Antiferromagnetic Spin Order Research Articles

Crystal Growth and Magnetic Properties of Topological Nodal-Line Semimetal GdSbTe with Antiferromagnetic Spin Ordering

We report crystal growth, AC and DC magnetic susceptibilities [χ(T, H)], magnetization [M(T, H)], and heat capacity [CP(T, H)] measurement results of GdSbTe single crystal. GdSbTe is isostructural to the confirmed nonmagnetic nodal-line semimetal ZrSiS of noncentrosymmetric tetragonal crystal structure in space group P4/nmm (No. 129), but it shows additional long-range antiferromagnetic spin ordering for the Gd spins of S = 7/2 below TN. Both χ(T, H) and CP(T, H) measurements confirm the existence of a long-range antiferromagnetic (AFM) spin ordering of Gd spins below TN ∼ 12 K, and an additional spin reorientation/recovery (sr) behavior is identified from the change of on-site spin anisotropy between Tsr1 ∼ 7 and Tsr2 ∼ 4 K. The anisotropic magnetic susceptibilities of χ(T, H) below TN clearly demonstrate that the AFM long-range spin ordering of GdSbTe has an easy axis parallel to the ab-plane direction. The field- and orientation-dependent magnetization of M(T, H) at 2 K shows two plateaus to indicate the spin-flop transition for H||ab near ∼2.1 T and a metamagnetic state near ∼5.9 T having ∼1/3 of the fully polarized magnetization by the applied field. The heat capacity measurement results yield Sommerfeld coefficient of γ ∼ 7.6(4) mJ/mol K2 and θD ∼ 195(2) K being less than half of that for the nonmagnetic ZrSiS. A three-dimensional (3D) AFM spin structure is supported by the ab initio calculations for Gd having magnetic moment of 7.1 μB and the calculated AFM band structure indicates that GdSbTe is a semimetal with bare density of states (0.36 states/eV fu) at the Fermi level, which is 10 times smaller than the measured one to suggest strong spin-fluctuation.

Mechanism of superconductivity and electron-hole doping asymmetry in \u03ba-type molecular conductors

Unconventional superconductivity in molecular conductors is observed at the border of metal-insulator transitions in correlated electrons under the influence of geometrical frustration. The symmetry as well as the mechanism of the superconductivity (SC) is highly controversial. To address this issue, we theoretically explore the electronic properties of carrier-doped molecular Mott system κ-(BEDT-TTF)2X. We find significant electron-hole doping asymmetry in the phase diagram where antiferromagnetic (AF) spin order, different patterns of charge order, and SC compete with each other. Hole-doping stabilizes AF phase and promotes SC with dxy-wave symmetry, which has similarities with high-Tc cuprates. In contrast, in the electron-doped side, geometrical frustration destabilizes the AF phase and the enhanced charge correlation induces another SC with extended-s + d_{x^2 - y^2}wave symmetry. Our results disclose the mechanism of each phase appearing in filling-control molecular Mott systems, and elucidate how physics of different strongly-correlated electrons are connected, namely, molecular conductors and high-Tc cuprates.

B-site disorder driven multiple-magnetic phases: Griffiths phase, re-entrant cluster glass, and exchange bias in Pr2CoFeO6

The magnetic spin ordering and the magnetization dynamics of a double perovskite Pr2CoFeO6 have been investigated by employing the (dc and ac) magnetization and neutron powder diffraction techniques. The study revealed that Pr2CoFeO6 adopted a B-site disordered orthorhombic structure (Pnma). Furthermore, ab initio band structure calculations suggested an insulating antiferromagnetic ground state. Magnetization measurements revealed that the system possesses a spectrum of competing magnetic phases, viz., long range canted antiferromagnetic (AFM) spin ordering (TN ∼ 269 K), Griffiths-like phase, re-entrant cluster glass (TG ∼ 34 K), and exchange bias effects. The neutron diffraction study divulged the exhibition of a long range G-type of canted AFM spin ordering. The random nonmagnetic dilution of magnetic Fe3+ (high spin) ions by Co3+ (low spin) ions due to B-site disorder essentially played a crucial role in manifesting such magnetic properties of the system.

