Magnetic Superexchange Interactions Research Articles

Controlling Noncollinear Ferromagnetism in van der Waals Metal-Organic Magnets.

Van der Waals (vdW) magnets both allow exploration of fundamental 2D physics and offer a route toward exploiting magnetism in next generation information technology, but vdW magnets with complex, noncollinear spin textures are currently rare. We report here the syntheses, crystal structures, magnetic properties and magnetic ground states of four bulk vdW metal-organic magnets (MOMs): FeCl2(pym), FeCl2(btd), NiCl2(pym), and NiCl2(btd), pym = pyrimidine and btd = 2,1,3-benzothiadiazole. Using a combination of neutron diffraction and bulk magnetometry we show that these materials are noncollinear magnets. Although only NiCl2(btd) has a ferromagnetic ground state, we demonstrate that low-field hysteretic metamagnetic transitions produce states with net magnetization in zero-field and high coercivities for FeCl2(pym) and NiCl2(pym). By combining our bulk magnetic data with diffuse scattering analysis and broken-symmetry density-functional calculations, we probe the magnetic superexchange interactions, which when combined with symmetry analysis allow us to suggest design principles for future noncollinear vdW MOMs. These materials, if delaminated, would prove an interesting new family of 2D magnets.

Novel ternary AgIICoIIIF5 fluoride: synthesis, structure and magnetic characteristics.

We present a new compound in the silver-cobalt-fluoride system, featuring paramagnetic silver (d9) and high-spin cobalt (d6), synthesized by solid-state method in an autoclave under F2 overpressure. Based on powder X-ray diffraction, we determined that AgIICoIIIF5 crystallizes in a monoclinic system with space group C2/c. The calculated fundamental band-gap falls in the visible range of the electromagnetic spectrum, and the compound has the character of charge-transfer insulator. AgCoF5 is likely a ferrimagnet with one predominant superexchange magnetic interaction constant between mixed spin cations (Ag-Co) of -62 meV (SCAN result). Magnetometric measurements conducted on a powdered sample allowed the identification of a transition at 128 K, which could indicate magnetic ordering.

First-Principles Studies on the Structural and Electronic Properties of α-Na2FePO4F with Strong Antisite Disorder.

Polyanionic Na2FePO4F is one of the most important cathode materials for sodium-ion batteries. The orthorhombic β-Na2FePO4F material has been studied extensively and intensively since it was proposed. In this article, a novel monoclinic sodium phosphate fluoride α-Na2FePO4F is concerned. Kirsanova's experiment showed that Na and Fe ions in α-Na2FePO4F are prone to antisite, leading to strong antisite disorder. Through first-principle calculations, we show that the steric effect, the magnetic exchange and superexchange interactions between transition-metal cations are shown to be the main driving forces for Na+/Fe2+ antisite disorder. We first calculated the crystal structures, electronic properties, and cohesive energies of all the 10 antisite phases of α-Na2FePO4F and β-Na2FePO4F. Then, we compared the difference charge densities, magnetism, binding energies, and electrostatic potentials of α-Na2FePO4F and β-Na2FePO4F materials in the antisite and pristine phases. In α-Na2FePO4F, the binding energy of the antisite phase with the lowest binding energy is almost degenerate with that of the pristine phase. Moreover, only small differences of the electrostatic potential and the charge density distribution are found between the antisite (with lowest energy) and the pristine phases of α-Na2FePO4F, which also helped elaborate the facile formation of Na+/Fe2+ antisite in the α-Na2FePO4F material. Our research contributes to the understanding of the mechanism of Na+/Fe2+ antisite and the development of high-performance polyanionic cathode materials.

Analysing the super exchange interaction of Fe3+-O2--Cr3+ on multiferroic structured BiFeO3/LaCrO3 nanocomposite

Herein, we report a successful attempt to synthesise a material based on the theory proposed by Goodenough and Kanamori et al. This report details the super-exchange magnetic interaction of Fe3+-O2--Cr3+ taking place between BiFeO3/LaCrO3 nanocomposite. The tilt in FeO6's octahedral sites cancels out the tilt in the CrO6 site, making it an ideal perovskite explained through nearest neighbouring interaction (NN), with the next nearest neighbour interaction (NNN) pointing the way to possible ferromagnetic exchange in Fe3+-O2--Cr3+. The significant enhancement of retentivity (residual magnetism) in BiFeO3/LaCrO3 nanocomposite is three times that of the pristine material. The material's high coercive nature (351.35 Oe) makes it suitable for hard magnet applications. The correlation between magnetic anisotropy value and the first-order derivative (dM/dH) of M − H data concludes the successful construction of Fe–O–Cr interaction for magnetic applications.

