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The induced Pt magnetization in a Pt/Co/Pt thin film structure is studied. The normally nonmagnetic Pt acquires a magnetic moment due to the magnetic proximity effect at the Co–Pt interfaces. Element specific Pt structural and magnetic properties are characterized by synchrotron-based resonant x-ray reflectivity and x-ray resonant magnetic reflectivity measurements. An advanced analysis method based on Bayesian inference is used for model fitting of the x-ray data. Using this method, we retrieve the best fit values of material parameters (e.g., thickness, interfacial roughness) from the data. Analysis of x-ray reflectivity data of this specific system shows that the Pt magnetization and Co–Pt interfacial roughness is significantly different between the top and bottom Pt layers, with both values being larger in the top Pt. The successful application of this Bayesian method to study the magnetic and structural properties of a thin film system demonstrates its effectiveness for x-ray reflectivity data analysis.

Two-dimensional triangular-lattice antiferromagnets are predicted under some conditions to exhibit a quantum spin liquid ground state with no energy barrier to create emergent, fractionalized spinon excitations that carry spin but no charge. Materials that realize this kind of spin liquid are expected to have a low-energy behaviour described by a spinon Fermi surface. Directly imaging the resulting spinons, however, is difficult due to their chargeless nature. Here we use scanning tunnelling spectroscopy to image density waves consistent with the predictions of spinon density modulation arising from a spinon Fermi surface instability in single-layer 1T-TaSe2. We confirm the existence of a triangular lattice of localized spins in this material by contacting it with a metallic 1H-TaSe2 substrate and measuring the Kondo effect. Spectroscopic imaging of isolated single-layer 1T-TaSe2 reveals long-wavelength super-modulations at Hubbard band energies, consistent with the predicted behaviour of itinerant spinons. These super-modulations allow the direct experimental measurement of the spinon Fermi wavevector, in good agreement with theoretical predictions for a two-dimensional quantum spin liquid. Some quantum spin liquids are expected to have an effective Fermi surface of fractionalized spinon excitations. The two-dimensional spin liquid candidate 1T-TaSe2 has charge density modulations that may be caused by an unstable spinon Fermi surface.

The interconversion of charge and spin currents via spin-Hall effect is essential for spintronics. Energy-efficient and deterministic switching of magnetization can be achieved when spin polarizations of these spin currents are collinear with the magnetization. However, symmetry conditions generally restrict spin polarizations to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced. Here, we show control of the spin polarization direction by using a non-collinear antiferromagnet Mn3GaN, in which the triangular spin structure creates a low magnetic symmetry while maintaining a high crystalline symmetry. We demonstrate that epitaxial Mn3GaN/permalloy heterostructures can generate unconventional spin-orbit torques at room temperature corresponding to out-of-plane and Dresselhaus-like spin polarizations which are forbidden in any sample with two-fold rotational symmetry. Our results demonstrate an approach based on spin-structure design for controlling spin-orbit torque, enabling high-efficient antiferromagnetic spintronics., In the typical spin-hall effect, spin-current, charge current, and spin polarisation are all mutually perpendicular, a feature enforced by symmetry. Here, using an anti-ferromagnet with a triangular spin structure, the authors demonstrate a spin-hall effect without a perpendicular spin alignment.

Of all parent compounds of iron-based high-temperature superconductors, ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ exhibits by far the largest magnetostructural coupling due to the sizable biquadratic interaction between Eu and Fe moments. While the coupling between Eu antiferromagnetic (AFM) order and Fe structural/AFM domains enables rapid field detwinning, this prevents simple magnetometry measurements from extracting the critical fields of the Eu metamagnetic transition. Here, we measure these critical fields by combining x-ray magnetic circular dichroism spectroscopy with in situ tunable uniaxial stress and applied magnetic field. The combination of two tuning knobs allows us to separate the stress detwinning of structural domains from the field-induced reorientation of Eu moments. Intriguingly, we find a spin-flip transition which can only result from a strongly anisotropic interaction between Eu planes. We argue that this anisotropic exchange is a consequence of the strong anisotropy in the magnetically ordered Fe layer, which presents a form of higher-order coupling between Eu and Fe magnetism.

