When suppressing the itinerant antiferromagnetism in chromium by doping with the isostructual itinerant ferromagnet iron, a dome of spin-glass behavior emerges around a putative quantum critical point at an iron concentration $x \approx 0.15$. Here, we report a comprehensive investigation of polycrystalline samples of Fe$_{x}$Cr$_{1-x}$ in the range $0.05 \leq x \leq 0.30$ using x-ray powder diffraction, magnetization, ac susceptibility, and neutron depolarization measurements, complemented by specific heat and electrical resistivity data for $x = 0.15$. Besides antiferromagnetic ($x < 0.15$) and ferromagnetic regimes ($0.15 \leq x$), we identify a dome of reentrant spin-glass behavior at low temperatures for $0.10 \leq x \leq 0.25$ that is preceded by a precursor phenomenon. Neutron depolarization indicates an increase of the size of ferromagnetic clusters with increasing $x$ and the Mydosh parameter $ϕ$, inferred from the ac susceptibility, implies a crossover from cluster-glass to superparamagnetic behavior. Taken together, these findings consistently identify Fe$_{x}$Cr$_{1-x}$ as an itinerant-electron system that permits to study the evolution of spin-glass behavior of gradually varying character in unchanged crystalline environment.
The Hubbard model in the Heisenberg limit is studied by Kondo-lattice theory. The Kondo temperature T_K or k_BT_K, which is an energy scale of low-energy local quantum spin fluctuations, is enhanced by the resonating valence bond (RVB) mechanism, so that T_K\simeq T_MF/(2D), where T_MF is the Neel temperature in the mean-field approximation of the corresponding Heisenberg model and D is the spatial dimensionality. Electrons certainly behave as localized spins at T\gg T_K, but they are still itinerant at T\ll T_K unless an antiferromagnetic complete gap opens. When the Neel temperature T_N is s…
The microscopic magnetism in the helical, the conical and the ferro-magnetically polarized phases in an itinerant helical magnet, MnSi, has been studied by an extended 29Si NMR at zero field and under external magnetic fields. The temperature dependence of staggered moment, M_Q(T), determined by the 29Si NMR frequency, nu(T), and nuclear relaxation rate, 1/T_1(T) is in general accord with the SCR theory for weak itinerant ferromagnetic metals and its extension. The external field dependence of resonance frequency, nu(H), follows a vector sum of the contributions from atomic hyperfine and macro…
Detailed analysis of the magnetic properties of the Hubbard model within dynamical mean-field theory (DMFT) is presented. Using a RPA-like decoupling of two-particle propagators we derive a universal form for susceptibilities, which captures essential aspects of localized and itinerant pictures. This expression is shown to be quantitatively valid whenever long-range coherence of particle-hole excitations can be neglected, as is the case in large parts of the phase diagram where antiferromag- netism is dominant. The applicability of an interpretation in terms of the two archetypical pictures of…
It is demonstrated that itinerant-localized transition of heavy electrons occurs inside the magnetically ordered phase of the Kondo-Heisenberg lattice. The phase diagram and electronic structure are derived by means of the continuous-time quantum Monte Carlo combined with the dynamical mean-field theory. Around the itinerant-localized transition, nearly flat bands appear on the Fermi surface with almost vanishing quasi-particle renormalization factor. At the same time, there emerges a strong local magnetic fluctuation with a minute energy scale. Considering both antiferromagnetic and ferromagn…
A dynamical generalisation of the nonlocal coherent-potential approximation is derived based upon the functional integral approach to the interacting electron problem. The free energy is proven to be variational with respect to the self-energy provided a self-consistency condition on a cluster of sites is satisfied. In the present work, calculations are performed within the static approximation and the effect of the nonlocal physics on the formation of the local moment state in a simple model is investigated. The results reveal the importance of the dynamical correlations.
