Antiferromagnetic Superexchange Interactions Research Articles

Study on the structural and magnetic properties of Fe/Co co-doped Bi4Ti3O12 ceramics

Polycrystalline Bi4(Ti3-xFex/2Cox/2)O12 (0 ≤ x ≤ 0.4) ceramics are prepared by solid state reaction method. The low temperature ferromagnetism is successfully induced by Fe/Co co-doping. Particularly, the x = 0.3 ceramics possess the largest remnant magnetization (2Mr) of about 0.272 emu/g at 100 K. Furthermore, all the doped ceramics undergo a ferromagnetic to paramagnetic transition, and show typical spin glass behavior, which is confirmed by the isothermal remnant magnetization and Memory Effect. The magnetic mechanisms probably originate from the coexistence and competition between ferromagnetic double exchange interactions and antiferromagnetic superexchange interactions, which lead to the appearance of the spin glass behavior in the doped ceramics and the strongest ferromagnetism in the x = 0.3 ceramics. The present work provides basic data and theoretical reference for the exploitation of the new multifunctional materials.

Magnetic Properties and Mössbauer Investigations of La0.67Sr0.33FexMn1−xO3 (x = 0 and 0.5) Perovskites

We report magnetic phase separation in pure La0.67Sr0.33FexMn1−xO3 (x = 0 and 0.5) ceramic samples. Rhombohedral crystal structures obtained with R3C space group show no significant changes in the lattice structures due to the close values of ionic radii of Fe3+ and Mn3+ ions. However, significant changes occur in the magnetic properties due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interactions because of the presence of Fe3+ substitution. The FM double exchange interactions (DE) (Mn3+–O–Mn4+) appears suppressed by AFM superexchange interactions (Fe3+–O–Mn4+, Fe3+–O–Fe3+, Mn4+–O–Mn4+). The saturation magnetization for x = 0 has the highest value of approximately 80 emu/g at 2 K, which dropped significantly to approximately 5 emu/g at 2 K for the x = 0.5 sample. The coercivity (Hc) for x = 0.5 increased from 161 Oe at 300 K to 786 Oe at 2 K compared with that of x = 0 which varied from 110 to 150 Oe. A value of Hc(0) = 1.5 ± 0.04 kOe was extrapolated from the model fit to the temperature dependence of Hc. The temperature-dependent 57Fe Mossbauer spectroscopy measurements show the presence of Fe in two different environments that have an AFM Fe3+–O–Mn4+ and Fe3+/2+–O–Fe3+/2+ coordination. The unequal fraction population of Fe ions at both AFM environments seems to increase the exchange bias effect.

Predicting diamond-like Co-based chalcogenides as unconventional high temperature superconductors

We predict Co-based chalcogenides with a diamond-like structure can host unconventional high temperature superconductivity (high-Tc). The essential electronic physics in these materials stems from the Co layers with each layer being formed by vertex-shared CoA4 (A=S, Se, Te) tetrahedra complexes, a material genome proposed recently by us to host potential unconventional high-Tc close to a d7 filling configuration in 3d transition metal compounds. We calculate the magnetic ground states of different transition metal compounds with this structure. It is found that (Mn, Fe, Co)-based compounds all have a G-type antiferromagnetic (AFM) insulating ground state while Ni-based compounds are paramagnetic metal. The AFM interaction is the largest in the Co-based compounds as the three t2g orbitals all strongly participate in AFM superexchange interactions. The abrupt quenching of the magnetism from the Co to Ni-based compounds is very similar to those from Fe to Co-based pnictides in which a C-type AFM state appears in the Fe-based ones but vanishes in the Co-based ones. This behavior can be considered as an electronic signature of the high-Tc gene. Upon doping, as we predicted before, this family of Co-based compounds favor a strong d-wave pairing superconducting state.

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.

Local structure and magnetic properties of Mn3+–O–Fe3+ superexchange interaction in an oxygen-vacant perovskite: Experimental and theoretical study

In this work we report an experimental and theoretical study on the local structure and magnetic properties of Mn3+–O–Fe3+ superexchange interaction for an oxygen-vacant perovskite PrBaMnFeO5+δ with δ ∼ 0.5. Structural studies were performed by high resolution X-Ray diffraction technique in normal and anomalous scattering, modeled using an orthorhombic biphasic system corresponding to an oxygen-rich and oxygen-poor stoichiometries. Mössbauer spectroscopy showed the presence of two orthorhombic iron sites, one ascribed to a fully octahedral coordination and another, highly distorted, ascribed to a pyramidal coordination, in full agreement with theoretical DFT + U + SO calculations. The samples showed a spin-glass transition at ∼20 K and mainly the presence of Mn3+–O–Fe3+ type-G antiferromagnetic superexchange interactions. However, our theoretical calculations using first principles also suggested that the presence of oxygen vacancies in certain conditions could be possibly promoting local ferromagnetic interactions.