In this study, we employ bulk electronic properties characterization and x-ray scattering/spectroscopy techniques to map the structural, magnetic and electronic properties of (Eu$_{1-x}$Ca$_{x}$)$_{2}$Ir$_{2}$O$_{7}$ as a function of Ca-doping. As expected, the metal-insulator transition temperature, $T_{MIT}$, decreases with Ca-doping until a metallic state is realized down to 2 K. In contrast, $T_{AFM}$ becomes decoupled from the MIT and (likely short-range) AFM order persists into the metallic regime. This decoupling is understood as a result of the onset of an electronically phase separated state, the occurrence of which seemingly depends on both synthesis method and rare earth site magnetism. PDF analysis suggests that electronic phase separation occurs without accompanying chemical phase segregation or changes in the short-range crystallographic symmetry while synchrotron x-ray diffraction confirms that there is no change in the long-range crystallographic symmetry. X-ray absorption measurements confirm the $J_{eff}$ = 1/2 character of (Eu$_{1-x}$Ca$_{x}$)$_{2}$Ir$_{2}$O$_{7}$. Surprisingly these measurements also indicate a net electron doping, rather than the expected hole doping, indicating a compensatory mechanism. Lastly, XMCD measurements show a weak Ir magnetic polarization that is largely unaffected by Ca-doping., 18 pages, 10 figures

The anomalous Hall effect (AHE) is an intriguing transport phenomenon occurring typically in ferromagnets as a consequence of broken time reversal symmetry and spin-orbit interaction. It can be caused by two microscopically distinct mechanisms, namely, by skew or side-jump scattering due to chiral features of the disorder scattering, or by an intrinsic contribution directly linked to the topological properties of the Bloch states. Here we show that the AHE can be artificially engineered in materials in which it is originally absent by combining the effects of symmetry breaking, spin orbit interaction and proximity-induced magnetism. In particular, we find a strikingly large AHE that emerges at the interface between a ferromagnetic manganite (La_(0.7)Sr_(0.3)MnO_3) and a semimetallic iridate (SrIrO_3). It is intrinsic and originates in the proximity-induced magnetism present in the narrow bands of strong spin-orbit coupling material SrIrO_3, which yields values of anomalous Hall conductivity and Hall angle as high as those observed in bulk transition-metal ferromagnets. These results demonstrate the interplay between correlated electron physics and topological phenomena at interfaces between 3d ferromagnets and strong spin-orbit coupling 5d oxides and trace an exciting path towards future topological spintronics at oxide interfaces. The anomalous Hall effect (AHE) occurs in ferromagnets caused by intrinsic and extrinsic mechanisms. Here, Yoo et al. report large anomalous Hall conductivity and Hall angle at the interface between a ferromagnet La_(0.7)Sr_(0.3M)nO_3 and a semimetallic SrIrO_3, due to the interplay between correlated physics and topological phenomena.

As a hallmark of electronic correlation, spin-charge interplay underlies many emergent phenomena in doped Mott insulators, such as high-temperature superconductivity, whereas the half-filled parent state is usually electronically frozen with an antiferromagnetic order that resists external control. We report on the observation of a new positive magnetoresistance that probes the staggered susceptibility of a pseudospin-half square-lattice Mott insulator built as an artificial SrIrO3/SrTiO3 superlattice. Its size is particularly large in the high-temperature insulating paramagnetic phase near the N\'eel transition. This novel magnetoresistance originates from a collective charge response to the large longitudinal spin fluctuations under a linear coupling between the external magnetic field and the staggered magnetization enabled by strong spin-orbit interaction. Our results demonstrate a magnetic control of the binding energy of the fluctuating particle-hole pairs in the Slater-Mott crossover regime analogous to the BCS-to-Bose-Einstein condensation crossover of ultracold-superfluids., Comment: Accepted by Nature communications

Chiral interactions in magnetic systems can give rise to rich physics manifested, for example, as nontrivial spin textures. The foremost interaction responsible for chiral magnetism is the Dzyaloshinskii-Moriya interaction (DMI), resulting from inversion symmetry breaking in the presence of strong spin-orbit coupling. However, the atomistic origin of DMIs and their relationship to emergent electrodynamic phenomena, such as topological Hall effect (THE), remain unclear. Here, we investigate the role of interfacial DMIs in 3d-5d transition metal-oxide-based LaMnO3/SrIrO3 superlattices on THE from a chiral spin texture. By additively engineering the interfacial inversion symmetry with atomic-scale precision, we directly link the competition between interfacial collinear ferromagnetic interactions and DMIs to an enhanced THE. The ability to control the DMI and resulting THE points to a pathway for harnessing interfacial structures to maximize the density of chiral spin textures useful for developing high-density information storage and quantum magnets for quantum information science.

