Antiferromagnetic Metallic Phase Research Articles

Onset of Antiferromagnetic Insulator‐to‐Ferromagnetic Half Metal‐to‐Antiferromagnetic Metal Transitions in La1−xCaxCrO3 and their Microscopic Origin

The magnetic ground state of LaCrO3 and CaCrO3 solid solution is investigated in the complete miscibility range, that is, La1−xCaxCrO3 with 0 ≤ x ≤ 1 at the interval of x = 0.25 to understand the magnetic phase transitions with Ca substitutional fraction. The density functional theory is used with Hubbard correction to account for Cr exchange interaction. The isostructural La1−xCaxCrO3 system exhibits antiferromagnetic insulator (x = 0, i.e., LaCrO3) to ferromagnetic half metal (x = 0.75, i.e., La0.25Ca0.75CrO3) to antiferromagnetic metal (x = 1, i.e., CaCrO3) phase transitions. Cr changes its oxidation state from 3+ in LaCrO3 to 4+ in CaCrO3, showing both 3+ and 4+ states for intermediate compositions. Interestingly, the introduction of Hubbard energy U and Ca substitution affects Cr–O hybridization, as observed in partial density of states. A compositional phase diagram with magnetic ground state is presented for the La1−xCaxCrO3 (0 ≤ x ≤ 1) system.

Antiferromagnetic metal phase in an electron-doped rare-earth nickelate

Long viewed as passive elements, antiferromagnetic materials have emerged as promising candidates for spintronic devices due to their insensitivity to external fields and potential for high-speed switching. Recent work exploiting spin and orbital effects has identified ways to electrically control and probe the spins in metallic antiferromagnets, especially in non-collinear or non-centrosymmetric spin structures. The rare-earth nickelate NdNiO3 is known to be a non-collinear antiferromagnet in which the onset of antiferromagnetic ordering is concomitant with a transition to an insulating state. Here we find that for low electron doping, the magnetic order on the nickel site is preserved, whereas electronically, a new metallic phase is induced. We show that this metallic phase has a Fermi surface that is mostly gapped by an electronic reconstruction driven by bond disproportionation. Furthermore, we demonstrate the ability to write to and read from the spin structure via a large zero-field planar Hall effect. Our results expand the already rich phase diagram of rare-earth nickelates and may enable spintronics applications in this family of correlated oxides. Films of the correlated oxide NdNiO3 form a metallic antiferromagnetic phase that can be identified using electrical currents, raising the prospect of applications in spintronics.

Interfacial charge transfer induced antiferromagnetic metals and magnetic phase transition in (CrO2) m /(TaO2) n superlattices

As a class of remarkable spintronic materials, intrinsic antiferromagnetic (AFM) metals are rare. The exploration and investigation of AFM metals are still in its infancy. Based on first-principles calculations, the interface-induced magnetic phenomena in the (CrO2) m /(TaO2) n superlattices are investigated, and a new series of AFM metals is predicted. Under different ratios of with varying valence states of Cr, the (CrO2) m /(TaO2) n superlattices exhibit three different phases, including the AFM metal, the AFM semiconductor, and the ferromagnetic (FM) metal. In the AFM semiconducting phases, the intra-CrO2-monolayer magnetic exchange interaction is systematically discussed, corresponding to m = 1 or m = 2. Both the localization of the Cr 3 d orbitals and the crystal-field splitting are crucial for magnetic ordering in super-exchange interactions. Based on the analyses of the AFM semiconducting phases with m = 1 and m = 2, the mechanisms of AFM metallic phases with radios of and are discussed in detail. Additionally, the AFM metallic superlattices can be tuned into a FM metallic phase by applying strain in the c-direction, such as a compression of 7% in the (CrO2)1/(TaO2)3 superlattice, and a tensile strain of 7% in the (CrO2)2/(TaO2)3 superlattice. The phase diagram of the (CrO2) m /(TaO2) n superlattices is obtained as a function of the layer thickness. This work provides new insights about realizing and manipulating AFM metals in artificial superlattices or heterostructures in experiments.