Evolution from Griffiths like phase to non-Griffiths like phase with Y doping in [formula omitted

Detailed analysis of the temperature dependent dc susceptibility data shows the modification from the Griffiths like phase (GP) to non-Griffiths like phase (non-GP) in (La1-xYx)0.7Ca0.3MnO3(x=0,0.3,0.4,0.5,0.6) compounds with x. The existence of non-Griffiths phase is revealed from the upward deviation of the inverse susceptibility (1/χ) versus temperature plot from the Curie-Weiss behavior for the compounds with x⩾0.5. Whereas Griffiths phase is identified from the downturn of 1/χ versus T plot from the Curie-Weiss line in the compounds with x=0.3,0.4. The increased lattice distortion (with x) decreases the M-O-Mn bond angle and at the same time increase the electron-phonon (e-ph) coupling which in turn decreases the ferromagnetic double exchange (DE) interactions and enhances the effective short-range antiferromagnetic superexchange interactions between Mn3+/Mn3+ and Mn4+/Mn4+ ions above the Curie temperature (TC). The enhancement of this short-range antiferromagnetic (SR-AFM) interactions have been attributed to the conversions from GP to non-GP phase. The increased SR-AFM interactions with Y doping has also been substantiated from the magnetocaloric study.

Site-Selective Probe of Magnetic Excitations in Rare-Earth Nickelates Using Resonant Inelastic X-ray Scattering

We use high-resolution resonant inelastic x-ray scattering (RIXS) to study a thin film of NdNiO3, a compound whose unusual spin- and bond-ordered electronic ground state has been of long-standing interest. Below the magnetic ordering temperature, we observe well-defined collective magnon excitations along different high-symmetry directions in momentum space. The magnetic spectra depend strongly on the incident photon energy, which we attribute to RIXS coupling to different local electronic configurations of the expanded and compressed NiO6 octahedra in the bond-ordered state. Both the noncollinear magnetic ground state and the observed site-dependent magnon excitations are well described by a model that assumes strong competition between the antiferromagnetic superexchange and ferromagnetic double-exchange interactions. Our study provides direct insight into the magnetic dynamics and exchange interactions of the rare-earth nickelates and demonstrates that RIXS can serve as a site-selective probe of magnetism in these and other materials.Received 3 August 2017Revised 20 June 2018DOI:https://doi.org/10.1103/PhysRevX.8.031014Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAntiferromagnetismExchange interactionMagnonsSpin wavesPhysical SystemsComplex materialsStrongly correlated systemsTechniquesExact diagonalizationHeisenberg modelResonant inelastic x-ray scatteringX-ray absorption spectroscopyCondensed Matter, Materials & Applied Physics

Influence of Ti-doping on the magnetic exchange interaction of La0.5Ca0.5MnO3 nanoparticles

The striking effect of Ti-doping on the magnetic properties of La0.5Ca0.5MnO3 nanoparticles have been investigated by different methods. The half doped manganites are at the boundary between the ferromagnetic metallic and antiferromagnetic insulating phases. There is a delicate balance between ferromagnetic double exchange and antiferromagnetic superexchange interactions. In half doped La0.5Ca0.5MnO3 charge ordered antiferromagnetic state is destabilized below a critical size followed by development of ferromagnetism. We have seen that slight disturbance in magnetic exchange interactions by Ti-substitution (>5%) drastically changes the fraction of ferromagnetic clusters. The ferromagnetic Mn3+—O—Mn4+ interaction is suppressed by Mn3+—O—Ti4+ type of interactions. For more than 10% Ti doping the surface ferromagnetic character of the nanoparticles is largely weighed down. This suppression of ferromagnetic component by diamagnetic Ti-doping eventually leads to phase separated states with lower magnetization value.

