Magnetic Coupling Properties Research Articles

Theoretical Study on the Structures and Magnetic Properties of Metal String Complexes [Ni<sub>3</sub>(L)<sub>4</sub>(NCS)<sub>2</sub>] (L = dpa<sup>-</sup>, mpta<sup>-</sup>, mdpa<sup>-</sup>, mppa<sup>-</sup>)

? ? ?nb ?¤ Density functional theory at the BP86 level and natural bond orbital theory were used to investigate the influence of bridging ligands on the Ni―Ni interactions and magnetic coupling properties of metal string complexes [Ni3(L)4(NCS)2] (L =1: dpa– (dipyridylamine),2: mpta– (4-methylpyridyl-thiazolylamine),3: mdpa– (4-methyl-dipyridylamine),4: mppa– (4-methylpyridyl-3H-pyrrolylamine)) with potential applications in molecular wires. The folowing conclusions can be drawn. (1) The ground states of the complexes are antiferromagnetic (AF) singlet states, which correspond to the quintet state (HS). The energy and structure of HS is similar to AF. There are three-center-four-electron bonds ( ) along the chains. (2) The Ni―Ni and Ni―N distances are unaffected by methyl substituents on the pyridine ring of dpa– ligands. However, substitution of the 3H-pyrrole ring or thiazole ring by the pyridine ring in mdpa– lengthens the N1―N2 and Ni―Ni distances but shortens the Ni2―N2 distance. These effects of the thiazole ring are weaker than those of the 3H-pyrrole ring. Therefore, the strength of the Ni―Ni interaction is1 ≈3 >2 >4. (3) The predictedJab values of3 and4n? are –103 and –88 cm–1, respectively. The AF magnetic coupling effects of the complexes increase with increasing Ni―Ni interaction strength: the stronger the Ni―Ni interaction, the greater the direct magnetic coupling in the orbitals along the chains. In addition, the stronger the Ni2―N2 interaction, the larger the indirect magnetic coupling involving the bridging ligand. The direct magnetic coupling is stronger than the indirect magnetic coupling.

Peculiar magnetism of BiFeO3 nanoparticles with size approaching the period of the spiral spin structure

Size effect of multiferroics is important for its potential applications in new type miniaturized multifunctional devices and thus has been widely studied. However, is there special size effect in the materials with spiral modulated spin structure (such as BiFeO3)? It is still an issue to be investigated. In this report, structural, magnetic and magnetoelectric coupling properties are investigated for sol-gel prepared BiFeO3 nanoparticles with various sizes. It is found that a structural anomaly arises for the particles with size close to the 62 nm period of the spiral modulated spin structure, which induces an obviously increased ferromagnetism. In addition, large magnetoelectric coupling effect is observed in 62 nm BiFeO3 nanoparticles. Our result provides another insight into the size effect of BiFeO3, and also a clue to the magnetic structure at nanoscale.

Magnetic coupling constants and vibrational frequencies by extended broken symmetry approach with hybrid functionals

The description of the electronic structure and magnetic properties of multi-centers transition metal complexes, especially of mixed-valence compounds, still represents a challenge for density functional theory (DFT) methods. The energies and the geometries of the correctly symmetrized low-spin ground state are estimated using the Heisenberg-Dirac-van Vleck spin Hamiltonian within the extended broken symmetry method introduced by Marx and co-workers [Nair et al., J. Chem. Theory Comput. 4, 1174-1188 (2008)]. In the present work we extend the application of this technique, originally implemented using the DFT+U scheme, to the use of hybrid functionals, investigating the ground-state properties of di-iron and di-manganese compounds. The calculated magnetic coupling and vibrational properties of ferredoxin molecular models are in good agreements with experimental results and DFT+U calculations. Six different mixed-valence Mn(III)-Mn(IV) compounds have been extensively studied optimizing the geometry in low-spin, high-spin, and broken-symmetry states and with different functionals. The magnetic coupling constants calculated by the extended broken symmetry approach using B3LYP functional presents a remarkable agreement with the experimental results, revealing that the proposed methodology provides a consistent and accurate DFT approach to the electronic structure of multi-centers transition metal complexes.