Diverse electronic and magnetic properties of CrS2 enabling strain-controlled 2D lateral heterostructure spintronic devices

Lateral heterostructures of two-dimensional (2D) materials, integrating different phases or materials into a single piece of nanosheet, have attracted intensive research interests for electronic devices. Extending the 2D lateral heterostructures to spintronics demands more diverse electromagnetic properties of 2D materials. In this paper, using density functional theory calculations, we survey all IV, V, and VI group transition metal dichalcogenides (TMDs) and discover that CrS2 has the most diverse electronic and magnetic properties: antiferromagnetic (AFM) metallic 1T phase, non-magnetic (NM) semiconductor 2H phase, and ferromagnetic (FM) semiconductor 1T′ phase with a Curie temperature of ~1000 K. Interestingly, we find that a tensile or compressive strain can turn the 1T′ phase into a spin-up or spin-down half-metal. Such strain tunability can be attributed to the lattice deformation under tensile/compressive strain that selectively promotes the spin-up/spin-down VBM (valence band bottom) orbital interactions. The diverse electromagnetic properties and the strain tunability enable strain-controlled spintronic devices using a single piece of CrS2 nanosheet with improved energy efficiency. As a demo, a prototypical design of the spin-valve logic device is presented. It offers a promising solution to address the challenge of high energy consumption in miniaturized spintronic devices.

Local moments versus itinerant antiferromagnetism: Magnetic phase diagram and spectral properties of the anisotropic square lattice Hubbard model

Using a cluster extension of the dynamical mean-field theory (CDMFT) we map out the magnetic phase diagram of the anisotropic square lattice Hubbard model with nearest-neighbor intrachain $t$ and interchain $t_{\perp}$ hopping amplitudes at half-filling. A fixed value of the next-nearest-neighbor hopping $t'=-t_{\perp}/2$ removes the nesting property of the Fermi surface and stabilizes a paramagnetic metal phase in the weak-coupling regime. In the isotropic and moderately anisotropic regions, a growing spin entropy in the metal phase is quenched out at a critical interaction strength by the onset of long-range antiferromagnetic (AF) order of preformed local moments. It gives rise to a first-order metal-insulator transition consistent with the Mott-Heisenberg picture. In contrast, a strongly anisotropic regime $t_{\perp}/t\lesssim 0.3$ displays a quantum critical behavior related to the continuous transition between an AF metal phase and the AF insulator. Hence, within the present framework of CDMFT, the opening of the charge gap is magnetically driven as advocated in the Slater picture. We also discuss how the lattice-anisotropy-induced evolution of the electronic structure on a metallic side of the phase diagram is tied to the emergence of quantum criticality.

Perovskite as a spin current generator

We theoretically show that materials with perovskite-type crystal structures provide a platform for spin current generation, taking advantage of a mechanism requiring neither the spin-orbit coupling nor a ferromagnetic moment, but is based on spin-split band structures in certain kinds of collinear antiferromagnetic states. By investigating a multiband Hubbard model for transition metal compounds by means of the Hartree-Fock approximation and the Boltzmann transport theory, we find that a pure spin current is induced by an electric field applied to a C-type antiferromagnetic metallic phase. The spin current generation originates from a cooperative effect of spatially anisotropic electron transfer integrals owing to the GdFeO$_3$-type lattice distortion, which is ubiquitous in many perovskites, and the collinear spin configuration. We discuss our finding from the symmetry point of view, in comparison with other spin current generator candidates with collinear antiferomagnetism. We also propose several ways to detect the phenomenon in candidate perovskite materials.

