Presence Of Antiferromagnetism Research Articles

THE INFLUENCE OF STRUCTURAL AND COMPOSITIONAL FACTORS ON THE REALIZATION OF THE EXCHANGE BIAS EFFECT IN (Cr-Mn)/Fe20Ni80

The influence of a number of physical factors on the structural and hysteresis properties of multilayer films (Cr-Mn)/Fe Ni has been studied. By indirect signs, the presence of antiferromagnetism in Cr-Mn layers with a Mn content in the range of 20-40 at.% has been established. It is shown that in such structures, the exchange bias effect can be observed, but only when the thickness of the antiferromagnetic layer is greater than 40 nm. The initial reason for the low "fixing" properties of the Cr-Mn layer is its weak magnetic anisotropy, which is superimposed with instability in the reproduction of the micro-structure. The use of substrate heating during film deposition increased the reproducibility of microstructure parameters and hysteresis characteristics but led to a weakening of the exchange bias effect, apparently due to changes in the structure and composition of the interlayer interface.

Study of superconducting gap in presence of antiferromagnetism in iron pnictide superconductors

A Hamiltonian model of the pnictide composite superconductors is presented to study the Superconducting(SC) gap. The total Hamiltonian has been solved by writing equations of motion for single particle Greens function of Zubarev type. Equations for appropriate single particle correlation functions are derived and the order parameters corresponding to superconductivity and antiferromagnetism are determined. The equations for superconducting gap and antiferromagnetic gap are solved self-consistently and numerically. The multi-gap superconductivity is studied for different values of interlayer coupling constant parameters and Debye frequencies.

Topological signatures of the coexistence of antiferromagnetism and odd-parity spin-triplet superconductivity

Strongly correlated systems exhibit a rich phenomenology due to the antagonism of a diversity of ordered phases. The aftermath of this interplay can lead to a coexistence which takes place at a microscopic level, or, a phase separation in which non-overlapping single-order domains extend throughout the material. In most cases it appears experimentally challenging to disentangle the two scenarios, unless there exist robust and measurable properties particular to only one of the two types of coexistence. This is for instance the case when the type of coexistence decides on the appearance of topologically protected excitations, such as, Majorana fermions. In this work, we explore a concrete example falling into this category of systems, and specifically, we investigate one-dimensional odd-parity spin-triplet superconductors in the presence of antiferromagnetism. We determine the symmetry conditions for the occurrence of Majorana edge states and explore their response to variations of the strength and orientation of the antiferromagnetic field M, as well as, the spin structure of the Cooper pairs controlled by the so-called d-vector.

Metal-insulator transitions, superconductivity, and magnetism in the two-band Hubbard model

We explore the ground-state properties of the two-band Hubbard model with degenerate electronic bands, parametrized by nearest-neighbor hopping $t$, intra- and inter-orbital on-site Coulomb repulsions $U$ and $U^\prime$, and Hund coupling $J$, focusing on the case with $J>0$. Using Jastrow-Slater wave functions, we consider both states with and without magnetic/orbital order. Electron pairing can also be included in the wave function, in order to detect the occurrence of superconductivity for generic electron densities $n$. When no magnetic/orbital order is considered, the Mott transition is continuous for $n=1$ (quarter filling); instead, at $n=2$ (half filling), it is first order for small values of $J/U$, while it turns out to be continuous when the ratio $J/U$ is increased. A significant triplet pairing is present in a broad region around $n=2$. By contrast, singlet superconductivity (with $d$-wave symmetry) is detected only for small values of the Hund coupling and very close to half filling. When including magnetic and orbital order, the Mott insulator acquires antiferromagnetic order for $n=2$; instead, for $n=1$ the insulator has ferromagnetic and antiferro-orbital orders. In the latter case, a metallic phase is present for small values of $U/t$ and the metal-insulator transition becomes first order. In the region with $1<n<2$, we observe that ferromagnetism (with no orbital order) is particularly robust for large values of the Coulomb repulsion and that triplet superconductivity is strongly suppressed by the presence of antiferromagnetism. The case with $J=0$, which has an enlarged SU(4) symmetry due to the interplay between spin and orbital degrees of freedom, is also analyzed.

