A-type Antiferromagnetic Phase Research Articles

The underlying mechanism of the strain-induced reversible magnetic state transition in MXene nanosheets

MXenes have emerged as one of the most promising members among the two-dimensional (2D) material family owing to their unique physical-chemical properties and outstanding application prospects. Here, we investigated the unique magnetic properties of Ti2C MXenes monolayer by means of the first-principles calculations combining with the Heisenberg spin Hamiltonian approach. A strain-induced reversible magnetic transition for the Ti2C monolayer could be found in our study. When the strain is applied to the Ti2C monolayer, the magnetic configuration of the Ti2C monolayer transforms from the A-type antiferromagnetic (A-AFM) phase to the ferromagnetic (FM) phase with the biaxial tensile strain larger than 6%. Furthermore, the transition from the A-AFM phase to the FM phase was found to be closely related with the change of the Ti and C atomic magnetic moments. The direct magnetic coupling between Ti and C atoms with the increased Ti and C magnetic moments is able to stabilize the structure due to the weakened superexchange interaction between Ti and C atoms under large tensile strain. This is the underlying driving force for the transition from the A-AFM phase to the FM phase. This work provides a promising pathway to better understand the existence of magnetic reversal behavior in the Ti2C MXenes monolayer and opens the door to future magnetic applications of MXenes.

Interplay of magnetism and band topology in Eu[formula omitted]Ca[formula omitted]Mg[formula omitted]Bi[formula omitted] (x=0, 0.5, 0.67) from first principles study

Recent discovery of the time reversal symmetry breaking magnetic Weyl semimetals (WSMs) has created a huge surge of activities in the field of quantum topological materials. In this work, we have studied systematically various magnetic orders, electronic structure and the interplay between the magnetic order and band topology in one such material, EuMg2Bi2 (EMB) and its Ca doped variant using first principle methods within the framework of density functional theory (DFT). Our thorough investigation reveals the existence of various magnetic order driven topological phases (for example, topological insulator in the A-type antiferromagnetic (A-AFM) phase with Eu moments aligned along the crystallographic a or b direction, Dirac semimetal in the A-AFM phase when Eu moments are parallel to c direction and Weyl semimetal in the ferromagnetic (FM) phase with Eu moments pointing along the c direction) in this material. These phases are found to be energetically very close and hence are expected to be tunable from one to the other by an external handle such as magnetic field or chemical substitution. Most importantly, we observe the existence of a single pair of Weyl points (WPs) connecting valence and conduction band very close to the Fermi level (FL) along Γ-A direction in the FM state of EuMg2Bi2 with Eu moments aligned along the c direction making it an ideal Weyl semimetal in its FM state exactly similar to EuCd2As2. On doping 50% and 67% Ca at Eu sites, we observe the single pair of WPs to move even closer to the FL which is highly desirable for application purposes. Further, we observe that the separation between the WPs in the pair decreases in doped compounds compared to that in the parent compound which has direct consequence on anomalous Hall conductivity (AHC). Our first principles calculation of AHC shows high peak values exactly at these WPs and the peak height decreases when we dope the system with Ca. Therefore, Ca doping can be a good external handle to tune AHC in this system.

High-pressure evolution of the magnetic order in LaMnO3

A relationship between evolution of the long-range magnetic order, crystal structure, and lattice distortions in $\mathrm{LaMn}{\mathrm{O}}_{3}$ was studied by a combination of neutron diffraction and Raman spectroscopy at high pressures up to 39 and 50 GPa, respectively, covering the temperature range 5 to 290 K. The Raman spectra reveal a gradual structural phase transformation evolving in the pressure range of 4 to 17 GPa, caused by a modification of the Jahn-Teller (JT) lattice distortions from the static cooperative character to the local one. A presence of residual JT-distorted regions associated with the initial phase is detected up to 32 GPa, where the insulator-metal transition occurs. At higher pressure, the local JT distortions also vanish completely at further compression up to 50 GPa. In the neutron diffraction data, a strong suppression of the A-type antiferromagnetic (AFM) phase is observed over the pressure region of the phase transformation. This is accompanied by a noticeable reduction in magnitude of the ${Q}_{2}$ and ${Q}_{3}$ JT local modes. The effective ordered magnetic manganese moment is reduced about twice at pressures up to 14 GPa. At higher pressures up to 30 GPa, residual regions of the A-type AFM phase coexist with the magnetically disordered phase. In the range 30 to 39 GPa, i.e., during the pressure-induced insulator-metal transition, these regions disappear and, finally, the magnetically disordered metallic phase becomes the only ground state. The possible models of the insulator-metal transition in $\mathrm{LaMn}{\mathrm{O}}_{3}$ are analyzed.

