Two-dimensional heisenberg antiferromagnets: syntheses, X-ray structures, and magnetic behavior of [Cu(pz)2](ClO4)2, [Cu(pz)2](BF4)2, and [Cu(pz)2(NO3)](PF6)

We report on the syntheses, crystal structures, and magnetic susceptibilities of a family of copper pyrazine (pz)-based antiferromagnets with moderate in-plane magnetic exchange. These materials fall into two classes: monoclinic complexes [Cu(pz)2]A2 for A = ClO4 (1) or BF4 (2) and the tetragonal complex [Cu(pz)2(NO3)]PF6 (3). Compound 1 and its deuterated version [Cu(pz-d4)2](ClO4)2 (1a) crystallize in the space group C2/m at room temperature with disordered perchlorate anions. For both 1 and 2, the C centering of the Cu(II), S = 1/2, site yields four equivalent nearest neighbors, producing layers of Cu(II) ions bridged by the pz molecules, which map onto a square magnetic lattice. The layers are offset such that Cu(II) ions lie above and below the holes of adjacent layers. Compound 3 crystallizes in the space group I4/mcm with a layer structure similar to those of 1 and 2 but with Cu(II) ions of adjacent layers stacked above each other and bridged by semicoordinate NO3- ions. The variable-temperature susceptibilities in these compounds approximate a two-dimensional Heisenberg antiferromagnet with J values within the layers of 17.5(3) K (1), 15.3(3) K (2), and 10.8(3) K (3). Ordering transitions are observed in the magnetic data at 4.2(3) and 4.3(5) K for 1 and 2, respectively.     

Review: A gentle introduction to magnetism: units,fields, theory,and experiment

We present an introduction to the workings, units of measure, and general properties of magnetic materials. This is intended as a “primer to interpretation of magnetic data” for those who are entering the field, or those who are encountering magnetic measurements in the literature. We expect this work will serve as an initial guide to the reader to familiarize them with the basics in the hope that those working in the field of magnetochemistry will wish to explore additional, more detailed literature as their specific investigations demand. Topics covered include: magnetic fields and units (SI and cgs), paramagnetism (magnetization and magnetic susceptibility), Curie and Curie–Weiss behavior, magnetic exchange interactions, magnetic anisotropy, dimeric systems and exchange-coupled networks (including chains, ladders, and layers), and long-range order.     

The detection of oxygen in magneto-optical layers with SIMS

The detection of oxygen in magneto-optical layers is of fundamental importance for the characterization of the stability of RE-TM material. The magnetic properties are directly influenced by oxide formation. Oxygen depth profiles are carried out by using the SIMS technique. A comparison with magnetic measurements showed a clear conformity. We were able to study the oxidation behaviour of various layers at room temperature and at higher values up to 250° C for several hours. It could be shown that aluminum is a successful material for the protection of RE-films against oxidation. The difficulties of translating SIMS counting rates into concentration values were overcome by using EPMA. Specially prepared reference samples were measured by this technique and could then be used as standard samples for SIMS.     

A First-Principles Study of C3N Nanostructures: Control and Engineering of the Electronic and Magnetic Properties of Nanosheets,Tubes and Ribbons

Using first-principles calculations we systematically investigate the atomic, electronic and magnetic properties of novel two-dimensional materials (2DM) with a stoichiometry C₃N which has recently been synthesized. We investigate how the number of layers affect the electronic properties by considering monolayer, bilayer and trilayer structures, with different stacking of the layers. We find that a transition from semiconducting to metallic character occurs which could offer potential applications in future nanoelectronic devices. We also study the affect of width of C₃N nanoribbons, as well as the radius and length of C₃N nanotubes, on the atomic, electronic and magnetic properties. Our results show that these properties can be modified depending on these dimensions, and depend markedly on the nature of the edge states. Functionalization of the nanostructures by the adsorption of H adatoms is found induce metallic, half-metallic, semiconducting and ferromagnetic behavior, which offers an approach to tailor the properties, as can the application of strain. Our calculations give insight into this new family of C₃N nanostructures, which reveal unusual electronic and magnetic properties, and may have great potential in applications such as sensors, electronics and optoelectronic at the nanoscale.     

Polymer mediated self-assembly of magnetic nanoparticles

We present a simple polymer-mediated process of assembling magnetic FePt nanoparticles on a solid substrate. Alternatively absorbing the PEI molecule and FePt nanoparticles on a HO-terminated solid surface leads to a smooth FePt nanoparticle assembly with controlled assembly thickness and dimension. Magnetic measurements show that the thermally annealed FePt nanoparticle assembly as thin as three nanoparticle layers is ferromagnetic. The magnetization direction of this thin FePt nanoparticle assembly is readily controlled with the laser-assisted magnetic writing. The reported process can be applied to various substrates, nanoparticles, and functional macromolecules and will be useful for future magnetic nanodevice fabrication.     

