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Multiferroic material bismuth ferrite (BiFeO3 = BFO) and Ho-substitution (Bi1-xHoxFeO3, x = 0.05, 0.10, 0.15, 0.20) thin films were successfully fabricated on silicon (110) substrates by the sol-gel approach. The doping-Ho effects on crystal structure, surface morphology and magnetic property were investigated. The X-ray diffraction (XRD) revealed that the Ho-substitution thin films underwent a phase transition from the rhombohedral to the orthorhombic structure. The changes in Ho-substitution and pure BFO surface appearance were analyzed using the Field Emission Scanning Electron Microscopy (SEM). Meanwhile, the saturation magnetization (Ms) of Ho-substitution samples obtained remarkable augment with the increase in Ho-doping due to the enhancement of superexchange interactions between Fe-O-Fe and spatial modulation for helical structure destruction. The heightening of superexchange interactions is attributed to the reduction of oxygen vacancies because of Ho3+ ions in a stable status.

Significance Antiferromagnets can host strong quantum fluctuations in their ground state if they combine both low dimensionality and low spin. Materials based on copper oxides (spin-1/2 ions in layered or 1D structures) are unique in optimizing the tendency to strong quantum fluctuations. As a bonus, they show extremely large magnetic interactions, which lead to interesting quantum effects at relatively high temperatures as anomalous transport properties and high-Tc superconductivity in doped systems. Obtaining similar features with other ions has been a long-standing goal. We show that silver and fluorine (which are next to copper and oxygen in the periodic table) in the commercial compound AgF2 reach the goal, paving the way for a different generation of quantum materials., The parent compound of high-Tc superconducting cuprates is a unique Mott insulator consisting of layers of spin-12 ions forming a square lattice and with a record high in-plane antiferromagnetic coupling. Compounds with similar characteristics have long been searched for without success. Here, we use a combination of experimental and theoretical tools to show that commercial AgF2 is an excellent cuprate analog with remarkably similar electronic parameters to La2CuO4 but larger buckling of planes. Two-magnon Raman scattering and inelastic neutron scattering reveal a superexchange constant reaching 70% of that of a typical cuprate. We argue that structures that reduce or eliminate the buckling of the AgF2 planes could have an antiferromagnetic coupling that matches or surpasses the cuprates.

Homogeneous solid solutions of sol–gel-prepared R2NiMnO6 (R = La, Pr, Nd, Sm, Gd, Tb, Dy, Y, and Ho) double perovskites crystallize in a B-site-ordered monoclinic structure (P21/n space group). Mon...

The development of new routes for the synthesis of the magnetic nanostructures with low cost, simple and rapid growth has inspired as alternative materials in a wide range of applications. In this report, 3D-hierarchical MnO2 microspheres with an ultrathin nanosheet structure have been synthesized by microwave heating method. X-ray diffraction analysis exhibit the microcrystalline nature of microspheres and (211) is the major pattern with the highest intensity. High purity and surface adsorbed oxygen species have been detected by valance state of microspheres. The magnetic properties of microsphere α-MnO2 investigated experimentally and combined with density functional theory (DFT) calculations, suggest the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) interaction in their nanostructures. The observed FM state is arising from noncollinear magnetic field that tilts two AFM Mn spins consequently, which make a distortion through intermediate oxygen ion in the crystal structure. While, the canted angle between two Mn sublattices lead them to a weak FM state. Similarly, the superexchange interaction is also responsible for magnetic ordering in the adjacent O ion, that can help to exchange magnetic information by direct electron hops of Mn–Mn chain. This is further confirmed from our DFT calculations. They may be found the strong potential for alternative materials other than partially oxidized metal nanoparticles.

Detailed analysis of the temperature dependent dc susceptibility data shows the modification from the Griffiths like phase (GP) to non-Griffiths like phase (non-GP) in ( La 1 - x Y x ) 0.7 Ca 0.3 MnO 3 ( x = 0 , 0.3 , 0.4 , 0.5 , 0.6 ) compounds with x . The existence of non-Griffiths phase is revealed from the upward deviation of the inverse susceptibility (1/ χ ) versus temperature plot from the Curie-Weiss behavior for the compounds with x ⩾ 0.5 . Whereas Griffiths phase is identified from the downturn of 1 / χ versus T plot from the Curie-Weiss line in the compounds with x = 0.3 , 0.4 . The increased lattice distortion (with x ) decreases the M - O - Mn bond angle and at the same time increase the electron-phonon (e-ph) coupling which in turn decreases the ferromagnetic double exchange (DE) interactions and enhances the effective short-range antiferromagnetic superexchange interactions between Mn 3 + / Mn 3 + and Mn 4 + / Mn 4 + ions above the Curie temperature ( T C ). The enhancement of this short-range antiferromagnetic (SR-AFM) interactions have been attributed to the conversions from GP to non-GP phase. The increased SR-AFM interactions with Y doping has also been substantiated from the magnetocaloric study.

The structural and magnetic properties of Al substituted yttrium-iron garnet (Y3AlxFe5-xO12, x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6 and 1.8) ceramic powders synthesized using solution combustion method were investigated. Post combustion, the samples were calcination at 1045 °C for 6 h and subsequently at 1200 °C for 6 h to obtain phase-pure garnets. X-ray diffraction (XRD) results confirm the formation of garnets with Ia 3 ¯ d structure. The occupancy of Y3+ ions in the dodecahedral site and the distribution of Al3+ and Fe3+ ions in the tetrahedral and octahedral sites in the bcc structure of the garnet were confirmed by Rietveld refinement of XRD patterns, Mossbauer spectroscopy and 57Fe internal field NMR spectroscopy. For low Al content, Al3+ ions have preference to occupy tetrahedral (Td) sites than the octahedral (Oh) sites. At higher Al content the distribution of Al tends towards a ratio of 3:2 at the tetrahedral:octahedral site. Increase in Al doping results in the decrease in the lattice parameter due to smaller size of Al3+ as compared to Fe3+ ion. All the studied samples show coral-network-like surface morphology. The saturation magnetization (MS) values decrease from ∼26.94 emu/g to ∼ 0.17 emu/g with increase in Al content from 0.0 to 1.8. Further addition of Al makes the sample paramagnetic at RT. Substitution of non-magnetic Al3+ reduces the saturation magnetization rapidly due to the decrease in the superexchange interaction in the crystal.

