Magnetovolume Effect Research Articles

First-principles based investigation on effects of magnetism on lattice dynamics in Fe 72Pd 28 alloy

A first-principles based investigation of influence of magnetism on lattice dynamics of Fe 72Pd 28 system near the Invar composition has been carried by computing the pressure dependence of the phonon frequencies, the Gru n ¨ eisen parameters, the disorder-induced widths and the elastic shear constant using a combination of transferable force constant model, density functional perturbation theory and the itinerant coherent potential approximation, an analytic tool for performing configuration averaging in disordered alloys. We find that with increasing pressure and collapse of magnetic moment, the TA 1[110] phonon frequencies harden along with the elastic shear constant. We do not observe any significant variation of the mode Gru n ¨ eisen parameter with change in magnetic moment. These results indicate that there is no magneto-volume effect on the lattice dynamics and the experimentally observed phonon softening with increasing magnetization has to be associated with the martensitic instability.

Magnetic phase transition in MnFeP0.5As0.4Si0.1

We have carried out a detailed investigation of the magnetic phase transition in MnFeP0.5As0.4Si0.1. Temperature hysteresis has been observed in the variable temperature magnetization curves (Bappl = 0.01 T) with TCW ~ 302 K on warming and TCC ~ 292 K on cooling. The first order nature of this transition in MnFeP0.5As0.4Si0.1 is confirmed by the negative slope obtained from isotherms of M2 versus B/M around the critical temperature. Linear thermal expansion measurements reveal a large volume change, ΔV/V~8.7×10−3 at the magnetic phase transition and that this magnetovolume effect is suppressed to ΔV/V ~ 5.5×10−3in an applied field of Bappl = 1.0 T. Analyses of 57Fe Mössbauer spectra (4.5 – 300 K) using a random distribution model and taking nearest-neighbour environments into account, indicate that the paramagnetic and ferromagnetic phases coexist over a temperature range of ~ 45 K around the Curie temperature. The Debye temperature for MnFeP0.5As0.4Si0.1 has been evaluated as θD = 350 ± 20 K from the temperature dependence of the average isomer shift.

Mechanical and Magnetic Properties of Rh and RhH: First-Principles Calculations

Mechanical and magnetic properties of Rh in bcc, fcc structures and RhH in cubic structure are investigated by using first-principles calculations. Theoretical strengths of these structures are given for the first time. The results show that the NaCl-type cubic RhH has a lower bulk modulus and a theoretical strength larger than those of Rh in bcc and fcc structures. A strong magneto-volume effect of a transition from “low magnetic moment-low cell volume" to “high magnetic moment-large cell volume" is also found for both the bcc and fcc Rh structures as well as RhH.

Spin-Lattice Coupling and Superconductivity in Fe Pnictides

We consider strong spin-lattice and spin-phonon coupling in iron pnictides and discuss its implications on superconductivity. Strong magneto-volume effect in iron compounds has long been known as the Invar effect. Fe pnictides also exhibit this effect, reflected in particular on the dependence of the magnetic moment on the atomic volume of Fe defined by the positions of the nearest neighbor atoms. Through the phenomenological Landau theory, developed on the basis of the calculations by the density functional theory (DFT) and the experimental results, we quantify the strength of the spin-lattice interaction as it relates to the Stoner criterion for the onset of magnetism. We suggest that the coupling between electrons and phonons through the spin channel may be sufficiently strong to be an important part of the superconductivity mechanism in Fe pnictides.

Spin–lattice coupling in iron-pnictide superconductors

The magnetic moment in the parent phase of the iron-pnictide superconductors varies with composition even when the nominal charge of iron is unchanged. We propose the spin–lattice coupling due to the magneto-volume effect as the primary origin of this effect, and formulate a Landau theory to describe the dependence of the moment to the Fe–As layer separation. We then compare the superconductive critical temperature of doped iron pnictides to the local moment predicted by the theory, and suggest that the spin–phonon coupling may play a role in the superconductivity of this compound.