A structurally perfect S = (1/2) kagomé antiferromagnet

The syntheses and magnetic susceptibilities of a pure series of rare copper minerals from the atacamite family with general formula ZnxCu4-x(OH)6Cl2 (0 相似文献   

Analytical solution for restricted diffusion in circular and spherical layers under inhomogeneous magnetic fields

We propose an analytical solution for restricted diffusion of spin-bearing particles in circular and spherical layers in inhomogeneous magnetic fields. More precisely, we derive exact and explicit formulas for the matrix representing an applied magnetic field in the Laplacian eigenbasis and governing the magnetization evolution. For thin layers, a significant difference between two geometrical length scales (thickness and overall size) allows for accurate perturbative calculations. In these two-scale geometries, apparent diffusion coefficient (ADC) as a function of diffusion time exhibits a new region with a reduced but constant value. The emergence of this intermediate diffusion regime, which is analogous to the tortuosity regime in porous media, is explained in terms of the underlying Laplace operator eigenvalues. In general, regions with constant ADCs would be reminiscent of multiscale geometries, and their observation can potentially be used in experiments to detect the length scales by varying diffusion time.     

Propriétés magnétiques des multicouches Tb/Fe amorphes recuites

The magnetic properties in the iron layers of Tb/Fe amorphous multilayers are investigated using ⁵⁷Fe Mössbauer spectrometry as a function of the nominal iron thickness and adequate heat treatment. We show that the perpendicular magnetic anisotropy is improved after homogenisation of the terbium-iron interface when the initial multilayer is of modulated composition Tb/Tb-Fe/Tb.     

Hybrid molecular magnets obtained by insertion of decamethyl-metallocenium cations into layered, bimetallic oxalate complexes:

A new series of hybrid organometallic - inorganic layered magnets with the formula [Z(III)Cp*2][M(II)M(III)(ox)3] (Z(III) = Co, Fe; M(III) = Cr, Fe; M(II) = Mn, Fe, Co, Cu, Zn; ox = oxalate; Cp* = pentamethylcyclopentadienyl) has been prepared. All of these compounds are isostructural and crystallize in the monoclinic space group C2/m, as found by X-ray structure analysis. Their structure consists of an eclipsed stacking of the bimetallic oxalate-based extended layers separated by layers of organometallic cations. These salts show spontaneous magnetization below To, which corresponds to the presence of ferro-, ferri-, or canted antiferromagnetism. Compounds in which the paramagnetic deca-methylferrocenium is used instead of the diamagnetic decamethylcobaltocenium are good examples of chemically constructed magnetic multilayers with alternating ferromagnetic and paramagnetic layers. The physical properties of this series have been thoroughly studied by means of magnetic measurements and ESR and Mossbauer spectroscopy. We have found that the two layers are electronically quasiindependent. As a consequence, the bulk properties of these magnets have not been significantly affected by the insertion of a paramagnetic layer of S = 1/2 spins in between the extended layers. In fact, the critical temperatures remain unchanged even when comparing [MCp*2]+ derivatives with [XR4]+ compounds (X = N, P; R = Ph, nPr, nBu). Nevertheless, the presence of the paramagnetic layer has been shown to have some influence on the hysteresis loops of these compounds. In the same context, the spin polarization of the paramagnetic units (which arises from the internal magnetic field created by the bimetallic layers in the ordered state) has been observed by Mossbauer and ESR spectroscopy.     

Lipase Immobilized in Organic-Inorganic Matrices

Enzyme lipase was immobilized with ferrite powder and deposited in layers on glass slides from lipase to a solution of silicone alkoxides. The highest hydrolytical activity was observed with the magnetic lipase prepared by mixing the paste of ferrite powder and lipase with tetramethoxysilane, 3-aminopropyltriethoxysilane and propyltrimethoxysilane. In a mixed reactor, the particles of the magnetic lipase were desintegrated by mechanical stirring which caused loosing the lipase linked to magnetic material and resulted in a significant drop of activity after magnetic separation. Transparent layers were prepared by dip- or spin-coating from partially hydrolyzed tetraethoxysilane and solutions containing methyltriethoxysilane with 3-aminopropyltriethoxysilane or tetraethoxysilane with 3-mercaptopropyltriethoxysilane. The lipases immobilized in films with magnetic particles were active in tests with 4-nitrophenyl butyrate and were not inhibited by 0,0-dimethyl-0-(2,2-dichlor-vinyl)-phosphate.     

