Antiferromagnetic Superexchange Interactions Research Articles

Low-temperature magnetocaloric effect of the polyoxovanadate molecular magnet {VIV/V12As8}: An experimental study

Magnetic measurements have been performed for the compound (NH(C2H5)3)4[VIV8VV4AsIII8 O40(H2O)]·H2O compound in order to determine its magnetocaloric effect in terms of the isothermal entropy change and compare it to the theoretical prediction based on the previously developed model. A closer inspection of the experimental magnetization and susceptibility data revealed that this model is incomplete neglecting a contribution from the off-center V4 subsystems, where the vanadium ions are in the mixed valence state. The probability that two valence electrons residing within the two off-center V4 subsystems are magnetically decoupled was estimated to be equal to 2 – 3 %. For most of the time they turn out to be magnetically correlated by the high antiferromagnetic superexchange interaction. The magnetocaloric effect calculated with the full model accounting for the off-center contribution was found to agree satisfactorily with the values derived from the measured magnetization data. A valuable byproduct of the analysis is the rigorous estimation of the errors in the isothermal entropy change due to the necessary numerical approximations.

Signature of elementary excitations in Se-substituted layered triangular-lattice antiferromagnet CuCrS2

Geometrical spin frustration in a layered triangular-lattice system prohibits the long-range magnetic order and could give rise to unusual spin-disordered states of matter in which spins are strongly correlated and highly entangled with low-energy excitations. Here we investigate the low-temperature magnetic and specific heat properties of the layered triangular-lattice system CuCr(S1-xSex)2 within the entire Se-for-S substitution range. In magnetization measurements, the x = 0 phase exhibits a sharp cusp-like antiferromagnetic transition at 38 K, while for all the Se-substituted samples a broad maximum along with a spin-glass-like freezing at low temperatures is seen. The Curie-Weiss temperature systematically increases from −122 K for x = 0 to + 15 K for x = 1. These observations suggest that the magnetic ground state is determined by the competition between antiferromagnetic direct exchange interactions and ferromagnetic super-exchange interactions. For x = 0, the magnetic contribution to heat capacity exhibits a sharp jump at the magnetic transition temperature indicative of 3D long-range magnetic order, while for the x > 0 samples it increases smoothly across the magnetic transition range following a power-law (∼T2.1) behavior. All these results are in line with an existence of unusual gapless spin-liquid like excitations originating from the spin frustrations and competing nearest-neighbor interactions in CuCr(S,Se)2.

Effect of Cr substitution on structural, magnetic, magnetocaloric and critical behaviors of La0.7Y0.05Ba0.25MnO3 perovskite manganite

A thorough investigation was conducted into the structural, magnetic, and critical properties of La0.7Y0.05Ba0.25Mn1-xCrxO3 (x = 0 and 0.15) perovskite manganite synthesized using the sol-gel method. The samples possess an orthorhombic structure with the Pnma space group according to X-ray diffraction (XRD) patterns. The average ion radii of the B-site cation (<rB>), the unit cell volume (V), and <θMn,Cr–O–Mn,Cr > bond angle decreased with Cr substitution. While the average bond distance, <dMn,Cr–O>, and Goldschmidt tolerance factor (tG) increased. The samples present a magnetic transition from a paramagnetic (PM) phase to a ferromagnetic (FM) phase at Curie temperature (TC). Addition of Cr to La0.7Y0.05Ba0.25MnO3 decreased the TC value from 310 K (for x = 0 sample) to 258 K (for x = 0.15 sample). Using Banerjee's criteria, second-order phase transitions were determined for the samples. Both samples showed significant maximum entropy change |ΔSMmax| and relative cooling power (RCP) whose values decreased from (6.40 J Kg−1 K−1 and 231 J Kg−1 for x = 0) to (4.45 J Kg−1 K−1 and 185 J Kg−1 for x = 0.15), respectively. These values indicate that the La0.7Y0.05Ba0.25Mn1-xCrxO3 manganites exhibit favourable magnetocaloric effect (MCE) and demonstrate promising potential for application as materials in magnetic refrigeration technology. The estimated critical exponents (β, γ, and δ) for the x = 0 sample align with the 3D-Ising model, while for x = 0.15, they closely resemble the 3D-Heisenberg model. This shift in the universality class can be attributed to the interplay between competing ferromagnetic double-exchange and antiferromagnetic super-exchange interactions that exist within the prepared samples.