The hybrid lattice of KxFe2−ySe2: where superconductivity and magnetism coexist

Much remains unknown of the microscopic origin of superconductivity in atomically disordered systems of amorphous alloys or in crystals riddled with defects. A manifestation of this conundrum is envisaged in the highly defective superconductor of KxFe2−ySe2. How can superconductivity survive under such crude conditions that call for strong electron localization? Here, we show that the Fe sublattice is locally distorted and accommodates two kinds of Fe valence environments giving rise to a bimodal bond-distribution, with short and long Fe bonds. The bimodal bonds are present even as the system becomes superconducting in the presence of antiferromagnetism, with the weight continuously shifting from the short to the long with increasing K content. Such a hybrid state is most likely found in cuprates as well while our results point to the importance of the local atomic symmetry by which exchange interactions between local moments materialize.

Effect of hybridization and dispersion of quasiparticles on the coexistent state of superconductivity and antiferromagnetism in RNi2B2C

The effect of hybridization of conduction electrons and f-level on superconductivity (SC) and antiferromagnetism (AFM) in the coexistent phase of rare-earth nickel borocarbide superconductors (RNi2B2C) is reported. The Hamiltonian of the system is a mean field one and has been solved by writing equations of motion for the single-particle Green functions. It is assumed that superconductivity arises due to BCS pairing mechanism in the presence of antiferromagnetism in nickel lattices of Ni2B2 plane. The expressions for superconducting and antiferromagnetic order parameters are derived using double time electron Green functions. The quasiparticle energy bands are plotted and the nature of band dispersion of the quasiparticles is studied.

X-ray absorption and emission spectroscopic investigation of Mn doped ZnO films

The electronic structure of (Zn,Mn)O films with different Mn concentrations has been investigated by element-selective soft X-ray absorption and emission spectroscopy. The band gap narrowing of (Zn,Mn)O with increase of Mn concentration (<20% Mn) is attributed to the Mn doping and sp– d exchange interactions. According to analysis of the O Kα and resonant Mn L 2,3 X-ray emission spectra, the splitting of Mn 3 d subbands is related to Mn-derived states. It indicates that ferromagnetic coupling in (Zn,Mn)O can be taken into account to be carrier-induced. The presence of antiferromagnetism in the heavier Mn-doped films can be explained in terms of the existence of MnO secondary phases.

Dispersion of quasi-particles in high T c cuprates

The interplay between superconductivity (SC) and antiferromagnetism (AFM) is studied in strongly correlated systems of high Tc Cuprate superconductors. It is assumed that superconductivity arises due to BCS pairing mechanism in presence of AFM in Cu lattices of Cu-O planes. The total Hamiltonian of the system is mean field one and has been solved exactly by writing the equations of motion for the single particle Green’s functions. Equations for the appropriate single particle co-relation functions are derived and the order parameters corresponding to SC and AFM are determined. It is assumed that the Fermi energy ∈F = 0 and the renormalized localized f energy level coincide with the Fermi level. All the quantities in the final equation for h and Δ are made dimensionless by dividing by 2t, where t is the hopping integral. The temperature dependent values of staggered magnetic field (h) and SC gap (Δ) were determined by solving self-consistent equations for h and Δ. The quasiparticle energy bands are function of AFM gap (h), SC gap (Δ) and hybridization (V). Then the dispersion of quasi-particles are studied at different temperatures by considering temperature dependent values of h and Δ and varying other different model parameters.

Mutual influence of superconductivity and magnetism in strongly correlated cuprates

The interplay between superconductivity (SC) and anti-ferromagnetism (AFM) is studied in strongly correlated systems: R 2− x M x CuO 4 (R=Nd, La, Pr, Gd; M=Sr, Ce). It is assumed that SC arises due to BCS pairing mechanism in the presence of AFM in Cu lattices of Cu–O planes. Temperature dependence of SC gap as well as staggered magnetic field are calculated analytically and solved self-consistently with respect to half-filled band situation for different model parameters ( λ 1 and λ 2 being the SC and AFM coupling parameters, respectively). The SC gap is studied in the co-existent phase of SC and AFM.

