Negative Exchange Interaction Research Articles

Role of biquadratic exchange on thermodynamic quantities in anisotropic Heisenberg model

The magnetic and thermodynamic properties of the spin-1 anisotropic Heisenberg system are studied within Oguchi approximation. The system contains both the bilinear and biquadratic exchange interactions between the nearest neighboring spins located on a centered honeycomb lattice. For the positive biquadratic exchange interaction parameters, the phase transition in the Heisenberg case is of both second and first order where there is a discontinuity in the thermodynamic properties at this transition temperature. For some negative biquadratic exchange interaction parameters, in both the Heisenberg and Ising cases, the ferromagnetic state changes in favor of the antiferromagnetic state. The frustrated antiferromagnetic structure created by the biquadratic exchange parameter with negative sign was shown significantly influence the magnetization, quadrupolar moment, internal energy, heat capacity, entropy and free energy of the system, where the system has an oscillating chaotic phase regime. A double peak behavior was observed in the specific heat, with one at the critical temperature and another at a low temperature.

Spin reorientation in GdMn2(Ge1-xSix)2 compounds

Structure and magnetic properties of layered GdMn2(Ge1-xSix)2 (0 ≤ x ≤ 1) compounds were studied. All the compounds crystallize in the tetragonal ThCr2Si2-type structure. It was shown by magnetization measurements at low temperature on quasi-single crystals that, with increasing Si concentration, the easy magnetization direction reorients from the c-axis to the basal plane. The spin reorientation occurs via an angular phase. A model of three magnetic sublattices coupled by negative intersublattice exchange interactions was used to describe the field dependences of the magnetization. For GdMn2Ge2 and GdMn2(Ge0.9Si0.1)2 in the fields applied along the c-axis, seven different magnetic structures were predicted, including two angular structures considered for the first time. The model explains formation of angular magnetic structures in zero field in GdMn2(Ge1-xSix)2 system by taking into account magnetic anisotropy of Mn sublattices with a positive anisotropy constant K1 and negative K2.

Electronic and Magnetic Interactions in 6-Oxoverdazyl Diradicals: Connection through N(1) vs C(3) Revisited.

Four isomeric di-6-oxoverdazyl diradicals connected at their N(1) or C(3) positions with either 1,3- or 1,4-phenylene linkers were obtained and characterized by spectroscopic, electrochemical, magnetic, and structural methods. These results were compared to those for the corresponding 6-oxoverdazyl monoradicals. UV-vis spectroscopy demonstrated that only the N(1)-connected para-through-benzene diradical has a distinct spectrum with significant bathochromic and hypsochromic shifts relative to the remaining species. Electrochemical analysis revealed two one-electron reduction processes in all diradiacals, while only the N(1)-connected para-through-benzene diradical exhibits two one-electron oxidation processes separated by 0.10 V. Variable temperature EPR measurements in polystyrene solid solutions gave negative mean exchange interaction energies J for all diradicals, suggesting the dominance of conformers with significant intramolecular antiferromagnetic interactions for the meta-through-benzene isomers. DFT calculations predict a small preference for the triplet state with the ΔES-T of about 0.25 kcal mol-1 for both meta-through-benzene connected diradicals.

Dilute iron‐doped polycrystalline strontium titanate: Tracking iron valence and local interactions

AbstractThe mixed‐valence states of dilute Fe (0.2, 0.5, and 1.2 mol%) in bulk polycrystalline (SrTiO3) (STO) were determined. Specimens were sintered in air or O2 at 1500°C or 1300°C with the intent to quantify the relative valence states of Fe in the context of understanding microstructure development of Fe‐doped STO. Mössbauer spectroscopy, electron paramagnetic resonance spectroscopy, and magnetometry were combined to track changes in Fe valence, and Raman spectroscopy and X‐ray diffraction were used to describe the associated structural changes. Small concentrations of defect associates were detected, but the majority of Fe′Ti acceptors are isolated from oxygen vacancies. As doping increases, negative exchange interactions increase, consistent with an increase in the concentration of configurations. Additionally, the fraction of dopants existing as isovalent increased with of the sintering atmosphere and with total Fe content.