Improved magnetic properties of Fe substituted La2CoMnO6 for spintronic applications

A detailed study of magnetic behaviour of double perovskites was carried out by developing La2Co1-xFexMnO6; (x = 0.0, 0.2, 0.5, 0.8 and 1.0) via sol–gel technique. All samples showed single magnetic transition except pure LCMO (x = 0.0) and LCFMO2 (x = 0.5). Temperature dependent inverse magnetic susceptibility (χ-1) was fitted in accordance with Curie-Weiss law. μeff (effective magnetic moment) was determined for all the developed samples. Pure LCMO showed a downward divergence from Curie-Weiss law below 230 K but above transition temperature which indicates the characteristics of Griffiths phase. Saturation magnetization (Ms) increased initially and then decreased, whereas coercive field (Hc) was found to be monotonous decreased with increase in Fe content. LCFMO1 (x = 0.2) was noticed to have maximum Ms (38.87 emu/g) which indicated that the magnetic properties of pure LCMO were improved with Fe substitution, but for lower substitution only. Magnetization vs. field (M−H) curves were not well saturated even at a magnetic field of 60 kOe, therefore, values of Ms for all the samples were estimated using law of approach to saturation. With increase in Fe content, an increased anti-site disorder among Mn and Co strengthened antiferromagnetic (AFM) super exchange interactions and weakened ferromagnetism (FM). Therefore, substitution of Fe on Co-site was responsible for systematic decrease in saturation magnetization when × exceeded 0.2.

Effect of Al substitution at the Ga site on the structural and magnetic properties of GaFeO3

GaFeO3 compound figures among those orthoferrites that are potential candidates for multiferroics. However, GaFeO3 suffers from a relatively low Nèel temperature, TN ≈ 200 K, and magnetic moment. Based on the results of a detailed investigation of Raman-active modes, structural and magnetic properties of the Ga1-xAlxFeO3 (x = 0.0, 0.05, 0.10, 0.20, 0.30) samples, we show that Al substitution at the Ga sites remedies these shortcomings to a large extent. This work brings out clearly that Al preferentially occupies the tetrahedral (Ga1) sites rather than the octahedral (Ga2, Fe1 and Fe2) sites. The effect of Al substitution in Ga1-xAlxFeO3 is to (i) decrease the lattice parameters and cation-O bond lengths which results in a shift in the positions of Raman peaks towards higher wavenumbers, and (ii) increase (decrease) the cation-O-cation bond-angles such that the antiferromagnetic (ferromagnetic) superexchange interactions grow in strength. An enhancement in these interactions accounts for the observed increase in the Nèel temperature, TN, as well as in the saturation magnetization. The radically different temperature variations of the ‘zero-field-cooled’ and ‘field-cooled’ magnetizations below the irreversibility temperature can be qualitatively understood in terms of the Nèel molecular-field theory for ferrimagnetism. The temperature dependence of inverse susceptibility in the paramagnetic state provides evidence for the non-linear behavior at temperatures well above TN and hence for the persistence of short-range magnetic order to temperatures as high as 1.75 TN.

High-pressure syntheses, crystal structures, and magnetic properties of novel quadruple perovskite oxides LaMn3Ru2Mn2O12 and LaMn3Ru2Fe2O12

Quadruple perovskite oxides LaMn3Ru2M2O12 (M = Mn, Fe) have been synthesized under high-pressure and high-temperature conditions of 8 GPa and 1373 K. Synchrotron X-ray powder diffraction study demonstrates that both oxides crystallized in AA'3B4O12-type cubic quadruple perovskite structures, where the Ru and M ions did not form the special ordering at B-site. X-ray absorption spectroscopy reveals that the valence states are represented as LaMn3+3Ru3+2M3+2O12 (M = Mn, Fe) in the ionic models, indicating that the isovalent states of Ru3+ and M3+ ions disturb the B-site ordering. Magnetization, heat capacity, and neutron diffraction data prove that short-range magnetic orderings are predominant for both compounds instead of long-range magnetic orderings. We conclude that the competition between ferromagnetic and antiferromagnetic superexchange interactions between Ru3+ and M3+ ions at B-site prevent the long-range magnetic ordering Mn3+ ions at A′-site, although the A'- and B-site magnetic sublattices are expected to be intrinsically independent in A′-Mn quadruple perovskite system. This finding represents that the atomic ordering of the B-site is essential for the precise design of magnetic structures for the LaMn3M2M′2O12 quadruple perovskite series.