DFT study of magneto-volume effects in iron and cobalt nitrides

Abstract Largely investigated, both theoretically and experimentally, the iron–nitrogen system (Fe–N) shows large magneto-volume effects, where N element plays an important role. On the other side, the cobalt–nitrogen (Co–N) system magnetic properties are less known. We present here a first principles comparative study, within density functional theory framework of these two systems in order to get further insights into the role of nitrogen within these insertion alloys. The properties of the cobalt nitride are firstly discussed with respect to Fe–N ones, and then the two systems are treated within Slater–Pauling–Friedel model whose limitations are shown.

Saturation magnetization in supersaturated solid solution of Co–Cu alloy

The magnetovolume effect has been investigated using a supersaturated solid solution of a Co–19 at. %Cu alloy processed by electrodeposition. The enhanced saturation magnetization of the Co–Cu alloy was attributed to both metastable fcc Co and lattice expansion. The density functional theory using the CASTEP code revealed that an enhanced magnetic moment due to the magnetovolume effect is obtained in fcc Co, but not in hcp Co.

DFT study of hydrogen instability and magnetovolume effects in CeNi

Changes in electronic and magnetic structure due to hydrogen uptake within CeNi leading to the experimental hydride CeNiH 3, are examined ab initio for identifying the hydrogen positions and establishing the equations of state. Analyses of the site projected density of states and of the chemical bonding point to modifications of the electronic structure whereby hydrogen brings new states within the valence band and is found to preferably bind with Ni rather than with Ce. From energy differences hydrogen binds weakly to the lattice, in agreement with the instability of the hydrided binary intermetallic.

Magnetism and anomalous properties of plutonium

It is shown that because of their high magnetic susceptibility δ plutonium and its alloys are close to magnetic instabilities; dynamic effects of short-range magnetic order which are associated with the fluctuating magnetization density of electrons – spin fluctuations – play a large role in them. Long-range ferro- and antiferromagnetic order is suppressed. A model is developed for the spin fluctuations in plutonium. In this model, because of strong magneto-elastic coupling the spin-fluctuation energy depends on the volume of the crystal and is independent of temperature and spatial dispersion. It is shown that the contribution of the zero-point quantum spin-fluctuations is largely temperature-independent, as a result of which saturation of local magnetic moments is absent, the Curie–Weiss law for the magnetic susceptibility breaks down, and below the melting temperature plutonium is a strongly quantum system. An explanation is given for the thermal expansion anomalies of the δ phase of plutonium and its alloys on the basis of the theory of magneto-volume effect with a negative magneto-elastic coupling constant.

Magnetic-field-induced effects in the electronic structure of itinerant d- and f-metal systems

A paramagnetic response of transition metals and itinerant d- and f-metal compounds in an external magnetic field is studied by employing ab initio full-potential LMTO method in the framework of the local spin density approximation. Within this method the anisotropy of the magnetic susceptibility in hexagonal close-packed transition metals is evaluated for the first time. This anisotropy is owing to the orbital Van Vleck-like paramagnetic susceptibility, which is revealed to be substantial in transition-metal systems due to hybridization effects in the electronic structure. It is demonstrated that compounds TiCo, Ni3Al, YCo2, CeCo2, YNi5, LaNi5, and CeNi5 are strong paramagnets close to the quantum critical point. For these systems the Stoner approximation underestimates the spin susceptibility, whereas the calculated field-induced spin moments provide a good description of the large paramagnetic susceptibilities and magnetovolume effects. It is revealed that an itinerant description of hybridized f electrons produces magnetic properties of the compounds CeCo2, CeNi5, UAl3, UGa3, USi3, and UGe3 in close agreement with experiment. In the uranium compounds UX3 the strong spin-orbit coupling together with hybridization effects give rise to peculiar magnetic states in which the field-induced spin moments are antiparallel to the external field, and the magnetic response is dominated by the orbital contribution.

