Changes in vibrational modes near the magnetic phase transition in magnetostrictive materials

Mössbauer studies of ⁵⁷Fe in RFe₂ (R = Y, Tb, Dy, Ho) at temperatures 300 to 800 K and in R(Co_0.98Fe_0.02)₂ (R = Tb, Dy) at temperatures 85 to 300 K have been performed. The spectra yield the temperature dependence of the easy axis of magnetization, the magnetic hyperfine fields, electric field gradients and isomer shift. From the total Mössbauer absorption area the recoil free fraction was determined. The relative intensities of the various Mössbauer absorption lines yield the anisotropy in the recoil free fraction. In all compounds, except in YFe₂, all measured quantities show large variations near the magnetic phase transition temperature (T_c). The variation in the recoil free fraction and in its anisotropy and the variation in the electric field gradient are all consistent, quantitatively, with anisotropic softening in the vibration modes of the Fe nucleus near T_c In RFe₂ the modes in which the iron vibrates along the local C_3h symmetry axes are softened less than the perpendicular modes. In RCo₂ it is the other way around. It seems, that these critical softening phenomena are due to the strong magnon-phonon coupling present in these materials and absent in YFe₂. This conclusion is confirmed by a theoretical analysis given in an adjacent paper. In it we use the Green function method to calculate the average of the square of vibrational amplitudes in a Debye solid with a strong magnon-phonon coupling at temperatures close to the magnetic phase transition. We show that these amplitudes may increase or decrease near T_c depending on the details of the coupled system.     

Ab initio calculations of magnetic properties of the interstitially doped YFe<Subscript>11</Subscript>Mo compound

The recent increase in the number of studies of RFe_11–xM_x compounds is related to their promising application as permanent magnets. However, the insufficiently high value of the Curie temperature T_C of these compounds is a barrier to their widespread use. The increase in the Curie temperature of these compounds is achieved by doping with the light nonmetallic atoms such as hydrogen, nitrogen, and carbon. In this paper, it is shown numerically that this doping leads to drastic changes of the electronic band dispersions in a wide energy region around the Fermi level. This in turn changes values of the magnetic moments of ions and Heisenberg exchange interaction parameters. The values of ab initio calculated magnetic moments and direct exchange interaction parameters make it possible to calculate the Curie temperatures for both parent and nitrogen-doped compounds within the mean-field approach to the Heisenberg model in the sample of YFe₁₁Mo, a typical representative of the R(Fe,M)₁₂L class. Theoretical values of T_C obtained for YFe₁₁Mo and YFe₁₁MoN (514 and 723 K respectively) are consistent with experimental ones (472 and 664 K) with an accuracy of 10%. Also, the calculated increase in T_C upon nitrogenization (about 200 K) is in good agreement with the experimental data.     

Magnetic properties of the ternary hydrides of Nd6Mn23 and Sm6Mn23

The intermetallic compounds Nd₆Mn₂₃ and Sm₆Mn₂₃ and their ternary hydrides were studied by X-ray diffraction. The crystal structure (Th₆Mn₂₃ type) is preserved after hydrogen absorption. From the change in lattice constants a volume increase of about 14% was deduced. The temperature dependence of the magnetization of Nd₆Mn₂₃, Sm₆Mn₂₃ and their hydrides was studied in the range 4–500 K. The uncharged compounds have magnetic ordering temperatures above 400 K while in the hydrides magnetic ordering occurs close to 200 K. Indications were obtained of a substantial weakening of the magnetic coupling between the rare earth and manganese sublattice magnetization.     

Effect of hydrogenation on the magnetic and magnetoelastic properties of R 2Fe14B compounds (R = Nd, Gd, Er, Lu)

The temperature dependences of the magnetization σ(T), magnetostriction λ(T), and linear thermal expansion coefficient α(T) of R ₂Fe₁₄B intermetallic compounds (R = Nd, Gd, Er, Lu) and of their hydrides R ₂Fe₁₄BH_2.5 are studied. The magnetization was measured with a pendulum magnetometer within the temperature interval 77–700 K in a magnetic field H = 500 Oe. Magnetostriction and thermal expansion were measured using the tensometric technique in the temperature interval 77–420 K. It was established that Gd₂Fe₁₄BH_2.5 undergoes a spin-reorientational (SR) transition at T _SR = 235 K. In compounds with Nd and Er, anomalies associated with the SR transition were found in the σ(T), λ(T), and α(T) curves. The SR transition temperatures were refined and magnetic phase diagrams were constructed for the compounds studied. The α(T) curves of the R ₂Fe₁₄BH_2.5 hydrides (R = Nd, Er) revealed anomalies of a nonmagnetic origin associated with hydrogen ordering in the crystal lattice of these compounds.     

