Itinerant Electron Ferromagnets Research Articles

The roles of spin wave and phonon in the temperature dependence of magnetization in an itinerant electron ferromagnet

We show that since the phonon frequency changes with magnetization in an itinerant electron ferromagnet, phonons can have an effect on magnetization that is similar to that of spin waves. From a model numerical analysis we find that the size of the phonon effect is generally of the same order of magnitude as that of the spin wave.

Phonon mechanism of the anomalous volume behavior of ferromagnetic metals

We explore the role of phonons in the volume behavior of an itinerant electron ferromagnet by taking into account the close relation between phonon and magnetic properties. From a model numerical calculation we find that phonon contribution to thermal expansion can be negative in the ferromagnetic state of a metal.

Magnetic-field effect on sound propagation as a probe of electronic structure and electron interaction in an itinerant-electron ferromagnet.

We carry out a systematic formulation and a model numerical calculation on the effect of a magnetic field on the sound velocity and attenuation of a ferromagnetic metal based on the Stoner model. We find the magnetic-field effect to be strongly exchange enhanced and to depend very sensitively on the details of the electronic structure, both above and below the Curie point ${\mathit{T}}_{\mathit{C}}$. According to our result, for T>${\mathit{T}}_{\mathit{C}}$ the magnetic-field effects on the sound velocity and attenuation are proportional to, respectively, ${\mathrm{\ensuremath{\chi}}}^{3}$ and ${\mathrm{\ensuremath{\chi}}}^{4}$, where \ensuremath{\chi} is the (Stoner) magnetic susceptibility, with coefficients very sensitively reflecting the electronic structure near the Fermi surface. We also obtain similar features of the magnetic-field effects for T${\mathit{T}}_{\mathit{C}}$; the magnetic-field effects reflect very sensitively the electronic structure and electron interaction of a ferromagnetic metal. The mechanism of such magnetic effect on sound is that the screening of the ion-ion interaction is affected by the (additional) spin splitting of the conduction electron bands induced by a magnetic field. These findings suggest that the magnetic-field effects on sound propagation can be a useful probe of the electronic structure and electron interaction of a metal, ferromagnetic one, in particular.

The role of phonons versus spin waves in the temperature dependence of magnetization in an itinerant electron ferromagnet

We show that since phonon frequencies change with magnetization in an itinerant ferromagnet, as do the spin waves, phonons can also contribute to the temperature dependence of magnetization. From a model calculation we show that such a contribution of phonons can be as large as that of spin waves.

A new theory of the Invar behavior considering the role of phonon

Abstract By treating the roles of electrons and phonons on the same footing, we numerically calculate thermal expansion coefficient of an itinerant electron ferromagnet for both T T c and T > T c with a simple model of the electronic density of 1758 0727 states, and find thermal phonons to play a crucial role in the Invar behavior.

Intrinsic Properties of Doped Lanthanum Manganite

AbstractAn investigation designed to display the intrinsic properties of perovskite manganites was accomplished by comparing the behavior of bulk samples with that of thin films; the results show the "colossal magneto resistance" at very low temperatures is not an intrinsic property of the thermodynamically stable 1/3 doped material. Epitaxial 1500 Å films of perovskite La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3 were grown by solid source chemical vapor deposition on LaA1O3 and post annealed in oxygen at 950°C. Crystals were prepared by LASER heated pedestal growth. The magnetic and electrical transport properties are essentially the same. Below Tc/2 the intrinsic magnetization decreases as T2 (as can be expected for itinerant electron ferromagnets) while the intrinsic resistivity increases proportional to T2. The constant and T2 coefficients of the resistivity are largely independent of magnetic field and alkaline earth element (Ca, Sr or Ba). We identify three distinct types of negative magnetoresistance. The largest effect is observed near the Curie temperature and is likely to be due to magnetic critical scattering. There is also magnetoresistance associated with the net magnetization of polycrystalline samples. The high temperature (above Tc) resistivity of La0.67Ca0.33MnO3 is consistent with small polaron hopping conductivity with a transition at 750K, while La0.67Sr0.33MnO3 does not exhibit activated conductivity until about 500K, well above Tc. The limiting low and high temperature resistivities may place a limit on the maximum possible magnetoresistance of these materials.

