Double Exchange Coupling Research Articles

Cr and Mn doping AgNbO3 for high Curie temperature ferromagnetic half-metals

The alterations in the electronic and magnetic properties of AgNbO3 induced by Cr and Mn doping at two concentrations (6% and 14%) are investigated within the framework of density functional theory (DFT), both with and without the Hubbard correction (U) as well as HSE method. Half-metallic ferromagnetic behavior with 100% polarization is observed for all dopants. Double exchange coupling is the mechanism responsible for the ferromagnetic interaction in these materials. Our results also indicates that the correction with U-term and HSE method provide consistent values that closely align with experimental measurements, and significantly improve the standard DFT description. Finally, the value of the Curie temperature, which reaches 425.49 (K), exceeds the ambient temperature, revealing the potential use of this doped material for new spintronic devices.

Double honeycomb symmetry: Effective-field theory study

The double hexagonal lattice model is a new statistical model constructed by connecting two-dimensional hexagonal structured materials such as Graphene and Lie symmetries. The double hexagonal structure of this model appears in the G2 symmetry of Lie algebras. To investigate the magnetic properties of this new lattice model, an Ising-1/2 model was constructed with spin values σ = ±1. An effective-field theory (EFT) method was used with the differential operator technique to formulate the identity of the spin system, based on Callen's work. Different phase diagrams were established by varying the double-exchange interaction couplings and the super-exchange interaction coupling present in the model. The accurate results obtained by the EFT method were compared with those found by the mean-field approximation (MFA) method. The symmetry of the MFA diagrams was broken in the EFT diagrams. The critical temperature behavior was found to depend on the type of super-exchange ferromagnetism.

(Ti, V, Fe and Ni)-doped LiMgP half Heusler-induced Ferromagnetism stability: DFT approach

This paper reports density functional theory (DFT) on LiMgP half Heusler alloy doped by the transition metal elements (Ti, V, Fe and Ni) determined by present results, in which the half-metallicity besides ferromagnetism behavior is estimated. The electron spin polarization of Ti(3[Formula: see text] is around the [Formula: see text] and the Ti[Formula: see text] will have two electrons, of which two electrons occupied the [Formula: see text]. Then, this orbital is set on the Fermi level. When the conduction band of the orbitals [Formula: see text], [Formula: see text] and [Formula: see text] is empty, LiMg[Formula: see text]Ti[Formula: see text]P displays the half-metallic characteristic, where the ferromagnetic (FM) conduct fits the double-exchange coupling. For LiMg[Formula: see text]V[Formula: see text]P alloy, the charge state is closer to V[Formula: see text] electron configuration, which predicts the following configuration, in which two electrons are occupied by [Formula: see text] and one electron is occupied by [Formula: see text] at the upside of [Formula: see text]. Based on LiMg[Formula: see text]Fe[Formula: see text]P alloy, the presence of the half metallicity at the downside of [Formula: see text] due to (3[Formula: see text][Formula: see text]orbital is clear. A hybridization between the Fe(3[Formula: see text] and the P(3[Formula: see text] orbitals is in the range −0.35 — −0.04 Ry. For LiMg[Formula: see text]Ni[Formula: see text]P alloy, the predicted charge is Ni[Formula: see text] and exhibits the half-metallic on the downside of the [Formula: see text] which is occupied by [Formula: see text] minority state. For LiMg[Formula: see text]TM[Formula: see text]P alloy, (TM = Ti, V, Fe and Ni), the half-metallic conduct is estimated with 100% spin polarized on the Fermi level. The spin moment of TM is [Formula: see text], where their total magnetic moments [Formula: see text] are in correlation for those Ni, Ti, V and Fe, where the trend also is [Formula: see text]. Likewise, based on LiMg[Formula: see text]TM[Formula: see text]P alloys, the crystal field splitting ([Formula: see text] observation stated that the doped elements conduct a trend having the following range [Formula: see text]. From these observations, it is predicted to have a correlation between magnetic moments of (Ti, V, Fe and Ni) and the crystal field splitting.

