The Curie temperature of the ferromagnetic transition metals and their compounds
TL;DR: In this article, the Curie temperature of Fe, Co and Ni as well as of their compounds with Y (including Y2Fe14B) are calculated by including the effects of spin fluctuations in the long wavelength limit via a renormalisation of Landau coefficients.
Abstract: The Curie temperature of Fe, Co and Ni as well as of their compounds with Y (including Y2Fe14B) are calculated by including the effects of spin fluctuations in the long wavelength limit via a renormalisation of Landau coefficients. The numerical values depend on the use of recent calculations of correlation effects and of the band structure. As a result, good agreement is found with the experimental values of TC of the materials covered.
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TL;DR: It is shown that together these effects can effectively manipulate electron and phonon transport at nanometre and mesoscopic length scales and thereby improve the thermoelectric performance of the resulting nanocomposites.
Abstract: The ability to control chemical and physical structuring at the nanometre scale is important for developing high-performance thermoelectric materials. Progress in this area has been achieved mainly by enhancing phonon scattering and consequently decreasing the thermal conductivity of the lattice through the design of either interface structures at nanometre or mesoscopic length scales or multiscale hierarchical architectures. A nanostructuring approach that enables electron transport as well as phonon transport to be manipulated could potentially lead to further enhancements in thermoelectric performance. Here we show that by embedding nanoparticles of a soft magnetic material in a thermoelectric matrix we achieve dual control of phonon- and electron-transport properties. The properties of the nanoparticles-in particular, their superparamagnetic behaviour (in which the nanoparticles can be magnetized similarly to a paramagnet under an external magnetic field)-lead to three kinds of thermoelectromagnetic effect: charge transfer from the magnetic inclusions to the matrix; multiple scattering of electrons by superparamagnetic fluctuations; and enhanced phonon scattering as a result of both the magnetic fluctuations and the nanostructures themselves. We show that together these effects can effectively manipulate electron and phonon transport at nanometre and mesoscopic length scales and thereby improve the thermoelectric performance of the resulting nanocomposites.
432 citations
TL;DR: In this paper, the band structure theory of magnetism in 3d-4f compounds is reviewed, and the cornerstones of the DFT are explicitly sketched, as well as recent developments needed to cope with the treatment of localized 4f and itinerant 3d magnetisms in one and the same framework.
Abstract: The band structure theory of magnetism in 3d-4f compounds is reviewed. Among the open-shell electrons, a hierarchy of interactions is present which governs the intrinsic magnetic properties of these materials. Density functional theory (DFT) is an appropriate tool to describe and quantitatively investigate both ground state properties and model interaction parameters, which are necessary to calculate the temperature-dependent behaviour. The cornerstones of the DFT are explicitly sketched, as well as recent developments needed to cope with the treatment of localized 4f and itinerant 3d magnetism in one and the same framework. This includes the open-core scheme, self-interaction corrected DFT, relativistic DFT, and orbital polarization. On this basis, the exchange coupling among itinerant and localized states can be understood, together with the size of Curie temperature and ground state spin and orbital magnetic moments. Finally, the problem of magnetocrystalline anisotropy is addressed, concerning both the band and the 4f crystal field contribution.
158 citations
TL;DR: In this paper, a comparative study of the band structure on the basis of local spin-density functional theory is presented for the ferrites Fe 3 O 4, CoFe 2 O 4, NiFe 2O 4, MnFe 2 OD 4 and ZnFe 2 OM 4.
126 citations
TL;DR: In this article, the magnetic properties of binary rare-earth 3d-transition-metal intermetallic compounds are discussed, and the basic concepts related to intrinsic magnetic properties related to the 3D-rich R n T m (R = rare earths; T = 3d heavy transition metals Mn, Fe, Co, Ni) intermetallics are discussed.
Abstract: Publisher Summary This chapter discusses the magnetic properties of binary rare-earth 3d-transition-metal intermetallic compounds The basic concepts related to the intrinsic magnetic properties of the 3d-rich R n T m (R = rare earths; T = 3d heavy transition metals Mn, Fe, Co, Ni) intermetallic compounds are discussed The study of rare-earth transition-metal intermetallic compounds has a number of interesting aspects Owing to the wide range of intermetallics and their different stoichiometries and variable rare-earth elements, modifications of magnetic properties of 3d transition-metal and 4f rare-earth ions can be investigated systematically These investigations illuminate the complex interactions in which the 3d and 4f electrons are involved The rare-earth metals in their ‘normal’ state where the magnetic properties of the ion cores are well defined The 4f electrons are positioned within the ion cores and hybridization with the conduction-band-electron states is negligible This situation is realized for most iron- and cobalt-based compounds with 4f elements The Ce and Yb ions, which sometimes demonstrate unusual properties connected with valence fluctuations, tend to behave quite normally in the compounds with iron or cobalt Nevertheless, there are some anomalies in the Ce compounds that can be ascribed to a mixed-valence state of the cerium ion
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TL;DR: In this paper, a new compound composed of Nd, Fe, and a small quantity of B (about 1 wt. %) has been found, which has a tetragonal structure with lattice constants a=0.880 nm and c=1.221 nm.
