Unusual Magnetic Behavior Research Articles

Study of Magnetic Behavior of Salts of Cationic Dinitrosyl Iron Complexes with Thiocarbamide and its Derivatives

We report magnetic characterization and theoretical modeling of a series of four salts of cationic dinitrosyl iron complexes with thiocarbamide and its derivatives. To explain unusual magnetic behavior of these complexes we have assumed that constituent nitrosyl groups possess not only spins but also unquenched orbital angular momenta. With the aid of this assumption we were able to explain the observed temperature dependences of the effective magnetic moment for all members of the series and to establish magneto-structural correlations.

Sintering Effect on Structural, Optical and Unusual Magnetic Behaviour in Zn0.95Co0.05O-Based DMS Materials

In this paper, the sinteringinduced unusual optical and magnetic behaviour in Co2+ doped ZnO nanoparticles is reported. Codoped ZnO (Zn0.95Co0.05O) powders were prepared using a co-precipitation method and sintered at three different temperatures (350, 500 and 650 °C). The single crystalline phase and Co2+ ion incorporation in the ZnO host were confirmed by X-ray diffraction (XRD) analysis. The surface morphology of prepared samples was analysed by scanning electron microscopy (SEM), and elemental analyses were performed by energy-dispersive X-ray spectroscopy. The UV–DRS spectrum was expressing the absorbing nature and confirms the Co2+ ion incorporation in the ZnO host. Photoluminescence spectrum reveals emission properties and the presence of lattice defects in the prepared samples. Further, the Raman studies also clearly indicate the structural disorder and dislocations present in these samples. Room-temperature magnetic properties are studied using vibrating sample magnetometers, and all the three samples exhibit ferromagnetic nature.

Non-equilibrium phase transition properties of disordered binary ferromagnetic alloy

Non-equilibrium dynamic phase transition features of a disordered binary ferromagnetic alloy consisting of spin-1/2 and spin-1 components under the presence of a time dependent oscillating magnetic field have been analyzed for a two dimensional square lattice. With the help of Glauber-type stochastic process, the kinetic equations of time dependent magnetizations have been derived based on the effective-field theory with single-site correlations. A systematic analysis for the whole range of the concentrations of randomly distributed components as well as other system parameters has been carried out. According to our numerical investigations, the considered system presents unusual thermal and magnetic field behaviors such as the existence of dynamic multi-critical behavior and also boundaries of the coexistence region, where both dynamically ordered and disordered phases overlap, sensitively depends on the studied parameter space.

Magnetic response of a disordered binary ferromagnetic alloy to an oscillating magnetic field

By means of Monte Carlo simulation with local spin update Metropolis algorithm, we have elucidated non-equilibrium phase transition properties and stationary-state treatment of a disordered binary ferromagnetic alloy of the type ApB1−p on a square lattice. After a detailed analysis, we have found that the system shows many interesting and unusual thermal and magnetic behaviors, for instance, the locations of dynamic phase transition points change significantly depending upon amplitude and period of the external magnetic field as well as upon the active concentration of A-type components. Much effort has also been dedicated to clarify the hysteresis tools, such as coercivity, dynamic loop area as well as dynamic correlations between time dependent magnetizations and external time dependent applied field as a functions of period and amplitude of field as well as active concentration of A-type components, and outstanding physical findings have been reported in order to better understand the dynamic process underlying present system.

Abnormal magnetic field effects on electrogenerated chemiluminescence.

We report abnormal magnetic field effects on electrogenerated chemiluminescence (MFEECL) based on triplet emission from the Ru(bpy)3Cl2-TPrA electrochemical system: the appearance of MFEECL after magnetic field ceases. In early studies the normal MFEECL have been observed from electrochemical systems during the application of magnetic field. Here, the abnormal MFEECL suggest that the activated charge-transfer [Ru(bpy)33+ … TPrA•] complexes may become magnetized in magnetic field and experience a long magnetic relaxation after removing magnetic field. Our analysis indicates that the magnetic relaxation can gradually increase the density of charge-transfer complexes within reaction region due to decayed magnetic interactions, leading to a positive component in the abnormal MFEECL. On the other hand, the magnetic relaxation facilitates an inverse conversion from triplets to singlets within charge-transfer complexes. The inverse triplet → singlet conversion reduces the density of triplet light-emitting states through charge-transfer complexes and gives rise to a negative component in the abnormal MFEECL. The combination of positive and negative components can essentially lead to a non-monotonic profile in the abnormal MFEECL after ceasing magnetic field. Nevertheless, our experimental studies may reveal un-usual magnetic behaviors with long magnetic relaxation from the activated charge-transfer [Ru(bpy)33+ … TPrA•] complexes in solution at room temperature.

