Antiferromagnetic-like Behavior Research Articles

Discovery of Magnetic Phase Transitions in Heavy-Fermion Superconductor CeRh_{2}As_{2}.

We report on the specific heat studies performed on a new generation of CeRh_{2}As_{2} single crystals. Superior quality of the samples and dedicated experimental protocol allowed us to observe an antiferromagneticlike behavior in the normal state and to detect the first-order phase transition of magnetic origin within the superconducting state of the compound. Although in the available literature the physical behavior of CeRh_{2}As_{2} is most often described with the use of quadrupole density wave scenario, we propose an alternative explanation using analogies to antiferromagnetic heavy-fermion superconductors CeRhIn_{5} and Ce_{2}RhIn_{8}.

Magnetic Properties of (1 − x)Bi0.5Na0.5TiO3 + xCaCoO3−δ Solid-Solution System

The (1 − x)Bi0.5Na0.5TiO3 + xCaCoO3−δ solid-solution system has been fabricated by a simple sol–gel route. X-ray diffraction analysis and Raman scattering measurements were performed to determine the structure of the solid-solution system. The samples were polycrystalline and had perovskite structure with rhombohedral symmetry. With increasing concentration of CaCoO3−δ solid solution in the host Bi0.5Na0.5TiO3, the optical bandgap reduced from 3.06 eV for pure Bi0.5Na0.5TiO3 to 2.15 eV for 9 mol.% CaCoO3−δ-modified Bi0.5Na0.5TiO3. This process also suppressed the photoluminescence of the host Bi0.5Na0.5TiO3. Weak ferromagnetism as well as diamagnetism were observed for pure Bi0.5Na0.5TiO3, changing to ferromagnetism and finally compensated ferromagnetism versus paramagnetism and/or antiferromagnetic-like behavior as the concentration of CaCoO3−δ was increased. Such introduction of ABO3 impurities containing alkaline-earth and transition metals into the host Bi0.5Na0.5TiO3 could therefore enhance the magnetic properties of this lead-free ferroelectric material to produce multiferroic materials for use in smart electronic devices.

Structural, Optical, and Magnetic Properties of the New (1-x)Bi0.5Na0.5TiO3 + xMgCoO3-δ Solid Solution System

A new (1-x)Bi0.5Na0.5TiO3 + xMgCoO3-δ solid solution system was synthesized using the simple sol–gel method. The structure that was investigated via X-ray diffraction and Raman scattering analysis indicated that the MgCoO3-δ materials were highly soluble into the host Bi0.5Na0.5TiO3 crystals. The random incorporation of Mg and Co cations into the host crystal of the Bi0.5Na0.5TiO3 materials reduced the optical band gap energy and induced a complex magnetic behavior as a function of MgCoO3−δ concentration solute in the host matrix. The optical band gap energy values decreased from 3.05 eV for pure Bi0.5Na0.5TiO3 materials to 2.22 eV with 9 mol% MgCoO3-δ solution for the Bi0.5Na0.5TiO3 compounds. A weak ferromagnetic property with strongly induced diamagnetism was obtained at room temperature for the pure Bi0.5Na0.5TiO3 materials, whereas ferromagnetic properties were observed for the MgCoO3-δ-modified Bi0.5Na0.5TiO3 compounds. The addition of paramagnetic and/or antiferromagnetic-like behavior was obtained for the MgCoO3-δ solid solution with over 5 mol% in the host Bi0.5Na0.5TiO3 materials. This work contributes to the current understanding of the interaction of transition metal cations in the host crystal ABO3 perovskite of lead-free ferroelectric materials due to the co-modification of A and B sites.

