Antiferromagnetic Insulator Phase Research Articles

Uniaxial Strain and Hydrostatic Pressure Engineering of the Hidden Magnetism in La1-xCaxMnO3 (0 ≤ x ≤ 1/2) Thin Films.

Here, using various substrates, we demonstrate that the in-plane uniaxial strain engineering can enhance the Jahn-Teller distortions and promote selective orbital occupancy to induce an emergent antiferromagnetic insulating (AFI) phase at x = 1/3 of La1-xCaxMnO3. Such an AFI phase depends not only on the magnitude of epitaxial strain but also on the symmetry of the substrates. Using the large uniaxial strain imparted by DyScO3(001) substrate, the AFI ground state is achieved in a wide range of doping levels (0 ≤ x ≤ 1/2), leaving an extended AFI phase diagram. Moreover, it is found that hydrostatic pressure can tune the AFI phase back to a hidden ferromagnetic metallic phase, accompanied by the formation of accommodation strain. The coaction of the accommodation strain, uniaxial strain, and hydrostatic pressure produces complex phase competition and evolution, and the result may shed light on phase space control of other functional perovskites with the competing magnetic interactions.

Magnetic ordering and topology in Mn2Bi2Te5 and Mn2Sb2Te5 van der Waals materials

Using density functional theory calculations we study atomic, electronic, and magnetic structures and their influence on the topological phase of ${\mathrm{Mn}}_{2}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{5}$ and ${\mathrm{Mn}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ van der Waals compounds. Our results show that the antiferromagnetic topological insulator (AFM TI) phase in ${\mathrm{Mn}}_{2}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{5}$ is robust both to details of the magnetic ordering within its structural units, nonuple layer (NL) blocks, and the type of atomic layer stacking, NaCl-type ABC or NiAs-type ABAC, within the (${\mathrm{MnTe})}_{2}$ sublattice. The structure with the NiAs-type stacking is energetically more favorable for both compounds. However, for ${\mathrm{Mn}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ the AFM TI phase is realized in the unstable structure with ABC stacking while it is a Dirac semimetal in favorable structure with NiAs stacking within a (${\mathrm{MnTe})}_{2}$ sublattice. We also show that imposing the overall ferromagnetic state by applying an external magnetic field can drive the ${\mathrm{Mn}}_{2}\mathrm{Bi}{(\mathrm{Sb})}_{2}{\mathrm{Te}}_{5}$ compounds into different topologically nontrivial phases like axion insulator or Weyl semimetal.

Antiferromagnetic excitonic insulator state in Sr3Ir2O7

Excitonic insulators are usually considered to form via the condensation of a soft charge mode of bound electron-hole pairs. This, however, presumes that the soft exciton is of spin-singlet character. Early theoretical considerations have also predicted a very distinct scenario, in which the condensation of magnetic excitons results in an antiferromagnetic excitonic insulator state. Here we report resonant inelastic x-ray scattering (RIXS) measurements of Sr3Ir2O7. By isolating the longitudinal component of the spectra, we identify a magnetic mode that is well-defined at the magnetic and structural Brillouin zone centers, but which merges with the electronic continuum in between these high symmetry points and which decays upon heating concurrent with a decrease in the material’s resistivity. We show that a bilayer Hubbard model, in which electron-hole pairs are bound by exchange interactions, consistently explains all the electronic and magnetic properties of Sr3Ir2O7 indicating that this material is a realization of the long-predicted antiferromagnetic excitonic insulator phase.

First-order metal–ferromagnetic insulator phase transition induced by Rashba spin-orbit coupling on the puckered honeycomb lattice

We investigate the first-order metal–ferromagnetic insulator phase transition on the puckered honeycomb lattice, combining the cellular dynamical mean field theory with the continuous-time quantum Monte Carlo method. By analyzing the interplay among intrinsic spin–orbit coupling (SOC), Rashba SOC and on-site interaction, we show that the ferromagnetic (FM) order and the antiferromagnetic (AFM) order occur in different regimes. Rashba SOC allows the electron spin flipping, which leads to the phase transition from the metal to FM insulator induced by the increasing on-site interaction. In contrast to the usual continuous metal–antiferromagnetic insulator phase transition, we find that the metal–ferromagnetic insulator transition is first-order by computing the double occupancy. Furthermore, the complete phase diagrams of the Rashba SOC, on-site interaction and temperature are also demonstrated.

