Direct Exchange Interaction Research Articles

ChemInform Abstract: Square Planar Coordinate Iron Oxides

AbstractReview: 69 refs.

Effect of the direct exchange interaction between magnetic impurities on magnetization in diluted magnetic semiconductors

We consider a model of III-V diluted magnetic semiconductors where both of the exchange interaction between carrier and impurity spins, and the direct exchange interaction between magnetic impurities are taken into account. The magnetization as a function of temperature for a wide range of model parameters is calculated and discussed. We show that for a degenerate carrier system the suppression of the magnetization is sensitive to the antiferromagnetic coupling constant and the impurity concentration.

Synthesis and characterization of ferromagnetic Zn1−xCoxO films

High-quality Zn1−xCoxO (ZnCoO) thin films with different Co concentrations (x=0−0.07) were synthesized on Al2O3 substrates using a radio-frequency magnetron sputtering process. The structural, chemical, optical, and electrical properties of the films were characterized using various techniques. High-resolution X-ray diffraction measurements showed that the ZnCoO films had a c-oriented hexagonal wurtzite structure with no extra phases. The lattice constants of the films increased slightly with increasing the Co concentration ratio. X-ray photoelectron spectra show the incorporation of Co2+ ions in the ZnO host without the formations of Co clusters. Hall measurements revealed that the films were n-type with a charge carrier density of ∼2.6×1018 per cm3. A neutral-donor bound exciton peak (D0X) from all specimens was observed at 3.36eV using photoluminescence measurements at 10K. The physical and chemical properties of the films suggested that their ferromagnetic properties originated from a direct exchange interaction between magnetic Co ions.

Instability of the ferromagnetic ground state in Lu2Fe17−XMnX [x = 0.5, 0.7

The magnetic properties of polycrystalline Lu2Fe16.5Mn0.5 and Lu2Fe16.3Mn0.7 compounds have been studied under hydrostatic pressure up to 10 kbar in the temperature range of 5–300 K and at magnetic fields up to 5 T. The Lu2Fe16.3Mn0.7 compound is in the ferromagnetic (FM) state, and the Lu2Fe16.5Mn0.5 is in the FM state at low temperatures and in the antiferromagnetic state at high temperatures at ambient pressure. We have revealed that the FM state is very sensitive to volume changes and even moderate pressure is sufficient to complete its suppression. The critical pressures are 4 kbar for Lu2Fe16.5Mn0.5 and 7 kbar for Lu2Fe16.3Mn0.7. The suppression of the FM ground state is observed at practically the same volume of the elementary cell. The pressure induced Néel temperature, TN, of both compounds decreases with increasing pressure, dTN/dp = −2.5 K/kbar and dTN/dp = −3.4 K/kbar, for Lu2Fe16.5Mn0.5 and Lu2Fe16.3Mn0.7, respectively. The remarkable strengthening of the FM ground state by the Mn substitution and its total suppression by external pressure can mainly be attributed to a subtle competition of positive and negative local direct exchange interactions between Fe atoms located at inequivalent structural sites. These interactions are strongly dependent on the interatomic Fe-Fe distances.

Effects of thermal annealing on the magnetic interactions in nanogranular Fe–Ag thin films

In this paper we have studied, by analysing the evolution of the magnetic behaviour during thermal treatment, the role of the interparticle magnetic interactions in FexAg100−x granular thin films prepared by sputtering deposition technique. Two compositions have been selected: x=25 and 35, below and around the magnetic percolation of the system, respectively, according to our previous works. The structure of these thin films has been studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements. To analyse the magnetic behaviour, DC magnetic measurements have been carried out after progressively annealing the samples at different temperatures (0≤Tann≤200°C). These measurements have revealed that, upon thermal treatment, the frustrated state at low temperatures (T<80K) for the x=25 sample tends to disappear, probably due to the weakening of RKKY interactions after the segregation of soluted Fe atoms in the Ag matrix. However, dipolar interactions are not affected by the annealing. On the contrary, at x=35, around the magnetic percolation, the annealing gives rise to an increasingly ordered interface, thereby enhancing the transfer of the direct exchange interactions.