Supreme enhancement of ferromagnetism in a spontaneous-symmetry-broken 2D nanomagnet

The ability to tune and control the magnetic phases of two-dimensional (2D) nanomagnets at room temperature is indispensable for the development of future spintronics and low-dimensional spin circuits. In this work, a first-principles-based investigation combined with a Monte Carlo simulation based on a 2D Ising model is used to investigate the electronic and magnetic behaviour of a recently discovered 2D material, Cr2Ge2Se6 over a large range of strain and electric field strength. This material offers ferromagnetic antiferromagnetic and semiconductor metallic phase transitions in different regimes. In the presence of strain, a colossal enhancement of the critical temperature (T c ) is observed, from 149 K to 885 K. The application of an electric field allows a further enhancement of the T c to a value of 919 K, offering a supreme enhancement (~517%), compared to its natural condition. The origin of this behavior can be traced to a super-exchange interaction between the Cr and Se atoms and the intrinsic magnetic anisotropy of Cr2Ge2Se6. The presence of external stimuli engenders spontaneous symmetry breaking with an enhanced magnetic moment (~4.36 /Cr atom), a significant intrinsic spin polarisation (~100%) in a half-metallic regime, and a very high critical temperature. The insights of the current investigation could be useful for future developments in multi-stimuli-assisted room-temperature ferromagnetism and electronic phase control, which are of great significance for future magneto-electronic applications.

Metallic states beyond the Tomonaga-Luttinger liquid in one dimension

In this paper, we propose some new strongly correlated gapless states (or critical states) of spin-1/2 electrons in 1+1-dimensions, such as doped anti-ferromagnetic spin-1/2 Ising chain. We find doped anti-ferromagnetic Ising chain to be a different metallic phase from the doped ferromagnetic Ising chain, despite the two have identical symmetry. The doped anti-ferromagnetic Ising chain has a finite energy gap for all charge-1 fermionic excitations even without pairing caused by attractive interactions, resembling the pseudo-gap phase of underdoped high Tc superconductors. Applying a transverse field to the ferromagnetic and anti-ferromagnetic metallic phases can restore the $Z_2$ symmetry, which gives rise to two distinct critical points despite that the two transitions have exactly the same symmetry breaking pattern. We also propose new chiral metallic states. All those new gapless states are strongly correlated in the sense that they do not belong to the usual Tomonaga-Luttinger phase of fermions, i.e., they cannot be smoothly deformed into the non-interacting fermion systems of the same symmetry. Our non-perturbative results are obtained by noticing that gapless quantum systems have emergent categorical symmetries, i.e., non-invertible gravitational anomalies), which are described by multi-component partition functions that are modular covariant. This allows us to calculate the scaling dimensions and quantum numbers of all the low energy operators for those strongly correlated gapless states. This demonstrates an application of emergent categorical symmetries in determining low energy properties of strongly correlated gapless states, which are hard to obtain otherwise.

Magnetic States and Metal—Insulator in Strongly Correlated Systems (Scientific Summary)

Various slave particle approaches to the consideration of electron correlations in many-electron models, in particular, in application to layered copper oxide systems, have been analyzed. The magnetic phase diagrams of the ground state have been considered within the t—t’ Hubbard and s—d exchange models for the square and cubic lattices depending on the degree of band filling and the interaction parameter with allowance for collinear ferromagnetic, antiferromagnetic, and incommensurate (helical) magnetic phases, as well as magnetic phase separation. The generalized Hartree—Fock and slave boson approaches including correlations have been used. The latter approach makes it possible to correctly include the contribution of doubly occupied sites to the energy and to adequately describe the paramagnetic phase. A criterion of a metal—insulator transition has been obtained by numerical calculations and the analytical expansion in the transfer integral between next-nearest neighbors t’ and in the direct antiferromagnetic gap Δ. The order of the transition in the square lattice changes from the second to the first with increasing t’ at a small value of t’ ∼ 0.05t, which is due to the presence of a van Hove singularity near the band center. For the simple and body-centered cubic lattices, the transition from the antiferromagnetic insulator state occurs to an antiferromagnetic metal phase, is a second-order transition, and is followed by a transition to a paramagnetic metal. The inclusion of the intersite Heisenberg interaction changes these results and the transition can become a first-order transition.