Single shot x-ray diffractometry in SACLA with pulsed magnetic fields up to 16 T

Single shot x-ray diffraction (XRD) experiments have been performed with a x-ray free electron laser (XFEL) under pulsed high magnetic fields up to 16 T generated with a nondestructive minicoil. The antiferromagnetic insulator phase in a perovskite manganaite, Pr$_{0.6}$Ca$_{0.4}$MnO$_{3}$, is collapsed at a magnetic field of $\approx 8$ T with an emergence of the ferromagnetic metallic phase, which is observed via the accompanying lattice changes in a series of the single shot XRD. The feasibility of the single shot XRD experiment under ultrahigh magnetic fields beyond 100 T is discussed, which is generated with a portable destructive pulse magnet.

Multi-messenger nanoprobes of hidden magnetism in a strained manganite.

The ground-state properties of correlated electron systems can be extraordinarily sensitive to external stimuli, offering abundant platforms for functional materials. Using the multi-messenger combination of atomic force microscopy, cryogenic scanning near-field optical microscopy, magnetic force microscopy and ultrafast laser excitation, we demonstrate both 'writing' and 'erasing' of a metastable ferromagnetic metal phase in strained films of La2/3Ca1/3MnO3 (LCMO) with nanometre-resolved finesse. By tracking both optical conductivity and magnetism at the nanoscale, we reveal how strain-coupling underlies the dynamic growth, spontaneous nanotexture and first-order melting transition of this hidden photoinduced metal. Our first-principles calculations reveal that epitaxially engineered Jahn-Teller distortion can stabilize nearly degenerate antiferromagnetic insulator and ferromagnetic metal phases. We propose a Ginzburg-Landau description to rationalize the co-active interplay of strain, lattice distortions and magnetism nano-resolved here in strained LCMO, thus guiding future functional engineering of epitaxial oxides into the regime of phase-programmable materials.

Magnetic phase profile in La0.7Ca0.3MnO3/Pr0.58Ca0.42MnO3 superlattices: Role of substrate inherent disorders

We report the impact of dissimilar substrate inherent twinning over the structural, electrical and magnetic properties of otherwise identical ferromagnetic/antiferromagnetic La0.7Ca0.3MnO3/Pr0.58Ca0.42MnO3 (LCMO/PCMO) superlattices (SLs). 12 layers of LCMO and PCMO each ~11 nm thick were deposited and then LCMO was deposited as the top layer. The SLs were fabricated by RF magnetron sputtering method over three LaAlO3 (0 0 1) substrates embodied with different twinning disorders. Structural analysis employing high resolution X-ray diffraction (HRXRD; 2θ, ω, and ϕ scans) demonstrates that substrate twin structures exactly get imprinted over all the superlattice peaks. Transport and magnetization measurements showed coupled insulator-metal (IM)/paramagnetic-ferromagnetic (PM-FM) transition around 220 K in all the three SLs. In the high temperature PM region all the samples behave similarly however at low temperatures where long-range ferromagnetic metal (FMM) and antiferromagnetic insulating (AFMI) phases start emerging in LCMO and PCMO respectively, SLs show deviant behaviors. This is ascribed to the substrate originated microstructural disorders in SLs which are distinct for all the samples and hence perturb FMM/AFMI interfacial interactions differently. Samples with prominently large substrate FWHM = 0.683° and 0.959° respectively show a second but weak IM transition and a strong hysteretic IM transition behavior. Magnetization displays a hysteretic behavior in field cooled data measured during cooling and warming cycles. The hysteresis gets stronger with the rise in FWHM which signifies an enhancement of phase competition between FMM and AFMI phases at the interfaces. This behavior of LCMO/PCMO SLs is attributed to the interfacial exchange interactions lead magnetic reconstruction at the interfaces.

