Range Antiferromagnetic Order Research Articles

Magnetic phases of XY model with three-spin terms: interplay of topology and entanglement

Magnetic and topological properties along with quantum correlations in terms of several entanglement measures have been investigated for an antiferromagnetic (AFM) spin-1/2 XY model in the presence of transverse magnetic field and XZX−YZY type of three-spin interactions. Symmetries of the spin Hamiltonian have been identified. Under the Jordan–Wigner transformation, the spin Hamiltonian converted into spinless superconducting model with nearest neighbor (NN) hopping and Cooper pairing terms in addition to next NN Cooper pairing potential. Long range AFM order has been studied in terms of staggered spin–spin correlation functions, while the topological orders have been characterized by winding numbers. Magnetic and topological phase diagrams have been prepared. Faithful coexistence of magnetic and topological superconducting phases is found in the entire parameter regime. Boundaries of various quantum phases have been marked and positions of bicritical points have been identified.

Electric current driven insulator–metal transition and large electroresistance in single crystal Sr2Ir1–xRuxO4

A series of single crystal Sr2Ir1–xRuxO4 (0 ≤ x ≤ 0.55) have been synthesized, and their physical properties have been investigated. Ru-substitution at Ir-site can drastically reduce the resistivity, which is accompanied by serious suppression of the long range antiferromagnetic order. With the application of electric current, remarkable electroresistance exceeding 90% is obtained at x ∼ 0.2, which may contain significant contribution from the strong magnetic competitions. Interestingly, insulator–metal transition driven by electric current is observed (e.g., below 100 K), which can be ascribed to the lattice modulation. The findings indicate that the electric current can be an efficient route to tune the physical properties of the Jeff = 1/2 Mott insulator.

A New Family of Layered Metal-Organic Semiconductors: Cu/V-Organophosphonates.

Herein, wereport the design and synthesis of a layered redox-active, antiferromagnetic metal organic semiconductor crystals with the chemical formula [Cu(H2 O)2 V(µ-O)(PPA)2 ] (where PPA is phenylphosphonate). The crystal structure of [Cu(H2 O)2 V(µ-O)(PPA)2 ] shows that the metal phosphonate layers are separated by phenyl groups of the phenyl phosphonate linker. Tauc plotting of diffuse reflectance spectra indicates that [Cu(H2 O)2 V(µ-O)(PPA)2 ] has an indirect band gap of 2.19eV. Photoluminescence (PL) spectra indicate a complex landscape of energy states with PL peaks at 1.8 and 2.2eV. [Cu(H2 O)2 V(µ-O)(PPA)2 ] has estimated hybrid ionic and electronic conductivity values between 0.13 and 0.6Sm-1 . Temperature-dependent magnetization measurements show that [Cu(H2 O)2 V(µ-O)(PPA)2 ] exhibits short range antiferromagnetic order between Cu(II) and V(IV) ions. [Cu(H2 O)2 V(µ-O)(PPA)2 ] is also photoluminescent with photoluminescence quantum yield of 0.02%. [Cu(H2 O)2 V(µ-O)(PPA)2 ] shows high electrochemical, and thermal stability.

Magnetic ground state of the Kitaev Na2Co2TeO6 spin liquid candidate

As a candidate Kitaev material, Na$_2$Co$_2$TeO$_6$ exhibits intriguing magnetism on a honeycomb lattice that is believed to be $C_3$-symmetric. Here we report a neutron diffraction study of high quality single crystals under $a$-axis magnetic fields. Our data support the less common notion of a magnetic ground state that corresponds to a triple-$\mathbf{q}$ magnetic structure with $C_3$ symmetry, rather than the multi-domain zigzag structure typically assumed in prototype Kitaev spin liquid candidates. In particular, we find that the field is unable to repopulate the supposed zigzag domains, where the only alternative explanation is that the domains are strongly pinned by hitherto unidentified structural reasons. If the triple-$\mathbf{q}$ structure is correct then this requires reevaluation of many candidate Kitaev materials. We also find that fields beyond about 10 Tesla suppress the long range antiferromagnetic order, allowing new magnetic behavior to emerge different from that expected for a spin liquid.