Magnetism and EPR Spectroscopy of Nanocrystalline and Amorphous TiO2: Fe upon Al Doping

This work is devoted to the study of the magnetic properties and Electron Paramagnetic Resonance (EPR) spectroscopy of TiO2:Fe nanoparticles doped with Al in different structural states. The sol-gel methods have been used to obtain the particles in both crystalline (average size from 3 to 20 nm) and X-ray amorphous states. The electron paramagnetic resonance spectra of crystalline samples TiO2:Fe doped with aluminum besides a resonance line with g-factor ~2 exhibit a small signal with a g-factor of 4.3 from Fe3+ ions with rhombohedral distortions. The fraction of Fe3+ with rhombohedral distortions increases with increasing aluminum content. For the amorphous state at Al doping, the resonance with a g-factor of 4.3 is completely dominant in the electron paramagnetic resonance spectrum. The density functional theory calculation shows that aluminum prefers to be localized near iron ions, distorting the nearest Fe3+ environment. The complex integral electron paramagnetic resonance spectrum of all samples was fitted with sufficient accuracy by three separate resonance lines with different widths and intensities. The temperature behavior of the electron paramagnetic resonance spectrum can be described by the coexistence of paramagnetic centers (isolated Fe3+ ions including dipole-dipole interactions) and iron clusters with negative exchange interactions.

Generalization properties of restricted Boltzmann machine for short-range order

A biased sampling algorithm for the restricted Boltzmann machine (RBM) is proposed, which allows generating configurations with a conserved quantity. To validate the method, a study of the short-range order in binary alloys with positive and negative exchange interactions is carried out. The network is trained on the data collected by Monte–Carlo simulations for a simple Ising-like binary alloy model and used to calculate the Warren–Cowley short-range order parameter and other thermodynamic properties. We demonstrate that the proposed method allows us not only to correctly reproduce the order parameters for the alloy concentration at which the network was trained, but can also predict them for any other concentrations.

Negative magnetisation and magnetic exchange interactions in Fe3+ substituted Ca2NdCr2NbO9

The crystal structure, magnetic properties, and magnetic exchange interactions of Fe3+-substituted Ca2NdCr2-xFexNbO9 single-phase perovskites were systematically studied in. The substituted ranges of 0 ≤ x ≤ 0.6 using a standard solid-state reaction. X-ray diffraction results indicated an orthorhombic structure for all the samples. Tunable negative magnetisation phenomena were observed for samples corresponding to x = 0.25 to x = 0.5 and the maximum compensation temperature was observed in the x = 0.3 sample. Detailed magnetic measurements demonstrated the coexistence of the normal and inverse magnetic moments. Furthermore, the complicated magnetic exchange interactions were described using the fitting model obtained in the low-temperature region of the field cooling curves. The magnetic switching effect and magnetocaloric effect were investigated based on fundamental physics and potential applications.

Spin-wave resonance of exchange-coupled three-layers FeNi/Cu/FeNi planar structures

The spectrums of standing exchange spin waves into magnetic sandwich Ni80Fe20/Cu(tCu)/Ni80Fe20 were studied by spin-wave resonance method. The layer of diamagnetic Cu depending on its thickness tCu provided both positive and negative exchange interaction between ferromagnetic layers. It was shown that spin waves modes of SWR spectra which are described by well-known relation Hacres(n)~ n2 are standing acoustic exchange spin oscillations of individual ferromagnetic layers, regardless of a sign of interlayer exchange couple. It was detected that spin modes which are optical satellites of acoustic SWR peaks are described by the dependence of the resonance field Hopres(n)~ n5/2 or Hopres(n)~ n2 depending on the boundary conditions established on inner interfaces. Keywords: spin-wave resonance, surface anisotropy constant, exchange interaction constant, interlayer exchange interaction.

