Exchange Interaction Constants Research Articles

Size and Ion-Doping Effects on Magnetic, Optical, and Phonon Properties of CuAlO2 Nanoparticles

The magnetic, optical, and phonon properties of ion-doped CuAlO2 nanoparticles on the Cu or Al site are theoretically investigated. The room temperature ferromagnetism in CuAlO2 nanoparticles can be due to the surface, size, and doping effects. The magnetization increases with the decreasing nanoparticle size. The different radii of the transition metal ion and the host Cu ion lead to compressive strain, to the enhancment of the exchange interaction constants, and to increased magnetization Ms and Curie temperature TC. By substitution with Mn or Cr on the Al site, tensile strain, a decrease in Ms, and an increase in dopants are observed. The size and ion-doping influence on the band-gap energy is also discussed. The phonon energy ω decreases, whereas the phonon damping γ increases with increasing temperature and decreasing NP size. They show a kink around TC ∼ 400 K. The behavior of ω and γ for different ion dopings is observed.

Magnetic, Electric and Optical Properties of Ion Doped CuCr2O4 Nanoparticles

The magnetic, electric and optical properties of pure and ion doped CuCr2O4 - bulk and nanoparticles are investigated theoretically. The magnetization Ms and the band gap Eg decrease with increasing particle size. By Co ion doping Ms and the polarization P show a maximum whereas by Pr ion doping they decrease with increasing the doping concentration. The dielectric constant decreases with enhancing Pr dopants. It is shown that the difference between the doping and host ions radii leads to appearing of a compressive or tensile strain and to different exchange interaction constants in the doped state. Eg decreases by Co doping, whereas it increases by Pr doping.

Manipulation of optical coherence of quantum-well excitons by transverse magnetic field

We have studied experimentally spin-dependent photon echoes from excitons in an InGaAs/GaAs quantum well subject to a transverse magnetic field (Voigt geometry). Larmor precession of the spins of the electron and heavy hole in an exciton leads to a periodic transfer of coherence between bright and dark exciton states. The increase in dephasing time due to the transition to the dark states could be useful for coherent control development. A comprehensive analysis shows a good agreement between a four-wave mixing experiment and the predictions of a theoretical treatment based on a five-level exciton model comprising a ground state and two pairs of bright and dark states. The extracted optical dephasing time of bright and dark excitons is equal to 30 and 130 ps, respectively. Exploiting the photon echo reveals evidence of electron Larmor precession with in-plane $g$ factor $|{g}_{e,\ensuremath{\perp}}|=0.44\ifmmode\pm\else\textpm\fi{}0.05$ and heavy hole precession as well. The precession frequency of the latter depends nonlinearly on the applied magnetic field, and the corresponding $g$ factor reaches a value of $|{g}_{h,\ensuremath{\perp}}|\ensuremath{\approx}0.3$ at $B=6$ T. Estimates for the heavy hole $g$-factor spreading as well as the isotropic exchange interaction constant are provided.

Spin-state transition of Co ion (S = 2 → S = 5/2) in hole substituted 1D chain of Ca3Co2O6

We have discovered spin-state transition (S = 2 to S = 5/2) of Co ions due to Mg substitution in the Ca3Co2O6 apparent in the magnetic susceptibility, x-ray photoelectron spectroscopy (XPS), and first-principles study. We also examine the effect of Mg substitution on the magnetic and electronic structure of Ca3Co2O6 by first-principles calculations. It involves generalized gradient approximation with Coulomb interaction (U) in exchange–correlation energy functional. Our study shows a reasonable agreement between effective magnetic moment (μ eff) determined from the Curie–Weiss fit with that from the XPS analysis and first-principles calculations study. We have attributed the decrease in positive intra-chain exchange interaction constant (J 1/k B) to the antiferromagnetically coupled induced Co4+ ions (S = 5/2) arising from the Mg2+ ions substitution. The in-field metamagnetic transitions in the isothermal M(H) curves below the critical field (H c) have been accurately mapped and successfully explained by the change in magnetic entropy (ΔS) calculations and Arrott plots. Electronic structure study reveals hole-type doping of Mg atom, and the Fermi level (E F) shifts below. Density of state and band structure calculation indicates strong hybridization between partial states of Co-3d and O-2p orbitals for the Mg-doped compound due to which the band crossing at Fermi level is observed, and a hole-type Fermi surface is formed.