A phenomenological approach to study the effect of uniaxial anisotropy on the magnetization of ferromagnetic nanoparticles

We study the effect of the uniaxial anisotropy in a system of ideal, noninteracting ferromagnetic nanoparticles by means of a thermodynamical model. We show that the effect of the anisotropy can be easily assimilated in a temperature shift Ta∗, in analogy to what was proposed by Allia et al. (2001) in the case of interacting nanomagnets. The phenomenological anisotropic Ta∗ parameter can be negative, indicating an antiferromagnetic-like behavior, or positive, indicating a ferromagnetic-like character as seen in the inverse susceptibility behavior as a function of temperature. The study is done considering an easy axis distribution to take into account the anisotropy axis dispersion in real samples (texture). In the case of a volumetric uniform distribution of anisotropy axes, the net effect makes Ta∗ to vanish, and the magnetic susceptibility behaves like a conventional superparamagnetic system, whereas in the others a finite value is obtained for Ta∗. When magnetic moment distribution is considered, the effect is to enhance the Ta∗ parameter, when the dispersion of the magnetic moments becomes wider.

Syntheses, crystal structures and magnetic properties of ternary rare-earth zirconium selenides, Ln2ZrSe5 (Ln = Ce–Nd)

Three new ternary rare-earth zirconium chalcogenides, Ce2ZrSe5 (1), Pr2ZrSe5 (2) and Nd2ZrSe5 (3), have been synthesized by a facile solid-state route with boron as the reducing reagent. They crystallize in the space group Pnma of the orthorhombic system, belonging to the U3Se5 structure type. Their 3-D structures are constructed by parallel polyanionic {[ZrSe5]6–}n chains and LnSe8 bicapped trigonal prisms. The magnetic susceptibilities' data indicate their antiferromagnetic-like behavior without magnetic order down to 2 K. The structural relationships among several similar structures are discussed, together with the lanthanide contraction in the Ln2ZrSe5 system.

Understanding the stability of Fe incorporation within Mn3N2(0 0 1) surfaces: An ab-initio study

We present first principles spin-polarized calculations of the adsorption and incorporation of iron in the Mn3N2(001) surfaces. By means of a surface formation energy criterion, it is demonstrated that Fe incorporation is energetically stable for all studied surfaces. An Fe bilayer formation is achieved after Fe atoms displace Mn atoms in the sub-surface N-vacancy layers. An analysis of the magnetic coupling shows an antiferromagnetic alignment along the [001] direction as in the clean, ideal surfaces. Also, the in-plane magnetic coupling between Fe–Fe and Fe–Mn shows a ferromagnetic tendency, similar to the clean, ideally terminated surfaces. These results clearly indicate that Fe behaves like Mn when adsorbed into the Mn3N2 surface. Density of states calculations of the stable structures show a slight deviation from the antiferromagnetic-like behavior, with the most important contribution around the Fermi level coming from the Fe-d and Mn-d orbitals.

A metalloporphyrinic compound with a high selectivity for N2 and CO2 separation

Using a hydrothermal reaction of SmCl3•6H2O, ZnBr2 and TPPS, a crystalline metalloporphyrinic compound (MPC), [SmZn(TPPS)H3O][Formula: see text]•2[Formula: see text]H2O (1), was synthesized. The crystal structure of the MPC exhibits a condensed and robust 3-D porous open framework. We studied the adsorption and desorption isotherms for N2, H2 and CO2 conducted at 77 K and 273 K. They show different isotherms such as Type I, II and III isotherm behaviors. We found that low temperature is propitious for the title complex to adsorb more H2, but less N2. Compound 1 showed remarkably high selectivity for CO2–N2 separation, and good thermal stability. The temperature dependent magnetic susceptibility shows an antiferromagnetic-like behavior for 1.

General study on the crystal, electronic and band structures, the morphological characterization, and the magnetic properties of the Sr2DyRuO6 complex perovskite

