Strong Ferromagnetic Coupling Research Articles

Tuning of Valence States, Bonding Types, Hierarchical Structures, and Physical Properties in Copper/Halide/Isonicotinate System

Seven cupric halide coordination polymers, namely [Cu5(OH)3Br3(ina)4] (1), [Cu5(OH)3Cl3(ina)4] (2), [Cu2(OH)Cl(ina)2] (3), [Cu3(OH)2Cl2(ina)2]·2H2O (4), [Cu3(OH)2Br2(ina)2]·2H2O (5), [Cu2Cl2(ina)2(H2O)2] (6), [Cu2Cl(ina)2(gca)(H2O)] (7), cupric complex templated cuprous halide [Cu(II)(Me-ina)2(H2O)][Cu(I)5Br7] (8), and organic templated cuprous halide Me2-ina[Cu2Br3] (9) (Hina = isonicotinic acid), were prepared from the starting materials of cupric halide and Hina via fine-tuning solvothermal reactions. According to valence states of copper, 1-7 are copper(II) complexes, 8 is a mixed-valent Cu(I,II) complex, while 9 is a Cu(I) compound. According to bonding types of halides, nine complexes can be classified as three types: complexes 1-3 include only normal X-Cu bond (X = halide); complexes 4-7 include normal X-Cu bond and X···Cu weak bond; complexes 8 and 9 include normal X-Cu bond and X···H-C halogen hydrogen bonds. Complexes 1 and 2 are isomorphic three-dimensional (3D) pcu topological metal organic frameworks (MOFs) with butterfly-like Cu4(μ3-OH)2X2 and steplike Cu6(μ3-OH)4 cores as nodes, showing strong ferromagnetic couplings. Complex 3 also is a pcu topological MOF with only butterfly-like Cu4(μ3-OH)2Cl2 clusters as nodes, presenting spin canting antiferromagnetic behavior. Isostructural 4 and 5 are Cu3(OH)2 clusters based two-dimensional (2D) (4,4) layers, which are extended into 3D eight-connected networks via weak Cu···X bonds, showing ferromagnetic coupling. Antiferromagnetic 6 is a simple one-dimensional coordination polymer, which is extended via weak Cu···Cl bonds into 3D (3,4)-connected networks. Paramagnetic 7 is a ladderlike polymer, which is extended into 2D (3,4)-connected layer via weak Cu···Cl bonds. The syntheses of polymeric cupric complexes 1-7 mainly result from differences in reactant ratio and pH value. Utilization of reducing methanol generated novel cubane-containing [Cu5Br7](2-) chain templated by paddlewheel-like [Cu(II)(Me-ina)2](2+) 8 and face-shared dimer-containing [Cu2Br3](-) chain templated by N-methylated and O-esterificated Me2-ina 9. Complex 9 exhibits a strong red emission and a weaker green emission upon excitation.

First Principles Calculations of Magnetic Exchange Parameters of Fe-Mn-Al Heusler Alloys

The composition dependences of magnetic exchange couplings for the cubic phase of Fe2+xMn1-xAl Heusler alloys, where 0.0 < x < 1.0 are obtained with the help of first principles calculations. Our simulations have shown that the Fe-Fe nearest neighbors present a strong ferromagnetic coupling. Moreover, these exchange interactions are larger than the other ones.

Inclusion of surface anisotropy in the micromagnetic analysis of exchange-coupled hard/soft bilayers

The effects of surface anisotropy and interface anisotropy in soft and hard layers have been considered in the frame of a previously developed micromagnetic continuum model for the study of demagnetization processes in exchange-coupled bi- and multi-layers of soft/hard-type with planar and perpendicular anisotropy, in order to extend and generalize it. In particular, fields applied along the easy direction and strong interface ferromagnetic coupling have been considered. The theoretical results have been applied to a bilayer with perpendicular anisotropy in the hard layer and positive perpendicular surface anisotropy in the soft layer, in order to compare them with those obtained by considering the equivalent volume anisotropy in the soft layer. Moreover, the separate effects of positive perpendicular surface and interface anisotropies in the bilayer have been compared. We have deduced the nucleation field equation and obtained plots of the nucleation field as a function of the soft layer thickness and of the surface and interface anisotropies. Demagnetization curves have been also calculated. The nucleation process can be reversible (exchange-spring (ES) regime) or irreversible (rigid magnet (RM) regime) depending on magnetic and geometrical parameters. The critical equation, which allows determining the boundary between RM and ES regions, has been deduced. Corresponding phase diagrams in the plane of soft and hard layer thickness have been obtained as a function of surface and interface anisotropies.