Unusual magnetic behaviors and electronic configurations driven by diverse Co(II) or Mn(II) MOF architectures.

Five novel metal organic frameworks were obtained by hydro-solvothermal reactions using the hexafluorisopropylidenebis(benzoic) acid (H2hfipbb) as linker and Co(II) or Mn(II) ions as connectors. [Co2(Hhfipbb)(TEA)], compound 1 (TEA = triethanolamine trianion) with a three-dimensional (3D) framework, and a tpu net; [Co1.5(hfipbb)2]·HN(CH2CH3)3 and [Co3(hfipbb)2]·2{HN(CH2CH3)3}, compounds 2 and 2-a, respectively, both with two-dimensional structure, sql topologies, and different layer packings. Compounds 3 and 4, having the general formula [M2(hfipbb)2]·C7H8, where M = Co (3) or Mn (4), have 3D frameworks with an sqc topology. A deep analysis of the magnetic measurements reveals different striking magnetic behaviors resulting from diverse secondary building unit and framework architectures. Compound 1 presents canted antiferromagnetic chains, compound 2 contains ferromagnetic linear trimeric clusters, and compound 3 exhibits ferromagnetic chains. For the three compounds, a 3D canted antiferromagnetic structure takes place at ∼8 K by means of weak magnetic interactions between the mentioned magnetic units. Such long-range magnetic order is precluded with the application of a high enough magnetic field. Compound 4 evidenced intrachain antiferromagnetic interactions.

Investigating Unexpected Magnetism of Mesoporous Silica-Supported Pd and PdO Nanoparticles

The synthesis and magnetic behavior of matrix-supported Pd and PdO nanoparticles (NPs) are described. Mesoporous silica with hexagonal columnal packing is selected as a template, and the impregnation method with thermal annealing is used to obtain supported Pd and PdO NPs. The heating rate and the annealing conditions determine the particle size and the phase of the NPs, with a fast heating rate of 30 °C/min producing the largest supported Pd NPs. Unusual magnetic behaviors are observed. (1) Contrary to the general belief that smaller Pd NPs or cluster size particles have higher magnetization, matrix-supported Pd NPs in this study maintain the highest magnetization with room temperature ferromagnetism when the size is the largest. (2) Twin boundaries along with stacking faults are more pronounced in these large Pd NPs and are believed to be the reason for this high magnetization. Similarly, supported PdO NPs were prepared under air conditions with different heating rates. Their phase is tetragonal (P42/mmc) with cell parameters of a = 3.050 Å and c = 5.344 Å, which are slightly larger than in the bulk phase (a = 3.03 Å, c = 5.33 Å). Faster heating rate of 30 °C/min also produces larger particles and larger magnetic hysteresis loop, although magnetization is smaller and few twin boundaries are observed compared to the supported metallic Pd NPs.

Discontinuous reactions in melt-spun Cu–10 at. %Co alloys and their effect on magnetic anisotropy

The occurrence of discontinuous reactions under isothermal annealing of melt-spun Cu–10 at. % Co alloys, consisting of ribbons (20 µm thick) with columnar grains in the as-solidified state, has been investigated. The microstructure of the ribbons for different annealing temperatures (723–923 K) and annealing times (5–60 min) was determined by transmission electron microscopy, including analysis by energy-dispersive X-ray spectroscopy. Magnetic properties at room temperature were measured by means of hysteresis curve measurements in a vibrating sample magnetometer. Different types of microstructure were observed within grains and at grain boundaries. The spinodal decomposition microstructure was observed during the early stages of annealing for all annealing temperatures. Spherical precipitates grew from the modulated structure at a later stage, forming homogenous distributions throughout the grains. Heterogeneous distributions of incoherent precipitates formed at T > 873 K. As result of discontinuous precipitation, all grain boundaries exhibited arrays of rod-like Co precipitates with diameters and inter-rod spacing of few nanometers. The coarsening of discontinuous precipitates is attributed to a grain boundary-controlled phenomenon, called discontinuous coarsening (DC). The columnar morphology of the grains in the as-solidified alloy was connected with Co rods that were primarily oriented along the ribbon plane. This structure is connected with magnetic anisotropy, which is later weakened by DC. These results elucidate the unusual magnetic behavior of melt-spun Cu–Co alloys and provide a key to understanding their higher magnetoresistance in comparison with other heterogeneous systems.