Lattice, magnetic and transport properties in antiperovskite [formula omitted] compounds

The temperature-dependent magnetization, lattice, and transport properties of Mn 3Sn 1− x Ge x C ( 0 ≤ x ≤ 0.5 ) compounds are systematically investigated. The Mn–Mn atomic distance decreases as Ge content is increased, and the transition temperature from ferromagnetic (or ferrimagnetic) to paramagnetic state decreases too. Mn 3SnC has a large magnetovolume effect (MVE). However, Ge-doping in Mn 3SnC gradually reduces the MVE, till the MVE disappears. Whether there is an abnormal lattice change or not, there always exists an anomalous increase in resistivity near the magnetic phase transition point with decreasing temperature.

Control of Magnetocaloric Effects by Partial Substitution in Itinerant-Electron Metamagnetic $\hbox{La}{(\hbox{Fe}_{\rm x}\hbox{Si}_{1-{\rm x}})}_{13}$ for Application to Magnetic Refrigeration

The influence of a partial substitution of Ce or Pr on the magnetocaloric effects due to the itinerant electron metamagnetic transition of La(Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> has been investigated. The partial substitution of R=Ce or Pr for La in La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-z</sub> R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.88</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.12</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> results in the decrease of the Curie temperature <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> with decreasing the lattice constant. For the partial substitution of Ce, the change in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> is mainly attributed to the magnetovolume effect caused by small radius of Ce atoms. The influence of Pr moment on <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> is also pointed out. The increase of the isothermal entropy change Delta <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> with decreasing <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> is confirmed by both the partial substitution and application of hydrostatic pressure, although the increment of Delta <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> by the latter is smaller than that by the former. The spin-wave dispersion coefficient <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sw</sub> is slightly increased after partial substitutions in contrast to the application of hydrostatic pressure, and the reduction of thermal demagnetization up to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> brings about the enhancement of Delta <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> . Accordingly, the increase in ferromagnetic stability due to the change in 3d band structure enhances Delta <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> , in addition to the magnetovolume effect. The hysteretic behavior of the IEM transition is reduced by decreasing Fe concentration, whereas Delta <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> is enhanced by the partial substitution. Consequently, a marked reduction of hysteresis loss with retaining the large magnitude of Delta <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> is demonstrated in La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-z</sub> R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.86</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.14</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> (R=Ce and Pr). We have also presented that <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> in the partially substituted compounds with Ce or Pr is increased up above room temperature by hydrogen absorption, keeping the enhancement of MCEs by the partial substitution.

Ultrasonic studies of the magnetic phase transition in MnSi

Measurements of the sound velocities in a single crystal of MnSi were performed in thetemperature range 4–150 K. Elastic constants, controlling propagation of longitudinal waves,reveal significant softening at a temperature of about 29.6 K and small discontinuities at∼28.8 K, which corresponds to the magnetic phase transition in MnSi. In contrast, theshear elastic moduli do not show any softening at all, reacting only to the smallvolume deformation caused by the magneto-volume effect. The current ultrasonicstudy exposes an important fact that the magnetic phase transition in MnSi,occurring at 28.8 K, is just a minor feature of the global transformation marked bythe rounded maxima or minima of heat capacity, thermal expansion coefficient,sound velocities and absorption, and the temperature derivative of resistivity.