Towards Design of High Spin Metal-free Materials

In this paper, in order to explore high-spin carbon-based magnetic materials with strong ferromagnetic coupling, geometric structure, electronic structure and magnetic properties of alternating stacks of \(\pi \)-radical-halogenated-hydrocarbons and diamagnetic molecules have been investigated based on density-functional theory with dispersion correction. These alternating stacks are predicted to avoid the typical antiferromagnetic spin-exchange of indentical face-to-face radicals via spin polarization of a diamagnetic molecule in between. Our results show that \(\pi\)-radical-halogenated-hydrocarbons like perchlorophenalenyl (C\(_{13}\)Cl\(_{9})\) is strong ferromagnetic coupling if alternatingly stacked with aromatics like fluorinated coronene (C\(_{24}\)F\(_{12})\) or coronene (C\(_{24}\)H\(_{12})\), while fluorinated perinaphthenyl (C\(_{13}\)F\(_{9})\) and perinaphthenyl (C\(_{13}\)H\(_{9})\) are not an equally good choice. The role of ligand configuration in determining exchange coupling in stacks is discussed. These results would give some hints for designing new high-spin carbon-based ferromagnetic materials.

Electronic structures and ferromagnetism of SnO2 (rutile) doped with double-impurities: First-principles calculations

The electronic and magnetic properties of double-impurities-doped SnO2 (rutile) are explored using first-principles calculations within the generalized gradient approximation to examine their potential use as spintronic system. Calculations are performed for double impurities (M1 and M2) from M1 = Cr, and M2 = Mn, and Re. The origins of ferromagnetism are shown to be different in the two cases. For Sn1-2xCrxMnxO2, the hybridization between Cr-3d and O-2p results in Cr becoming ferromagnetic with a magnetic moment of about 5.0 μB per supercell. The Cr-and Mn-doped SnO2 system exhibits half-metallic ferromagnetism. The strong ferromagnetic couplings between local magnetic moments can be attributed to p-d hybridization. In contrast, in (Cr, Re) codoped TiO2, the local magnetic moments of the impurities and their oxidation states agree with the charge transfer between Cr and Re, which would lead to the ferromagnetic through the double-exchange mechanism in transition metal oxides. Since there are two possible couplings between the impurities, we studied both configurations (ferromagnetic and antiferromagnetic (AF)) for double-impurities-doped SnO2. Our calculations show that a ferromagnetic alignment of the spins is energetically always more stable than simple AF arrangements, which makes these materials possible candidates for spin injection in spintronic devices.

Prediction of half-metallic ferromagnetism in C-doped CdS nanowire

The geometric, energetic, and magnetic properties of nonmagnetic carbon (C)-doped cadmium sulfur (CdS) nanowires (NWs) are investigated based on first-principles calculations. We find that the two C dopants are most stable when they are close to each other in the surface sites, exhibiting half-metallic (HM) behavior with a net magnetic moment of 2.0 μB C−1. The magnetic interaction between the nearest and next-nearest C dopants results in a strong ferromagnetic (FM) coupling. However, the ground state of the system tends to be paramagnetic when the distance between the C dopants is larger than 6.2 A. The HMFM order in C-doped CdS NWs, which is sensitive to the confinement of electrons in the radial direction and the curvature of the NW's surface, can be attributed to hole-mediated double exchange through the strong p–p interaction between carbon dopants. These predicted results endow CdS NWs potential applications in spintronics.

A family of 3D lanthanide–organic frameworks constructed from parallelogram secondary building units: synthesis, structures and properties

A family of lanthanide–organic frameworks, {Ln2(pyip)(ox)2.5(H2O)5}n (Ln = La (1), Pr (2), Sm (3), Eu (4), Gd (5), Tb (6), Dy (7)) has been synthesized based on mixed bridging ligands, H2pyip and oxalate anion (H2pyip = 5-(4-pyridyl)-isophthalic acid, H2ox = oxalic acid) and characterized by elemental analysis, Fourier transform infrared spectroscopy, steady state and time-resolved luminescence spectroscopy, thermogravimetric analysis, powder X-ray diffraction and single-crystal X-ray diffraction. The single-crystal X-ray diffraction analysis results show that complexes 1–7 are isostructural. They crystallize in the same P21/c space group and exhibit a (3,4)-connected {42·6·82·10}{82·10} topology, being a 2-nodal net topology. The luminescence properties of 3, 4, 6, and 7 were investigated and related to the structural properties. The magnetic studies reveal that strong ferromagnetic coupling exists in compound 6.

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.