First-principles study on electronic structure, optical properties and doping-induced half metallicity in double perovskite Bi2CuCrO6

We performed a study on Bi2CuCrO6 by the density functional theory. The calculated results suggest that Bi2CuCrO6 is a A-type antiferromagnet and a semiconductor. The absorption coefficient of Bi2CuCrO6 can reach up to 105–106 ​cm−1, and the peak reflectivity to 0.6. After doping with Pb, the system experiences a transition from A-type antiferromagnetic phase to ferrimagnetic phase, and the d-orbital occupancy of Cr changes due to the injection of holes. Therefore, BiPbCuCrO6 displays a half metallic nature, and it would be a promising material for spintronics.

Pressure-Tuned Intralayer Exchange in Superlattice-Like MnBi2Te4/(Bi2Te3)n Topological Insulators.

The magnetic structures of MnBi2Te4(Bi2Te3)n can be manipulated by tuning the interlayer coupling via the number of Bi2Te3 spacer layers n, while the intralayer ferromagnetic (FM) exchange coupling is considered too robust to control. By applying hydrostatic pressure up to 3.5 GPa, we discover opposite responses of magnetic properties for n = 1 and 2. MnBi4Te7 stays at A-type antiferromagnetic (AFM) phase with a decreasing Néel temperature and an increasing saturation field. In sharp contrast, MnBi6Te10 experiences a phase transition from A-type AFM to a quasi-two-dimensional FM state with a suppressed saturation field under pressure. First-principles calculations reveal the essential role of intralayer exchange coupling from lattice compression in determining these magnetic properties. Such magnetic phase transition is also observed in 20% Sb-doped MnBi6Te10 because of the in-plane lattice compression.

Pressure-induced phase transition in magnetoresistance perovskite La0.75Ca0.25MnO3: a GGA + U study

Density functional theory calculations within a DFT+U approach have been carried out to determine the effect of hydrostatic pressure and strain on the structural, electronic and magnetic properties of La0.75Ca0.25MnO3. We investigate the opportunuty of the appearence of the A-type antiferromagnetic phase under pressure as observed from experiments. At ambient pressure and in the hydrostatic pressure regime (4–25 GPa), the system remains ferromagnetic in its ground state, and the A-type antiferromagnetic coupling is tied to Mn e g orbital order while the ferromagnetic behavior refers to orbital disorder one. No magnetic transition was found when increasing the hydrostatic pressure up to 25 GPa. Applying a tetragonal distortion along the b direction of the pnma orthorhombic structure, the system exhibits a transition from the ferromagnetic orbitally disordered phase to the A-type antiferromagnetic orbitally ordered phase at about 22.5 GPa, in agreement with experiment. Projected density of states (PDOS) calculations predict the pressure and strain effects that are discussed in terms of different electronic occupations through and orbitals.

The crystal and magnetic structure of nanostructured manganite La0.53Sr0.47MnO3 at high pressure

The crystal and magnetic structure of the nanostructured manganite La0.53Sr0.47MnO3 has been studied using the neutron diffraction method in the pressure range 0–5.5 GPa and the temperature range 15–300 K. At ambient pressure, the ferromagnetic ordering in the studied compound is formed below T ≈ 335 K. With an increase of the pressure above 2.1 GPa and cooling below T < 250 K, an A-type antiferromagnetic phase appears, which coexists with the initial ferromagnetic one. Further increase of the pressure leads to the growth of the volume fraction of the antiferromagnetic phase, while the ferromagnetic ordering is gradually suppressed. The structural origins of the pressure-induced magnetic phase separation in the nanostructured manganites are discussed.

Neutron Diffraction Study on the Magnetic Structure of 153EuMnO3-δ: One Way to Assess the Magnetic Structure of EuMnO3-δ

Owing to the strong neutron absorption of 151Eu, 151Eu free 153EuMnO3-δ has been synthesized to collect the neutron diffraction data for analyzing the magnetic structure of EuMnO3-δ. The obtained neutron diffraction data of 153EuMnO3-δ indicates that the magnetic diffraction peaks corresponding to cAAFM (canted A-type antiferromagnetic) phase can be observed, but the magnetic diffraction peaks corresponding to expected ICAFM (incommensurate antiferromagnetic) phase may be too weak to be observed.