Thermal expansion of ZrFe2 and some rare-earth iron compounds

The thermal expansion coefficient of the compounds ZrFe₂, TiFe₂, YFe₂, ErFe₂, ErFe₃, Er₆Fe₂₃ and Er₂Fe₁₇ has been determined from room temperature up to 700°C. The anomalous thermal expansion of some of these compounds observed at temperatures below their Curie temperature (T_c) is shown to be related to a strong pressure dependence of T_c. (dT_c/dP is large and negative).     

Sign reversal of the magnetoplastic effect in C60 single crystals during the sc-fcc phase transition

It is found that the magnetoplastic effect in C₆₀ single crystals in a pulsed magnetic field with induction larger than 10 T changes its sign in the vicinity of the phase transition at T _c =250–260 K: crystal strengthening is observed for T<T _c , and softening occurs for T>T _c . This indicates a change in the crystal lattice structure in the magnetic field.     

Synthesis of superconducting compounds by thermolysis of volatile hydrides and organometallic compounds on glowing wires

The synthesis of high temperature superconducting phases in the NbGe, NbSn, VSi, VGe, VSn, NbC and MoC systems is described by method consisting in the thermolysis of volatile hydrides or organometallic compounds on resistively heated wires. For face-centred cubic NbC a higher transition temperature than previously reported was obtained. The A15 phase boundary of NbGe is extended towards the stoichiometric 3:1 composition, affording samples of Nb₃ Ge with a T_c onset of 15.8°K.     

Creation of ferromagnetic properties of V–Fe and Zr–Fe alloys by hydrogen absorption

The results of investigations of V_1?y Fe _y H _x and Zr_1?y Fe _y H _x alloys by ⁵⁷Fe Mössbauer spectroscopy are presented and discussed in view of hydrogen ability to create ferromagnetic properties of the alloy. The results indicate two different possibilities of hydrogen influence on the hyperfine magnetic field. Hydrogen absorption causes the ferromagnetic behaviour of the alloys at significant lower iron concentration compared to the concentration of magnetic transition in binary alloys. The main reason for such behaviour is the anisotropic lattice expansion in hydrogenated V–Fe and Zr–Fe alloys as well as the decomposition of paramagnetic Zr-rich intermetallic compounds in the aftermath of the strong electron affinity of hydrogen for zirconium. These trends give rise to growth of magnetic clusters of Fe atoms so strong that they can participate in the overall magnetic properties of the system under investigation.     

Cluster Method Calculation of the Curie Temperature and Exchange Parameters for the Magnetocaloric Compounds MnFeAs x P1 − x (0.25 ≤ x ≤ 0.65) and Hexagonal MnFeAs

A wealth of experimental and theoretical data on the crystallographic and magnetic properties of the magnetocaloric compounds MnFeAs _x P_1???x (0.25?≤?x?≤?0.65) and MnFeAs has become available in the last decade. By analyzing the data and treating the spin interactions with Callen’s cluster expansion method, we extrapolate first-principle results for the exchange-coupling constants of MnFeAs to the P-substituted compounds and find Curie temperatures that agree, within 5 % deviation, with experiment. Simulations with different coupling parameters show that T _c is weakly dependent on the Fe–Fe interactions. Analysis of lattice expansion as a function of composition shows that changes in the lattice parameters a and c have opposite effects upon the strength of the magnetic interactions between ions. The results indicate that the cluster expansion method provides reliable estimates of magnetic properties, even for metallic compounds characterized by multiple interactions among ions with distinct magnetic moments.     

Magnetoresistance anomalies at the Curie point of some FeCr spinels

Magnetoresistance measurements on three sintered FeCr spinels up to H = 14.5 koe show clear peaks at the Curie points T_c. It is suggested that this effect is associated with Jahn-Teller lattice distortions and possibly with a non Néel spin configuration leading to large spin disorder at T_c.     