On some ground state properties of extended t-J-U model induced by Next-Nearest-Neighbor (NNN) pair hopping interaction

Considering an 8-site tilted square cluster, effect of NNN pair hopping interaction Y' on some ground state parameters like ground state energy E0, hole-hole correlation Phh, and spin–spin correlation C(l) of high-Tc superconductors (HTSC) have been examined at a hole doping concentration < h> = 0.25. The system has been described by two-dimensional (2D) extended t-J-U Hamiltonian. Calculations are based on Lanczos method. Slight decrease in E0 is observed with Y'. Phh between NNN sites increases with Y'. A short range antiferromagnetic (AF) order emerges when Y' is greater than one third of NN pair hopping interaction (Y). Correlated hopping interaction renormalizes Y'.

Phase diagram of the Su-Schrieffer-Heeger-Hubbard model on a square lattice

The Hubbard and Su-Schrieffer-Heeger Hamiltonians (SSH) are iconic models for understanding the qualitative effects of electron-electron and electron-phonon interactions respectively. In the two-dimensional square lattice Hubbard model at half filling, the on-site Coulomb repulsion, $U$, between up and down electrons induces antiferromagnetic (AF) order and a Mott insulating phase. On the other hand, for the SSH model, there is an AF phase when the electron-phonon coupling $\lambda$ is less than a critical value $\lambda_c$ and a bond order wave when $\lambda > \lambda_c$. In this work, we perform numerical studies on the square lattice optical Su-Schrieffer-Heeger-Hubbard Hamiltonian (SSHH), which combines both interactions. We use the determinant quantum Monte Carlo (DQMC) method which does not suffer from the fermionic sign problem at half filling. We map out the phase diagram and find that it exhibits a direct first-order transition between an antiferromagnetic phase and a bond-ordered wave as $\lambda$ increases. The AF phase is characterized by two different regions. At smaller $\lambda$ the behavior is similar to that of the pure Hubbard model; the other region, while maintaining long range AF order, exhibits larger kinetic energies and double occupancy, i.e. larger quantum fluctuations, similar to the AF phase found in the pure SSH model.

Coexistence of antiferro- and ferrimagnetism in the spinel ZnFe2O4 with an inversion degree δ lower than 0.3

Samples with inversion parameter values (δ) ranging from 0.27 to 0.14 while maintaining the crystallite size value have been successfully fabricated from commercially available powders by mechanical grinding and thermal annealing treatments at temperatures ranging between 400 and 600 °C. Detailed characterization studies of these samples using X-ray, neutron diffraction and magnetic measurements have confirmed for the first time the simultaneous coexistence at 2 K of short range antiferromagnetic and ferrimagnetic ordering for a wide range of the inversion parameter. The magnetic phase diagram obtained is different from the one previously reported, which shows at 2 K the coexistence of long range antiferromagnetic order and short range order for values of inversion parameters less than 0.1 and the presence of a ferrimagnetic order only for values of δ > 0.2. At room temperature, the Rietveld analysis of NPD patterns and the magnetization curves showed a paramagnetic behavior in the samples with δ ≤ 0.1. For the samples with higher cationic inversion, typical hysteresis curves of ferrimagnetic materials were observed and the saturation magnetization values obtained agree quite well with the net magnetic moment obtained from the Rietveld refinement of the neutron diffraction patterns.

High pressure effects on the crystal and magnetic structure of 160Gd metal

Structural and magnetic properties of the isotopically substituted 160Gd metal (97.5 % enrichment) were studied by means of neutron powder diffraction at pressures up to 9 GPa in the temperature range 7 – 290 K. At ambient conditions, a coexistence of hexagonal close packed (hcp) and Sm-type rhombohedral phases was observed. Under high pressure, the hcp phase was suppressed gradually and above 6 GPa only the Sm-type phase is observed. The long range ferromagnetic order formed in the hcp phase is also suppressed rapidly with a pressure coefficient of the Curie temperature dTC/dP = -6.3(1) K/GPa. In the Sm-type phase below the Néel temperature of about 100 K (evaluated for 9 GPa), the long range antiferromagnetic order with a propagation vector kAF = (0 0 1/2) is formed. The lattice parameters and unit cell volume as functions of pressure in the hcp and Sm-type phases were determined. A partial phase transformation of the Sm-type phase to the double hexagonal close packed (dhcp) phase was observed at 9 GPa.