The Co2＋-Ir5＋ orbital hybridization in LaCaCoIrO6 double perovskite

Here, we report on the structural, electronic, and magnetic properties of a polycrystalline sample of the LaCaCoIrO6 double-perovskite investigated by means of synchrotron x-ray powder diffraction, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism at the Co and Ir L2,3 edges, magnetometry, and electrical transport. Our results indicate a configuration of nearly Co2+/Ir5+ configuration for the transition-metal ions, with spin canting within the Co antiferromagnetic superstructure responsible for the ferromagnetic-like behavior observed below 100 K. The highly insulating character of LaCaCoIrO6 and its positive magnetoresistance further suggest that this antiferromagnetic superexchange interaction occurs through an indirect hybridization between the Co eg orbitals.

X-ray magnetic circular dichroism and anomalous Hall effect in proton-doped spinel NiCo2O4 thin films

Element-specified x-ray magnetic circular dichroism (XMCD) and the anomalous Hall effect (AHE) have been investigated to provide deep insights into the magnetic and electronic structural evolutions of different proton-doping spinel ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ (NCO) films. Our results show that the magnetic dichroism at the Co ${L}_{23}$-edge is mainly contributed by the spin magnetic moment at the tetrahedral (${T}_{d}$)-site ${\mathrm{Co}}^{2+}$ in pristine NCO. The spin magnetic moments at Co and Ni sites have antiparallel alignments. The magnetic anisotropy is related to the considerable orbital magnetic moments therein. Upon proton-doping in ${\mathrm{NCO}}_{\text{int}}$ films (the intermediate state with the mixed phases of ${\mathrm{H}}_{x}\mathrm{NCO}$ and NCO), the magnetic dichroism at the Co ${L}_{23}$-edge still exists. The doping of one extra electron resides at the octahedral (${O}_{h}$)-site ${\mathrm{Co}}^{3+}$ partially and changes ${\mathrm{Co}}^{3+}$ ($S=0)$ to ${\mathrm{Co}}^{2+}$ ($S=3/2$). This ${O}_{h}$-site ${\mathrm{Co}}^{2+}$ aligns in an antiparallel manner with ${T}_{d}$-site ${\mathrm{Co}}^{2+}$ through O-ions mediated antiferromagnetic superexchange interaction, resulting in a reduced spin magnetic moment in ${\mathrm{NCO}}_{\text{int}}$ compared to pristine NCO films. Moreover, the reduced magnetic anisotropy in ${\mathrm{NCO}}_{\text{int}}$ is due to the decreasing of the orbital magnetic moment, which can be related to the charge redistribution at Co $3d$ energy levels and the crystal-field effect. In the two-proton doped ${\mathrm{H}}_{2}\mathrm{NCO}$ films, the magnetic dichroism is absent where the antiferromagnetism arises from the antiparallel spin alignments of ${\mathrm{Co}}^{2+}$ in ${T}_{d}$ and ${O}_{h}$ sites. The intrinsic ferromagnetism is also verified by the AHE, which shows a square-shaped hysteresis as a function of the magnetic fields for pristine NCO and ${\mathrm{NCO}}_{\text{int}}$. Interestingly, the proton-doped ${\mathrm{NCO}}_{\text{int}}$ films exhibit an enhanced coercive field and the exchange bias effect with an asymmetric hysteresis loop at low temperatures, which can be ascribed to the antiferromagnetic phase stemming from partial Co valence state reduction by proton doping.

