Itinerant Electron Model Research Articles

Study of magnetic ordering in the perovskite manganites Pr0.6Sr0.4CrxMn1-xO3

Powder samples of the ABO3 perovskite manganites Pr0.6Sr0.4CrxMn1-xO3 (0.00≤x≤0.30) were synthesized using the sol-gel method. X-ray diffraction analyses showed that all the samples had a single-phase orthorhombic structure. By analyzing magnetic parameters on the basis of the O2p itinerant electron model, we found that there are two magnetic transition temperatures, TCM and TCP, corresponding to changes in the magnetic ordering for the Mn and Pr cations, respectively. The magnetic moments of Mn3+ and Cr3+ cations within the B sublattice show canted ferromagnetic coupling, and the magnetic moments of the Pr cations within the A sublattice also show canted ferromagnetic coupling. However, the total magnetic moment of the A sublattice shows antiferromagnetic coupling against that of the B sublattice. The assumption of the canted ferromagnetic coupling within the B sublattice was confirmed using magnetoresistance experimental results.

Partial breakdown of quantum thermalization in a Hubbard-like model

We study the possible breakdown of quantum thermalization in a model of itinerant electrons on a one-dimensional chain without disorder, with both spin and charge degrees of freedom. The eigenstates of this model exhibit peculiar properties in the entanglement entropy, the apparent scaling of which is modified from a "volume law" to an "area law" after performing a partial, site-wise measurement on the system. These properties and others suggest that this model realizes a new, non-thermal phase of matter, known as a quantum disentangled liquid (QDL). The putative existence of this phase has striking implications for the foundations of quantum statistical mechanics.

Dependences of valence electronic structure on magnetic moment and electrical resistivity of metals

Conventionally, the energy band theory is used to explain the magnetic and electrical transport properties of metals. However, so far, there has been no quantitative explanation of the relations between the average magnetic moment per atom and the resistivity for Fe, nor Ni, nor Co metals. In this paper, a new itinerant electron model for magnetic metal is proposed on the basis of electron distribution theory at the energy level. 1) In the process of free atoms forming the metal solid, most of the 4s electrons of Fe, Ni and Co enter into the 3d orbits subjected to the Pauli repulsive force, and the remaining 4s electrons form free electrons. 2) Since the average number of 3d electrons is not an integer, a part of atoms have one 3d electron more than the other atoms. These excess 3d electrons have a certain probability to itinerate between the 3d orbits of the adjacent atoms as itinerant electrons; and the other 3d electrons are local electrons. 3) The transition probability of itinerant electrons is very low, thus the contribution to metal resistivity from itinerant electrons is far lower than that from free electrons. Resistivity of metal decreases with increasing the number of free electrons. Therefore, using the observed values of average atomic magnetic moments, 2.22, 0.62 and 1.72 B, the average numbers of free electrons in Fe, Ni and Co can be calculated to be 0.22, 0.62 and 0.72, respectively. This is the reason why the electrical resistivities of Fe, Ni and Co (8.6, 6.14 and 5.57 -cm) decease successively. In addition, according to this model, the average number of 3d electrons per atom in Ni metal is 9.38. This indicates that 38% of atoms in Ni metal have ten 3d electrons, forming a full 3d sub-shell, as in Cu or Zn atoms. The 3d electrons in these atoms are difficult to itinerate or exchange. This may be the reason why the Curie temperature of Ni metal (631 K) is far lower than those of Fe and Co metals (1043 and 1404 K). On the basis of the energy band theory, the numbers of 3d electrons in Fe, Ni and Co metals are 7.4, 9.4 and 8.3, which are close to our results (7.78, 9.38 and 8.28), respectively. This indicates that our model is consistent with the energy band theory. Compared with the complex energy band theory, a simple and effective method on investigating valence electron structures through the experimental average magnetic moments per atom in a metal is presented based on our model. Therefore, the new itinerant electron model may be a new clue to understanding the electronic structure of metals and alloys.