Evidence of discrete energy states and cluster-glass behavior in Sr2−xLaxCoNbO6

We report the detailed analysis of specific heat [C$_{\rm P}$(T)] and ac-susceptibility for magnetically frustrated Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0--1) double perovskites to understand low temperature complex magnetic interactions and their evolution with $x$. Interestingly, the observed Schottky anomaly in the $x\leqslant$ 0.4 samples shifts gradually towards higher temperature with magnetic field as well as $x$, and the analysis reveal the persistence of the discrete energy states in these samples resulting from the spin-orbit coupling and octahedral distortion. Moreover, the extracted values of Land\'e g--factor indicate the existence of high-spin state Co$^{3+}$ ions close to non-magnetic low-spin state. The specific heat data show the $\lambda$-type anomaly for the $x\geqslant$ 0.6 samples due to evolution of the long range antiferromagnetic ordering. Our analysis of low temperature C$_{\rm P}$(T) data for the $x\geqslant$ 0.6 samples demonstrate the 3D isotropic Heisenberg antiferromagnetic (AFM) interactions and the temperature induced second order AFM--paramagnetic phase transition. More interestingly, we demonstrate the presence of the free Co$^{2+}$ like Kramers doublet ground state in the $x =$1 sample. Further, the ac susceptibility and time evolution of the magnetization data reveal the low temperature cluster-glass like behavior in the $x=$ 0--0.4 samples, where spin-spin correlation strength decreases with $x$.

Role of f−d exchange interaction and Kondo scattering in the Nd-doped pyrochlore iridate (Eu1−xNdx)2Ir2O7

We report study of magnetization, resistivity, magnetoresistance and specific heat of the pyrochlore Iridate (Eu1-xNdx)2Ir2O7 with x=0.0, 0.5 and 1.0, where spin orbit coupling, electronic correlation, magnetic frustration and Kondo scattering coexists. Metal insulator transition temperature (T_MI) decrease with increase in Nd content but always coincides with magnetic irreversibility temperature (field induced moment). Resistivity below T_MI do not fit with either activated (gap) or to any power law (gapless) dependence. The Curie constant show surprising result, that Nd induces singlet correlation (reduction of para-moment) in Ir sublattice. Magnetoresistance is negative at low temperatures below 10 K and increases strongly with increase in x and vary quadratically with field switching over to linear dependence above 50 kOe. Low temperature specific heat shows Schottky peak, coming from Nd moments, showing existence of doublet split in Nd energy level, arising from f-d exchange interaction. All materials show presence of a linear specific heat in the insulating region. The coefficient of linear specific heat for x= 0.0 does not vary with external magnetic field but varies superlinearly for x = 1.0 materials. We argue that linear specific heat probably rules out weakly correlated phases like Weyl fermions. We propose that with the introduction of Nd at Eu site the system evolves from chiral spin liquid with gapless spinon excitations with a very small charge gap to Kondo type interaction superposed on chiral spin liquid coexisting with long range antiferromagnetic ordering. Huge increase of magnetoresistance with increase in Nd concentrations shows importance of Kondo scattering in the chiral spin liquid material by rare earth moments.

