Appearance Of Antiferromagnetism Research Articles

Effect of Ta capping layer on the magnetic coupling oscillation of L10-MnGa/Co/Ta films

The magnetic exchange interactions between L10-MnGa and ultrathin Co film with Ta capping layer were studied using the first-principles calculations. An oscillation of magnetic coupling from antiferromagnetic (AFM) to ferromagnetic (FM) couplings was observed with increasing the thickness of Co layers. The oscillation period is two monolayers, which was attributed to the quantum well states mainly confined in the Co layers. The appearance of AFM and FM states of L10-MnGa/Co/Ta correspond to the odd and even number of Co layers, respectively. In contrast, the oscillation was completely reversible with that in L10-MnGa/Co without Ta layers, clearly indicating Ta capping layers strongly affect magnetic coupling behavior between L10-MnGa and Co. The related mechanism was discussed by analyzing the spin resolved energy bands and charge densities. The results are useful to understand the magnetic coupling interactions and the confinement behavior of spin electrons in multilayer films, benefiting to develop new magnetic nanostructures.

Mn2VAl Heusler alloy thin films: appearance of antiferromagnetism and exchange bias in a layered structure with Fe

Mn2VAl Heusler alloy films were epitaxially grown on MgO(1 0 0) single-crystal substrates by ultra-high-vacuum magnetron sputtering. A2- and L21-type Mn2VAl order was controlled by the deposition temperature. A2-type Mn2VAl films showed no spontaneous magnetization, while L21-type Mn2VAl films showed ferrimagnetic behaviour with a maximum saturation magnetization of 220 emu cm−3 at room temperature (RT). An antiferromagnetic reflection was observed with neutron diffraction at RT for an A2-type Mn2VAl film deposited at 400 °C. A bilayer sample of the antiferromagnetic A2-type Mn2VAl and Fe showed an exchange bias of 120 Oe at 10 K.

Appearance of antiferromagnetism and superconductivity in superconducting cuprates

We represent the superconducting ceramic compounds by the single band extended Hubbard model. We use this model for solving the simultaneous presence of antiferromagnetism and the d-wave superconductivity in the Hartree–Fock (H–F) and in the coherent potential (CP) approximation, which is applied to the on-site Coulomb repulsion U.The hopping interaction used in addition to the Coulomb repulsion causes rapid expansion of the band at carrier concentrations departing from unity. This expansion shifts the d-wave superconductivity away from the half filled point reducing its occupation range approximately to the experimental range in the YBaCuO compound.The CP approximation used for the Coulomb repulsion is justified by the large ratio of Coulomb repulsion to the effective width of perturbed density of states being reduced by the hopping interaction.The experimentally observed fast disappearance of antiferromagnetism is supported in calculations by the relatively large value of the total width of unperturbed density of states and the high values of the hopping interaction.It is shown that our model is capable of describing the electron doped compounds as well.

Complex magnetic phase separation induced by Li-doping in multiferroic CaBaCo4O7

The doping of lithium at the cobalt sites in the multiferroic orthorhombic cobaltite CaBaCo4O7 has been investigated. The oxides CaBaCo4-2xLixGaxO7 and CaBaCo4-xLixO7 keep the same polar space group as the parent phase. In contrast, they show a spectacular decrease of their ferrimagnetic properties for very low doping levels (0 < x ≤ 0.10), with the appearance of antiferromagnetism below TN ∼ 80 K and magnetic frustration at around Tf ∼ 20 K, which should have a great impact upon multiferroism in this system. This behavior is different from that of the Ga-doped phases CaBaCo4-xGaxO7, but remarkably similar to that of the Zn-doped phases CaBaCo4-xZnxO7. The actual role of the valence of the diamagnetic cation, Li+ is interpreted from its ability to sit in the Co2+ zig-zag chain, breaking the ferromagnetism along those chains in both series of oxides. Importantly, the Li-doping in CaBaCo4-xLixO7 oxides, introduces an excess Co3+ with respect to the other two series, CaBaCo4-xZnxO7 and CaBaCo4-2xLixGaxO7. This additional Co3+ is thought to be at the origin of the complex magnetic phase separation and especially of new magnetic interactions that appear around 52 K in competition with magnetic frustration and antiferromagnetism.