High entropy effect on double exchange interaction and charge ordering in half doped Nd0.5Sr0.5MnO3 manganite

The entropy stabilized oxides i.e., the so-called high-entropy oxides are of great interest in recent time due to their intriguing physical properties. This new category of advanced materials is stabilized by the high configurational entropy of randomly mixed elements, where a specific lattice site is populated by five or more elements. The present study is devoted to explore the high entropy effect in well-known charge ordered manganite Nd0.5Sr0.5MnO3 where a delicate balance exists between the ferromagnetic double exchange and the antiferromagnetic superexchange interactions. Powder X-ray diffraction analyses confirm phase purity of the samples. FE-SEM imaging revealed the microstructure and elemental mapping confirmed the chemical homogeneity of the samples as well as nominal cationic composition. Bond valence sum (BVS) calculation using refinement parameters supports the expected cationic valence in all the samples. Replacement of Nd3+ by five trivalent cations as in (La0.1Pr0.1Nd0.1Gd0.1Bi0.1)Sr0.5MnO3 with larger average A-site ionic radius,<rA>and smaller size variance, σ2 compared with Nd0.5Sr0.5MnO3 dramatically suppressed the ferromagnetic double exchange interaction. This observation is in contrast with the expected variation in<rA>and σ2. In (La0.1Nd0.1Gd0.1Bi0.1Y0.1)Sr0.5MnO3 with smaller<rA> and marginally higher σ2 the ferromagnetic state ceases to exist. In both the multicationic compositions, a long-range antiferromagnetic state is evolved around 200 K with the retention of charge ordering. In the high entropy (La1/6Nd1/6Dy1/6Ca1/6Ba1/6Sr1/6)MnO3 composition with nominal<rA> and larger σ2 values the high temperature long-range ordering is completely suppressed and a new antiferromagnetic state is evolved at 45 K without manifestation of any charge ordering. This composition also avoids magnetic frustration unlike the dopant variant with larger σ2. The present investigation thus highlights the role of local lattice distortion or high entropy effect apart from the<rA>and σ2 in manganites.

Evolution of complex magnetic phases and metal-insulator transition through Nb substitution in La0.5Sr0.5Co1−xNbxO3

We report the evolution of structural, magnetic, transport, and electronic properties of bulk polycrystalline ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ ($x=$ 0.025--0.25) samples. The Rietveld refinement of the x-ray diffraction patterns with $R\overline{3}c$ space group reveals that the lattice parameters and rhombohedral distortion monotonously increase with the ${\mathrm{Nb}}^{5+}(4{d}^{0}$) substitution ($x$). The magnetic susceptibility exhibits a decrease in the magnetic ordering temperature and net magnetization with $x$, which manifests that the Nb substitution dilutes the ferromagnetic (FM) double-exchange interaction and enhances the antiferromagnetic (AFM) superexchange interaction. Interestingly, for the $x&gt;$ 0.1 samples the FM order is completely suppressed and the emergence of a glassy state is clearly evident. Moreover, the decrease in the coercivity (${H}_{C}$) and remanence (${M}_{r}$) with $x$ in the magnetic isotherms measured at 5 K further confirms the dominance of AFM interactions and reduction of FM volume fraction for the $x&gt;$ 0.1 samples. More interestingly, we observe resistivity minima for the $x=$ 0.025 and 0.05 samples, which are analyzed using the quantum corrections in the conductivity, and found that the weak localization effect dominates over the renormalized electron-electron interactions in the three-dimensional (3D) limit. Further, a semiconducting resistivity behavior is obtained for $x&gt;$ 0.05, which follows the Arrhenius law at high temperatures ($\ensuremath{\sim}160$--320 K), and the 3D variable range hopping prevails in the low-temperature region ($&lt;160$ K). The core-level photoemission spectra confirm the valence state of constituent elements and the absence of ${\mathrm{Co}}^{2+}$ is discernible.