Reversible thermally controlled spontaneous magnetization switching in perovskite-type manganite

Reversible, thermally controlled, spontaneous magnetization switching (reversible TCSMS) is demonstrated in the absence and presence of an external magnetic field in polycrystalline perovskite-type CaMn0.95Sb0.05O3. The spontaneous magnetization value is retained throughout a cyclic process even in bias magnetic fields. Neutron diffraction and theoretical studies indicate that two weak ferromagnetic phases, namely, spin-canted G- and A-type antiferromagnetic phases, coexist and that their ratio changes during magnetization switching. This spin structure transition is attributable to growth of eg orbital ordering clusters. The coexistence of two weak ferromagnetic phases plays a key role in exhibiting reversible TCSMS.

Ferromagnetic cluster-glass phase in Ca(Co1−xIrx)2−yAs2 crystals

Single crystals of Ca[Co_(2-x)Ir_(x)]_(2-y)As2 with 0 <= x <= 0.35 and 0.10 <= y <= 0.14 have been grown using the self-flux technique and characterized by single-crystal x-ray diffraction (XRD), energy-dispersive x-ray spectroscopy, magnetization M and magnetic susceptibility chi measurements versus temperature T, magnetic field H, and time t, and heat capacity Cp(H,T) measurements. The XRD refinements reveal that all the Ir-substituted crystals crystallize in a collapsed-tetragonal structure as does the parent CaCo_(2-y)As2 compound. A small 3.3% Ir substitution for Co in CaCo_(1.86)As2 drastically lowers the A-type antiferromagnetic (AFM) transition temperature TN from 52 to 23 K with a significant enhancement of the Sommerfeld electronic heat-capacity coefficient. The positive Weiss temperatures obtained from Curie-Weiss fits to the chi(T>TN) data indicate that the dominant magnetic interactions are ferromagnetic (FM) for all x. A magnetic phase boundary is inferred to be present between x = 0.14 and x = 0.17 from a discontinuity in the x dependences of the effective moment and Weiss temperature in the Curie-Weiss fits. FM fluctuations that strongly increase with increasing x are also revealed from the chi(T) data. The magnetic ground state for x >= 0.17 is a spin glass as indicated by hysteresis in chi(T) between field-cooling and zero-field-cooling measurements and from the relaxation of M in a small field that exhibits a stretched-exponential time dependence. The spin glass has a small FM component to the ordering and is hence inferred to be comprised of small FM clusters. A logarithmic T dependence of Cp at low T for x = 0.14 is consistent with the presence of significant FM quantum fluctuations. This composition is near the T = 0 boundary at x = 0.16 between the A-type AFM phase containing ferromagnetically-aligned layers of spins and the FM cluster-glass phase.

Electronic States and Magnetic Response of MnBi2Te4 by Scanning Tunneling Microscopy and Spectroscopy.

Exotic quantum phenomena have been demonstrated in recently discovered intrinsic magnetic topological insulator MnBi2Te4. At its two-dimensional limit, the quantum anomalous Hall effect and axion insulator state were observed in odd and even layers of MnBi2Te4, respectively. Here, we employ low-temperature scanning tunneling microscopy to study the electronic properties of MnBi2Te4. The quasiparticle interference patterns indicate that the electronic structures on the topmost layer of MnBi2Te4 are different from those of the expected out-of-plane A-type antiferromagnetic phase. The topological surface states may be embedded in deeper layers beneath the topmost surface. Such novel electronic structure is presumably related to the modification of crystalline structure during sample cleaving and reorientation of the magnetic moment of Mn atoms near the surface. Mn dopants substituted at the Bi site on the second atomic layer are observed. The electronic structures fluctuate at atomic scale on the surface, which can affect the magnetism of MnBi2Te4.

Magnetic phase transition in La0.8Sr0.2Mn0.9Sb0.1O3 manganite under pressure

The high-pressure evolution of structural and magnetic properties of the single valence Mn3+ manganite La0.8Sr0.2Mn0.9Sb0.1O3 was studied by means of neutron powder diffraction up to 4.5 GPa in the temperature range of 10–300 K. At ambient pressure, a ferromagnetic ground state with the magnetic ordering temperature TC ≈ 175 K is stabilized. Application of high pressure suppresses the initial FM phase and leads to the appearance of a new A-type antiferromagnetic AFM phase with TN ~ 130 K at P = 2.2 GPa. Under compression, both TC and TN almost linearly increases with the pressure coefficient of 1.9(2) and 0.9(3) K/GPa, respectively. It is assumed that the appearance of the A-type AFM state is mediated by the strong anisotropic distortions of MnO6 octahedra, coupled to the egdx2-z2 orbital polarization at high pressures.