An investigation of La and Ca doped Sr4Ru3O10, featuring a coexistence of interlayer ferromagnetism and intralayer metamagnetism, is presented. La doping readily changes magnetism between ferromagnetism and metamagnetism by tuning the density of states. It also results in different Curie temperatures for the c-axis and the basal plane, highlighting a rare spin-orbit coupling with the crystal field states. In contrast, Ca doping enhances the c-axis ferromagnetism and the magnetic anisotropy. La doping also induces a dimensional crossover in the interlayer transport whereas Ca doping exhibits a …
We study the dynamical magnetic susceptibility of a strongly correlated electronic system in the presence of a time-dependent hopping field, deriving a generalized Bethe-Salpeter equation which is valid also out of equilibrium. Focusing on the single-orbital Hubbard model within the time-dependent Hartree-Fock approximation, we solve the equation in the non-equilibrium adiabatic regime, obtaining a closed expression for the transverse magnetic susceptibility. From this, we provide a rigorous definition of non-equilibrium (time-dependent) magnon frequencies and exchange parameters, expressed in…
We report the discovery of \CMA, a metal with strong magnetic anisotropy and moderate electronic correlations. Magnetization measurements find a Curie-Weiss moment of $0.83\,\mathrm{μ_B}$/Mn, significantly reduced from the Hund's rule value, and the magnetic entropy obtained from specific heat measurements is correspondingly small, only $\approx 9$ \% of $R \mathrm{ln}\,2$. These results imply that the Mn magnetism is highly itinerant, a conclusion supported by density functional theory calculations that find strong Mn-Al hybridization. Consistent with the layered nature of the crystal structu…
A magnetic Bragg reflection corresponding to the wave vector k13 = (2pi/a)[1/2,1/2,1/2] of the antiferro-quadrupolar ordering is found in CeB6 in zero magnetic field below the Neel temperature TN. Its intensity is two orders of magnitude weaker than those due to the basic magnetic structure [O. Zaharko et al., Phys. Rev. B 68, 214401 (2003)]. The peak has a width of the other Bragg reflections below TN, but widens abruptly at T = TN with simultaneous increase of intensity. Correlation length just above TN is of the order of 70 A. The peak intensity decreases to zero at T = 7 K with no visible …
The electronic structure of the ferromagnetic superconductor URhGe in the paramagnetic phase has been studied by angle-resolved photoelectron spectroscopy using soft x rays (hn=595-700 eV). Dispersive bands with large contributions from U 5f states were observed in the ARPES spectra, and form Fermi surfaces. The band structure in the paramagnetic phase is partly explained by the band-structure calculation treating all U 5f electrons as being itinerant, suggesting that an itinerant description of U 5f states is a good starting point for this compound. On the other hand, there are qualitative di…
The electronic structure and magnetism of LiFeO$_{2}$Fe$_{2}$Se$_{2}$ are investigated using the first-principle calculations. The ground state is N$\acute{e}$el antiferromagnetic (AFM) Mott insulating state for Fe1 with localized magnetism in LiFeO$_{2}$ layer and striped AFM metallic state for Fe2 with itinerant magnetism in Fe$_{2}$Se$_{2}$ layer, accompanied with a weak interlayer AFM coupling between Fe1 and Fe2 ions, resulting in a coexistence of localized and itinerant magnetism. Moreover, the layered LiFeO$_{2}$ is found to be more than an insulating block layer but responsible for enh…
FeCrAs displays an unusual electrical response that is neither metallic in character nor divergent at low temperatures, as expected for an insulating response, and therefore it has been termed a "nonmetal-metal". We carried out neutron scattering experiments on powder and single crystal samples to study the magnetic dynamics and critical fluctuations in FeCrAs. Magnetic neutron diffraction measurements find Cr3+ magnetic order setting in at 115 K with the mean-field critical exponent. Neutron spectroscopy, however, observes gapless stiff magnetic fluctuations emanating from magnetic positions …
We study the interplay between correlation, itinerant ferromagnetism and local moment formation on the electron doped triangular lattice of sodium cobaltates Na$_x$CoO$_2$. We find that strong correlation renormalizes the Stoner criterion and stabilizes the paramagnetic state for $x<x_c\simeq0.67$. For $x>x_c$, ferromagnetic (FM) order emerges. The enhanced Na dopant potential fluctuations play a crucial role in the sodium rich phases and lead to an inhomogeneous FM state, exhibiting nonmagnetic Co$^{3+}$ patches, antiferromagnetic (AF) correlated regions, and FM clusters with AF domains. Hole…
