Coexistence Of Antiferromagnetic Order Research Articles

Coexistence of zigzag antiferromagnetic order and superconductivity in compressed NiPSe3

NiPSe3 is regarded as a bandwidth-controlled Mott insulator, distinct from the widely studied Mott insulating magnetic graphene MPSe3 (M = Mn and Fe) family. By employing high-pressure synchrotron X-ray diffraction, we observe two structural transitions as a function of pressure. With the help of first-principles calculations, we discover the antiferromagnetic (AFM) moment directions of NiPSe3 switch from out-of-plane to in-plane and the honeycomb layers slide relative to each other at the first structural transition. The in-plane AFM order persists until the second structural transition, whereupon the two-dimensional (2D) structure assumes a more three-dimensional (3D) character. A bandwidth-controlled Mott insulator-metal transition (IMT) occurs between the two structural transitions at Pc ~ 8.0 GPa, concomitant with the emergence of superconductivity with Tc ~ 4.8 K. The superconductivity in NiPSe3 emerging in the 2D monoclinic phase coexists with the in-plane AFM order and continues into the 3D trigonal phase. Our electronic structure calculations reveal that the IMT and superconductivity in NiPSe3 are both related to the enhanced Se2− 4p and Ni2+3d electronic hybridizations under pressure. From these results, we construct a temperature-pressure electronic phase diagram of NiPSe3, revealing interesting relationships between magnetism and superconductivity.

Magnetotransport property of oxygen-annealed Fe1+y Te thin films

Fe-based superconductors are one of the current research focuses. FeTe is unique in the series of FeSe1−x Te x , since it is nonsuperconducting near the FeTe side in the phase diagram in contrast to the presence of superconductivity in other region. However, FeTe thin films become superconducting after oxygen annealing and the mechanism remains elusive. Here, we report the temperature dependences of resistivity, Hall effect and magnetoresistance (MR) of a series of FeTe thin films with different amounts of excess Fe and oxygen. These properties show dramatic changes with excess Fe and oxygen incorporation. We found the Hall coefficients are positive for the oxygen-annealed samples, in contrast to the transition from positive to negative below 50 K for the vacuum-annealed samples. For all samples, both the resistivity and Hall coefficient show a dramatic drop, respectively, at around 50 K–75 K, implying coexistence of superconductivity and antiferromagnetic order for the oxygen-annealed samples. The vacuum-annealed samples show both positive and negative values of MR depending on temperature, while negative MR dominates for the oxygen-annealed samples. We also found that oxygen annealing reduces the excess Fe in FeTe, which has been neglected before. The results are discussed in terms of several contributions, and a comparison is made between the oxygen-annealed FeTe thin films and FeSe1−x Te x . This work is helpful for shedding light on the understanding of oxygen-annealed FeTe thin films.

An unusual magnetic phenomenon in Mg doping [(CH3)2NH2] CoxMg1−x[HCOO]3 frameworks

In this study, Mg doping metal-organic frameworks (MOFs) [(CH3)2NH2] Co1−x Mg x [HCOO]3 single crystals were synthesized by solvothermal method with normal ratios x = 0, 0.1, 0.2, 0.3, 0.4, 0.6. Powder X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) results showed that Mg ion is uniformly doped into Co-based MOFs crystal. With the increase of diamagnetic magnesium ions doping, the antiferromagnetic phase transition temperature of Co-based MOFs for pure Co-MOF decreased from 15 K to 7.5 K for x = 0.4. Abnormal larger magnetic hysteresis was obtained for Mg doping crystals with a large coercive field 3100 Oe (x = 0.1), 3700 Oe (x = 0.2), 2900 Oe (x = 0.3) compared with 600 Oe coercive field for pure Co-MOFs (x = 0). At the same time, step-like behavior was first observed on Mg2+ doping sample with x = 0.1–0.3. This unusual magnetic phenomenon is well interpreted based on the coexistence of long-range canted antiferromagnetic order and isolated single-ion quantum magnets. The diamagnetic magnesium ions doping decreases the strong exchange coupling between neighboring Co ions and step-like behavior stemmed from the quantum tunneling of isolated Co ions.

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.