Indirect Exchange Coupling in Carbon Nanotubes

Nanostructured magnetic composites based on carbon nanotubes (CNTs) and ferromagnetic nanoparticles (FNPs) are of great interest both from an applied and fundamental point of view. In particular, one of the features of CNTs with FNPs is the possibility of magnetic interaction of nanoparticles through the conducting medium of CNTs. For a detailed description of this special type of interaction, which is the indirect exchange coupling, it is necessary to establish the relationship between the macroscopic and microscopic parameters of the physical system. In nanostructured ferromagnets, these dependences are described within the framework of a random magnetization model in which the spin system and, consequently, the main macroscopic characteristics (coercivity, susceptibility, and saturation magnetization) are determined by such microscopic parameters as the exchange interaction constant, the FNP magnetization, the local magnetic anisotropy constant, and the grain size. In this paper, on the basis of the previously obtained microscopic parameters of CNT – FNP nanocomposites, the possibility of obtaining long-range magnetic correlations through the indirect exchange coupling (IEC) between FNP embedded inside a multi-wall CNT (MWCNT) is considered. A model Hamiltonian is used that takes into account the diameter, chirality, chemical potential and spin-orbit interaction (SOI) in the system. The reason for the appearance of a noticeable SOI in CNTs is the curvature of the tubes, which significantly increases the SOI compared to graphene, as well as possible defects and the presence of FNP. IEC is realized by means of p-electrons of the inner wall of the MWCNT. The propagation of the spin susceptibility along the MWCNT axis is calculated and it is shown that a long-range magnetic order is realized under the condition that the chemical potential enters the gap opened by the SOI. Coherence is realized at distances up to micrometers. The proposed approach also made it possible to estimate the energy of the exchange interaction between the FNP belonging to one CNT. The results obtained indicate the prospects for the use of CNT– FNP nanocomposites in carbon spintronics.

Emission of electromagnetic radiation due to spin-flip transitions in a ferromagnet

We theoretically analyze the probability of electromagnetic wave emission due to electron transitions between spin subbands in a ferromagnet. Different mechanisms of such spin-flip transitions are considered. One mechanism is electron transitions caused by the magnetic field of the wave. Another mechanism is due to the Rashba spin–orbit interaction. While the two mentioned mechanisms contribute in a homogeneously magnetized ferromagnet, there are two other mechanisms that occur for a non-collinearly magnetized medium. The first mechanism is known and is due to the dependence of the exchange interaction constant on the quasimomentum of the conduction electrons. The second one is due to the minimal coupling. It follows from the connection of spin and spatial degrees of freedom in any non-collinearly magnetized medium. We study these mechanisms in a non-collinear ferromagnet with a helicoidal magnetization distribution. Estimations of the probabilities of electron transitions due to different mechanisms are made for realistic parameters, and we compare the mechanisms. We also estimate the radiation power and the threshold current in a simple model in which spin is injected into the ferromagnet by a spin-polarized electric current through a tunnel barrier.

Theoretical Investigation on Electron Transport Capabilities of Helically Twisted Molecules Based on Decay Constants of Exchange Interaction

Abstract The electron transport capabilities of helically twisted molecules were theoretically evaluated based on the decay constant of the exchange interaction (βJ) between terminal nitronyl nitroxide radicals using broken-symmetry unrestricted density functional theory (UDFT) calculations. A small βJ value (βJ = 0.16 Å−1) was estimated for a homogeneously π-extended helicene consisting of a helically fused oligo-phenanthrenes, which is less than half that of the original carbohelicene (βJ = 0.39 Å−1) and comparable to that of rylene (βJ = 0.13 Å−1). The excellent electron transport capability suggested by the small βJ of the π-extended helicene can be attributed to the olefinic electronic nature found inside the helical framework.

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.

Температурные зависимости межслойной обменной константы трехслойных пленок FeNi/Dy/FeNi, исследованные динамическим методом

Three-layer FeNi/Dy/FeNi films were studied by the ferromagnetic resonance method in the temperature range from 4 to 300 К. The detected acoustic and optical peaks in the microwave absorption spectra demonstrate that there is exchange coupling between the ferromagnetic FeNi layers of the planar system, which allows us to define the sign and value of the interlayer exchange interaction constant. The temperature dependences of the interlayer exchange constant of the three-layer films with an interlayer thickness of Dy equal to 5 and 10 nm show a number of features (change of the sign and extreme point) which reflect the transformation of the Dy magnetic structure