PHONON ANOMALY IN RAMAN ACTIVE PHONONS OF CUPRATES IN NORMAL STATE

We present here a microscopic theory to explain phonon anomaly in Raman active phonons of the cuprates R 2-x M x CuO 4 in normal state. We used electronic Hamiltonion of Fulde in presence of antiferromagnetism. We incorporated phonon interaction to the hybridisation between the conduction electrons of the system and the f-electrons. The spectral density is calculated by the Green fuction technique of Zubarev type at zero wave vector and low temperature limit. Temperature dependence of Raman active phonon frequencies are studied numerically to discern phonon anomalies by varying model parameters like position of f-level (∊f), the effective electron–phonon coupling strength (g), the staggered magnetic field (h) and the hybridisation parameter (V). The phonon anomalies are explained on the basis of softening, hardening and temperature dependence of different Raman modes.

Fermi Level Pinning of a Massive Electron State in YbBiPt

The electronic structure of the heavy fermion compound YbBiPt is investigated using energy band calculations which account for strong on-site f -electron Coulomb correlations. We find that in the very low free-electron-like density near the Fermi level one extremely narrow 4f state becomes pinned at the Fermi energy E{sub F} . The pinned 4f state gives, in the presence of antiferromagnetism, rise to massive electron behavior for which we calculate an unenhanced Sommerfeld constant of 0.25Jmol{sup -1}K{sup -2} maximally. {copyright} {ital 1997} {ital The American Physical Society}

Neutron scattering from the geometrically frustrated pyrochlore antiferromagnet Tb2Mo2O7 (abstract)

The magnetic metal ions in the cubic pyrochlore Tb2Mo2O7 form an infinite three-dimensional network of corner-sharing tetrahedra with a very high potential for frustration in the presence of antiferromagnetism. Previous bulk susceptibility measurements show strong irreversibilities below a temperature of Tf∼25 K. We have performed neutron scattering measurements that show short range spatial correlations that develop continuously with decreasing temperature, while the characteristic time scale for the fluctuating moments decreases dramatically below Tf. Further, the results indicate extremely slow spin dynamics are relevant near Tf. The low temperature state of this pure material, which exhibits frustration that is solely geometrical in origin, closely resembles that of a spin glass.

Spin freezing in the geometrically frustrated pyrochlore antiferromagnet Tb2Mo2O7.

The magnetic metal ions in the cubic pyrochlore ${\mathrm{Tb}}_{2}$${\mathrm{Mo}}_{2}$${\mathrm{O}}_{7}$ form an infinite three-dimensional network of corner-sharing tetrahedra with a very high potential for frustration in the presence of antiferromagnetism. We have performed neutron scattering measurements which show short-range spatial correlations that develop continuously with decreasing temperature, while the characteristic time scale for the fluctuating moments decreases dramatically below ${\mathit{T}}_{\mathit{f}}$\ensuremath{\sim}25 K. Therefore, this pure material, which possess frustration that is purely geometrical in origin, displays a spin-glass state at low temperatures.

A mean field approach for an extended Hubbard model with correlated hopping: interplay of antiferromagnetism and superconductivity

An extended Hubbard model, including Coulomb repulsion and correlated hopping between nearest neighbors, is studied using a mean field approach. Superconducting solutions are tested in the presence of antiferromagnetism, near to and away from half filling. Competition between on-site and intersite repulsions, modulated by correlated hopping, yields a covalent bond configuration with partial intrasite and intersite pairings. In addition, antiferromagnetism induces mixed s- and p-type pairing, and superconductivity is completely suppressed by the saturation of the staggered magnetic moment.

