Antiferromagnetic Insulator Phase Research Articles

Multiferroic tunnel junctions and ferroelectric control of magnetic state at interface (invited)

As semiconductor devices reach ever smaller dimensions, the challenge of power dissipation and quantum effect place a serious limit on the future device scaling. Recently, a multiferroic tunnel junction (MFTJ) with a ferroelectric barrier sandwiched between two ferromagnetic electrodes has drawn enormous interest due to its potential applications not only in multi-level data storage but also in electric field controlled spintronics and nanoferronics. Here, we present our investigations on four-level resistance states, giant tunneling electroresistance (TER) due to interfacial magnetoelectric coupling, and ferroelectric control of spin polarized tunneling in MFTJs. Coexistence of large tunneling magnetoresistance and TER has been observed in manganite/(Ba, Sr)TiO3/manganite MFTJs at low temperatures and room temperature four-resistance state devices were also obtained. To enhance the TER for potential logic operation with a magnetic memory, La0.7Sr0.3MnO3/BaTiO3/La0.5Ca0.5MnO3 /La0.7Sr0.3MnO3 MFTJs were designed by utilizing a bilayer tunneling barrier in which BaTiO3 is ferroelectric and La0.5Ca0.5MnO3 is close to ferromagnetic metal to antiferromagnetic insulator phase transition. The phase transition occurs when the ferroelectric polarization is reversed, resulting in an increase of TER by two orders of magnitude. Tunneling magnetoresistance can also be controlled by the ferroelectric polarization reversal, indicating strong magnetoelectric coupling at the interface.

Controlling phase separation in La5/8−yPryCa3/8MnO3 (y = 0.45) epitaxial thin films by strain disorder

Present study reveals that the length-scale of phase separation in La5/8−yPryCa3/8MnO3 thin films can be controlled by strain disorder invoked during the growth and relaxation process of film. Strain disorder provides an additional degree of freedom to tune colossal magnetoresistance. Magneto-transport measurements following cooling and heating in unequal fields protocol demonstrate that coherent strain stabilizes antiferromagnetic insulating phase, while strain disorder favors ferromagnetic metallic phase. Compared to bulk, antiferromagnetic-insulating phase freezes at lower temperatures in strain disordered films. Raman spectroscopy confirms the coexistence of charge-ordered-insulating and ferromagnetic-metallic phases which are structurally dissimilar and possess P21/m and R-3C like symmetries, respectively.

Antiferromagnetic metal and Mott transition on Shastry-Sutherland lattice.

The Shastry-Sutherland lattice, one of the simplest systems with geometrical frustration, which has an exact eigenstate by putting singlets on diagonal bonds, can be realized in a group of layered compounds and raises both theoretical and experimental interest. Most of the previous studies on the Shastry-Sutherland lattice are focusing on the Heisenberg model. Here we opt for the Hubbard model to calculate phase diagrams over a wide range of interaction parameters, and show the competing effects of interaction, frustration and temperature. At low temperature, frustration is shown to favor a paramagnetic metallic ground state, while interaction drives the system to an antiferromagnetic insulator phase. Between these two phases, there are an antiferromagnetic metal phase and a paramagnetic insulator phase (which should consist of a small plaquette phase and a dimer phase) resulting from the competition of the frustration and the interaction. Our results may shed light on more exhaustive studies about quantum phase transitions in geometrically frustrated systems.

Axionic Antiferromagnetic Insulator Phase in a Correlated and Spin–Orbit Coupled System

We study theoretically a three-dimensional correlated and spin-orbit coupled system, the half-filled extended Fu-Kane-Mele-Hubbard model on a diamond lattice, focusing on the topological magnetoelectric response of the antiferromagnetic insulator phase. In the antiferromagnetic insulator phase, the Dirac-like low-energy effective Hamiltonian is obtained. Then the theta term, which results in the magnetoelectric response, is derived as a consequence of the chiral anomaly. The realization of the dynamical axion field in our model is discussed. The relation with a symmetry broken phase induced by interactions in lattice quantum chromodynamics is also discussed.

Anisotropic resistivities in anisotropic-strain-controlled phase-separated La0.67Ca0.33MnO3/NdGaO3(100) films

The anisotropic resistivities (AR) in La0.67Ca0.33MnO3 films grown on orthorhombic NdGaO3(100) substrates were investigated. In this epitaxial system, the large anisotropic misfit strain was demonstrated to induce an antiferromagnetic insulator (AFI) phase transition near ∼250 K, leading to phase separation (PS) with the coexistence of AFI and ferromagnetic-metal (FM) phases at the temperature below. In the PS regime, the resistivity measured along the highly strained b-axis is greater than that along c-axis, giving rise to a huge AR. It can reach ∼12 500% and shows strong dependence on the amplitude and orientation of the magnetic field. We ascribed this unusual AR to the anisotropic-strain-controlled MnO6 octahedral deformations which can organize the competing AFI and FM phases into orientation-preferred PS pattern, thus resulting in the anisotropic percolative transport.