Superconductivity and unusual magnetic behavior in amorphous carbon

Traces of superconductivity (SC) at elevated temperatures (up to 65 K) were observed by magnetic measurements in three different inhomogeneous sulfur doped amorphous carbon (a-C) systems: (a) in commercial and (b) synthesized powders and (c) in a-C thin films. (a) Studies performed on a commercial (a-C) powder, which contains 0.21% sulfur, revealed traces of non-percolated superconducting phases below Tc = 65 K. The SC volume fraction is enhanced by the sulfur doping. (b) The a-C powder obtained by pyrolytic decomposition of sucrose did not show any sign of SC above 5 K. This powder was mixed with sulfur and synthesized at 400 °C (a-CS). The inhomogeneous products obtained show traces of SC phases at Tc = 17 and 42 K. (c) Non-superconducting composite a-C-W thin films were grown by electron-beam induced deposition. SC emerged at Tc = 34.4 K only after heat treatment with sulfur. Other parts of the pyrolytic a-CS powder show unusual magnetic features. (i) Pronounced irreversible peaks around 55–75 K appear in the first zero-field-cooled (ZFC) sweep only. Their origin is not known. (ii) Unexpectedly, these peaks are totally suppressed in the second ZFC runs measured a few minutes later. (iii) Around the peak position the field-cooled (FC) curves cross the ZFC plots (ZFC > FC). These peculiar magnetic observations are also ascribed to an a-CS powder prepared from the commercial a-C powder and are connected to each other. All SC and magnetic phenomena observed are intrinsic properties of the sulfur doped a-C materials. It is proposed that the a-CS systems behave similarly to well-known high Tc curates and/or pnictides in which SC emerges from magnetic states.

Unusual magnetic behaviors and electrical properties of Nd-doped BiFeO3 nanoparticles calcined at different temperatures

Bi1−x Nd x FeO3 (x = 0, 0.05, 0.1, 0.15, 0.2) nanoparticles (about 20–50 nm) calcined at 500 and 600 °C, respectively, were prepared by an ethylene glycol-based sol–gel. The XRD analysis reveals that the BiFeO3 samples are in single phase, and their crystal structure is varied with the Nd content. Due to the small particle size, the uncompensated spin moments on the surface and the suppression of spin helical ordering structure result in a ferromagnetic phase of the BiFeO3 nanoparticles. The magnetization of the Nd-doped samples calcined at 600 °C is improved with the increase of Nd content, but for the Nd-doped samples calcined at 500 °C, it shows an opposite trend, which is ascribed to the interplay of size effect and the ratio of Fe2+:Fe3+ of samples calcined at different temperatures via XPS analysis. The dielectric properties were measured and analyzed for the samples calcined at 500 and 600 °C. Moreover, the leakage current value of the Bi1−x Nd x FeO3 samples can be modulated by the Nd doping, and it reaches a minimum at the Nd content around 0.1.

Non-equilibrium dynamics of a ferrimagnetic core–shell nanocubic particle

The non-equilibrium dynamics of a single cubic core–shell ferrimagnetic nanoparticle system under a time dependent oscillating magnetic field is elucidated by making use of a classical Monte Carlo simulation technique with a standard Metropolis algorithm. Many interesting and unusual thermal and magnetic behaviors are observed, for instance, the locations of dynamic phase transition points change significantly depending upon amplitude and period of the external magnetic field as well as other Hamiltonian parameters in related planes. Much effort has also been devoted to the influences of the varying shell thickness on the thermal and magnetic properties of the particle, and outstanding physical findings are reported in order to better understand the dynamic process of the studied nanoparticle system.

