Antiferromagnetic Superexchange Research Articles

Tailoring Bi1−x−yBaxFeyFeO3 for desired properties: insights into structural, dielectric, and magnetic responses

In present study, samples of Bi1−x−yBaxFeyFeO3 have been synthesized, where x = 0.15, y = 0; x = 0.10, y = 0.10; and x = 0.15, y = 0.10 utilizing a swift two-stage solid-state reaction method. X-ray diffraction (XRD) data has been Rietveld refined to evaluate the structural parameters. Micro-strain is also calculated using Williamson Hall method. Temperature (300 K to 660 K) dependent measurements of the dielectric constant have been conducted at various frequencies (100 kHz, 500 kHz, and 1000 kHz). The dielectric constant (ε′) rises as the temperature increases. Two dielectric anomalies around 450 K and 613 K have been noticed in ε′ versus T curves for all the samples which might be related with defect dipoles and the magnetic transition respectively. Further, an insignificant value of loss tangent (0.2) specifically at around 300 K is a signal of small leakage current in the samples. The source of high dielectric constant is discussed by considering the structural distortions in the ceramics. A clear hysteresis loop has been observed for all the samples which is a sign of collapse of antiferromagnetic nature of BiFeO3. Further, in case of co-doped samples, almost a saturation in magnetization with magnetization value 5.9718 emu g−1 has been noticed in hysteresis curve indicating a major contribution of ferromagnetic interaction. Enhancement in the net magnetization is briefly discussed by considering the ferromagnetic type direct interaction among Fe3+ ions and suppressing the anti-ferromagnetic type super exchange interaction.

Evidence of multifunctional properties of ferromanganite La0.5Ca0.5Mn0.5Fe0.5O3

Synthesis and characterization of the magnetic and electrical properties of the perovskite-type ferro-magnetite La0.5Ca0.5Mn0.5Fe0.5O3 are reported. Structural analysis reveals crystallization of the material in an orthorhombic structure, space group Pbnm (#62). The optical response suggests semiconductor-type behavior with bandgap Eg=2.3 eV. Susceptibility curves as a function of temperature and magnetization as a function of applied field suggest a soft ferromagnetic type response below room temperature, with evidence of disorder characterized by magnetic irreversibility at low temperatures and low applied fields. Resistivity measurements as a function of temperature and corroborate the semiconductor nature of the material and hysteretic I-V curves enhance its applicability in programmable logic memory devices. Band structure calculations around the Fermi level result in two different band gap values for the two spin up and spin down polarizations, as expected in ferromagnetic semiconductor materials. The hybridizations observed in the density of states curves as a function of energy suggest that the ferromagnetic character is due to double exchange mechanisms, coexisting with antiferromagnetic super-exchange, which is essentially due to the disorder of Fe3+ and Mn4+ cations in the octahedra along the three crystallographic axes.

Origins of multi-sublattice magnetism and superexchange interactions in double–double perovskite CaMnCrSbO6

The multi-sublattice magnetism and electronic structure in double–double perovskite compound CaMnCrSbO6 is explored using density functional theory. The bulk magnetization and neutron diffraction suggest a ferrimagnetic order ( TC∼ 49 K) between between Mn2+ and Cr3+ spins. Due to the non-equivalent Mn atoms (labelled as Mn(1) and Mn(2) which have tetrahedral and planar oxygen coordinations, respectively) and the Cr atom in the centre of distorted oxygen octahedron in the unit cell, the exchange interactions are more complex than that expected from a two sublattice magnetic system. The separations between the on-site energies of the d-orbitals of Mn(1), Mn(2) and Cr obtained from Wannier function analysis are in agreement with their expected crystal field splitting. While the DOS obtained from non spin-polarized calculations show a metallic character, starting from Hubbard U = 0 eV the spin-polarized electronic structure calculations yield a ferrimagnetic insulating ground state. The band gap increases with Ueff (U − J), thereby showing a Mott–Hubbard nature of the system. The inclusion of anti-site disorder in the calculations show decrease in band-gap and also reduction in the total magnetic moment. Due to the ∼90∘ superexchange, nearest neighbour exchange constants obtained from DFT are an order of magnitude smaller than those reported for various magnetic perovskite and double-perovskite compounds. The Mn(1)–O–Mn(2) (out of plane and in-plane), Mn(1)–O–Cr and Mn(2)–O–Cr superexchange interactions are found to be anti-ferromagnetic, while the Cr–O–O–Cr super-superexchange is found to be ferromagnetic. The Mn(2)–O–Cr superexchange is weaker than the Mn(1)–O–Cr super-exchange, thus effectively resulting in ferrimagnetism. From a simple 3-site Hubbard model, we derived expressions for the antiferromagnetic superexchange strength JAFM and also for the weaker ferromagnetic JFM . The relative strengths of JAFM for the various superexchange interactions are in agreement with those obtained from DFT. The expression for Cr–O–O–Cr super-superexchange strength ( J~SS ), which has been derived considering a 4-site Hubbard model, predicts a ferromagnetic exchange in agreement with DFT. Finally, our mean field calculations reveal that assuming a set of four magnetic sub-lattices for Mn2+ spins and a single magnetic sublattice for Cr3+ spins yields a much improved T C , while a simple two magnetic sublattice model yields a much higher T C .