Impact of the indium content on structural, magnetic, and electrodynamic properties of nanocomposites based on In-substituted Sr hexaferrite and Ni–Zn spinel ferrite with excellent absorption characteristics

SrInxFe12-xO4/Ni0.5Zn0.5Fe2O4 (0.00 ≤ x ≤ 0.04) nanocomposites (SIFO/NZFO NCs) were synthesized using a one-pot sol-gel auto-combustion technique. The EDX method was used to verify the chemical composition. An X-ray diffractometer (XRD) was used for the structural analysis. Scanning electron microscopy (SEM) was used to study the morphological characteristics. The impact of indium doping on the magnetic properties of the nanocomposites was investigated using magnetization measurements at T = 300. A kink was observed in the M − H curves for lower indium content (x ≤ 0.02), which was suppressed with further increasing indium content until it vanished at a higher content. A single peak at a higher indium content replaced two peaks at a lower indium content (x 0.02), according to the switching field distribution (i.e., dM/dH vs. H). When the indium concentration increased, the saturation, remanence magnetization, and coercive field increased until x = 0.01, and then decreased, according to the analysis of the M − H loops. The obtained results reveal that an exchange interaction at the surface of the SIFO and NZFO nanoparticles (NPs) exists. The degree of intergranular dipolar interactions diminishes with increasing indium content within the hard-soft ferrite SIFO/NZFO NCs and becomes insignificant at higher indium contents. Moreover, the core of the SIFO and NZFO NPs also contains magnetic super-exchange interactions between Fe and O ions. At high frequencies (2–20 GHz), a correlation was found between the hard phase composition (indium content) and electrodynamic characteristics. Analyses were performed on the evolution of essential parameters (real and imaginary permittivity and permeability) in relation to the frequency. In this frequency range, the effect of electromagnetic radiation absorption was observed. A significant level of electromagnetic absorption (>-12 dB) in the frequency range was observed for investigated nanocomposites (NCs). The NCs with lower indium content (0.00 ≤ x ≤ 0.01) have higher reflection looses coefficients (−18 … -36 dB). This opens broad perspectives for the practical application of magnetic media for intensive electromagnetic absorption.

Influence of Ba-doping on the structural and physical properties of Sr2−xBaxFeVO6 double perovskites

The influence of A-site Ba-doping on the structural, dielectric, magnetic, and optical properties of double perovskite (BaxSr2−x)FeVO6 (BSFV(x): x = 0.0–2.0) ceramics prepared by solid-state reaction method, is reported. X-ray diffraction patterns and Rietveld refinements revealed that the BSFV(x) samples with x = 0.0–1.0 crystallized in a cubic crystal structure with an Fm3̅m symmetry whereas the samples with x = 1.5 and 2.0 crystallized in a hexagonal crystal structure with R3c symmetry. Their unit cell volumes increased progressively with the Ba-doping content due to the steric effect generated by the progressive increase of the average ionic radius at A-site, 〈rA〉. SEM images revealed the BSFV(x) samples were highly dense with ceramic grains of polyhedral morphology. Their average grain sizes increased from 2.0(2) μm (x = 0.0) to 5.0(3) μm (x = 2.0). X-ray energy dispersive spectroscopy data gave out the chemical compositions of these samples, which were close to the nominal stoichiometric values. FTIR spectra verified the vibrating modes of the [FeO6] and [VO6] octahedrons in the present samples. XPS spectra confirmed dual chemical valence states of Fe element (Fe2+ and Fe3+) and a triple oxidation states of V element (V3+, V4+ and V5+) as well as two species of oxygens in the BSFV(x) samples. The BSFV ceramics exhibited a strongly frequency-dependent dielectric behavior and a diffuse phase transition, which were resulted from the dielectric response of oxygen vacancies and the electron hopping between the variable cations at B-site as well as the B-site cationic random occupancy. Magnetic Curie temperature (TC) and effective magnetic moment (μeff) decreased as increasing the Ba-doping content due to the weakening of superexchange magnetic interactions between the Fe and V ions. The optical band gaps (Eg) of the BSFV(x) samples extracted from UV-Visible spectra, were in the range of 1.94 – 2.42 eV, exhibiting a blue shift towards semiconductor range as increasing the Ba-doping content. The overlapping between the O-2p and Fe/V-3d orbitals contributes to the Eg in the BSFV samples.