The magnetic phase transition in ZrZn 2

The critical behaviour of the ferro-paramagnetic phase transformation has been analysed in the weak itinerant-electron ferromagnet, ZrZn 2. The critical exponents β, γ and γ′ describe the temperature dependence of the spontaneous polarization J S and the inverse zero-field susceptibility χ -1, whereas the exponent δ determines the field dependence of the polarization at the Curie temperature, T C. To complete the description of the critical behaviour the reduced critical amplitudes J 0/ J S(0), μ h 0/( k B T C) and DJ δ 0/ h 0 were also determined. The exponent values obtained are in good agreement with the theoretical prediction of the mean field theory, which should be valid in the case of magnetic long-range interactions. In the case of the critical amplitudes, the values cannot be correlated to a particular model.

Negative mode-mode coupling among spin fluctuations and the magneto-volume effect in an itinerant-electron ferromagnet

By making use of the Landau-Ginzburg energy expanded up to the sixth power of magnetization density, the spontaneous volume magnetostriction and dependence on pressure P of the Curie temperature Tc of an itinerant-electron ferromagnet with negative mode-mode coupling among spin fluctuations are discussed. The spontaneous moment is found to show first-order and second-order transitions at Tc, according to the values of the Landau coefficients. It is shown that the difference between the spontaneous volume magnetostrictions at T=0 and Tc is large and the P-dependence of Tc becomes anomalously large when a certain condition is satisfied by the Landau coefficients. The results obtained in the present theory could explain the anomalous T-dependence of the thermal expansion and the strong P-dependence of Tc observed in Invar alloys, some Laves phase Co compounds and others.

The effect of the electron-phonon interaction on the spin splitting of electron energy bands in an itinerant electron ferromagnet

We report our discovery that the exchange splitting of the electron energy bands in the ferromagnetic state of a metal can be affected by the effect of the electron-phonon interaction and that, whereas the sign and magnitude of such a phonon effect depend sensitively on the electronic structure around the Fermi surface of the metal, the magnitude can be comparable with that of the ordinary Stoner exchange splitting. This conclusion was obtained by examining how the phonon frequencies, and therefore the phonon free energy, change with the spin splitting of the energy bands of the screening electrons in an itinerant electron ferromagnet. By carrying out a numerical calculation with a model electronic density of states, we demonstrate the importance of considering this effect in accounting for the observed magnetization and its temperature dependence in a ferromagnetic metal.

Sound propagation and its magnetic field dependence in an itinerant electron ferromagnet

In this paper first we derive general expressions for the velocity and attenuation constant of longitudinal acoustic sound and their magnetic field dependence in an itenerant electron ferromagnet both for T<T c and T>T c. Then by carrying out numerical calculation on these quantities with a simple model electronic density of states we find them to reflect very sensitively the electronic structure and the exchange interaction between electrons of a metal. We emphasize the possibility of using such ultrasonic observation as a useful probe of electronic structure and electron interaction in a metal. Finally we point out that from these magnetic field effects on sound velocity we can infer the effect of the electron-phonon interaction on the magnetic properties of a metal.

The role of the electron-phonon interaction in the magnetization behavior of metallic ferromagnets

We show that the effect of the electron-phonon interaction in the ferromagnetic state of an itinerant electron ferromagnet results in an effective magnetic field and modifies the spin splitting of the electron energy bands. From a numerical calculation with a simple model electronic structure we find that such phonon effect can be much larger than ordinarily anticipated; the effect on the saturation magnetization can be as large as ∼1μ B/atom, and that on the temperature dependence of magnetization can significantly reduce the Curie temperature.

On the origin of itinerant electron behaviour and long-range ferromagnetic order in La1-xSrxCoO3

The itinerant electron ferromagnetism in La1-xSrxCoO3 (0.1<or=x<or=0.5) has been reinvestigated. AC magnetic susceptibility studies on these systems have been carried out for the first time. The nature of the susceptibility indicates the freezing of superparamagnetic clusters at low values of x (<0.3) and long-range ferromagnetic order at higher values of x (>or=0.3). Electrical resistivity studies show that itinerant electron behaviour with a positive temperature coefficient of resistance at low temperatures is obtained for x >or=0.2. Metallization and long-range ferromagnetic order thus seems to be associated with different origins. Chemical systematics obtained from the changes in lattice parameters show that the expansion of LaCoO3 on the substitution of La by Sr stabilizes the paramagnetic states of Co ions which are in predominantly diamagnetic low-spin state in LaCoO3. The core S=1 spins of trivalent Co seems to be coupled ferromagnetically by the itinerant electron in a manner similar to the Zener double-exchange mechanism.

Ferromagnetic properties of potassium clusters incorporated into zeolite LTA.