Handling magnetic and structural properties of EuMnO3 thin films by the combined effect of Lu doping and substrate strain

This work aims to understand the alterations induced by film/substrate lattice mismatch in structure, lattice dynamics and magnetic response of orthorhombic Eu1-xLuxMnO3 thin films within the range 0 ≤ x ≤ 0.4, when compared to results reported for ceramics with analogous composition. Thin films, which have been deposited onto Pt/Ti/SiO2/Si(100) oriented substrates via chemical method, exhibit noteworthy modifications in the magnetic ordering properties and, contrary to ceramics, do not show any sharp phase transition to the paramagnetic state. This reveals an induced ferromagnetic response in the films which is stable up to 100 K. X-ray diffraction and Raman spectroscopy measurements have been performed to identify the mechanically compressive state induced by the substrate and Lu doping. This facilitates insight into the magnetoelastic coupling effect in these films which is driven by alterations in electronic orbital overlapping and the associated antiferromagnetic superexchange interactions.

Collective Superexchange and Exchange Coupling Constants in the Hydrogenated Iron Oxide Particle Fe8O12H8.

Motivated by the fact that Fe2O3 nanoparticles are used in the treatment of cancer, we have examined the role of ligands on the magnetic properties of these particles by focusing on (Fe2O3)4 as a prototype system with H as ligands. Using the Broken-Symmetry Density Functional Theory, we observed a strong collective superexchange in the hydrogenated Fe8O12H8 cluster. The average antiferromagnetic exchange coupling constant between the four iron-iron oxo-bridged pairs was found to be -178 cm-1, whereas coupling constants between hydroxo-bridged pairs were much smaller. We found that despite the apparent symmetry of the iron atom framework, it is not reasonable to assume this symmetry when fitting the exchange coupling constants. We also analyzed the geometrical and magnetic properties of Fe8O12H n for n = 0-12 and found that hydrogenating oxo-bridges would generally inhibit the Fe-O-Fe antiferromagnetic superexchange interactions. Antiferromagnetic lowest total energy states become favorable only when specific distributions of hydrogen atoms are realized. The (HO)4-Fe4(all spin-up)-O4-Fe4(all spin-down)-(OH)4 configuration in Fe8O12H8 presents such an example. This symmetric configuration can be considered a superdiatomic system.

Structural and magnetic studies of the ruthenium perovskites Ba2-xSrxHoRuO6

The series of ruthenate double perovskites Ba2-xSrxHoRuO6 (0 ≤ x ≤ 2) have been synthesized using solid state methods. The crystal structures of the series have been determined by high resolution synchrotron X-ray diffraction and show the sequence of structures to be Fm3̅m (a0a0a0) (0 ≤ x ≤ 0.6) → I4/m (a0a0c-) (x = 0.8) → I2/m (a0b-b-) (1.0 ≤ x ≤ 1.2) → P21/n (a-a-c+) (1.4 ≤ x ≤ 2.0). A similar progression of structures is observed in the sample BaSrHoRuO6 with increasing temperature. Magnetic characterisation of these materials was undertaken utilising variable temperature bulk magnetic susceptibility, isothermal magnetisation and low temperature neutron powder diffraction measurements. All members of the series order antiferromagnetically with the Ru and Ho sublattices ordering at different temperatures between 50 and 16 K. The presence of the magnetic Ho3+ at the perovskite B site stabilises the antiferromagnetic (AFM) ordering of the Ru sublattice. The addition of Sr for Ba beyond x = 1.2 weakens the AFM superexchange interactions through induced structural distortions, resulting in a canted AFM ground state.

Evolution from non-Griffiths phase to Griffiths phase: Giant enhancement of magnetoresistance in nanocrystalline (La0.4Y0.6)0.7Ca0.3MnO3 compound

The particle size driven modification of the non-Griffiths phase to Griffiths phase in (La0.4Y0.6)0.7Ca0.3MnO3 (LYCMO) compound have been presented here. In the nanocrystal, the decreased lattice distortions together with the quenched disorder arising from the ionic size mismatch of the different A-site ions are the possible reason for the occurrence of Griffiths phase. On the other hand, though in the bulk compound quenched disorder is present but the higher distortions enhance the effective antiferromagnetic superexchange interactions and may be the probable reason for the non-Griffiths phase which is identified by the upturn of the inverse susceptibility versus temperature plot from the Curie-Weiss line. An enhancement of Colossal magnetoresistance (CMR) in the minimal surface disorder nanoparticle (∼120nm) has been observed. The study shows that for LYCMO compound, the enhancement of CMR in 120 nm nanoparticle is due to the conversion from non-Griffiths phase to Griffiths phase.