We have elucidated the spin, lattice, charge, and orbital coupling mechanism underlying the multiferroic character in tensile-strained $\mathrm{EuTi}{\mathrm{O}}_{3}$ films. Symmetry determined by oxygen octahedral tilting shapes the hybridization between the Eu $4f$ and the Ti $3d$ orbitals, and this inhibits predicted Ti displacement proper ferroelectricity. Instead, phonon softening emerges at low temperatures within the pseudocube (110) plane, orthogonal to the anticipated ferroelectric polarization symmetry. Additionally, the magnetic anisotropy is determined by orbital distortion through hybridization between the Ti $3d$ and the typically isotropic $\mathrm{E}{\mathrm{u}}^{2+}\phantom{\rule{4pt}{0ex}}4f$ states. This unique scenario demonstrates the critical role symmetry plays in the coupling of order parameters defining multiferroic behavior.

We report on the epitaxial strain-driven electronic and antiferromagnetic modulations of a pseudospin-half square-lattice realized in superlattices of (SrIrO_{3})_{1}/(SrTiO_{3})_{1}. With increasing compressive strain, we find the low-temperature insulating behavior to be strongly suppressed with a corresponding systematic reduction of both the Neel temperature and the staggered moment. However, despite such a suppression, the system remains weakly insulating above the Neel transition. The emergence of metallicity is observed under large compressive strain but only at temperatures far above the Neel transition. These behaviors are characteristics of the Slater-Mott crossover regime, providing a unique experimental model system of the spin-half Hubbard Hamiltonian with a tunable intermediate coupling strength.

Correlated electrons give rise to both exotic electronic and magnetic properties in rare-earth nickelates. Here we present evidence of the interfacial coupling between two nickelate systems, EuNiO_{3} (ENO) and LaNiO_{3} (LNO), with different electronic and magnetic properties but with compatible structural registry giving rise to an electrostructural transition, unobserved in each constituent. Nominally, LNO remains in a paramagnetic-metallic R3[over ¯]c phase while orthorhombic ENO undergoes antiferromagnetic and insulating transitions. However, the ENO/LNO heterostructure displays a uniform rotational symmetry set by an entwined interface. This leads to an anomalous reduction of bond disproportionation in the ENO layer through the metal to insulator transition and concomitantly charge disproportionation opens the gap accompanied by antiferromagnetic ordering. Our results resolve a long-standing question in the physics of rare-earth nickelates, herein demonstrating that charge and bond disproportionation are competing mechanisms for the charge localization process in the rare-earth nickelate system.

Rare-earth pyrochlore iridates $(R{E}_{2}\mathrm{I}{\mathrm{r}}_{2}{\mathrm{O}}_{7})$ consist of two interpenetrating cation sublattices, the $\mathit{RE}$ with highly frustrated magnetic moments, and the iridium with extended conduction orbitals significantly mixed by spin-orbit interactions. The coexistence and coupling of these two sublattices create a landscape for discovery and manipulation of quantum phenomena such as the topological Hall effect, massless conduction bands, and quantum criticality. Thin films allow extended control of the material system via symmetry-lowering effects such as strain. While bulk $\mathrm{P}{\mathrm{r}}_{2}\mathrm{I}{\mathrm{r}}_{2}{\mathrm{O}}_{7}$ shows a spontaneous hysteretic Hall effect below 1.5 K, we observe the effect at elevated temperatures up to 15 K in epitaxial thin films on (111) yttria-stabilized zirconia (YSZ) substrates synthesized via solid-phase epitaxy. Similar to the bulk, the lack of observable long-range magnetic order in the thin films points to a topological origin. We use synchrotron-based element-specific x-ray diffraction and x-ray magnetic circular dichroism to compare powders and thin films to attribute the spontaneous Hall effect in the films to localization of the Ir moments. We link the thin-film Ir local moments to lattice distortions absent in the bulklike powders. We conclude that the elevated-temperature spontaneous Hall effect is caused by the topological effect originating either from the Ir or Pr sublattice, with interaction strength enhanced by the Ir local moments. This spontaneous Hall effect with weak net moment highlights the effect of vanishingly small lattice distortions as a means to discover topological phenomena in metallic frustrated magnetic materials.

