Change In Magnetic Behavior Research Articles

A Combined Experimental and Theoretical Study on the Magnetic Properties of a Family of Bis(μ‐phenoxido)dicopper(II) Complexes Bearing ω‐[Bis(2‐hydroxy‐3,5‐dimethylbenzyl)amino]alkan‐1‐ol Ligands

AbstractFive new neutral bis(μ‐phenoxido)dicopper(II) complexes, [Cu2(μ‐HL1)2]·3EtOH·H2O (1), [Cu2(μ‐HL2)2]·1.65H2O (2), [Cu2(μ‐HL3)2(μ‐H2O)] (3), [Cu2(μ‐HL4)2] (4) and [Cu2(μ‐HL5)2(μ‐H2O)] (5), were prepared from a family of ω‐[bis(2‐hydroxy‐3,5‐dimethylbenzyl)amino]alkan‐1‐ol ligands (H3L1–H3L5 derived from 2‐aminoethanol, 3‐aminopropanol, 4‐aminobutanol, 5‐aminopentanol and 6‐aminohexanol, respectively) bearing a [O,N,O,O′] donor set. In complexes 3 and 5, there is also a bridging water molecule between the metallic centres. The copper(II) coordination planes of all these complexes form a roof‐like structure (the bridging O atoms are located at the top of the roof). The structural differences found for the different complexes and their relation with the magnetic properties is discussed below. Magnetic studies of these dinuclear complexes showed that J values vary from –470.8 to –91.2 as the Cu–O–Cu angles (θ) vary from 100.66(9) to 92.76(7)°. DFT theoretical calculations produced the corresponding magnetic exchange coupling constants, finding that these values are quite near to the experimental ones. A linear relationship between the calculated J values and θ was observed, clearly supporting that the major factor controlling the magnetic exchange coupling in this series of complexes is, by far, the Cu–O–Cu bridging angle. The crossover point below which the magnetic behaviour changes from antiferromagnetic to ferromagnetic coupling is predicted at ≈ 89°. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Doping dependent room-temperature ferromagnetism and structural properties of dilute magnetic semiconductor ZnO:Cu 2+ nanorods

Copper doped ZnO nanoparticles were synthesized by the chemical technique based on the hydrothermal method. The crystallite structure, morphology and size were determined by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) for different doping percentages of Cu 2+ (1–10%). TEM/SEM images showed formation of uniform nanorods, the aspect ratio of which varied with doping percentage of Cu 2+. The wurtzite structure of ZnO gradually degrades with the increasing Cu 2+ doping concentration and an additional CuO associated diffraction peak was observed above 8% of Cu 2+ doping. The change in magnetic behavior of the nanoparticles of ZnO with varying Cu 2+ doping concentrations was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong room-temperature ferromagnetic behavior, however at higher doping percentage of copper the ferromagnetic behavior was suppressed and paramagnetic nature was enhanced.

Photochemical modification of magnetic properties in organic low-dimensional conductors

Magnetic properties of organic charge transfer salts Ag( DX) 2 ( DX=2,5-dihalogeno- N, N′-dicyanoquinonediimine; X=Cl, Br, I) were modified by UV irradiation from paramagnetism to diamagnetism in an irreversible way. The temperature dependence of susceptibility revealed that such change in magnetic behavior could be continuously controlled by the duration of irradiation. The observation with scanning electron microprobe revealed that the original appearance of samples, e.g. black well-defined needle-shaped shiny single crystals, remained after irradiation irrespective of the irradiation conditions and the duration. Thermochemical analysis and X-ray diffraction study demonstrated that the change in the physical properties were due to (partial) decomposition of Ag( DX) 2 to Ag X, which was incorporated in the original Ag( DX) 2 lattices. Because the physical properties of low-dimensional organic conductors are very sensitive to lattice defects, even a small amount of Ag X could effectively modify the electronic properties of Ag( DX) 2 without making the original crystalline appearance collapse.

Room-temperature ferromagnetism in Sn1−xMnxO2 nanocrystalline thin films prepared by ultrasonic spray pyrolysis

Abstract Sn 1− x Mn x O 2 ( x =0.01–0.05) thin films were synthesized on quartz substrate using an inexpensive ultrasonic spray pyrolysis technique. The influence of doping concentration and substrate temperature on structural and magnetic properties of Sn 1− x Mn x O 2 thin films was systematically investigated. X-ray diffraction (XRD) studies of these films reflect that the Mn 3+ ions have substituted Sn 4+ ions without changing the tetragonal rutile structure of pure SnO 2 . A linear increase in c- axis lattice constant has been observed with corresponding increase in Mn concentration. No impurity phase was detected in XRD patterns even after doping 5 at% of Mn. A systematic change in magnetic behavior from ferromagnetic to paramagnetic was observed with increase in substrate temperature from 500 to 700 °C for Sn 1− x Mn x O 2 ( x =0.01) films. Magnetic studies reveal room-temperature ferromagnetism (RTFM) with 3.61×10 −4 emu saturation magnetization and 92 Oe coercivity in case of Sn 1− x Mn x O 2 ( x =0.01) films deposited at 500 °C. However, paramagnetic behavior was observed for the films deposited at a higher substrate temperature of 700 °C. The presence of room-temperature ferromagnetism in these films was observed to have an intrinsic origin and could be obtained by controlling the substrate temperature and Mn doping concentration.