Appearance of glassy ferromagnetic behavior in [formula omitted] ([formula omitted]) thin films: A revised phase diagram

We report on results of electrical transport properties of Gd1-xCaxMnO3 (0⩽x⩽1) thin films fabricated on top of SrTiO3 substrates by pulsed laser deposition. The lowest studied doping level (x<0.1) was enough to trigger a transition from the antiferromagnetic state present in the parent compound (x = 0) to a ferrimagnetic state. Up to x = 0.3, the system exhibits a high temperature insulator paramagnetic state and a low temperature insulator ferrimagnetic one. At higher concentrations, the electrical and magnetical properties of the films differ drastically from the bulk form which shows a strong antiferromagnetic behavior. At mid Ca substitution (0.4⩽x⩽0.7), samples show a high temperature charge ordered state with ordering temperatures around room temperature and a low temperature glassy state which was not observed in their bulk counterparts. In contrast, high x samples show a high temperature conductive paramagnetic state and a low temperature insulator antiferromagnetic phase. More interestingly, sample x = 0.9 shows a distinct minimum in the resistivity versus temperature plots and a region of thermal hysteresis which indicate a first order transition. Magnetoresistance measurements display a strong irreversibility which we attributed to phase separation effect of a kinetically arrested ferromagnetic metallic phase and a stable antiferromagnetic insulator phase. Based on the above observations and previous magnetic results, we have reconstructed a phase diagram and characterized the properties of each phase.

Identification of ferrimagnetic orbitals preventing spinel degradation by charge ordering in LixMn2O4

Spinel Li$_x$Mn$_2$O$_4$ is a key cathode material that is used extensively in commercial Li-ion batteries. A challenge with this material has been that the capacity of the battery fades with cycling, an effect that can be traced to the presence of an anti-ferromagnetic insulator phase in the fully lithiated LiMn$_2$O$_4$ (LMO) and the associated charge disproportionation that drives distortions of the MnO$_6$ octahedra. Here, by combining x-ray magnetic Compton scattering experiments with parallel first-principles computations, we show that the anti-ferromagnetic phase of LMO is surrounded by a robust ferrimagnetic metallic phase, which becomes stable when even a small amount of Li is removed from or added to the charge-ordered LMO. In this surprising ferrimagnetic state, charge-ordering and octahedral distortions are found to be strongly suppressed. We identify the nature of the ferrimagnetic orbitals involved through theoretical and experimental analyses of the magnetic Compton scattering spectra.

Tuning of Al doping on martensitic-like transition in phase separated manganites La0.6Ca0.4Mn1-xAlxO3-δ

The tuning effects of Al3+ doping on the magnetic and magnetotransport properties of La0·6Ca0·4MnO3-δ have been studied by preparing series La0·6Ca0·4Mn1-xAlxO3-δ (x = 0, 0.1, 0.125 and 0.15). Al3+ doping suppresses the ferromagnetic metal phase of La0·6Ca0·4MnO3-δ by weakening the double-exchange interaction of Mn3+-O2--Mn4+, leading to a phase separation state. Al3+ doping dependent irreversible metamagnetic transitions are observed at lower temperatures, especially a martensitic-like transition appears in x = 0.125 and 0.15 system below 4.2 K. Phase diagrams of La0·6Ca0·4Mn1-xAlxO3-δ (0.1 ≤ x ≤ 0.15) in the B-T plane are presented, in which the antiferromagnetic insulator and ferromagnetic metal phases boundary is figured out. The antiferromagnetic matrix and magnetic disorders caused by the random distribution of Al3+ ions and the spontaneous oxygen vacancies grow into a pinning center to pin the Mn moments. As magnetic field is increased to the critical field, the localized magnetic moments rotate to parallel to the magnetic field direction, leading to a martensitic-like transition.

