Heavy-mass fermi liquid near a ferromagnetic instability in layered ruthenates

Low temperature magnetic, thermal, and transport measurements in Ca2-xSrxRuO4 clarify the appearance of a cluster glass phase, after the evolution of a nearly ferromagnetic heavy-mass Fermi liquid from the spin-triplet superconductor Sr2RuO4. As the Mott transition is approached across a 2nd-order structural transition, both the magnetization and specific heat decrease considerably while the transport scattering rate diverges. A metamagnetic transition to a highly spin polarized state, with a local moment S=1/2, is observed. We argue that an orbital rearrangement with Ca substitution changes itinerant ferromagnetism to antiferromagnetism of localized moments.     

Appearance of spontaneous ferromagnetism in non-stoichiometric ZrCo2

The cubic Laves phase compound ZrCo₂ can accommodate excess cobalt up to the composition ZrCo₃.ZrCo_x for 2 ? x ? 2.5 is an exchange-enhanced Pauli paramagnet. Ferromagnetism is observed in the system for 2.8 ? x ? 3.0 with Curie temperatures ranging up to 160 K. Ferromagnetism appears to develop by the Stoner-Edwards-Wohlfarth mechanism involving the itinerant Co 3d electrons associated with the ZrCo₂ matrix. The excess Co, thought to reside on the Zr sites, appears to carry a local moment and to experience antiferromagnetic exchange as evidenced by its negative Weiss constant. The observed magnetic moment of the ferromagnetic ZrCo_x systems is the sum of two, roughly equal, contributions, one from the local moments associated with the excess cobalt and the other from the itinerant d electrons.     

On structural instabilities in itinerant magnets and superconductors

The influence of a structural distortion on the itinerant antiferromagnetic (IAF) phase and on the superconducting (S) phase is investigated using a free electron band structure for the electron and hole pockets. For equal concentrations of electrons and holes a metastable phase is found in which the lattice distortion coexists with the IAF or S phase. For unequal concentrations of electrons and holes the critical temperature and the value of the order parameter for the IAF or S phase (in the coexistence region) will always be enhanced by the onset of the structural distortion. The theoretical predictions are compared in the IAF case with the occurrence of a spin flip transition in chromium accompanied by a tetragonal distortion, and in the S case with experimental results on A-15 compounds exhibiting a martensitic phase transition.     

Itinerant ferromagnetism in the C-15 Laves phase - TiBe2−xCux

By substituting copper for part of the beryllium in the C-15 Laves phase, TiBe₂, the phase is converted into an itinerant electron ferromagnet. The stable C-15 phase forms over the general composition range, TiBe_2?xCu_x where 0 ? x <.50 and the lattice parameter expands from an a_o=6.4532 Å for TiBe₂ to an a_o=6.554 Å for the composition TiBe_1.5Cu_.5. The Curie temperature increases with increasing copper content. Since none of the constituent elements have a local moment in their metallic state, this material joins the other two truly itinerant ferromagnets, Sc₃In and ZrZn₂.     

Complete Fermi surface in BaFe2As2 observed via Shubnikov-de Haas oscillation measurements on detwinned single crystals

We show that the Fermi surface (FS) in the antiferromagnetic phase of BaFe(2)As(2) is composed of one hole and two electron pockets, all of which are three dimensional and closed, in sharp contrast to the FS observed by angle-resolved photoemission spectroscopy. Considerations on the carrier compensation and Sommerfeld coefficient rule out existence of unobserved FS pockets of significant sizes. A standard band structure calculation reasonably accounts for the observed FS, despite the overestimated ordered moment. The mass enhancement, the ratio of the effective mass to the band mass, is 2-3.     

Nuclear orientation and nuclear structure

The purpose of this paper is to demonstrate how recent experimental results of¹⁵¹Eu and⁵⁷Fe high pressure Mössbauer studies in 4f and 3d metallic magnetic systems can contribute to a deeper understanding of the nature of local moment (4f) and itinerant (3d) magnetism in these systems. Special emphasis is given on the comparison of the experimental results with related theoretical models.     

Numerical investigation of spin waves in ferromagnetic iron

Spin wave energies and intensities have been calculated along three principal symmetry directions for ferromagnetic iron. These calculations are based on an itinerant model which incorporates band and wave-vector dependence of the relevant Coulomb matrix elements. The results indicate that iron's spin waves can be described completely by an itinerant model without recourse to additional assumptions about strong Hund's rule coupling or local moment behavior.     