Molecular precursor route for the phase selective synthesis of α-MnS or metastable γ-MnS nanomaterials for magnetic studies and deposition of thin films by AACVD

Heterocyclic dithiocarbamate complex of manganese and the adducts of the complex with 1,10-phenanthroline and 2,2-bipyridyl respectively were synthesized and characterized. The complexes were used as single-source precursors for the preparation of MnS nanoparticles via thermolysis in oleylamine (OLA) and dodecanethiol (DT). The optimized reaction parameters have resulted in the formation of α-MnS and γ-MnS nanoparticles depending on the dispersion medium and the nature of the precursor. The same precursors were also used for the deposition of MnS thin films by the aerosol-assisted chemical vapour deposition (AACVD) method in varying solvents and temperatures. The morphology and phase of the synthesized nanoparticles were shown to be affected by the precursor type and the nature of the surfactants. TEM images show the formation of nanoparticles of various morphologies ranging from spherical monodispersed to large elongated pointed rods, depending on the precursor type. SEM images showed that the morphology of the deposited thin films is affected by precursor type and the deposition temperature. Worm-like to rod-like films were observed on TEM images at 350 °C, which changes to spherical or cubic films at 450 °C. Magnetic measurements at room temperature showed the α-MnS and γ-MnS nanoparticles in paramagnetic states due to Mn2+ moments. The susceptibility for the γ-phase is at least double that of the α-phase. Finite and small hysteretic effects in low fields are indicative of small ordered antiferromagnetic phases due to negative exchange interaction between Mn2+ ions.

Magnetism and temperature dependence of nano-TiO2: Fe EPR spectra

Nanocrystalline TiO2 samples of various compositions (up to 5 at.% Fe) with anatase structure and an average particle size of about 20 nm were synthesized using sol-gel technology. The magnetic properties of nano-TiO2: Fe was studied by electron paramagnetic resonance (EPR) and Faraday balance. The existence of an inhomogeneous magnetic state in TiO2: Fe samples of different compositions were revealed by EPR spectroscopy in a wide temperature range. The analysis of the EPR spectra in the L -, X - and Q-bands allowed us to calculate the quadratic fine structure (D, E) parameters, “axial” and “rhombic” ones, respectively. The value of D turned out to be quite small, which indicates an insignificant anisotropy, which can be ignored describing the magnetic properties of TiO2:Fe. It was shown that the temperature behavior of different separate components of the integral EPR spectra can be qualitatively interpreted in the model of coexistence in the TiO2: Fe system, mainly, dimers with a strong negative exchange interaction and isolated paramagnetic centers. No ferromagnetic state in TiO2: Fe-based samples after etching of aE-prepared state were detected.

Спин-волновой резонанс в обменно-связанных трехслойных FeNi/Cu/FeNi планарных структурах

The spectrums of standing exchange spin waves into magnetic sandwich Ni80Fe20/Cu(tCu)/Ni80Fe20 were studied by spin-wave resonance method. The layer of diamagnetic Cu depending on its thickness tCu provided both positive and negative exchange interaction between ferromagnetic layers. It was shown that spin waves modes of SWR spectra which are described by well-known relation Hres(n)~n^2 are standing acoustic exchange spin oscillations of individual ferromagnetic layers, regardless of a sign of interlayer exchange couple. It was detected that spin modes which are optical satellites of acoustic SWR peaks are described by the dependence of the resonance field Hres(n)~n^(5/2) or Hres(n)~n^2 depending on the boundary conditions established on inner interfaces.

Spanning leader–subordinate relationships through negative interactions: An examination of leader–member exchange breaches

The study aims to discover the dynamic and processual nature of a supervisory relationship (here a leader–member exchange) through a novel, theoretical concept termed leader–member exchange breach, and by examining the characteristics of negative exchange interactions within the leader–member exchange relationship. The notion of the leader–member exchange breach is empirically defined through data on 336 responses to open-ended questions on negative interactions between leaders and subordinates, analyzed through qualitative analysis methods. The results of the study raise questions about the complexity of leader–member exchange relationships and show that breaches of the leader–member exchange relationship might lead to a reassessment of the dyadic relationship between leaders and their subordinates, spanning or even breaking the exchange relationships. This study extends current knowledge on leader–member exchange relationships by providing a viewpoint on the leader–member exchange breach that helps explain the processual and dynamic nature of those relationships through interpersonal interactions and exchanges.