QED3theory of underdoped high-temperature superconductors

The low-energy theory of d-wave quasiparticles coupled to fluctuating vortex loops that describes the loss of phase coherence in a two-dimensional d-wave superconductor at $T=0$ is derived from first principles. The theory has the form of (2+1) dimensional quantum electrodynamics $({\mathrm{QED}}_{3}),$ and is proposed as an effective description of the $T=0$ superconductor-insulator transition and of the pseudogap phase in underdoped cuprates. The coupling constant (``charge'') in this theory is proportional to the dual order parameter of the $\mathrm{XY}$ model, which is assumed to describe fluctuations of the phase of the superconducting order parameter. Finiteness of the charge is then tantamount to the appearance of infinitely large vortex loops, i.e., to the loss of phase coherence in the system. The principal result is that the destruction of the superconducting phase coherence in the d-wave superconductors typically, and immediately, leads to the appearance of antiferromagnetism. This transition can be understood in terms of the spontaneous breaking of an approximate ``chiral'' ${\mathrm{SU}}_{c}(2)$ symmetry, which may be discerned at low enough energies in the standard d-wave superconductor. The mechanism of this spontaneous symmetry breaking is formally analogous to the dynamical mass generation in ${\mathrm{QED}}_{3},$ with the ``mass'' here being proportional to staggered magnetization. Other phases with broken chiral symmetry include the translationally invariant $``d+ip''$ and $``d+is$'' insulators, and the one-dimensional charge-density and spin-density waves, which are all insulating descendants of the d-wave superconductor. All the insulating states have neutral spin-$1/2$ excitations that one can identify in the superconductor confined by the logarithmic potential. Electron repulsion is in this formalism represented by a particular quartic perturbation to the ${\mathrm{QED}}_{3}$ action, which breaks the chiral symmetry and selects the antiferromagnet as the preferred broken symmetry state. I formulate the mean-field theory of the antiferromagnetic instability in presence of a short-range repulsive interaction, and find the staggered magnetization to be significantly enhanced deeper inside the insulating state. The theory offers an explanation for the rounded d-wave-like dispersion seen in angle-resolved photoemission spectroscopy experiments on the insulating ${\mathrm{Ca}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}$ [F. Ronning et al., Science 282, 2067 (1998)]. Relations to other theoretical approaches to the high-${T}_{c}$ problem are discussed.

Complex competition between ferromagnetism and antiferromagnetism in the CMR manganites Pr1−x Ca x MnO3

The systematic investigation of the magnetic susceptibility of the CMR manganites Pr1−x Ca x MnO3 versus temperature has been performed for 0.25 ≤ x ≤ 0.50. Due to the similar size of calcium and praseodymium, these results show the important role of the mixed valence of manganese upon the complex magnetic behaviour of these compounds. They demonstrate that the appearance of antiferromagnetism coincides with charge ordering, T N = T CO varying with x, from 250K for x = 0.50 to 225K for x = 0.35. A strong competition between ferromagnetism and antiferromagnetism is observed for 0.35 ≤ x 170 K) and going through canted antiferromagnetic or weak ferromagnetic states for intermediate temperatures (70 K < T < 170 K). For 0.25 ≤ x ≤ 0.30, a strong ferromagnetic state is observed for 95 K ≤ T ≤ 150 K, with a transition to a spin glass like state below 95–110 K.

Magnetic and transport properties of Na doped SrRuO3 and CaRuO3

The magnetic susceptibility χ(T) (2≤T≤400 K) and electrical resistivity ρ(T) (2≤T≤300 K) of undoped and Na doped SrRuO3 and CaRuO3 single crystals were measured. These two perovskites are orthorhombic with space group Pbnm. While both of the undoped systems exhibit metallic conductivity, SrRuO3 is ferromagnetic with Tc≤160 K, but CaRuO3 is paramagnetic. When a small concentration of Na (≤12%) replaces Sr in the structure, SrRuO3 becomes an insulator with a depressed ferromagnetic response. In contrast, Na doped CaRuO3 (≤5%) remains metallic with the appearance of antiferromagnetism below 70 K. The dramatic changes in magnetic and transport properties resulting from Na replacement of Sr and Ca reflects the very subtle differences driving the magnetism in SrRuO3 and CaRuO3.

Thermal properties of γ-Mn (abstract)

Mn can exist in a fcc environment in many alloys with other elements of the 3d-transition series. Some of these alloys, which can contain more than 50 at. % Mn, are classified as Invar. They show large magnetovolume effects and at the same time have ferromagnetic, antiferromagnetic, or mixed magnetic ground states. In order to gain a better understanding of Invar systems incorporating Mn, a closer examination of this element in a fcc environment is necessary. The γ state of Mn (fcc or fct Mn) is relatively little known, since its stability range lies only between 1364 and 1410 K. However, it is possible to stabilize γ-Mn in a temperature interval ranging from low temperatures up to about 600 K by the addition of small amounts of Cu. We have measured the specific heat (300&amp;lt;T&amp;lt;600 K) and thermal expansion (4&amp;lt;T&amp;lt;600 K and 1100&amp;lt;T&amp;lt;1300 K) of three MnCu alloys containing 4.42, 6.75, and 8.55 at. % Cu. The specific heat shows a first-order transition at Tt, which is the temperature below which the lattice undergoes a tetragonal distortion. This distortion is accompanied by an abrupt appearance of antiferromagnetism. The same effect is seen in the thermal expansion which shows a volume increase associated with the lattice distortion and the magnetic ordering. The high-temperature value of the thermal expansion coefficient is the largest so far observed in any fcc 3d system (45×10−6 1/K). From this data we estimate the thermal expansion behavior of pure γ-Mn. We obtain the atomic volume in the nonmagnetic and antiferromagnetic state and compare it to theoretical calculations.