Perovskite rhodates are characterized by intermediate strengths of both electronic correlation as well as spin-orbit coupling (SOC) and usually behave as moderately correlated metals. A recent publication [Phys. Rev. B 95, 245121 (2017)] on epitaxial ${\mathrm{SrRhO}}_{3}$ thin films reported a bad-metallic behavior and suggested the occurrence of antiferromagnetism below 100 K. We have further studied this ${\mathrm{SrRhO}}_{3}$ thin film by hard x-ray photoemission spectroscopy and found a very small density of states (DOS) at the Fermi level (${E}_{F}$), which is consistent with the previously reported bad-metallic behavior. However, the absence of DOS persists up to room temperature, which contradicts the explanation of antiferromagnetic transition at $\ensuremath{\sim}100$ K. We also employed electronic structure calculations within the framework of density functional theory and dynamical mean-field theory. In contrast to the photoemission results, our calculations indicate metallic behaviors of both bulk ${\mathrm{SrRhO}}_{3}$ and the ${\mathrm{SrRhO}}_{3}$ thin film, and a stronger correlation effect was observed in the thin film than that in the bulk. The calculated uniform magnetic susceptibility is substantially larger in the thin film than that in the bulk. We also investigated the role of SOC and found only a moderate modulation of the band structure. Hence SOC is not expected to significantly affect the electronic correlation in ${\mathrm{SrRhO}}_{3}$. Extrinsic effects of finite-thickness effects beyond our calculations and localization effects may play important roles to induce the negligible spectral weight at ${E}_{F}$.

In this work, we analyze the role of a thin Cr spacer between Fe and Gd layers on the structure and magnetic properties of a [Fe(35 Å)/Cr(tCr)/Gd(50 Å)/Cr(tCr)]12 superlattice. Samples without the Cr spacer (tCr = 0) and with a thin spacer (tCr = 4 Å) are investigated using X-ray diffraction, polarized neutron and resonance X-ray magnetic reflectometry, static magnetometry, magneto-optical Kerr effect, and ferromagnetic resonance techniques. Magnetic properties are studied experimentally in a wide temperature range 4–300 K and analyzed theoretically using numerical simulation on the basis of the mean-field model. We show that a reasonable agreement with the experimental data can be obtained considering temperature dependence of the effective field parameter in gadolinium layers. The analysis of the experimental data shows that besides a strong reduction of the antiferromagnetic coupling between Fe and Gd, the introduction of Cr spacers into Fe/Gd superlattice leads to modification of both structural and magnetic characteristics of the ferromagnetic layers. © 2018, Pleiades Publishing, Inc.

A fundamental difference between antiferromagnets and ferromagnets is the lack of linear coupling to a uniform magnetic field due to the staggered order parameter. Such coupling is possible via the Dzyaloshinskii-Moriya (DM) interaction but at the expense of reduced antiferromagnetic (AFM) susceptibility due to the canting-induced spin anisotropy. We solve this long-standing problem with a top-down approach that utilizes spin-orbit coupling in the presence of a hidden SU(2) symmetry. We demonstrate giant AFM responses to sub-Tesla external fields by exploiting the extremely strong two-dimensional critical fluctuations preserved under a symmetry-invariant exchange anisotropy, which is built into a square-lattice artificially synthesized as a superlattice of SrIrO3 and SrTiO3. The observed field-induced logarithmic increase of the ordering temperature enables highly efficient control of the AFM order. As antiferromagnets promise to afford switching speed and storage security far beyond ferromagnets, our symmetry-invariant approach unleashes the great potential of functional antiferromagnets., Comment: 3 figures