Ferromagnetic properties are reported for K clusters incorporated into the \ensuremath{\alpha} cage (supercage) of zeolite LTA. In LTA, \ensuremath{\alpha} cages are arrayed in a simple cubic structure with a lattice constant of 12.3 \AA{}. The average 4s-electron number per cluster, n, is systematically changed from 0.8 to 7.3 by controlling the K loading density. Ferromagnetism is observed for 2--3n6--7. The magnetization shows a peak at n\ensuremath{\sim}5, and the average magnetic moment amounts to 0.24${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ per cluster at 100 Oe and 1.7 K. The Curie temperature is n dependent and less than 7 K. Magnetic properties are basically interpreted in terms of the model of itinerant electron ferromagnetism. Narrow energy bands due to the electron transfer between adjacent clusters play an important role in the construction of a model. The magnetic properties exhibit spin-glass and reentrant spin-glass phenomena depending on the K loading density.

Disorder effects in transport properties of weak itinerant-electron ferromagnets.

The electric and thermal conductivity of weak itinerant-electron ferromagnets in the presence of spin fluctuations and disorder is calculated and the validity of the Wiedemann-Franz law in these systems is discussed. We used the Keldysh-diagram technique to obtain the expression for the conductivities of the clean system as well as the correction due to disorder. The electric conductivity shows a maximum at a low temperature, which characterizes the crossover behavior from the disorder-dominated region to the spin-fluctuation region. The thermal conductivity (κ/T) also has a maximum at the crossover temperature. The contribution due to spin fluctuations and the quantum correction due to disorder violate the Wiedemann-Franz law while the Drude term restores it as disorder increases

High-Pressure Synthesis, Crystal Structure, and Magnetic Properties of Mn1-xTxGe4 (T: Co, Fe)

Mn1-xTxGe4 (T: Co, Fe) solid solution was synthesized at 6 GPa and 650-750°C using a belt-type high-pressure apparatus. The crystal structure of the Mn1-xTxGe4 solid solution is related to the β-NiHg4 type. The following structural changes were observed in the Mn1-xCoxGe4 system: tetragonal (x = 0) to orthorhombic (0 < x < 0.2) and orthorhombic to pseudotetragonal (0.2 ≦ x ≦ 0.4). Similar structural changes were also observed at 0 < x < 0.4 (tetra. → ortho.) and x = 0.4 (ortho. → pseudotetra.) in the Mn1-xFexGe4 system. These results indicate that the structural changes in Mn1-xTxGe4 strongly depend on the average valence electron number of the transition metal atoms. Mn1-xTxGe4 solid solutions are itinerant electron ferromagnets and the Curie temperature decreases linearly with decreasing unit-cell volume.

Ferromagnetic properties of Ni-rich Ni 1− xNb x alloys

In this paper, we investigate the paramagnetic ( T ⪢ T c) and ferromagnetic ( T ⪡ T c) properties of Ni-rich Ni 1− x Nb x (crystalline and atomically disordered) fcc solid solutions with 0 < x ≤ 0.0632. We re-analysed the high-temperature paramagnetic susceptibility data reported by Arajs et al. at temperatures T significantly higher than T c, where the effects of magnetic short-range order could be neglected. From this re-analysis, we derived new values of the Curie-Weiss parameters of the Ni 1− x Nb x alloys with 0 < x < 0.0632. The low-temperature magnetization σ = σ( H, T) of the investigated alloys has been measured by the induction method in (internal) fields H up to 54 kOe, at different temperatures ranging from 4.2 to 90 K, even up to 155 K (depending on the sample). The corresponding σ( H, T) data were analysed by means of a simple expression in which we took into account a magnetization contribution due to spin waves, together with that arising from the superimposed paramagnetic high-field susceptibility. This analysis led to the following main results. (i) The decrease in σ s( T) (the ferromagnetic saturation magnetization) with T, could be represented by the Bloch law σ s( T) = σ so(1 − α T 3 2 ) only over narrow T-ranges in the limit T → 0. (ii) From the comparison of the spin-wave stiffness constant (at T ⋍ 0 K) D 0M (as derived from the Bloch coefficients α) with that D 0N (resulting from neutron scattering experiments available in the literature), it could be inferred that other excitations (for instance, Stoner single-particle excitations), in addition to the excitations of thermal spin waves, are responsible for the decrease in σ s( T) magnetization in Ni-rich Ni 1− x Nb x alloys (as has been shown by other authors for other Ni-based alloys). (iii) The decrease in the absolute ferromagnetic moment μ (in μ B per mean atom of alloy) with atomic (solute) Nb content x, is consistent with Friedel's virtual bound-state (VBS) model: the initial rate of moment decrease, (dμ / d x) 0 ≅ -5.664μ B/Nb roughly agrees with the value of -5.0μ B/solute atom predicted by the VBS model for V, Nb and Ta solutes in Ni. (iv) From the values of the Rhodes-Wohlfarth ratio, q c / q s ⋍ 1.2–1.4 ( q c / q s increases with decreasing T c), it may be concluded that the ferromagnetism of t he investigated Ni 1− x Nb x alloys is close to the localized limit of itinerant-electron ferromagnetism. (v) The relative changes y in absolute sarturation magnetization σ so of Ni 1− x Nb x alloys y = [σ so( x) - σ so(0)]/σ so(0), have been found to agr ee satisfactorily with the phenomenological expression of y recently proposed by Hunter et al., in which the parameter values were taken from the calculations of Stephanou et al.

