Appearance Of Weak Ferromagnetism Research Articles

Magnetic and electrical properties of Bi 0.8Ca 0.2Fe 1 −xMn xO 3 (0 ≤ x ≤ 0.5)

Mn-substituted Bi 0.8Ca 0.2FeO 3 ceramics Bi 0.8Ca 0.2Fe 1 −x Mn x O 3 (0 ≤ x ≤ 0.5) were synthesized by a solid-state reaction method. Powder X-ray diffraction investigations performed at room temperature show that the crystal structure is rhombohedral for x ≤ 0.1 and orthorhombic for 0.2 ≤ x ≤ 0.5. Compared to the undoped Bi 0.8Ca 0.2FeO 3 compound, enhanced magnetization and electric polarization were observed in the samples with x ≤ 0.1. A further increase in the magnetization with increasing x took place in the samples with 0.2 ≤ x ≤ 0.5. All the Mn-substituted samples studied are basically antiferromagnetic accompanied by the appearance of weak ferromagnetism, which is similar to the BiFeO 3 compound. The conductivity of the samples with x ≥ 0.3, measured between 140 and 380 K, is of the semiconducting type.

Correlation between Ionic Radius of Substituting Element and Magnetic Properties of Bi<sub>1-x</sub>A<sub>x</sub>FeO<sub>3-x/2</sub> (A= Ca, Sr, Pb, Ba) Multiferroics

Investigation of crystal structure and magnetic properties of the diamagnetically- substituted Bi1-xAxFeO3-x/2 (A= Ca, Sr, Pb, Ba; x= 0.2, 0.3) polycrystalline samples has been carried out. It has been shown that the heterovalent A2+ substitution result in the formation of oxygen vacancies in the host lattice. The solid solutions have been found to possess a rhombohedrally distorted perovskite structure described by the space group R3c. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO3 giving rise to the appearance of weak ferromagnetism.

Weak ferromagnetism in diamagnetically-doped Bi 1 − xA xFeO 3 (A = Ca, Sr, Pb, Ba) multiferroics

Investigation of crystal structure, magnetic and local ferroelectric properties of the diamagnetically-doped Bi 1 − x A x FeO 3 (A = Ca, Sr, Pb, Ba; x = 0.2, 0.3) ceramic samples has been carried out. It has been shown that the solid solutions have a rhombohedrally distorted perovskite structure described by the space group R3c. Piezoresponse force microscopy data have revealed the existence of the spontaneous ferroelectric polarization in the samples at room temperature. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO 3 and to result in the appearance of weak ferromagnetism. The magnetic properties have been discussed in terms of doping- induced changes in the magnetic anisotropy.

Effect of Diamagnetic A2+ Substitution on the Magnetic and Ferroelectric Properties of the Bi1−xAxFeO3 Multiferroics

AbstractInvestigation of crystal structure, magnetic and local ferroelectric properties of the diamagnetically-doped Bi1−xAxFeO3 (A= Ca, Sr, Pb, Ba; x= 0.2, 0.3) ceramic samples has been carried out. It has been shown that the solid solutions have a rhombohedrally distorted perovskite structure described by the space group R3c. Piezoresponse force microscopy data have revealed the existence of the spontaneous ferroelectric polarization in the samples at room temperature. Magnetization measurements have shown that the magnetic state of these compounds is determined by the ionic radius of the substituting elements. A-site substitution with the biggest ionic radius ions has been found to suppress the spiral spin structure of BiFeO3 and to result in the appearance of weak ferromagnetism. The magnetic properties have been discussed in terms of doping- induced changes in the magnetic anisotropy.

Structure and Physical Properties of Molecule-Based Magnets Including Transition Metal Complexes of Crown Thioethers

Abstract A new series of molecule-based magnets, including crown thioether (9S3 = 1,4,7-trithiacyclononane) complexes of transition metals (Ni, Co, Cu), are presented. TCNQ salts [M(9S3)2](TCNQ)2 (M = Ni, Co) are one-dimensional (1D), antiferromagnetic (AF) complexes with an intra-chain interaction J = −3.9 K and −1.3 K for M = Ni and Co, respectively, despite its long intermolecular distance, for which the large spin density on the sulfur atom in the magnetic [M(9S3)2]2+ cation is responsible. The mixed valence salt [M(9S3)2](TCNQ)3 (M = Ni, Co) is a paramagnetic semiconductor. 1D AF magnet [Cu(9S3)Br2] does not undergo any magnetic transition because of the weak inter-chain interaction. Substituting a very small amount (∼5%) of Cu with Ni causes a structural change. The change decreases the distances of inter-chain S–S contacts, resulting in the generation of an AF transition at TN = 4.5 K. [M(9S3)2][Ni(bdt)2]2 (M = Ni, Co; bdt = 1,2-benzenedithiolato) are weak-ferromagnets with TN = 6.2 K and 2.6 K for M = Ni and Co, respectively. In the crystals, [M(9S3)2]2+–[Ni(bdt)2]− alternate chains and [Ni(bdt)2]− uniform chains coexist. The appearance of weak-ferromagnetism is associated with a competition between two kinds of inter-chain AF interactions between [M(9S3)2]2+–[Ni(bdt)2]− alternate chains, where the stronger one is an indirect inter-chain interaction through [Ni(bdt)2]− uniform chains, while the weaker is a direct inter-chain interaction.