Modulating the antiferromagnetic metallic and insulating states by lattice distortion for lightly-doped La0.92Sr0.08MnO3 films

In order to modulate the magnetic and electronic properties of antiferromagnetic (AFM) materials, La0.92Sr0.08MnO3 (LSMO) films were deposited on SrTiO3 (STO), LaAlO3 (LAO) and MgO substrates, which present the AFM metallic, AFM insulating and ferromagnetic (FM) insulating states, respectively. The temperature-dependent Raman spectra demonstrate that the FM state of LSMO/MgO results from the strong lattice distortion induced by large lattice mismatch (δ = −7.2%), which breaks the AFM superexchange interactions of the magnetic ground state. Interestingly, the LSMO/STO (δ = 0.13%) shows two metal–insulator transitions (MIT) at 218 and 53 K, respectively. Meanwhile, the LSMO/LAO (δ = 3.1%) exhibits the MIT at 195 K, while the LSMO/MgO presents the insulating behavior. This is attributed to the strong electron–phonon coupling induced by lattice distortion and thus the localization of itinerant electrons. With the increasing lattice distortion from LSMO/STO to LSMO/MgO, the localization of itinerant electrons is enhanced. Therefore, the insulating behavior occurs for LSMO/MgO. The dramatic change from AFM metallic to FM insulating state for lightly-doped LSMO films has potential application in AFM spintronic devices.

Origin of ferromagnetic interactions in NaMnCl3 : How the response theory reconciles with Goodenough-Kanamori-Anderson rules

The on-site Coulomb repulsion $U$ is the key ingredient for describing the magnetic properties of Mott insulators, leading to a popular believe that many limitations of the density-functional theory based methods can be cured by artificially incorporating such on-site interactions for localized electrons in the model form. The layered antiferromagnet NaMnCl$_3$ reveals quite a different story: while the Coulomb $U$ on the Mn sites controls the strength of antiferromagnetic superexchange interactions, an equally important parameter is the Stoner coupling ${\cal I}_{\rm Cl}$ on the ligand sites. The latter is responsible for large ferromagnetic contributions to the interatomic exchange interactions, which in NaMnCl$_3$ nearly cancel the effect of the superexchange interactions. Although such behavior is anticipated from the phenomenological Goodenough-Kamanori-Anderson rules, the quantitative description of the ligand-related contributions remains disputable. Considering NaMnCl$_3$ as an example, we discuss how they can be generally taken into account in the linear response theory to regain the dependence of the exchange interactions on ${\cal I}_{\rm Cl}$. The problem is complicated by the fact that, for the nearly filled Cl $3p$ shell, the parameters ${\cal I}_{\rm Cl}$ are sensitive to the model assumptions.

Synthesis of Ruddlesden-Popper LaSrFe1-Cr O4 phases (x = 0.0, 0.2, 0.4, 0.6) by glycine-nitrate combustion process: Effect of Cr doping on magnetic, optical and photocatalytic properties

This work presents the study of new Ruddlesden-Popper phases, LaSrFe 1- x Cr x O 4 ( x = 0.0, 0.2, 0.4 and 0.6), synthesized by glycine-nitrate combustion method. In this research, we are for the first time reporting on a complete Ruddlesden-Popper series ( n = 1) for the photocatalytic degradation of a dye. The effect of Cr 3+ doping on the crystal structure, optical and magnetic properties of the LaSrFeO 4 phase has also been systematically explored via different characterization techniques. Rietveld refinement of PXRD data indicates the formation of single phase compounds with tetragonal symmetry in the space group I 4/ mmm . The results show a decrease in the lattice parameters a and c , as well as the unit cell volume V , with increase in Cr 3+ doping. The energy band gap ( E g ) values calculated using the DRS spectra decreases with increase in Cr concentration. The magnetic measurements revealed dominant anti-ferromagnetic (AFM) interactions in all the phases and the appearance of weak ferromagnetic (FM) interactions with Cr 3+ doping. The results are discussed by considering the increase in Fe 3+ ─O─Cr 3+ FM interaction in addition to the Fe 3+ ─O─Fe 3+ and Cr 3+ ─O─Cr 3+ AFM super-exchange interactions induced due to random distribution of Fe 3+ and Cr 3+ ions at the octahedral site. Rhodamine B, as a dye pollutant model, was used for evaluation of photocatalytic activity of the prepared phases. Enhanced photocatalytic activity was observed with the addition of Cr 3+ substitution. The results show ~99 % degradation for x = 0.6 while only ~76 % degradation is shown for x = 0.0 sample in 40 min under visible light irradiation. • New RP-type LaSrFe 1- x Cr x O 4 phases synthesized by glycine-nitrate combustion method. • E g values decreased with increase in Cr concentration. • Dominant AFM interactions in all the phases and the appearance of weak FM interactions with Cr doping. • Enhanced photocatalytic activity was observed by Cr doping.