Synthesis, structural, dielectric and peculiar magnetic behaviour of Pb2Mn2Si2O9

Polycrystalline ceramic sample of novel lead manganese silicate (Pb2Mn2Si2O9: Kentrolite) was prepared by solid state reaction at 860°C. Structural, morphological, dielectric and magnetic properties of the prepared product were investigated using XRD, EDS & SEM, impedance spectroscopic and VSM techniques. Rietveld refinement of XRD and EDS & SEM analysis confirmed that stoichiometric Pb2Mn2Si2O9 crystallizes in the orthorhombic structure (space group P21221, No.: 18). Frequency dependence of impedance and optical absorption studies establishes semiconducting nature of Pb2Mn2Si2O9. Dispersion in the dielectric parameters is observed due to hopping of electrons between Mn3+↔Mn2+ and Pb2+↔Pb3+. Pb2Mn2Si2O9 displays paramagnetic character at 300K and, it undergoes multiple magnetic transitions at TN = ~67K and TG = ~17K. This compound exhibits Curie-Weiss (θP) = −70K and the best fitted μeff = 3.76μB/Mn ion shows close agreement with the magnetic moment of free Mn3+ ion. Further, this study unveils co-existence of in plane ferromagnetic order (high coercivity = ~1000Oe and low retentivity = ~0.05emu/g) and long range antiferromagnetic order in Pb2Mn2Si2O9 at low temperatures.

Observation of a C-type short-range antiferromagnetic order in layer spacing expanded FeS.

We report neutron diffraction studies of FeS single crystals obtained from Rb x Fe2-y S2 single crystals via a hydrothermal method. While no iron vacancy order or block antiferromagnetic order typical of Rb x Fe2-y S2 is found in our samples, we observe C-type short-range antiferromagnetic order with moments pointed along the c axis hosted by a different phase of FeS with an expanded interlayer spacing. The Néel temperature for this magnetic order is determined to be 170 ± 4 K. Our finding of a variant FeS structure hosting this C-type antiferromagnetic order demonstrates that the known FeS phase synthesized in this method is in the vicinity of a magnetically ordered ground state, providing insights into understanding a variety of phenomena observed in FeS and the related FeSe1-x S x iron chalcogenide system.

Magnetic ordering of the martensite phase in Ni–Co–Mn–Sn-based ferromagnetic shape memory alloys

The magnetic state of low temperature martensite phase in Co-substituted Ni–Mn–Sn-based ferromagnetic shape memory alloys (FSMAs) has been investigated, in view of numerous conflicting reports of occurrences of spin glass (SG), superparamagnetism (SPM) or long range anti-ferromagnetic (AF) ordering. Combination of DC magnetization, AC susceptibility and small angle neutron scattering (SANS) studies provide clear evidence for AF order in the martensitic phase of Ni45Co5Mn38Sn12 alloy and rule out SPM and SG orders. Identical studies on another alloy of close composition, Ni44Co6Mn40Sn10, point to the presence of SG order in the martensitic phase and the absence of SPM behavior, contrary to earlier reports. SANS results do show the presence of nanometre-sized clusters but they are found to grow in size from 3 nm at 30 K to 11 nm at 300 K, and do not correlate with magnetism in these alloys.

Investigations on Magnetization and Electron Magnetic Resonance Properties of Nd0.65Ca0.35Mn1–xZnxO3(x = 0, 0.1, 0.3) Nanomanganite

Zinc substitution effects on the magnetic ordering of nanosized Nd0.65Ca0.35MnO3 particles prepared by sol–gel method have been investigated by electron magnetic resonance (EMR) technique complimented by magnetization measurements. Bulk Nd0.65Ca0.35 MnO3 exhibits a charge ordering transition at 240 K and an antiferromagnetic transition at 150 K followed by a transition to a ferromagnetic (FM) phase below 50 K. In contrast, the nano Nd0.65Ca0.35MnO3 exhibits only a single transition from paramagnetic to ferromagnetic phase at 102 K which supports the general phenomenon of vanishing phase complexities in nano materials. In this paper, we discuss the effect of 10% and 30% zinc doping on the nano sized Nd0.65Ca0.35Mn1–x ZnxO3. Magnetization and EMR studies confirm the existence of ferromagnetism in all the samples which weakens with increasing doping of zinc. Though magnetization measurements do not show the charge order fluctuations and short range antiferromagnetic (AFM) order in the zinc-doped samples, EMR measurements evidence it. Electron magnetic resonance line width increases with zinc doping over a wide range of temperature due to the destruction of motional narrowing.