Coupling between antiferromagnetic and spin-glass orders in the quasi-one-dimensional iron tellurideTaFe1+xTe3(x=0.25)

Understanding the interplay among different magnetic exchange interactions and its physical consequences, especially in the presence of itinerant electrons and disorders, remains one of the central themes in condensed matter physics. In this vein, the coupling between antiferromagnetic and spin glass orders may lead to large exchange bias, a property of potential broad technological applications. In this article, we report the coexistence of antiferromagnetic order and spin glass behaviors in a quasi-one-dimensional iron telluride TaFe$_{1+x}$Te$_3$ ($x$=0.25). Its antiferromagnetism is believed to arise from the antiferromagnetic interchain coupling between the ferromagnetically aligned FeTe chains along the $b$-axis, while the spin glassy state stems from the disordered Fe interstitials. This dichotomic role of chain and interstitial sublattices is responsible for the large exchange bias observed at low temperatures, with the interstitial Fe acting as the uncompensated moment and its neighboring Fe chain providing the source for its pinning. This iron-based telluride may thereby represent a new paradigm to study the large family of transition metal chalcogenides whose magnetic order or even the dimensionality can be tuned to a large extent, forming a fertile playground to manipulate or switch the spin degrees of freedom thereof.

Coexistence of Charge Order and Antiferromagnetic Order in an Extended Periodic Anderson ModelSupported by the National Natural Science Foundation of China (Grant Nos. 11974048 and 11974049), and the Beijing Science Foundation (Grant No. 1192011).

The competition between the RKKY interaction and the Kondo effect leads to a magnetic phase transition, which occurs ubiquitously in heavy fermion materials. However, there are more and more experimental evidences indicating that the valence fluctuation plays an essential role in the Ce- and Y-based compounds. We study an extended periodic Anderson model (EPAM) which includes the onsite Coulomb repulsion Ucf between the localized electrons and conduction electrons. By employing the density matrix embedding theory, we investigate the EPAM in the symmetric case at half filling. By fixing the onsite Coulomb repulsion U of the localized electrons to an intermediate value, the interplay between the RKKY interaction, the Kondo effect and the Coulomb repulsion Ucf brings rich physics. We find three different phases, the antiferromagnetic phase, the charge order phase and paramagnetic phase. When the hybridization strength V between the localized orbital and the conduction orbital is small, the Kondo effect is weak so that the AF phase and the CO phase are present. The phase transition between the two long-range ordered phase is of first order. We also find a coexistence region between the two phases. As V increases, the Kondo effect becomes stronger, and the paramagnetic phase appears between the other two phases.

Coexistence of superconductivity and antiferromagentic order in Er2O2Bi with anti-ThCr2Si2 structure

We investigated the coexistence of superconductivity and antiferromagnetic order in the compound Er2O2Bi with anti-ThCr2Si2-type structure through resistivity, magnetization, specific heat measurements and first-principle calculations. The superconducting transition temperature Tc of 1.23 K and antiferromagnetic transition temperature TN of 3 K are observed in the sample with the best nominal composition. The superconducting upper critical field Hc2(0) and electron-phonon coupling constant λe−ph in Er2O2Bi are similar to those in the previously reported non-magnetic superconductor Y2O2Bi with the same structure, indicating that the superconductivity in Er2O2 Bi may have the same origin as in Y2O2Bi. The first-principle calculations of Er2O2Bi show that the Fermi surface is mainly composed of the Bi 6p orbitals both in the paramagnetic and antiferromagnetic state, implying minor effect of the 4f electrons on the Fermi surface. Besides, upon increasing the oxygen incorporation in Er2OxBi, Tc increases from 1 to 1.23 K and TN decreases slightly from 3 K to 2.96 K, revealing that superconductivity and antiferromagnetic order may compete with each other. The Hall effect measurements indicate that hole-type carrier density indeed increases with increasing oxygen content, which may account for the variations of Tc and TN with different oxygen content.

Low temperature ferromagnetism in CaCu3Ti4O12 * *Project supported by the National Natural Science Foundation of China (Grant Nos. 11504227 and 51971128), the Program of Shanghai Academic/Technology Research Leader, China (Grant No. 20XD1401800), and the Project of the Science and Technology Commission of Shanghai Municipality, China (Grant No. 19020501000).