Square planar coordinate iron oxides

We will provide an overview of the synthesis, structures, chemical and physical properties of novel iron oxides bearing FeO(4) square planar coordination, such as SrFeO(2) and Sr(3)Fe(2)O(5). The preparation of these materials relies on topotactic low-temperature reduction using metal hydrides. For instance, a simple 3D perovskite structure SrFeO(3) converts to a 2D structure SrFeO(2)via SrFeO(2.5). SrFeO(2) shows a remarkable stability against temperature and chemical substitution (for both A- and B-sites) and also tolerates distortions of square planes toward tetrahedra to adapt different A sites. Such structural stability and flexibility arise from strong covalent interactions not only through the in-plane Fe-O-Fe superexchange interactions but also through the out-of-plane Fe-Fe direct exchange interactions, and explains why SrFeO(2) exhibits magnetic order far beyond room temperature. The application of pressure on SrFeO(2) and Sr(3)Fe(2)O(5) further enhances the Fe-Fe direct exchange interactions and eventually induces striking transitions at around 34 GPa: spin-state transition from S = 2 to S = 1, insulator-to-metal transition, and antiferro-to-ferromagnetic transition. The high mobility of oxide ions at relatively low temperatures, during the reduction and reoxidation reaction process would offer an important challenge to tailor and design new solid oxide fuel cells/membranes toward lowering working temperatures.

Molecular magnets based on chain polymer complexes of copper(II) bis(hexafluoroacetylacetonate) with isoxazolyl-substituted nitronyl nitroxides

First isoxazolyl-substituted nitronyl nitroxides (L and $$L^{Me_2 }$$ ) were synthesized and characterized. Their reactions with Cu(hfac)2 and Mn(hfac)2 (hfac is hexafluoroacetylacetonate) afford the heterospin complexes [Cu(hfac)2L] n , [Cu2(hfac)4L] n , $$\left[ {Cu_2 (hfac)_4 L^{Me_2 } } \right]_n$$ , $$\left[ {Cu(hfac)_2 L^{Me_2 } } \right]_n$$ , $$\left[ {Cu(hfac)_2 L^{Me_2 } _2 } \right]$$ , $$\left[ {Cu(hfac)_2 L^{Me_2 } (MeCN)} \right]$$ , [Mn(hfac)2]3L4, and $$\left[ {Me(hfac)_2 L^{Me_2 } } \right]_2$$ . In the ligand L, the N atom of the isoxazole ring (NIz) has weak electron-donating properties. For example, the paramagnetic ligand in the chain polymer complex [Cu(hfac)2L] n acts as a bidentate bridging ligand coordinated through both O atoms of the nitronyl nitroxide group (ON-O); the NIz and OIz atoms are not involved in the coordination. The introduction of Me groups into the isoxazole substituent results in an increase in the electron density on the NIz atom and enables the synthesis of the chain polymer complex $$\left[ {Cu(hfac)_2 L^{Me_2 } } \right]_n$$ , in which the bidentate bridging ligand $$L^{Me_2 }$$ is coordinated through the ON-O and NIz atoms. In the mononuclear complexes $$\left[ {Cu(hfac)_2 L^{Me_2 } _2 } \right]$$ and $$\left[ {Cu(hfac)_2 L^{Me_2 } (MeCN)} \right]$$ , the paramagnetic ligand is coordinated only through the NIz atom. The solid heterospin Mn complexes [Mn(hfac)2]3L4 and $$\left[ {Mn(hfac)_2 L^{Me_2 } } \right]_2$$ have a molecular structure. In these complexes, strong antiferromagnetic intracluster exchange interactions arise. The residual magnetic moments of the exchange clusters in the complex [Mn(hfac)2]3L4 are ferromagnetically coupled, resulting in the increase in the effective magnetic moment (μeff) of the complex with decreasing temperature in the range of 300—30 K. The thermomagnetic study of the complexes [Cu(hfac)2L] n , [Cu2(hfac)4L] n , and $$\left[ {Cu_2 (hfac)_4 L^{Me_2 } } \right]_n$$ in the range of 2–300 K revealed the ferromagnetic ordering at temperatures below 5 K. The ESR study of the solid complex $$\left[ {Cu(hfac)_2 L^{Me_2 } } \right]_n$$ showed that the decrease in its μeff in the temperature range of 30–300 K is associated with the direct exchange interaction between the unpaired electrons of the nitronyl nitroxides of adjacent chains, whereas at temperatures below 30 K, only Cu2+ ions contribute to the magnetic susceptibility of the complex.