Asperomagnetic order in diluted magnetic semiconductor (Ba,Na)(Zn,Mn)2As2

We report an investigation of magnetic ordering in (Ba,Na)(Zn,Mn)2As2 (BNZMA) single crystals, a type of II-II-V magnetic semiconductor based on BaZn2As2, with a combination of magnetic, electron transport, and spin polarization measurements. At temperatures above 90 K, the susceptibility is found to follow the Curie-Weiss law with a Curie-Weiss temperature of 16 K. At low temperatures, both the dc and ac susceptibilities exhibit spin-glass-like features and the electron spin polarization is determined to be 48 ± 8% using Andreev reflection spectroscopy. Our experimental results suggest that the ground state of BNZMA has an asperomagnetic order, an intermediate regime between a ferromagnet and a canonical spin glass. Such a partially spin polarized glassy phase can be attributed to the competition between the short range antiferromagnetic superexchange interaction and the long range ferromagnetic/antiferromagnetic exchange interactions mediated by high-density holes.

First-principles DFT study new series of ruthenates double perovskites Ba2MRuO6 with M = Sc, Ti, V, Cr, Mn, Fe and Co

First-principles density functional theory (DFT) within generalized gradient approximation (GGA) and GGA incorporated with Coulomb correlation interaction U (GGA + U) are used to investigate the crystal, electronic, magnetic and optical properties of a new series containing barium-ruthenates double perovskites Ba2MRuO6, with M is being 3d transition-metal ion (M = Sc, Ti, V, Cr, Mn, Fe and Co). All these compounds crystallize in a cubic structure at room-temperature fitting in space-group Fm-3m, with a = b = c ≈ 2a0 (a0 = 4 Å). Ba2MRuO6 oxides show a long-range antiferromagnetic superexchange interaction between neighboring 3d and 4d ions achieved via M3+ (3dn)↑– O2−(2p6)–Ru5+ (4d3)↓. Metallic state exists in Ba2MRuO6 within GGA for M = Sc, Ti, V, Mn, Fe and Co, and semimetallic exists in M = Cr case. While, GGA + U produces half-metallic (HM) state in M = Ti, V, Mn and Co, and insulating state in M = Sc, Cr and Fe cases. In the rest of the paper, total and partial density of states, magnetic moments, bond-valence sums, charge transfer in double perovskites Ba2MRuO6 are computed and discussed in detail. In addition, the real and imaginary parts of the optical dielectric function and energy-loss spectra are computed and the main features shown by their spectra are ascertained on the base of the investigation of density of states.

Charge injection driven switching between ferromagnetism and antiferromagnetism in transitional metal-doped MoS2 materials

Using first-principles calculations, we report on charge injection induced switching between ferromagnetic (FM) and antiferromagnetic (AFM) in a 2H- monolayer. monolayers doped with different transition metals (TM)—Fe and Mn—initially demonstrate FM and AFM magnetic ground state, respectively. Once the injected charge approaches e/unit, the systems respectively tend to AFM and FM states, due to the modulation effect of the exchange splitting of spins via injected charge. The interesting switch between FM and AFM can be explained by the competition between FM double-exchange and AFM super-exchange interaction. In contrast, the / heterojunction, because of the direct bonding between dopant TM atoms, remains in the AFM state even under charge injection. These findings point toward the possible development of spintronic switch devices using charge injection in TM doped materials, which could be pivotal to information storage and spintronic applications.

Competing ferromagnetic and anti-ferromagnetic interactions in iron nitride ζ-Fe2N

The paper discusses the magnetic state of zeta phase of iron nitride viz. ζ-Fe2N on the basis of spin polarized first principles electronic structure calculations together with a review of already published data. Results of our first principles study suggest that the ground state of ζ-Fe2N is ferromagnetic (FM) with a magnetic moment of 1.528μB on the Fe site. The FM ground state is lower than the anti-ferromagnetic (AFM) state by 8.44 meV and non-magnetic (NM) state by 191 meV per formula unit. These results are important in view of reports which claim that ζ-Fe2N undergoes an AFM transition below 10 K and others which do not observe any magnetic transition up to 4.2 K. We argue that the experimental results of AFM transition below 10 K are inconclusive and we propose the presence of competing FM and AFM superexchange interactions between Fe sites mediated by nitrogen atoms, which are consistent with Goodenough-Kanamori-Anderson rules. We find that the anti-ferromagnetically coupled Fe sites are outnumbered by ferromagnetically coupled Fe sites leading to a stable FM ground state. A Stoner analysis of the results also supports our claim of a FM ground state.