Thermal expansion of two-dimensional itinerant nearly ferromagnetic metal

Thermal expansion of two-dimensional itinerant nearly ferromagnetic metal is investigated according to the recent theoretical development of magneto-volume effect for the three-dimensional weak ferromagnets. We particularly focus on the T2-linear thermal expansion of magnetic origin at low temperatures, so far disregarded by conventional theories. As the effect of thermal spin fluctuations we have found that the T-linear thermal expansion coefficient shows strong enhancement by assuming the double Lorentzian form of the non-interacting dynamical susceptibility justified in the small wave-number and low frequency region. It grows faster in proportional to y-1/2 as we approach the magnetic instability point than two-dimensional nearly antiferromagnetic metals with ln(1/ys) dependence, where y and ys are the inverses of the reduced uniform and staggered magnetic susceptibilities, respectively. Our result is consistent with the Grüneisen's relation between the thermal expansion coefficient and the specific heat at low temperatures. In 2-dimensional electron gas we find that the thermal expansion coefficient is divergent with a finite y when the higher order term of non-interacting dynamical susceptibility is taken into account.

Local magnetovolume effects inFe65Ni35alloys

A systematic ab initio study of static ionic displacements in a face-centered-cubic ${\text{Fe}}_{65}{\text{Ni}}_{35}$ alloy has been carried out. Theoretical results for magnitudes of average Fe-Fe, Fe-Ni, and Ni-Ni $⟨110⟩$ bond vectors agree well with experimental measurements. In addition, we have observed that in collinear ferrimagnetic states, iron-iron nearest-neighbor pairs with antiparallel local magnetic moments are shorter on average than those with parallel moments. Furthermore, having considered different states (ferromagnetic, nonmagnetic, and collinear ferrimagnetic states) for the same lattice spacing, we have shown that the magnetic structure strongly influences local geometrical properties of the alloy. For example, a transition from a ferromagnetic state to a collinear ferrimagnetic state induces a significant contraction of the volume associated with an iron site where the moment flips. A model system based on a Hamiltonian written as the sum of Lennard-Jones energies and a classical Heisenberg Hamiltonian has been introduced. It yields structural properties which are qualitatively similar to those obtained ab initio. We have found that some of the phenomena can be classified as magnetovolume effects.

Structure and magnetic properties of Sm2Fe17-xCrxcompound prepared by arc melting

The structural and magnetic properties of Sm2Fe17-xCrx compound have been investigated by means of x-ray diffraction and magnetization measurements. The results show that the Sm2Fe17-xCrx （x=1—3） compounds annealed at 1050 ℃ for 5 days have a rhomhedral Th2Zn17-type structure，and the Sm2Fe17-xCrx（x=25, 30） compounds annealed at 1050 ℃ for 7 days have a monoclinic Nd3（Fe，Ti）29-type structure. Long annealing time and high content of Cr can make the Sm-Fe-Cr compound with a Nd3（Fe，Ti）29-type structure become very stable. This is quiet different from the R2Fe17-xCrx compound （R is a heavy rare earth element）. The analysis of unit-cell volume and unit-cell parameters show that there exists a strong anisotropic magneto-volume effect in Sm2Fe17-xCrx compounds. The result of magnetization measurement shows that the effects of the substituting Cr for Fe at dumbbell sites on the Curie temperature and the magneto-crystalline anisotropy of the compound with Th2Ni17-type structure are greater than that of the compound with Th2Zn17-type structure.

Magnetic structures and phase transitions in PrMn2−xFexGe2

The magnetic properties and magnetic structures of PrMn2−xFexGe2 compounds (space group I4/mmm) have been investigated using magnetic, F57e Mössbauer effect (x=1.0,1.3,1.6), and neutron diffraction measurements (x=0.4,0.6,0.8,1.3) over the temperature range of 3–410 K. This has enabled the existing magnetic phase diagram for PrMn2−xFexGe2 to be extended from Fe concentration x=0–1 to the full range x=0–2 in terms of concentration and dMn–Mn, the intralayer distance. Analysis of the Mössbauer spectra (4.5–300 K) using a model which takes nearest-neighbor environments into account confirms the nonmagnetic nature of Fe atoms in these compounds, and leads to hyperfine parameters which deviate around the magnetic transition temperatures derived from the magnetic and neutron investigations while also enabling the Debye temperatures of PrMn2−xFexGe2 (x=0.4–1.6) to be determined. The experimental values for TCinter are found to decrease rapidly with increasing Fe concentration in the range x=0.0–0.6 compared with calculated TCinter values due to pressure (and therefore geometric) effects only. This behavior demonstrates that electronic effects and replacement of the magnetic Mn atoms with nonmagnetic Fe atoms contribute to the overall magnetic behavior of PrMn2−xFexGe2 compounds. Compared with intralayer Mn–Mn interactions, the interlayer Mn–Mn interactions play the major role in the anomalous thermal expansion observed at magnetic transition in these layered systems, with the interlayer Mn–Mn interactions governing the significant magnetovolume effects.