In materials where two or more ordering degrees of freedom are closely matched in their free energies, coupling between them, or multiferroic behavior can occur. These phenomena can produce a very rich phase behavior, as well as emergent phases that offer useful properties and opportunities to reveal novel phenomena in phase transitions. The ordered alloy FeRh undergoes an antiferromagnetic to ferromagnetic phase transition at ~375 K, which illustrates the interplay between structural and magnetic order mediated by a delicate energy balance between two configurations. We have examined this transition using a combination of high-resolution x-ray structural and magnetic imaging and comprehensive x-ray magnetic circular dichroism spectroscopy. We find that the transition proceeds via a defect-driven domain nucleation and growth mechanism, with significant return point memory in both the structural and magnetic domain configurations. The domains show evidence of inhibited growth after nucleation, resulting in a quasi-2nd order temperature behavior.

The depth-resolved magnetic configuration of a Ga0.97Mn0.03As/GaAs:Be multilayer with an antiferromagnetic interlayer exchange coupling was investigated using surface-sensitive soft x-ray resonant magnetic reflectivity (XRMR). We observed intriguing opposite increments in the XRMR intensities at the half-Bragg peak position between two different remanent states with opposite field sweep directions. A quantitative analysis shows that these opposite intensity increments result from two different anti-parallel spin configurations in such a way that the magnetization of top-most magnetic layer is pinned along the saturation magnetization direction before the remanent state. This surface pinning effect is important for understanding spin-dependent transport in semiconducting magnetic multilayers.

Single crystals of a honeycomb lattice antiferromagnet, ${\mathrm{Tb}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Ga}}_{9}$, were synthesized, and the physical properties have been studied. From magnetometry, a long-range antiferromagnetic ordering at $\ensuremath{\approx}12.5$ K with highly anisotropic magnetic behavior was found. Neutron powder diffraction confirms that the Tb spins lie along the $\mathbf{a}$-axis, parallel to the shortest Tb-Tb contact. Two field-induced spin-flip transitions are observed when the field is applied parallel to this axis, separated by a plateau corresponding roughly to $M\ensuremath{\approx}{M}_{\mathrm{s}}/2$. Transport measurements show the resistivity to be metallic with a discontinuity at the onset of N\'eel order. Heat capacity shows a $\ensuremath{\lambda}$-like transition confirming the bulk nature of the magnetism. We propose a phenomenological spin Hamiltonian that describes the magnetization plateau as a result of strong Ising character arising from a quasidoublet ground state of the ${\mathrm{Tb}}^{3+}$ ion in a site of ${C}_{s}$ symmetry and expressing a significant bond-dependent anisotropy.

Significance Heterojunctions between dissimilar materials provide an opportunity to generate emergent properties not present in their individual components. These properties are often linked to interfacial charge transfer, an important mechanism allowing one to access and control novel quantum states. Here, we have grown unit-cell scale superlattices composed of 5 d S r I r O 3 and 3 d L a N i O 3 . We have discovered a massive interfacial charge transfer from Ir to Ni, triggering dramatic electronic and magnetic reconstructions at the interface that suppress strong spin–orbit coupling effects normally present in the iridates. These findings call for careful evaluation and reinterpretation of experiments on spin–orbit-driven physics in thin films and heterostructures based on prototypical 5 d transition metal oxides.

This study explores a filling-controlled metal-insulator transition in the pyrochlore-based iridium oxide Nd${}_{2}$Ir${}_{2}$O${}_{7}$. Through measurements of the local site symmetry, average structure, and electronic properties, the authors comment on the interplay of the lattice, charge gap, and magnetism in this material. The work holds important implications for tuning this compound toward its anticipated quantum critical regime, which has been predicted to host a Weyl semimetal state.

National Science Foundation (U.S.) (Through Massachusetts Institute of Technology Materials Research Science and Engineering Center, DMR-1419807)

Significance Interfaces of transition-metal oxides (TMOs) offer a fertile platform to uncover emergent states, which has been extensively explored in 3 d TMOs with strong electron correlations. Recently research on 5 d TMOs with pronounced spin–orbit coupling (SOC) is flourishing due to the emergence of new topological states and potential application in spintronics. Interfaces between 3 d and 5 d TMOs provide a unique test bed to combine the merits of these two fundamental interactions. However, so far research is limited. Here we present results on one model system comprising the ferromagnet La 2/3 Sr 1/3 MnO 3 and the strong SOC paramagnet SrIrO 3 . We observe a manipulation of the magnetic anisotropy by tuning the SrIrO 3 dimensionality, which is accompanied by a novel SOC state in SrIrO 3 .