A thermodynamic and transport study of Sr4Ru3O10 as a function of temperature and magnetic field is presented. The central results include a growing specific heat C with increasing field B, a magnetic contribution to C/T at low temperatures proportional to -log(T), an abrupt jump and a peak in C/T at 2.90 T and 7 T for B||ab-plane and B||c-axis, respectively, and corresponding changes in the low T power laws of the resistivity. The novelty of this work lies in the fact that this system is strongly anisotropic displaying spontaneous ferromagnetism along the c-axis and an intralayer metamagnetic…
Do electrons become ferromagnetic just because of their repulisve Coulomb interaction? Our calculations on the three-dimensional electron gas imply that itinerant ferromagnetim of delocalized electrons without lattice and band structure, the most basic model considered by Stoner, is suppressed due to many-body correlations as speculated already by Wigner, and a possible ferromagnetic transition lowering the density is precluded by the formation of the Wigner crystal.
The weak antiferromagnetism of URu${}_{2}$Si${}_{2}$ is discussed on the basis of a duality model which takes into account salient features of both itinerant fermions and "localized" component of spin degrees of freedom. The problem is analyzed in the framework of induced-moment mechanism by taking a singlet-singlet crystal field scheme together with the nesting property of partial Fermi surface of itinerant fermions . It is shown that the extremely small ordered moment $m$ of ${\cal O}$($10^{-2}$$\times$$μ_{B}$) can be compatible with the large specific-heat jump at the transition temperature…
The influence of uncorrelated (nonmagnetic) overlayers on the magnetic properties of thin itinerant-electron films is investigated within the single-band Hubbard model. The Coulomb correlation between the electrons in the ferromagnetic layers is treated by using the spectral density approach (SDA). It is found that the presence of nonmagnetic layers has a strong effect on the magnetic properties of thin films. The Curie temperatures of very thin films are modified by the uncorrelated overlayers. The quasiparticle density of states is used to analyze the results. In addition, the coupling betwe…
We consider electronic and magnetic properties of chromium, a well-known itinerant antiferromagnet, by a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). We find that electronic correlation effects in chromium, in contrast to its neighbours in the periodic table, are weak, leading to the quasiparticle mass enhancement factor ${m^*/m \approx 1.2}$. Our results for local spin-spin correlation functions and distribution of weigths of atomic configurations indicate that the local magnetic moments are not formed. Similarly to previous results of DFT at ambient …
We study the order-from-disorder transition and reentrant magnetism in La(2-x)Sr(x)Cu(1-z)Zn(z)O(4) within the framework of a long-wavelength nonlinear sigma model that properly incorporates the Dzyaloshinskii-Moriya and XY anisotropies. Doping with nonmagnetic impurities, such as Zn, is considered according to classical percolation theory, whereas the effect of Sr, which introduces charge carriers into the CuO(2) planes, is described as a dipolar frustration of the antiferromagnetic order. We calculate several magnetic, thermodynamic, and spectral properties of the system, such as the antifer…
In this contribution we would like to revisit the problems of ferromagnetism (F) and antiferromagnetism (AF) in the pure itinerant model. These methods can be extended later to the superconducting materials. In our model we assume the extended Hubbard Hamiltonian. Transition from the paramagnetic state to the ordered state of magnetic nature is decided by the competition between kinetic and potential energy in which there is an increase in the kinetic energy moderated by the inter-site interactions, and a decrease in the potential energy. The competition between these two energies results in t…
A Kondo-lattice theory is applied to the crossover between local-moment magnetism for almost half fillings of electrons and itinerant-electron magnetism away from the half filling. In clean systems with no disorder, the bandwidth W^* of quasiparticles is non-zero and of the order of |J| at T=0K even in the limit of the half filling, with J the superexchange interaction constant between nearest neighbors. The so called Gutzwiller's term also contributes to W^* away from the half filling; it is approximately proportional to doping concentrations measured from the half filling. Magnetism is enhan…