Ultrafast Spin Dynamics in Photodoped Spin-Orbit Mott Insulator Sr2IrO4

Ultrafast photodoping of the Mott insulators, possessing strong correlation between electronic and magnetic degrees of freedom, holds promise for launching an ultrafast dynamics of spins which cannot be described in terms of conventional models of ultrafast magnetism. Here we study the ultrafast laser-induced dynamics of the magnetic order in a novel spin-orbit Mott insulator Sr2IrO4 featuring an uncompensated pattern of antiferromagnetic spin ordering. Using the transient magneto-optical Kerr effect sensitive to the net magnetization, we reveal that photodoping by femtosecond laser pulses with photon energy above the Mott gap launches melting of the antiferromagnetic order seen as ultrafast demagnetization with a characteristic time of 300 fs followed by a sub-10-ps recovery. Nonequilibrium dynamical mean-field theory calculations based on the single-band Hubbard model confirm that ultrafast demagnetization is primarily governed by the laser-induced generation of electron-hole pairs, although the precise simulated time dependencies are rather different from the experimentally observed ones. To describe the experimental results, here we suggest a phenomenological model which is based on Onsager’s formalism and accounts for the photogenerated electron-hole pairs using the concepts of holons and doublons.

Antiferromagnetic NiO atomic chains for information storage and the enhancement of inter-chain spin coupling by CO adsorption

One-dimensional antiferromagnetic atomic chains have potential applications in information storage and spintronics devices. NiO chains have been assembled on the Ni(110) single crystal surface, and the antiferromagnetic spin order has been determined by the first-principles calculations. Scanning tunneling microscope simulation clearly distinguishes two types of Ni with opposite spin-orientations. Therefore, the information carried by such system is possible to be read out. Furthermore, we have introduced the CO molecules, which zipper the neighboring NiO chains and greatly enhance the inter-chain spin coupling. The incorporation of CO has been verified by the low energy electron diffraction and the infrared spectroscopy.

Emergence of Griffiths phase and magnetocaloric behavior in electron doped Ca0.85Nd0.15MnO3

A detailed study of temperature and magnetic field dependent magnetic properties of electron doped polycrystalline manganite Ca0.85Nd0.15MnO3 (CNMO) has been reported. Nd doping at the Ca site induces ferromagnetism through the interruption in the long range antiferromagnetic spin ordering of pristine CaMnO3. Room temperature paramagnetic CNMO sample becomes ferromagnetic at 115 K. Analysis of isothermal magnetization data through Arrott plot reveals that CNMO sample undergoes 2nd order magnetic phase transition. Study on magnetic susceptibility exposes that Griffiths phase is present in CNMO manganite. Magnetic entropy changes as a function of temperature at different values of magnetic field changes have been evaluated by integrating Maxwell’s thermodynamic relation numerically and using isothermal magnetization data. For a magnetic field change of 30 kOe the value of maximum magnetic entropy change is found to be 0.816 J kg− 1k− 1. After rescaling, coincidence of all temperature dependent magnetic entropy change curves for different magnetic field leads to a universal curve. This further establishes that the magnetic phase transition in polycrystalline CNMO sample is a 2nd order phase transition.

Theoretical study of anisotropy in orbital and antiferromagnetic spin orderings in CMR manganites

We report here a tight binding model for doped rare earth manganites consisting of Kubo-Ohata type double exchange (DE) interaction between the eg and t2g band electron spins of same site. A Heisenberg type antiferromagnetic spin-spin interaction is considered among the core band electrons. Both nearest and next nearest neighbouring interactions are taken in the Heisenberg coupling and kinetic energy interaction. The presence of DE interaction induces antiferromagnetism (AFM) in eg band. The Jahn-Teller (JT) distortion leads to anisotropy in the manganite systems. We have considered JT distortion as an extra mechanism. The model Hamiltonian is solved using Zubarev's Green's function technique. The interplay between temperature dependent lattice strain and transverse spin fluctuations is studied. We have studied the in-plane anisotropy observed in the AFM spin fluctuation and its effect on the lattice strain.

