Magnetovolume Properties Research Articles

Nitrided FeNi: Chemical versus magnetovolume effects from ab initio

Abstract Magnetic, magneto-volume properties and bonding analyses of binary intermetallic FeNi and its nitride FeNiN have been investigated within density functional theory DFT. From total energy calculations and establishing the energy-volume equations of states (EOS) the ground state of both compounds is ferromagnetic with a reduced magnetization of the nitrided intermetallic due to spin pairing between Fe/Ni and N. The volume expansion indicates that iron shows itinerant magnetism, whereas Ni exhibits a localized behavior. Important magneto-volume effects are observed whereby saturation of the magnetic moment is reached in FeNi but not in FeNiN. The chemical bonding shows Ni-N and Fe-N contributions to the stability of the nitride systems. However, this bonding, which reduces magnetization around experimental volume, loses its effect at large volumes where magneto-volume influence prevails again as in FeNi.

Effect of dimensionality crossover on magnetovolume properties of quasi one-dimensional weakly antiferromagnetic metals

Magnetic properties and thermal expansion of quasi one-dimensional itinerant electron magnets close to the antiferromagnetic instability are studied. By controlling the relative spectral dispersions of spin fluctuation spectra depending on the direction of crystal axes, we show that the three dimensional properties are changed with increasing the one-dimensional anisotropy. Crossover behaviors are discussed on the temperature dependence of the magnetic susceptibility and the magnetovolume expansion around their critical regions.

Magnetic and magneto-volume properties of FexM100−x solid solutions with M = Pd and Pt

Band structure calculations were performed on FexM100−x solid solutions with M = Pd and Pt as well as on ordered phases present in the above systems. The equilibrium values of lattice parameters, magnetic moments, Curie temperatures, and spontaneous volume magnetostrictions were computed. A good agreement with experimental results has been shown. The magnetic properties, in particular Curie temperatures, were correlated with distances between iron atoms, as suggested by Néel-Slater curve.

Role of Electronic Band Structure and Lattice Parameters in Magnetism of the R<sub>2</sub>(Fe,M)<sub>17</sub>, М = Si, Al Compounds

Magnetic, electronic, structural and magnetovolume properties of the R2Fe17-xMx (R = Y, Lu, Ce; M = Si or Al) compounds were studied. We pioneered to apply electronic band parameters determination to account for the anomalous volume dependence of ferromagnetism in these compounds.

Grüneisen’s Approach to Magnetovolume Effect of Itinerant Electron Ferromagnets

Interplay between magnetism and crystal volume in itinerant electron ferromagnets is reexamined paying particular attention on effects of spin fluctuations. By introducing Gruneisen parameters, magnetovolume properties are derived from the explicit volume dependence of the free energy. As the result, we have clarified the presence of new thermal expansions, giving rise to the enhanced T-linear coefficient of the thermal expansion coefficient at low temperatures around the magnetic instability point. We have also found that the magnetovolume coupling constant is temperature dependent and vanishes at the critical point. These results are compared with experiments.

Magnetovolume effect of itinerant electron ferromagnets

The magnetovolume effect of itinerant electron magnets is discussed by the explicitexamination of the volume dependence of the spin fluctuation free energy. MagneticGrüneisen parameters are introduced instead of magnetovolume coupling constants, thatenable us to describe all the magnetovolume properties in terms of these parameters.We have particularly found the presence of a new thermal expansion, showingT2-like temperature dependence. It explains the appreciable electronic volume thermalexpansions observed experimentally. We also show that the magnetovolume couplingconstants for spontaneous and forced magnetostrictions, of different magnitudes, aretemperature dependent. Analysis of the pressure effect on the Curie temperatureTc and the spontaneousmagnetic moment M shows that the linear relation between them, , is generally violated.

Correlation Between Magnetovolume and Colossal Magnetoresistance Effects in Terbium Doped La-Sr-Mn-O Perovskite

(La0.67Tb0.33)2/3Sr1/3MnO3 has been studied in order to probe mechanisms responsible for the giant magnetoresistance ratios and the lattice effect in this kind of compound. The experiment has shown a strong connection between the magnetotransport and magnetovolume properties. An applied magnetic field not only gives rise to a large negative magnetoresistance (-900%) but also produces two different magnetovolume effects which reflect two different magnetostriction mechanisms in the compound.

Relation between anomalous magnetovolume behavior and magnetic frustration in Invar alloys.

We present an exactly solvable microscopic model of the T=0 K magnetovolume properties (spontaneous volume magnetostriction, forced volume magnetostriction, compressibility, pressure-induced magnetization response, and paraprocess susceptibility) of a local moment binary alloy with two magnetic species, chemical disorder, and the possibility of magnetic frustration. The calculated properties are shown to have an explicit dependence on both the ground-state spin structure and the degree of magnetic frustration, evaluated as the fraction of magnetic exchange bonds that are energetically not satisfied. We apply this model to fcc ${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Ni}}_{\mathit{y}}$ alloys (including classical Invar at y=0.35) in which we determine the spin structures by adjusting Monte Carlo simulation results to the measured magnetic properties. We find that Invar (${\mathrm{Fe}}_{65}$${\mathrm{Ni}}_{35}$) is a highly frustrated system and that this effectively changes the sign of the resulting magnetovolume expansion. Invar alloys based on high-moment \ensuremath{\gamma}-Fe are therefore understood as arising from an optimized blend of ferromagnetism, that causes most of the Fe-Fe bonds to be unsatisfied, and the antiferromagnetism of the magnetovolume active Fe-Fe bonds. \textcopyright{} 1996 The American Physical Society.