A systematic study of oxygen vacancy mediated short range and long range magnetic ordering in BaTi1-xFexO3(x = 0.02)

A systematic study on the magnetic properties of the BaTi1-xFexO3-δ (x = 0.02) system with decrease in grain sizes is presented. The combined study of long range and short range magnetic orderings together was carried out. The samples were prepared in single tetragonal phase. The grains of the pristine sample were refined from micron to nanometer range by mechanical milling. Oxygen vacancies in different charge states gave rise to ferromagnetic and antiferromagnetic orderings among the Fe3+ ions whereas oxygen atoms acted as the medium for antiferromagnetic superexchange among the Fe3+ ions. Magnetization was found to increase significantly as a result of milling and eventually the saturation magnetization became four-fold in the sample with grain size around 18 nm. The increase in magnetization is attributed to the increase in oxygen vacancies as a consequence of increase in milling time. Clear sextet patterns in the Mössbauer spectra were observed in the sample wherein 57Fe–enriched iron was used in place of natural iron. These sextets were attributed to the presence of FM and AFM orderings among Fe3+ ions. Along with the sextet patters, we also observed a doublet pattern in Mössbauer spectra because of quadrupolar interaction of the isolated Fe3+ ions arranged in non-cubic environment and lacking any kind of magnetic interactions among them. Mössbauer Spectroscopic (MS) study revealed that FM ordering increased while AFM ordering disappeared after milling. Drastic fall of coercivity in samples with grain sizes below around 40 nm indicated that they became single magnetic domain.

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.

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.

Possible routes to superconductivity in the surface layers of V-doped Mg1−δTi2O4 through multiple charge transfers and suppression of Jahn-Teller activity

Superconductivity in the family of spinel oxides is very rare owing to their robust Mott-insulating nature. About half a century ago, ${\mathrm{LiTi}}_{2}{\mathrm{O}}_{4}$ became the first reported spinel compound to show superconductivity with a 12 K transition temperature. Since then, several unsuccessful attempts were made to enhance the ${T}_{\mathrm{c}}$ of this family of materials. However, a very recent experiment [A. Rahaman et al., arXiv:2209.02053] has reported superconductivity at a higher temperature (below $\ensuremath{\sim}16$ K) in the V-doped ${\mathrm{Mg}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ thin surface layer while its bulk counterpart remains Mott insulating. The superconducting ${T}_{\mathrm{c}}$ of this material is significantly higher compared to other engineered ${\mathrm{MgTi}}_{2}{\mathrm{O}}_{4}$ thin films grown on different substrates. From our first-principles analysis, we have identified that Mg depletion significantly reduces Jahn-Teller (JT) activity and antiferromagnetic superexchange at the surface layer of V-doped ${\mathrm{Mg}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ due to considerable charge transfer between various ions. The combined effect of a degraded antiferromagnetic order and reduced JT activity weakens the ``Mottness'' of the system, leading to the emergence of superconductivity at higher temperatures.

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 $x>$ 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 $x>$ 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 $x>$ 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 ($<160$ K). The core-level photoemission spectra confirm the valence state of constituent elements and the absence of ${\mathrm{Co}}^{2+}$ is discernible.

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.

Role of Magnetic Defects in Tuning Ground States of Magnetic Topological Insulators.