Face-sharing octahedral dinuclear Cr(III) compounds with d3-d3 electronic configurations represent nontrivial examples of electronic complexity, posing particular challenges for theoretical and computational studies. A tris-hydroxy-bridged Cr(III)-Cr(III) system has proven to be a richly rewarding target for studies of magnetism and electron paramagnetic resonance spectroscopy. It was also reported to be a peculiarly difficult system to treat with density functional theory (DFT). In this work the magnetic coupling problem for this dimer is approached with broken-symmetry (BS)-DFT and multireference calculations that utilize the density matrix renormalization group (DMRG) to handle full-valence active spaces. BS-DFT is shown to recover the correct ordering and energy spacing of Heisenberg spin states if used in conjunction with appropriate spin projection procedures, albeit with pronounced functional sensitivity. The contrasting conclusions of previous studies are traced to incorrect inclusion of electronically excited configurations. Analysis of the direct and differential overlap of corresponding orbital pairs from the BS-DFT solution indicates that metal-metal through-space interaction is the dominant contributor to antiferromagnetic coupling. At the DFT level a procedure that utilizes pseudopotential substitution is demonstrated that allows evaluation of the direct exchange vs superexchange contributions. A complementary description is obtained with DMRG-SCF calculations that enable state-averaged CASSCF calculations with both metal and bridge orbitals in the active space. A localized orbital subspace analysis supports the DFT conclusions that in contrast to doubly bridged isoelectronic analogues, antiferromagnetic coupling in the chromium dimer arises primarily from direct metal-metal interaction but is significantly enhanced by ligand-mediated superexchange.

Here we report a new three-dimensional nickel-based hybrid structure [Ni3(BTB)2(BPE)4(H2O)2]·2DMF·2H2O, 1 [where BTB = 1,3,5-tris(4-carboxyphenyl)benzene and BPE = 1,2-bis((4-pyridyl)ethane)], which exhibits long-range ferromagnetism and semiconductor behavior. The dipolar interaction between the magnetic spins is suggested to explain the development of long-range ferromagnetic ordering, where the superexchange interaction can be unwanted due to the large distance between the magnetic spins. Optical band gap and resistance vs. temperature measurements reveal the semiconductor nature of this compound. The density of states calculations shed light towards the origin of the low band gap value.

A novel family of two-dimensional (2D) crystalline metal superhalogens V3X8 (X = F, Cl, Br, I) with intrinsic magnetism was predicted using first-principles calculations in the framework of density functional theory (DFT). The calculation results show that the V3Cl8 monolayer is an intrinsic anti-ferromagnetic semiconductor (AFS) with an indirect bandgap of 0.086 eV at the PBE functional level, while the V3X8 (X = F, Br, I) monolayer exhibits a fascinating ferromagnetic half-metal (FH) property with 100% spin-polarization at the Fermi level. Such 2D materials possess robust dynamical stability as well as thermal stability at room temperature except for V3Br8. In addition, Monte Carlo simulations based on the Ising model with the classical Heisenberg model estimate a Curie temperature of approximately 77 K and 103 K for the V3F8 and V3I8 systems, respectively. Furthermore, we predict an extraordinary phenomenon induced by biaxial compressive strain from the ferromagnetic half-metal (FH) to the ferromagnetic semiconductor (FS) at 2% strain and then to the antiferromagnetic metal (AM) with the biaxial strain increasing up to about 6.3% in the 2D V3I8 monolayer. In the case of 2D V3F8, as the strain varies from -10% to 8%, a series of electronic and magnetic phase transitions of AFS-AM-FH-AFS will occur. These tunable magnetic and electronic properties of 2D halides originate from the competition between AFM direct nearest-neighbor d-d exchange and FM superexchange via halogen p states, which leads to a variety of magnetic states. The explored controllable magnetic properties, electronic properties and the high Curie temperature render the 2D V3I8 monolayer a promising candidate for applications in magnetic logic devices and strain sensor devices.

We have investigated here the structural and magnetic properties of polycrystalline La1.5Ca0.5CoMnO6 double perovskite. The room temperature structural analysis reveals good crystallinity of the sample without presence of any impurity phase. Reitveld refinement of obtained x-ray diffraction and neutron powder diffraction data using Fullprof Suite programme shows that our double perovskite system has monoclinic (P21/n) crystal structure. The presence of antisite disorders (either, Co-O-Co or Mn-O-Mn) in this system suggest antiferromagnetic ordering at ∼200 K due to the presence of Co2+/Co3+-O2--Co2+/Co3+ or/and Mn3+/Mn4+-O2--Mn3+/Mn4+ magnetic interactions along with ferromagnetic ordering below 157 K due to the presence of Co2+–O2-–Mn4+ superexchange interactions.

Polycrystalline rare earth orthochromite LaCo0.5Cr0.5O3 has been synthesized by soft chemical method with the rhombohedral (R-3c space group) structure and physical properties such as magnetism has been compared with the prototype perovskite LaCo0.5Fe0.5O3 (orthorhombic: Pbnm). The magnetic transition is much lower (295 K) than the orthoferrite LaCo0.5Fe0.5O3, although it is also a canted antiferromagnet at low temperature. However, this perovskite faintly resembles the behavior of the parent LaCrO3 (orthorhombic: Pnma), which is reported to be weak ferromagnet at room temperature. The magnetic behavior of LaCo0.5Cr0.5O3 perovskite could be explained by the superexchange mechanism gives rise to antiferromagnetic interactions between the metal cations: Co3+-O-Cr3+, Co3+-O-Co3+ and Cr3+-O-Cr3+.

Three new divalent cobalt coordination polymers containing anionic cyclohexyldicarboxylate and neutral 3-pyridylnicotinamide (3-pna) tethering ligands have been prepared and structurally characterized via single-crystal X-ray diffraction. These phases show a diversity of layered topologies and differences in cobalt atom aggregation. [Co(cis-12chedc)(3-pna)(H2O)]n (1, cis-12chedc = 4-cyclohexene-cis-1,2-dicarboxylate) shows a standard (4,4) grid topology with magnetically isolated cobalt atoms. [Co(cis-13cdc)(3-pna)]n (2, cis-13cdc = cis-cyclohexane-1,3-dicarboxylate) shows a 2D 3,5-connected net with a previously unreported (3.52)(3253647) topology featuring “infinite” anti–syn bridged {Co(OCO)}n chain motifs pillared by 3-pna ligands. [Co(cis-14cdc)(3-pna)]n (3, cis-14cdc = cis-cyclohexane-1,4-dicarboxylate) shows a 2D 6-connected triangular net with (3,6) topology built from {Co2(OCO)2} dimeric units linked by cis-14cdc and 3-pna tethers. Magnetic susceptibility studies of compound 2 revealed a decrease in χmT product upon cooling, ascribed to antiferromagnetic coupling along the {Co(OCO)}n chain motifs concomitant with Kramers doublet formation (g = 2.24(3), D = 12.8(8) cm−1, J = –0.46(6) cm−1). For compound 3, Kramers doublet formation was also observed, but with ferromagnetic superexchange within its {Co2(OCO)2} dimeric units (g = 2.27(1), D = 24(1) cm−1, J = 0.35(2) cm−1). Thermal decomposition behavior of the three new phases is also discussed.