The low-temperature magnetic order behaviors of perovskite oxide CaCu3Ti4O12 (CCTO) ceramics prepared by different methods are discussed. X-ray diffraction, scanning electron microscope, x-ray photoelectron spectroscopy, and direct current (DC) magnetization are used to characterize the structures, microscopic morphologies, valence states, and magnetic properties of the samples. The results show that the magnetic behaviors of CCTO ceramics are very sensitive to the preparation process. The quenched CCTO ceramic and CCTO powders grown in a molten salt crystal, which contain much more oxygen vacancies and Ti3+, show the coexistence of weak ferromagnetic order and antiferromagnetic order below the Neel temperature. It suggests that the bound magnetopolaron formed by oxygen vacancies and Ti3+ ion composite defects are responsible for the weak ferromagnetic order at low temperature.

Impact of hydrogenation on the structural, dielectric and magnetic properties of La0.5Ca0.5MnO3

Impact of hydrogenation on the structural, dielectric and magnetic properties of half-doped La0.5Ca0.5MnO3 manganate have been investigated. Polycrystalline sample of La0.5Ca0.5MnO3 was prepared through solid state reaction at ~ 1300 °C. Subsequently, the prepared sample was annealed at 600 °C for 6 h in a continuous flow of hydrogen gas in a tubular reduction furnace. Single-phase orthorhombic structure of the hydrogenated La0.5Ca0.5MnO3 in the space group Pnma was confirmed by Rietveld refinement of PXRD and FTIR analysis. SEM micrographs revealed well resolved grains of ~ 2 μm in a good crystalline sample. Hydrogenated La0.5Ca0.5MnO3 showed a sharp optical absorption peak in the UV range with a bandgap energy of 4.96 eV. Hydrogenation of La0.5Ca0.5MnO3 leads to significant reduction in the magnitude of dielectric constant and tangent loss at room temperature. Paramagnetic character of La0.5Ca0.5MnO3 at 300 K is retained upon the hydrogenation but the low temperature magnetic properties get modified dramatically. Temperature and field dependent magnetization measurements showed that hydrogenated La0.5Ca0.5MnO3 undergoes paramagnetic to antiferromagnetic transition at TN = ~ 110 K and in a disordered magnetic phase below 50 K. Systematic enhancements in the magnitude of saturation magnetization (Ms), coercivity (Hc), remanence (Mr) and squareness ratio at 50 K and 20 K indicate for the coexistence of antiferromagnetic and weak ferromagnetic order due to competitive magnetic exchange interactions between Mn3+ and Mn4+ ions. This study shows that hydrogenation plays key role in the modification of dielectric and magnetic properties of La0.5Ca0.5MnO3.

Effect of antiferromagnetic order on topological electronic structure in Eu-substituted Bi2Se3 single crystals

Topological antiferromagnetic spintronics is an emerging field due to advantages such as no stray field effect and high-speed dynamics. In bismuth chalcogenide topological insulators, the coexistence of long-range antiferromagnetic order and topologically protected states has scarcely been observed. Here, an antiferromagnetic order, which sets in the direction of magnetic moments perpendicular to the c axis, is introduced by Eu substitution in Bi2Se3, and therefore it influences the topological electronic properties of Bi2Se3. Despite Eu substitution, Shubnikov–de Haas (SdH) oscillations are observed. The angle dependence of SdH oscillations shows a signature of anisotropic 3D Fermi surfaces with a nontrivial Berry phase by Eu substitution, and the temperature dependence of SdH oscillations reveals that the effective mass is comparable to the pristine Bi2Se3. These results suggest that the nontrivial topological state can survive in the antiferromagnetic order of Eu-substituted Bi2Se3. Our work expands the base of topological materials available for antiferromagnetic spintronics applications.

Anisotropic fluoride nanocrystals modulated by facet-specific passivation and their disordered surfaces

Rutile-type fluorides have been proven to be active components in the context of emerging antiferr-omagnetic devices. However, controlled synthesis of low-dimensional, in particular two-dimensional (2D), fluorides in a predictable and deterministic manner remains unrealized because of a lack of efficient anisotropic control, which impedes their further development in reduced dimensions. We report here that altered passivation of {110} growing facets can direct the synthesis of rutile-type fluoride nanocrystals into well-defined zero-dimensional (0D) particulates, one-dimensional (1D) rods and 2D sheets in a colloidal approach. The obtained nanocrystals show positive exchange bias and enhanced magnetic transition temperature from the coexistence of long-range antiferromagnetic order and disordered surface spins, making them strong alternatives for flexible magnetic devices and sensors.