Pressure Control of the Structure and Multiferroicity in a Hydrogen-Bonded Metal-Organic Framework.

Multiferroic materials with the cross-coupling of magnetic and ferroelectric orders provide a new platform for physics study and designing novel electronic devices. However, the weak coupling strength of ferroelectricity and magnetism is the main obstacle for potential applications. The recent research focuses on enhancing the coupling effect via synthesizing novel materials in a chemical route or tuning the multiferroicity in the physical way. Among them, pressure is an effective method to modify multiferroic materials, especially when the chemical doping has reached its tuning limit. In this work, we systemically studied the multiferroic properties in a hydrogen-bonded metal-organic framework (MOF) [(CH3)2NH2]Ni(HCOO)3 under high pressure. X-ray diffraction and Raman scattering reveal that a structural phase transition occurs in a pressure region of 6-9 GPa, and the crystal structure is greatly modified by pressure. With the ac magnetic susceptibility, pyroelectric current, and dielectric constant measurements, we obtain the multiferroic property evolution under high pressure and create a temperature-pressure phase diagram. Our study demonstrates that the pressure can modify the magnetic superexchange interaction and hydrogen bonding simultaneously in these perovskite-like MOFs. The multiferroic phase region has been expanded to higher temperature due to the pressure-enhanced spin-phonon coupling effect.

Sorption and Magnetic Properties of Oxalato-Based Trimetallic Open Framework Stabilized by Charge-Assisted Hydrogen Bonds.

We report a new structure of {[Co(bpy)2(ox)][{Cu2(bpy)2(ox)}Fe(ox)3]}n·8.5nH2O NCU-1 presenting a rare ladder topology among oxalate-based coordination polymers with anionic chains composed of alternately arranged [Cu2(bpy)2(ox)]2+ and [Fe(ox)3]3− moieties. Along the a axis, they are separated by Co(III) units to give porous material with voids of 963.7 Å3 (16.9% of cell volume). The stability of this structure is assured by a network of stacking interactions and charge-assisted C-H…O hydrogen bonds formed between adjacent chains, adjacent cobalt(III) units, and alternately arranged cobalt(III) and chain motifs. The soaking experiment with acetonitrile and bromobenzene showed that water molecules (8.5 water molecules dispersed over 15 positions) are bonded tightly, despite partial occupancy. Water adsorption experiments are described by a D’arcy and Watt model being the sum of Langmuir and Dubinin–Serpinski isotherms. The amount of primary adsorption sites calculated from this model is equal 8.2 mol H2O/mol, being very close to the value obtained from the XRD experiments and indicates that water was adsorbed mainly on the primary sites. The antiferromagnetic properties could be only approximately described with the simple CuII-ox-CuII dimer using H = −J·S1·S2, thus, considering non-trivial topology of the whole Cu-Fe chain, we developed our own general approach, based on the semiclassical model (SC) and molecular field (MF) model, to describe precisely the magnetic superexchange interactions in NCU-1. We established that Cu(II)-Cu(II) coupling dominates over multiple Cu(II)-Fe(III) interactions, with JCuCu = −275(29) and JCuFe = −3.8(1.6) cm−1 and discussed the obtained values against the literature data.

CoMOF5(pyrazine)(H2O)2 (M = Nb, Ta): Two-Layered Cobalt Oxyfluoride Antiferromagnets with Spin Flop Transitions.