Correlation between magnetovolume and giant magnetoresistance effects in doped La2/3Ca1/3MnO3 perovskites

La0.62Tb0.05Ca0.33MnO3 and La0.6Y0.07Ca0.33MnO3 have been studied in order to probe into the mechanisms responsible for the giant magnetoresistance ratios observed in this kind of compound. The experiments have shown a strong connection between the magnetotransport and magnetovolume properties. A large volume effect appears above Tc which collapses with applied magnetic field or when the long range magnetic order sets in. Above Tc the magnetostriction and magnetoresistance isotherms are highly correlated. Charge localization with local distortion appears to be responsible for the transport and volume properties at zero field. Charge mobility and a more normal volume dependence on temperature are restored by applying high enough magnetic fields.

Noncollinear spin fluctuations in Fe3Pt Invar

Abstract The magnetic properties of Fe 3 Pt Invar are determined using a noncollinear fixed spin moment procedure. Here the total energy is obtained as a function of the volume and the magnetic moment arranged in noncollinear spin structures. This on a microscopic basis establishes the connection of the volume with longitudinal and transverse spin fluctuations. The thermal expansion coefficient of Fe 3 Pt is determined and an explanation of the anomalous behaviour of magnetovolume properties is given.

Spin fluctuations in gamma -Fe and in Fe3Pt Invar from local-density-functional calculations.

The electronic structure and the ensuing magnetic properties of \ensuremath{\gamma}-Fe and ${\mathrm{Fe}}_{3}$Pt Invar are determined using the local approximation to spin-density-functional theory and the augmented-spherical-wave method. A noncollinear constrained-moment procedure is described and employed to obtain the total energy as a function of the volume and the magnetic moment arranged both in collinear and noncollinear spiral structures. This, on a microscopic basis, establishes the connection of the volume with longitudinal and transverse spin fluctuations. We determine the thermal expansion coefficients of \ensuremath{\gamma}-Fe and ${\mathrm{Fe}}_{3}$Pt and give an explanation of their anomalous magnetovolume properties.

Magnetovolume effects in strong paramagnets

We present calculations of magnetovolume properties for different strong paramagnetic systems such as ZrV2, TiBe2, and ZrZn2 in the C15 structure. These materials show indications of anti-Invar behavior, i.e., enhanced magnetostriction, and thermal expansion.

The magnetovolume properties of Y(Co1-xAlx)2 compounds

The magnetovolume properties of Y(Co1-xAlx)2 compounds have been investigated by means of magnetization, magnetostriction and thermal expansion measurements. A complete magnetic phase diagram is presented for the whole range of the considered Al concentration (0 <x < 0.20). The pressure dependence of magnetization and volume effects on the onset of ferromagnetism are discussed on the basis of a model of itinerant electron metamagnetism which supposes an interplay between electronic and elastic energies.

Magnetic and magnetovolume properties of RCo2 compounds and alloys from energy band structure

A quantitative description of the magnetic behaviour is obtained for the itinerant electrons of the rare-earth-cobalt Laves-phase compounds. This is accomplished by fitting the results of a fixed-spin-moment calculation of the electronic energy of YCo2 to a power series in the magnetization for several different volumes and by coupling to the rare earth spins in molecular field theory. The T=0 itinerant moments are obtained for the RCo2 series and the alloy Y1-xGdxCo2. The spontaneous volume magnetostriction of RCo2 is also estimated. The results are compared with experiment.

Magnetic and magnetovolume properties of Fe-Pt alloys around γ-α phase boundary

Magnetization and thermal expansion have been measured for both the ordered and disordered Fe-Pt alloys around the γ-α phase boundary. The fcc alloys display Invar characteristics and their magnetic ground state is strong ferromagnetism. A martensitic transformation with a small hysteresis has been detected at low temperatures for both ordered and disordered alloys in a critical composition range.

Magnetovolume instability in FCC iron-nickel alloys

The possibility of magnetic and elastic instabilities is pointed out from the thermodynamic aspects of the magnetovolume properties of a ferromagnet, and the instabilities, Invar anomalies and martensitic transformations in fcc iron-nickel alloys are discussed.

Volume effects in itinerant ferromagnets

Magneto-volume experiments (including: pressure dependence of the magnetization, volume magnetostriction and magnetic contribution to the thermal expansion) on strongly paramagnetic and weakly ferromagnetic Ni 3Al and NiPt alloys can be described with one single magneto-volume parameter. Its value is discussed in the Stoner-Edwards-Wohlfarth model for very weak itinerant ferromagnetism. Alloys that follow this model have some characteristic magneto-volume properties. In view of these properties the transition from ferro- to paramagnetism at the critical concentration for ferromagnetism is studied for three pseudo binary alloy systems: Y 6(Fe, Mn) 23, Zr(Fe, Co) 2 and Ti(Fe, Co).