Strong Ferromagnetic Coupling Research Articles

Magnetic properties in Pd doped ZnS from ab initio calculations

First-principles calculations based on density functional theory within the general gradient approximation (GGA) are performed to study the electronic structure and magnetic properties of Pd doped ZnS. It is found that an isolated Pd atom doped 2 × 2 × 2 ZnS supercell shows half-metallic ferromagnetic character with a total magnetic moment of 2.0μB per supercell, which is significantly enhanced compared with the pure ZnS supercell. The strong ferromagnetic coupling of the local magnetic moments can be explained in terms of strong hybridisation between Pd-4d and S-3p states. The hybridisation between Pd and the neighbouring S atoms leads to a strong coupling chain Pd(4d)-S(3p)-Zn(3d)-S(3p)-Pd(4d), which induces strong indirect long range FM coupling between Pd dopants. The results of several doping configurations demonstrate that ferromagnetic coupling exists between the two doped palladium atoms. These results suggest that Pd doped ZnS can also be considered as suitable candidates for exploring new half-metallic ferromagnetism in semiconductors.

Mixed ligand binuclear alkoxo-bridged copper(II) complexes derived from aminoalcohols and nitrogen ligands

Six binuclear alkoxo-bridged complexes have been obtained and cystallographically characterized. The binuclear species are spontaneously assembled by reacting copper perchlorate with an aminoalcohol (monoethanolamine, Hmea, or propanolamine, Hpa) in the presence of various co-ligands: 4-phenyl-pyridine (4-phpy) 4-amino-pyridine (4-apy) dipyridylamine (dpyam), 2,3-bis(2-pyridyl)pyrazine (dpp). The six new compounds have the following formulas: [Cu(mea)(4-phpy)(ClO4)]21, [Cu(pa)(4-phpy)2]2(ClO4)22, [Cu(pa)(4-apy)(ClO4)]23, [Cu(mea)(dpyam)]2(ClO4)24, [Cu(pa)(dpyam)]2(ClO4)25, and [Cu(mea)(dpp)]2(ClO4)2·CH3OH 6. Except compound 1, the binuclear entities in crystals 2 – 6 are centrosymmetric, with pentacoordinated copper ions. In compound 1, one copper is pentacoordinated, while the other one is hexacoordinated. The perchlorate ions play different functions (monodentate and bridging in 1, monodentate in 3, uncoordinated in 2, 4, 5 and 6. The magnetic properties of compound 1 have been investigated and reveal a quite strong ferromagnetic coupling between the copper ions (J= +81.9cm−1, H=−JS1S2).

Manipulation of half-metallicity and ferromagnetism in N-doped CdS nanowire

The spin-polarization and magnetic coupling of N-doped CdS nanowires (NWs) with a diameter size of 12.33 Ǻ have been investigated through first-principles calculations. The doped N prefers surface S sites and leads to a half-metallic property. The magnetic interaction between the nearest and the next-nearest N dopants results in a strong ferromagnetic (FM) coupling, while, when the distance between N dopants is larger than 8.0 Ǻ, the ground state of the system tends to be paramagnetic. Also, a nitrogen concentration threshold to produce the FM order is estimated to be 8.3 %. The results of incorporation of N dopants to CdS NWs pave a promising way to realize potential applications in spintronics.

Multiferroicity in vanadium-doped La2Ti2O7: insights from first principles

We explore an unconventional route in the search for multiferroicity investigating the magnetic doping of La2Ti2O7 (LTO), a ferroelectric layered perovskite, by density functional calculations. We find that substitution of Ti (in the 3d 0 configuration) by V (3d 1) produces: (i) robust ferromagnetic (FM) order, achieving multiferroicity; (ii) structural and magnetic anisotropy, as the dopants cluster in magnetic chains. Moreover, the FM V-chains possess antiferro orbital ordering. We show that the origin of the strong FM coupling lies in the fruitful host-dopant combination: the layered structure of the host prewires the orbitals t 2g with d xy and d xz lower in energy, the V dopants provide the full occupancy of the bonding levels. We further address the stability of chains vs. temperature and the role of oxygen vacancies in undoped as well as Sc-doped and V-doped LTO.