The polar and nonpolar interfaces influenced of magnetism in LaMnO3-based superlattices.

The interface of perovskite heterostructures has been shown to exhibit various electronic and magnetic phases such as two-dimensional electron gas, magnetism, superconductivity, and electronic phase separation. These rich phases are expected due to the strong interplay between spin, charge, and orbital degree of freedom at the interface. In this work, the polar and nonpolar interfaces are designed in LaMnO3-based (LMO) superlattices to investigate the difference in magnetic and transport properties. For the polar interface in a LMO/SrMnO3 superlattice, a novel robust ferromagnetism, exchange bias effect, vertical magnetization shift, and metallic behaviors coexist due to the polar catastrophe, which results in a double exchange coupling effect in the interface. For the nonpolar interface in a LMO/LaNiO3 superlattice, only the ferromagnetism and exchange bias effect characteristics exist due to the polar continuous interface. This is attributed to the charge transfer between Mn3+ and Ni3+ ions at the interface. Therefore, transition metal oxides exhibit various novel physical properties due to the strong correlation of d electrons and the polar and nonpolar interfaces. Our observations may provide an approach to further tune the properties using the selected polar and nonpolar oxide interfaces.

Stabilization of intermediate spin states in mixed-valent diiron dichalcogenide complexes

The electronic structure and ground spin states, S, observed for mixed-valent iron–sulfur dimers (FeII-FeIII) are typically determined by the Heisenberg exchange interaction, J, that couples the magnetic interaction of the two metal centres either ferromagnetically (J > 0, S = 9/2) or antiferromagnetically (J < 0, S = 1/2). In the case of antiferromagnetically coupled iron centres, stabilization of the high-spin S = 9/2 ground state is also feasible through a Heisenberg double-exchange interaction, B, which lifts the degeneracy of the Heisenberg spin states. This theorem also predicts intermediate spin states for mixed-valent dimers, but those have so far remained elusive. Herein, we describe the structural, electron paramagnetic resonance and Mössbauer spectroscopic, and magnetic characterization of a series of mixed-valent complexes featuring [Fe2Q2]+ (Q = S2–, Se2–, Te2–), where the Se and Te complexes favour S = 3/2 spin states. The incorporation of heavier chalcogenides in this series reveals a delicate balance of antiferromagnetic coupling, Heisenberg double-exchange and vibronic coupling.

High-Sensitivity Tunnel Magnetoresistance Sensors Based on Double Indirect and Direct Exchange Coupling Effect* Supported by the Framework Project of SGCC (Grant No. 5700-202058381A-0-0-00), and the National Key Research and Development Program of China (Grant No. 2017YFA0206200).

Detection of ultralow magnetic field requires magnetic sensors with high sensitivity and low noise level, especially for low operating frequency applications. We investigated the transport properties of tunnel magnetoresistance (TMR) sensors based on the double indirect exchange coupling effect. The TMR ratio of about 150% was obtained in the magnetic tunnel junctions and linear response to an in-plane magnetic field was successfully achieved. A high sensitivity of 1.85%/Oe was achieved due to a designed soft pinned sensing layer of CoFeB/NiFe/Ru/IrMn. Furthermore, the voltage output sensitivity and the noise level of 10.7 mV/V/Oe, 10 nT/Hz1/2 at 1 Hz and 3.3 nT/Hz1/2 at 10 Hz were achieved in Full Wheatstone Bridge configuration. This kind of magnetic sensors can be used in the field of smart grid for current detection and sensing.