Magnetism, spin-orbit coupling, and superconducting pairing in UGe2

A consistent picture on the mean-field level of the magnetic properties and electronic structure of the superconducting itinerant ferromagnet UGe2 requires inclusion of correlation effects beyond the local density approximation (LDA). The " LDA+U" approach reproduces both the magnitude of the observed moment and the magnetocrystalline anisotropy. The largest Fermi surface sheet is composed primarily of spin majority states with orbital projection m(l) = 0, suggesting a much simpler picture of the pairing than is possible for general strong spin-orbit coupled materials. The quasi-two-dimensional geometry of the Fermi surface supports the likelihood of magnetically mediated p-wave triplet pairing.     

Effect of pressure on itinerant magnetism and spin disorder in cubic FeGe

The results of ab initio calculations of the pressure dependence of Fe magnetism in cubic FeGe are presented. We find that when the pressure-volume scale is set by means of generalized gradient approximation total energies and magnetism is described by means of the local density approximation, the critical pressure at which the magnetic phase transition occurs is estimated at ≈18 GPa, which is in good agreement with experiments. Using the disordered local moment method we find a localized to itinerant model cross-over of Fe magnetism in cubic FeGe, as a function of volume. Moreover, our calculations also suggest subtle signatures of longitudinal spin fluctuations in cubic FeGe, and that the stiffness parameter softens with increasing pressure. We associate the retention of metallicity in FeGe under pressure with the spin-disorder scattering. The effect of spin-orbit coupling on the electronic structure is also discussed.     

Theory of two-magnon Raman scattering in iron pnictides and chalcogenides

Although the parent iron-based pnictides and chalcogenides are itinerant antiferromagnets, the use of local moment picture to understand their magnetic properties is still widespread. We study magnetic Raman scattering from a local moment perspective for various quantum spin models proposed for this new class of superconductors. These models vary greatly in the level of magnetic frustration and show a vastly different two-magnon Raman response. Light scattering by two-magnon excitations thus provides a robust and independent measure of the underlying spin interactions. In accord with other recent experiments, our results indicate that the amount of magnetic frustration in these systems may be small.     

Covalent magnetism, exchange interactions and anisotropy of the high temperature layered antiferromagnet MnB?

The investigation of the electronic structure and magnetism for the compound MnB(2) with crystal structure type AlB(2) has been revisited to resolve contradictions between various experimental and theoretical results present in the literature. We find that MnB(2) exhibits an interesting example of a Kübler's covalent magnetism (Williams et al 1981 J. Appl. Phys. 52 2069). The covalent magnetism also appears to be the source of some disagreement between the calculated values of the magnetic moments and those given by neutron diffraction experiments. We show that this shortcoming is due to the atomic sphere approximation applied in earlier calculations. The application of the disordered local moment approach and the calculation of the inter-atomic exchange interactions within the Liechtenstein formalism reveal strong local moment antiferromagnetism with a high Néel temperature predicted from Monte Carlo simulations. A fully relativistic band structure calculation and then the application of the torque method yields a strong in-plane anisotropy of the Mn magnetic moments. The agreement of these results with neutron diffraction studies rules out any possible weak itinerant electron magnetism scenarios as proposed earlier for MnB(2).     

Quantum phase transitions in the itinerant ferromagnet ZrZn2

We report a study of the ferromagnetism of ZrZn2, the most promising material to exhibit ferromagnetic quantum criticality, at low temperatures T as a function of pressure p. We find that the ordered ferromagnetic moment disappears discontinuously at p(c)=16.5 kbar. Thus a tricritical point separates a line of first order ferromagnetic transitions from second order (continuous) transitions at higher temperature. We also identify two lines of transitions of the magnetization isotherms up to 12 T in the p-T plane where the derivative of the magnetization changes rapidly. These quantum phase transitions (QPT) establish a high sensitivity to local minima in the free energy in ZrZn2, thus strongly suggesting that QPT in itinerant ferromagnets are always first order.     

Magnetic properties of ZrZn2 calculated by the functional integral method

The functional integral method based upon the Stratonovitch-Hubbard transformation is used to calculate the temperature dependence of magnetization, amplitude of the local moment, paramagnetic susceptibility and specific heat of the weak itinerant ferromagnet ZrZn₂. Calculations are performed within the static approximation, using the alloy-analogy approximation and single-site CPA.     