120∘helical magnetic order in the distorted triangular antiferromagnetα-CaCr2O4

$\ensuremath{\alpha}$-CaCr${}_{2}$O${}_{4}$ is a distorted triangular antiferromagnet. The magnetic Cr${}^{3+}$ ions, which have spin-3$/$2 and interact with their nearest neighbors via Heisenberg direct exchange interactions, develop long-range magnetic order below ${T}_{N}=42.6$ K. Powder and single-crystal neutron diffraction reveal a helical magnetic structure with ordering wave vector $\mathbf{k}=(0,\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}1/3,0)$ and angles close to ${120}^{\ensuremath{\circ}}$ between neighboring spins. Spherical neutron polarimetry unambiguously proves that the spins lie in the $ac$ plane perpendicular to $\mathbf{k}$. The magnetic structure is therefore that expected for an ideal triangular antiferromagnet where all nearest-neighbor interactions are equal, in spite of the fact that $\ensuremath{\alpha}$-CaCr${}_{2}$O${}_{4}$ is distorted with two inequivalent Cr${}^{3+}$ ions and four different nearest-neighbor interactions. By simulating the magnetic order as a function of these four interactions, it is found that the special pattern of interactions in $\ensuremath{\alpha}$-CaCr${}_{2}$O${}_{4}$ stabilizes ${120}^{\ensuremath{\circ}}$ helical order for a large range of exchange interactions.

A First Principles Study of O2/Ag(111) –Adsorption and Magnetic Properties–

We have investigated the O 2 molecular and dissociative adsorption on Ag(111) surfaces. To clarify the adsorption properties, we performed first principles calculation based on spin-polarized density functional theory (DFT) with the generalized gradient approximation for the exchange–correlation energy. We calculated the potential energy surfaces (PES) for thirteen trajectories of O 2 , which gives us information with respect to the relevant six dimensional PES. From these results, we found that the effective energy barrier of O 2 dissociative adsorption is about 1.24 eV. We also found that upon O 2 molecular and dissociative adsorption, the magnetic moments per O atom were 0.52 µ B and 0.32 µ B , respectively. For reference, the isolated O 2 and Ag(111) surfaces have the magnetic moments of 2.0 µ B and 0.0 µ B . In O 2 dissociative adsorption, ferromagnetic behaviors were induced by ferromagnetic direct exchange interactions and superexchange interactions. Superlattice structures of O atoms which have an effective ferromagnetic superexchange interaction made ferromagnetic states feasible.

Ferromagnetic Behavior of Ultrathin Manganese Nanosheets

Ferromagnetic behaviour has been observed experimentally for the first time in nanostructured Manganese. Ultrathin ($\sim$ 0.6 nm) Manganese nanosheets have been synthesized inside the two dimensional channels of sol-gel derived Na-4 mica. The magnetic properties of the confined system are measured within 2K-300K temperature range. The confined structure is found to show a ferromagnetic behaviour with a nonzero coercivity value. The coercivity value remains positive throughout the entire temperature range of measurement. The experimental variation of susceptibility as a function of temperature has been satisfactorily explained on the basis of a two dimensional system with a Heisenberg Hamiltonian involving direct exchange interaction.

First-principles studies for magnetism in Cu-doped GaN

Hole carrier mediated magnetization in Cu-doped GaN is investigated by using the first-principles calculations. By studying the sp-d interaction and the direct exchange interaction among the dopants, we obtain an equation to determine the spontaneous magnetization as a function of the Cu dopant concentration and the hole carrier density. It is demonstrated that nonmagnetic Cu doped GaN can be of room-temperature ferromagnetism. The system’s Curie temperature Tc can reach about 345 K with Cu concentration of 1.0% and hole carrier density of 5.0×1019 cm−3. The results are in good agreement with experimental observations and indicate that ferromagnetism in this systems is tunable by controlling the Cu concentration and the hole carrier density.

Direct exchange interaction of localized spins associated with unpaired sp electrons in Be-doped low-temperature-grown GaAs layers

Beryllium-doped GaAs layers grown at low temperatures by molecular-beam epitaxy contain localized spins associated with unpaired sp electrons of AsGa+ ions. Interactions of these localized spins are investigated by measuring the magnetization with a superconducting quantum interference device and the peak-to-peak width of electron paramagnetic resonance (EPR) spectra for samples with different spin concentrations ranging from 3 × 1018 to 2.0 × 1019 cm−3. The results show that localized spins in this material antiferromagnetically interact on each other via direct exchange. From the analysis of the temperature dependence and field dependence of the magnetization on the basis of the Curie–Weiss law and the molecular-field approximation, exchange energy of each sample was derived. The dependence of the exchange energy on the concentration of localized spins is reasonably explained by a model of direct exchange, which results from the overlapping of wave functions of unpaired electrons at AsGa+ ions. The peak-to-peak width of EPR spectra increases with an increase in the spin concentration at low temperatures, whereas it decreases with an increase in the temperature for samples with high spin concentrations. These EPR results also show that significant exchange interactions indeed occur between localized spins in this material. These effects of direct exchange interactions between localized spins can clearly be observed at their average distances of around 4 nm, which implies a considerably large spatial extension of the wave function of an unpaired sp electron around an AsGa+ ion.