The duality between localized and itinerant nature of magnetism in $5\textit{f}$ electron systems has been a longstanding puzzle. Here, we report inelastic neutron scattering measurements, which reveal both local and itinerant aspects of magnetism in a single crystalline system of UPt$_{2}$Si$_{2}$. In the antiferromagnetic state, we observe broad continuum of diffuse magnetic scattering with a resonance-like gap of $\approx$ 7 meV, and surprising absence of coherent spin-waves, suggestive of itinerant magnetism. While the gap closes above the Neel temperature, strong dynamic spin correlations…
Magnetic ordering of itinerant fermionic systems is at the forefront of condensed matter physics dating back to Stoner's instability. Spin-orbit coupling (SOC) which couples two essential ingredients of an itinerant fermionic system, namely spin and orbital motion, opens up new horizons to this long-standing problem. Here we report that the itinerant ferromagnetism is absent in 3D Fermi gas with a Weyl SOC and various itinerant spin density waves emerge instead, which is deeply rooted in the unique symmetry and spin-momentum locking effect in spin-orbit coupled systems. What is more appealing …
Intricacies of Frustrated Magnetism in the Kondo Metal YbAgGe
PSI Center for Neutron and Muon Sciences（保罗谢勒研究所中子与μ子科学中心）；Department of Physics and Astronomy, Iowa State University（爱荷华州立大学物理与天文系）；Ames National Laboratory（艾姆斯国家实验室）；Neutron Scattering Division, Oak Ridge National Laboratory（橡树岭国家实验室中子散射部）；Dresden High Magnetic Field Laboratory (HLD-EMFL), Helmholtz-Zentrum Dresden-Rossendorf（德累斯顿强磁场实验室（HLD-EMFL），赫尔霍兹德累斯顿-罗斯多夫中心）；ISIS Facility, STFC Rutherford Appleton Laboratory（STFC卢瑟福阿普尔顿实验室ISIS设施）；Institut Laue-Langevin（朗之万研究所）
Itinerant and local moment magnetism have substantively different origins, and require distinct theoretical treatment. A unified theory of magnetism has long been sought after, and remains elusive, mainly due to the limited number of known itinerant magnetic systems. In the case of the two such examples discovered several decades ago, the itinerant ferromagnets ZrZn_2 and Sc_3In, the understanding of their magnetic ground states draws on the existence of 3d electrons subject to strong spin fluctuations. Similarly, in Cr, an elemental itinerant antiferromagnet (IAFM) with a spin density wave (S…
Electron correlations tend to generate local magnetic moments that usually order if the lattices are not too frustrated. The hexagonal compound SrRu$_2$O$_6$ has a relatively high Neel temperature but small local moments, which seem to be at odds with the nominal valence of Ru$^{5+}$ in the $t_{2g}^3$ configuration. Here, we investigate the electronic property of SrRu$_2$O$_6$ using density functional theory (DFT) combined with dynamical-mean-field theory (DMFT). We find that the strong hybridization between Ru $d$ and O $p$ states results in a Ru valence that is closer to $+4$, leading to the…
The magnetism of ordered and disordered La$_2$NiMnO$_6$ is explained using a model involving double exchange and superexchange. The concept of majority spin hybridization in the large coupling limit is used to explain the ferromagnetism of La$_2$NiMnO$_6$ as compared to the ferrimagnetism of Sr$_{2}$FeMoO$_{6}$. The ferromagnetic insulating ground state in the ordered phase is explained. The essential role played by the Ni-Mn superexchange between the Ni $e_{g}$ electron spins and the Mn $t_{2g}$ core electron spins in realizing this ground state, is outlined. In presence of antisite disorder,…
We study the competition of Coulomb interaction and hybridization effects in the five-orbital Anderson impurity model by means of continuous time quantum Monte Carlo, exact diagonalization, and Hartree Fock calculations. The dependence of the electronic excitation spectra and thermodynamic ground state properties on hybridization strength and the form of the Coulomb interaction is systematically investigated for impurity occupation number $N \approx 6$. With increasing hybridization strength, a Kondo resonance emerges, broadens and merges with some of the upper and lower Hubbard peaks. Concomi…