EXAFS Studies and Magnetic Behavior of FeCuZr Ball-Milled Alloys

Metastable alloys of nominal composition (Fe0.5Cu0.5)100-xZrx (x=0-17 at.%) have been synthesized by high energy ball milling. In spite of the high and positive enthalpy of mixing between Fe and Cu, nanocrystalline or amorphous alloys have been obtained depending on the Zr content. Alloys exhibit a ferromagnetic behavior with a Curie temperature below room temperature. The thermal dependence of the thermorremanence shows a anomalous increase above the Curie temperature. This behavior seems to be related with a magnetovolume effect. Extended X-ray absorption fine structure (EXAFS) measurements have been performed to explain this effect. Preliminary results seem to indicate an almost negligible thermal expansion at temperatures below Tc, while normal thermal expansion takes place at higher temperatures.

Reduction of hysteresis loss and large magnetocaloric effects in substituted compounds of itinerant-electron metamagnets La(FexSi1−x)13

The maximum value of hysteresis loss EhMAX due to the itinerant-electron metamagnetic (IEM) transition of La(FexSi1−x)13 and the partially substituted compounds La1−zCez(Fe0.86Si0.14)13 and La1−z′Prz′(Fe0.86Si0.14)13 increases when the magnetocaloric effects (MCEs) become large. It should be noted that the reduction of EhMAX without the decrease of large MCEs is achieved in La1−zCez(Fe0.86Si0.14)13 and La1−z′Prz′(Fe0.86Si0.14)13. For both the compound systems mentioned above, the critical temperature T0 for the IEM transition decreases and the difference between T0 and the Curie temperature TC becomes larger with decreasing TC. These results are consistent with the magnetic phase diagram of La(Fe0.86Si0.14)13 under hydrostatic pressure. Consequently, the reduction of EhMAX in La1−zCez(Fe0.86Si0.14)13 and La1−z′Prz′(Fe0.86Si0.14)13 is closely related with the magnetovolume effects.

Neutron Diffraction and Isotropic Volume Expansion Caused by Deuterium Absorption into La(Fe0.88Si0.12)13

The crystal structures and magnetic properties of La(Fe 0.88 Si 0.12 ) 13 D 1.5 and La(Fe 0.88 Si 0.12 ) 13 H 1.6 have been investigated. The powder neutron diffraction declared that the compound La(Fe 0.88 Si 0.12 ) 13 D 1.5 has a cubic NaZn 13 -type structure of the F m \bar3 c space group in analogy with La(Fe 0.88 Si 0.12 ) 13 . The deuterium atoms in the compound La(Fe 0.88 Si 0.12 ) 13 D 1.5 occupy the interstitial 24 d site surrounded by two La atoms and four icosahedral clusters. The x-ray diffraction data of La(Fe 0.88 Si 0.12 ) 13 H 1.6 are also consistently explained by the same site of hydrogen atom. The deuterium or hydrogen absorption brings out a notable increase of the Curie temperature T C . The neutron diffraction of the compound La(Fe 0.88 Si 0.12 ) 13 D 1.5 shows that the magnetic structure is ferromagnetic and the average magnetic moment per Fe is about 2.2 µ B , almost the same as that of La(Fe 0.88 Si 0.12 ) 13 . Such behaviors can be explained as characteristic magnetovolume effect...