Two cobalt oxyfluoride antiferromagnets CoMOF5(pyz)(H2O)2 (M = Nb 1, Ta 2; pyz = pyrazine) have been synthesized via conventional hydrothermal methods and characterized by thermogravimetric (TGA) analysis, FTIR spectroscopy, electron spin resonance (ESR), magnetic susceptibility, and magnetization measurements at both static low field and pulsed high field. The single-crystal X-ray diffraction indicates both compounds 1 and 2 are isostructural and crystallize in the monoclinic space group C2/m with a two-dimensional Co2+ triangular lattice in the ab plane, separated by the nonmagnetic MOF5 (M = Nb 1, Ta 2) octahedra along the c-axis with large intertriangular-lattice Co···Co distance. Because of low dimensionality together with frustrated triangular lattice, compounds 1 and 2 exhibit no long-range antiferromagnetic order until ∼3.7 K. Moreover, a spin flop transition is observed in the magnetization curves at 2 K for both compounds, which is further confirmed by ESR spectra. In addition, the ESR spectra suggest the presence of a zero-field spin gap in both compounds. The high field magnetization measured at 2 K saturates at ∼7 T with Ms = 1.55 μB for 1 and 1.71 μB for 2, respectively, after subtracting the Van Vleck paramagnetic contribution, which is usually observed for Co2+ ions with pseudospin spin of 1/2 at low temperature. Powder-averaged magnetic anisotropy of g = 3.10 for 1 (3.42 for 2) and magnetic superexchange interaction J/kB = -3.2 K for 1 (-3.6 K for 2) are obtained.

Observation of magnetoelectric effect in CuO-doped SrBi4Ti4O15 ceramics

Room temperature magnetoelectric coupling effect was observed in CuO-doped SrBi4Ti4O15 lead-free ferroelectric ceramics. Interstitial doping of CuO resulted in the formation of secondary phases at lower angles, which was confirmed from X-ray diffraction studies. Due to the strong magnetic superexchange interactions of CuO, weak room temperature ferromagnetic hysteresis curve was observed. The highest saturation magnetization of 1.61 memu/g and remnant polarization of 6.21 µC/cm2 was achieved for x = 1 mol% of CuO-doped ceramic. Further, the existence of weak magnetoelectric effect (χE = 0.1352 mVcm-1Oe-1) was also achieved. The obtained weak magnetoelectric effect is due to the presence of spontaneous magnetization and oxygen deficiency in the prepared samples.

High temperature spin-driven multiferroicity in ludwigite chromocuprate Cu2CrBO5

We report spin-driven multiferroicity above 100 K in the ludwigite Cu(II) oxyborate Cu2CrBO5. Spontaneous polarization, which reaches 35 μC m−2 at 5 K, appears below 120 K, concomitantly with an incommensurate antiferromagnetic order and complex magnetodielectric effects. In magnetically induced ferroelectrics, multiferroicity usually appears at low temperature because of the competing magnetic exchanges needed to stabilize a magnetic spiral. The remarkably high transition temperature observed in Cu2CrBO5 originates from the presence of strong Cu–O–Cu magnetic super-exchange interactions, which are not weakened by Cu/Cr cationic disorder. Our result provides an important contribution to the search for high temperature spin-driven multiferroics among low-dimensional cuprates.

Role of Mg2+ and In3+ substitution on magnetic, magnetostrictive and dielectric properties of NiFe2O4 ceramics derived from nanopowders.

The effect of Mg2+ and In3+ substitution on the structural, magnetic, magnetostrictive and dielectric properties of NiFe2O4 samples derived through sintering of nanocrystalline ceramic powders is investigated. Namely, NiFe2O4, NiMg0.2Fe1.8O4 and NiIn0.2Fe1.8O4 nanopowders were synthesized by a tartrate-gel route followed by calcination at 500 °C, pelletization and sintering at 1200 °C for 2 h. The average particle size was found to decrease from ∼13 nm (for NiFe2O4) to ∼9 nm and ∼7 nm for as-synthesized Mg2+ and In3+ substituted NiFe2O4 samples (after calcination), respectively. However, after sintering a better grain growth occurs for the In3+ substituted sample, as confirmed through the microscopy and dielectric results. Due to the replacement of smaller Fe3+ cations by larger In3+ and Mg2+ cations at the tetrahedral (Td) and octahedral (Oh) interstitial positions, respectively, in the spinel structure of NiFe2O4, the lattice parameters increase in both the cases. The Td site occupation of In3+ leads to higher magnetization for the NiIn0.2Fe1.8O4 sample, but Oh site occupation of Mg2+ leads to lower magnetization for the NiMg0.2Fe1.8O4 sample, in comparison to the pure NiFe2O4 sample. Furthermore, the Curie temperatures (TC) and magnetocrystalline anisotropy constants (K1) for both the samples are considerably lower than the parent compound, with all the parameters being the lowest for the In3+ substituted sample because the Td occupation of non-magnetic In3+ drastically decreases the A-O-B magnetic superexchange interactions. The above alteration to the magnetic interactions alters the magnitude of maximum magnetostriction (λmax), which is explained based on the occupation of the Mg2+ and In3+ cations in our samples. The obtained magnetostriction properties of our samples are very useful for magnetostriction based sensor application.