Prediction of Weyl semimetal and antiferromagnetic topological insulator phases in Bi2MnSe4

Three-dimensional materials with strong spin–orbit coupling and magnetic interactions represent an opportunity to realize a variety of rare and potentially useful topological phases with broken time-reversal symmetry. In this work, we use first principles calculations to show that the recently synthesized material Bi2MnSe4 displays a combination of spin–orbit-induced band inversion, also observed in non-magnetic topological insulator Bi2PbSe4, with magnetic interactions, leading to several topological phases. In bulk form, the ferromagnetic phase of Bi2MnSe4 has symmetry protected band crossings at the Fermi level, leading to either a nodal line or Weyl semimetal, depending on the direction of the spins. Due to the combination of time reversal symmetry plus a partial translation, the ground state layered antiferromagnetic phase is instead an antiferromagnetic topological insulator. The surface of this phase intrinsically breaks time-reversal symmetry, allowing the observation of the half-integer quantum anomalous Hall effect. Furthermore, we show that in thin film form, for sufficiently thick slabs, Bi2MnSe4 becomes a Chern insulator with a band gap of up to 58 meV. This combination of properties in a stoichiometric magnetic material makes Bi2MnSe4 an excellent candidate for displaying robust topological behavior.

Static and dynamic signatures of anisotropic electronic phase separation in La2/3Ca1/3MnO3 thin films under anisotropic strain

The electronic phase separation (EPS) of optimally doped ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) thin films under various degrees of anisotropic strain is investigated by static magnetotransport and dynamic relaxation measurements. Three LCMO films were grown simultaneously on (001) $\mathrm{NdGa}{\mathrm{O}}_{3}$ substrates by pulsed laser deposition, and then postgrowth annealed at $780{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ in ${\mathrm{O}}_{2}$ for different durations of time. With increasing annealing time, the films developed significant strains of opposite signs along the two orthogonal in-plane directions. The static temperature-dependent resistivity, $\ensuremath{\rho}(T)$, was measured simultaneously along the two orthogonal directions. With increasing annealing time, both zero-field-cooled and field-cooled $\ensuremath{\rho}(T)$ show significant increases, suggesting strain-triggered EPS and appearance of antiferromagnetic insulating (AFI) phases in a ferromagnetic metallic (FMM) ground state. Meanwhile, $\ensuremath{\rho}(T)$ along the tensile-strained [010] direction becomes progressively larger than that along the compressive-strained [100]. The enhanced resistivity anisotropy indicates that the EPS is characterized by phase-separated FMM entities with a preferred orientation along [100], possibly due to the cooperative deformation and rotation/tilting of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra under the enhanced anisotropic strain. The anisotropic EPS can also be tuned by an external magnetic field. During a field cycle at several fixed temperatures, the AFI phases are melted at high fields and recovered at low fields, resulting in sharp resistance changes of ratio as high as ${10}^{4}$. Furthermore, the resistivity was found to exhibit glasslike behavior, relaxing logarithmically in the phase-separated states. Fitting the data to a phenomenological model, the resulting resistive viscosity and characteristic relaxation time are found to evolve with temperature, showing a close correlation with the static measurements in the EPS states.

Direct detection of metal-insulator phase transitions using the modified Backus-Gilbert method

The detection of the (semi)metal-insulator phase transition can be extremely difficult if the local order parameter which characterizes the ordered phase is unknown. In some cases, it is even impossible to define a local order parameter: the most prominent example of such system is the spin liquid state. This state was proposed to exist in the Hubbard model on the hexagonal lattice in a region between the semimetal phase and the antiferromagnetic insulator phase. The existence of this phase has been the subject of a long debate. In order to detect these exotic phases we must use alternative methods to those used for more familiar examples of spontaneous symmetry breaking. We have modified the Backus-Gilbert method of analytic continuation which was previously used in the calculation of the pion quasiparticle mass in lattice QCD. The modification of the method consists of the introduction of the Tikhonov regularization scheme which was used to treat the ill-conditioned kernel. This modified Backus-Gilbert method is applied to the Euclidean propagators in momentum space calculated using the hybrid Monte Carlo algorithm. In this way, it is possible to reconstruct the full dispersion relation and to estimate the mass gap, which is a direct signal of the transition to the insulating state. We demonstrate the utility of this method in our calculations for the Hubbard model on the hexagonal lattice. We also apply the method to the metal-insulator phase transition in the Hubbard-Coulomb model on the square lattice.