Semimetal-antiferromagnetic insulator transition in graphene induced by biaxial strain

We report first-principles calculations on the antiferromagnetic spin ordering in graphene under biaxial strain. Using hybrid functional calculations, we found that the semimetallic graphene sheets undergo a transition to antiferromagnetic insulators at a biaxial strain of 7.7$%$ and that the band gap rapidly increases after the onset of this transition before reaching 0.9 eV at a biaxial strain of 12$%$. We examined the competition of the antiferromagnetic spin ordering with two-dimensional Peierls distortions upon biaxial strain, and found that the preceding antiferromagnetic insulator phase impedes the Peierls insulator phase. The antiferromagnetic insulator phase is destabilized upon carrier filling but robust up to moderate carrier densities. This work indicates that biaxially strained graphene represents a noble system where the electron-electron and electron-lattice interactions compete with each other in a simple but nontrivial way.

Annealing induced coherent evolutions of biaxial strain and antiferromagnetic-insulator phase in La0.625Ca0.375MnO3 films

La0.625Ca0.375MnO3 (LCMO) films with thicknesses between 7 and 54 nm were epitaxially grown on (LaAlO3)0.3(Sr2AlTaO6)0.35 (001) [LSAT (001)] substrates by using pulsed laser deposition. For this epitaxial system, antiferromagnetic-insulator (AFI) state can be controlled by changing the film thickness and annealing time with various epitaxial strain states, although this phenomenon is absent in the relatively thick films or bulk samples. The consistency between magnetization and resistivity data suggests all these interesting transport behaviors are attributed to the fluctuation of AFI volume fractions and their instability. Especially, there are huge low-field magnetoresistance over −54% (32 nm) at 0.1 T and enhanced magnetoresistance over a broad temperature range. Based on these above results, annealing induced coherent evolutions of biaxial strain and AFI phase in LCMO epitaxial films is a consequence of the strain-driven orbital ordered state, and this may make an approach for a possible application of strongly correlated electron devices.

Tuning the ground state of La0.67Ca0.33MnO3films via coherent growth on orthorhombic NdGaO3substrates with different orientations

The commensurate La${}_{0.67}$Ca${}_{0.33}$MnO${}_{3}$ films, of which the bulk is a ferromagnetic-metal (FM), can show strikingly different ground states when grown on NdGaO${}_{3}$ substrates of various orientations. With a larger orthorhombic lattice distortion in the substrates, the (110) films can grow essentially strain-free, but the (100) and (001) films are anisotropically strained, accompanied by an increased orthorhombicity. As a result, while the (110) films show constantly a bulklike FM ground state, the others behave dramatically different, i.e., they show a tunable antiferromagnetic-insulator (AFI) phase below \ensuremath{\sim}250 K with strong phase instability in the wide temperature range and at low magnetic fields. Considering that in La${}_{1\ensuremath{-}x}$Ca${}_{x}$MnO${}_{3}$ system the AFI phase is highly involved with the orthorhombic lattice distortion, the different ground state in (100) and (001) films could be ascribed to the epitaxial anisotropic strain, which enhances the orthorhombicity of the films via increasing the rotation and deformation of the MnO${}_{6}$ octahedra.

Paramagnetic Metal–Antiferromagnetic Insulator Transition of λ-BETS2FexGa1-xCl4 System

The quasi-two-dimensional organic conductor λ-BETS 2 FeCl 4 [BETS = bis(ethylenedithio) tetraselenafulvalene] transforms from a paramagnetic metal (PM) to an antiferromagnetic insulator (AFI) at a transition temperature T MI of 8.3 K under a zero magnetic field. To investigate the mechanism of this PM–AFI phase transition, we studied the Fe 3 d spin content dependence of the AFI ground state by measuring the specific heat of the mixed crystal λ-BETS 2 Fe x Ga 1- x Cl 4 at temperatures as low as 0.2 K. The increasing Fe 3 d spin density enhances the internal magnetic field caused by π spin antiferromagnetic (AF) ordering. It was confirmed that the Fe 3 d spin provides favorable conditions for this mysterious PM–AFI phase transition. We propose that this phase transition originates from the magnetic anisotropy introduced by the π– d interaction, which suppresses the low-dimensional fluctuation in the π spin system.