The effect of mechanical stress on Ni63.8Mn11.1Ga25.1 microwire crystalline structure and properties

We studied structure and magnetic properties of Heusler alloy microwires. The dependence of the phase composition of the annealed microwires on the presence of glass coating has been found. The crystalline structure of the glass-coated samples is composed of two cubic crystalline phases of different density. The samples annealed without coating are in the single-phase state. The two-phase state is supposed to be caused by intrinsic stresses resulted from the action of opposite sign stresses in different microwire sections. The magnetic properties of the fabricated Heusler-type microwires differ from their bulk and thin film counterparts. Annealing of microwires considerably affect magnetic properties at room temperature. Unusual magnetic behavior is connected with two-phase structure of microwires.

Ferromagnetic Exchange Anisotropy from Antiferromagnetic Superexchange in the Mixed3d−5dTransition-Metal CompoundSr3CuIrO6

We report a combined experimental and theoretical study of the unusual ferromagnetism in the one-dimensional copper-iridium oxide Sr(3)CuIrO(6). Utilizing Ir L(3) edge resonant inelastic x-ray scattering, we reveal a large gap magnetic excitation spectrum. We find that it is caused by an unusual exchange anisotropy generating mechanism, namely, strong ferromagnetic anisotropy arising from antiferromagnetic superexchange, driven by the alternating strong and weak spin-orbit coupling on the 5d Ir and 3d Cu magnetic ions, respectively. From symmetry consideration, this novel mechanism is generally present in systems with edge-sharing Cu(2+)O(4) plaquettes and Ir(4+)O(6) octahedra. Our results point to unusual magnetic behavior to be expected in mixed 3d-5d transition-metal compounds via exchange pathways that are absent in pure 3d or 5d compounds.

IMPROVED AND UNUSUAL MAGNETIC PROPERTIES OF ZnO NANORINGS

We report the novel result regarding the observation of unusual magnetic behavior with high magnetic moment (higher than reported so far) at room temperature in highly pure ZnO nanorings synthesized by a simple chemical route using cetyltrimethyl ammonium bromide. The enhanced magnetization is attributed to the huge number of surface spins associated with defects created due to the larger surface to volume ratio of nanorings. The present study demonstrates that ZnO samples can be made highly magnetic just by tailoring the nanostructure even without doping transition-metal. Some new magnetic features have been observed and possible explanations are delineated here.

Detailed EPR Study of Spin Crossover Dendrimeric Iron(III) Complex

The unusual magnetic behavior of the first dendritic Fe(3+) complex with general formula [Fe(L)2](+)Cl(-)·H2O based on a branched Schiff base has been investigated by electron paramagnetic resonance (EPR) and Mössbauer spectroscopy. EPR displays that complex consists of the three types of magnetically active iron centers: one S = 1/2 low-spin (LS) and two S = 5/2 high-spin (HS) centers with strong low-symmetry and weak distorted octahedral crystal fields. Analysis of the magnetic behavior reflected by I versus T (where I is the EPR lines integrated intensity of the spectrum) demonstrates that the dendritic Fe(3+) complex has sufficiently different behavior in three temperature intervals. The first (4.2-50 K) interval corresponds to the antiferromagnetic exchange interactions between LS-LS, LS-HS, and HS-HS centers. The appearance of a presumable magnetoelectric effect is registered in the second (50-200 K) temperature interval, whereas a spin transition process between LS and HS centers occurs in the third (200-330 K) one. The coexistence of the magnetic ordering, presumable magnetoelectric effect, and spin crossover in one and the same material has been detected for the first time. The Mössbauer spectroscopy data completely confirm the EPR results.

Stoner enhanced paramagnetic influence on superconductivity in a superconductor/metallic heterostructure

The unusual magnetic behaviour of a Pd/Nb67Ti33/Pd heterostructure was investigated using polarised neutron reflectivity technique. On application of a large in-plane magnetic field, Stoner enhanced paramagnet Pd was found to influence the magnetic state of the Nb67Ti33 layer both above and below the superconducting transition temperature Tc significantly. Unlike the case of a conventional proximity effect for a superconductor/metallic heterostructure, the pair correlation in the superconducting state has been found to be more stable in the higher field limit compared to its low field counterpart, possibly signaturing a ‘novel’ coupled state in the system. The superconducting state in Nb67Ti33 has been found to be diamagnetic in nature at all the fields that can be fitted using a Meissner kind of behaviour in the high field limit. The magnetic properties of Pd and Nb67Ti33 are in excellent agreement with those measured using bulk magnetisation measurements.