The magnetic coupling in manganese-based dinuclear superhalogens and their analogues. A theoretical characterization from a combined DFT and BS study

The structures, relative stabilities, vertical detachment energies and magnetic coupling properties of a series of manganese-based dinuclear superhalogens and their isoelectronic analogues are explored via a combined density functional theory and broken symmetry study. Both the capabilities of various exchange-correlation functionals and basis set effects are investigated. The large magnitudes of the calculated exchange coupling constants indicate clearly the apparent molecular magnetism of these new types of superhalogen. Encouragingly, the high possibility of the coexistence of both high stability and strong magnetic coupling in these new polynuclear superhalogens is also confirmed. Besides these, the larger magnitudes of the calculated coupling constants of iron-based clusters here, compared with the homodinuclear [Mn(2)Cl(5)](-) cluster, demonstrate the possibility of the existence of strong magnetic coupling in potential iron-based homo- and heterodinuclear superhalogens. The analysis of spin density distribution is also performed in order to understand the coupling mechanisms.

Effect of Strain on Ferroelectric and Magnetic Behavior in Pb(Zr0.52Ti0.48)O3-Based Magnetoelectric Heterostructures

In this paper, the "sandwich" structured magnetoelectric composite films of Pb(Zr0.52Ti0.48)O3/ NiFe2O4/Pb(Zr0.52Ti0.48)O3 and Pb(Zr0.52Ti0.48)O3/CoFe2O4/Pb(Zr0.52Ti0.48)O3 are epitaxially grown on SrRuO3/SrTiO3 substrates by pulsed-laser deposition. The crystalline quality and microstructures of these heterostructures are investigated by X-ray diffraction technique. The effects of strain on the ferroelectric, magnetic and magnetoelectric coupling properties of these thin films are systematically studied. The results show that the strain effect induced by lattice mismatch between the ferroelectric/ferromagnetic layers plays an important role in the ferroelectric and magnetic properties of these composite films. Compared to the strained Pb(Zr0.52Ti0.48)O3/ CoFe2O4/Pb(Zr0.52Ti0.48)O3 heterostructure, improved ferroelectric properties with an out-of-plane polarization (2P(r)) of 34.2 microC/cm2 and electric coercivity field of 158 kV/cm are obtained in the strain-free Pb(Zr0.52Ti0.48)O3/NiFe2O4/Pb(Zr0.52Ti0.48)O3 heterostructure. The ME measurement results not only show that the strain induced by lattice mismatch has great influence on the ME behavior, but also provide an understanding of the multilayers with full control over the interface structure at the atomic-scale.

The influence of magnetic and electric coupling properties on the magnetocaloric effect in quantum paraelectric EuTiO3

We report on the magnetic and magnetocaloric effect calculations in antiferromagnetic perovskite-type EuTiO3. From the isothermal magnetic entropy change calculated upon low magnetic field changes (below 1T) several results were predicted: inverse magnetocaloric effect, latent heat associated to spin AFM-FM reorientation transition and a temperature interval (controlled by magnetic field) where the EuTiO3 does not change heat in an isothermic process. The magnetocaloric effect described through magnetic entropy change was correlated with magnetocapacitance formula. The theoretical investigation was carried out using a Heisenberg Hamiltonian considering the G-type antiferromagnetic structure with exchange interactions, in mean field approximation, between nearest-neighbor and next-nearest-neighbor magnetic Eu+2 ions.

Electronic structure and magnetic coupling properties of EuFe 2P 2: First-principles calculations

We have investigated the electronic structure and magnetic properties of EuFe 2P 2 using first-principles density functional theory within the generalized gradient approximation (GGA)+U schemes. Our calculated ground state magnetic configurations of EuFe 2P 2 is ferromagnetic which Eu 2 + spins order along c axis. We argue that this kind of magnetic structure of Eu is determined by the indirect RKKY interactions between Eu and direct coupling interaction between Eu 4f with Fe 3d state by our spin-polarized density of states calculations. From the charge density and the Laplace charge density of EuFe 2P 2, we believe that the magnetic moment of Fe is determined by not only Fe–P coupling interactions but also Fe–Fe directly exchange interactions.

Ferromagnetic coupling in Mg‐doped passivated AlN nanowires: A first‐principles study

AbstractBased on density functional theory calculations, we have studied the electronic structures and magnetic properties of passivated AlN nanowires (NWs) with Mg dopants. The calculated results show that double Mg atom doped passivated AlN NWs display ferromagnetic properties, and the total magnetic moment is 1.80 µB per 96‐atom unit cell excluding the pseudohydrogen atoms. However, a couple of Mg atom doped bare AlN nanowire unit cells display anti‐ferromagnetic (AFM) properties. Unequal properties of magnetic coupling in different Mg‐doped AlN structures are due to the different localization and overlapping of impurity wave functions. It is also found that the ferromagnetic stability and Curie temperature of the passivated AlN NWs are much higher than those of bulk structures.