Ferromagnetic evolution in Nd3+, Ti4+ co-doped antiferromagnetic YFeO3

The structure-property correlation in polycrystalline powders of Y1-xNdxFe1-(4/3)yTiyO3 (x = 0.05, 0.10, 0.15; y = 0.05) was investigated. Diffuse reflectance spectra analysis revealed a reduction in the bandgap from 2.14 to 1.92 eV on increasing x. The remnant magnetization (Mr) and coercivity (Hc) increased from 0.145 to 7.50 emu/g and 0.099–8.46 kOe, respectively. The infusion of Nd3+ at Y3+ and Ti4+ at Fe3+ sites suggested that the magnetic behavior of the chosen composition of Y1-xNdxFe1-(4/3)yTiyO3 tilts away from the G-type anti-ferromagnetism. The magnetic orientation is achieved largely due to the Fe3+- O2− - Fe3+, Fe4+- O2− - Fe4+ super-exchange coupling, and Fe3+- O2− - Fe4+ double exchange coupling interactions. These materials in the present study with the enhanced magnetic and optical properties thus exhibit potential for varied applications.

Creating Ferromagnetic Insulating La0.9Ba0.1MnO3 Thin Films by Tuning Lateral Coherence Length.

In this work, heteroepitaxial vertically aligned nanocomposite (VAN) La0.9Ba0.1MnO3 (LBMO)-CeO2 films are engineered to produce ferromagnetic insulating (FMI) films. From combined X-ray photoelectron spectroscopy, X-ray diffraction, and electron microscopy, the elimination of the insulator–metal (I–M) transition is shown to result from the creation of very small lateral coherence lengths (with the corresponding lateral size ∼ 3 nm (∼7 u.c.)) in the LBMO matrix, achieved by engineering a high density of CeO2 nanocolumns in the matrix. The small lateral coherence length leads to a shift in the valence band maximum and reduction of the double exchange (DE) coupling. There is no “dead layer” effect at the smallest achieved lateral coherence length of ∼3 nm. The FMI behavior obtained by lateral dimensional tuning is independent of substrate interactions, thus intrinsic to the film itself and hence not related to film thickness. The unique properties of VAN films give the possibility for multilayer spintronic devices that can be made without interface degradation effects between the layers.

Local disorder and structure relation induced by magnetic exchange interactions in A2(Mo1−yMny)2O7 pyrochlores

We present an extended X-ray absorption fine structure study at the Mo K-edge of A2(Mo1−yMny)2O7 (A = Gd, Ho; y = 0.05 and 0.10) pyrochlores, as a function of temperature and composition, coupled to diffractometric and magnetic characterizations. Extending the study reported in our previous paper on the pristine A2Mo2O7 compounds to these Mo/Mn partially substituted samples, where we hypothesize a competition between double-exchange and superexchange couplings, we aim to check which structure parameters are related to the nature of the nearest-neighbor magnetic interactions, looking for the presence and evolution of order and structure anomalies. Two Ho samples Ho2(Mo1−yMny)2O7 (y = 0.05 and 0.10) keep the spin-glass nature of the parent compositions with a strong distortion of the MoO(1)6 octahedron and of the Mo‒Ho and Mo‒Mo second shell. On the other hand, two Gd2(Mo1−yMny)2O7 samples, ferromagnetic at low temperature in the undoped case, show the appearance on a local scale of a spin-glass transition mirrored by a quite high structure disorder and by a distortion coherent with a lattice frustration after Mn-doping. Therefore, the Gd and Ho Mn-doped samples display a similar frustrated behavior, differently from the corresponding undoped ones.

Magnetoresistance, magnetothermopower, magnetothermal conductivity and magnetostriction in La1-xNaxMnO3[formula omitted

We report the influence of the magnetic field on electrical resistivity (ρ), thermal conductivity (κ), thermopower (S), and linear thermal expansion (ΔL/L0) in La1-xNaxMnO3 series (x = 0, 0.01, 0.02, and 0.05) over a wide range of temperature. While LaMnO3 is an antiferromagnetic insulator, the doping with Na weakens the inter-planner antiferromagnetic coupling and transforms it into a ferromagnetic metal by promoting the double exchange coupling between Mn3+ and Mn4+ ions. ρ and S show peaks, whereas, κ shows a sudden increase at the onset of ferromagnetic spin ordering in the samples. Dominant charge carriers are holes in these doped compounds as indicated by the positive sign of S. High-temperature transport is explained in the context of the small polaron hopping model. External magnetic field influences thermal properties (S and κ), and ΔL/L0 mostly around the Curie temperature (TC), while it affects ρ to as low as 10 K. Enhancement of κ at the insulator-metal transition, on the application of external magnetic field, suggests a decrease in spin-wave scattering. The highest value of negative magnetoresistance (~69.7%) and negative magnetothermopower (~80.4%) is obtained for x = 0.02 and the value decreases for higher dopant concentration.