Molecular field origin for magnetic ordering of magnetic materials

In 1907, Weiss proposed that there is a molecular field to explain the magnetic ordering of magnetic materials. However, it has not been clarified where the molecular field comes from so far. In recent decades, the magnetic ordering of metals and alloys were explained by using the direct exchange interaction of between electrons on neighboring atoms, while magnetic ordering of oxides were explained by using the super exchange interaction and double exchange interaction models. The intrinsic relation between those exchange interactions has not been well explained. This resulted in the fact that there are many puzzles for magnetic ordering of the magnetic materials. For example, what role the Cr cations play in spinel ferrite CrFe2O4; why the calculated molecular magnetic moment (3.85B) for La0.85Sr0.15MnO3 by using double exchange interaction model is lower than its experimental value (4.20B); whether there is a relation between the average atom magnetic moment and their electrical resistivity for each of Fe, Co and Ni metals. These several puzzles have been explained recently by our group through using an O 2p itinerant electron model for magnetic oxides and a new itinerant electron model for magnetic metals. In this paper, a model for the molecular field origin is proposed. There are three states for the electrons rotating with high speed at the outer orbits of two adjacent ions of magnetic oxides or metals and alloys. 1) There is a probability with which form the electron pairs with opposite spin directions and a certain life time, named Weiss electron pairs (WEP); the static magnetic attraction energy between two electrons of WEP is the elementary origin of Weiss molecular field. 2) There is a probability with which two electrons with the same spin direction exchange mutually. 3) If there are two electrons at the outer orbit of an ion, then for its adjacent ion whose orbit has only one electron, the excess electron will itinerates between the ions. Furthermore, the energy equation of WEP, equilibrium distance, re0, and maximum distance, rem, between electrons of WEP are derived. The probability with which WEP forms in each of several perovskite manganites is investigated. For perovskite manganites La0.8Ca0.2MnO3, La0.75Ca0.25MnO3, La0.70Sr0.30MnO3, the crystal cell constants increase linearly with temperature when the temperature is much lower than the Curie temperature, TC, while they show a rapid increase nonlinearly near TC. We then calculate the difference in MnO bond length at TC between the linear and the nonlinear variation, △dobs. Obviously, when the distance between the two electrons of WEP, re, is larger than the rem, WEP and the magnetic ordering energy both disappear. Assuming △dobs=rem-re0, the probabilities with which WEP appears in La0.8Ca0.2MnO3, La0.75Ca.25MnO3, La0.70Sr0.30MnO3, are calculated to be 0.07%, 0.31% and 3.13%, respectively. These results indicate that the WEP model for the magnetic ordering energy is qualitatively reasonable.

Study of cation distributions in spinel ferrites M xMn1-xFe2O4 (M=Zn, Mg, Al)

Powder samples of the ferrites MxMn1-xFe2O4 (M = Zn, Mg, Al) were prepared using a chemical co-precipitation method. X-ray diffraction analysis showed that the three series of samples had a single-phase cubic spinel structure and that there was a decrease in the lattice parameters with increasing x. There were different dependences on the doping level x of the magnetic moments (μexp) for the three series of samples measured at 10 K. We found a non-monotonic behavior for μexp as a function x for the Zn doped samples with a maximum at x = 0.4, while μexp decreased monotonically with increasing x for the Mg and Al doped samples. On the basis of the O2p itinerant electron model, the magnetic moment direction of the Mn3+ cations is expected to be antiparallel to those of the Mn2+ and Fe cations in these samples. With this assumption, the curves of μexp versus x for the three series of samples were fitted using a quantum-mechanical potential barrier model earlier proposed by our group, and the cation distributions were obtained.

Stability of a spin-triplet nematic state near to a quantum critical point

We analyze a model of itinerant electrons interacting through a quadrupole density-density repulsion in three dimensions. At the mean field level, the interaction drives a continuous Pomeranchuk instability towards $d$-wave, spin-triplet nematic order, which simultaneously breaks the SU(2) spin-rotation and spatial rotational symmetries. This order results in spin antisymmetric, elliptical deformations of the Fermi surfaces of up and down spins. We show that the effects of quantum fluctuations are similar to those in metallic ferromagnets, rendering the nematic transition first-order at low temperatures. Using the fermionic quantum order-by-disorder approach to self-consistently calculate fluctuations around possible modulated states, we show that the first-order transition is pre-empted by the formation of a nematic state that is intertwined with a helical modulation in spin space. Such a state is closely related to $d$-wave bond density wave order in square-lattice systems. Moreover, we show that it may coexist with a modulated, $p$-wave superconducting state.

Relation between oxygen stoichiometry and thermodynamic properties and the electronic structure of nonstoichiometric perovskite La0.6Sr0.4CoO3-δ.