Probing the quantum phase transition in Mott insulator BaCoS2 tuned by pressure and Ni substitution

We present a muon spin relaxation study of the Mott transition in BaCoS_2 using two independent control parameters: (i) pressure p to tune the electronic bandwidth and (ii) Ni-substitution x on the Co site to tune the band filling. For both tuning parameters, the antiferromagnetic insulating state first transitions to an antiferromagnetic metal and finally to a paramagnetic metal without undergoing any structural phase transition. BaCoS_2 under pressure displays minimal change in the ordered magnetic moment S_ord until it collapses abruptly upon entering the antiferromagnetic metallic state at p_cr ~ 1.3 GPa. In contrast, S_ord in the Ni-doped system Ba(Co_{1-x}Ni_{x})S_{2} steadily decreases with increasing x until the antiferromagnetic metallic region is reached at x_cr ~ 0.22. In both cases, significant phase separation between magnetic and nonmagnetic regions develops when approaching p_cr or x_cr, and the antiferromagnetic metallic state is characterized by weak, random, static magnetism in a small volume fraction. No dynamical critical behavior is observed near the transition for either tuning parameter. These results demonstrate that the quantum evolution of both the bandwidth- and filling-controlled metal-insulator transition at zero temperature proceeds as a first-order transition. This behavior is common to magnetic Mott transitions in RENiO_3 and V_2O_3, which are accompanied by structural transitions without the formation of an antiferromagnetic metal phase.

Formation of an Antiferromagnetic Metal Phase in the Electron-Doped Sr0.98La0.02MnO3 Oxide According to 17O NMR Data

The spin density distribution of itinerant electrons, neg, and their effect on pairwise correlations of localized spins S(t2g) of Mn4+ ions in cubic Sr1 − xLaxMnO3 antiferromagnet (x = 0.02, TN = 230 K, G type magnetic structure) is studied experimentally by nuclear magnetic resonance on 17O nuclei. The regions with neg > x, in which the local spin susceptibility of pairs of Mn atoms follows the dependence χ ∼ (T − Θ)−1 with Θ = 20(5) K, indicating the growth of ferromagnetic spin correlations of neighboring magnetic ions in these domains, are found in the paramagnetic phase. The fraction of Mn-O-Mn bonds (neg > x) increases with the decrease in the temperature. The interpenetrating meshes of Mn-O-Mn bonds with different densities of itinerant eg electrons form an antiferromagnetic metal phase below TN. The role of weak localization effects is discussed as the main origin of the magnetic inhomogeneity of the antiferromagnetic metal phase in Sr0.98La0.02MnO3.

Effect of electron doping on the magnetic properties and electronic structures of Ba2MnMoO6

The electron-carrier-doping effects in an antiferromagnetic insulator Ba2MnMoO6 were investigated by La doping using the first-principles calculation. We examined the magnetic properties and the electronic structures for different doping ratios of La. The results indicate that a transition from an antiferromagnetic (AFM) insulating Ba2MnMoO6 to a AFM half-metallic Ba2-xLaxMnMoO6 (x ≤ 1.0) and then a AFM metallic phase (x = 1.5, 2.0) occurs upon La doping. The injected electron accommodated into the Mo 4d states accounts for the metallic properties. Moreover, the electron correlation plays a crucial role in the determination of the electronic structure. The evaluated magnetic ordering temperatures for the doping systems are all higher than room temperature revealing the potential applications.