Two-dimensional moire superlattices have recently emerged as a fertile ground for creating novel electronic phases of matter with unprecedented control. Despite intensive efforts, theoretical investigation of correlated moire systems has been challenged by the large number of atoms in a superlattice unit cell and the inherent difficulty of treating electron correlation. The physics of correlated moire systems is governed by low-energy electrons in a coarse-grained long-wavelength potential, unlike the singular Coulomb potential of atomically-spaced ions in natural solids. Motivated by the sepa…
Within the framework of density functional theory we investigate the nature of magnetism in various families of Fe-based superconductors. (i) We show that magnetization of stripe-type antiferromagnetic order always becomes stronger when As is substituted by Sb in LaOFeAs, BaFe$_2$As$_2$ and LiFeAs. By calculating Pauli susceptibilities, we attribute the magnetization increase obtained after replacing As by Sb to the enhancement of an instability at $(π,π)$. This points to a strong connection between Fermi surface nesting and magnetism, which supports the theory of the itinerant nature of magne…
We consider a system of 2D fermions on a triangular lattice with well separated electron and hole pockets of similar sizes, centered at certain high-symmetry-points in the Brillouin zone. We first analyze Stoner-type spin-density-wave (SDW) magnetism. We show that SDW order is degenerate at the mean-field level. Beyond mean-field, the degeneracy is lifted and is either $120^{\circ}$ "triangular" order (same as for localized spins), or a collinear order with antiferromagnetic spin arrangement on two-thirds of sites, and non-magnetic on the rest of sites. We also study a time-reversal symmetric …
Metallic ferromagnets with strongly interacting electrons often exhibit remarkable electronic phases such as ferromagnetic superconductivity, complex spin textures, and nontrivial topology. In this report, we discuss the synthesis of a layered magnetic metal NiTa$_4$Se$_8$ (or Ni$_{1/4}$TaSe$_{2}$) with a Curie temperature of 58 Kelvin. Magnetization data and \textit{ab initio} calculations indicate that the nickel atoms host uniaxial ferromagnetic order of about 0.7$μ_{B}$ per atom, while an even smaller moment is generated in the itinerant tantalum conduction electrons. Strong correlations a…
The electronic structure and magnetism of a new magnetic intercalation compound (Li0.8Fe0.2)OHFeSe are investigated theoretically. The electronic structure calculations predict that the Fe in the (Li,Fe)OH intercalated layer is in +2 valence state, i.e. there is electron doping to the FeSe layer, resulting in the shrinking of the Fermi surface (FS) pocket around Gamma and a strong suppression of dynamical spin susceptibility at M in comparison with the bulk FeSe compound. The ground state of the FeSe layer is a striped antiferromagnetic (SAFM) metal, while the (Li,Fe)OH layer displays a very w…
We report the results of ac and dc magnetic susceptibility and isothermal magnetization measurements (T= 2-300 K) on the single crystals of a metallic kagome lattice, Tb3Ru4Al12, reported recently to undergo reentrant magnetism with the onset of long range antiferromagnetic order below (TN=) 22K. The magnetization data obtained on the crystal with the c-axis orientation along magnetic-field reveal spin-glass-like characteristics near 17 K (below TN). However, for the orientation along basal plane, such glassy anomalies are not observable above 2 K. In this respect, this compound behaves like a…
Neutron scattering measurements were performed to investigate magnetic excitations in a single-crystal sample of the ternary iron arsenide BaFe2As2, a parent compound of a recently discovered family of Fe-based superconductors. In the ordered state, we observe low energy spin-wave excitations with a gap energy of 9.8(4) meV. The in-plane spin-wave velocity v_ab and out-of-plane spin-wave velocity v_c measured at 12 meV are 280(150) and 57(7) meV A, respectively. At high energy, we observe anisotropic scattering centered at the antiferromagnetic wave vectors. This scattering indicates two-dimen…
Collective excitations in magnetic materials can be investigated by means of inelastic neutron scattering. We show that this experimental method gives access to the complete spectrum of magnetic fluctuations through the energy- and momentum-dependence of the dynamical susceptibility. We focus on the dynamical properties of magnets with localized spin densities and of metals. From such studies, microscopic parameters like exchange integrals, spin-wave stiffness and relaxation times can be determined. This is of great help to test current theories in magnetism.