Continuous phase diagram 3 - δ - log pO2 - T of the nonstoichiometric perovskite La0.6Sr0.4CoO3-δ was obtained in a gas flow reactor by means of the quasi-equilibrium oxygen release technique. The thermodynamic properties of oxides were determined as a function of oxygen nonstoichiometry. Within the framework of the itinerant electron model, the dependence of the oxide nonstoichiometry on the oxygen activity was related to the density of electronic states near the Fermi level.

Influence of thermal treatment on the ionic valence and the magnetic structure of perovskite manganites La0.95Sr0.05MnO3

In traditional views, the magnetic ordering of oxides may be explained using magnetic superexchange (SE) or double exchange (DE) interaction models. Both models are based on an assumption that the valences of all oxygen ions be -2. For example, both La and Mn in LaMnO3 are assumed to be trivalent, in which antiferromagnetic spin structure is explained using the SE interaction between Mn3+ cations mediated by oxygen anions. In La1-xSrxMnO3, there exists a part of Mn4+ cations with the content ratio of Mn4+/Mn3+ being x/(1-x), in which spin structure and electronic transport properties are explained by DE interaction. However, there is a part of monovalent oxygen ions existing in oxides. Cohen [Nature 358 136] has calculated the densities of states for valence electrons in the perovskite oxide BaTiO3 using density functional theory. Results indicate that the average valence of Ba is +2, being the same as that in the traditional one, but the average valences of Ti and O are +2.89 and -1.63 respectively, agreeing with the results obtained using ionicity investigation [Rev. Mod. Phys. 42 317] and X-ray photoelectron spectra (XPS) analysis, but different from the conventional results +4 and -2. In this paper, three samples with the nominal composition La0.95Sr0.05MnO3 are prepared by different thermal-treatments. Likewise, there are only Mn2+ and Mn3+ cations, but no Mn4+ cations in La0.95Sr0.05MnO3, a result obtained by XPS analysis, and the average valence of Mn in La0.95Sr0.05MnO3 samples increases with increaseing thermal-treatment. Although the crystal structures of the samples are the same, the magnetic moments per formula are obviously different. This magnetic structure cannot be explained using the conventional SE and DE interaction models. Using the O 2p itinerant electron model for spinel ferrites proposed recently by our group, we can explain this magnetic structure. The variation trend of the average valences of Mn cations calculated using the magnetic moments per formula of the samples at 10 K, is in accordance with the experiment results of XPS. The O 2p itinerant electron model is based on an assumption that there is a part of monovalent oxygen ions in the oxides, which is the fundamental difference from SE and DE interaction models.

Quantum Phases of a Deformed Itinerant Electron Model in One Dimension

Quantum Phases of a Deformed Itinerant Electron Model in One Dimension

Interplay of magnetic order, pairing, and phase separation in a one-dimensional spin-fermion model

We consider a lattice model of itinerant electrons coupled to an array of localized classical Heisenberg spins. The nature of the ground state ordered magnetic phases that result from the indirect spin-spin coupling mediated by the electrons is determined as a function of density and the spin-fermion coupling $J$. At a fixed chemical potential, spiral phases exist only up to values of $J$ which are less than roughly half the electronic bandwidth. At a fixed electron density and near half filling, the system phase separates into a half-filled antiferromagnetic phase and a spiral phase. The ferromagnetic phases are shown to be fully polarized, while the spiral phases have equal admixture of up and down spins. Phase separation survives in the presence of weak pairing field $\Delta$ but disappears when $\Delta$ exceeds a critical value $\Delta_c$. If pairing fields are large enough, an additional spiral state arises at strong coupling $J$. The relevance of this study, especially the phase separation, to artificially engineered systems of adjacent itinerant electrons and localized spins is discussed. In particular, we propose a method which might allow for the braiding of Majorana fermions by changing the density and moving their location as they are pulled along by a phase separation boundary.

Antiferromagnetic coupling between Mn3+and Mn2+cations in Mn‐doped spinel ferrites