A comprehensive investigation of the general properties of the Sr2DyRuO6 complex perovskite was undertaken. Crystal structure characterization performed by X-ray diffraction measurements and Rietveld analysis allowed establishing that the material crystallizes in a distorted monoclinic perovskite-like structure belonging to the P21/n (#14) space group, with alternating distribution of Dy3+ (2c: 0, 0.5, 0) and Ru5+ (2d: 0.5, 0, 0). Because of the mismatch in the ionic radii, the DyO6 and RuO6 octahedra are forced to tilt around the cubic directions so as to optimize the Sr–O inter-atomic bond lengths. Morphological characterization carried out by scanning electron microscopy indicated a particle size D=37.17nm and an activation energy Q=109.8kJ/mol. Semi-quantitative compositional study, performed through energy-dispersive X-ray experiments, corroborated that the pure phase of the Sr2DyRuO6 was correctly obtained. Magnetic properties determined from the fit of the Curie–Weiss law to the curves of magnetic susceptibility as a function of temperature showed that Sr2DyRuO6 exhibits an antiferromagnetic-like behavior at low temperatures as a consequence of a magnetic transition at T=38K. Data collected with respect to the field dependence of the magnetization showed the existence of a weak ferromagnetic moment relationship with antiferromagnetic-like behavior. Density functional theory allowed establishing the optimum electronic structure for Sr2DyRuO6, and the study of the density of states showed that Dy3+ and Ru5+ are responsible for the magnetic character of the compound, with the prediction that at T=0K it behaves as a half-metallic material. The spin magnetic moment of the cell is close to 16μB, and the integer number of Bohr magneton is a signature of half-metallic character. Evolution of crystal structure at high temperature revealed that Sr2DyRuO6 does not undergo any structural phase transitions, since the Bragg reflections [hkl] do not undergo changes, and the structure is preserved.

Synthesis and structure of a new family of 3d–4f heterometallic compounds Rb7LnFe6O2(PO4)8 (Ln=Sm, Eu, Gd, Dy): Magnetic properties of the Sm-, Gd-, Dy-derivatives

A new family of mixed lanthanide(III) and iron(III) oxo-phosphate phases, Rb7LnFe6O2(PO4)8 (Ln=Sm 1, Eu 2, Gd 3, Dy 4), was isolated by using a high-temperature, solid-state method in molten-salt media. The X-ray single-crystal structure analysis shows that these isomorphic derivatives crystallize in a triclinic space group P–1 (no. 2); Z=1. The 3-D framework of these 3d–4f oxo-phosphates are comprised of LnO6 octahedral, FeO5 trigonal bipyramidal (tbp), and μ3-oxo [Fe4O18] tetrameric units interconnected through PO4 tetrahedra. The preliminary results of the temperature-dependent magnetic susceptibility measurements for selected compounds (1, 3, 4) reveal antiferromagnetic-like behavior. 1 shows a weak antiferromagnetric ordering at TN=∼7K while others show little evidence of long-range magnetic order down to 2K. All three compounds have measured magnetic moments significantly smaller than the expected values.

Density-functional characterization of antiferromagnetism in oxygen-deficient anatase and rutileTiO2

We present theoretical evidence for local magnetic moments on ${\text{Ti}}^{3+}$ ions in oxygen-deficient anatase and rutile ${\text{TiO}}_{2}$ observed in a recent experiment [S. Zhou et al., Phys. Rev. B 79, 113201 (2009)]. Results of our first-principles $\text{GGA}+U$ calculations reveal that an oxygen vacancy converts two ${\text{Ti}}^{4+}$ ions to two ${\text{Ti}}^{3+}$ ions in anatase phase, which results in a local magnetic moment of $1.0{\ensuremath{\mu}}_{B}$ per ${\text{Ti}}^{3+}$. The two ${\text{Ti}}^{3+}$ ions, however, form a stable antiferromagnetic state, and similar antiferromagnetism is also observed in oxygen-deficient rutile phase ${\text{TiO}}_{2}$. The calculated results are in good agreement with the experimentally observed antiferromagneticlike behavior in oxygen-deficient Ti-O systems.

Element-Specific Magnetic Properties of Di-Erbium Er2@C82 and Er2C2@C82 Metallofullerenes: A Synchrotron Soft X-ray Magnetic Circular Dichroism Study

The magnetic properties of mono- and dierbium(Er) metallofullerenes have been studied by means of a synchrotron soft X-ray magnetic circular dichroism (SXMCD) technique. The effective magnetic moment of the Er ions has been estimated to be 8.48 μB and is independent of the number of Er ions encapsulated inside the fullerene cage. These Er metallofullerenes exhibit an antiferromagnetic-like behavior at low temperatures that is stronger in Er@C82(C2v isomer) monometallofullerene than in Er2@C82(C2v) and Er2C2@C82(C2v) dimetallofullerenes. The result suggests that the unpaired spin on the monometallofullerene cages causes an indirect magnetic interaction between Er 4f spins of neighboring metallofullerenes. Furthermore, we have found that the Weiss temperatures of the metallofullerene remain the same even when one of the Er ions in Er2C2@C82(Cs) is replaced by a nonmagnetic Y ion (i.e., ErYC2@C82). The results indicate that interaction between the two Er ions in the same fullerene cage does not contribute si...

