Intrinsic magnetic properties of SmFe12−xVx alloys with reduced V-concentration

International audienceIn this work, we present experimental and theoretical results on SmFe 12−x V x (x = 0.5-2.0) alloys with the ThMn 12 (1:12) structure as possible candidates for rare earth-lean permanent magnets. The compound with x = 2 has been previously reported to have a Curie temperature of 330 • C, saturation magnetization of about 80 Am 2 /kg, and anisotropy field around 9 T. We have synthesized the SmFe 11 V compound with a nearly pure 1:12 phase; the x = 0.5 compound couldn't be synthesized. The stability of the x = 1 compound was also confirmed theoretically by calculations of their formation enthalpies using first principles. The newly synthesized SmFe 11 V compound has a Curie temperature of 361 • C and saturation magnetization of 115 Am 2 /kg (1.12 T). The anisotropy field has been obtained in magnetically-oriented fine powders, and is around 11 T. These parameters make SmFe 11 V a good candidate for a new kind of high energy, rare earth-lean permanent magnets

Atomic contributions to the valence band photoelectron spectra of metal-free, iron and manganese phthalocyanines

The present work reports a photoelectron spectroscopy study of the low-energy region of the valence band of metal-free phthalocyanine (H2Pc) compared with those of iron phthalocyanine (FePc) and manganese phthalocyanine (MnPc). Density Functional Theory calculations have been used to resolve the atomic orbital composition of the valence spectra of all the phthalocyanines (Pcs) analyzed in this study. Moreover we show how the atomic character of the Highest Occupied Molecular Orbital (HOMO) is reflected on the outermost valence band binding energy region. The intensity related to the C 2p contributions, resulting in the HOMO for H2Pc and FePc and in the HOMO-1 for MnPc as described by the theoretical predictions is in very good agreement with the experimental results. The DFT simulations, discerning the atomic contribution to the density of states, indicate how the central metal atom interacts with the C and N atoms of the molecule, giving rise to different partial and total density of states for these three different Pc molecules

Database of novel magnetic materials for high-performance permanent magnet development

This paper describes the open Novamag database that has been developed for the design of novel Rare-Earth free/lean permanent magnets. Its main features as software technologies, friendly graphical user interface, advanced search mode, plotting tool and available data are explained in detail. Following the philosophy and standards of Materials Genome Initiative, it contains significant results of novel magnetic phases with high magnetocrystalline anisotropy obtained by three computational high-throughput screening approaches based on a crystal structure prediction method using an Adaptive Genetic Algorithm, tetragonally distortion of cubic phases and tuning known phases by doping. Additionally, it also includes theoretical and experimental data about fundamental magnetic material properties such as magnetic moments, magnetocrystalline anisotropy energy, exchange parameters, Curie temperature, domain wall width, exchange stiffness, coercivity and maximum energy product, that can be used in the study and design of new promising high-performance Rare-Earth free/lean permanent magnets. The results therein contained might provide some insights into the ongoing debate about the theoretical performance limits beyond Rare-Earth based magnets. Finally, some general strategies are discussed to design possible experimental routes for exploring most promising theoretical novel materials found in the database.European Horizon 2020 Framework Programme for Research and Innovation (2014-2020) under Grant Agreement No. 686056, NOVAMAG. European Regional Development Fund in the IT4Innovations national supercomputing center – path to exascale project, project number CZ 02.1.01/0.0/0.0/16–013/0001791 within the Operational Programme Research, Development and Educatio

Instability of the rhodium magnetic moment as origin of the metamagnetic phase transition in alpha-FeRh

Based on ab initio total energy calculations we show that two magnetic states of rhodium atoms together with competing ferromagnetic and antiferromagnetic exchange interactions are responsible for a temperature induced metamagnetic phase transition, which experimentally is observed for stoichiometric alpha-FeRh. A first-principle spin-based model allows to reproduce this first-order metamagnetic transition by means of Monte Carlo simulations. Further inclusion of spacial variation of exchange parameters leads to a realistic description of the experimental magneto-volume effects in alpha-FeRh.Comment: 10 pages, 13 figures, accepted for publication in Phys. Rev.

Ab-initio-Untersuchung magnetischer und struktureller Eigenschaften von 3d-Uebergangsmetallen und ihren Legierungen