Abstract: A new compound composed of Nd, Fe, and a small quantity of B (about 1 wt. %) has been found, which has a tetragonal structure with lattice constants a=0.880 nm and c=1.221 nm. This phase, which has the approximate composition, 12 at. % Nd, 6 at. % B and balance Fe, possesses remarkable magnetic properties. From the approach to saturation an anisotroy constant of about 3.5 MJ/m3 can be calculated, while saturation magnetization amounts to 1.35 T. The magnetization versus temperature curve shows a Curie temperature of 585 K, which is much higher than those of the Fe and light rare earth binary compounds. Based on the new compound, sintered permanent magnets have been developed which have a record high energy product. Permanent magnet properties and physical properties of a typical specimen which has the composition Nd15B8Fe77 are as follows: Br =1.23 T, HcB =880 kA/m, HcI =960 kA/m, (BH)max =290 kJ/m3, temperature coefficient of Br =−1260 ppm/K, density=7.4 Mg/m3, specific resistivity=1.4 μΩm, Vickers hardn...
2,525 citations
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04 Sep 1985TL;DR: In this paper, a general theory of spin fluctuations and thermodynamical properties of itinerant electron magnets is developed, interpolating between the weakly and strongly ferromagnetic limits, and a unified expression is given for the Curie temperature and the physical meaning of the curie-Weiss magnetic susceptibility is discussed.
Abstract: A general theory of spin fluctuations and thermodynamical properties of itinerant electron magnets is developed, interpolating between the weakly and strongly ferromagnetic limits. A unified expression is given for the Curie temperature and the physical meaning of the Curie-Weiss magnetic susceptibility is discussed. As new phenomena derived from this theory the temperature-induced local magnetic moments as observed in CoS2, CoSe2, etc. and peculiar magnetic and thermal properties of nearly ferromagnetic semiconductors such as FeSi are discussed.
1,321 citations
TL;DR: In this article, a conceptual model and a calculational procedure for the study of the electronic structure of metallic compounds are presented, which consists of spherical atoms compressed into finite volumes appropriate to the solid.
Abstract: We present a conceptual model and calculational procedure for the study of the electronic structure of metallic compounds. The model consists of spherical atoms compressed into finite volumes appropriate to the solid. The model involves no adjustable or experimentally derived parameters. All contributions to the total energy (other than the Madelung energy) are obtained from independent compressed-atom calculations. Interatomic interactions enter the calculations through the electronic configuration (the distribution of the valence charge among $s$, $p$, $d$, etc., states) and boundary conditions which give the atomic valence levels a finite width. These environmental constraints, which specify the state of the compressed atoms, are obtained from energy-band calculations. For the latter we introduce a new method, which we call the augmented-spherical-wave (ASW) method to stress its conceptual similarity to Slater's augmented-plane-wave (APW) method. The ASW method is a direct descendant of the linear-muffin-tin-orbitals technique introduced by Andersen; when applied to pure metals, it yields results which closely approximate those of the much more elaborate Korringa-Kohn-Rostoker calculations of Moruzzi, Williams, and Janak. The combined ASW compressed-atom procedure is tested on (i) the empty lattice, (ii) the pure metals Na, Al, Cu, and Mo, and (iii) the ordered stoichiometric compounds NaCl, NiAl, and CuZn. Finally, we demonstrate the utility of the procedure by using it to study the anomalous tendency of Ni and Pd (as compared to their Periodic Table neighbors Co, Cu, Rh, and Ag) to form hydride phases. We have calculated the total energies of the six pure metals and their monohydrides. The total energy differences exhibit the anomaly and an analysis of quantities internal to the calculation reveals its origin.
881 citations
TL;DR: In this paper, the authors present a quantitative model for the magnetic equation of state of nearly or weakly ferromagnetic metals at low temperatures which includes corrections to conventional Stoner theory arising from enhanced fluctuations in the local magnetisation.
Abstract: The authors present a quantitative model for the magnetic equation of state of nearly or weakly ferromagnetic metals at low temperatures which includes corrections to conventional Stoner theory arising from enhanced fluctuations in the local magnetisation. The model takes account of both longitudinal and transverse fluctuations in terms of four physically transparent parameters which may be determined independently from the equation of state in the T=0 limit and from inelastic neutron scattering, or calculated directly from a semi-empirical band structure model near the Fermi level fitted for example to experimental Fermi surface areas. For parameters of the same order of magnitude as those recently determined in the weakly spin-polarised metal Ni3Al, the model yields approximately a quadratic temperature dependence of the spontaneous magnetisation over a wide range well below the Curie temperature (Tc), a nearly linear inverse susceptibility well above Tc, and nearly linear magnetic isotherms (Arrott plots) at high magnetic fields. These results are qualitatively consistent with the behaviour observed in many magnetic metals near the ferromagnetic instability at low temperatures. For Ni3Al the model yields good quantitative agreement with experiment for the magnitude of the Curie temperature Tc, for the ratio peff/p0 of the high- to low-temperature effective magnetic moments, and for the coefficient of the quadratic (T2) variation of the magnetisation with temperature well below Tc, without the use of any free adjustable parameters. Finally the authors show that the model also provides a good quantitative description of the paramagnetic susceptibility and transition temperature of the more complex magnetic system MnSi, the only other unsaturated magnetic metal for which all of the microscopic parameters are well known.
337 citations