Coupled magnetic-elastic and metal-insulator transition in epitaxially strained SrMnO3/BaMnO3 superlattices.

The spin–phonon coupling and the effects of strain on the ground-state phases of artificial SrMnO3/BaMnO3 superlattices were systematically investigated using first-principles calculations. The results confirm that this system has antiferromagnetic order and an intrinsic ferroelectric polarisation with the P4mm space group. A tensile epitaxial strain can drive the ground state to another antiferromagnetic–ferroelectric phase and then to a ferromagnetic–ferroelectric phase with the Amm2 space group, accompanied by a change in the ferroelectric polarisation from an out-of-plane direction to an in-plane direction. In contrast, a compressive strain could induce a transition from the antiferromagnetic insulator phase to the ferromagnetic metal phase.

Comparative study on the roles of anisotropic epitaxial strain and chemical doping in inducing the antiferromagnetic insulator phase in manganite films

Epitaxial strain and chemical doping are two different methods that are commonly used to tune the physical properties of epitaxial perovskite oxide films, but their cooperative effects are less addressed. Here we try to tune the phase separation (PS) in ${(\mathrm{L}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{P}{\mathrm{r}}_{x})}_{2/3}\mathrm{C}{\mathrm{a}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ $(0\ensuremath{\le}x\ensuremath{\le}0.4$, LPCMO) films via cooperatively controlling the anisotropic epitaxial strain (AES) and the Pr doping. These films are grown simultaneously on $\mathrm{NdGa}{\mathrm{O}}_{3}(110),{(\mathrm{LaAl}{\mathrm{O}}_{3})}_{0.3}{(\mathrm{SrA}{\mathrm{l}}_{0.5}\mathrm{T}{\mathrm{a}}_{0.5}{\mathrm{O}}_{3})}_{0.7}(001)$, and $\mathrm{NdGa}{\mathrm{O}}_{3}(001)$ substrates with progressively increased in-plane AES, and probed by x-ray diffraction, magnetotransport, and magnetic force microscopy (MFM) measurements. Although it is known that for $x=0$ the AES can enhance the orthorhombicity of the films yielding a phase diagram with the antiferromagnetic charge-ordered insulator (AF-COI) state induced, which is quite different from the bulk one, we illustrate that the Pr doping can further drive the films towards a more robust COI state. This cooperative effect is reflected by the increasing magnetic fields needed to melt the COI phase as a function of AES and the doping level. More strikingly, by directly imaging the phase competition morphology of the $\mathrm{LPCMO}/\mathrm{NdGa}{\mathrm{O}}_{3}(001)$ films via MFM, we find that during COI melting the PS domain structure is subject to both AES and the quenched disorder. However, in the reverse process, as the magnetic field is decreased, the COI phase reappears and the AES dominates leaving a crystalline-orientation determined self-organized microstructure. This finding suggests that the PS states and the domain configurations can be selectively controlled by the AES and/or the quenched disorder, which may shed some light on the engineering of PS domains for device fabrications.