Temperature dependences of the interlayer exchange constant of three-layer FeNi/Dy/FeNi films studied by the resonance method

Three-layer FeNi/Dy/FeNi films were studied by ferromagnetic resonance method in the temperature range from 4 to 300 K. The acoustic and optical peaks recorded in microwave spectra demonstrate the presence of exchange coupling between the ferromagnetic FeNi layers of the planar system and allow us to establish the sign and value of the constant of the interlayer exchange interaction. Temperature dependences of the interlayer exchange interaction constant for three-layer films with intermediate layer thicknesses Dy 5 and 10 nm show a number of features (change in sign and extremum point), which reflect transformations of the magnetic structure Dy. Keywords: interlayer exchange coupling, helical magnetic structure, ferromagnetic resonance, three-layer films.

Multiferroic, Phonon and Optical Properties of Pure and Ion-Doped YFeO3 Nanoparticles

The magnetic, electric, phonon and optical properties of pure and ion-doped orthorhombic YFeO nanoparticles are studied for the first time theoretically. The spontaneous magnetization in YFeO decreases with decreasing particle size. is also shape dependent. The magnetization increases by Co and Er ion doping and decreases by Ti doping, which is caused by the different strain which appears in the nanoparticles and changes the exchange interaction constants in the doped states. The phonon energy for the mode = 149 cm and their damping decreases or increases with increasing temperature, respectively. Both show a kink near the Neel temperature, , which disappears by applying an external magnetic field. The influence of different ion doping on the band gap energy is also discussed. The doping effects can be used for different applications.

THz radiation spectra in magnetic Fe/Mo and Fe/Co2FeAl junctions

An investigation was made of THz radiation spectra in the Fe/Mo and Fe/Co2FeAl magnetic junctions arising from the pumping by a high-density current of the upper spin subband at the Fe/Mo interface. The tensor character of the exchange interaction constant at the Fe/Mo interface, associated with the presence of the DMI-like interaction, promotes radiative relaxation.

Multiferroic and phonon properties near the phase transitions of pure and ion-doped Ca3Mn2O7

ABSTRACT The magnetic, dielectric and phonon properties of CaMnO are investigated using a microscopic model and the Green's function technique. Electrically induced decrease of the magnetization M is observed which is an indirect evidence that CaMnO is a multiferroic material. The dielectric function has an anomaly at the Nel temperature and a broad peak at the Curie temperature . increases with increasing magnetic field. M and are enhanced by Li ion doping, whereas reduced by Ti ion doping due to different strain caused by the doped ions which changes the exchange interaction constants. increases with increasing Ti concentration. The temperature and magnetic field dependence of the phonon energy and damping for the A mode = 624 cm are studied. Both show a kink at due to the strong spin–phonon interaction. The kink disappears applying a magnetic field.

Impact of the positive electron–hole exchange interaction constant on the binding energy of neutral donor bound excitons in AlN

Low-temperature photoluminescence spectroscopy is performed for unintentionally doped and silicon-doped aluminum nitride (AlN) films grown on AlN substrates. Considering the positive electron–hole exchange interaction constant substantially changes the neutral silicon-donor bound exciton binding energy from 28.5 to 15.3 meV. The silicon-donor binding energy is also experimentally deduced as 64.8 meV from a two-electron transition, which is justified by a theoretical calculation considering the crystal anisotropy and electron–phonon coupling. The estimated binding energies are discussed with a previous theoretical result within the effective mass approximation and the experimentally known binding energy of elementary four-particle (three-particle) systems.

ПЕРВОПРИНЦИПНЫЕ ИССЛЕДОВАНИЯ ФАЗОВЫХ ПРЕВРАЩЕНИЙ В СПЛАВАХ Fe-Si

In this paper, the structural and magnetic properties of Fe100-xSix alloys (10 ≤ x ≤ 25,0 at. %) were calculated. The structural phase transition temperatures for the crystal structures A2, B2, and D03 were estimated from the geometry optimization. The Curie temperatures were calculated in a molecular-field approximation using the constants of magnetic exchange interaction calculated ab initio. For all the considered concentrations, with the temperature increase, we observed the structural transitions from the ordered cubic phase to a disordered structure, with the intermediate stage of a partially disordered state. The ferromagnet–paramagnet transition was observed for all the compositions, though in various crystal phases.