Differentiation of non-volatile strain and ferroelectric field effects in (011)- and (001)-La0.67Sr0.33MnO3/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures

La0.67Sr0.33MnO3 (LSMO) thin films were fabricated on (011)- and (001)-Pb(Mg1/3Nb2/3)1 − xTixO3 (PMN-PT) substrates to investigate the mechanism of electric field regulation. Both (011)- and (001)-LSMO/PMN-PT heterostructures exhibit electric-field-dependent resistance curves with an asymmetric butterfly shape under a bipolar electric field (±10 kV/cm). Confirmed through a mathematical method, the “butterfly” and “asymmetric” characteristics indicate the volatile strain and non-volatile ferroelectric field effects, respectively. However, by applying a unipolar electric field, the electric-field-dependent resistance curves of the (011)-LSMO/PMN-PT heterostructures present a larger loop shape than those in the (001)-LSMO/PMN-PT heterostructure, which is related to the non-volatile strain effect. The volatile strain, non-volatile strain, and ferroelectric field effects on the resistance of the LSMO films are associated with the enhanced or weakened double exchange coupling.

High Curie temperature in halfmetallic ferromagnets (Zn, Cr, Ti)Se and (Zn, Cr, Ti)Te for spintronic devices: Ab initio and Monte Carlo treatments

The current study is purposed to investigate the magnetic- and electronic-properties of Zn1-2xCrxTixSe and Zn1-2xCrxTixTe systems by means of Ab initio and Monte Carlo calculations. We have predicted that the two systems show ferromagnetic halfmetallic behavior with 100% spin polarization at the Fermi level and their ferromagnetic stability is attributed to the double exchange coupling. Our calculations suggest further that these compounds exhibit a 2nd order ferromagnetic transition with high Curie temperature. Thus, (Zn, Cr, Ti)Se and (Zn, Cr, Ti)Te compounds are strong candidates for spintronic devices, especially for magnetic random access memories (MRAM) based on the spin transfer torque.

Room Temperature Blocked Magnetic Nanoparticles Based on Ferrite Promoted by a Three-Step Thermal Decomposition Process.

Exchange coupled nanoparticles that combine hard and soft magnetic phases are very promising to enhance the effective magnetic anisotropy while preserving sizes below 20 nm. However, the core-shell structure is usually insufficient to produce rare earth-free ferro(i)magnetic blocked nanoparticles at room temperature. We report on onion-type magnetic nanoparticles prepared by a three-step seed mediated growth based on the thermal decomposition method. The core@shell@shell structure consists of a core and an external shell of Fe3-δO4 separated by an intermediate Co-doped ferrite shell. The double exchange coupling at both core@shell and shell@shell interfaces results in such an increased of the magnetic anisotropy energy, that onion-type nanoparticles of 16 nm mainly based on iron oxide are blocked at room temperature. We envision that these results are very appealing for potential applications based on permanent magnets.

Ferrimagnetism and ferromagnetism behavior in (C, Mn) co-doped SnO2 for microwave and spintronic: Ab initio investigation

Abstract In this paper, the challenge is to investigate the possibility to have the ferromagnetic or/and ferrimagnetic behavior connected to Sn0.98Mn0.02(O1-xCx)2 system at low concentration (x = 0.02, 0.03, 0.04 and 0.05) at room temperature (RT). We utilized the Korringa-Kohn-Rostoker method combined with the Coherent Potential Approximation (KKR-CPA). The C-doped SnO2 induced the half-metallic characteristic located on the Fermi level and the presence of the double-exchange coupling type belongs to the ferromagnetic (FM). The Sn0.98Mn0.02O2 shows the antiferromagnetic (AFM) stability belongs to super-exchange coupling type. For Sn0.98Mn0.02(O1-xCx)2 system, we predicted two statements, the presence of the FM stability where (x = 0.03) and the ferrimagnetic stability where (x = 0.02, 0.04, 0.05). The Curie temperature (TC) can be estimated within the mean field approximation (MFA), our calculations predicted 358.45 K for Sn(O0.95C0.05)2 and 402.26 K for Sn0.98Mn0.02(O0.95C0.05)2. The ferrimagnetic materials are useful for many applications.