Effect of nickel doping concentration on structural and magnetic properties of ultrafine diluted magnetic semiconductor ZnO nanoparticles

The ZnO:Ni 2+ nanoparticles of mean size 2–12 nm were synthesized at room temperature by the simple co-precipitation method. The crystallite structure, morphology and size were determined by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Ni doping concentration and an additional NiO-associated diffraction peak was observed above 15% of Ni 2+ doping. The change in magnetic behavior of the nanoparticles of ZnO with varying Ni 2+ doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially, these nanoparticles showed strong ferromagnetic behavior, however, at higher doping percentage of Ni 2+, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Ni–Ni ions suppressed the ferromagnetism at higher doping concentrations of Ni 2+.

Crystal-water-induced switching of magnetically active orbitals inCuCl2

The dehydration of ${\text{CuCl}}_{2}\ensuremath{\cdot}2{\text{H}}_{2}\text{O}$ to ${\text{CuCl}}_{2}$ leads to a dramatic change in magnetic behavior and ground state. Combining density-functional electronic structure and model calculations with thermodynamical measurements, we reveal the microscopic origin of this unexpected incident\char22{}a crystal-water-driven switching of the magnetically active orbitals. This switching results in a fundamental change in the coupling regime from a three-dimensional antiferromagnet to a quasi-one-dimensional behavior. ${\text{CuCl}}_{2}$ can be well described as a frustrated ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ Heisenberg chain with ferromagnetic exchange ${J}_{1}$ and ${J}_{2}/{J}_{1}\ensuremath{\sim}\ensuremath{-}1.5$ for which a helical ground state is predicted.

A Thermally and Hydrolytically Stable Microporous Framework Exhibiting Single‐Chain Magnetism: Structure and Properties of [Co2(H0.67bdt)3]⋅20 H2O

Fixing a hole: Hydrothermal chemistry has been exploited in the preparation of a 3D framework material exhibiting 48% accessible void volume and 1.5% hydrogen uptake by weight at 120 kPa (see picture). The title compound also exhibits single-chain magnetic behavior and reversible changes in magnetic properties upon solvation and desolvation.

Crystal water induced alteration of magnetic exchange interactions

Cuprates show an intimate interplay between the crystal structure and their magnetism, particularly with respect to frustration and dimensionality. On the other hand, it is a widespread belief that crystal water has just a moderate and quantitative influence to the magnetic properties of these compounds caused by the modification of interatomic distances. In contrast, the hydration of CuCl2 leads to a dramatic change in magnetic behaviour and ground state. While CuCl2·2H2O is a classic example for a three-dimensional antiferromagnet (TN 4.3 K) [1] with small exchange couplings, CuCl2 is a quasi-one dimensional chain compound that exhibits long-range order at T 24 K [2].

Switching pairwise exchange interactions to enhance SMM properties

Abstract A simple change in the identity of “R” in the family of complexes of general formula [Mn 6 O 2 (R-sao) 6 (O 2 C-th) 2 L 4-6 ] (where saoH 2 = salicylaldoxime) causes subtle changes in the magnetic core of the compounds that leads to dramatic changes in the magnetic behaviour, switching the pairwise exchange interactions from anti- to ferromagnetic, greatly enhancing the barrier for magnetization relaxation.

Structures and magnetic ordering of Mn nFe ( n=1–12) clusters

The structures and magnetic ordering of Mn n Fe ( n=1–12) clusters have been investigated using all-electron density functional theory. The results indicate that the Mn n Fe clusters undergo a change in magnetic behavior from ferromagnetic ordering for the smallest size to ferrimagnetic ordering for intermediate sizes and beyond. Ferromagnetic ordering is clearly favored for n = 1 , but ferromagnetic and ferrimagnetic states are nearly degenerate for n = 2 , 3, and 4. A radical change occurs at n = 5 where the ferrimagnetic states completely prevail. The transition range of magnetic ordering from ferromagnetic to ferrimagnetic for Mn n clusters occurs early with the doping of one Fe atom.