Spin-orbital resonating valence bond liquid on a triangular lattice: Evidence from finite-cluster diagonalization

We investigate the ground state of the $d^1$ spin-orbital model for triply degenerate $t_{2g}$ orbitals on a triangular lattice which unifies intrinsic frustration of spin and orbital interactions with geometrical frustration. Using full or Lanczos exact diagonalization of finite clusters we establish that the ground state of the spin-orbital model which interpolates between the superexchange and direct exchange interactions on the bonds is characterized by valence-bond correlations. In the absence of Hund's exchange the model describes a competition between various possible valence-bond states. By considering the clusters with open boundary conditions we demonstrate that orbital interactions are always frustrated, but this frustration is removed by pronounced spin singlet correlations which coexist with supporting them dimer orbital correlations. Such local configurations contribute to the disordered ground states found for the clusters with periodic boundary conditions which interpolate between a highly resonating, dimer-based, entangled spin-orbital liquid phase, and a valence-bond state with completely static spin-singlet states. We argue that these states are also realized for the infinite lattice and anticipate that pronounced transitions between different regimes found for particular geometries will turn out to smooth crossovers in the properties of the spin-orbital liquid in the thermodynamic limit. Finally, we provide evidence that the resonating spin-orbital liquid phase involves entangled states on the bonds. In such a phase classical considerations based on the mean-field theory cannot be used, spin exchange interactions do not determine spin bond correlations, and quantum fluctuations play a crucial role in the ground states and magnetic transitions.

Appearance of a Large Magnetization at Elevated Temperatures in Nearly Antiferromagnetic α-Fe2O3

A significant magnetization of about 10–20 emu/g has been observed under a magnetic field of 5 kOe in nearly antiferromagnetic α-Fe2O3 when heated at 350–500 °C in air. The present outstanding effect that contrasts with the temperature dependence of the Curie's law has been investigated on the basis of various temperature-dependent measurements of magnetization, diffuse reflectance spectra, electrical conductivity, Seebeck potential, X-ray and neutron diffractions, differential scanning calorimetry, thermogravimetry, and X-ray photoelectron spectra. The color of α-Fe2O3 is found to drastically change from vivid red to black at about 380 °C, and this is closely correlated with the appearance of the magnetization. The color change is due to the formation of oxygen defects called a color center (i.e., F-center). Then, the absence of the intervening O2- ions greatly disturbs the superexchange interaction that is responsible for the anti-parallel spin alignment in the Fe–O–Fe (cation–anion–cation) system. As a result, the uncoupled spins can be aligned in parallel due to the direct exchange interaction between various nearest-neighboring Fe3+ ions, resulting in the appearance of the large magnetization at elevated temperatures.

Modification of Magnetic Stability of Co Single Atoms on Pt(111) by Dimer Formation

Novel magnetic properties of very small nanostructures have attracted broad interests in the fields of nanoscience and nanotechnology. The magnetic stability of such nanostructures relies on the magnetic anisotropy and magnetization dynamics. These properties were investigated for single Co atoms on Pt(111) using low-temperature scanning tunneling microscopy. The atomic manipulation capabilities of scanning tunneling microscopy allow to extend the study to Co dimers. Information on the magnetic anisotropy and lifetime of magnetically excited states of single atoms and dimers was achieved simultaneously with inelastic tunneling spectroscopy. Manufactured dimers consist of adatoms in nearest-neighbor fcc-hcp sites. For adatoms this close, a strong direct exchange interaction of 14 meV/h2 was found, much larger than the weak long-range indirect Ruderman-Kittel-Kasuya-Yoshida interaction. [DOI: 10.1380/ejssnt.2011.237]

Exchange interaction function for spin-lattice coupling in bcc iron

We performed first-principle calculations on the exchange interaction (EI) between atoms in BCC-Fe strained volumetrically. Our results show that the volume-dependence of the EI deviates considerably from the Bethe-Slater curve. This behavior is discussed in terms of the on-site and/or inter-site direct exchange interactions between electrons.