Compensation point phenomena in nanoscale Ising particles with high critical temperature

ABSTRACT The phase diagrams and magnetizations of Ising nanoparticles with high critical temperature, consisting of atoms with a negative exchange interaction between the surface and the core, are investigated by using the effective-field theory with correlations. They show a compensation point below their critical temperatures, although they have not ferrimagnetic spin configuration. The effects of an applied transverse field to the nanoparticles are also discussed. One of them has exhibited the reentrant phenomena, although other does not show the phenomena. The reason is also discussed. The decoration for one of the nanoparticles is investigated. A compensation point is also found for the decoration.

Unconventional magnetism of non-uniform distribution of Co in TiO2 nanoparticles

High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) analysis, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), magnetic methods, and density-functional theory (DFT) calculations were applied for the investigations of Co-doped anatase TiO2 nanoparticles (∼20 nm). It was found that high-spin Co2+ ions prefer to occupy the interstitial positions in the TiO2 lattice which are the most energetically favourable in compare to the substitutional those. A quantum mechanical model which operates mainly on two types of Co2+ – Co2+ dimers with different negative exchange interactions and the non-interacting paramagnetic Co2+ ions provides a satisfactorily description of magnetic properties for the TiO2:Co system.

Correlating the size and cation inversion factor in context of magnetic and optical behavior of CoFe2O4 nanoparticles.

Herein, the size dependent behavior of cobalt ferrite nanoparticles was investigated using synchrotron radiation based techniques. Scanning electron micrographs revealed the enhancement of particle/crystallite size with increase of annealing temperature. Moreover, the shape of these particles also changed with increase of crystallite size. Saturation magnetization increased with increase of crystallite size. The higher saturation magnetization for larger crystallite size nanoparticles was attributed to a cation distribution similar to that of bulk CoFe2O4. The optical band-gap of these nanoparticles decreased from 1.9 eV to 1.7 eV with increase of crystallite size. The enhancement of the optical band-gap for smaller crystallites was due to phenomena of optical confinement occurring in the nanoparticles. Fe L Co L-edge near edge extended X-ray absorption fine structure (NEXAFS) measurements showed that Fe and Co ions remain in the 3+ and 2+ state in these nanoparticles. The results obtained from Fe & Co K-edge X-ray absorption near edge structure (XANES)-imaging experiments further revealed that this oxidation state was possessed by even the crystallites. Extended X-ray absorption fine structure (EXAFS) measurements revealed distribution of Fe and Co ions among tetrahedral (A) and octahedral (B) sites of the spinel structure which corroborates the results obtained from Rietveld refinement of X-ray diffraction patterns (XRD). X-ray magnetic circular di-chroism (XMCD) measurements revealed negative exchange interaction among the ions situated in tetrahedral (A) and octahedral (B) sites. Theoretical and experimental calculated magnetic moments revealed the dominancy of size effects rather than the cation redistribution in the spinel lattice of CoFe2O4 nanoparticles.

Dimerization and low-dimensional magnetism in nanocrystalline TiO2 semiconductors doped by Fe and Co

The report is devoted to an analysis of the structural and magnetic state of the nanocrystalline diluted magnetic semiconductors based on TiO2 doped with Fe and Co atoms. Structural and magnetic characterization of samples was carried out using X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR) spectroscopy, SQUID magnetometry, and the density functional theory (DFT) calculations. Analysis of the experimental data suggests the presence of non-interacting paramagnetic Fe3+ and Co2+ ions in the high-spin state and negative exchange interactions between them. The important conclusions is that the distribution of dopants in the TiO2 matrix, even at low concentrations of 3d-metal dopant (less than one percent), is not random, but the 3d ions localization and dimerization is observed both on the surface and in the nanoparticles core. Thus, in the paper the quantum mechanical model for describing the magnetic properties of TiO2:(Fe, Co) was suggested. The model operates only with two parameters: paramagnetic contribution of non-interacting 3d-ions and dimers having different exchange interactions between 3d magnetic carriers.