Ru-Doping-Induced Spin Frustration and Enhancement of the Room-Temperature Anomalous Hall Effect in La2/3 Sr1/3 MnO3 Films.

In transition-metal-oxide heterostructures, the anomalous Hall effect (AHE) is a powerful tool for detecting the magnetic state and revealing intriguing interfacial magnetic orderings. However, achieving a larger AHE at room temperature in oxide heterostructures is still challenging due to the dilemma of mutually strong spin-orbit coupling and magnetic exchange interactions. Here, Ru-doping-enhanced AHE in La2/3 Sr1/3 Mn1-x Rux O3 epitaxial films is exploited. As the B-site Ru doping level increases up to 20%, the anomalous Hall resistivity at room temperature can be enhanced from nΩ cm to µΩcm scale. Ru doping leads to strong competition between the ferromagnetic double-exchange interaction and the antiferromagnetic superexchange interaction. The resultant spin frustration and spin-glass state facilitate a strong skew-scattering process, thus significantly enhancing the extrinsic AHE. The findings can pave a feasible approach for boosting the controllability and reliability of oxide-based spintronic devices.

Tricopper(II)bis(2-((hydrogen phosphonato)methyl)benzylphosphonate) as a layered oxo-bridged copper(II) coordination polymer: Synthesis, structure, magnetic property, and catalytic activity

A new copper diphosphonate framework with the formula [Cu3(C8H9O6P2)2]n has been synthesized using a trianion of (1,2-phenylenebis(methylene))bis(phosphonic acid) (H4L, C8H12O6P2) under hydrothermal conditions. The coordination polymer was fully characterized using elemental analysis, FTIR, ICP-OES, TG-FTIR and single crystal X-ray analysis. The X-ray crystallographic analysis shows that title compound forms a two-dimensional coordination network in an ionic portion, giving an alternating organic/inorganic bilayer structure. From the analysis of magnetic susceptibility measurement between 1.8 and 100 K, relatively weak superexchange antiferromagnetic interactions were characterized, which is compatible with the geometries of the oxo-bridged Cu2+ ions. It can be used as an efficient catalyst for oxidation of thymol to thymoquinone. In addition, it shows high selectivity under the applied catalytic conditions and can be recycled at least three times without any significant degradation.

Oxygen controlled perpendicular magnetic anisotropy in LaCoO3−δ/La0.7Sr0.3MnO3/LaCoO3−δ heterostructures

Large data storage at reduced dimension is of a great deal of interest in spintronic devices in which magnetic oxide films lead for realizing the modulation of perpendicular magnetic anisotropy (PMA). Herein, we report the modulation of PMA in oxide heterostructures composed of one manganite layer sandwiched between two cobaltite layers, i.e., LaCoO3−δ/La0.7Sr0.3MnO3/LaCoO3−δ via oxygen vacancies. The PMA is originated by the orbital reconstruction and ferromagnetic exchange interaction at the interface between the CoO6 and MnO6 octahedra layers. The modulation ratio up to a significant value ∼200% of PMA can be realized through the phase transition of LaCoO3−δ layers between perovskite and brownmillerite via deprivation or replenishment of oxygen sites. The antiferromagnetic superexchange interaction at the interface between the oxygen-deficient CoO4 tetrahedra layer and the adjacent MnO6 octahedral layer with lattice distortion may suppress the magnetization and PMA. This work ensures promising outcomes in situ modulation of PMA in oxide films such as ferroelectric polarization or ionic liquid gating via the transport of oxygen vacancies.