Nonmonotonic temperature dependence of the spontaneous magnetization of the antiferromagnetic crystal LiCoPO4

The temperature dependence of the magnitude of the weak ferromagnetic moment is measured in the many-sublattice antiferromagnetic magnetoelectric crystal LiCoPO4. The dependence is found to be nonmonotonic—as the temperature is decreased below the Neel temperature TN, the ferromagnetic moment initially increases and then decreases markedly. It reaches its highest value, close to 0.12 G, at a temperature of around 10.5 K, which is roughly TN/2. The possible mechanisms for the appearance of weak ferromagnetism in LiCoPO4 and the causes of the nonmonotonic temperature dependence of the spontaneous magnetic moment are discussed. Among the possible mechanisms, there is a preference for one in which the weak ferromagnetism is due to a nonuniform antiferromagnetic structure which is proposed for this crystal.

Magnetic structures ofY2SrCu0.6Co1.4O6.5andY2SrCuFeO6.5

The nuclear and magnetic structures of two layered mixed copper transition-metal oxides ${\mathrm{Y}}_{2}{\mathrm{SrCu}}_{0.6}{\mathrm{Co}}_{1.4}{\mathrm{O}}_{6.5}$ and ${\mathrm{Y}}_{2}{\mathrm{SrCuFeO}}_{6.5}$ have been determined from time-of-flight neutron powder diffraction. Both compounds consist of apex-linked pyramidal Cu/M-${\mathrm{O}}_{5}$ double layers, which alternate with oxygen defective ${\mathrm{Y}}_{2}{\mathrm{O}}_{1.5}$ fluorite layers. The Co-containing compound has a simple magnetic structure with a Shubnikov group ${\mathrm{Ib}}^{\ensuremath{'}}{a}^{\ensuremath{'}}m.$ The magnetic moments $(2.2{\ensuremath{\mu}}_{B})$ of this compound are aligned along the crystallographic b axis with antiferromagnetic order between neighboring Cu/Co ions. In contrast, the Fe-analog possesses a complicated noncolinear magnetic structure ${(Pc}^{\ensuremath{'}}{c}^{\ensuremath{'}}n,$ with moments $m\ensuremath{\perp}c),$ which can also be considered as comprising two components with ${I}^{\ensuremath{'}}{\mathrm{bam}}^{\ensuremath{'}}$ and ${\mathrm{Ib}}^{\ensuremath{'}}{a}^{\ensuremath{'}}m$ symmetry. The combination of the two components results in an $82\ifmmode^\circ\else\textdegree\fi{}$ angle between the spins of Cu/Fe ions in neighboring Cu/Fe-${\mathrm{O}}_{5}$ planes within a given double layer. The approximate $90\ifmmode^\circ\else\textdegree\fi{}$ rotation of the moments results from a ferromagnetic interaction between the neighboring Cu/Fe ions along the c axis through the apical oxygen atoms. The ferromagnetic component may originate from the $\mathrm{Cu}{d}_{{z}^{2}}^{2}$-$\mathrm{O}2p$-$\mathrm{Fe}{d}_{{z}^{2}}^{1}$ type exchange interaction. The implication of such an interaction to related layered cuprates is also discussed. By symmetry analysis, the magnetic structure of the Co compound can be classified as a ${G}_{y}g$ type and the appearance of weak ferromagnetism can be attributed to the Dzyaloshinsky-Moria interaction which couples the ${G}_{y}g$ mode and ${F}_{z}f$ mode in the same representation of the $\mathrm{Ibam}$ space group.

Magnetic properties and weak ferromagnetism of Eu 2CuO 4+δ

Results for X-ray diffraction and dc magnetic susceptibility for Eu 2CuO 4+δ synthesized under high oxygen pressure are reported. A magnetic anomaly with weak ferromagnetism(WF) behavior occured at ∼28K has been first found in oxygen-doped Eu 2CuO 4+δ sample. The results show that the appearance of WF can not be attributed to the size effects, but to the distortion in CuO 2 planes caused by the doped-oxygen.

