnetic moments alternate direction from layer to layer, and an antiferromagnetic coupling between Fe and Cr at the interfacial layer is seen. The magnetic moments of the Cr layers are suppressed because there is a strong hybridization between d-states of both Fe and Cr atoms. Only a small moment is found in the Cr layer. The Cr moment alignment is determined by a delicate balance between the different magnetic interaction.     

Response of Mn overlayers on Fe to external magnetic fields: Electronic structure calculations

The behavior of Mn overlayers on Fe(0 0 1) under the influence of external magnetic fields is investigated. The electronic charge distribution, local magnetic moments as well as their couplings are determined as a function of the external field by solving self-consistently a tight binding Hamiltonian, parameterized to ab initio TBLMTO calculations. Our method allows to trace back the field-dependent average magnetization of the system to its electronic structure and magnetic configuration. We show how in the non-collinear framework the response of the system is markedly different to what is found in the collinear framework. If metastable magnetic configurations exist, the external field can be used for tuning the system between some of them because the system stays in some of those metastable states even after switching off the external field.     

Magnetic properties of Eu-Sc-Fe garnets studied with the Mössbauer effect

Magnetic properties of the garnet system Eu_3-ySc_2+yFe₃O₁₂ for 0 ? y ? 0.5 have been studied with ⁵⁷Fe and ¹⁵¹ Eu Mössbauer spectroscopy. The system is shown to have a ferrimagnetic structure. Both, the ordering temperatures and the hyperfine magnetic fields at the ⁵⁷Fe nuclei are found to increase with an increase in y. The iron magnetic moments at the octahedral and the tetrahedral sites are non-collinear. The average canting angles at these sites are determined and used for the calculation of the magnetic moments. Good agreement is obtained between the calculated and the experimental moment for the y = 0.0 sample. The determined magnetic properties are consistent with the recently proposed cation distribution in this system.     

Magnetic anisotropy and magnetization dynamics of Fe nanoparticles embedded in Cr and Ag matrices

Static and dynamical magnetic properties of Fe nanoparticles (NPs) embedded in non-magnetic (Ag) and antiferromagnetic (Cr) matrices with a volume filling fraction (VFF) of 10% have been investigated. In both Fe@Ag and Fe@Cr nanocomposites, the Fe NPs have a narrow size distribution, with a mean particle diameter around 2 nm. In both samples, the saturation magnetization reaches that of Fe bulk bcc, suggesting the absence of alloying with the matrices. The coercivity at 5 K is much larger in Fe@Cr than in Fe@Ag as a result of the strong interaction between the Fe NPs and the Cr matrix. Temperature-dependent magnetization and ac-susceptibility measurements point out further evidence of the enhanced interparticle interaction in the Fe@Cr system. While the behaviour of Fe@Ag indicates the presence of weakly interacting magnetic monodomain particles with a wide distribution of blocking temperatures, Fe@Cr behaves like a superspin glass produced by the magnetic interactions between NPs.     

Interface magnetism of Fe/Nd multilayers studied by Mössbauer spectroscopy

Fe/Nd multilayers with⁵⁷Fe enriched interfaces are prepared to investigate crystal structure and magnetism at the interface by Mössbauer spectroscopy. The intermixture at the interface is less than two atomic layers. The magnetic moments of interface Fe atoms align collinear and turn at a certain temperature or at a certain magnetic field with keeping the collinear structure. By annealing, the interface component with smaller hyperfine field decreases and the perpendicular magnetic anisotropy increases.     

Ab-initio investigation of electronic properties and magnetism of half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In)

The electronic structures and magnetism of the half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In) have been investigated to search for new candidate half-metallic materials. Here, we predict that NiCrAl, and NiCrGa and NiCrIn are possible half-metals with an energy gap in the minority spin and a completely spin polarization at the Fermi level. The energy gap can be attributed to the covalent hybridization between the d states of the Ni and Cr atoms, which leads to the formation of bonding and antibonding peaks with a gap in between them. Their total magnetic moments are 1μ_B per unit cell; agree with the Slater-Pauling rule. The partial moment of Cr is largest in NiCrZ alloys and moments of Ni and Al are in antiferromagnetic alignment with Cr. Meanwhile, it is also found that FeCrAl is a normal ferromagnetic metal with a magnetic moment of 0.25μ_B per unit cell and CoCrAl is a semi-metal and non-magnetic.