The study of the thermal properties of γ-Mn from Mn-Cu alloys

The addition of small amounts of Cu to Mn stabilizes the fcc phase in Mn (γ-Mn) in a broad temperature range which is otherwise stable in a narrow limit around 1400 K. These alloys undergo an fcc-fct transition on cooling which is accompanied by an abrupt appearance of antiferromagnetism. In this paper we present specific heat and thermal expansion data of three Mn-Cu alloys containing 4.42, 6.75 and 8.55 at% Cu throughout their stability range. The specific heat data reflects the features of the structural magnetic transition and suggests a high temperature anharmonic part which is supported by the thermal expansion data.

CPA treatment of the extended Hubbard model

AbstractThe Hamiltonian for a completely degenerate d‐band including intra‐atomic Coulomb and exchange interactions is studied using the two‐sublattice alloy analogy method. The density of states and the criterion for the appearance of antiferromagnetism are calculated by this method. At the transition from paramagnetism to antiferromagnetism the energy gap appears. The interaction constant required to produce antiferromagnetism tends to zero at the maxima of the state density function.

Magnetic and optical study of NaCrO 2

Magnetic and optical measurements in the layer compound NaCrO 2 are reported. This compound is antiferromagnetic with T n = 46±2 K and θ = −400±30 K. Broad antiferromagnetic resonances are observed which cannot be interpreted on any simple model. Optical absorption data are consistent with the appearance of antiferromagnetism. Non-intrinsic fluorescence has also been observed.

Electrical Properties of Ni1-xS

Hexagonal nickel sulfide NiS shows a metal-nonmetal transition with the appearance of antiferromagnetism as temperature is lowered below 265 K ( T t ). Measurements of conductivity, Hall coefficient, thermoelectric power and magnetoresistance were made on polycrystalline samples of hexagonal Ni 1- x S (0.004≦ x ≦0.050). The low temperature phase is found to be a degenerate semiconductor with a high hole density from 3.13×10 20 cm -3 ( x =0.004) to 1.60 ×10 21 cm -3 ( x =0.020), which is essentially due to ionized nickel vacancies. Carriers are mainly scattered by ionized and neutral impurities associated with nickel vacancies. The effective mass is estimated from Seebeck coefficients to have a value between 1.50 m 0 and 2.90 m 0 . Above T t , the compound is a normal metal without a magnetic moment. The low temperature resistivity has revealed the Debye temperature to be 175 K for the metallic phase. There is no apparent evidence of a strong correlation effect although the electronic state densities are l...

Magnetism of γ Fe-Ni Invar Alloys with Low Nickel Concentration

Magnetization, Mossbauer absorption spectra and lattice constants have been measured on the γ phase (f.c.c.) Fe-Ni alloys of 20∼30 at% Ni using finely grained samples in which the γ→α martensitic transformation can be suppressed even at liq. He temperature. The magnetic moment decreases very sharply from 1.3µ B in 30 at% Ni to 0.2µ B in 20 at% Ni. The Mossbauer absorption spectra observed at liq. He temperature show the appearance of antiferromagnetism in the low nickel concentration range. These experimental facts indicate that in these composition range the alloys consist magnetically of two phases, namely ferromagnetic and antiferromagnetic ones. This magnetic heterogeneity is caused by the statistical concentration fluctuation and is discussed quantitatively by considering a Gaussian type concentration distribution. The temperature and composition variations of the lattice constant and the thermal expansion coefficient can also be interpreted quantitatively on the basis of the same Gaussian type conce...

Nuclear magnetic resonance in chromium-vanadium alloys

Observations were made of the V51 magnetic resonance shift and line width in chromium -vanadium solid solutions over a wide composition range. The Knight shift varies considerably with composition reaching a maximum value of 0.687% for an alloy cortaining 62% chromium. This may be connected with the appearance of magnetic moments on the Cr atoms. A slight dependence of shift on magnetic field is found and is attributed to second order quadrupole effects. The line width decreases to a value of about 1.5 gauss as the concentration of chromium increases due to the reduction dipolar broadening. A discussion of the field dependence of the line width is given. For vanadium concentrations less than 5% broad transitions appear at which the intensity of the resonance signal decreases. This is attributed to the appearance of antiferromagnetism.