In dieser Arbeit werden die strukturellen und magnetischen Eigenschaften von 3d-Uebergangsmetallen und ihren Verbindungen mit Hilfe verschiedener Methoden der Dichtefunktionaltheorie behandelt. Dabei werden insbesondere Magnetovolumen-Instabilitaeten und martensitische Phasenuebergaenge in Eisen-Nickel und Eisen-Mangan-Verbindungen diskutiert. Einen zentralen Punkt der Arbeit bildet die Untersuchung von elementarem Eisen, da viele Eigenschaften der Legierungen bereits dadurch verstanden werden koennen. Die Verknuepfung der ab-initio-Ergebnisse mit einem einfachen thermodynamischen Ansatz ermoeglicht eine Diskussion bei endlichen Temperaturen. Die im Rahmen dieses Modells berechneten thermischen Ausdehnungskoeffizienten zeigen bereits eine gute Uebereinstimmung mit den experimentellen Daten. Neben den Eisenlegierungen wird das nichtmagnetische System Al-Cu-Zn untersucht, welches in bestimmten Konzentrationsbereichen ebenfalls martensitische Phasenuebergaenge aufweist und auf der Al-Zn-Seite spinodale Entmischung zeigt, die mit zunehmender Substitution von Aluminium durch Kupfer in inkommensurable Ordnung uebergeht. An Hand von ab-initio-Berechnungen der elektronischen Struktur geordneter Verbindungen wird das Mischungsverhalten sowie die strukturelle Phasenstabilitaet des Systems fuer T = 0 K untersucht. Die Ergebnisse spiegeln die experimentell gefundenen Tendenzen gut wieder.In the present thesis the structural and magnetic properties of 3d transition metals and their alloys have been investigated within the framework of density functional methods. The calculations have been made mainly to understand magneto-volume instabilities and martensitic phase transformations in iron-nickel and iron-manganese alloys. Many aspects of the alloys can already be understood from the discussion of elemental iron, which forms the essential part of this work. A simple thermodynamical approach combined with the ab-initio results allows the discussion of the physical properties to be extended to finite temperatures. The thermal expansion coefficients calculated within this model coincide well with experiment. Besides the magnetic iron alloys the nonmagnetic Al-Cu-Zn system has been investigated. This ternary alloy undergoes several martensitic phase transformations and spinodal decomposition has also been observed on the Al-Zn-rich side of the phase diagram. The spinodal decomposition is relieved by incommensurable ordering if the aluminium atoms are increasingly replaced by copper. The mixing behavior and the structural phase stability at T = 0 K are obtained from ab initio electronic structure calculations for ordered compounds. The experimental trends are well reproduced.SIGLEAvailable from: http://www.iwi-iuk.org/dienste/TheO/ / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Ab Initio Investigations of Iron-Based Martensitic Systems

We apply a full-potential first-principles method to investigate the electronic-structure and elastic features of pure iron and ordered transition metal systems like Fe-Ni and Fe-Mn. The martensitic phase transformations in these compounds mainly depend on the properties of iron. Therefore the phase diagram of iron has been studied in detail in view of structural transformations and magnetic order. To this the total energy and magnetic moment have been investigated as function of the atomic volume. In agreement with experimental results, we find a ferromagnetic bcc ground-state and an antiferromagnetic fcc phase having a higher energy of 7 mRy. The resulting pressure for the bcc → hcp transition is about 11.5 GPa in agreement with experiments. Besides that, total energy calculations have been done for stoichiometric Fe-Mn and Fe-Ni compounds. The resulting alloy phase diagrams are in a good agreement with the experimental data

Electron-Phonon Interaction and Phonon Softening in Systems with Magnetovolume Instabilities

The interrelation of Invar and martensitic behaviour in transition metal alloys like Fe65Ni35 is discussed on the basis of ab initio calculations for stoichiometric systems like Fe3Ni. We also examined face centered cubic iron as a model system for Antiinvar which like Invar shows a pronounced phonon softening. Neutron scattering experiments on Invar systems have shown that the TA1 phonon mode softens for small wavevectors (vector q) along the [110] direction in the fcc lattice. To day there exist only a few ab initio calculations for martensitic transformations in nonmagnetic compounds. We have extended these kinds of investigations to the magnetic case. Our numerical calculations are based on the first principles theory of Varma and Weber who considered the electron-phonon interaction in second order perturbation theory. We have calculated renormalized phonon dispersion curves with the help of an appropriate set of tight-binding states fitted to ab initio band structure results. Our numerical results reproduce the experimental findings. In particular we observe that phonon softening in systems with magnetovolume instabilities is caused by the long-range contribution to the electronphonon interaction

A first-principles study aided with Monte Carlo simulations of carbon doped iron-manganese alloys

We have investigated the complex magnetic properties of Fe1−xMnxCy alloys by using an iterative combination of ab initio calculations and Monte Carlo simulations. The latter gives insight into finite temperature magnetism and allows to determine the critical temperature of magnetic phase transitions. We restrict the investigation to ordered systems with 25, 50 and 75% manganese and study the influence of carbon at octahedral interstitial sites on the magnetic properties. The combination of ab initio calculations with Monte Carlo simulations turns out to be a powerful tool to determine the complex magnetic structures, which originate from the competition of ferro- and antiferromagnetic interactions in the FeMn alloys

Structural Phase Transformation and Phonon Softening in Iron-Based Alloys

The interrelation of Invar and Martensite in iron-based alloys like Fe3Ni is discussed on the basis of ab initio calculations for different crystal structures. Inelastic neutron scattering and ultrasonic experiments have shown that a soft transverse phonon mode in [110] direction acts as a precursor in the close packed fcc phase of these systems. Unfortunately there are no experimental results for phonon modes in the transformed bct phase. We have examined in detail the changes in the phonon dispersions of these systems by varying the ratio of the lattice constants along the Bain path. In recent calculations we have used the method of Varma and Weber in combination with the full potential LAPW (FLAPW) method to calculate frozen phonons for the tetragonal phase. Our new numerical results reproduce the experimental findings for the fcc phase and give an overview of the changes of the phonon energies when varying the c/a ratio towards the bct structure. In particular we observe that phonon softening is caused by the long-range electron-phonon interaction in systems with magnetovolume instabilities, and vanishes after the martensitic transformation