Spontaneous magnetostriction in R2Fe14B (R=Y,Nd, Gd,Tb, Er)

Thermal expansion anomalies of R₂Fe₁₄B (R=Y, Nd, Gd, Tb, Er) stoichiometric compounds were studied by X-ray diffraction with high-energy synchrotron radiation using a Debye–Scherrer geometry in temperature range of ∼10–1000 K. A large invar effect with a corresponding large temperature dependence of lattice parameters ∼10–15 K above their Curie temperatures (T_c) are observed. The a-axes show a larger invar effect than the c-axes in all compounds. The spontaneous magnetostrain of the lattices and bonds are calculated. The iron sublattice is shown to dominate the volumetric spontaneous magnetostriction of the compounds and the contribution from the rare-earth sublattice is roughly proportional to the spin magnetic moment of the rare earths. The bond-length changes are consistent with the theoretical spin-density calculation. The average bonds magnetostrain around Fe sites is approximately proportional to their magnetic moments.     

Note on the magnetic properties of Y-Mn compounds and their hydrides

The lattice constants and the magnetic properties of YMn₂ and Y₆Mn₂₃ were studied before and after hydrogen absorption. The compound YMn₂ is an exchange-enhanced Pauli paramagnet. Upon hydrogen absorption it becomes a magnetically ordered compound with an ordering temperature of 285 K. Conversely, the compound Y₆Mn₂₃ is a ferromagnet (T_c=498), in which hydrogen absorption leads to a loss of magnetic moment of the Mn atoms.     

Microwave absorption in 2HNbSe2 and critical fluctuations above Tc

Large three-dimensional critical fluctuations above T_c have been observed in 2HNbSe₂ compounds by microwave absorption.     

Influence of hydrogen absorption on magnetic properties of Zr(Fe1−xVx)2 ternaries

Measurements of magnetization, susceptibility and Mössbauer effect were made on Zr(Fe_1?xV_x)₂ ternaries and their hydrides. Absorbed hydrogen leads to a large increase (20–30%) in volume without a change in the crystal structure. Ferromagnetism in the Fe-rich region is enhanced by hydrogen absorption, whereas hydrogenation leads to suppression of superconductivity in the V-rich range. The Fe moments in the Zr(Fe_1?xV_x)₂ hydrides are remarkably larger than those in the corresponding host compounds. The Fe moment in the β-ZrFe₂ hydride extrapolated reaches to 2.9μ_B, which exceeds the saturation value in bcc Fe. The hyperfine fields of ⁵⁷Fe in both Zr(Fe_0.8V_0.2)₂ and the hydride distribute widely, indicating that the Fe moments are very sensitive to the local environment arround the Fe atoms. Arguments are presented that it is possible to interprete the Fe moment increase by hydrogenation in terms of a decrease in occupancy of the 3d-band state due to electron transfers from Fe to hydrogen and/or vanadium.     

Magnetic properties of titanium vanadium hydrides

Measurements have been made of the magnetic susceptibility X in ternary titanium-vanadium hydrides Ti_1?yV_yH_x in the range 0<y< 0.5 and with a hydrogen to metal ratio from 1.6 to 1.92; they cover the temperature interval from 80 to 440 K. The x^?T curves of some of the ternary hydrides possess a maximum similar to that seen in binary hydrides. Its occurrence depends upon both hydrogen and vanadium concentration. Comparison with the lattice constants shows that this dependence is related to the transition from the cubic γ-phase to the tetragonal δ-phase. The x^?T curves of those samples showing the phase transition were extrapolated from the cubic phase down to lower temperatures. Conclusions were drawn from the measured and extrapolated susceptibility values, as to the influence of both hydrogen and vanadium concentrations on the electronic structure of these hydrides. The dependence of the magnetic susceptibility on the vanadium concentration shows that, in the cubic phase, the fermi energy lies on the increasing side of a maximum in the density of states curve. The degree of occupation of the conduction band does not depend on the hydrogen concentration. The rigid band model cannot be used in the hydrides studied here to account for the effects of varying their hydrogen concentration.     