Reduction kinetics, magnetic behavior and morphological changes during reduction of magnetite single crystal

The true fundamental trends of the different phase transformation steps (Fe 2O 3–Fe 3O 4–FeO–Fe) during iron oxide reduction process are necessary requested. Both of the macroscopic and microscopic changes during these steps have a direct bearing on the overall reduction kinetics. Magnetite single crystals were isothermally reduced in pure hydrogen at 900–1100 °C. The oxygen weight loss during reduction process was recorded as a function of time. The magnetite samples and its reduction products were characterized by X-ray diffraction analysis, reflected light microscope, scanning electron microscope and vibrating sample magnetometer to reveal the effect of hydrogen reduction at different temperatures on reduction behavior, composition, microstructure and magnetic properties of the produced samples. The reduction rate increases with reaction temperature. The reduction at 900 and 950 °C was stopped at 83 and 89% extent, respectively. Depending on the kinetics obtained data from reduction process, the reduction rate was found to be controlled by the solid-state diffusion mechanism. The magnetic characterizations of the reduced magnetite at 900–1100 °C were measured whereas the magnetization value increased from 114.1 to 132.1 emu/g with increasing temperature. Magnetite single crystal was partially reduced at 20, 40, 60, and 80%, its magnetic properties was found to be dependent on the morphology, core–shell like structure is formed at 40% reduction extent.

Magnetic behaviour of iron nanoparticles passivated by oxidation

AbstractThis study is to understand the effect of oxidation, especially magnetically, on iron nanoparticles. According to generation of the oxidated iron nanoparticles, mechanical alloying technique was used and nanosized magnetite (Fe3O4), maghemite (γ‐Fe2O3) and hematite (α‐Fe2O3) particles were obtained as the resultant samples. The reactance to the thermal treatment was determined by differential thermal analysis and thermogravimetric (DTA‐TG) measurements. X‐ray powder diffractions (XRD) helped to exhibit the structure of the sample by ICDD cards and to determine the size of nanoparticles by using the Scherrer formula. On the other hand, VSM (vibrating sample magnetometer) measurements were determined to understand the magnetic behaviour. Through the transformation of Fe3O4 to other iron‐oxides, two exothermic peaks were observed at around 169.11 °C and 562.61 °C by DTA analysis. Beside of this, the experimental results demonstrate the effects of mechanical milling parameters, atmosphere and lubricant, to the structure and to the size of the resultant particles and the change of magnetic behavior of the iron‐oxide and iron nanoparticles when they approach to superparamagnetic region, especially in single domain region. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic resonance imaging of microstructure transition in stainless steel

Magnetic resonance images are prone to artifacts caused by metallic objects. Such artifacts may not only hamper image interpretation, but also have been shown to provide information about the magnetic properties of the substances involved. In this work, we aim to explore the potential of MRI to detect, localize and characterize changes in magnetic properties that may occur when certain alloys have been exposed to a thermomechanical stress. For this purpose, stainless steel 304 L wires were drawn to induce a change from paramagnetic austenitic into ferromagnetic martensitic microstructure. The changes in magnetic behavior were quantified by analyzing the geometric distortion in spin echo and the geometric distortion and intravoxel dephasing in gradient echo images at 0.5, 1.5 and 3 T. The results of both imaging strategies were in agreement and in accordance with independent measurements with a vibrating sample magnetometer. Drawing wire to 2% of its cross-sectional area was found to increase the volume fraction of the ferromagnetic martensite from 0.3% to 80% and to enhance the magnetization up to two or three orders of magnitude. The results demonstrate the potential of MRI to locate and quantify stress-induced changes in the magnetic properties of alloys in a completely noninvasive and nondestructive way.

The effects of Co-metal clusters on exchange bias for Co-doped NiO/FeNi bilayers

Co-doped NiO inhomogeneous films were synthesized by sputtering metallic Co chips and NiO together and the exchange bias of bilayers Co-doped NiO/FeNi was investigated. When Co content was up to 25.2%, the exchange bias field H E at the room temperature increased to the maximum which was about three times compared to the undoped-bilayers. With further increase of Co content, the exchange bias field H E and blocking temperature T B decreased. Analysis suggests that the configuration of nanometer-sized Co-metal clusters enchased into NiO matrix played an important role in the change of magnetic behavior for the bilayers.

Modeling of sharp change in magnetic hysteresis behavior of electrical steel at small plastic deformation

In 2.2% Si electrical steel, the magnetic hysteresis behavior is sharply sheared by a rather small plastic deformation (0.5%). A modification to the Jiles–Atherton hysteresis model makes it possible to model magnetic effects of plastic deformation. In this paper, with this model, it is shown how a narrow hysteresis with an almost steplike hysteresis curve for an undeformed specimen is sharply sheared by plastic deformation. Computed coercivity and hysteresis loss show a sharp step to higher values at small strain due to an n=1∕2 power law dependence on residual strain. The step is seen experimentally.