Electronic and magnetic properties of Fe clusters inside finite zigzag single-wall carbon nanotubes

Density functional calculations of the electronic structure of the Fe${}_{12}$ cluster encapsulated inside finite single-wall zigzag carbon nanotubes of indices $(11,0)$ and $(10,0)$ have been performed. Several Fe${}_{12}$ isomers have been considered, including elongated shape isomers aimed to fit well inside the nanotubes, and the icosahedral minimum energy structure. We analyze the structural and magnetic properties of the combined systems, and how those properties change compared to the isolated systems. A strong ferromagnetic coupling between the Fe atoms occurs both for the free and the encapsulated Fe${}_{12}$ clusters, but there is a small reduction (3\char21{}$7.4\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$) of the spin magnetic moment of the encapsulated clusters with respect to that of the free ones ($\ensuremath{\mu}=38\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$). The reduction of the magnetic moment is mostly due to the internal redistribution of the spin charges in the iron cluster. In contrast, the spin magnetic moment of the carbon nanotubes, which is zero for the empty tubes, becomes nonzero (1\char21{}$3\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$) because of the interaction with the encapsulated cluster. We have also studied the encapsulation of atomic Fe and the growth of small Fe${}_{n}$ clusters ($n=2$, 4, 8) encapsulated in a short $(10,0)$ tube. The results suggest that the growth of nanowires formed by distorted tetrahedral Fe${}_{4}$ units will be favorable in $(10,0)$ nanotubes and nanotubes of similar diameter.

Strong magnetic enhancement in self-assembled multiferroic-ferrimagnetic nanostructures

In the past decade, self-assembled vertical nano-heterostructures have drawn considerable attention because a high interface-to-volume ratio can be used to tailor or create functionalities. We have systematically investigated the magnetic properties of oxide heterostructures consisting of the CoFe2O4 nanopillars embedded in the BiFeO3 matrix using macroscopic magnetization measurements and element-selective soft X-ray absorption magnetic circular dichroism (XMCD) at the Co- and Fe-L2,3 edge. The magnetization and XMCD data show that the total ordered magnetic moment of Co(2+) in CoFe2O4-BiFeO3 nano-heterostructures is strongly enhanced. This study clearly indicates that the high interface-to-volume ratio vertical nanostructure creates a strong ferromagnetic and antiferromagnetic magnetic coupling via an interface. Furthermore, the magnetic coupling can be tuned in the multiferroic-ferrimagnetic self-assembled heterostructures by controlling the spacing between nanopillars.

Ab initio study of magnetic properties of Fe-Mn-Al Heusler alloys

ABSTRACTDensity functional theory (DFT) based on the spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) method is used to investigate the magnetic properties of nonstoichiometric Fe2+xMn1-xAl Heusler alloys, where 0 ≤ x ≤ 0.9. The composition dependences of the magnetic exchange couplings and the Curie temperature for the cubic L21 phase are obtained. Our simulations have shown that the Fe-Fe nearest neighbors present a strong ferromagnetic coupling. Moreover, these exchange interactions are larger than other interactions. The substitution of Mn by Fe in Fe2+xMn1-xAl (0 ≤ x ≤ 0.9) leads to an increase in the Curie temperature. This tendency and the values of Curie temperatures are in agreement with the experimental results for Fe2+xMn1-xAl (x = 0, and 0.1). The highest Curie temperature was observed for the Fe-richer alloy.