Exploring two-dimensional (2D) materials with both ferromagnetic and ferroelectric properties is scientifically interesting and of great technical importance to numerous functionalities in nanoscale devices. In this work, we have demonstrated a strong magnetoelectric coupling that appeared in the 2D FeI2/In2Se3 van der Waals heterostructure. FeI2 layers undergo a transition from ferromagnetic to antiferromagnetic by reversing the direction of ferroelectric polarization. First-principles calculation predicts a new magnetoelectronic coupling mechanism which is completely different from the Dzyaloshinskii-Moriya (DM) effect in multiferroic materials. Because of the polarization discontinuity at the interface, the valence states of Fe ions change between +2 and +3 for two different polarization directions, leading to the magnetic interaction variation between the direct exchange and I ion mediated superexchange. Moreover, metallic 2D electron gas (2DEG) transfers from the surface of FeI2 to In2Se3 when the polarization reverses, which induces the spin polarization of the heterostructure varying from 93% to 0%. Our work is the first realization of manipulation magnetism by an electric field in full 2D van der Waals heterostructures.

In this study, the structural, magnetic, electric and optical properties in X (X=F, N, S) and Cr co-doped BiFeO3 (BFO) are calculated using the density functional theory. For Cr–X co-doping case, the structure of BFO undergo a phase transition from monoclinic to triclinic structure accompanying net magnetic moments of 5.92 μB, 6.04 μB and 7.80 μB, for Cr–F co-doping, Cr–N co-doping and Cr–S co-doping, respectively. The underlying physical mechanisms are the lattice distortions tunned by doping. The decreased Fe–O–Fe bond angles and Fe–O bond lengths will bring weak antiferromagnetism superexchange interaction. In addition, the band gaps of Cr–X co-doping cases are decreased from 2.20 eV (BFO bulk) to 1.31 eV and 1.80 eV for Cr–F co-doping and Cr–S co-doping, respectively, which are well for photovoltaic applications according to the well-known Shockley-Queisser criterion, suggesting a possible great improvement optical properties in Cr–X co-doped samples.

Photoinduced electron transfer via hopping is generally considered to have a stronger temperature dependence than electron transfer via superexchange. However, in this work, an opposite trend of the temperature dependence is observed. This unexpected result is rationalized by considering the specific geometrical and electronic structure of the Ru-bis(terpyridine) photosensitizer.

In this work we report an experimental and theoretical study on the local structure and magnetic properties of Mn3+–O–Fe3+ superexchange interaction for an oxygen-vacant perovskite PrBaMnFeO5+ δ with δ ∼ 0.5. Structural studies were performed by high resolution X-Ray diffraction technique in normal and anomalous scattering, modeled using an orthorhombic biphasic system corresponding to an oxygen-rich and oxygen-poor stoichiometries. Mössbauer spectroscopy showed the presence of two orthorhombic iron sites, one ascribed to a fully octahedral coordination and another, highly distorted, ascribed to a pyramidal coordination, in full agreement with theoretical DFT + U + SO calculations. The samples showed a spin-glass transition at ∼20 K and mainly the presence of Mn3+–O–Fe3+ type-G antiferromagnetic superexchange interactions. However, our theoretical calculations using first principles also suggested that the presence of oxygen vacancies in certain conditions could be possibly promoting local ferromagnetic interactions. Fil: Romero, Mariano. Universidad de la República; Uruguay Fil: Faccio, Ricardo. Universidad de la República; Uruguay Fil: Pardo, Helena. Universidad de la República; Uruguay Fil: Montenegro, Benjamín. Universidad de la República; Uruguay Fil: Richard, Diego. Instituto de Física la Plata (conicet- Universidad Nacional de la Plata); Argentina Fil: Martínez, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional del Noroeste de la Provincia de Buenos Aires; Argentina Fil: Mudarra Navarro, Azucena Marisol. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina Fil: Mombrú, Alvaro W.. Universidad de la República; Uruguay

Two-dimensional (2D) magnetic semiconductors with room-temperature ferromagnetism are very desirable. Despite the great progress made recently, the Curie temperature is still very low (∼45 K), originating from the weak ferromagnetic superexchange interaction. Here, based on first-principles calculations, we propose a general route to achieve 2D magnetic semiconductors with enhanced Curie temperature in organometallic frameworks by incorporating antiaromatic rings as organic linkers. Antiaromatic rings usually possess low-energy multiple spin states, which can be easily induced by adjacent magnetic moments of transition metals and subsequently coupled with them through the strong d-p direct exchange interaction, producing high-temperature ferrimagnetic ordering. The design of 2D organometallic frameworks with typical antiaromatic rings such as pentalene, which show Curie temperatures well above room temperature from classic Heisenberg model Monte Carlo simulations, confirms our proposal.

A new oxide NaCu3Fe2Os2O12 is synthesized using high pressure and temperature conditions. The Rietveld structural analysis shows that the compound possesses both A- and B-site ordered quadruple perovskite structure in Pn3 symmetry. The valence states of transition metals are confirmed to be Cu2+/Fe3+/Os5.5+. The three transition metals all take part in magnetic interactions and generate strong Cu2+(↑)Fe3+(↑)Os5.5+(↓) ferrimagnetic superexchange interactions with a high Curie temperature about 380 K. Electrical transport measurements suggest its half-metallic properties. The first-principles theoretical calculations demonstrate that the compound has a spin-down conducting band and a spin-up insulating band with a wide energy gap.

The solutions of the Bardeen–Cooper–Schrieffer equation are found within the model of the lower Hubbard subband taking into account three-site correlations and the superexchange, Coulomb, phonon, and spinfluctuation mechanisms of quasiparticle pairing. The Pr0.89LaCe0.11CuO4 compound is considered as an example. The dependence of the superconducting gap on the wave vector along the Fermi contour is approximated by the expression Δφ = Δ0 (B cos(2φ) + (1 − B) cos(6φ)) where the angle φ is measured from the edge of the Brillouin zone. The calculated parameters Δ0 and B correspond to the experimental data. The role of the phonon mechanism is relatively small. The competition of other specified mechanisms in the formation of Δ0 is quite strong. The effect of their interference is important and is different in different parts of the Fermi surface. The main contribution to the formation of the component proportional to cos(6φ) (the highest harmonic of the gap) is due to the spin-fluctuation and Coulomb interactions. It is numerically and analytically proved that the role of three-site correlations is reduced to weakening the superexchange mechanism.