Magnetic defects play an important, but poorly understood, role in magnetic topological insulators (TIs). For example, topological surface transport and bulk magnetic properties are controlled by magnetic defects in Bi2 Se3 -based dilute ferromagnetic (FM) TIs and MnBi2 Te4 (MBT)-based antiferromagnetic (AFM) TIs. Despite its nascent ferromagnetism, the inelastic neutron scattering data show that a fraction of the Mn defects in Sb2 Te3 form strong AFM dimer singlets within a quintuple block. The AFM superexchange coupling occurs via Mn-Te-Mn linear bonds and is identical to the AFM coupling between antisite defects and the FM Mn layer in MBT, establishing common interactions in the two materials classes. It is also found that the FM correlations in (Sb1-x Mnx )2 Te3 are likely driven by magnetic defects in adjacent quintuple blocks across the van der Waals gap. In addition to providing answers to long-standing questions about the evolution of FM order in dilute TI, these results also show that the evolution of global magnetic order from AFM to FM in Sb-substituted MBT is controlled by defect engineering of the intrablock and interblockcoupling.

Robust ferromagnetism in a cubic perovskite oxide with Curie temperature above 600 K

Robust ferromagnetism with high Curie temperatures (TC), especially above room temperature, in perovskite oxides is extremely rare because the oxygen-mediated antiferromagnetic superexchange generally dominates in these nearly cubic perovskite lattices. Fe4+ ions show potential to promote magnetism responses due to more apparent hybridization between Fe4+ and oxygen orbitals with Jahn-Teller distortion, while this distortion also sacrifices the stability of Fe4+-based oxides. Here, by engineering oxygen vacancies, we report robust ferromagnetism with TC above 600 K in cubic perovskite BaFeO3-δ films epitaxially grown on KTaO3 substrates. A mixture of Fe4+ and Fe3+ cations, mediated by the oxygen vacancies, was identified to be responsible for the robust magnetism as revealed by atomic-scale studies and element-selective circularly polarized X-ray absorption spectra. This study indicates that spin-state ordering could be manipulated in a simple perovskite oxide by engineering oxygen stoichiometry and would be of significance toward future technological applications.

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.

Nematic superconductivity in a one-dimensional system of massless fermions

The superconducting properties of the one-dimensional model of “relativistic” fermions with attraction generated by antiferromagnetic (Heisenberg) pair superexchange spin interaction are studied. Namely, we demonstrate that such a pairing in this system takes place in the nematic channel, with extended s-wave symmetry, where the attraction between fermions mostly takes place when the fermions occupy the nearest sites. It is demonstrated, that the zero-temperature properties of such a system are rather different from the “standard” case of superconductivity with local attraction. For instance, the order parameter has an unusual helical momentum dependence, ∼e−ika, where a is the lattice parameter and the dependence of the gap on doping has a bell shape, qualitatively similar to cuprate high-Tc superconductors. Finally, the smooth transition from the overlapping pair to the local pair regime (or BCS–BEC crossover) in the nematic phase takes place at much lower values of doping as compared to the local pairing case, i.e., the “relativistic 1D” nematic superconductor is much less “friendly” to the local pairs. We also discuss the possible relation of the properties of this model to the superconducting properties of twisted graphene.

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.

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.

Unconventional Pressure-Driven Metamagnetic Transitions in Topological van der Waals Magnets.

Activating metamagnetic transitions between ordered states in van der Waals magnets and devices bring great opportunities in spintronics. We show that external pressure, which enhances the interlayer hopping without introducing chemical disorders, triggers multiple metamagnetic transitions upon cooling in the topological van der Waals magnets Mn(Bi1-xSbx)4Te7, where the antiferromagnetic interlayer superexchange coupling competes with the ferromagnetic interlayer coupling mediated by the antisite Mn spins. The temperature-pressure phase diagrams reveal that while the ordering temperature from the paramagnetic to ordered states is almost pressure-independent, the metamagnetic transitions show nontrivial pressure and temperature dependence, even re-entrance. For these highly anisotropic magnets, we attribute the former to the ordering temperature being only weakly dependent on the intralayer parameters and the latter to the parametrically different pressure and temperature dependence of the two interlayer couplings. Our independent probing of these disparate magnetic interactions paves an avenue for efficient magnetic manipulations in van der Waals magnets.