Global phase diagram and momentum distribution of single-particle excitations in Kondo insulators

Kondo insulators are emerging as a simplified setting to study both magnetic and metal-to-insulator quantum phase transitions. Here, we study a half-filled Kondo lattice model defined on a magnetically frustrated Shastry-Sutherland geometry. We determine a "global" phase diagram that features a variety of zero-temperature phases; these include Kondo-destroyed antiferromagnetic and paramagnetic metallic phases in addition to the Kondo-insulator phase. Our result provides the theoretical basis for understanding how applying pressure to a Kondo insulator can close its hybridization gap, liberate the local-moment spins from the conduction electrons, and lead to a magnetically correlated metal. We also study the momentum distribution of the single-particle excitations in the Kondo insulating state, and illustrate how Fermi-surface-like features emerge as a precursor to the actual Fermi surfaces of the Kondo-destroyed metals. We discuss the implications of our results for Kondo insulators including SmB$_6$.

Tailoring colossal magnetoresistance and magnetoresistive memory effect by two-dimension-like phase competition in electron-doped manganite superlattices

Colossal magnetoresistance and giant magnetoresistive memory effect have been demonstrated in the electron-doped manganite superlattices composed of alternatively of G-type antiferromagnetic insulator (G-AFI) layers of CaMnO3 and C-type antiferromagnetic insulator (C-AFI) layers of Ca0.92Ce0.08MnO3. The transport properties of the superlattices strongly depend on the periodicity and the unit cells of each layer. The charge transfer through the interface of the constituent insulators induces appearance of the canted G-type antiferromagnetic metal (cG-AFM) phase in the superlattices. Enhanced colossal magnetoresistance and memory effect emerge in the superlattices composed of the thinner G-AFI layers, which can be attributed to the artificially generated two-dimension-like phase competition between cG-AFM and C-AFI in the charge transfer region. The effective charge transfer through interface provides us an opportunity to control phase separation and thus tailoring electronic properties of electron-doped manganites.

Revealing out-of-equilibrium hidden phases in Sr3Ru2O7 by applying stress

We predict that the application of biaxial tensile strain will uncover an antiferromagnetic metallic hidden phase in ${\mathrm{Sr}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$. By using hybrid density functional theory we applied uniaxial and biaxial strains and identified a variety of phases. We found that tensile strain can reduce by up to half the stress needed to expose hidden phases compared to the use of uniaxial compressive strain. In addition, our results demonstrate that while elongation and compression of the octahedra induce magnetic phase transitions, tilting of the octahedra leads to a metal-to-insulator transition, indicating that the different hidden phases can be accessed by applying strain.

Phase separation and local lattice distortions analysis of charge-ordered manganese films La1-xCaxMnO3-δ by Raman spectroscopy

Polycrystalline films La1-xCaxMnO3-δ (LCMO) deposited on (001) MgO substrates for three different Ca concentrations (x = 0.67, 0.75, 0.85) were studied by Raman spectroscopy, X-ray diffraction and scanning electron microscope (SEM). Images of the surface and cross-section (edge) of samples of all compositions were obtained using SEM with high (up to 1 nm) spatial resolution. It was demonstrated that changes occur in the morphology and the formation of preferential orientation films x = 0.75, 0.85 relative to the substrate. It was founded that with an increase in the quantity of calcium there occurs a decrease in parameter pseudocubic unit cell. We have investigated the temperature (80K-350K) dependence of the Raman spectra of LCMO films. We have shown that Raman response of polycrystalline films below 150K corresponds to the microscopic coexistence of isostructural phases, with crystallographic symmetry Pnma. Detailed analysis of spectra allows to detect the occurrence of several different local lattice distortions: structural phases that have weak Jahn–Teller or/and spin crossover induced structural distortions with distorted octahedra Mn3+O6 and symmetric or nearly symmetric Mn4+O6 octahedra and the octahedral tilts in charge-ordered manganese films LCMO with x = 0.67, 0.75 at low temperature. Our results confirm the coexistence of charge–ordered antiferromagnetic state and ferromagnetic metallic phases with the competition between super-exchange and double-exchange interactions.