Weak ferromagnetic resonance of Gd2CuO4 small particles

Microwave-absorption measurements have been performed in Gd2CuO4 particles of controlled size. We have measured the microwave absorption associated with a magnetic resonance of the weak-ferromagnetic component of the magnetization. The onset temperature of this signal, related to the appearance of weak ferromagnetism, is depressed when the particle size is decreased. A simultaneous depression of the Néel temperature is suggested as the origin of the observed dependence, due to the limitation on the two-dimensional correlation length imposed by the particle size.

Relationship between weak ferromagnetism, superconductivity, and lattice parameter in the A2−x−y>BxCeyCuO4 (A,B≡La, Pr, Nd, Sm, Eu, Gd, Y) compounds

We report the results obtained by x-ray, dc magnetization, ac magnetic susceptibility, microwave magnetic absorption, and electron paramagnetic resonance of the magnetic and superconductive properties of a large series of compounds of the form A2−x−yBxCeyCuO4 with A,B≡La, Pr, Nd, Sm, Eu, Gd, and Y. For y=0 antiferromagnetic ordering of the copper moments is present at TN≊280 K for all the compounds formed in the tetragonal T′ phase. Weak ferromagnetism (WF) associated with the canting of the copper moments is observed in all the compounds with a lattice parameter, a<a0, a0≊3.91 Å, regardless of the rare-earth (R) ions present in the system. Superconductivity is observed only in the Ce-doped compounds with a≳a1, a1≊3.92 Å. The maximum onset temperature Tc is found for a≊3.95 Å. A phase diagram was obtained by varying the lattice parameter size as a function of A, B, and x. The correlation between the appearance of WF in the undoped compounds and the disappearance of superconductivity in the Ce-doped systems is discussed.

Raman study of T'-phase distortion in R2CuO4 compounds (R=Nd,Sm,Eu,Gd).

We present a Raman study of the appearance of structural distortions in T'-phase cuprates ${\mathit{R}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (R=Sm,Eu,Gd) upon R atomic number increase. Besides some anomalies in the frequencies of the allowed modes, the distortion from the T' phase is evidenced in ${\mathrm{Gd}}_{2}$${\mathrm{CuO}}_{4}$ by the appearance of an additional mode with ${\mathit{B}}_{1\mathit{g}}$ symmetry, ${\mathit{B}}_{1\mathit{g}}^{\mathrm{*}}$, that we assign to the motion of the in-plane oxygen atoms, O(1). This mode, which appears incipiently in ${\mathrm{Eu}}_{2}$${\mathrm{CuO}}_{4}$ and ${\mathrm{SmGdCuO}}_{4}$, is formally forbidden by symmetry, and would be activated by O(1) displacement resulting in a distorted ${\mathrm{CuO}}_{2}$ plane. This interpretation is in agreement with previous experimental results either of structural or magnetic type. The appearance of weak ferromagnetism for R=Gd and beyond, for instance, is incompatible with perfect T'-phase structure. We compare the results in pure compounds with those that are Ce doped and show that the ${\mathit{B}}_{1\mathit{g}}^{\mathrm{*}}$ activity decreases with respect to that of the ${\mathit{B}}_{1\mathit{g}}$ mode as increasing amounts of Ce are introduced in the lattice; this is interpreted as a consequence of T'-phase stabilization upon doping. Temperature-dependence spectra are also presented and discussed.

Magnetostriction and magnetoelastic effects in certain amorphous alloys

Magnetoelastic phenomena in amorphous transition metal-metalloid alloys have been studied. It was found that the magnetoelastic coupling coeficient B(0) of amorphous FeSiB alloys depends mostly on the Fe content, and does not depend significantly on the B/Si ratio. The value of B(0) exhibits a maximum at about 80 at% Fe. The forced volume magnetostriction is a strongly increasing function of iron concentration above 75 at% Fe. The ΔE effect of as-quenched amorphous alloys is not a simple function of saturation magnetostriction λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> . The ΔE effect is proportional to the magnetomechanical coupling coefficient. The observed magnetoelastic effects may be related to the appearance of weak ferromagnetism in Fe-base amorphous alloys.

Thermal Expansion of NiS2, Ni0.9Co0.1S2, Ni0.9Cu0.1S2 and Ni0.95Cu0.05S2

The thermal expansion of NiS 2 , Ni 0.9 Co 0.1 S 2 , Ni 0.9 Cu 0.1 S 2 and Ni 0.95 Cu 0.05 S 2 has been measured from 4.2 K to room temperature. The anomalies, which correspond to antiferromagnetic ordering and to appearance of weak ferromagnetism, have been observed in all specimens. The lattice, electronic and magnetic Gruneisen constans are determined. The magnetic Gruneisen constant changes its sign from negative to positive corresponding to the metal-semiconductor transition which occurs between Ni 0.9 Co 0.1 S 2 and Ni 0.93 Co 0.07 S 2 .