Theoretical study of anisotropy in orbital and antiferromagnetic spin orderings in CMR manganites

Theoretical study of anisotropy in orbital and antiferromagnetic spin orderings in CMR manganites

B-Site Disorder and Spin States Driven Multiple-Magnetic Phases: Griffiths Phase, Re-Entrant Cluster Glass and Exchange Bias in Pr <sub>2</sub>CoFeO <sub>6</sub>

The magnetic spin ordering and the magnetization dynamics of a double perovskite Pr2CoFeO6 have been investigated by employing the (dc and ac) magnetization, neutron powder diffraction (NPD) and X-ray magnetic circular dichroism (XMCD) techniques. Structural study revealed that Pr2CoFeO6 adopts a B-site disordered orthorhombic structure (Pnma). Furthermore, ab initio band structure calculations suggested an insulating anti-ferromagnetic ground state. Magnetization measurements revealed the system to possess a spectrum of competing magnetic phases viz., long range antiferromagnetic (canted) spin ordering (TN ~269 K), Griffiths phase, re-entrant cluster glass (RCG) (TG ~34 K) and exchange bias effects. The Neutron diffraction divulged the exhibition of a long range canted G-type spin ordering by Fe spins. Moreover, the electronic structure probed by the X-ray absorption spectroscopy (XAS) study suggested a nominal valance state of +3 for both the B-site ions (Co/Fe). The random non-magnetic dilution of magnetic Fe3+ (HS) ions by Co3+ (LS) ions due to B-site disorder essentially played a crucial role in manifesting the magnetic properties of the system.

First principles study on the electronic and magnetic properties in Zn doped BiFe0.9375Mg0.0625O3 with intrinsic defects

Based on first principles spin-polarized density functional theory plus U calculations, the electronic structure and magnetic properties of BiFe0.9375Mg0.0625O3 (BFMO) and Zn doped BiFe0.9375Mg0.0625O3 (BFMZO) are investigated. Compared with the BFMO, the magnetism of BFMZO is greatly enhanced and the calculated net magnetic moment of the supercell is 10.08 μB. The improved magnetic moment originates from the destroyed antiferromagnetic spin ordering of Fe3+–O–Fe3+ and the spin-polarized of O atoms. Electronic and magnetism of BFMZO with intrinsic defects (Bi and O vacancy) are also investigated. The existence of Bi vacancy favors the excitation of the Bi-6 s and O-2p impurity states into the band gap, in contrast, these in-gap impurities are passivated completely with the appearance of O vacancy in BFMZO. The defect of Bi vacancy imposes significant influence on the electronic structure and magnetic behaviors, inducing a reduced net magnetic moment of about 7.98 μB, while the O vacancy almost has no effect on the magnetism of BMFZO. The improved ferromagnetism of Zn and Mg codoping suggesting a potential way to enhance the multiferroic properties of BiFeO3.

Double perovskite Ba2CaIrO6: A Slater-type antiferromagnet system

Abstract A systematic study, using first-principles methods, is performed to understand the nature and origin of electronic gap and magnetism in the double perovskite antiferromagnetic insulator Ba2CaIrO6. By virtue of its perfectly aligned IrO6, motifs the system may be anticipated as an ideal spin-orbit driven J eff system. However, different from the Ir + 4 and Ir + 5 iridates, our calculations reveal that the electronic gap in Ba2CaIrO6 is implicitly associated with an unconventional antiferromagnetic ordering of Ir spins, thereby classifying this Ir + 6 iridate as a Slater-type antiferromagnetic insulator. On the other hand, spin-orbit coupling enhances the Ir 5 d - O 2 p orbital hybridization, due to which both the magnitude of the electronic gap and the local Ir magnetic moment decreases in comparison to the scalar relativistic calculations. Our results not only affirm the validity of local approximations to the exchange-correlation potential in the scalar relativistic Kohn-Sham Hamiltonian, but also strongly convey that the insulating nature of iridates may be intimately linked with magnetism.

Magnetism and exchange-bias effect at the MnN/Fe interface

Based on ab initio calculations and spin dynamics simulations, we perform a detailed study on the magnetic properties of bulk MnN and the MnN/Fe interface. We determine the spin model parameters for the $\ensuremath{\theta}$-phase of bulk MnN, and we find that the competition between the nearest and the next-nearest-neighbor interactions leads to antiferromagnetic ordering of the Mn spins, in agreement with previous theoretical and experimental results. At the MnN/Fe interface, a sizable Dzyaloshinskii-Moriya interaction appears leading to a stable exchange-bias effect. We study the dependences of the exchange-bias effect on the thicknesses of the ferromagnetic and the antiferromagnetic layers, and we compare them to experimentally obtained results [Meinert et al., Phys. Rev. B 92, 144408 (2015)].