Observation of room temperature multiferroicity in CuO-doped Sr3Bi4Ti6O21 lead-free ferroelectric ceramics

Room temperature multiferroicity was achieved in CuO-doped Sr3Bi4Ti6O21 (SBT6) lead-free bismuth layered structure ferroelectric ceramics. On addition of CuO, a feeble magnetization with highest saturation magnetization of ~ 3 memu/g and least coercivity of 676 Oe was obtained for x = 0.5 mol% sample. The effect of magnetic field on the ferroelectric properties was investigated, which reveals the presence of magnetoelectric (ME) coupling indirectly and the maximum remnant polarization of 12.16 μC/cm2 at 1 kOe was obtained. Moreover, from direct measurements (ME studies), coupling between ferromagnetic and ferroelectric phases was also achieved at room temperature. Further, the presence of feeble ME effect decreases upon increasing CuO doping concentration, especially at higher doping concentration. The induced spontaneous magnetization, oxygen vacancies and strong magnetic superexchange interaction of CuO played a key role in achieving room temperature multiferroicity.

Super-exchange interaction model in DMOs: Co doped TiO2 thin films

This work presents a study of the structural, morphological, and magnetic properties of TiO2/Co thin films deposited by DC magnetron co-sputtering of Co and TiO2 targets, varying the concentration of the Co and TiO2 at room temperature. The formation of Co oxides binary phases was not observed. From XRD, μXRD, and Raman measurements were identified rutile and anatase phases, which evidence the formation of diluted magnetic oxide semiconductor (DMOs). Additionally, the TiO2 polymorphous and random location of Co ions are induced when the substrate is at room temperature, and in situ annealing process is applied. SEM, AFM, and MFM micrographs evidenced the formation of smaller grains (< 10 nm) in the surface without relationship with magnetic domains; values of the magnetic roughness were obtained between 199 and 442 pm. Magnetic measurements showed a ferromagnetic-like behavior with the appearance of a magnetic hysteresis loop. The hysteresis curves were associated with a combination of dipolar and super-exchange interactions between Co ions and mediated for oxygen ions based on Anderson’s model; this behavior was described from numerical simulations of isolated magnetic moments (Co atoms) diluted into a non-magnetic matrix (TiO2 structure). In this model, a random distribution of Co ions, contributing to dipolar magnetic interactions and super-exchange interaction, is considered, due to the hybridization sp3d2 of the Co–O bond and located randomly of Co ions. The experimental and theoretical results of this study present an explanation of ferromagnetic-like behavior for Co doped TiO2 thin films, as diluted magnetic semiconductor (DMS) for spintronic applications.

Oxygen-vacancy induced magnetic phase transitions in multiferroic thin films

Multiferroics in which giant ferroelectric polarization and magnetism coexist are of tremendous potential for engineering disruptive applications in information storage and energy conversion. Yet the functional properties of multiferroics are thought to be affected detrimentally by the presence of point defects, which may be abundant due to the volatile nature of some constituent atoms and the high temperatures involved in the synthesis of materials. Here, we demonstrate with theoretical methods that oxygen vacancies may enhance the functionality of multiferroics by radically changing their magnetic interactions in thin films. Specifically, oxygen vacancies may restore missing magnetic super-exchange interactions in large axial ratio phases, leading to full antiferromagnetic spin ordering, and induce the stabilization of ferrimagnetic states with considerable net magnetizations. Our theoretical study should help to clarify the origins of long-standing controversies in bismuth ferrite and improve the design of technological applications based on multiferroics.