Atomic-scale coexistence of short-range magnetic order and superconductivity in Fe1+ySe0.1Te0.9

The ground state of the parent compounds of many high temperature superconductors is an antiferromagnetically (AFM) ordered phase, where superconductivity emerges when the AFM phase transition is suppressed by doping or application of pressure. This behaviour implies a close relation between the two orders. Understanding the interplay between them promises a better understanding of how the superconducting condensate forms from the AFM ordered background. Here we explore this relation in real space at the atomic scale using low temperature spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy. We investigate the transition from antiferromagnetically ordered $\mathrm{Fe}_{1+y}\mathrm{Te}$ via the spin glass phase in $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.1}\mathrm{Te}_{0.9}$ to superconducting $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.15}\mathrm{Te}_{0.85}$. In $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.1}\mathrm{Te}_{0.9}$ we observe an atomic-scale coexistence of superconductivity and short-ranged bicollinear antiferromagnetic order.

Superconductivity of strongly correlated electrons on the honeycomb lattice

A microscopic theory of the electronic spectrum and of superconductivity within the t-J model on the honeycomb lattice is developed. We derive the equations for the normal and anomalous Green functions in terms of the Hubbard operators by applying the projection technique. Superconducting pairing of d + id′-type mediated by the antiferromagnetic exchange is found. The superconducting Tc as a function of hole doping exhibits a two-peak structure related to the van Hove singularities of the density of states for the two-band t-J model. At half-filling and for large enough values of the exchange coupling, gapless superconductivity may occur. For small doping the coexistence of antiferromagnetic order and superconductivity is suggested. It is shown that the s-wave pairing is prohibited, since it violates the constraint of no-double-occupancy.Graphical abstract

Absence of a spontaneous long-range order in a mixed spin-(1/2, 3/2) Ising model on a decorated square lattice due to anomalous spin frustration driven by a magnetoelastic coupling

The mixed spin-(1/2, 3/2) Ising model on a decorated square lattice, which takes into account lattice vibrations of the spin-3/2 decorating magnetic ions at a quantum-mechanical level under the assumption of a perfect lattice rigidity of the spin-1/2 nodal magnetic ions, is examined via an exact mapping correspondence with the effective spin-1/2 Ising model on a square lattice. Although the considered magnetic structure is in principle unfrustrated due to bipartite nature of a decorated square lattice, the model under investigation may display anomalous spin frustration driven by a magnetoelastic coupling. It turns out that the magnetoelastic coupling is a primary cause for existence of the frustrated antiferromagnetic phases, which exhibit a peculiar coexistence of antiferromagnetic long-range order of the nodal spins with a partial disorder of the decorating spins with possible reentrant critical behavior. Under certain conditions, the anomalous spin frustration caused by the magnetoelastic coupling is responsible for unprecedented absence of spontaneous long-range order in the mixed-spin Ising model composed from half-odd-integer spins only.

Dielectric and Magnetic Properties of NixPb1-xTiO3 Solid Solution and Composite: Coexistence of Ferroelectric and Antiferromagnetic Order

A complete serie of NixPb1-xTiO3 samples were synthesized by solid state reaction and their structural, vibrational, dielectric and magnetic properties were properly characterized using X-ray diffraction, Raman scattering, dielectric permittivity, magnetic susceptibility and magnetization loops. The XRD and Raman results showed the formation of solid solutions for x below 0.3, while for higher values of x there is preferentially the formation of a PbTiO3 and NiTiO3 composite. The dielectric and magnetic measurements exhibited the coexistence of the ferroelectric and antiferromagnetic orders for Ni concentrations from 0.3 to 0.9, a very interesting result for magnetoelectric device application.