Oxygen stoichiometry and magnetic properties of LuFe2O4+δ

We report a comprehensive investigation of the magnetic properties of LuFe2O4 (LFO) samples with different oxygen stoichiometries. Samples with excess oxygen exhibit spin glass behavior without long-range magnetic order, while three-dimensional ferrimagnetic order exists in a stoichiometric sample. Dissimilar experimental observations reported in several papers can be understood consistently when oxygen stoichiometry is taken into account. The stoichiometric sample orders magnetically below TN = 243 K, and a monoclinic lattice distortion sets in below TL = 175 K. This structural change is sensitive to the applied magnetic field, indicating strong spin-lattice coupling in this material. Unusual low-field thermal magnetization behavior was observed near TL, and its origin is discussed.

Unusual evolution of the magnetism on oxygen in La1−χSrχCoO3

AbstractWe present X‐ray absorption spectroscopy (XAS) and X‐ray magnetic circular dichroism (XMCD) data of La1−χSrχCoO3, for χ= 0.10‐0.30, at the O K edge and then compare the results to local density approximation (LDA) and generalized gradient approximation (GGA) calculations using a 2x2x2 supercell. Increases in the XAS and XMCD are nonlinear with χ, instead appearing correlated with the overall system magnetism, which changes from glass‐like to metallic ferromagnetism around χ=0.18. The magnetic states observed in XMCD only occur at the beginning of the pre‐edge; higher energy states show no orbital moment, suggesting two distinct types of holes exist. This is consistent with the formation of ferromagnetic clusters growing with χ and eventually overlapping for χ>0.18.Subsequent LDA and GGA calculations find a significant spin magnetic moment on O atoms (which are spinpolarized) and indicate that the doping‐induced holes are nearly equally distributed over the Co and O atoms. Calculations of the magnetization for individual O sites indicate that they are not all equivalent, but actually have different moments, depending on the proximity to Sr. This variation in the small fraction of oxygen state with angular momentum may explain in part the unusual magnetic behavior of these materials. Further, the orbitally decomposed density of states reveals that the bands from the traditional picture (localized eg and t2g states) overlap considerably and extend from ‐6 to 4 eV around E F, suggesting that the local picture is not the best for La1−χSrχCoO3 (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ferromagnetic Cu–O–Cu coupling in CaCu3Sn4O12 probed by neutron diffraction

The A-site ordered perovskite oxide with the formula CaCu3Sn4O12 has been synthesized in polycrystalline form under moderate pressure conditions (3.5 GPa) in combination with high temperature (1000 °C). This oxide crystallizes in the cubic space group (no. 204) with the unit-cell parameter a = 7.64535(6) Å at 300 K. The SnO6 network is extremely tilted, giving rise to a square planar coordination for Cu2+ cations. The non-magnetic character of Sn4+ offers an excellent opportunity to probe the magnetism of Cu2+ at the A sublattice in CaCu3Sn4O12. Magnetic susceptibility shows that this compound is ferromagnetic below TC = 10 K, which is an unusual magnetic behaviour in cuprates. This peculiar aspect has been examined by neutron powder diffraction. The refinement of the magnetic structure at 4 K indeed indicates a parallel coupling between Cu2+ spins with a magnetic moment of 0.5 μB/Cu atom.

Studies on structural and magnetic properties of hydrochloride crystals based on 2-(imidazo[1,2–a]pyridin-2-yl)-2-oxoacetic acid radical

We herein report two hydrochloride crystals obtained by slow diffusion of acetone into methanol solution of 2-(imidazo[1,2-a]pyridin-2-yl)-2-oxoacetic acid radical which was acidified with different concentration of hydrochloric acid. The relationship between magnetic properties and crystal-stacking structures was also investigated. 2-(Imidazo[1,2-a]pyridin-2-yl)-2-oxoacetic acid radical in two crystals mainly exists as diamagnetic dimers formed by intermolecular close contacts or supramolecular interactions, giving rise to low magnetic susceptibilities. One crystal exhibits quasi-one-dimensional columnar stacking chain and weak antiferromagnetic behavior. Another one consists of dimers assembled by supramolecular interactions (hydrogen bonds and anion–π interactions), and reveals unusual magnetic behavior.