Covalent bond pairing in an extended Hubbard model with correlated hopping : Interplay of antiferromagnetism and superconductivity

An extended one-band Hubbard model, including Coulomb repulsion and correlated hopping between nearest neighbors, is studied for the square lattice. The onset of superconductivity in the presence of antiferromagnetism is tested, near to and away from half filling, using a mean field approach. Competition between on-site and inter-site repulsion, modulated by correlated hopping, yields a covalent bond configuration with partial intrasite and intersite pairings. In addition, antiferromagnetism induces a mixed p- and s-type pairing, and completely suppresses superconductivity when the magnetic moment gets saturation. We discuss the relevance of the present model in relation to copper oxide superconductors.

Penetration depths of antiferromagnetic superconductors: NdRh4B4 and SmR4B4 (abstract)

We have investigated two antiferromagnetic superconductors, NdRh4B4 and SmRh4B4, which exhibit two quite different temperature dependencies of the upper critical field arising from two different mechanisms1 of the modification of superconductivity by the presence of antiferromagnetism. In order to further study these mechanisms, we have investigated the electromagnetic properties and have measured the penetration depths λ(T) of NdRh4B4 (Tc=5.45 K, TN1=1.31 K, and TN2=0.89 K) and SmRh4 B4 (Tc=2.2 K, TN=0.9 K), employing a tunnel diode oscillator at ∼30 MHz. Above the antiferromagnetic transition temperature (TN1 for NdRh4B4 and TN for SmRh4B4), the temperature dependencies of the penetration depths of these two superconductors fit the BCS theory well. We have found λ(0)=2170 and 980 Å for NdRh4B4 and SmRh4B4, respectively. However, as temperature is lowered, λ(T) for NdRh4B4 increases near TN1 and then saturates near TN2, presumably owing to the molecular field effect.2 The spin-fluctuation effects,2 expected to be significant near TN2 for NdRh4B4 and TN for SmRh4B4, have not been observed within our experimental accuracy.

Synthesis and Characterization of Metal Derivatives of a Carboxylic Group Containing Organoaminophosphonic Acid

Abstract Polynuclear metal complexes have been synthesized using 1-amino-2-carboxyethane-1,1-diphosphonic acid as ligand with bi- and tri-valent transition metal ions. I.R. (far and near), electronic and ESR spectral data, magnetic moment values and thermal analyses have been used for structural characterisation. Elemental analyses suggests the general formula of the complexes M5L2.xH2O (x [dbnd] 2–5) and Fe5L3.2H2O. The complexes have polymeric six-coordinate structure. Subnormal magnetic moment values showed the presence of antiferromagnetism due either to direct metal-metal interactions or via super-exchange.

Lowest energy Cooper pairing in the presence of antiferromagnetism

Cooper pairing of electron eigenstates of an antiferromagnet involves considerable complexity in spin space and in phases of the order parameters. With a BCS interaction the pairing scheme with lowest free energy has anisotropic spin-matrix order parameter ▪( k); however ▪ +( k) ▪( k)=|△| 2 ▪ . Magnitude of the anisotropic order parameter satisfies the simple BCS gap equation but with electron energies of the antiferromagnet eigenstates appearing instead of Bloch state energies of the nonmagnetic crystal.

Cooper pairing in the presence of antiferromagnetism

With a BCS interaction, the free energy for usual BCS pairing of electrons but in the presence of antiferromagnetism is shown to be lower than for a different pairing scheme where pairs are formed from electron eigenstates of the antiferromagnet. In both pairing schemes, super-conductivity in the presence of antiferromagnetism is always a mixture of spin-singlet-even-parity-orbital and spin-triplet-odd-parity-orbital electron pairs.

Preparation and investigation of a mixed valence complex of potassium. Manganese and cyanide containing manganese-(II) and -(III)

A crystalline complex has been prepared which contains both MnII and MnIII. The chemical composition corresponds to 1·2[K4Mn(CN)6]0·7[K3Mn(CN)6], ¼ H2O and u.v., i.r., and visible spectral characteristics are consistent with those of a class II mixed valence compound. The effective magnetic moment for the manganese ion was found to be comparatively low indicating the presence of antiferromagnetism in the complex which is also consistent with the properties of a class II compound. The unit cell dimensions, space group, and number of formula weights in the unit cell were also determined.