Magnetic and structural properties of Fe-doped layered cobaltite TbBaCo1.91Fe0.09O5.5 at high pressures

The crystal and magnetic structures of the perovskite-like, oxygen deficient layered cobalt oxide TbBaCo1.91Fe0.09O5.5 have been studied by means of neutron diffraction at high pressures up to 6.2 GPa in the temperature range 5–300 K and X-ray diffraction at high pressures up to 30 GPa and ambient temperature. The structural anomalies in lattice compression at P = 20–25 GPa were revealed, which could be associated with the insulator-metal transition. At ambient pressure below TN = 300 K a formation of the complex magnetic structure on the Co/Fe sublattice, comprising the G-type antiferromagnetic (AFM) and ferromagnetic (FM) components, was observed. Below T ≈ 150 K the FM component vanished. At T = 5 K a presence of the long range AFM order on the Tb sublattice was also revealed. At high pressures, the FM component has become suppressed and the only G-type AFM order was found for the Co/Fe sublattice. The Néel temperature decreased noticeably down to 275 K at P = 6.2 GPa with a pressure coefficient dTN/dP = −4 K/GPa. The intrinsic mechanisms of the observed pressure-induced magnetic phenomena are discussed.

Microscopic spinon-chargon theory of magnetic polarons in the t−J model

The interplay of spin and charge degrees of freedom, introduced by doping mobile holes into a Mott insulator with strong anti-ferromagnetic (AFM) correlations, is at the heart of strongly correlated matter such as high-Tc cuprate superconductors. Here we capture this interplay in the strong coupling regime and propose a trial wavefunction of mobile holes in an AFM. Our method provides a microscopic justification for a class of theories which describe doped holes moving in an AFM environment as meson-like bound states of spinons and chargons. We discuss a model of such bound states from the perspective of geometric strings, which describe a fluctuating lattice geometry introduced by the fast motion of the chargon. This is demonstrated to give rise to short-range hidden string order, signatures of which have recently been revealed by ultracold atom experiments. We present evidence for the existence of such short-range hidden string correlations also at zero temperature by performing numerical DMRG simulations. To test our microscopic approach, we calculate the ground state energy and dispersion relation of a hole in an AFM, as well as the magnetic polaron radius, and obtain good quantitative agreement with advanced numerical simulations at strong couplings. We discuss extensions of our analysis to systems without long range AFM order to systems with short-range magnetic correlations.

Size dependent morphology, magnetic and dielectric properties of BiFeO3 nanoparticles

AbstractNano-sized BiFeO3 were synthesized by sol-gel auto combustion method and report the effect of different annealing temperature (400 °C, 500 °C, 600 °C) on phase formation, morphology, magnetic and dielectric properties of synthesized bismuth ferrite (BiFeO3) nanoparticles. The phase formation of BFO nanoparticles were confirmed by X-ray diffraction pattern. Further, significant increment in particle size with increasing annealing temperature was estimated by field emission electron microscopy (FESEM). Magnetization curve showed the soft ferromagnetic behavior of the samples annealed at 400 OC and 500 OC that was explained on the basis of disturbance of spiral modulated long range antiferromagnetic order of bulk BFO. Dielectric response revealed decrease in dielectric constant with increasing annealing temperature. BFO is a room-temperature multiferroic material so it is potential candidate for various applications viz. Water waste treatment, gas sensors and photovoltaic cells in rural areas.

Neutron diffraction study of a metallic kagome lattice, Tb3Ru4Al12

We present the results of neutron diffraction studies on polycrystals of a metallic kagome lattice, Tb3Ru4Al12, reported recently to undergo reentrant magnetism, with the onset of long range antiferromagnetic order below (TN=) 22 K and spin-glass features below about 17 K. The present results reveal long-range antiferromagnetic order of an incommensurate type with the moments oriented along c-axis at all temperatures below TN. There are however notable changes in the T dependence of propagation vector along b-axis across 17 K. An observation of interest is that there is no decrease of intensity of magnetic Bragg peaks on entrance into the glassy phase (that is, below 17 K). This finding suggests that the magnetism of this compound is an exotic one and we wonder whether this compound is an example for dynamical spin-glass freezing phenomenon, as a consequence of geometrical frustration.

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.

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

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.