We report in this paper the dynamical properties of ${\mathrm{GdMn}}_{2}{\mathrm{O}}_{5}$ studied by inelastic neutron scattering and infrared spectroscopy assisted by ab initio calculations. Our work sheds light on the electromagnon, a magnetic mode that can be excited by an electric field. Combining spin-wave measurements, simulations, and ab initio calculations of the single-ion anisotropies and the superexchange interactions, we describe in detail the magnetic contribution to this mode. An exhaustive study of the temperature and polarization dependence of its electroactivity completes this comprehensive study of the complex ${\mathrm{GdMn}}_{2}{\mathrm{O}}_{5}$ system.

Using magneto-optical Faraday and Kerr measurements, we investigate the magnetic and magneto-optical properties of a thick $\mathrm{Bi}$-substituted gadolinium iron garnet film over a broad range of wavelengths (250--850 nm) and temperatures (150--300 K), including the magnetization compensation point, ${T}_{M}$. We observe an exchange-bias-like effect in the vicinity of ${T}_{M}$. By slightly changing the sample temperature, we can precisely tune the bias field, which reaches a magnitude 6 times higher than the coercive field. We explain this phenomenon by considering the short-range superexchange interaction and a change in the magnetic behavior when moving from the surface to the bulk of the film. This finding may lead to the development of single-film magneto-optical devices based on the exchange-bias effect.

Fe/Ga- and Fe/Zn-codoped titanium oxide nanoparticles were prepared by co-precipitation technique. The samples were characterized by several methods: energy-dispersive X‐ray fluorescence, X‐Ray Diffraction, UV–visible absorption, and magnetization methods. The host Fe-doped TiO2 nanoparticles were co-doped either with Ga3+ or Zn2+ ions aiming to compare their effects on the possible creation of long-range (LR) interaction between Fe2+−Fe2+ dopant ions. The hydrogenation of the samples was necessary to fabricate the dilute magnetic semiconductor based on TiO2 by its creation of O-vacancies (VO). It was established that Zn/VO defects are energetically favourable for the LR Fe3+−Fe3+ ferromagnetic coupling in Fe/Zn-codoped anatase TiO2 by the superexchange interaction through Zn ions.

The antiferromagnetic behavior of Fe3+ oxides of composition RE1.2Ba1.2Ca0.6Fe3O8, RE2.2Ba3.2Ca2.6Fe8O21, and REBa2Ca2Fe5O13 (RE = Gd, Tb) is highly influenced by the type of oxygen polyhedron around the Fe3+ cations and their ordering, which is coupled with the layered RE/Ba/Ca arrangement within the perovskite-related structure. Determination of the magnetic structures reveals different magnetic moments associated with Fe3+ spins in the different oxygen polyhedra (octahedron, tetrahedron, and square pyramid). The structural aspects impact on the strength of the Fe-O-Fe superexchange interactions and, therefore, on the Néel temperature (TN) of the compounds. The oxides present an interesting transition from three-dimensional (3D) to two-dimensional (2D) magnetic behavior above TN. The 2D magnetic interactions are stronger within the FeO6 octahedra layers than in the FeO4 tetrahedra layers., The antiferromagnetic behavior of Fe3+ oxides is highly influenced by the type of oxygen polyhedron around the Fe3+ cations and their ordering, which is coupled with the layered RE/Ba/Ca arrangement, within the perovskite-related structure in this striking (RE, Ba, Ca)3m+5nFe3m+5nO8m+13n (m, n) homologous series.

The emergence of ferromagnetism in materials where the bulk phase does not show any magnetic order demonstrates that atomically precise films can stabilize distinct ground states and expands the phase space for the discovery of materials. Here, the emergence of long-range magnetic order is reported in ultrathin (111) LaNiO3 (LNO) films, where bulk LNO is paramagnetic, and the origins of this phase are explained. Transport and structural studies of LNO(111) films indicate that NiO6 octahedral distortions stabilize a magnetic insulating phase at the film/substrate interface and result in a thickness-dependent metal–insulator transition at t = 8 unit cells. Away from this interface, distortions relax and bulk-like conduction is regained. Synchrotron x-ray diffraction and dynamical x-ray diffraction simulations confirm a corresponding out-of-plane unit-cell expansion at the interface of all films. X-ray absorption spectroscopy reveals that distortion stabilizes an increased concentration of Ni2+ ions. Evidence of long-range magnetic order is found in anomalous Hall effect and magnetoresistance measurements, likely due to ferromagnetic superexchange interactions among Ni2+–Ni3+ ions. Together, these results indicate that long-range magnetic ordering and metallicity in LNO(111) films emerges from a balance among the spin, charge, lattice, and orbital degrees of freedom.

Using density functional theory within the generalized gradient approximation, we predict that monolayer vanadium oxyhydroxide (VOOH) and chromium oxyhydroxide (CrOOH) are two-dimensional (2D) ferromagnetic semiconductors. These two kinds of oxyhydroxides possess excellent thermal and dynamical stability as the free-standing 2D monolayer films. Our results show that the monolayer VOOH and CrOOH remain ferromagnetic ordering with the critical temperature up to $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and the ferromagnetic ordering is due to a superexchange interaction in the near-${90}^{\ensuremath{\circ}}$ V(Cr)-O-V(Cr) bonds in monolayer VOOH(CrOOH). However, the different occupancy of $d$ electrons between the ${\mathrm{V}}^{3+}$ and ${\mathrm{Cr}}^{3+}$ ions gives rise to the different axes or planes of easy magnetization and the different magnetic anisotropy energies between the monolayer VOOH and CrOOH. Our study demonstrates that monolayer transition metal oxyhydroxides could be one category of testing ground for the investigation of 2D ferromagnets.

Magnetic and crystallographic transitions in the Cairo pentagonal magnet ${\mathrm{Bi}}_{2}{\mathrm{Fe}}_{4}{\mathrm{O}}_{9}$ are investigated by means of infrared synchrotron-based spectroscopy as a function of temperature (20--300 K) and pressure (0--15.5 GPa). One of the phonon modes is shown to exhibit an anomalous softening as a function of temperature in the antiferromagnetic phase below 240 K, highlighting spin-lattice coupling. Moreover, under applied pressure at 40 K, an even larger softening is observed through the pressure-induced structural transition. Lattice dynamical calculations reveal that this mode is indeed very peculiar as it involves a minimal bending of the strongest superexchange path in the pentagonal planes, as well as a decrease in the distances between second-neighbor irons. The latter confirms the hypothesis made by Friedrich et al., [J. Phys.: Condens. Matter 24, 145401 (2012)] about an increase in the oxygen coordination of irons being at the origin of the pressure-induced structural transition. As a consequence, one expects a new magnetic superexchange path that may alter the magnetic structure under pressure.