Ferrite powder samples of MnxNi1–xFe2O4(0.0 ≤ x ≤ 1.0) with single phase (A)[B]2O4spinel structure were prepared using a sol–gel method. Following the successful proposal by our group, that the magnetic moment directions of Cr2+(3d4) cations are antiparallel to those of Fe3+(3d5) and Fe2+(3d6) cations in a given sublattice of the Cr‐doped spinel ferrites due to the constraints imposed by Hund's rules, we extend here the same idea and assume that the magnetic moment directions of Mn3+(3d4) cations are also antiparallel to those of Mn2+(3d5) and divalent and trivalent Fe (Ni) cations in a given sublattice of Mn‐doped spinel ferrites. We have thereby obtained cation distributions for the samples by fitting the magnetic moments of the samples at 10 K. The results indicate that 72% of the Mn cations occupy the [B] sites in MnFe2O4, which is close to the results for Ni (82%) in NiFe2O4, but is different from the result obtained using the conventional view which yields 80% of the Mn cations in the (A) sites of MnFe2O4. On the basis of the present analyses of the magnetic structure of MnxNi1–xFe2O4(0.0 ≤ x ≤ 1.0), we propose here a new model for spinel ferrites that is distinctly different from both the super‐ and the double‐exchange model and that we refer to as the O2p itinerant electron model. Using this model, not only can the magnetic structure of the spinel ferritesMFe2O4(M = Fe, Co, Ni, and Cu) be explained better than by using the super‐ and double‐exchange interaction models, but also the magnetic structure and the cation distributions of Cr‐, Mn‐, and Ti‐doped spinel ferrites can be explained.

Electronic structure, spin excitations, and orbital ordering in a three-orbital model for iron pnictides

A three-orbital itinerant-electron model involving dxz, dyz and dxy Fe 3d orbitals is proposed for iron pnictides towards understanding the () ordered magnetism and magnetic excitations in these materials. It is shown that this model at half filling yields a gapped () magnetic state, and simultaneously reproduces several experimentally observed features such as the electronic structure, spin excitations, as well as the ferro orbital order between the dxz and dyz orbitals.

The role of orbital order in the stabilization of the (π, 0) ordered magnetic state in a minimal two-band model for iron pnictides

Spin wave excitations and stability of the (π, 0) ordered magnetic state are investigated in a minimal two-band itinerant-electron model for iron pnictides. Presence of hopping anisotropy generates a strong ferro-orbital order in the dxz and dyz Fe orbitals in the (π, 0) state. The orbital order sign is as observed in experiments. The induced ferro-orbital order strongly enhances the spin wave energy scale and stabilizes the magnetic state by optimizing the strength of the emergent antiferromagnetically and ferromagnetically spin couplings through optimal band fillings in the two orbitals. The calculated spin-wave dispersion is in quantitative agreement with neutron scattering measurements. Finite inter-orbital Hund's coupling is shown to further enhance the spin wave energies state by coupling the two magnetic sub-systems. A more realistic two-band model with less hopping anisotropy is also considered, which yields not only the circular hole pockets but also correct ferro-orbital order and emergent F spin coupling.

Pressure dependence of magnetic properties in Fe–Mn–B amorphous alloys: evidence for inhomogeneous ferromagnetism

The pressure dependence of the saturation magnetization and Curie temperature was studied in melt-spun Fe60Mn20B20, Fe56Mn24B20 and Fe75B25 amorphous alloys up to 0.9 GPa, corresponding to volume changes up to 0.45%. In addition, in situ high-pressure (up to 40 GPa) x-ray diffraction was performed to determine the compressibility of the latter two alloys. Both the Curie temperature TC (at atmospheric pressure TC = 201 ± 3 and 159 ± 3 K) and the low-temperature saturation magnetization M5 K,5 T decrease remarkably with increasing pressure: dTC/dp =− 31 ± 0.5 and −32 ± 5 K GPa−1 and dlnM5 K,5 T/dp =− 0.15 ± 0.02 and −0.13 ± 0.03 GPa−1 for xMn = 20 and 24 at.%, respectively. Compared to dlnM5 K,5 T/dp =− 0.016 ± 0.003 GPa−1 measured for Fe75B25, the pressure dependence of M5 K,5 T is one order of magnitude larger in the ternary alloys. The bulk moduli for the Fe56Mn24B20 and Fe75B25 glasses were measured to be 152 GPa and 173 GPa, respectively. These data are also compared with the pressure dependence of the hyperfine field and theoretical calculations of the saturation moment for Fe–B alloys reported in the literature. The results were interpreted within an inhomogeneous itinerant-electron model of ferromagnetism.