Renormalization Group Approach for the Double Exchange Ferromagnets

The discovery of the colossal magnetoresistance (CMR) in the manganese oxides with perovskite structures T1−xDMnO3 (T = La, Pr, Nd; D=Sr, Ca, Ba, Pb) and its potential technological application motivated theoretical and experimental researchers to study the itinerant ferromagnetism. A rst theoretical description of this phenomenon in terms of the double-exchange mechanism was given a long time ago by Zener. In this model, the spin orientation of adjacent Mn-moments is associated with kinetic exchange of conduction eg electrons. Consequently, alignment of the core Mn-spins by an external magnetic eld causes higher conductivity. The Mn ions are considered as localized forming a spin of S = 3 2 and they are coupled to the itinerant electrons by a strong ferromagnetic Hund coupling, JH > 0. We apply the ow equation technique (nonperturbative method, based on continuous canonical transformation) to the double-exchange model for ferromagnetism described by the Kondo type Hamiltonian. We want to eliminate the interaction term responsible for non-conservation of magnon number and to take into account fermion and magnon degrees of freedom. We express the spin operators of Mn ions via the magnon operators (the Holstein Primako transformation) and investigate the magnon excitation spectrum determined by Green's function.

First-principles study of electronic structure and magnetic properties of Cu-doped CeO2

We performed first-principles calculations within density-functional theory to study the origin of the magnetism in Cu-doped CeO2. We show that the electron spin-polarization and the magnetic coupling are sensitive to the defect structures in Cu-doped CeO2. The substitution of a Cu atom for a Ce atom (CuCe) induces a local magnetic moment of 3.00 μB around per impurity, but the magnetic coupling between local magnetic moments is very weak. The defect complex consisting of a CuCe and a nearest-neighbor oxygen vacancy (VO) has low formation energy and thus high plausibility in Cu-doped CeO2. Although the local magnetic moment triggered by the CuCe-VO complex is only 1.00 μB per complex, strong ferromagnetic coupling between the defect complexes is achieved which can be attributed to a magnetic coupling chain formed by the strong p-d interaction between Cu and host O atoms.

Competing exchange interactions in Co-doped ZnO: Departure from the superexchange picture

We report the results of a comprehensive study of the exchange interactions in Co-doped ZnO using inelastic neutron scattering, electron paramagnetic resonance, and magnetic property measurements. In particular, we observe an unprecedentedly strong spatial anisotropy of the two nearest-neighbor exchanges, ${J}^{(1)}=\ensuremath{-}25.6\ifmmode\pm\else\textpm\fi{}0.3$ K and ${J}^{(2)}=\ensuremath{-}8.5\ifmmode\pm\else\textpm\fi{}0.4$ K, along with the distant-neighbor $J$ values of ferromagnetic sign. We argue that the superexchange mechanism alone cannot account for the obtained data and we suggest that an additional mechanism leading to a strong ferromagnetic spin coupling is responsible for these findings. We also discuss the origin of this ferromagnetic mechanism.

Synthesis, structure, and magnetic properties of a cyanidebridged Fe(III)-Cu(II) bimetallic double-zigzag chain with slow relaxation of the magnetization

Using Bu4N[Fe(Tp*)(CN)3]− (Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate) as the building block to react with CuII and N-methylimidazole, we obtained a one-dimensional (1D) heterobimetallic cyano-bridged chain, [Fe(Tp*)(CN)3]2Cu(N-methylimidazole)2·2H2O (1). The crystal structures and magnetic studies demonstrate that complex 1 exhibits slow relaxation of the magnetization due to strong intrachain ferromagnetic coupling and weak interchain interactions.

Magnetic impurities in graphane with dehydrogenated channels

We have investigated the electronic and magnetic response of a single Fe atom and a pair of interacting Fe atoms placed in patterned dehydrogenated channels in graphane within the framework of density functional theory. We have considered two channels: ``armchair'' and ``zigzag'' channels. Fully relaxed calculations have been carried out for three different channel widths. Our calculations reveal that the response to the magnetic impurities is very different for these two channels. We have also shown that one can stabilize magnetic impurities (Fe in the present case) along the channels of bare carbon atoms, giving rise to a magnetic insulator or a spin gapless semiconductor. Our calculations with spin-orbit coupling shows a large in-plane magnetic anisotropy energy for the case of the armchair channel. The magnetic exchange coupling between two Fe atoms placed in the semiconducting channel with an armchair edge is very weakly ferromagnetic whereas a fairly strong ferromagnetic coupling is observed for reasonable separations between Fe atoms in the zigzag-edged metallic channel with the coupling mediated by the bare carbon atoms. The possibility of realizing an ultrathin device with interesting magnetic properties is discussed.