Atomistic simulation for the phase stability, site preference and thermal expansion of YFe12−xTx (T=Ti, V, Cr, Mn, Zr, Nb, Mo, W)

The effect of the ternary element on the structural properties of YFe_12−xT_x (T=Ti, V, Cr, Mn, Zr, Nb, Mo, W) has been studied by using interatomic pair potentials based on ab initio method and lattice inversion equations. Calculated results show that adding ternary element T makes the crystal cohesive energy of YFe_12−xT_x decrease markedly, which indicates that T helps stabilize the 1:12 phases. The ternary elements T prefer to occupy the 8i sites in these compounds. The calculated results are found in good agreement with the values deduced from experiments. Furthermore, we have calculated the thermal expansion of YFe₁₁V compounds with the ThMn₁₂ structure. The method utilized in the present investigation offers a rather easy and direct way to study the structural properties of YFe_12−xT_x.     

Properties suggesting H3+-type clusters in some metallic hydrides

The p-c-T relationships for the hydrogen-rich phase of some metallic hydrides are calculated supposing the formation of H₃⁺ -type clusters in the metal lattice in equilibrium with the hydrogen gas. The experimental data for the hydride phase of LaNi₅, Pd, Nb (H?Nb? 0.75) and U are consistent with this model. Other properties reported in literature which seem to support this model are discussed.     

Mößbauereffektmessungen im titanreichen Eisen-Titan-System

Using the Mößbauer-effect of⁵⁷Fe the magnetic moments of iron-atoms dissolved in titanium were measured. This problem is of interest because the superconducting transition temperatureT _c increases with the Fe-concentration. The superconducting high-temperature phaseβ-Ti-Fe and the other stable and metastable phases were studied up to the intermetallic compound FeTi. It was found that in not any of this phases a localized magnetic moment exists with the Fe-atoms. Therefore the theory of Gangulyet al. for the increasingT _c is applicable. The only phase showing quadrupolinteraction isβ-Ti-Fe; the electric field gradient must be caused by a distortion of the cubic lattice. Two different chemical shifts are found inβ-Ti-Fe which cannot be understood by the phase diagram: one forβ-Ti-Fe withx≧ 8 at% Fe the other forx≦8 at% Fe. The former is equal to the value of FeTi, the latter to theω-phase. The difference is 0.10 mm/s.     

Superconductivity in zirconium-nickel glasses

The superconducting transition temperatures (T_c) and magnetic susceptibilities of amorphous Zr_100?xNi_x alloys have been measured. T_c decreases linearly with increasing x. The results are compared to those for amorphous ZrPd and ZrCu alloys and discussed in terms of changes in the electron to atom ratio on alloying.     

Effects of interstitial H and/or C atoms on the magnetic and magnetocaloric properties of La(Fe, Si)13-based compounds

La(Fe, Si)₁₃-based compounds have been considered as promising candidates for magnetic refrigerants particularly near room temperature. Herein we review recent progress particularly in the study of the effects of interstitial H and/or C atoms on the magnetic and magnetocaloric properties of La(Fe, Si)₁₃ compounds. By introducing H and/or C atoms, the Curie temperature T _C increases notably with the increase of lattice expansion which makes the Fe 3d band narrow and reduces the overlap of the Fe 3d wave functions. The first-order itinerant-electron metamagnetic transition is conserved and the MCE still remains high after hydrogen absorption. In contrast, the characteristic of magnetic transition varies from first-order to second-order with the increase of C concentration, which leads to remarkable reduction of thermal and magnetic hysteresis. In addition, the introduction of interstitial C atoms promotes the formation of NaZn₁₃-type (1:13) phase in La(Fe, Si)₁₃ compounds, and thus reducing the annealing time significantly from 40 days for LaFe_11.7Si_1.3 to a week for LaFe_11.7Si_1.3C_0.2. The pre-occupied interstitial C atoms may depress the rate of hydrogen absorption and release, which is favorable to the accurate control of hydrogen content. It is found that the reduction of particle size would greatly depress the hysteresis loss and improve the hydrogenation process. By the incorporation of both H and C atoms, large MCE without hysteresis loss can be obtained in La(Fe, Si)₁₃ compounds around room temperature, for instance, La_0.7Pr_0.3Fe_11.5Si_1.5C_0.2H_1.2 exhibits a large |ΔS _M| of 22.1 J/(kg·K) at T _C = 321 K without hysteresis loss for a field change of 0–5 T.