Antiferromagnetic successive superexchange interactions underlying ferromagnetic couplings in codoped diluted magnetic semiconductors

Our recent works have revealed that the magnetic coupling among the magnetic codopants in diluted magnetic semiconductors and doped transition metal oxides has a strong local feature. This was attributed to successive spin polarizations induced by the codopants to their neighboring anion ligands. In the present work, we analyze and refine the successive spin polarization based magnetic coupling using results of ab initio calculations and assign the magnetic coupling among the magnetic codopants to a combination of superexchange and double-exchange interactions. In particular, it is shown that antiferromagnetic successive superexchange interactions can lead to a ferromagnetic coupling between two magnetic dopants mediated by a suitable codopant with the latter forming a ferromagnetic double exchange coupling with its first nearest neighbor anions which couple it with the magnetic cations. This is exemplified by ab initio results for the magnetic coupling of two Co-dopants in the presence of a mediated Cu codopant in the environment of various hosts, namely ZnO, GaN, GaP, TiO2, CdS and SnO2. Additional results for other codopant pairs in various hosts are also presented.

Orbitronic effect on the magnetic reconstruction of monocell SrMnO3 on ferroelectric BaTiO3 substrate

The magnetic reconstruction of doped LaMnO3 on the ferroelectric substrate intrinsically involves sizeable carrier inhomogeneity on the scale of one-unit cell due to the charge screening effect at the interface. The small variation in Mn-d valence caused by the inhomogeneity may be responsible for the conflicting magnetic patterns predicted for thick magnetic films. To avoid the complexity resulting from the carrier inhomogeneity, we consider in this paper the magnetic reconstruction of a rigorously carrier homogeneous monocell SrMnO3 film on a BaTiO3 substrate. Stoichiometric SrMnO3 is used to further prevent the film from structural defects and associated Anderson's localization. Our study showed that magnetic reconstruction takes place between nearest-neighbor antiferromagnetic to ferromagnetic structures during polarization reversal of the substrate even if the ground state of bulk SrMnO3 is far away from the magnetic phase boundary. Furthermore, the estimated effective magnetoelectric coupling coefficient is 6.1 × 10−2 Oe cm V−1. It is more than doubled than those values obtained for thick magnetic films. Our detailed analysis suggested that the magnetic reconstruction is dictated by the modulated double-exchange ferromagnetic coupling mediated by Mn-d(eg)-orbitals against the superexchange antiferromagnetic coupling mediated by d(t2g)-orbitals. The nearest-chain antiferromagnetic structure also appears during the polarization reversal process.

Tuning the interfacial charge, orbital, and spin polarization properties in La0.67Sr0.33MnO3/La1−xSrxMnO3 bilayers

The possibility of controlling the interfacial properties of artificial oxide heterostructures is still attracting researchers in the field of materials engineering. Here, we used surface sensitive techniques and high-resolution transmission electron microscopy to investigate the evolution of the surface spin-polarization and lattice strains across the interfaces between La0.66Sr0.33MnO3 thin films and low-doped manganites as capping layers. We have been able to fine tune the interfacial spin-polarization by changing the capping layer thickness and composition. The spin-polarization was found to be the highest at a critical capping thickness that depends on the Sr doping. We explain the non-trivial magnetic profile by the combined effect of two mechanisms: On the one hand, the extra carriers supplied by the low-doped manganites that tend to compensate the overdoped interface, favouring locally a ferromagnetic double-exchange coupling. On the other hand, the evolution from a tensile-strained structure of the inner layers to a compressed structure at the surface that changes gradually the orbital occupation and hybridization of the 3d-Mn orbitals, being detrimental for the spin polarization. The finding of an intrinsic spin-polarization at the A-site cation observed in x-ray magnetic circular dichroism (XMCD) measurements also reveals the existence of a complex magnetic configuration at the interface, different from the magnetic phases observed at the inner layers.