Magnetic coupling properties of Mn-doped AlN nanowires: First-principles calculations

Based on first-principles within the framework of the density functional theory, we have studied the magnetic coupling properties of Mn-doped AlN nanowires. By analyzing the results of different Mn-doped AlN nanowires, we found that for the passivated nanowire, ferromagnetic state is more stable, while for the unpassivated nanowire, the favorable state transits into anti-ferromagnetic state, which can be well explained by the band coupling model. The results indicate that the degree of surface passivation of dangling bonds is an important factor in the magnetic properties of doped nanowires.

Magnetic, ferroelectric and magnetoelectric coupling properties of Fe-implanted BaTiO 3 films

BaTiO 3 (BTO) nano-films were implanted with iron by means of ion-implantation. According to the results of X-ray diffraction (XRD) and Raman analysis, all the films are single-phase with tetragonal structure. Upon the increase of Fe dosage, on one hand, there is a decrease in c/ a ratio of the unit cells. As the grain size and surface roughness of the Fe-implanted films decrease, the well crystallized film was obtained. Consequently, both decreasing of coercivity and boosting of magnetization were observed simultaneously. With the decline of the grain size and surface roughness during thermal treatment of the Fe-implanted films, there is an improvement of Fe distribution across the BTO matrix. Consequently, there is a decrease of coercivity and rise of magnetization. Upon the decrease of Fe dosage, on the other, there is a decline in magnetization, and a ferroelectric hysteresis loop is observed at a Fe dosage of 3.0 × 10 14 cm − 2 . Additionally, the dependence of capacitance on the applied magnetic field and its frequency were also monitored.

Electronic structure and magnetic coupling properties of Gd-doped AlN: first-principles calculations

In this work, the electronic structure and magnetic coupling properties of Gd doped AlN have been investigated using first-principles method. We found that in the AlN:Gd system, due to the s-f coupling allowed by the symmetry, the exchange splitting of the conduction band is much larger than that of the valence band, which makes the electron-mediated ferromagnetism possible in this material. This property is also confirmed by the energy differences between anti-ferromagnetic and ferromagnetic phase for Al14Gd2N16 with different concentrations of electrons (holes), as well as by the calculated exchange constants. The result indicates that Gd-doped AlN is a promising candidate for the applications in future spintronic devices.

Enhanced electron-mediated ferromagnetism in Co-doped ZnO nanowires

We perform density functional calculations to investigate the magnetic coupling properties of Co-doped ZnO nanowires (NWs). The ferromagnetism of NWs is strongly affected by the position of the minority Co ta levels and their population that is controlled by additional electron doping. While the antiferromagnetic state is energetically more favorable than the ferromagnetic state in carrier-free NWs, electron doping greatly enhances the stability of ferromagnetism. Compared with bulk ZnO, the minority ta levels relative to the conduction band edge have a tendency to decrease with decreasing of the wire diameter, indicating that electron concentrations to achieve the ferromagnetism are much reduced. The short-range nature of the magnetic coupling between two Co ions suggests that sufficiently high doping levels of the Co ions are needed to yield ferromagnetic NWs.

Magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO: First-principles calculations

The electronic structure and magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO have been investigated using first-principles methods. We show that the magnetic coupling between Gd or Nd ions in the nearest neighbor sites is ferromagnetic. The stability of the ferromagnetic coupling between Gd ions can be enhanced by appropriate electron doping into ZnO:Gd system and the room-temperature ferromagnetism can be achieved. However, for ZnO:Nd system, the ferromagnetism between Nd ions can be enhanced by appropriate holes doping into the sample. The room-temperature ferromagnetism can also be achieved in the n-conducting ZnO:Nd sample. Our calculated results are in good agreement with the conclusions of the recent experiments. The effect of native defects (VZn,VO) on the ferromagnetism is also discussed.

Spin crossover in iron(II) complexes: Recent advances

In this review article, several representative multifunctional SCO materials exhibiting interplay/synergy between the spin transition and magnetic coupling or liquid crystalline properties together with the present pioneering works on nano-structuration of SCO materials are illustrated. As the Mössbauer spectroscopy has been decisive in the study of the physical properties of these multifunctional materials, special attention is given to their corresponding Mössbauer investigations.