Structure and magnetic properties in mixed valent LaCr0.7Cu0.3O3 ceramics

Single-phase LaCr0.7Cu0.3O3 ceramics was fabricated successfully using high pressure sintering method. A mixed valent state with the coexistence of Cr2+，Cr3+ and Cr4+ was confirmed by X-ray photoelectron spectroscopy measurement. Magnetic measurement results indicate that in addition to antiferromagnetic superexchange interaction of Cr3+ -O2- -Cr3+ accompanied by weak ferromagnetism at high temperature, two types of short-range order ferromagnetism can be observed at low temperature, which could be attributed to double exchange interactions of Cr2+ -O2- -Cr3+ and Cr3+ -O2- -Cr4+, respectively. The results here would enrich the understanding on magnetic interactions in rare-earth chromites.

Temperature dependent magnetic Compton profiles and first-principles strategies of quaternary half-Heusler alloy Co1−xCuxMnSb(0 ⩽ x ⩽ 0.8)

Temperature dependent experimental Compton profiles of quaternary alloys Co1−xCuxMnSb (x = 0.0, 0.2, 0.6 and 0.8) when decomposed into constituent profiles show that the sp-electron spin polarization is antiferromagnetically coupled to Mn-3d moments. The orbital magnetic moments derived from combination of magnetic Compton profiles (MCPs) and magnetization measurements are found to be small. Moreover, the first-principles full potential-linearized augmented plane wave (FP-LAPW) calculations have been performed to validate the experimental investigations of spin moments and half-Heusler properties. Present experimental and theoretical work show major role of Mn atoms in building-up the absolute spin moments. Our MCP data and spin-projected density of states derived from FP-LAPW computations show an increase in sp-d interaction in conduction region on increasing the Cu concentration. Further, Ruderman–Kittel–Kasuya–Yosida-type hybridization and antiferromagnetic superexchange interactions are witnessed in the reported alloys.

Ab initio prediction of the electronic, magnetic and topological properties of Ln2O3 clusters

The structural, electronic and magnetic properties of the lanthanide oxide Ln2O3 clusters, where Ln signifies lanthanides from La to Lu, have been calculated using spin-polarized density functional theory with the B3LYP hybrid functional. The intensities of ferromagnetic RKKY interaction are found comparable with that of antiferromagnetic superexchange interaction in Ce2O3 / Pr2O3 / Nd2O3 / Gd2O3 / Tb2O3 / Tm2O3 clusters, while the other Ln2O3 clusters prefer ferromagnetic states to antiferromagnetic states in energy, except nonmagnetic La2O3 and Lu2O3 clusters. The theoretical spin magnetic moments, calculated three-dimensional spin density maps and dipole moments of Ln2O3 clusters suggest that the induced polarizations of oxygen atoms in Sm2O3, Eu2O3 and Yb2O3 clusters remarkably lead to the elongated Ln–O bond lengths in these clusters. The partial density of states of Ln2O3 clusters reveals that Sm3+ /Eu3+ /Yb3+ ions are distinctive from other Ln3+ ions in that their Ln-4f electrons are strongly hybrid with O-2 p electrons. The topological analysis of the electron density was also performed with quantum theory of atoms in molecules, which indicates the ionic Ln–O bonds have partial covalent characteristics.

Antiferromagnetic CsCrF5 and canted antiferromagnetism in RbCrF5 and KCrF5

In ACrF5 (A=Cs, Rb, K), Cr(IV) ions are coordinated by six fluoride ligands where the resulting CrF6 octahedra share cis vertexes to form infinite chains of ([CrIVF5]−)n. The geometry of the latter in Cs compound differs from that in K and Rb compounds. The results of investigations of the magnetic behaviour of these compounds have shown that an antiferromagnetic superexchange interaction is present within the chains with JCs=−10.2cm−1, JRb=−13.3cm−1, and JK=−13.1cm−1. Additional ferromagnetic-like long-range ordering has been observed in KCrF5 and RbCrF5 below 6K which can be explained, in a correlation with their crystal structures, as canted antiferromagnetism.