Structure, electrical and magnetic properties of La0.67Ca0.33−x K x MnO3 polycrystalline ceramic

A systematic investigation of structure, electrical and magnetic properties of polycrystalline ceramics La0.67Ca0.33−x K x MnO3 (x = 0.05, 0.10, 0.15, 0.20, 0.25) samples, prepared by sol–gel method had been undertaken. As K content increases the crystal structures were transformed from orthorhombic to rhombohedral structure identified by X-ray diffraction, and the effect of increasing K ion is to increment the Mn–O–Mn bond angle. The surface morphology was investigated by scanning electron microscope, which indicates that grain size decreasing with increasing of K+. Temperature dependence of resistivity (ρ − T) was measured by standard four-probe method. The insulator–metal transition temperature (T P ) shifted to higher temperature and the temperature coefficient of resistivity decreased sharply with the substitution K+ for Ca2+ ion. The temperature dependence of magnetization (M–T) shown that Curie temperature (T C ) was increasing with the increase of K content, which can be explained by enhancement of double–exchange interaction. The data of resistivity on low-temperature (T < T P ) had been fitted with the relation ρ(T) = ρ 0 + ρ 2T2 + ρ 4.5T4.5; the high-temperature (T > T P ) resistivity data were explained using small-polaron hopping and variable-range hopping models. Resistivity data in whole temperature range (100–320 K) could be fitted by percolation model. Polaron activation energy E a was found to decrease with the content K+ increasing, which suggested that K doping increase bond angle Mn–O–Mn, thereby the effective band gap was decreased and the double exchange coupling was increased of, this is the reason for the decrease of resistivity.

Exchange couplings calculation in A2CrMoO6(A= Sr, Ca) Double Perovskites

Exchange couplings in A2CrMoO6 (A = Sr, Ca) Double Perovskites have been calculated for the first time by using a Monte Carlo Simulation and a relation between internal energy per site and magnetization per site. This work shows how effects of A-site substitution could be captured by an overall scaling factor 'alpha', which allows to deduce a numerical estimation of exchange couplings in question. Thermal dependence of magnetization per site, magnetic susceptibility, internal energy per site and specific heat have been investigated. Super Exchange coupling has been found equal to −7,488meV in the presence of Sr and −1,977meV in the presence of Ca. Double Exchange couplings corresponding respectively to Cr and Mo sublattices have been found equal to 17.473meV and 5.611meV in the presence of Sr and 4.614meV and 1.481meV in the presence of Ca. Internal energy per site for both materials at low temperatures has been given.

Structural and magnetic properties of perovskite SrMnO3 thin films grown by molecular beam epitaxy

The structural and magnetic properties of the epitaxial (0 0 1) SrMnO3 (SMO) thin film grown on (0 0 1) SrTiO3 (STO) substrate by molecular beam epitaxy (MBE) are reported. In-situ reflection high energy electron diffraction (RHEED) intensity oscillation is utilized to precisely adjust the stoichiometry by tuning the atom source shuttered time during the growth. The surface of the thin film with obvious steps and terraces is observed to be atomically flat. The epitaxial single crystal SMO thin film which is grown along the direction of (0 0 1) STO substrate is confirmed to be four-fold symmetric along the 〈0 0 1〉 azimuth by X-ray diffraction φ scan. The XRD reciprocal space map indicates that the SMO thin film is effectively strained in the present thickness and is typical tetragonal crystal structure. It is found that the present SMO thin film is ferromagnetic. The factors, namely Mn3+-Mn4+ double exchange coupling, oxygen vacancies and the strain, lead the SMO thin films to produce ferromagnetism.