THz-излучение в магнитном переходе Fe/Mo

An investigation was made of THz radiation in the Fe/Mo magnetic junction arising from the pumping by a high-density current of the upper spin subband in the induced ferromagnetism layer in Mo. The tensor character of the exchange interaction constant at the Fe / Mo interface, associated with the presence of the Dzyaloshinsky-Moriya interaction, promotes radiative relaxation.

Microscopic theory of the specific absorption rate for self-controlled magnetic hyperthermia

Despite the efforts of many research teams, to optimize the process of thermal heating of magnetic nanoparticles interacting with an alternating magnetic field for self-controlled magnetic hyperthermia remains a challenge. Macroscopic models that quantify the process of heat generation could not combine the biocompatibility requirements of magnetic nanoparticles in human medicine on one hand with high values of specific absorption rate (SAR) on the other. For the first time, we propose a microscopic model using Kubo formalism, a modified Heisenberg Hamiltonian and the method of Green's functions for calculating the absorbed power (thermal energy absorbed by a magnetic nanoparticle per unit time in an alternating magnetic field). From the observed transverse magnetic susceptibility, elementary excitation energy, and damping is investigated the SAR as a function of the microscopic parameters of the systems: exchange interaction constants and single-ion magnetic anisotropy. The calculations are made for a magnetic complex heterogeneous nanoparticle consisting of core, shell, intermediate layer between them, and surface (the so called core/shell model). The proposed nanoparticle model allows for each region to define different exchange interaction and magnetic anisotropy constants as well as different magnetic configurations and thickness. The dependence of SAR on the microscopic parameters of the system is analyzed and qualitatively explained by the behavior of the elementary excitations (spin energy and damping). The results are in good qualitative agreement with the experimental data and show that using complex magnetic nanoparticles the heating properties for hyperthermia could be maximized.

Multiferroic and phonon properties of pure and ion doped CoCr2O4 - Bulk and nanoparticles

Using a microscopic model we have studied different properties of pure and ion doped CoCr2O4 - bulk and NPs. The magnetization Ms and the Curie temperature TC decrease with decreasing particle size. By Fe doping Ms and TC increase with enhancing Fe concentration x whereas by Mg doping we obtain the opposite effect. In the first case there is a compressive strain, in the second one - a tensile strain due to the different radii of the doping ions compared to the host ones. This leads to different exchange interaction constants in the doped states. The polarization Ps increases with increasing Fe doping concentration x. Two anomalies are observed in the specific heat Cp, at TC = 98 K, and TS = 25 K, which are not so sharp applying h. The temperature, magnetic field, size and doping dependence of the phonon energy ω0 = 685 cm−1 and damping are also investigated.

Microwave Giant Magnetoresistance and Ferromagnetic and Spin-Wave Resonances in (CoFe)/Cu Nanostructures

The microwave phenomena that occur in magnetic multilayer (CoFe)/Cu nanostructures, which have a giant magnetoresistance, are studied. The transmission of waves through a nanostructure is used to investigate the microwave giant magnetoresistance effect. The changes in the transmission coefficient at frequencies of 29–38 GHz are found to exceed the relative magnetoresistance, which distinguishes the system under study from the nanostructures studied earlier. Ferromagnetic and spin-wave resonances are used to study the angular dependences of the microwave absorption spectra of a multilayer (CoFe/Cu)n nanostructure. The following parameters are determined: the critical angle that determines the boundaries of the ranges of excitation of uniform and nonuniform spin modes, the type of boundary conditions describing the pinning of spins on the outer nanostructure surfaces, and the surface anisotropy and exchange interaction constants.

The magnetic properties of the LaCr3(BO3)4 crystal

This article investigates the magnetic susceptibility, χ(T), of the two-phase (R32 + C2/c) crystal LaCr3(BO3)4 in the temperature range 2–300 K for two orientations of an external magnetic field. It has been established that both structural modifications of lanthanum chromium borate are ordered antiferromagnetically, with each of them having its own Néel temperature (6.5 and 8.5 K). It has been shown that at T > 60 K, the magnetic susceptibility is well described by the Curie–Weiss law with an effective magnetic moment of μeff = 3.86 μB/Cr3+ and a Curie–Weiss temperature of θ = –23 K. Within the Heisenberg spin chain model, taking into account the interchain exchange interaction, the authors have obtained the following exchange-interaction constants: an antiferromagnetic intrachain exchange (J/k = 4.8 K) and a ferromagnetic interchain exchange (J'/k = –0.28 K).