Interplay of dipoles and spins in κ−(BEDT−TTF)2X , where X=Hg(SCN)2Cl, Hg(SCN)2Br, Cu[N(CN)2]Cl, Cu[N(CN)2]Br , and Ag2(CN)3

We combine first principles density functional calculations with empirical relationships for the Coulomb interactions in the `monomer' model of $\kappa$-(BEDT-TTF)$_2X$. This enables us to calculate the parameters for the model of coupled dipolar and spin degrees of freedom proposed by Hotta [Phys. Rev. B $\textbf{82}$, 241104 (2010)], and Naka and Ishihara [J. Phys. Soc. Japan $\textbf{79}$, 063707 (2010)]. In all materials studied, retaining only the largest interactions leads to a transverse field Ising model of the dipoles. This quantifies, justifies and confirms recent claims that the dipoles are of crucial importance for understanding these materials. We show that two effects are responsible for a range of behaviors found in the dipoles in different $\kappa$-(BEDT-TTF)$_2X$ salts. (i) The inter-dimer hopping, $t_{b1}$, which gives rise to the "transverse field" in the Ising model for the dipoles ($H^T=2t_{b1}$), is between a third and a tenth smaller in the mercuric materials than for the mercury-free salts. (ii) The Ising model of dipoles is in the quasi-one-dimensional limit for the mercuric salts, but quasi-two-dimensional (between the square and isotropic triangular limits) for the mercury-free materials. Thus, the dimensionless critical field is much smaller in the mercuric compounds. Effect (ii) is much larger than effect (i). Simple explanations of both effects based on the band structures of the different salts are given. We show that dipolar order and even short-range dipolar correlations have a profound impact on the nature of the interdimer magnetic (superexchange) interactions. For example, dipole crystallization drives the materials towards quasi-one-dimensional magnetic interactions, which could be important for understanding the spin liquids found in some of these materials.

Investigation of the crystal/magnetic structure, magnetic and optical properties of SrYxNbxFe12−2xO19 (x ≤ 0.05) hexaferrites

Series of Sr hexaferrites (HFs) co-substituted with Nb3+ and Y3+ ions (SrYxNbxFe12−2xO19 (x ≤ 0.05) were prepared via sol-gel combustion method. In addition to their optical, low temperature, magnetic properties and hyperfine interactions were investigated in detail. X-ray diffraction (XRD) patterns were considered proving both the formation and pureness of products. Mossbauer analysis indicated that the hyperfine field on iron nuclei at all sites decrease with substitution of Y3+–Nb3+ ions into Sr-hexaferrite as a result of changing the magnetic Fe3+ (5 μB) ions by nonmagnetic Y3+ and Nb3+ ions. This leads to decreasing of the magnetic super exchange interaction between the ions. Measurements of AC susceptibility and ZFC-FC magnetizations implemented in a range from room temperature (RT) down to around 2 K. The analysis of MFC–MZFC data proves the ferrimagnetic performance of different prepared HFs in the whole range of 2–325 K and a super-spin glass-like behavior was identified at minimal temperatures. The investigation of AC susceptibility revealed a weakening in the magnetic exchange interactions with the rise in the Nb3+ and Y3+ ions ratios.

Correlations between microstructural and magnetic properties of Gd3+-doped spinel ferrite nanoparticles

We report the effect of rare-earth Gd3+ doping in Ni–Zn ferrite nanoparticles with a generic formula Zn0.2Ni0.8GdxFe2–xO4 (x = 0.00, 0.05, 0.10 and 0.15), synthesized using chemical co-precipitation technique. The cubic spinel phase formation and phase purity were confirmed using X-ray diffraction with crystallite size in the range of 16 ± 7 nm obtained from Scherrer’s formula and Williamson–Hall (W–H) plots analysis. The lowering of octahedral cubic symmetry was also observed from Raman spectra with increasing Gd3+ ions content. Increase in Gd3+ ions concentration results in enhancement of A–O–B superexchange magnetic interaction, confirmed by hysteresis curves at 5 K. The room temperature M–H plots showed superparamagnetic behavior due to small particle size. The blocking temperature was found to decrease with increase in Gd3+ ions concentration in Ni–Zn ferrite nanoparticles, which makes it a potential candidate for biomedical applications such as targeted drug delivery and hyperthermia.