Emergent magnetism, heavy electrons and pressure-induced reentrant superconductivity in the iron substituted 4d transition-metal sulfides
Zhejiang University of Technology（浙江工业大学）；Fudan University（复旦大学）；Qufu Normal University（曲阜师范大学）；Nanjing University of Posts and Telecommunications（南京邮电大学）；Suzhou University of Technology（苏州科技大学）；Hangzhou Dianzi University（杭州电子科技大学）；National Taiwan Normal University（台湾师范大学）；Academia Sinica（中央研究院）；Zhejiang University（浙江大学）；Ningbo University（宁波大学）；Hangzhou Normal University（杭州师范大学）；Shanghai Jiao Tong University（上海交通大学）
Ground state properties of the spin$-1/2$ Falicov-Kimball model on a triangular lattice in the presence of uniform external magnetic field are explored. Both the orbital and the Zeeman field-induced effects are taken into account and in each unit cell only rational flux fractions are considered. Numerical results, obtained with the help of Monte Carlo simulation algorithm, reveal that the ground state properties strongly depend on the onsite Coulomb correlation between itinerant and localized electrons, orbital magnetic field as well as the Zeeman splitting. Strikingly, for the on-site Coulomb…
The intermetallic semiconductor FeGa$_{3}$ acquires itinerant ferromagnetism upon electron doping by a partial replacement of Ga with Ge. We studied the electron spin resonance (ESR) of high-quality single crystals of FeGa$_{3-x}$Ge$_x$ for $x$ from 0 up to 0.162 where ferromagnetic order is observed. For $x = 0$ we observed a well-defined ESR signal, indicating the presence of pre-formed magnetic moments in the semiconducting phase. Upon Ge doping the occurrence of itinerant magnetism clearly affects the ESR properties below $\approx 40$~K whereas at higher temperatures an ESR signal as seen …
The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe$_{1-x}$Co$_x$Ga$_3$ was studied by means of electrical resistivity, magnetization, ab-initio band structure calculations, and nuclear spin-lattice relaxation $1/T_1$ of the $^{69,71}$Ga nuclei. Temperature dependencies of the electrical resistivity reveal that the evolution from the semiconducting to the metallic state in the Fe$_{1-x}$Co$_x$Ga$_3$ system occurs at $0.025<x<0.075$. The $^{69,71}(1/T_1)$ was studied as a function of temperature in a wide temperature range of $2…
The paper critically overviews the recent developments of the theory of spin fluctuations (SF) in itinerant electron magnetism with particular emphasis on spin-fluctuation coupling or spin anharmonicity. It is argued that the conventional self-consistent renormalized (SCR) theory of spin fluctuations is usually used aside of the range of its applicability actually defined by the constraint of weak spin anharmonicity based on the random phase approximation (RPA) arguments. An essential step in understanding SF in itinerant magnets beyond RPA-like arguments was made recently within the soft-mode…
We have investigated the temperature (T)-dependent evolution of electronic structures and magnetic properties of an itinerant ferromagnet SrRuO3, employing the combined scheme of the density functional theory and the dynamical mean-field theory (DFT+DMFT). The inclusion of finite dynamical correlation effects beyond the DFT well describes not only the incoherent hump structure observed in the photoemission experiment but also the T-dependent magnetic properties in accordance with experiments. We have shown that the magnetization of SrRuO3 evolves with the Stoner behavior below the Curie temper…
Quantum materials that feature magnetic long-range order often reveal complex phase diagrams when localized electrons become mobile. In many materials magnetism is rapidly suppressed as electronic charges dissolve into the conduction band. In materials where magnetism persists, it is unclear how the magnetic properties are affected. Here we study the evolution of the magnetic structure in Nd(1-x)Ce(x)CoIn(5) from the localized to the highly itinerant limit. We observe two magnetic ground states inside a heavy-fermion phase that are detached from unconventional superconductivity. The presence o…