Variational cluster approach to superconductivity and magnetism in the Kondo lattice model

We investigate in detail antiferromagnetic (AF) and superconducting (SC) phases as well as their coexistence in the two-dimensional Kondo lattice model on a square lattice, which is a paradigmatic model for heavy fermion materials. The results presented are mainly obtained using the variational cluster approximation (VCA) and are complemented by analytical findings for the equations of motion of pairing susceptibilities. A particularly interesting aspect is the possibility to have s-wave SC near half filling as reported by Bodensiek \textit{et al.} [Phys. Rev. Lett. \textbf{110}, 146406 (2013)]. When doping the system, we identify three regions which correspond to an AF metallic phase with small Fermi surface at weak coupling, an AF metal with a different Fermi surface topology at intermediate coupling, and a paramagnetic metal with a large Fermi surface at strong coupling. The transition between these two AF phases is found to be discontinuous at lower fillings, but turns to a continuous one when approaching half-filling. In the quest for s-wave superconductivity, only solutions are found which possess mean-field character. No true superconducting solutions caused by correlation effects are found in the s-wave channel. In contrast, we clearly identify robust d-wave pairing away from half-filling. However, we show that only by treating antiferromagnetism and superconductivity on equal footing artificial superconducting solutions at half-filling can be avoided. Our VCA findings support scenarios previously identified by variational Monte Carlo approaches and are a starting point for future investigations with VCA and further approaches such as cluster-embedding methods.

Half-metallic ferromagnetism in Sr3Ru2O7

The bilayered member of the Ruddesden-Popper family of ruthenates, Sr$_3$Ru$_2$O$_7$, has received an increasing attention due to its interesting properties and phases. By using first principle calculations we find that the ground-state is characterized by a ferromagnetic (FM) half-metallic state. This state strongly competes with an antiferromagnetic metallic phase, which indicates the possible presence of a particular state characterized by the existence of different magnetic domains. To drive the system towards a phase transition we studied the electronic and magnetic properties as a function of RuO$_6$ octahedra rotations and found that the magnetic phase does not couple with the rotation angle. Our results provide accurate electronic, structure, and magnetic ground-state properties of Sr$_3$Ru$_2$O$_7$ and stimulate the investigation of other type of octahedra rotations and distortions in the search of phase transitions.

Doping an antiferromagnetic insulator: A route to an antiferromagnetic metallic phase

We have explored doping electrons into an antiferromagnetic (AFM) insulator as a route to realizing an AFM metal within a multiband Hubbard model. Considering parameters relevant for a 5d transition metal oxide with a half-filled t2g band we find that an AFM metallic phase is stabilized for occupancies up to 3.375 electrons per transition metal site for values of U up to 1.75 eV. At higher values of U, one has a charge-ordered insulating state. In contrast, the large Hund coupling associated with the 3d transition metal oxides does not allow for an AFM metallic phase for the concentrations examined. One has a ferromagnetic metallic phase for the 3d oxide for small values of U at 25% doping, however, at large values, one again finds charge ordering. Orbital degeneracy is found to play an important role, introducing the charge-ordered insulating phase into the phase diagram.

Electron interactions, spin-orbit coupling, and intersite correlations in pyrochlore iridates

We perform combined density functional and dynamical mean-field calculations to study the pyrochlore iridates Lu$_2$Ir$_2$O$_7$, Y$_2$Ir$_2$O$_7$ and Eu$_2$Ir$_2$O$_7$. Both single-site and cluster dynamical mean-field calculations are performed and spin-orbit coupling is included. Paramagnetic metallic phases, antiferromagnetic metallic phases with tilted Weyl cones and antiferromagnetic insulating phases are found. The magnetic phases display all-in/all-out magnetic ordering, consistent with previous studies. Unusually for electronically three dimensional materials, the single-site dynamical mean-field approximation fails to reproduce qualitative material trends, predicting in particular that the paramagnetic phase properties of Y$_2$Ir$_2$O$_7$ and Eu$_2$Ir$_2$O$_7$ are almost identical, although in experiments the Y compound has a much higher resistance than the Eu compound. This qualitative failure is attributed to the importance of intersite magnetic correlations in the physics of these materials.