MAGNETIZATION DEPENDENCE OF PHONON EFFECT ON VOLUME OF AN ITINERANT ELECTRON FERROMAGNET

Volume behavior is often anomalous for T&lt;Tc of an itinerant electron ferromagnet. Recently Kim pointed out that phonons also can be responsible for such anomalous behavior. Further to explore such possible role of phonons, in this paper we carry out a numerical calculation on the temperature dependences of phonon Debye temperature and Grüneisen constant of an itinerant electron ferromagnet by using a model electronic density of states. Our numerical results support our earlier conclusion and demonstrate the importance of taking into account the effect of the electron-phonon interaction in understanding the anomalous volume and elasticity behavior of an itinerant electron ferromagnet.

MAGNETIC FIELD EFFECT ON SOUND VELOCITY AND ATTENUATION IN AN ITINERANT ELECTRON FERROMAGNET

In this paper we present the result of a systematic numerical calculation on the temperature dependence of magnetic field effects on the velocity and attenuation constant of longitudinal acoustic sound for both T&lt;Tc and T&gt;Tc of an itinerant electron ferromagnet. Attributing the origin of these magnetic field effects to the change in the screening of the ion-ion interaction which is caused by a magnetic field, first we reformulate this problem in a unified way. Then we make numerical calculation with a simple model electronic density of states and see how sensitively the sign, size, and temperature variation of these magnetic field effects depend on the details of electronic structure and electron interaction. We find that these magnetic field effects can be very strongly exchange enhanced. These findings suggest the possibility of using such magnetic field effects as a probe of the electronic structure and electron interaction in an itinerant electron ferromagnet. Finally we point out that by measuring the magnetic field effect on sound velocity we can infer the effect of the electron-phonon interaction on the magnetic properties.

Magnetic and Mössbauer Study of Sputtered Amorphous Alloys Hf1-xTaxFe2

We have measured the magnetization and Mössbauer effect on vapor-quenched amorphous Hf 1- x Ta x Fe 2 alloys prepared by sputtering for x =0, 0.3, 0.5, 0.7, 0.9 and 1 from 4.2 to 300 K. We observed the spontaneous magnetization for all the present samples except for x =1, where the ferromagnetic moment and Curie temperature monotonically decrease with increasing Ta concentration. A comparison between the ferromagnetic and paramagnetic moments suggests that the alloys are an itinerant-electron ferromagnet. The reduced low-temperature magnetization falls characteristically below that of crystalline HfFe 2 and follows a T 3/2 law. From the broad Mössbauer spectrum below the Curie temperature we estimate the average hyperfine field. Ta-concentration and temperature dependence of the average hyperfine field agrees with that of the magnetization.

Preparation and characterization of stainless steel filters

The enhancement factor due to the magnetovolume coupling in the compressibility, high-field susceptibility and forced volume magnetostriction and the magnetovolume coupling constant are estimated from the experimental data for various itinerant electron ferromagnets, ZrZn2, Sc3In, Ti(Fe-Co)2 and Zr(Fe-Co)2 compounds, Ni-Al, Ni-Pd, Ni-Cu, Fe-Pt and Fe-Ni alloys, Fe, Co and Ni, and they are compared with each other. It is shown that the enhancement factor is large in Fe-Ni Invar alloys, Zr(Fe-Co)2 compounds and Fe-Pt Invar alloys at high temperatures. The spontaneous volume magnetostriction and the pressure dependence of the Curie temperature dTc/dP are calculated in the simple model and compared with the experimental results. Anomalies of the magnetovolume effects in Fe-Ni and Fe-Pt Invar alloys and the relation between dTc/dP and Tc are discussed.