D−p model and spin-orbital order in vanadium perovskites

Using the multi-band $d-p$ model and unrestricted Hartree-Fock approximation we investigate the electronic structure and spin-orbital order in three-dimensional VO$_3$ lattice. The main aim of this investigation is testing if simple $d-p$ model, with partly filled $3d$ orbitals (at vanadium ions) and $2p$ orbitals (at oxygen ions), is capable of reproducing correctly nontrivial coexisting spin-orbital order observed in the vanadium perovskites. We point out that the multi-band $d-p$ model has to include partly filled $e_g$ orbitals at vanadium ions. The results suggest weak self-doping as an important correction beyond the ionic model and reproduce the possible ground states with broken spin-orbital symmetry on vanadium ions: either $C$-type alternating orbital order accompanied by $G$-type antiferromagnetic spin order, or $G$-type alternating orbital order accompanied by $C$-type antiferromagnetic spin order. Both states are experimentally observed and compete with each other in YVO$_3$ while only the latter was observed in LaVO$_3$. Orbital order is induced and stabilized by particular patterns of oxygen distortions arising from the Jahn-Teller effect. In contrast to time-consuming \textit{ab-initio} calculations, the computations using $d-p$ model are very quick and should be regarded as very useful in solid state physics, provided the parameters are selected carefully.

Electronic structure and magnetic properties of Ca2IrO4, using first principles

A comprehensive set of electronic structure calculations are performed to understand the origin of the insulating gap and magnetic properties of hexagonal Ca2IrO4. Although, isoelectronic with Sr2IrO4, the spin-orbit coupling driven Jeff model is anticipated to be less appropriate for Ca2IrO4 following its structural considerations. We find that the local density approximation (LDA), and those including the effects of Coulomb correlations and spin-orbit coupling fail to reproduce the experimental results. Moreover, the calculations employing the modified Becke-Johnson formalism seems to provide sufficiently good results, by predicting Ca2IrO4 to be an antiferromagnetic insulator. The origin of electronic gap is attributed to the antiferromagnetic ordering of Ir spins and the effects of spin-orbit coupling is found marginal. However, due to the anisotropy in the IrO bonding within the distorted IrO6 octahedra we deduce large magneto-anisotropic energy in Ca2IrO4. Further, our analysis shows that Ca2IrO4 is an itinerant material, suggesting that band structure effects play an important role in determining the ground state properties of iridates.

Coexistence of magnetic phases in La(Mn0.70Ga0.30)O3 under high pressure: A neutron powder diffraction investigation

Neutron powder diffraction was used to analyse the La(Mn0.70Ga0.30)O3 compound under pressures of up to 6.1 GPa and temperatures down to 1.3 K. At ambient pressure, the magnetic ground state corresponds to a canted antiferromagnetic AxFy-type spin ordering. As the pressure increases, the Fy component is suppressed and a novel Ay-type ordering takes place coexisting with the Ax-type.The simultaneous occurrence of these spin orderings can be ascribed to a demixing of the pristine nuclear phase into a Jahn-Teller distorted phase (where the Ax-ordering takes place) and an undistorted nuclear phase which grows under pressure (where the Ay-ordering develops).

Domain imaging across the magneto-structural phase transitions in Fe1+yTe

The investigation of the magnetic phase transitions in the parent compounds of Fe-based superconductors is regarded essential for an understanding of the pairing mechanism in the related superconducting compounds. Even though the chemical and electronic properties of these materials are often strongly inhomogeneous on a nanometer length scale, studies of the magnetic phase transitions using spatially resolved experimental techniques are still scarce. Here, we present a real space spin-resolved scanning tunneling microscopy investigation of the surface of Fe1+yTe single crystals with different excess Fe content, y, which are continuously driven through the magnetic phase transition. For Fe1.08Te, the transition into the low-temperature monoclinic phase is accompanied by the appearance of a chevron-patterned structural ordering due to the four 90° rotational domains of the monoclinic lattice. Each of the structural domains contains locally commensurate nanoscale diagonal double stripe antiferromagnetic spin order domains with π-phase slips accross domain boundaries. In the low-temperature phase of Fe1.12Te, on the other hand, the chevron pattern gets rather narrow and less well-defined, and an additional 90° rotated component of the spin-order with local plaquette order emerges. The simultaneous imaging of spin and structural order we show here gives valuable insights into the nature of the magneto-structural domains of Fe1+yTe near the tricritical point, which presumably add to the understanding of the mechanism of superconductivity in the related Fe1+yTexSe1−x material.