Stabilization of E-type antiferromagnetic ordering in La and Y substituted orthorhombic LuMnO3: A first-principles study

We carry out first principles density functional theory calculations of non-lone pairs, namely, La and Y substituted orthorhombic LuMnO3, to verify the generality of symmetry breaking effects observed in the lone pair cations substituted orthorhombic rare earth manganites. Our calculations revealed that similar to lone pair cations ordering at the A-site of LuMnO3, non-lone pair cations ordering also results in the lowering of the symmetry thereby proving that the symmetry breaking with A-site ordering is a generic effect. Interestingly, we were able to stabilize the large radius La substituted LuMnO3 into an E-type antiferromagnetic (E-AFM) phase, in contrast to the normally expected A-type antiferromagnetic (A-AFM) phase of large radius rare earth manganites such as LaMnO3.

Magnetic orders and electronic behaviours of new chalcogenides Cu3MnCh4 (Ch = S, Se and Te): An ab initio study

ABSTRACTThe ternary copper-based chalcogenides Cu3MnCh4 (Ch = S, Se and Te), which have simple cubic (SC) crystal structure and conform to space group with 215 space number, have been investigated by spin-polarised Generalised Gradient Approximation (GGA) in the framework of Density Functional Theory (DFT). First, all systems have been considered in ferromagnetic (FM) and three different type antiferromagnetic (AFM) orders which are A-type (A-AFM), G-type (G-AFM) and C-type (C-AFM) to investigate most stable magnetic phase for each system. The energetically favoured magnetic phase is A-type AFM for Cu3MnS4 and Cu3MnSe4 compounds, while Cu3MnTe4 compound has FM ground state. The electronic band structures of these systems have been investigated, after the well-optimised structural parameters in their stable magnetic phase have been obtained. The calculated spin-polarised electronic band structure of FM Cu3MnTe4 compound exhibits half-metallic behaviour with a band gap in minority spin channel (Eg = 0.99 eV) while metallic band structure is observed for majority spin. The calculated electronic band structures in A-type AFM phase for Cu3MnS4 and Cu3MnSe4 systems show that Cu3MnS4 compound has semiconducting behaviour with a small band gap (Eg = 0.21 eV) whereas Cu3MnSe4 compound has metallic character. The calculated negative formation enthalpies indicate the thermodynamic stability and structural synthesisability of these compounds.

Strain-induced insulating ferromagnetism in LaMnO3 thin films from first-principles investigations

Previous experiments have found that LaMnO3 thin films grown on SrTiO3 substrate exhibit insulating ferromagnetism different from that of the bulk LaMnO3. Utilizing first-principles calculations based on density functional theory, magnetic transitions, energy gaps and spin exchange interactions of LaMnO3 thin films with different epitaxial strains are investigated, and the effect of the Jahn-Teller distortion, rotation and tilt of oxygen octahedrons on magnetic and electrical properties are analyzed in detail. A relative narrow insulating ferromagnetic area without structural phase transition is obtained during the transition from metallic ferromagnetic phase to insulating A-type antiferromagnetic phase. If the magnetic transition of LaMnO3 film is attributed to Jahn-Teller distortion, then the metal–insulator transition should be ascribed to the tilt of the oxygen octahedron and large change of the O1-Mn-O2 angle in ab plane, which is helpful to understand the insulating ferromagnetic characters during the preparations of the LaMnO3 superlattices or heterostructures, also indicates that the insulating ferromagnetic properties of LaMnO3 films can be tuned by epitaxial strain-controlled oxygen octahedrons distortions.

Spin-polaron formation and magnetic state diagram in La-doped CaMnO3

$La_xCa_{1-x}MnO_3$ (LCMO) has been studied in the framework of density functional theory (DFT) using Hubbard-U correction. We show that the formation of spin-polarons of different configurations is possible in the G-type antiferromagentic phase. We also show that the spin-polaron (SP) solutions are stabilized due to an interplay of magnetic and lattice effects at lower La concentrations and mostly due to the lattice contribution at larger concentrations. Our results indicate that the development of SPs is unfavorable in the C- and A-type antiferromagnetic phases. The theoretically obtained magnetic state diagram is in good agreement with previously reported experimental results