Ionic size effect in charge-orderedLa0.5Ca0.5MnO3

Magnetization, resistivity, and neutron diffraction measurements have been carried out on a series of samples ${\mathrm{La}}_{0.5\ensuremath{-}x}{\mathrm{Y}}_{x}{\mathrm{Ca}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ ($x=0$, 0.1, 0.18), to study the influence of ionic radii in charge-ordered manganites. ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ exhibits ferromagnetic (FM) transition at ${T}_{C}\ensuremath{\sim}230\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and antiferromagnetic (AFM) transition at ${T}_{N}\ensuremath{\sim}170\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Our experiments suggest presence of ferromagnetic clusters in charge-ordered (CO) AFM matrix at $T<{T}_{N}$. The CO state melts under the influence of magnetic field 5 T and a large (99% at 50 K) magnetoresistance (MR) is observed. Substituting with $\mathrm{Y}$ leads to decrease in unit cell volume and increase in substitution induced disorder ${\ensuremath{\sigma}}^{2}$ due to ionic size mismatch between $\mathrm{Y}$ and La. Phase separation effects as a result of the disorder are observed in all the samples studied. Significant changes in long-range magnetic ordering and MR behavior are observed as a result of substitution of La with $\mathrm{Y}$. For $x=0.1$ long-range AFM ordering is suppressed while FM clusters grow in size. Additionally, the MR is found to be negligibly small in comparison with $x=0$. This and other behavior observed as a result of disorder are discussed within the context of phase separation effects in charge ordered systems.

Magnetic Behavior of Novel Intermetallics CePt3X

A series of isostructural CePt3X intermetallics (X=Si, Al, Ge) has been synthesized and characterized by X-ray powder diffraction and microprobe analysis. All three compounds crystallize in a non-centrosymmetric CePt3B type structure with a space group P 4mm. CePt3Si is a heavy-fermion superconductor with antiferromagnetic (AF) ordering. The other two compounds also exhibit magnetic ordering at low temperatures, however, superconductivity is missing down to 0.4 K. A strong influence of heat treatment on structural disorder yielding a dramatic change of low temperature magnetic behavior has been observed for both novel compounds, respectively. Results of a systematic study of specific heat, electrical resistivity and magnetization are discussed from the view of technological and structural aspects.

Antiferromagnetic Coupling Driven by Bond Length Contraction near theGa1−xMnxNFilm Surface

Using first principles calculations based on gradient corrected density functional theory we show that Mn atoms, which couple ferromagnetically in bulk Ga1-xMnxN, couple antiferromagnetically on its surface. This change in magnetic behavior is brought about by a contraction of the Mn-Mn and Mn-N bond lengths, which is significantly greater on the surface than in the bulk. The present study provides new insight to the numerous conflicting experimental observations in Mn doped GaN systems.

Magnetic properties of Ising thin films with cubic lattices

We have used Monte Carlo simulations and mean-field analysis to observe the magnetic behavior of Ising thin films with cubic lattice structures as a function of temperature and thickness, especially in the critical region. Magnetization and magnetic susceptibility, including layer variation, are investigated. We find that the magnetic behavior changes from two-dimensional to three-dimensional character with increasing film thickness. Both the crossover of the critical temperature from a two-dimensional to a bulk value and the shift exponent are observed. Nevertheless, with support from a scaling function, the simulations show that the effective critical exponents for films with large enough layer extents only vary a little from their two-dimensional values. This, in particular, provides an indication of two-dimensional universality in the thin films.

Effect of magnetic anisotropy on soft magnetization of Fe, Fe50Co50 and Fe70Co30 films

An FeNiMo underlayer with magnetic anisotropy is shown to readily induce magnetic anisotropy in an overlying FeCoV or Fe50Ni50 layer and suppress the coercive force of these overlayers by about 90–97%. The exact nature of the change in magnetic behavior of the overlayers and the degree of coercive force reduction are confirmed to be dependent on the existence and direction of magnetic anisotropy in those layers upon deposition. The soft magnetization of Fe, Fe50Co50 and Fe70Co30 films with isotropic magnetization processes deposited on an Fe20Ni80 underlayer with magnetic anisotropy is successfully achieved by introducing magnetic anisotropy into the semi-hard magnetic overlayers by oblique-incidence evaporation. The coercive forces of Fe20Ni80/Fe, Fe20Ni80/Fe50Co50 and Fe20Ni80/Fe70Co30 bilayer films are reduced remarkably by this process, and the strength of the anisotropic magnetic field is demonstrated to be readily controllable by this method.