The molecular cluster system [Fe19(metheidi)10(OH)14O6(H2O)12]NO3·24H2O, abbreviated as Fe19, contains nineteen Fe(III) ions arranged in a disc-like structure with the total spin S = 35/2. For the first order, it behaves magnetically as a single molecule magnet with a 16 K anisotropy barrier. The high spin value enhances weak intermolecular interactions for both dipolar and superexchange mechanisms and an eventual transition to antiferromagnetic order occurs at 1.2 K. We used neutron diffraction to determine both the mode of ordering and the easy spin axis. The observed ordering was not consistent with a purely dipolar driven order, indicating a significant contribution from intermolecular superexchange. The easy axis is close to the molecular Fe1–Fe10 axis. Inelastic neutron scattering was used to follow the magnetic order parameter and to measure the magnetic excitations. Direct transitions to at least three excited states were found in the 2 to 3 meV region. Measurements below 0.2 meV revealed two low energy excited states, which were assigned to S = 39/2 and S = 31/2 spin states with respective excitation gaps of 1.5 and 3 K. Exchange interactions operating over distances of order 10 Å were determined to be on the order of 5 mK and were eight-times stronger than the dipolar coupling.

${\mathrm{Ba}}_{4}{\mathrm{Ru}}_{3}{\mathrm{O}}_{10}$, the quasi-one-dimensional spin-1 ($S$ = 1) compound, has rather intricate magnetic properties. The compound consists of structural Ru trimers, which together form the zig-zag chains where the Ru has two inequivalent crystallographic sites. While at high temperature both the inequivalent Ru ions stay in the $4+$ state with effective $S=$ 1 spin state, upon lowering the temperature, the magnetic moment of the central Ru atom is completely quenched accompanied by a change in the octahedral environment. This effectively gives rise to a bond-alternating chain and provides an opportunity to study the excitation of such a spin network in a real material. We have used microscopic tools such as neutron scattering and muon spin relaxation along with the density functional theory based calculations to address the spin state of the two inequivalent Ru ions in this material. From our neutron powder diffraction, on lowering of temperature, we find a large tetragonal distortion of the central ${\mathrm{RuO}}_{6}$ octahedra of the trimer. The splitting of the ${t}_{2g}$ level of the central Ru due to this distortion is found to be significant leading to the quenching of the moment underscoring the Hund's exchange. The nonmagnetic central Ru promotes a strong antiferromagnetic superexchange between the other two Ru ions in the trimer, which gives rise to a dimeric state. The presence of spin dimers is reflected by the manifestation of a gap in the spin excitation spectra. The spin-dimer formation in ${\mathrm{Ba}}_{4}{\mathrm{Ru}}_{3}{\mathrm{O}}_{10}$ is at par with the effective model proposed by Affleck and Haldane for the $S$ = 1 bond alternating chains in the light of valence bond solid formalism. Eventually, at a lower temperature, a long-range ordered antiferromagnetic state emerges from the gapped dimer state due to the significant interdimer interactions.

We discover magnetic phases hosting highly elusive and technologically important antiferromagnetic skyrmion quasiparticles in a model for a Rashba-coupled Hund insulator. The results are based on unbiased simulations of a classical superexchange model derived, in this work, from a realistic microscopic electronic Hamiltonian. We also discover a skyrmion density wave ground state characterized by a $(Q,Q)$ modulation in the local skyrmion density map. A unique inhomogeneous state characterized by a circular pattern in the spin structure factor and filamentary real-space textures is identified as the parent of sparse antiferromagnetic skyrmions. We predict that the magnetic states reported here can be realized in thin films of multiorbital systems involving $4d$ or $5d$ transition metals.

A spinless Bose-Hubbard model in a one-dimensional (1D) double-chain tilted lattice is numerically studied at unit filling per cell. When each atom is localized in a two-site cell, the low-energy effective model gives rise to an exact textbook model of the 1D transverse Ising model via superexchange interaction. To validate the effective transverse Ising model, we calculate the energy spectrum and the nearest-neighbor correlation functions of the states in a subspace of the Bose-Hubbard model which is equivalent to the Hilbert space of a spin-1/2 magnetic model. The results show good consistency with the effective transverse Ising model, and we show that it is possible to simulate the dynamical quantum phase transition of the 1D transverse Ising model exactly with such a double-chain Bose-Hubbard model. Our results may provide some inspirations for realizing and exploring an exact 1D transverse Ising model in ultracold neutral atom systems.

The nature of order in low-temperature phases of some materials is not directly seen by experiment. Such "hidden orders" (HO) may inspire decades of research to identify the mechanism underlying those exotic states of matter. In insulators, HO phases originate in degenerate many-electron states on localized f or d shells that may harbor high-rank multipole moments. Coupled by inter-site exchange, those moments form a vast space of competing order parameters. Here, we show how the ground state order and magnetic excitations of a prototypical HO system, neptunium dioxide NpO$_2$, can be fully described by a low-energy Hamiltonian derived by a many-body ab initio force-theorem method. Superexchange interactions between the lowest crystal-field quadruplet of Np$^{4+}$ ions induce a primary non-collinear order of time-odd rank-5 (triakontadipolar) moments with a secondary quadrupole order preserving the cubic symmetry of NpO$_2$. Our study also reveals an unconventional multipolar exchange-striction mechanism behind the anomalous volume contraction of the NpO$_2$ HO phase., Final version, 12 pages, 4 figures. For Supporting information etc. see the published version (Open Access)