Topological and magnetic phase transition in silicene-like zigzag nanoribbons

Spin–orbital interactions (SOI) in silicene results in the quantum-spin-Hall effect, while the Hubbard-induced Coulomb interaction in zigzag nanoribbons often generates a band gap with the anti-ferromagnetic (AF) spin orders on two edges. In this paper we systematically study these two joint contributions to the zigzag silicene-like nanoribbons (zSiNR). Some topological and magnetic phase transitions are investigated with different material parameters and external fields. We find when the ribbon width or the SOI value exceeds some critical value, the SOI may overcome the Coulomb interaction and the system transits from a band insulator to a topological insulator: the quantum-spin-Hall or the spin quantum-anomalous Hall state. We also find some magnetic phase transition exists in the Hubbard-dominated zSiNR systems when the exchange field or the electric field goes beyond some critical values. Lastly we observe a double topological/magnetic phase transition in a Hubbard–SOI-balanced zSiNR system before the magnetic and topological phases are destroyed by a strong electric field.

Comparative Study of Nano- and Bulk La0.5Sr0.5Ti0.5Co0.5O3 Perovskite: Structural, Magnetic, and Transport Properties

In this paper, we report a detailed study of the structural, magnetic, and electrical transport properties of nano- and bulk La0.5Sr0.5Ti0.5Co0.5O3 perovskite with the aim to study the effect of size reduction on resistivity and the antiferromagnetic spin order that is seen in the bulk samples of the cobaltites. Both the phases crystallize with orthorhombic structure in the Pbnm space group. Size reduction causes an increase in both the lattice parameters and unit cell volume. The M–H curve of nanosample shows appearance of hysteresis and gives a clear evidence of the formation of ferromagnetic (FM) moments while almost no RT-FM behavior was found for bulk sample. The resistivity of the nanocrystalline sample has been found to be much lower than that of the bulk sample in the entire temperature range of investigation which could be due to the formation of ferromagnetic moments in the nanocrystallites.

Strain effect on magnetic property of antiferromagnetic insulator SmFeO3

Thin films and heterostructures of antiferromagnetic insulator SmFeO3 were fabricated on LaAlO3 (001) substrates by magnetron sputtering, and their structural, magnetic properties were investigated. It was found that epitaxially strained thin films showed a pronounced magnetic anisotropy with the enhanced magnetization up to 65 emu/cc, which is approximately ten times larger than the bulk value. The observed enhancement of magnetization was considered to be due to the lattice distortion and the non-collinear antiferromagnetic spin ordering of SmFeO3.

Indirect K-edge bimagnon resonant inelastic x-ray scattering spectrum of α-FeTe

We calculate the K-edge indirect bimagnon resonant inelastic x-ray scattering (RIXS) intensity spectra of the bicollinear antiferromagnetic order known to occur in the α-FeTe chalcogenide system. Utilizing linear spin wave theory for this large-S spin system we find that the bimagnon spectrum contains four scattering channels (two intraband and two interband). We find from our calculations that for suitable energy-momentum combination the RIXS spectra can exhibit a one-, two- or three- peak structure. The number of peaks provides a clue on the various bimagnon excitation processes that can be supported both in and within the acoustic and optical magnon branches of the bicollinear antiferromagnet. Unlike the RIXS response of the antiferromagnetic or the collinear antiferromagnetic spin ordering, the RIXS intensity spectrum of the bicollinear antiferromagnet does not vanish at the magnetic ordering wave vector . It is also sensitive to next-next nearest neighbor and biquadratic coupling interactions. Our predicted RIXS spectrum can be utilized to understand the role of multi-channel bimagnon spin excitations present in the α-FeTe chalcogenide.