Effects of Eu doping on SmB6single crystals

The various phases in Sm${}_{1\ensuremath{-}x}$Eu${}_{x}$B${}_{6}$ are investigated based on magnetic susceptibility, resistivity, and Hall effect measurements. The end compounds are a Kondo insulator (SmB${}_{6}$) and a polaronic ferromagnet (EuB${}_{6}$). For $x$ \ensuremath{\approx} 0.2, the ground state undergoes a transition from a Kondo insulator to an antiferromagnetic (AF) insulator phase. Further doping induces a transition to an AF metal at $x$ \ensuremath{\approx} 0.4. The spin gap is reduced rapidly with the Eu substitution, while there is a charge gap up to $x$ \ensuremath{\approx} 0.4. The Hall effect indicates a dramatic decrease in the carrier density at low temperatures ($T$) for the Kondo insulator regime, whereas, the carrier density is almost independent of $T$ in the AF metallic phase.

Phase separation and enhanced magnetoresistance in the strained epitaxial La0.625Ca0.375MnO3 (001) films

La0.625Ca0.375MnO3 films have been epitaxially grown on the (LaAlO3)0.3(Sr2AlTaO6)0.35 (001) substrates at different oxygen pressures and show varying tetragonal distortion. Results indicate a tendency that larger tetragonal distortion can stabilize the antiferromagnetic-insulator phase in the thin films. Colossal magnetoresistance (CMR) and anisotropic magnetoresistance (AMR) have been greatly enhanced only in the strained films over a broad temperature, probably responding to the anisotropic fluctuation of the nanoscale orientation-ordered patterns near the temperature of the phase separation region or metal-insulator transition. Hence, the inhomogeneous phase competition may contribute to the CMR and the extraordinary AMR effect for manganite devices.

Evidence for the constancy ofUin the Mott transition of V2O3

We have performed high-resolution hard-x-ray photoemission spectroscopy for the metal-insulator transition (MIT) system (V${}_{1\ensuremath{-}x}$Cr${}_{x}$)${}_{2}$O${}_{3}$ in the paramagnetic metal, paramagnetic insulator, and antiferromagnetic insulator phases. The quality of the spectra enables us to conclude that the on-site Coulomb energy $U$ does not change through the MIT, which eliminates all but one theoretical MIT scenario in this paradigm material.

Resonant photoemission spectroscopy studies of the magnetic phase transitions in [formula omitted

The valence band electronic structure of Pr 0.5Sr 0.5MnO 3 has been investigated using resonant photoemission spectroscopies across its paramagnetic metallic (PMM)–ferromagnetic metallic (FMM)–antiferromagnetic insulator (AFMI) transition. Using bulk sensitive soft X-ray photoemission, we have confirmed the presence of a gap in the near Fermi level electronic spectrum in the charge ordering (CO) antiferromagnetic insulator (AFMI) phase. The Fermi level exhibits a rigid-band-like shift of about ∼ 0.4 eV across the PMM to AFMI transition. Further, we have observed a similar shift of the Pr 4f states to higher binding energies when the sample goes to the CO-AFMI state. The spectral behavior indicates a strong charge carrier localization associated with a charge-density-wave gap related to the CO. On the other hand, the growth of spectral weight at the Fermi level in the PMM state could well be connected to the microscopic phase-separation model.

Pseudogap behavior in Pr 0.5Sr 0.5MnO 3: A photoemission study

The valence band electronic structure of Pr 0.5Sr 0.5MnO 3 has been investigated across its paramagnetic metallic (PMM)–ferromagnetic metallic (FMM)–antiferromagnetic insulator (AFMI) transition. Using surface sensitive high resolution photoemission we have conclusively demonstrated the presence of a pseudogap of magnitude 80 meV in the near Fermi level electronic spectrum in the PMM and FMM phases and finite intensity at the Fermi level in the charge ordering (CO)-AFMI phase. The pseudogap behavior is explained in terms of the strong electron–phonon interaction and the formation of Jahn Teller (JT) polarons, indicating the charge localizations. The finite intensity at the Fermi level in the insulating phase showed a lack of charge ordering in the surface of the Pr 0.5Sr 0.5MnO 3 samples.