We report Electron Spin Resonance of the itinerant ferromagnets LaCrGe3, CeCrGe3, and PrCrGe3. These compounds show well defined and very similar spectra of itinerant Cr 3d spins in the paramagnetic temperature region. Upon cooling and crossing the Cr-ferromagnetic ordering (below around 90 K) strong spectral structures start to dominate the resonance spectra in a quite different manner in the three compounds. In the Ce- and Pr-compounds the resonance is only visible in the paramagnetic region whereas in the La-compound the resonance can be followed far below the ferromagnetic ordering tempera…
We analyze the origin of the three dimensional (3D) magnetism observed in nonhydrated Na-rich Na$_x$CoO$_2$ within an itinerant spin picture using a 3D Hubbard model. The origin is identified as the 3D nesting between the inner and outer portions of the Fermi surface, which arise due to the local minimum structure of the $a_{1g}$ band at the $Γ$-A line. The calculated spin wave dispersion strikingly resembles the neutron scattering result. We argue that this 3D magnetism and the spin fluctuations responsible for superconductivity in the hydrated systems share essentially the same origin.
A central question in a large class of strongly correlated electron systems, including heavy fermion compounds and iron pnictides, is the identification of different phases and their origins. It has been shown that the antiferromagnetic (AFM) phase in some heavy fermion compounds is induced by Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between localized moments, and that the competition between this interaction and Kondo effect is responsible for quantum criticality. However, conclusive experimental evidence of the RKKY interaction in pnictides is lacking. Here, using high resolution $^{…
Whereas magnetic frustration is typically associated with local-moment magnets in special geometric arrangements, here we show that SrCo$_{2}$As$_{2}$ is a candidate for frustrated itinerant magnetism. Using inelastic neutron scattering (INS), we find that antiferromagnetic (AF) spin fluctuations develop in the square Co layers of SrCo$_{2}$As$_{2}$ below $T\approx100$ K centered at the stripe-type AF propagation vector of $(\frac{1}{2},~\frac{1}{2})$, and that their development is concomitant with a suppression of the uniform magnetic susceptibility determined via magnetization measurements. …
The microscopic basis for the stability of itinerant ferromagnetism in correlated electron systems is examined. To this end several routes to ferromagnetism are explored, using both rigorous methods valid in arbitrary spatial dimensions, as well as Quantum Monte Carlo investigations in the limit of infinite dimensions (dynamical mean-field theory). In particular we discuss the qualitative and quantitative importance of (i) the direct Heisenberg exchange coupling, (ii) band degeneracy plus Hund's rule coupling, and (iii) a high spectral density near the band edges caused by an appropriate latti…
By calculating orbitally resolved Pauli susceptibilities within maximally localized Wannier orbital basis transformed from first principles band structures, we find that magnetism in Fe$_{1+x}$Te still has its itinerant origin even without Fermi surface nesting, provided orbital modulation of particle-hole excitations are considered. This leads to strong magnetic instabilities at wave vector (0,$π$)/($π$,0) in d$_{xz}$/d$_{yz}$ orbitals that are responsible for the bicollinear antiferromagnetic state as extra electrons donated from excess Fe are considered. Magnetic exchange coupling between e…