Gauss's law dictates that the net electric field inside a conductor in electrostatic equilibrium is zero by effective charge screening; free carriers within a metal eliminate internal dipoles that may arise owing to asymmetric charge distributions. Quantum physics supports this view, demonstrating that delocalized electrons make a static macroscopic polarization, an ill-defined quantity in metals--it is exceedingly unusual to find a polar metal that exhibits long-range ordered dipoles owing to cooperative atomic displacements aligned from dipolar interactions as in insulating phases. Here we describe the quantum mechanical design and experimental realization of room-temperature polar metals in thin-film ANiO3 perovskite nickelates using a strategy based on atomic-scale control of inversion-preserving (centric) displacements. We predict with ab initio calculations that cooperative polar A cation displacements are geometrically stabilized with a non-equilibrium amplitude and tilt pattern of the corner-connected NiO6 octahedral--the structural signatures of perovskites--owing to geometric constraints imposed by the underlying substrate. Heteroepitaxial thin-films grown on LaAlO3 (111) substrates fulfil the design principles. We achieve both a conducting polar monoclinic oxide that is inaccessible in compositionally identical films grown on (001) substrates, and observe a hidden, previously unreported, non-equilibrium structure in thin-film geometries. We expect that the geometric stabilization approach will provide novel avenues for realizing new multifunctional materials with unusual coexisting properties.

The recent discovery of two-dimensional (2D) magnets, with a number of interesting magnetic properties, has drawn much interest due to their potential for future 2D spintronic device applications. CrI3, a van der Waals magnet, exhibits two-dimensional ferromagnetism even in monolayer form, stabilized by strong magnetic anisotropy. Its interlayer magnetic ordering is coupled to structural layer stacking, implying that the charge density distribution mediating van der Waals interactions plays a key role in the magnetic interaction between the layers. Using polarization-dependent x-ray spectroscopy, we investigated the response of the electronic environment around Cr and I sites to structural changes of layer stacking order. The highly anisotropic nature of the Cr local environment is significantly enhanced and is accompanied by changes in the valence band, in the rhombohedral phase. Magnetic spectroscopy measurements reveal a sizable iodine orbital moment, indicating the iodine contribution to magnetic anisotropy. Our results uncover an important role for the extended nature of anisotropic Cr orbital states in dictating interlayer magnetic interactions and the iodine contribution to magnetic anisotropy.

We report the observation of multiple phonon satellite features in ultra thin superlattices of form $n$SrIrO$_3$/$m$SrTiO$_3$ using resonant inelastic x-ray scattering. As the values of $n$ and $m$ vary the energy loss spectra show a systematic evolution in the relative intensity of the phonon satellites. Using a closed-form solution for the cross section, we extract the variation in the electron-phonon coupling strength as a function of $n$ and $m$. Combined with the negligible carrier doping into the SrTiO$_3$ layers, these results indicate that tuning of the electron-phonon coupling can be effectively decoupled from doping. This work showcases both a feasible method to extract the electron-phonon coupling in superlattices and unveils a potential route for tuning this coupling which is often associated with superconductivity in SrTiO$_3$-based systems., 4 pages, 5 figures

The application of pressure reveals a rich phase diagram for the quantum $S=1/2$ spin chain material TiOCl. We performed x-ray diffraction on single-crystal samples in a diamond-anvil cell down to $T=4$ K and pressures up to 14.5 GPa. Remarkably, the magnetic interaction scale increases dramatically with increasing pressure, as indicated by the high onset temperature of the spin-Peierls phase. The spin-Peierls phase was probed at $\ensuremath{\sim}6$ GPa up to 215 K but possibly extends in temperature to above $T=300$ K, indicating the possibility of a quantum singlet state at room temperature. Near the critical pressure for the transition to the more metallic phase, coexisting phases are exemplified by incommensurate order in two directions. Further comparisons are made with the phase diagrams of related spin-Peierls systems that display metallicity and superconductivity under pressure.