The role of 3d electrons in the appearance of ferromagnetism in the antiferromagnetic Ru2MnGe Heusler compound: a magnetic Compton scattering study

The antiferromagnetism in Ru(2)MnGe can be suppressed by the substitution of V by Mn and ferromagnetism appears. Synchrotron-based magnetic Compton scattering experiments are used in order to investigates the role of 3d electrons in the indirect/direct exchange interactions for the appearance of ferromagnetism. A small spin moment for the itinerant electron part on the magnetic Compton profile indicates that the metallic ferromagnet Ru(2)Mn(0.5)V(0.5)Ge has a weak indirect exchange interaction between the d-like and sp-like (itinerant) electrons. This suggests that the appearance of ferromagnetism is caused by the enhancement of the direct exchange interactions between d-d electrons in the Ru(2)MnGe Heusler compound. These findings indicate that the indirect exchange interaction between itinerant electrons and localized electrons is a significant key point for the appearance of ferromagnetism in this system.     

Coexistence of ferromagnetism and superconductivity close to a quantum phase transition: the Heisenberg- to Ising-type crossover

A microscopic mean-field theory of the phase coexistence between ferromagnetism and superconductivity in the weakly ferromagnetic itinerant electron system is constructed, while incorporating a realistic mechanism for superconducting pairing due to the exchange of critical spin fluctuations. The self-consistent solution of the resulting equations determines the superconducting transition temperature which is shown to depend strongly on the exchange splitting. The effect of phase crossover from isotropic (Heisenberg-like) to uniaxial (Ising-like) spin fluctuations near the quantum phase transition is analyzed and the generic phase diagram is obtained. This scenario is then applied to the case of itinerant ferromagnet ZrZn2, which sheds light on the proposed phase diagram of this compound. A possible explanation of superconductivity in UGe2 is also discussed.     

Theory of finite-temperature magnetism of NiMn. III

The magnetic properties of NiMn alloys at finite temperatures are investigated on the basis of an improved theory of the local environment effect (LEE). Magnetization vs. concentration curves, magnetization-temperature curves, phase diagram, high-field susceptibility and atomic short-range order dependence of the magnetization vs. temperature curves are calculated. The asymmetric behaviour of the high-field susceptibility around the critical concentration of ferromagnetism is explained by the LEE, as well as the itinerant character of the amplitude of the Ni local moment, for the first time. Two self-consistent solutions, corresponding to the high magnetization state and the low magnetization state, are found for a certain intermediate region of atomic order in Ni₃Mn. Therefore, the existence of metamagnetism is suggested in that region. The anomalous double-stage magnetization-temperature curves in Ni₃Mn alloys are explained by the transition from the high magnetization state to the low magnetization state.     

Localized magnetics states in itinerant electron antiferromagnet

The influence of the itinerant electron antiferromagnetic state on the conditions for the existence or nonexistence of localized moments is investigated. Assuming a non-pertubed antiferromagnetic state, simple conditions are given for the occurence of a localized moment and the phase diagram is determined. The main result is that due to the modified density of states, in the antiferromagnetic phase the standard Anderson conditions are slightly modified even in the limit |E| ～ Δ.     

External field influence on the magnetic hyperfine fields in alpha-manganese

We report a study on the magnetic nature of the two hyperfine fields observed in α-Mn by PAC. Both of the field sites behave completely differently under the influence of an external field ranging from zero to 4 Tesla. The so-called high field originates from the local moment antiferromagnetism of the matrix, while the low field should be associated with an itinerant magnetism.     

Instabilities of a Fermi Gas with Nested Fermi Surfaces

The nesting of the Fermi surfaces of an electron and a hole pocket separated by a vector Q commensurate with the lattice in conjunction with the interaction between the quasiparticles can give rise to a rich phase diagram. Of particular importance is itinerant antiferromagnetic order in the context of pnictides and heavy fermion compounds. By mismatching the nesting the order can gradually be suppressed and as the Néel temperature tends to zero a quantum critical point is obtained. A superconducting dome above the quantum critical point can be induced by the transfer of pairs of electrons between the pockets. The conditions under which such a dome arises are studied. In addition numerous other phases may arise, e.g. charge density waves, non‐Fermi liquid behavior, non‐s‐wave superconductivity, Pomeranchuk instabilities of the Fermi surface, nematic order, and phases with persistent orbital currents.