Opportunity and challenge for study of valence electron structure in typical magnetic materials

The conventional magnetic ordering models, exchange interaction, super-exchange (SE) interaction and double exchange (DE) interaction models relating to the valence electron structure in the materials, were proposed about in or before the 1950's, the time when there was little experimental evidence. Since the 1970's, more and more experimental results for the valence electron states have been reported. These experimental results suggested that the conventional magnetic ordering models need improving. i) Many experimental results, including the electron energy-loss spectra (EELS), X-ray absorption spectra (XAS), and X-ray photoelectron spectra (XPS), indicate that there are O&lt;sup&gt;–&lt;/sup&gt; anions in addition to O&lt;sup&gt;2–&lt;/sup&gt; anions in oxides, and that the percentage of O&lt;sup&gt;–&lt;/sup&gt; anions may reach 30% or more. This suggests that the SE model and DE model both need to improving, in which all oxygen anions are assumed to be O&lt;sup&gt;2–&lt;/sup&gt; anions. ii) Several experimental results, including gamma radiation diffraction, XAS and magnetic circular dichroism spectra (XMCD), suggest that part of 4s electrons enter into 3d orbits and transit into the 3d electrons in the process of forming metals from free atoms. The effect of the orbital magnetic moment on the magnetic moment of a bulk metal is far smaller than the spin magnetic moments. These provide the evidence of exploring the relation between magnetic moment and electrical resistivity of the magnetic metal. iii) Using density function theory (DFT) to fit physical properties yields plenty of results for many materials, but there exist serious difficulties for magnetic materials. This is due to magnetic ordering energy is included in the exchange correlation energy, which has been find no phenomenological expression so far, and has to be fitted using various models in DFT calculation. These investigations provide an opportunity to improve magnetic ordering models. Therefore, our group proposed three models of magnetic ordering in typical magnetic materials, they including an O 2p itinerant electron model for magnetic oxides (IEO model), a new itinerant electron model for magnetic metal (IEM model), and a Weiss electron pair (WEP) model for the origin of magnetic ordering energy. Replacing the SE model and DE model with the IEO model, the magnetic structures of Co, Ni, Cu doped spinel ferrites as well as Cr and Ti doped spinel ferrites can be explained. The dependence of the magnetic moment on the Sr content in perovskite manganites La&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Sr&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt; can also be explained, for which there have been many ongoing disputes about the cation distributions. With the IEM model, we can explain qualitatively the relation of the magnetic moment with the resitivity for each of Fe, Co, Ni, Cu metals, and fit the curves of the resistivity of NiCu alloy versus test temperature and the Cu doped level. With the WEP model, we can explain why Fe, Co, Ni metal, NiCu alloys, Fe&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; and La&lt;sub&gt;0.7&lt;/sub&gt;Sr&lt;sub&gt;0.3&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt; oxides have different Curie temperature values. The new itinerant electron model is different from the classical model in the following three elementary characteristics. First, the s electrons in free 3d transition metal atoms are divided into two parts when they form a metal or alloy. One part of these s electrons enter into the d orbits and change into the d electrons. and the other part of those electrons are the free electrons which are no longer called the s electrons. Second, only the d electrons occupying the outer orbit of an ion core in a metal or alloy may form itinerant electrons with a certain probability, while the remaining d electrons are local electrons. Third, whether in a magnetic metal or in a magnetic oxide, the transition of the itinerant electrons is the spin-dependent transition below the Curie temperature, and the transition probability decreases with test temperature increasing. The transition of the itinerant electrons turns into the spin-independent transition when the temperature is above the Curie temperature. In this paper, first, we introduce several typical experimental results of the valence electron states. Then, we present the new magnetic ordering models proposed by our group and analyze the elementary differences between the new models and the conventional models. Finally, we point out the challenge to the future work.

Insight into the influence of terminal ligands on magnetic exchange coupling in a series of dimeric copper(II) acetate adducts

AbstractA combination of experimental (SQUID magnetometry) and quantum‐chemical (BS‐DFT, CASSCF(n,m)/NEVPT2, DDCI3) methods is used to probe the influence of terminal ligands on magnetic exchange coupling in the series of [Cu2(μ2‐OAc)4L2] complexes with O‐donor terminal ligands extended by two novel complexes containing phosphine oxide ligands. The possibilities and limitations of these approaches are discussed. The influence of terminal ligands on magnetic superexchange interaction was quantified by two contributions—the first contribution is associated with structural changes within the {Cu(μ2‐OAc)4Cu} core, while the second contribution is due to the difference in the electronic influence of different terminal ligands at a given core geometry. Both contributions were found to be approximately of the same order of magnitude. These results indicate that magnetic exchange coupling, known to be highly sensitive to the distortions in core geometry, is not solely determined by the core structure. Our findings are consistent with the fact that various one‐ and multiparameter magnetostructural correlations reported in the literature have only a qualitative predictive capability.