Two-, three-, and four-component magnetic multilayer onion nanoparticles based on iron oxides and manganese oxides

Magnetic multilayered, onion-like, heterostructured nanoparticles are interesting model systems for studying magnetic exchange coupling phenomena. In this work, we synthesized heterostructured magnetic nanoparticles composed of two, three, or four components using iron oxide seeds for the subsequent deposition of manganese oxide. The MnO layer was allowed either to passivate fully in air to form an outer layer of Mn(3)O(4) or to oxidize partially to form MnO|Mn(3)O(4) double layers. Through control of the degree of passivation of the seeds, particles with up to four different magnetic layers can be obtained (i.e., FeO|Fe(3)O(4)|MnO|Mn(3)O(4)). Magnetic characterization of the samples confirmed the presence of the different magnetic layers.     

An indication of magnetic-field-induced superconductivity in a bifunctional layered organic conductor,kappa-(BETS)(2)FeBr(4)

Hybrid systems consisting of the conducting layers of organic donor molecules and the magnetic layers of inorganic anions have been focused on as possible bifunctional materials, whose conducting properties can be tuned by controlling the magnetic state of the anion layers on an application of magnetic field. Here we report the magnetoresistance of the antiferromagnetic organic superconductor, kappa-(BETS)2FeBr4 [BETS = bis(ethylenedithio)tetraselenafulvalene], consisting of the two-dimensional superconducting layers of the BETS semications and the insulating layers of the FeBr4- anions. Due to the metamagnetic nature of the Fe3+ spin system, characteristic resistivity decrease was observed just below the antiferromagnetic superconductor-to-ferromagnetic metal transition at 1.6 T. Furthermore, an indication of the onsets of the magnetic-field-induced superconductivity was discovered around 12.5 T.     

Analytical investigations of tunnel magnetoresistance layers

The basic elements of tunnel magnetoresists are two magnetic layers separated by an insulating barrier layer. The uniformity of this only 1–2 nm thick barrier layer up to dot edges and the chemical composition of the layers are properties important for the efficiency of tunnel magnetoresistance devices. These key-properties have been investigated by analytical TEM methods like high resolution TEM imaging and energy-filtered imaging. With regard to the chemical composition the TEM results have been confirmed by XPS investigations. The subsequent oxidation of the barrier is one of the most critical steps of the deposition procedure of the layer stacks. An undersized oxygen dose leads to an incomplete oxidation of the barrier layer with uncontrollable tunnel behaviour. An overdose of oxygen leads to oxygen diffusion in the layers beneath the barrier and uncontrollable magnetic hardness of the lower magnetic electrode.     

Superconductivity and structure of the multilayer Cu/Nb/Nb-Ti nanocomposites

The multilayer composite tapes containing layers of superconducting alloy of niobium with 31 or 50 wt % of Ti separated by niobium layers were investigated. The thickness of a layer is from ∼140 to ∼10 nm. Composites were produced by three-step rolling of multilayer packages. Effective fixation of the superconducting vortex filaments occurred at the interlayer boundaries of Nb-NbTi, as evidenced by the large anisotropy of the critical current density. The second critical magnetic field was found also dependent on the orientation of the plane of rolling of the composite relative to the external magnetic field, and decreases with decreasing thickness of the layers.     

Competing magnetic interactions in Na10Co4O10, studied by neutron diffraction

Na(10)Co(4)O(10) was investigated by neutron powder diffraction at 230, 70, and 4 K. The crystal structure, determined previously by X-ray diffraction on single crystals, was confirmed. Na(10)Co(4)O(10) orders magnetically below 37 K. All observed magnetic reflections could be indexed by integers (hkl) with respect to the chemical unit cell and the magnetic propagation vector q=0. The refinement was performed in the Shubnikov space group C2/c and indicated a collinear antiferromagnetic spin structure. The determined spin arrangement is consistent with the magnetic intratetramer interactions suggested previously from the analysis of magnetic susceptibility data: the magnetic moments of the central Co(III) ions of the Co(4)O(10) tetramer lie parallel to each other and couple in an antiparallel fashion to the terminal Co(II) moments. The Rietveld analysis shows that the net moments of 0.64 mu(B) per tetramer form ferromagnetic layers parallel to the ab plane. Adjacent layers are coupled antiferromagnetically along c. The spins are aligned in the ac plane along the line connecting adjacent Co(II) and Co(III) ions of the tetramer. We have determined unusually low values for the ordered magnetic moments of 2.43(5) mu(B) and 2.11(6) mu(B) for Co(III) and Co(II), respectively. The occurrence of spontaneous magnetization below 37 K indicates a slight canting of 2.2 degrees of the antiferromagnetic structure. A representation analysis shows that a weak ferromagnetic component along b is compatible with the determined antiferromagnetic structure.     