O efeito exchange bias (EB) é de grande interesse acadêmico devido as suas aplicações na spintrônica. Recentemente foram descobertos alguns materiais que apresentam tal efeito espontaneamente, isto é, mesmo após serem resfriados na ausência de campo magnético externo. O foco deste trabalho foi o aprofundamento dos estudos da influência das estruturas cristalina e eletrônica no efeito EB espontâneo. Para tanto, aqui descrevemos a síntese e as caracterizações estrutural, eletrônica e magnética dos compostos policristalinos da série La1,5(Sr0,5-xBax)CoMnO6. O principal objetivo foi investigar a evolução da magnetização do sistema ao substituirmos gradualmente Sr por Ba. As amostras policristalinas foram sintetizadas por reação de estado sólido convencional. As técnicas de caracterização estrutural utilizadas foram difração de raios-X de pó e posteriormente o Refinamento Rietveld. Já para o estudo das propriedades magnéticas, foram feitas medidas de magnetização em função da temperatura [M(T)] e de magnetização em função do campo magnético [M(H)]. Os compostos se formaram em fase única, no grupo espacial romboédrico 3̅c. Além disso, observou-se que os parâmetros de rede aumentam à medida que a concentração de Ba aumenta. As medidas M(T) indicaram que todos os compostos apresentam ordenamentos ferromagnético (FM) e antiferromagnético (AFM), onde foi observado um aumento da proporção da fase AFM à medida que a concentração de Ba aumenta. Também, observou-se uma tendência de mudança no estado de spin médio dos íons de Co3+, que varia da configuração low spin (LS) para a configuração high spin (HS) à medida que a concentração de Ba aumenta. As medidas M(H) realizadas após resfriar as amostras a campo zero mostraram um robusto efeito EB espontâneo, tendo na concentração x = 0.25 o maior efeito já observado. Medidas M(H) realizadas após o resfriamento dos materiais na presença de campo externo apresentaram comportamento qualitativo similar àquelas realizadas após o resfriamento a campo zero. As variações no efeito exchange bias observadas ao longo da série podem ser explicadas pelas variações na fase AFM e no estado de spin do Co3+. The exchange bias (EB) effect is of great academic interest due to its potential applications in spintronics. Recently there were discovered some material exhibiting the EB effect spontaneously, i.e., even after being cooled in the absence of an external magnetic field. The main focus of this work is to investigate the influence of crystalline and electronic structures on the spontaneous EB effect. Therefore, here we describe the synthesis and structural, electronic and magnetic characterizations of polycrystalline samples of the La1.5(Sr0.5-xBax)CoMnO6 series. The main objective was to investigate the evolution of the magnetization of the system when Sr is gradually substituted by Ba. Polycrystalline samples were synthesized by conventional solid state reaction, and their structural properties were characterized by powder X-ray diffraction and the Rietveld Refinement. The magnetic properties were investigated by means of magnetization as a function of temperature [M(T)] and magnetization as a function of magnetic field [M(H)] measurements. The compounds were formed in a single phase, in the rhombohedral space group 3̅c. In addition, it was observed that the lattice parameters increase as the Ba concentration increases. The M(T) measurements indicate that all compounds have ferromagnetic (FM) and antiferromagnetic (AFM) ordering, where an increase in the proportion of the AFM phase was observed as the Ba concentration increases. Also, a trend of change is observed in the average spin state of the Co3+ ions, which varies from the low spin (LS) configuration to the high spin (HS) configuration as the Ba concentration increases. The M(H) measurements performed after cooling the samples in zero field showed a robust spontaneous EB effect, with the concentration x = 0.25 having the greatest effect ever observed. M(H) measurements taken after cooling the materials in the presence of an external field showed a qualitative behavior similar to those taken after cooling in zero field. The variations in the exchange bias effect observed throughout the series can be explained by the variations in the AFM phase and in the spin state of Co3+. Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES

International audience; 1s2p resonant inelastic X-ray scattering (1s2p RIXS) has proven successful in the determination of the differential orbital covalency (DOC, the amount of metal vs ligand character in each d molecular orbital) of highly covalent centrosymmetric iron environments including heme models and enzymes. However, many reactive intermediates have noncentrosymmetric environments, e.g., the presence of strong metal-oxo bonds, which results in the mixing of metal 4p character into the 3d orbitals. This leads to significant intensity enhancement in the metal K-pre-edge and as shown here, the associated 1s2p RIXS features, which impact their insight into electronic structure. Binuclear oxo bridged high spin Fe(III) complexes are used to determine the effects of 4p mixing on 1s2p RIXS spectra. In addition to developing the analysis of 4p mixing on K-edge XAS and 1s2p RIXS data, this study explains the selective nature of the 4p mixing that also enhances the analysis of L-edge XAS intensity in terms of DOC. These 1s2p RIXS biferric model studies enable new structural insight from related data on peroxo bridged biferric enzyme intermediates. The dimeric nature of the oxo bridged Fe(III) complexes further results in ligand-to-ligand interactions between the Fe(III) sites and angle dependent features just above the pre-edge that reflect the superexchange pathway of the oxo bridge. Finally, we present a methodology that enables DOC to be obtained when L-edge XAS is inaccessible and only 1s2p RIXS experiments can be performed as in many metalloenzyme intermediates in solution.

There is very limited repertoire of organic ambipolar semiconductors to date. Electron donor-acceptor alternative stacking is a unique and important binary motif for 1D mixed-stack cocrystals, opening up possibilities for the development of organic ambipolar semiconductors. Herein, four 1D mixed-stack cocrystals using N,N'-bis(perfluorobutyl)-1,7-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDICNF) as the acceptor and anthracene, pyrene, perylene, and meso-diphenyl tetrathia[22]annulene[2,1,2,1] (DPTTA) as the donors are achieved in a stoichiometric ratio (D:A = 1:1) through solution or vapor processed methods. Their packing structures, energy levels, charge transfer interactions, coassembling behaviors, and molecular orientations are systematically investigated by single-crystal X-ray analysis, absorption spectra, fluorescence quenching, Job's curve plot, and polarized photoluminescence measurements with the help of theoretical calculations. The donor-acceptor alternative stacking direction coincides with the long axis for all the four cocrystals. The field-effect transistors based on Pyrene-PDICNF show the electron mobility up to 0.19 cm2 V-1 s-1 , which is the highest value among perylene diimide-based cocrystals. Moreover, DPTTA-PDICNF cocrystals possess well-balanced electron and hole mobility with 1.7 × 10-2 and 2.0 × 10-2 cm2 V-1 s-1 respectively due to both hole and electron strong superexchange interactions, shedding light on the design of 1D mixed-stack cocrystals with excellent ambipolar transport behaviors.

The discovery of superconductivity in a d(9-delta) nickelate has inspired disparate theoretical perspectives regarding the essential physics of this class of materials. A key issue is the magnitude of the magnetic superexchange, which relates to whether cuprate-like high-temperature nickelate superconductivity could be realized. We address this question using Ni L-edge and O K-edge spectroscopy of the reduced d(9-1/3) trilayer nickelates R4Ni3O8 (where R = La, Pr) and associated theoretical modeling. A magnon energy scale of similar to 80 meV resulting from a nearest-neighbor magnetic exchange of J = 69(4) meV is observed, proving that d(9-delta) nickelates can host a large superexchange. This value, along with that of the Ni-O hybridization estimated from our O K-edge data, implies that trilayer nickelates represent an intermediate case between the infinite-layer nickelates and the cuprates. Layered nickelates thus provide a route to testing the relevance of superexchange to nickelate superconductivity.