Half-metallicity in Rh-doped TiO2 from ab initio calculations

First-principles calculations based on density functional theory (DFT) are performed to study the electronic structures and magnetic properties of Rh-doped TiO2 crystals. The hybridization between Rh-4d and O-2p results in Rh becoming ferromagnetic with a magnetic moment of about 1.0 μ B per supercell. The Rh-doped TiO2 system exhibits half-metallic ferromagnetism based both DFT and DFT + U. The strong ferromagnetic couplings between local magnetic moments can be attributed to both the p-d hybridization and double-exchange mechanisms, as well as superexchange interaction. These results suggest an alternative approach to achieve promising dilute magnetic semiconductors by doping non-magnetic transition metals in a TiO2 host.

Ferromagnetism induced by oxygen-vacancy complex in (Mn, in) codoped ZnO

Mn doped Zinc oxide (ZnO) thin films were prepared by metal organic chemical vapor deposition (MOCVD) technique. Structural characterizations by X-ray diffraction technique (XRD) and photoluminescence (PL) indicate the crystal quality of ZnO films. PL and Raman show a large fraction of oxygen vacancies (VO2+) are generated by vacuum annealed the film. The enhancement of ferromagnetism in post-annealed (Mn, In) codoped ZnO could result from VO2+ incorporation. The effect of VO2+ on the magnetic properties of (Mn, In) codoped ZnO has been studied by first-principles calculations. It is found that only In donor cannot induce ferromagnetism (FM) in Mn-doped ZnO. Besides, the presence of VO2+ makes the Mn empty 3d-t2g minority state broadened, and a t2g-VO2+ hybrid level at the conduction band minimum forms. The presence of VO2+ can lead to strong ferromagnetic coupling with the nearest neighboring Mn cation by BMP model based on defects reveal that the ferromagnetic exchange is mediated by the donor impurity state, which mainly consists of Mn 3d electrons trapped in oxygen vacancies.

First-Principles Study of Electronic Structure and Hydrogen Adsorption of 3d Transition Metal Exposed Paddle Wheel Frameworks

Open-site paddle wheels, comprised of two transition metals bridged with four carboxylate ions, have been widely used for constructing metal–organic frameworks with large surface area and high binding energy sites. Using first-principles density functional theory calculations, we have investigated atomic and electronic structures of various 3d transition metal paddle wheels before and after metal exposure and their hydrogen adsorption properties at open metal sites. Notably, the hydrogen adsorption is impeded by covalent metal–metal bonds in early transition metal paddle wheels from Sc to Cr and by the strong ferromagnetic coupling of diatomic Mn and Fe in the paddle wheel configurations. A significantly enhanced H2 adsorption is predicted in the nonmagnetic Co2 and Zn2 paddle wheel with the binding energy of ∼0.2 eV per H2. We also propose the use of two-dimensional Co2 and Zn2 paddle wheel frameworks that could have strongly adsorbed dihydrogen up to 1.35 wt % for noncryogenic hydrogen storage applications.

First-principles characterization of ferromagnetism in N-doped SrTiO3 and BaTiO3

The spin-polarization and magnetic coupling character of N-doped SrTiO3 (STO) and BaTiO3 (BTO) are studied through first-principles calculations. The substitutional N doping at O sites leads to a half-metallic property and produces a magnetic moment of 1.0 μB. The magnetic interaction between the nearest and next-nearest N dopants results in a strong ferromagnetic (FM) coupling. When the distance between the N dopants is larger than 7 Å, the ground state of the system tends to be paramagnetic. A nitrogen-concentration threshold to produce the ferromagnetism is estimated. The calculated results give a good explanation for the experimentally observed ferromagnetism in N-doped STO and BTO.