Valence-bond states: Link models

An isotropic anti-ferromagnetic quantum state on a square lattice is characterized by symmetry arguments only. By construction, this quantum state is the result of an underlying valence bond structure without breaking any symmetry in the lattice or spin spaces. A detailed analysis of the correlations of the quantum state is given (using a mapping to a 2D classical statistical model and methods in field theory like mapping to the non-linear sigma model or bosonization techniques) as well as the results of numerical treatments (regarding exact diagonalization and variational methods). Finally, the physical relevance of the model is motivated. A comparison of the model to known anti-ferromagnetic Mott–Hubbard insulators is given by means of the two-point equal-time correlation function obtained (i) numerically from the suggested state and (ii) experimentally from neutron scattering on cuprates in the anti-ferromagnetic insulator phase.

Correlated polaron transport and metal-insulator transition in La1−xSrxMnO3

The transport behavior of La1−xSrxMnO3 in the antiferromagnetic insulator, ferromagnetic insulator, and ferromagnetic metallic phases is shown to follow from the correlated polaron mechanism and a single transport equation accounts for the temperature dependence of resistivity for 0⩽x⩽0.40 in the 0–400K range. In the low temperature range zero-point lattice vibration plays a dominant role in the transport in the metallic phase.

Local structural changes in paramagnetic and charge-ordered phases ofSm0.2Pr0.3Sr0.5MnO3: an EXAFS study

Sm0.5−xPrxSr0.5MnO3 exhibits a varietyof ground states as x is varied from 0 to 0.5. At an intermediate doping ofx = 0.3 a charge-ordered CE-type antiferromagnetic insulating (AFI) ground state is seen. Thetransition to this ground state is from a paramagnetic-insulating (PMI) phase through aferromagnetic-metallic phase (FMM). Local structures in PMI and AFI phases of thex = 0.3 sample have been investigated using Pr K-edge and Sm K-edge extended x-rayabsorption fine structure (EXAFS). It can be seen that the tilting and rotation of theMnO6 octahedraabout the b-axis are responsible for the charge-ordered CE-type antiferromagnetic ground state at lowtemperatures. In addition a shift in the position of the rare-earth ion along thec-axis has to be considered to account for observed distribution of bond distances aroundthe rare-earth ion.

Enhancement of equilibrium fraction inLa0.5Ca0.5MnO3 by recrystallization

The nature of the coexisting ferromagnetic–metallic (FMM) and antiferromagnetic–insulating (AFI) phasesin a La0.5Ca0.5MnO3 sample is explored through magnetization and resistivity studies. Like many first-ordermagnetic transitions, in this case also the FMM–AFI transformation is interrupted byglass-like arrest of kinetics. As a result, a fraction of the high-temperature FMM phasepersists in a glassy state to the lowest temperature. We show that this glassy FMM phasedevitrifies to equilibrium AFI phase on heating. Significantly, it can be ‘recrystallized’ toAFI state by annealing and more easily by successive annealing, presumably byheterogeneous nucleation. We show that the recrystallization is easier when the startingglass fraction is larger.

Novel Dynamical Effects and Glassy Response in a Strongly Correlated Electronic System

We find an unconventional nucleation of a low-temperature paramagnetic metal phase with a monoclinic structure from the matrix of a high-temperature antiferromagnetic insulator phase with a tetragonal structure in a strongly correlated electronic system BaCo0.9Ni0.1S1.97. Such unconventional nucleation leads to a decrease in resistivity by several orders with relaxation at a fixed temperature. The novel dynamical process could arise from the competition of strain fields, Coulomb interactions, magnetic correlations, and disorders. Such competition may frustrate the nucleation, giving rise to a slow, nonexponential relaxation and "physical aging" behavior.

Charge transfer at the metal-insulator transition inV2O3thin films by resonant inelastic x-ray scattering

The metal-insulator transition in ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ thin films has been studied by resonant inelastic x-ray scattering (RIXS) at the V ${L}_{3}$ edge. Features belonging to $d\text{\ensuremath{-}}d$ excitations, charge transfer between $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ and $\mathrm{V}\phantom{\rule{0.2em}{0ex}}3d$ states, and normal x-ray emissionlike decay indicate that intraband, interband, and intersite transitions are involved. In particular, our findings are compatible with a more pronounced density of unoccupied nondegenerate ${a}_{1g}$ states in the paramagnetic insulator (PI) phase compared to the paramagnetic metal (PM) or antiferromagnetic insulator (AFI) phases. The $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ to $\mathrm{V}\phantom{\rule{0.2em}{0ex}}3d$ charge transfer is reduced in the PI phase compared to both the PM and AFI phases. The RIXS spectra also provide an estimate of the crystal-field splitting for ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$.