Magnetic coupling properties of Mn-doped ZnO nanowires: First-principles calculations

Based on the density functional theory, we study the magnetic coupling properties of Mn-doped ZnO nanowires. For the nanowires with passivated surfaces, the antiferromagnetic state is found and the Mn atoms have a clustering tendency. When the distance between two Mn atoms is large, the system energetically favors the paramagnetic or spin-glass state. For the nanowires with unpassivated surfaces, the ferromagnetic (FM) coupling states appear between the two nearest Mn atoms, and the zinc vacancies can further stabilize the FM states between them. The electrons with enough concentration possibly mediate the FM coupling due to the negative exchange splitting of conduction band minimum induced by the s-d coupling, which could be useful in nanomaterial design for spintronics.

Spin coupling properties for the diradicals of 2,2′-bipyridine and its derivatives

Geometrical structures of the diradicals of 2,2'-bipyridine and its derivatives in high and low spin states are optimized by UB3LYP method at 6-311G (d) basis set. The results show that the change rule of the effective exchange integral is agreement with that of the split energy difference of part occupied molecular orbitals with the different substituted groups and different conformations for all systems. The magnetic coupling properties of all systems are in line with spin polarization rule. The diradicals of trans-4,4'-dicarboxy-2,2'-bipyridine is a better ferromagnetic molecule.

Ferromagnetic Coupling Behavior in Oxo-Bridged Binuclear Bis(η5-cyclopentadienyl)titanium(III) Complex (Cp2Ti)2(μ-O): A Density Functional Theory Combined with Broken-Symmetry Approach

The ferromagnetic coupling behavior in oxo-bridged bis(bis(cyclopentadienyl)titanium(III)) complex (Cp 2Ti)2(I-O) is investigated on the basis of calculations of density functional theory combined with the brokensymmetry approach. The magnetic coupling constants calculated for the experimental and optimized geometries are 11.41 and 1.29 cm -1 , respectively, comparable with the experimentally measured J value (8.3 cm -1 ). The calculated results show that the magnetic coupling constant J slightly decreases with the increase of the Ti-(I-O)-Ti angle and decreases exponentially with the increase of the Ti-(I-O) distance. In variation of the dihedral angle ‚ between the two Cp2Ti fragments the transition of the magnetic coupling property occurs near ‚ ) 45°. For ‚ > 45°, the coupling is ferromagnetic, and it is antiferromagnetic for ‚ < 45°. The ferromagnetic coupling interaction between the magnetic centers is almost not affected by the protonation of the oxo-bridge ligand. Molecular orbital analysis reveals that, because of the nonbonding character and electronic location character of the single-occupied molecular orbitals (SOMO), there is no antiferromagnetic coupling pathway through superexchange via the bridging atom. However, when ‚ varies from 90° to 0°, the throughspace interaction between the two magnetic centers in the SOMOs occurs, leading to a gradually increscent antiferromagnetic contribution, JAF. As soon as the antiferromagnetic contribution exceeds the ferromagnetic contribution, JF, the transition of the magnetic coupling properties for the molecule occurs. The spin population analysis is briefly discussed.

Magnetic coupling and magnetoresistance in Fe/Si1−xAgx multilayers

In this letter, we present a study on the magnetic coupling and magnetoresistance (MR) properties in Fe/Si1−xAgx multilayers (MLs) with a granular Si1−xAgx spacer layer. We found that with increasing silver content (x) in a silicon matrix, the magnetic state of MLs varies from a nonmagnetic-coupling state to a weak-antiferromagnetic state around the percolation point of the ∼24-Å-thick granular spacer Si1−xAgx. The MR measurements also reveal an abrupt increase of MR near the same percolation point. These variations are ascribed to the formation of the percolation path in the granular spacer.

Percolation-related magnetic coupling and magnetoresistance properties in Fe/Si 1- x Ag x multilayers

In this paper, we present a study of the magnetic coupling and magnetoresistance (MR) properties in Fe/Si1-xAgx multilayers with a granular Si1-xAgx spacer layer. We have found that, with increasing silver content (x) in a silicon matrix, the magnetic state of multilayers changes from a nonmagnetic coupling state to weak antiferromagnetic around the percolation point of the ~2.4 nm thick spacer Si1-xAgx. The MR measurements also reveal an abrupt increase of MR near the same percolation point. These changes are ascribed to the formation of the percolation path in the granular spacer.

Aurivillius Ceramics: Focus on Symmetry

Structure--physical properties relationships for Aurivillius ceramics are discussed, with emphasis in symmetry considerations. Single-crystal materials and polycrystal ceramics are analysed. Electric and magnetic coupling properties are considered. Colour Symmetry Groups and Texture Analysis tools are employed. Symmetry conditions for polarisation vectors and inverse pole figures related to Aurivillius phases are given.