Interlayer exchange interaction driven topological phase transition in antiferromagnetic electride Gd2O

Based on first-principles calculations, we discover a two-dimensional layered antiferromagnetic (AFM) electride Gd$_2$O, where anionic excess electrons exist in the interstitial spaces between positively charged cationic layers. It is revealed that each cationic layer composed of three-atom-thick Gd$-$O$-$Gd stacks has in-plane ferromagnetic and out-of-plane AFM superexchange interactions between the localized Gd 4$f$ spins through O 2$p$ orbitals. Furthermore, the interlayer superexchange mediated by the hybridized Gd-5$d$ and interstitial-$s$-like states involves intimate couplings between the spin, lattice, and charge degrees of freedom, thereby inducing simultaneous magnetic, structural, and electronic phase transitions. The resulting ground state with the simple hexagonal lattice hosts massless Dirac fermions protected by nonsymmorphic magnetic symmetry, as well as massive Dirac fermions. We thus demonstrate that the anionic excess electrons in Gd$_2$O play a crucial role in the emergence of magnetic Dirac semimetal states, therefore offering an intriguing interplay between 2D magnetic electrides and topological physics.

From magnetic cluster glass state to giant vertical magnetization shift induced by ferromagnetic cluster growth in SrFe0.25Co0.75O2.63

A charge ordering phenomenon within the crystallographic sites as following Sr4(Fe0.143+Co0.363+)48h(Fe0.114+Co0.144+Co0.253+)48fO10.52 was previously reported thanks to neutron diffraction coupled with Mossbauer spectroscopy studies. Such distribution supports a natural magnetic layered structure combining both in-plane ferromagnetic super-exchange interactions mainly on the octahedron “8 f” sub-layer alternating and in-plane antiferromagnetic super-exchange interactions on the “8 h” sub-layer containing tetrahedral sites and five-fold symmetry polyhedra (i.e. squared based pyramid and/or trigonal bipyramid). Because of the interfacial magnetic interactions between the two types of layers, we report a detailed study of the intriguing magnetic properties. AC magnetic susceptibility shows a frequency dependent peak suggesting a cooperative character due to inter-cluster interactions resulting in a magnetic cluster glass state. When temperature decreases and/or applied magnetic field increase, the ferromagnetic clusters growth is promoted and results in a large vertical hysteretic shift on the Field Cooled magnetization isotherm in relation with minor loop.

Antiferromagnetic metallic state and low-temperature magnetoresistance in epitaxial La0.85Sr0.15MnO3 films

We report the magnetic coupling and magneto-transport properties of the epitaxial La0.85Sr0.15MnO3 films, which are fabricated on the (001)-oriented SrTiO3 substrate by RF magnetron sputtering. The anti-ferromagnetism and metallic state were observed in sequence, and both coexist below a critical temperature of 40 K. Meanwhile, the hysteresis loop exhibits the spin-flop transition. The antiferromagnetic phase originates from the A-type spin structure, in which the antiferromagnetic superexchange interaction between the adjacent MnO2 planes is along the c-axis direction. The metallic behavior stems from the double exchange interaction in the MnO2 plane in-plane. Furthermore, the spin scattering at phase boundaries between the ferromagnetic and antiferromagnetic phases causes a comparatively large magnetoresistance value of about −30% around 100 K. This effect, together with the colossal magnetoresistance, causes the large magnetoresistance values in a wide temperature range. This work helps to understand the magnetic coupling of the antiferromagnetic metallic state and magneto-transport properties in lightly doped manganite films.

Ferroelectricity and Ferromagnetism Achieved via Adjusting Dimensionality in BiFeO3/BiMnO3 Superlattices.

Integrating characteristics of materials through constructing artificial superlattices (SLs) has raised extensive attention in multifunctional materials. Here, we report the synthesis of BiFeO3/BiMnO3 SLs with considerable ferroelectric polarizations and tunable magnetic moments. The polarization of BiFeO3/BiMnO3 SLs presents a decent value of 12 μC/cm2, even as the dimensionality of BiFeO3 layers per period is reduced to about five-unit cells when keeping the BiMnO3 layers same. Moreover, it is found that the tunable magnetic moments of SLs are linked intimately to the dimensionality of BiFeO3 layers. Our simulations demonstrate that the superexchange interaction of Fe-O-Mn tends to be antiferromagnetic (AFM) with a lower magnetic domain formation energy rather than ferromagnetic (FM). Therefore, as the dimensionality of BiFeO3 per period is reduced, the AFM superexchange interaction between BiFeO3 and BiMnO3 in the SLs becomes weak, promoting a robust magnetization. This interlayer modulation effect in SLs presents an alluring way to accurately control the multiple order parameters in a multiferroic oxide system.