Nonmonotonic inelastic tunneling spectra due to surface spin excitations in ferromagnetic junctions

The paper addresses inelastic spin-flip tunneling accompanied by surface spin excitations (magnons) in ferromagnetic junctions. The inelastic tunneling current is proportional to the magnon density of states which is energy-independent for the surface waves and, for this reason, cannot account for the bias-voltage dependence of the observed inelastic tunneling spectra. This paper shows that the bias-voltage dependence of the tunneling spectra can arise from the tunneling matrix elements of the electron-magnon interaction. These matrix elements are derived from the Coulomb exchange interaction using the itinerant-electron model of magnon-assisted tunneling. The results for the inelastic tunneling spectra, based on the nonequilibrium Green's function calculations, are presented for both parallel and antiparallel magnetizations in the ferromagnetic leads.

Origin of the Frequency Branch ε of the de Haas–van Alphen Effect in UAl3

On the basis of the itinerant 4 f electron model, the electronic structure is calculated for CeIr 2 by a relativistic linear augmented-plane-wave method. It is found in common that the energy bands in the vicinity of the Fermi level are mainly due to the hybridization between Ce 4 f and Ir 5 d electrons. An itinerant electron model assures that this compound is compensated metal. The 21st, 22nd, and 23rd bands form the Fermi surface. The Fermi surface produces many de Haas–van Alphen frequencies in the wide frequency range between 1.0 and 200 MOe.

Defect structure, electronic conductivity and expansion of properties of [formula omitted

This study reports on oxygen nonstoichiometry, electronic conductivity and lattice expansion of three compositions as function of T and P O 2 in the ( La 1 − x Sr x ) s Co 1 − y Ni y O 3 − δ ( x=0.1, y=0.4; x=0.1, y=0.3; x=0.2, y=0.2) materials system. The nonstoichiometry data were successfully fitted using the itinerant electron model which indicates the existence of delocalized electronic states. This was also reflected in the high electronic conductivities, above 1000 S cm −1, measured for all three compositions. The electronic conductivity was shown to decrease linearly with the oxygen nonstoichiometry parameter, δ , supporting that the conductivity is dependent on p-type charge carriers. Comparing calculated p-type mobilities with data reported in literature on La 1−xSr x CoO 3 indicated that Ni-substitution into ( La 1 − x Sr x ) s CoO 3 − δ increases the p-type mobility. The electronic conductivity was also found to be dependent on intrinsic charge related to spin excitations and Ni substitution rather than the p-type charge. A conductivity mechanism is hypothesized including a metallic like conductivity of the p-type charge and a small polaron conductivity of the intrinsic charge. Lattice expansion as function of T and δ was successfully described using first and second order thermal and chemical expansion coefficients. Substituting 10% Co with Ni in ( La 0.6 Sr 0.4 ) 0.99 CoO 3 − δ was found to decrease the apparent thermal expansion with about 25%.

Quantum Melting of Charge Ice and Non-Fermi-Liquid Behavior: An Exact Solution for the Extended Falicov-Kimball Model in the Ice-Rule Limit

An exact solution is obtained for a model of itinerant electrons coupled to ice-rule variables on the tetrahedron Husimi cactus, an analogue of the Bethe lattice of corner-sharing tetrahedra. It reveals a quantum critical point with the emergence of non-Fermi-liquid behavior in melting of the "charge ice" insulator. The electronic structure is compared with the numerical results for the pyrochlore-lattice model to elucidate the physics of electron systems interacting with the tetrahedron ice rule.

Magnon softening in a ferromagnetic monolayer: A first-principles spin dynamics study

We study the Fe/W(110) monolayer system through a combination of first principles calculations and atomistic spin dynamics simulations. We focus on the dispersion of the spin waves parallel to the [001] direction. Our results compare favorably with the experimental data of Prokop et al. [Phys. Rev. Lett. 102, 177206], and correctly capture a drastic softening of the magnon spectrum, with respect to bulk bcc Fe. The suggested shortcoming of the itinerant electron model, in particular that given by density functional theory, is refuted. We also demonstrate that finite temperature effects are significant, and that atomistic spin dynamics simulations represent a powerful tool with which to include these.

Effect of thermal fluctuations of spin density wave order parameter on the quasiparticle spectral function

The two-dimensional model of itinerant electrons coupled to an anti-ferromagnetic order parameter is considered. In the mean field solution the Fermi surface undergoes reconstruction, and breaks into disconnected ``pockets''. We have studied the effect of the thermal fluctuations of the order parameter on the spectral density in such system. These fluctuations lead to a finite width of the spectral line scaling linearly with temperature. Due to the thermal fluctuations the quasi-particle spectral weight is transfered into a magnetic Brillouin zone. This can be interpreted as restoration of ``arcs'' of the non-interacting Fermi surface.