Impact on Interface Spin Polarization of Molecular Bonding to Metallic Surfaces

We have studied the repercussion of the molecular adsorption mechanism on the electronic properties of the interface between model nonmagnetic or magnetic metallic surfaces and metallo-organic phthalocyanines molecules (Pcs). Our intertwined x-ray absorption spectroscopy experiments and computational studies reveal that manganese Pc (MnPc) is physisorbed onto a Cu(001) surface and retains the electronic properties of a free molecule. On the other hand, MnPc is chemisorbed onto Co(001), leading to a dominant direct exchange interaction between the Mn molecular site and the Co substrate. By promoting an interfacial spin-polarized conduction state on the molecule, these interactions reveal an important lever to tailor the spintronic properties of hybrid organic-metallic interfaces.

Crossover from superspin glass to superferromagnet inFexAg100−xnanostructured thin films(20≤x≤50)

${\text{Fe}}_{x}{\text{Ag}}_{100\ensuremath{-}x}$ granular thin films, with $20\ensuremath{\le}x\ensuremath{\le}50$, have been prepared by the dc-magnetron sputtering deposition technique. With this technique we have been able to obtain samples comprising small Fe nanoparticles (2.5--3 nm) embedded in a Ag matrix, remaining their size practically constant with increasing Fe content. Their magnetic behavior has been fully characterized by dc magnetic measurements between 5--350 K. They have revealed a crossover in the collective magnetic behavior of the Fe nanoparticles around a $35\text{ }\text{at}\text{.}\text{ }\mathrm{%}$. Below such a concentration, a collective freezing of the magnetic moments is observed at low temperatures, while at high temperatures a transition, mainly mediated by dipolar interactions, to a magnetically disordered state is obtained. Above this concentration, direct exchange interactions overcome the dipolar magnetic interactions and a long-range order tends to prevail in the range of temperatures analyzed. ac magnetic measurements have indicated a crossover from a superspin glass $(xl35)$ to a superferromagnetic $(xg35)$ behavior for the magnetic moments of the Fe nanoparticles.

Polariton-polariton interaction constants in microcavities

Resonant transmission of light through a microcavity in the strong coupling regime is used to estimate the strength of the interaction between polaritons with parallel $({\ensuremath{\alpha}}_{1})$ or antiparallel $({\ensuremath{\alpha}}_{2})$ spins. The ratio ${\ensuremath{\alpha}}_{2}/{\ensuremath{\alpha}}_{1}$ is found to be strongly dependent on the detuning between exciton and photon modes. From negative to zero detuning it changes from about 0 to less than $\ensuremath{-}1$. Our observations indicate that at certain conditions the polaritons might rather condense in the real space than form a Bose-Einstein condensate. We analyze theoretically different mechanisms of polariton-polariton interaction including the mean-field electrostatic interaction, the direct exchange interaction, the Van-der-Waals interaction and the indirect exchange interaction via dark exciton and biexciton states.

Magnetism in ZnO nanowire with Fe/Co codoping: First-principles density functional calculations

Using first-principles density functional calculations, at both generalized gradient approximation (GGA) and $\text{GGA}+U$ levels we have investigated the electronic structure and magnetic properties of Fe/Co codoped ZnO nanowire. Here we have addressed some of the key issues such as, the preferable sites that Fe/Co can occupy, the coupling mechanism, and role of defects in coupling. We found that the spin alignment between the transition-metal atoms depends on their location. When Fe and Co atoms are nearest neighbors on the outer surface of the nanowire along [0001] direction is the lowest energy configuration with ferrimagnetic (FiM) ground state. At GGA level of description ferromagnetic ordering is observed when impurity atoms sit at surface and subsurface interface forming Fe-O-Co magnetic path, however at $\text{GGA}+U$ level of description antiferromagnetic superexchange interaction dominates and all configuration leads to FiM ground state. $\text{GGA}+U$ are found to give more realistic description of electronics structure of Fe/Co codoped ZnO nanowire. Interestingly ${\text{Fe-V}}_{\text{O}}\text{-Co}$ defect configurations formed by removing the O atom from Fe-O-Co magnetic path are ferromagnetic when Fe-Co separation is less than 2.596 at GGA and $2.801\text{ }\text{\AA{}}$ at $\text{GGA}+U$ irrespective of the location of transition ions. We have also found that Co atom has a tendency to form clusters around Fe atom leading to inhomogeneous doping concentrations. O vacancy is found to be crucial in case of promoting ferromagnetism in this system. Two competing factors are the Ruderman-Kittel-Kasuya-Yosida (RKKY) type of exchange interaction in bulk environment due to O vacancy and direct exchange interaction of carriers due to ${\text{Fe-V}}_{\text{O}}\text{-Co}$ defect configuration on the surface.