An antiferromagnet offers many important functionalities such as opportunities for electrical control of magnetic domains, immunity from magnetic perturbations, and fast spin dynamics. Introducing some of these intriguing features of an antiferromagnet into a low dimensional semiconductor core-shell nanowire offers an exciting pathway for its usage in antiferromagnetic semiconductor spintronics. Here, using a quantum mechanical approach, we predict that the Cr-doped Ge-core/Si-shell nanowire behaves as an antiferromagnetic semiconductor. The origin of antiferromagnetic spin alignments between Cr is attributed to the superexchange interaction mediated by the pz orbitals of the Ge atoms that are bonded to Cr. A weak spin-orbit interaction was found in this material, suggesting a longer spin coherence length. The spin-dependent quantum transport calculations in the Cr-doped nanowire junction reveals a switching feature with a high ON/OFF current ratio (∼41 times higher for the ON state at a relatively small bias of 0.83 V).

We investigated the ground state of monolayer 1T-XCl$_{2}$ (X: Fe, Co, and Ni) using the generalized Bloch theorem, which can generate ferromagnetic, spiral, and antiferromagnetic states. Each state was represented by a unique spiral vector that arranges the magnetic moment of magnetic atom in the primitive unit cell. We found the ferromagnetic ground state for the FeCl$_{2}$ and NiCl$_{2}$ while the spiral ground state appears for the CoCl$_{2}$. We also showed that the ground state depends sensitively on the lattice constant. When the hole-electron doping was taken into account, we found the phase transition, which involves the ferromagnetic, spiral, and antiferromagnetic states, for all the systems. Since the spin-spin interaction in the monolayer metal dichlorides is influenced by the competition between the direct exchange and the superexchange, we justify that the carrier concentration determines which interaction should dominate.

${\mathrm{Sr}}_{2}{\mathrm{CuTe}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{O}}_{6}$ is a square-lattice magnet with superexchange between $S=\frac{1}{2}{\mathrm{Cu}}^{2+}$ spins mediated by randomly distributed Te and W ions. Here, using sub-K temperature and $20\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$ energy resolution neutron scattering experiments we show that this system transits from a gapless disorder-induced spin liquid to a new quantum state below ${\mathbit{T}}_{\mathbit{f}}=1.7(1)\text{ }\text{ }\mathrm{K}$, exhibiting a weak frozen moment of $⟨\mathbit{S}⟩/\mathbit{S}\ensuremath{\sim}0.1$ and low energy dynamic susceptibility, ${\mathbit{\ensuremath{\chi}}}^{\ensuremath{'}\ensuremath{'}}(\ensuremath{\hbar}\mathbit{\ensuremath{\omega}})$, linear in energy which is surprising for such a weak freezing in this highly fluctuating quantum regime.

Recently, the analysis of single-orbital entropy and mutual information has been introduced as a tool for the investigation of contributions to the exchange (J) coupling between open-shell metal ions [Stein et al. J. Phys. Chem. Lett. 2019, 10, 6762-6770]. Here, we show that this analysis may lead to an incorrect interpretation of the J-coupling mechanism. Instead, we propose an orbital-entanglement analysis that is based on the two-electron density and that provides a coherent picture of the contributing exchange pathways, which seems fully consistent with the available J values. For this purpose, we used a prototypical bis-μ-oxo binuclear manganese complex ([Mn2O2(NH3)8]4+) and demonstrated that its antiferromagnetism (J < 0), calculated by using the active space composed of all valence pO and dMn orbitals, correlates well with the largest elements in the differential low-spin vs high-spin entanglement map. These elements correspond to interactions between the pairs of dMn orbitals mediated by the oxo-bridging out-of-plane p orbitals, representing the π superexchange pathway. We also show that the reduction of active space to manifold of the singly occupied magnetic orbitals does not lead to discrepancy between the calculated J values and entanglement maps. This contrasts to analysis of mutual information, which suggests the "direct" dMn-dMn interactions to play a dominant role for the J coupling, irrespective of the size of active space as well as of the antiferromagnetism expected. The failure is attributed to the large contribution of spin entanglement contained in the mutual information of the low-spin state, which may be regarded as the origin of the different complexity of its wave function and electron density.

A site-diluted classical Heisenberg model on the bcc lattice, with only isotropic nearest-neighbor exchange interactions, is used to describe the phase diagram of ${\mathrm{Fe}}_{p}\ensuremath{-}{\mathrm{Al}}_{q}$ alloys ($p+q=1$) in the structurally disordered phase. Due to the larger size of Al atoms, the Fe-Fe exchange interactions along the border of the Al clusters are assumed to increase since the distance between Fe atoms becomes smaller in this region and remains constant for the rest of the lattice. However, for higher Al concentrations the Fe-Fe exchange interaction should decrease as a result of the increase of the lattice size parameter. Considering these assumptions, we have employed Monte Carlo simulations to obtain the thermodynamic properties of the model and obtain quite good fits to the experimental data of the transition temperature as a function of $q$. The results are still better than those recently obtained by assuming superexchange interactions induced by Al between next-nearest-neighbor Fe atoms.

Nominally 45 nm GaN:Mg/ 5 nm (Ga,Mn)N / 45 nm GaN:Mg trilayers structures prepared by molecular beam epitaxy on GaN-buffered Al2O3 substrates are investigated to verify whether the indirect co-doping by holes from the cladding layers can alter the spin-spin interaction in (Ga,Mn)N. The four investigated structures, differing with the Mg doping level, are carefully characterized at the nanoscale by HRTEM, EDX, and by SIMS. HRTEM decisively excluded a presence of foreign Mn-rich phases. The structures, up to medium Mg doping, show no Mg over-doping effects. Magnetic studies of these structures are aided by the employment of a dedicated experimental approach of the in situ compensation of the magnetic contribution from the substrate, allowing up to about fifty-fold reduction of this contribution. This technique, dedicated to these structures, simultaneously provides a tenfold reduction of temporal instabilities of the magnetometric unit and lowers the experimental jitter to merely $5 \times 10^{-7}$~emu at 70~kOe, vastly increasing the precision and the credibility of the results of the standard integral SQUID magnetometry in high magnetic fields. The magnetic characteristics of the trilayers structures established here prove identical with the already known properties of the thick (Ga,Mn)N single layers, namely (i) the low temperature ferromagnetism among Mn$^{3+}$ ions driven by superexchange and (ii) purely paramagnetic response at higher temperatures. The possible cause of the lack of any effects brought about by the adjacent Mg-doping is a presence of residual Mn in the cladding layers, resulting in the deactivation of the p-type doping intended there. This finding points out that a more intensive technological effort has to be exerted to promote the co-doping-driven carrier-mediated ferromagnetic coupling in Mn-enriched GaN, especially at elevated temperatures., 16 pages, 6 figures