Structure and transport properties of La1−xSrxMnO3 granular ceramics

Two granular ceramics were prepared by spark plasma sintering (SPS) at 600–800 °C and classical ceramic sintering (CCS) at 900 °C using molten salt synthesized nanoparticles of the composition La0.53Sr0.47MnO3 and ≈40 nm size. Extensive study of the structural, magnetic, and electric transport properties showed that the SPS and CCS products essentially retain the two-phase magnetic structure of the starting nanoparticles, which consist of a ferromagnetic (FM) core and an A-type antiferromagnetic (AFM) shell. After the sintering, the AFM phase forms a 10–15 nm thick spacer between neighbouring FM granules, which represents a barrier for the transmission of spin-polarized eg carriers. This assembly retains reasonable conductivity down to the lowest temperatures, without marked localization, and it still gives rise to a large negative magnetoresistance, which is treated theoretically in terms of low- and high-field positive magnetoconductance. In a detailed analysis of these low-field magnetoconductance (LFMC) and high-field magnetoconductance (HFMC) effects, which are related to the field-induced alignment of the FM granules and spin canting in the AFM matrix, respectively, we conclude that the bulk conductivity is governed by resonant tunnelling, i.e. the second-order transmission via Mn4+ sites in the intergranular space. The experimental data on the SPS product confirm the theoretically predicted scaling of the LFMC effect with squared reduced magnetization, and also provide also a quantitative comparison between the linear coefficient of the HFMC and the high-field paraprocess seen in the magnetization measurement.

Magnetic phases and unusual topological electronic structures of Weyl semimetals in strong interaction limit

The interplay of electronic band structures and electron-electron interactions is known to brew new phases in condensed matter. In this paper, we investigate thermodynamic phases and corresponding electronic structures of the Weyl semimetal in the strong onsite Coulomb interaction limit. Based on a minimum model of the Weyl semimetal with two linear Weyl nodes, it is shown that generically the Weyl semimetal becomes magnetic in the presence of interactions. In particular, it is shown that the Dzyaloshinskii-Moriya exchange interaction is generally induced so that the A-type antiferromagnetic (A-AFM) phase and the spiral spin density wave (SSDW) states are two generic phases. Furthermore, we find that Weyl nodes proliferate and it is possible to doubly enhance the unusual properties of non-interacting Weyl semimetals through the realization of double-Weyl nodes in strong correlation limit. Specifically, it is shown that in the SSDW phase, linear Weyl nodes are tuned into double-Weyl nodes with the corresponding charges being $\pm 2$. As the spin-orbit coupling increases, a quantum phase transition occurs with the SSDW phase being turned into an A-AFM phase and at the same time, double-Weyl nodes are disintegrated into two pairs of linear Weyl nodes. Our results reveal the unusual interplay between the topology of electronic structures and magnetism in strongly correlated phases of Weyl semimetals.

Pressure induced antiferromagnetism in the manganite La0.7Sr0.3Mn0.83Nb0.17O3

The crystal and magnetic structure of a manganite La0.7Sr0.3Mn0.83Nb0.17O3 have been studied by means of a neutron diffraction method at pressures up to 4.5 GPa and in temperature range 4–290 K. At ambient pressure and temperature below TC = 150 K a ferromagnetic FM state is stabilized in this compound. At pressures P ≥ 1.5 GPa and temperature below TN∼110 K appearance of a new A-type antiferromagnetic AFM phase in La0.7Sr0.3Mn0.83Nb0.17O3 has been observed. A further pressure increasing leads to an increase in the volume fraction of the A-type antiferromagnetic AFM phase, which coexists with the suppressed ferromagnetic phase in the whole experimental pressure range. The Curie and Néel temperature increase upon lattice contraction with comparable pressure coefficients dTC/dP = 2.1 K/GPa and dTN/dP = 1.6 K/GPa. The structural mechanisms of the changes in magnetic structure of La0.7Sr0.3Mn0.83Nb0.17O3 manganite are discussed.

High pressure effects on the crystal and magnetic structure of nanostructured manganites La0.63Sr0.37MnO3 and La0.72Sr0.28MnO3

The crystal and magnetic structure of nanostructured manganites La0.63Sr0.37MnO3 and La0.72Sr0.28MnO3 has been studied by means of a neutron diffraction method in the pressure range 0–4.5 GPa and the temperature range 5–300 K. In both samples the FM ordering is formed close above the room temperature and at cooling below T < 270 K the ferromagnetic FM phase coexists with an A-type antiferromagnetic AFM phase. At high pressures the volume fraction of AFM phase increases, while FM is gradually suppressed. The structural aspects of the magnetic phase separation and pressure effects on the studied nanostructured manganites are discussed.