The magnetic map at the V/Co interface

Huttel et al. (Phys. Rev. B 68 (2003) 174405) using X-ray magnetic circular dichroism have depicted an induced magnetic moment for V atoms deposited on FCC Co(001). In the present communication, we discuss the onset of magnetic polarization of very thin V films atop Co(001) within classical density functional theory. Both local density approximation and gradient corrected approaches are used. For one V monolayer on Co(001), a ferromagnetic coupling is obtained whereas for thicker V slabs, an antiferromagnetic-like behavior is shown to be the ground state. It is clearly shown that the Co-induced polarization on V atoms is short-ranged and mainly limited to the Co–V interface. A very different magnetic behavior is observed when alloying at the V/Co interface is present.

Thermodynamics of alternate Ising chains of spins 1 and 3/2 with dipolar, biquadratic, and single ion interactions

Within a recently developed extended decoration-transformation formalism we study the thermodynamic properties of a linear chain of alternate Ising σ=1 and S=3/2 spins. We allow for different anisotropy fields on each subchain of different spins. For some range of the parameter space we show the existence of a crossover from a ferromagnetic to an antiferromagnetic-like behavior of the model, as explicitly captured in the susceptibility results.

Anisotropic transport properties of U4Cu4P7

The electrical resistivity of U4Cu4P7 for the current flowing in the basal plane of this tetragonal antiferromagnetic compound has been measured from 2 to 1000 K. Contrary to the case where the current flows along thec axis, we find an increasing resistivity with increasing temperature over the whole temperature range. This ferromagnetic-like behavior in the basal plane and the antiferromagnetic-like behavior perpendicular to it, is explained with the complex magnetic structure of the compound. The Hall effect forB parallel to thec axis shows an antiferromagnetic-like temperature dependence. The ordinary part of the Hall effect in the paramagnetic phase gives in an one-band model a carrier concentration of 0.7 holes per uranium atom, which is quite unique in uranium compounds by its sign and which results from the presence of two kinds of uranium atoms with different bonding, valence, magnetic moment and exchange. Quantitative differences between various samples of U4Cu4P7 are related to a mixed-layer stacking polytype of U4Cu4P7 and UCuP2 units.

Anomalous Magnetic Properties of Ti5O9

A magnetic susceptibility study from 77° to almost 900°K and an electron-spin-resonance study from 77° to 160°K of Ti5O9 shows an anomalous magnetic behavior. The magnetic susceptibility gives a peak in its temperature dependence which resembles a Néel temperature and can be interpreted to indicate antiferromagnetic-like behavior. The magnetic susceptibility peak occurs at about 130°K. An x-ray study of Ti5O9 indicates face sharing anion octahedra with a short metal-metal distance which lends support to a cation-cation interaction model. The electron-spin-resonance (ESR) study gives spectra which yield g values of 1.98±0.02. This indicates little orbital contribution to the magnetic moment. The spectral linewidth increases sharply with increasing temperature in the region of the magnetic susceptibility maximum, showing effects of electron-phonon interactions. The signal intensity decreases rapidly above this susceptibility peak. The line shape is asymmetric below this temperature region, however, it becomes symmetrical above it. The behavior presented in these results reflects a complex effect undoubtedly involving delocalized electrons in which a low-temperature electron localization may occur. The value of the magnetic moment of Ti3+ in Ti5O9 is, therefore, not clear. While some of these results indicate an apparent antiferromagnetic behavior and appear to support the cation-cation interaction model, they cannot be completely resolved as such. The crystalline distortion present may contribute to the trapping of the electrons at low temperatures, and thus facilitate the cation-cation interactions and influence the spin-lattice relaxation times.