Exploration and understanding of the spinterface between magnetic electrodes and organic semiconductors are vital to get an insight into interfacial science and develop spintronic devices. Intrigued by the fact that the modification of chemical states may affect interfacial exchange coupling, magnetic interactions between a series of metallic phthalocyanines ($M\mathrm{Pcs}, M=\mathrm{Cr}$, Mn, Fe, Co) and surface oxidized Co were studied. Interestingly, these four $M\mathrm{Pcs}$ are all antiferromagnetically coupled with the Co substrate via the interfacial O atomic layer, but the coupling strength strongly relies on the type of metallic ion in the $M\mathrm{Pcs}$. The physical mechanism for different exchange strengths is explored and discussed in terms of a superexchange model via O atoms. It is also noted that the adsorption energy and work function of the surface oxidized Co were sensitive to the metallic ions of $M\mathrm{Pcs}$. The valence and spin state of central $3d$ metallic ions could be tuned by the surface oxidized Co. Very interestingly, high negative spin-polarized spinterfaces are obtained in these $M\mathrm{Pcs}$. The spin polarizations are up to \ensuremath{-}86% and \ensuremath{-}80% for CrPc and MnPc, respectively. Additionally, high spin polarization can be maintained beyond room temperature because of the very large interfacial coupling energy, demonstrating a promising application of $M\mathrm{Pcs}$ in spintronic devices.

We investigate the magnetic excitations in view of the recent reports suggesting that the spin-wave energy may exhibit a significant dependence on the in-plane strain of a thin film of La$_2$CuO$_4$. The nature of dependence, as we find, can be explained naturally within a two-orbital model based on the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals. In particular, as the orbital-splitting energy between the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals increases with compressive strain, the zone-boundary spin-wave energy hardens. However, the hardening persists only until the orbital splitting reaches $\sim$ 2eV, beyond which there is no significant change. The behavior of zone-boundary spin-wave energy is explained in terms of the extent of hybridization between one of the exchange-split $d_{x^2-y^2}$ band which is nearly half filled and the $d_{3z^2-r^2}$ band. The role of second-order antiferromagnetic superexchange process involving the inter-orbital hopping is also discussed., 6 pages, 5 figures

Using density functional theory calculations, we found that recently high-pressure synthesized double perovskite oxide $$\text {Lu}_2 \text {NiIrO}_6$$ Lu 2 NiIrO 6 exhibits ferrimagnetic (FiM) Mott-insulating state having an energy band gap of 0.20 eV which confirms the experimental observations (Feng et al. in Inorg Chem 58:397–404, 2019). Strong antiferromagnetic superexchange interactions between high-energy half-filled $$\text {Ni}^{+2}$$ Ni + 2 -$$e_g^2\uparrow$$ e g 2 ↑ and low-energy partially filled $$\text {Ir}^{+4}\,t_{2g}^3\uparrow t_{2g}^2\downarrow$$ Ir + 4 t 2 g 3 ↑ t 2 g 2 ↓ orbitals, results in a FiM spin ordering. Besides, the effect of 3d transition metal (TM = Cr, Mn, and Fe) doping with 50% concentration at Ni sites on its electronic and magnetic properties is explored. It is established that smaller size cation-doping at the B site enhances the structural distortion, which further gives strength to the FiM ordering temperature. Interestingly, our results revealed that all TM-doped structures exhibit an electronic transition from Mott-insulating to a half-metallic state with effective integral spin moments. The admixture of Ir 5d orbitals in the spin-majority channel are mainly responsible for conductivity, while the spin minority channel remains an insulator. Surprisingly, a substantial reduction and enhancement of spin moment are found on non-equivalent Ir and oxygen ions, respectively. This leads the Ir ion in a mixed-valence state of $$+4$$ + 4 and $$+5$$ + 5 in all doped systems having configurations of $$5d^5$$ 5 d 5 ($$t_{2g}^3\uparrow t_{2g}^2\downarrow$$ t 2 g 3 ↑ t 2 g 2 ↓ ) and $$5d^4$$ 5 d 4 ($$t_{2g}^2\uparrow t_{2g}^2\downarrow$$ t 2 g 2 ↑ t 2 g 2 ↓ ), respectively. Hence, the present work proposes that doping engineering with suitable impurity elements could be an effective way to tailor the physical properties of the materials for their technological potential utilization in advanced spin devices.

Because of their long coherence times and potential for scalability, semiconductor quantum-dot spin qubits hold great promise for quantum information processing. However, maintaining high connectivity between quantum-dot spin qubits, which favor linear arrays with nearest neighbor coupling, presents a challenge for large-scale quantum computing. In this work, we present evidence for long-distance spin-chain-mediated superexchange coupling between electron spin qubits in semiconductor quantum dots. We weakly couple two electron spins to the ends of a two-site spin chain. Depending on the spin state of the chain, we observe oscillations between the distant end spins. We resolve the dynamics of both the end spins and the chain itself, and our measurements agree with simulations. Superexchange is a promising technique to create long-distance coupling between quantum-dot spin qubits., Comment: 6+9 pages, 4+7 figures

We report a survey of the magnetic properties of metal-organic complexes coupled to ferromagnetic and antiferromagnetic surfaces. Using element-resolved X-ray magnetic circular dichroism, we investigate the magnetism of single, double, and triple-decker phthalocyanines focusing on MnPc, TbPc2, and Tb2Pc3 deposited on Ni, Mn, and CoO thin films. Depending on the number of Pc ligands, we find that the metal ions within the molecules couple either parallel or antiparallel to a ferromagnetic substrate. Whereas single-decker complexes such as MnPc form a unique magnetic entity with ferromagnetic films, the intrinsic single molecule magnet properties of TbPc2 and Tb2Pc3 remain largely unaltered. TbPc2 deposited on perpendicularly magnetized Ni films exhibits enhanced magnetic stability compared to TbPc2 in molecular crystals, opposite to TbPc2 deposited on in-plane magnetized Ni. Depending on the competition between uniaxial anisotropy, superexchange, and Zeeman interaction, the magnetic moment of TbPc2 can be aligned parallel or antiparallel to that of the substrate by modulating the intensity of an external magnetic field. This occurs also for Tb2Pc3, but the substrate-induced exchange coupling in triple-decker molecules is found to be short-ranged, that is, limited to the Tb ion closer to the ferromagnetic surface. Finally, we discuss the conditions required to establish exchange bias between molecules and antiferromagnetic substrates. We show that TbPc2 deposited on antiferromagnetic Mn thin films exhibits both exchange bias and enhanced coercivity when field cooled parallel to the out-of-plane easy axis. However, exchange bias does not extend to all molecules on the surface. On oxide antiferromagnets such as CoO we find no evidence of exchange bias for either TbPc2 or MnPc., We received financial support from the European Research Council (StG 203239 NOMAD), the Ministerio de Ciencia e Innovación (MAT2010-15659), the Agència de Gestió d'Ajuts Universitaris i de Recerca (2009 SGR 695), and the Swiss Competence Centre for Materials Science and Technology (CCMX).