Electronic orders and phase transitions in a honeycomb Kondo lattice system

We study the electronic orders in a honeycomb-Kondo lattice. For the ground state, we use variational quantum Monte Carlo to find the transition from antiferromagnetic insulator to Kondo insulator is continuous, in contrast to the discontinuous transition in mean-field theory. Moreover, the hybridization parameter between the conduction electron and the Kondo spin is nonzero even within the antiferromagnetic phase. At finite temperatures, we resort to dynamical mean-field theory, which not only captures local quantum fluctuations but also accesses the thermodynamic limit directly. There are three phases, namely, antiferromagnetic insulator, Kondo insulator, and paramagnetic phase. The transition from antiferromagnetic phase to paramagnetic phase is likely discontinuous, while that from antiferromagnetic phase to Kondo insulator phase remains to be continuous at finite temperatures. There is a crossover from the paramagnetic phase, where spin excitations are gapless, to the Kondo insulator phase, where spin excitations are gapped. Our results indicate a significant effect of fluctuations beyond mean-field theory in the honeycomb-Kondo lattice. Since the transition from the antiferromagnetic phase to the Kondo insulating phase occurs at a sizable Kondo coupling, where the Kondo lattice model is inequivalent to the Anderson lattice model, our results are complementary to that for a honeycomb-Anderson lattice.

Model Hamiltonian and time reversal breaking topological phases of antiferromagnetic half-Heusler materials

In this work, we construct a generalized Kane model with a new coupling term between itinerant electron spins and local magnetic moments of anti-ferromagnetic ordering in order to describe the low energy effective physics in a large family of anti-ferromagnetic half-Heusler materials. Topological properties of this generalized Kane model is studied and a large variety of topological phases, including Dirac semimetal phase, Weyl semimetal phase, nodal line semimetal phase, type-B triple point semimetal phase, topological mirror (or glide) insulating phase and anti-ferromagnetic topological insulating phase, are identified in different parameter regions of our effective models. In particular, we find that the system is always driven into the anti-ferromagnetic topological insulator phase once a bulk band gap is open, irrespective of the magnetic moment direction, thus providing a robust realization of anti-ferromagentic topological insulators. Furthermore, we discuss the possible realization of these topological phases in realistic anti-ferromagnetic half-Heusler materials. Our effective model provides a basis for the future study of physical phenomena in this class of materials.

Chiral Magnetic Effect and Anomalous Hall Effect in Antiferromagnetic Insulators with Spin-Orbit Coupling.

We search for dynamical magnetoelectric phenomena in three-dimensional correlated systems with spin-orbit coupling. We focus on the antiferromagnetic insulator phases where the dynamical axion field is realized by the fluctuation of the antiferromagnetic order parameter. It is shown that the dynamical chiral magnetic effect, an alternating current generation by magnetic fields, emerges due to such time dependences of the order parameter as antiferromagnetic resonance. It is also shown that the anomalous Hall effect arises due to such spatial variations of the order parameter as antiferromagnetic domain walls. Our study indicates that spin excitations in antiferromagnetic insulators with spin-orbit coupling can result in nontrivial charge responses. Moreover, observing the chiral magnetic effect and anomalous Hall effect in our system is equivalent to detecting the dynamical axion field in condensed matter.

Visualization of Melting of Antiferromagnetic Insulator Phase in Phase-Separated Manganite Film using Magnetic Force Microscopy

The phase separation and magnetic-field-induced transition of the antiferromagnetic charge-ordered insulator (AFI) phase into the ferromagnetic metal (FM) phase in an anisotropically-strained manganite thin film is directly imaged using a home-built magnetic force microscope (MFM). The MFM images at 10 K show that the two competing phases already coexist in zero magnetic field. Remarkably anisotropic distribution of the stripe-like phase domains are observed, which can qualitatively account for the anisotropic transport. Above 2.2 T, the AFI phase starts to transform into FM phase gradually. The melting of AFI phase is completed at 3.2 T. The FM phase can be retained after the magnetic field is removed, suggesting the metastable nature of the AFI phase at this temperature.

Magnetoelectric response of the antiferromagnetic insulator phase in a three-dimensional correlated system with spin-orbit coupling

We theoretically investigate the antiferromagnetic insulator phase in a threedimensional correlated system with spin-orbit coupling, the Fu-Kane-Mele-Hubbard model at half-filling. We focus on the topological magnetoelectric effect which is described by the theta term. A low-energy effective Hamiltonian is derived in the antiferromagnetic insulator phase. Then with the use of a field-theoretical method, the theta term is derived as a consequence of the chiral anomaly.