Characteristics of Each Bonded Layer of a Polymer-Based Magnetic Tape Using Rule-of-Mixtures and Dynamic Mechanical Analysis

A rule-of-mixtures approach is used with dynamic mechanical analysis (DMA) to determine the viscoelastic characteristics of individual layers of a fourth-generation advanced magnetic tape used for archival storage. Such tapes are comprised of three layers: a 0.92 µm thick poly(ester-urethane)-based front coat that binds the magnetic particles together, a 5.00 µm thick poly(ethylene naphthalate) substrate, and a protective 0.64 µm thick back coat with cellulose nitrate as a constituent. The storage modulus, loss modulus, and loss tangent determined for each layer are discussed in terms of their influence on magnetic tape properties and characteristics.     

Formation of walls in cylindrical smectic C layers in the presence of a tilted magnetic field

A theoretical study of the elastic properties of a smectic C liquid-crystalline system consisting of cylindrical smectic layers is presented. We show that the ground state configuration of the c-director for such a system depends on the signs of two combinations of the relevant elastic constants. With these configurations as a starting point, we describe how a set of walls under certain conditions must be formed in the system if a magnetic field is applied at an angle to the cylinder axis. An estimate of the thickness of these walls is given. We also show that, depending on whether the angle between the magnetic field and the cylinder axis is larger or smaller than the tilt of the director with respect to the smectic layer normal, the system will exhibit two qualitatively different behaviours.     

Pulse magnetoelectrolysis

We present a novel technique consisting in the combination of pulse plating and magnetoelectrolysis. This technique is applied to copper electrolysis between two vertical copper-electrodes. The modulation of the current inside an inhomogeneous magnetic field generates a modulated Lorentz force driving an oscillating convection studied using particle image velocimetry. The resulting changes in the concentration boundary layers are analysed by means of a Mach-Zehnder interferometer.     

Topotactic oxidative and reductive control of the structures and properties of layered manganese oxychalcogenides

Topotactic modification, by both oxidation and reduction, of the composition, structures, and magnetic properties of the layered oxychalcogenides Sr4Mn3O7.5Cu2Ch2 (Ch=S, Se) is described. These Mn3+ compounds are composed of alternating perovskite-type strontium manganese oxide slabs separated by anti-fluorite-type copper chalcogenide layers and are intrinsically oxide deficient in the central layer of the perovskite slabs. The systems are unusual examples of perovskite-related compounds that may topotactically be both oxidized by fluorination and reduced by deintercalation of oxygen from the oxide-deficient part of the structure. The compounds exhibit antiferromagnetic ordering of the manganese magnetic moments in the outer layers of the perovskite slabs, while the other moments, in the central layers, exhibit spin-glass-like behavior. Fluorination has the effect of increasing the antiferromagnetic ordering temperature and the size of the ordered moment, whereas reduction destroys magnetic long-range order by introducing chemical disorder which leads to both further disorder and frustration of the magnetic interactions in the manganese oxide slab.     

Optimized Liquid-Phase Exfoliation of Magnetic van der Waals Heterostructures: Towards the Single Layer and Deterministic Fabrication of Devices

Van der Waals magnetic materials are promising candidates for spintronics and testbeds for exotic magnetic phenomena in low dimensions. The two-dimensional (2D) limit in these materials is typically reached by mechanically breaking the van der Waals interactions between layers. Alternative approaches to producing large amounts of flakes rely on wet methods such as liquid-phase exfoliation (LPE). Here, we report an optimized route for obtaining monolayers of magnetic cylindrite by LPE. We show that the selection of exfoliation times is the determining factor in producing a statistically significant amount of monolayers while keeping relatively big flake areas (~1 µm²). We show that the cylindrite lattice is preserved in the flakes after LPE. To study the electron transport properties, we have fabricated field-effect transistors based on LPE cylindrite. Flakes are deterministically positioned between nanoscale electrodes by dielectrophoresis. We show that dielectrophoresis can selectively move the larger flakes into the devices. Cylindrite nanoscale flakes present a p-doped semiconducting behaviour, in agreement with the mechanically exfoliated counterparts. Alternating current (AC) admittance spectroscopy sheds light on the role played by potential barriers between different flakes in terms of electron transport properties. The present large-scale exfoliation and device fabrication strategy can be extrapolated to other families of magnetic materials.