Design of zinc ferrite thin films with excess tetrahedrally coordinated Fe3+ ions and their magnetic interactions

In the present work, zinc ferrite thin films on glass substrates were grown using radio-frequency sputtering method in oxygen environment at base pressure of 2 × 10−6 Torr. As-grown films were annealed at 200 °C for 1, 3 and 5 h. X-ray diffraction study envisaged the existence of peaks corresponding to (311) and (440) planes of cubic spinel structure. Both the lattice parameter and crystallite size increased with annealing time leading to crystal perfection with annealing time. Fourier transform infrared spectroscopic study showed presence of bands corresponding to metal-oxygen vibrations in the spinel lattice. Fe K-edge extended X-ray absorption fine structure (EXAFS) measurements revealed that the tetrahedral site occupancy of Fe3+ ions are 1.2 for as-grown zinc ferrite thin film. This occupancy is 1.18, 1.12 and 1.09 for annealing times of 1, 3 and 5 h, respectively. These observations were supported by Zn K-edge EXAFS investigations. X-ray magnetic circular dichroism measurements performed for these films show negative exchange interaction among the Fe3+ ions situtated at different sites. Thus, zinc ferrite thin films having excess occupancy at tetrahedral site was successfully designed on glass substrate.

Study of RExFe100-x (RE = Tb, Dy, Gd) ferrimagnets for SOT application

Antiferromagnetic magnetization dynamics is more effective than ferromagnetic magnetization; however, it is extremely complex. Rare earth (RE) ferrimagnets, on the other hand, have tunable magnetization with associated negative exchange interaction in sublattices. Here, we study different RE-Fe ferrimagnets while changing the RE-content. We obtain various magnetic properties like saturation magnetization MS, coercivity HC, magnetic anisotropy energy constant Ku, exchange coupling constant λ, and compensation temperature Tcomp for different RE compositions. We suggest that along with MS and HC, λ is another important parameter that is easy to measure and has information that is similar to that in the spin-orbit torque effective field and the canting angle.

Magnetization reversal and inverse exchange bias phenomenon in the ferrimagnetic polycrystalline compound Er2CoMnO6

${\mathrm{Er}}_{2}{\mathrm{CoMnO}}_{6}$ is a ferrimagnetic oxide consisting of both $4f$ and $3d$ magnetic ions in a monoclinic double perovskite crystal structure (space group $P{2}_{1}/n$). The present work probes some novel magnetic features associated with its ferrimagnetic state on a polycrystalline sample of ${\mathrm{Er}}_{2}{\mathrm{CoMnO}}_{6}$. A clear low temperature compensation point followed by negative magnetization is observed in the field-cooled state of the sample. The sample shows a large field-induced change in entropy at very low temperature, which is related to the observed metamagnetism. Interestingly, ${\mathrm{Er}}_{2}{\mathrm{CoMnO}}_{6}$ is found to show remarkable feature of inverse hysteresis loop which is quite rare among bulk ferrimagnets. Presence of three different magnetic ions, with $4f$ moment of Er having larger uniaxial anisotropy, is likely to be responsible for the observed magnetization reversal and inverse hysteresis loop. The sample also shows inverse exchange bias effect, which is presumably related to the negative exchange interaction at the interface of ordered and antisite disordered ferrimagnetic phases. Such antisite disorder is quite common among the metal oxides having double perovskite structure.

Negative exchange interactions in coupled few-electron quantum dots

It has been experimentally shown that negative exchange interactions can arise in a linear three-dot system when a two-electron double quantum dot is exchange coupled to a larger quantum dot containing on the order of one hundred electrons. The origin of this negative exchange can be traced to the larger quantum dot exhibiting a spin triplet-like rather than singlet-like ground state. Here, we show using a microscopic model based on the configuration interaction (CI) method that both triplet-like and singlet-like ground states are realized depending on the number of electrons. In the case of only four electrons, a full CI calculation reveals that triplet-like ground states occur for sufficiently large dots. These results hold for symmetric and asymmetric quantum dots in both Si and GaAs, showing that negative exchange interactions are robust in few-electron double quantum dots and do not require large numbers of electrons.