Perpendicular magnetic anisotropy (PMA) plays a critical role in the development of spintronics, thereby demanding new strategies to control PMA. Here we demonstrate a conceptually new type of interface induced PMA that is controlled by oxygen octahedral rotation. In superlattices comprised of La_{1-x}Sr_{x}MnO_{3} and SrIrO_{3}, we find that all superlattices (0≤x≤1) exhibit ferromagnetism despite the fact that La_{1-x}Sr_{x}MnO_{3} is antiferromagnetic for x>0.5. PMA as high as 4×10^{6} erg/cm^{3} is observed by increasing x and attributed to a decrease of oxygen octahedral rotation at interfaces. We also demonstrate that oxygen octahedral deformation cannot explain the trend in PMA. These results reveal a new degree of freedom to control PMA, enabling discovery of emergent magnetic textures and topological phenomena.

We report an experimental investigation of the two-dimensional ${J}_{\text{eff}}=1/2$ antiferromagnetic Mott insulator by varying the interlayer exchange coupling in [${({\mathrm{SrIrO}}_{3})}_{1}$, ${({\mathrm{SrTiO}}_{3})}_{m}$] ($m=1$, 2 and 3) superlattices. Although all samples exhibited an insulating ground state with long-range magnetic order, temperature-dependent resistivity measurements showed a stronger insulating behavior in the $m=2$ and $m=3$ samples than the $m=1$ sample which displayed a clear kink at the magnetic transition. This difference indicates that the blocking effect of the excessive ${\mathrm{SrTiO}}_{3}$ layer enhances the effective electron-electron correlation and strengthens the Mott phase. The significant reduction of the N\'eel temperature from 150 K for $m=1$ to 40 K for $m=2$ demonstrates that the long-range order stability in the former is boosted by a substantial interlayer exchange coupling. Resonant x-ray magnetic scattering revealed that the interlayer exchange coupling has a switchable sign, depending on the ${\mathrm{SrTiO}}_{3}$ layer number $m$, for maintaining canting-induced weak ferromagnetism. The nearly unaltered transition temperature between the $m=2$ and the $m=3$ demonstrated that we have realized a two-dimensional antiferromagnet at finite temperatures with diminishing interlayer exchange coupling.

The perovskite SrIrO3 is an exotic narrow-band metal owing to a confluence of the strengths of the spin-orbit coupling (SOC) and the electron-electron correlations. It has been proposed that topological and magnetic insulating phases can be achieved by tuning the SOC, Hubbard interactions, and/or lattice symmetry. Here, we report that the substitution of nonmagnetic, isovalent Sn4+ for Ir4+ in the SrIr1-xSnxO3 perovskites synthesized under high pressure leads to a metal-insulator transition to an antiferromagnetic (AF) phase at TN > 225 K. The continuous change of the cell volume as detected by x-ray diffraction and the lamda-shape transition of the specific heat on cooling through TN demonstrate that the metal-insulator transition is of second-order. Neutron powder diffraction results indicate that the Sn substitution enlarges an octahedral-site distortion that reduces the SOC relative to the spin-spin exchange interaction and results in the type-G AF spin ordering below TN. Measurement of high-temperature magnetic susceptibility shows the evolution of magnetic coupling in the paramagnetic phase typical of weak itinerant-electron magnetism in the Sn-substituted samples. A reduced structural symmetry in the magnetically ordered phase leads to an electron gap opening at the Brillouin zone boundary below TN in the same way as proposed by Slater., 15 pages, 4 figures

In spintronic devices consisting of ferromagnetic/nonmagnetic systems, the ferromagnet-induced magnetic moment in the adjacent nonmagnetic material significantly influences the spin transport properties. In this study, such magnetic proximity effect in a Pd/Co/Pd trilayer system is investigated by x-ray magnetic circular dichroism and x-ray resonant magnetic reflectivity, which enables magnetic characterizations with element and depth resolution. We observe that the total Pd magnetic moments induced at the top Co/Pd interface are significantly larger than the Pd moments at the bottom Pd/Co interface, whereas transmission electron microscopy and reflectivity analysis indicate the two interfaces are nearly identical structurally. Such asymmetry in magnetic proximity effects could be important for understanding spin transport characteristics in ferromagnetic/nonmagnetic systems and its potential application to spin devices.