LiFePO4–Fe2P–C composite cathode: An environmentally friendly promising electrode material for lithium-ion battery

In this investigation, the synthesis strategy is involved the creation of LiFePO4–Fe2P–C composites with a porous conductive architecture, which includes distinct regions or clusters containing antiferromagnetic LiFePO4 in close proximity to ferromagnetic Fe2P. The microstructure is achieved by using a simple ultra-fast solvent assisted manual grinding method, combined with solid state reaction, which can replace the time-consuming high energy ball milling method. The crystalline structure, morphology, and electrochemical characterization of the synthesised product are investigated systematically. The electrochemical performance is outstanding, especially the high C rate. The composite cathode is found to display specific capacity of 167mAhg−1 at 0.2C and 146mAhg−1 at 5C after 100 cycles, respectively. At the high current density of 1700mAg−1 (10C rate), it exhibits long-term cycling stability, retaining around 96% (131mAhg−1) of its original discharge capacity beyond 1000 cycles, which can meet the requirements of a lithium-ion battery for large-scale power applications. The obtained results have demonstrated that the fabrication of samples with strong and extensive antiferromagnetic and ferromagnetic interface coupling of LiFePO4/Fe2P provides a versatile strategy toward improving the electrochemical properties of LiFePO4 materials and also opens up a new window for material scientists to further study the new exchange bias phenomenon and its ability to enhance the electrochemical performance of lithium-ion battery electrode.

Room temperature ferromagnetism in Teflon due to carbon dangling bonds

The ferromagnetism in many carbon nanostructures is attributed to carbon dangling bonds or vacancies. This provides opportunities to develop new functional materials, such as molecular and polymeric ferromagnets and organic spintronic materials, without magnetic elements (for example, 3d and 4f metals). Here we report the observation of room temperature ferromagnetism in Teflon tape (polytetrafluoroethylene) subjected to simple mechanical stretching, cutting or heating. First-principles calculations indicate that the room temperature ferromagnetism originates from carbon dangling bonds and strong ferromagnetic coupling between them. Room temperature ferromagnetism has also been successfully realized in another polymer, polyethylene, through cutting and stretching. Our findings suggest that ferromagnetism due to networks of carbon dangling bonds can arise in polymers and carbon-based molecular materials.

Touching the upper limit for ferromagnetic interactions in hetero-bridged dinuclear [Cu2II] complexes using a novel N5-dinucleating ligand bearing an endogenous monoatomic amido(R–NH−)-bridging group

A novel N(5)-dinucleating ligand 4-amino-3,5-bis(bipyridine-2-yl)-1,2,4-triazole allows the preparation for the first time, and under mild conditions, of single and mixed amido(R-NH(-))-bridged copper(II) complexes, the latter exhibiting very strong ferromagnetic coupling.

Influence of Interlayer Thickness on High-Frequency Magnetic Properties of FeCoSiN/AlO/FeCoSiN Trilayers

In this paper, we investigate the static magnetic properties and high-frequency permeability spectra of as-sputtered FeCoSiN/AlO/FeCoSiN trilayer films and discuss their dependences on the thickness of AlO interlayer, based on the phenomenological Landau-Lifshitz-Gilbert equation. The hysteresis loops show that the interlayer effectively decreases the coercivities of trilayers in the thickness range between 3 and 10 nm. The effective anisotropy field obtained from the fitting of permeability spectra increases with the interlayer thickness to saturation. The dependence of damping factor on the interlayer thickness has a maximum. It is attributed to the Néel couplings at the thicker interlayer range and the strong ferromagnetic couplings at the thinner interlayer range.