Spin dynamics of antiferromagnets in the presence of a homogeneous magnetization

We use general hydrodynamic equations to determine the long-wavelength spin excitations in isotropic antiferromagnets in the presence of a homogeneous magnetization. The latter may be induced, such as in antiferromagnets in an external magnetic field, or spontaneous, such as in ferrimagnetic or canted phases that are characterized by the coexistence of antiferromagnetic and ferromagnetic order. Depending on the physical situation, we find propagating spin waves that are gapped in some cases and gapless in others, diffusive modes, or relaxational modes. The excitation spectra turn out to be qualitatively different depending on whether or not the homogeneous magnetization is a conserved quantity. The results lay the foundation for a description of a variety of quantum phase transitions, including the transition from a ferromagnetic metal to an antiferromagnetic one, and the spin-flop transitions that are observed in some antiferromagnets. They also are crucial for incorporating weak-localization and Altshuler-Aronov effects into the descriptions of quantum phases in both clean and disordered magnetic metals.

Entropy Evolution in the Magnetic Phases of Partially Frustrated CePdAl.

In the heavy-fermion metal CePdAl, long-range antiferromagnetic order coexists with geometric frustration of one-third of the Ce moments. At low temperatures, the Kondo effect tends to screen the frustrated moments. We use magnetic fields B to suppress the Kondo screening and study the magnetic phase diagram and the evolution of the entropy with B employing thermodynamic probes. We estimate the frustration by introducing a definition of the frustration parameter based on the enhanced entropy, a fundamental feature of frustrated systems. In the field range where the Kondo screening is suppressed, the liberated moments tend to maximize the magnetic entropy and strongly enhance the frustration. Based on our experiments, this field range may be a promising candidate to search for a quantum spin liquid.

Coexistence of long-range antiferromagnetic order and slow relaxation of the magnetization in the first lanthanide complex of a 1,2,4-benzotriazinyl radical.

The first lanthanide complex of a 1,2,4-benzotriazinyl radical (1), Dy(1)(tbacac)3 (2, tbacac = 2,2,6,6-tetramethyl-3,5-heptane-dionato), was synthesised and found to have an antiferromagnetically ordered ground state with a metamagnetic phase diagram and a critical field of 0.91 T at 1.85 K. The application of a small dc field revealed the single-molecule magnet behaviour of 2, illustrating the coexistence of long-range antiferromagnetic order and slow relaxation of the magnetization.

Doping evolution of the superconducting gap structure in the underdoped iron arsenideBa1−xKxFe2As2revealed by thermal conductivity

The thermal conductivity kappa of the iron-arsenide superconductor Ba1-xKxFe2As2 was measured for heat currents parallel and perpendicular to the tetragonal c axis at temperatures down to 50 mK and in magnetic fields up to 15 T. Measurements were performed on samples with compositions ranging from optimal doping (x = 0.34; Tc = 39 K) down to dopings deep into the region where antiferromagnetic order coexists with superconductivity (x = 0.16; Tc = 7 K). In zero field, there is no residual linear term in kappa(T) as T goes to 0 at any doping, whether for in-plane or inter-plane transport. This shows that there are no nodes in the superconducting gap. However, as x decreases into the range of coexistence with antiferromagnetism, the residual linear term grows more and more rapidly with applied magnetic field. This shows that the superconducting energy gap develops minima at certain locations on the Fermi surface and these minima deepen with decreasing x. We propose that the minima in the gap structure arise when the Fermi surface of Ba1-xKxFe2As2 is reconstructed by the antiferromagnetic order.

Antiferromagnetism and superconductivity in the half-Heusler semimetal HoPdBi

We observed the coexistence of superconductivity and antiferromagnetic order in the single-crystalline ternary pnictide HoPdBi, a plausible topological semimetal. The compound orders antiferromagnetically at TN = 1.9 K and exhibits superconductivity below Tc = 0.7 K, which was confirmed by magnetic, electrical transport and specific heat measurements. The specific heat shows anomalies corresponding to antiferromagnetic ordering transition and crystalline field effect, but not to superconducting transition. Single-crystal neutron diffraction indicates that the antiferromagnetic structure is characterized by the propagation vector. Temperature variation of the electrical resistivity reveals two parallel conducting channels of semiconducting and metallic character. In weak magnetic fields, the magnetoresistance exhibits weak antilocalization effect, while in strong fields and temperatures below 50 K it is large and negative. At temperatures below 7 K Shubnikov-de Haas oscillations with two frequencies appear in the resistivity. These oscillations have non-trivial Berry phase, which is a distinguished feature of Dirac fermions.