In materials undergoing spontaneous symmetry breaking transitions, the emergence of multiple competing order parameters is pervasive. Employing in-field x-ray diffraction, we investigate the temperature and magnetic field dependence of the crystallographic structure of MnBi, elucidating the microscopic interplay between lattices and spin. The hexagonal phase of MnBi undergoes a spin reorientation transition (TSR), whereby the easy axis direction changes from the c axis to the basal plane. Across TSR, an abrupt symmetry change is accompanied by a clear sign change in the magnetostrictive coefficient, revealing that this transition corresponds to the onset of the spin reorientation. In the vicinity of TSR, a significantly larger in-plane magnetostrictive effect is observed, presenting the emergence of an intermediate phase that is highly susceptible to an applied magnetic field. X-ray linear dichroism shows that asymmetric Bi and Mn p orbitals do not play a role in the spin reorientation. This work suggests that the spin reorientation is caused by structural modification rather than changes in the local electronic configuration, providing a strategy for manipulating the magnetic anisotropy by external strain.

Heisenberg interactions are ubiquitous in magnetic materials and have been prevailing in modeling and designing quantum magnets. Bond-directional interactions offer a novel alternative to Heisenberg exchange and provide the building blocks of the Kitaev model, which has a quantum spin liquid (QSL) as its exact ground state. Honeycomb iridates, A2IrO3 (A=Na,Li), offer potential realizations of the Kitaev model, and their reported magnetic behaviors may be interpreted within the Kitaev framework. However, the extent of their relevance to the Kitaev model remains unclear, as evidence for bond-directional interactions remains indirect or conjectural. Here, we present direct evidence for dominant bond-directional interactions in antiferromagnetic Na2IrO3 and show that they lead to strong magnetic frustration. Diffuse magnetic x-ray scattering reveals broken spin-rotational symmetry even above Neel temperature, with the three spin components exhibiting nano-scale correlations along distinct crystallographic directions. This spin-space and real-space entanglement directly manifests the bond-directional interactions, provides the missing link to Kitaev physics in honeycomb iridates, and establishes a new design strategy toward frustrated magnetism., Comment: Nature Physics, accepted (2015)

We have investigated the microscopic origin of temperature and magnetic-field actuated magnetization reversal in Cu0.73Mn0.77[Fe(CN)6].zH2O, using XMCD. Our results show a fair deviation from the mean-field-theory in the form of different ordering temperatures of Fe and Mn sublattices. A preferential sign reversal of Mn spin under magnetic field and different spin cant angles for the two sublattices have also been observed. An antiferromagnetic coupling between the Fe and Mn sublattices along with different ordering temperatures (sublattice decoupling) for these sublattices explain the temperature-dependent magnetization reversal. Whereas, Mn spin reversal alone (under external magnetic field) is responsible for the observed field-dependent magnetization reversal. The dissimilar magnetic behavior of Fe and Mn sublattices in this cubic 3d-orbital system has been understood by invoking disparity and competition among inter-sublattice magnetic control parameters, viz. magnetic Zeeman energy, exchange coupling constant and magnetic anisotropy constant. Our results have significant design implications for future magnetic switches, by optimizing the competition among these magnetic control parameters.

International audience; The pressure behavior of an amorphous Si-rich SiGe alloy (α-SixGe1−x , x = 0.75) has been investigated up to about 30 GPa, by a combination of Raman spectroscopy, x-ray absorption spectroscopy, and x-ray diffraction measurements. The trends of microscopic structural properties and of the Raman-active phonon modes are presented in the whole pressure range. Nucleation of nanocrystalline alloy particles and metallization have been observed above 12 GPa, with a range of about 2 GPa of coexistence of amorphous and crystalline phases. Transformations from the amorphous tetrahedral, to the crystalline tetragonal (β-Sn) and to the simple hexagonal structures have been observed around 13.8 and 21.8 GPa. The recovered sample upon depressurization, below about 4 GPa, shows a local structure similar to the as-deposited one. Inhomogeneities of the amorphous texture at the nanometric scale, probed by high-resolution transmission electron microscopy, indicate that the recovered amorphous sample has a different ordering at this scale, and therefore the transformations can not be considered fully reversible. The role of disordered grain boundaries at high pressure and the possible presence of a highdensity amorphous phase are discussed.