Antiferromagnetic MnO Research Articles

Spinteract: a program to refine magnetic interactions to diffuse scattering data

Magnetic diffuse scattering—the broad magnetic scattering features observed in neutron-diffraction data above a material’s magnetic ordering temperature—provides a rich source of information about the material’s magnetic Hamiltonian. However, this information has often remained under-utilised due to a lack of available computer software that can fit values of magnetic interaction parameters to such data. Here, an open-source computer program, Spinteract, is presented, which enables straightforward refinement of magnetic interaction parameters to powder and single-crystal magnetic diffuse scattering data. The theory and implementation of this approach are summarised. Examples are presented of refinements to published experimental diffuse-scattering data sets for the canonical antiferromagnet MnO and the highly-frustrated classical spin liquid Gd3Ga5O12. Guidelines for data collection and refinement are outlined, and possible developments of the approach are discussed.

Photoelectron diffraction, multi-atom resonant photoemission, pioneered by Fadley

We discuss two important subjects pioneered by Fadley. As the first topic, we discuss the energy dependence of the intensity ratio observed in Mn 3s multiplet splitting excited from antiferromagnetic MnO, MnF2 and KMnF3. We apply the many-body scattering theory developed by Hedin in order to incorporate the extrinsic effects. We find that the extrinsic effects play an important role to explain the energy dependence of the branching ratio. Preliminary calculations show the smaller energy dependence than that observed. As the second topic we briefly discuss some features in multi-atom resonant photoemission (MARPE) based on the first principle quantum theory. Importance of the local symmetry around an emitter is stressed.

Testing the Green's function coupled cluster singles and doubles impurity solver on real materials within the framework of self-energy embedding theory

We apply the Green's function coupled cluster singles and doubles (GFCCSD) impurity solver to realistic impurity problems arising for strongly correlated solids within the self-energy embedding theory (SEET) framework. We describe the details of our GFCC solver implementation, investigate its performance, and highlight potential advantages and problems on examples of impurities created during the self-consistent SEET for antiferromagnetic MnO and paramagnetic SrMnO$_{3}$. GFCCSD provides satisfactory descriptions for weakly and moderately correlated impurities with sizes that are intractable by existing accurate impurity solvers such as exact diagonalization (ED). However, our data also shows that when correlations become strong, the singles and doubles approximation used in GFCC could lead to instabilities in searching for the particle number present in impurity problems. These instabilities appears especially severe when the impurity size gets larger and multiple degenerate orbitals with strong correlations are present. We conclude that to fully check the reliability of GFCCSD results and use them in fully {\em ab initio} calculations in the absence of experiments, a verification from a GFCC solver with higher order excitations is necessary.

Nonadiabatic Atomic-Like State Stabilizing Antiferromagnetism and Mott Insulation in MnO

This paper reports evidence that the antiferromagnetic and insulating ground state of MnO is caused by a nonadiabatic atomic-like motion, as is evidently the case in NiO. In addition, it is shown that experimental findings on the displacements of the Mn and O atoms in the antiferromagnetic phase of MnO corroborate the presented suggestion that the rhombohedral-like distortion in antiferromagnetic MnO, as well as in antiferromagnetic NiO is an inner distortion of the monoclinic base-centered Bravais lattice of the antiferromagnetic phases.

Spin-phonon interaction increased by compressive strain in antiferromagnetic MnO thin films

MnO thin films with various thicknesses and strains were grown on MgO substrates by pulsed laser deposition technique, then characterized using x-ray diffraction and infrared reflectance spectroscopy. Films grown on (0 0 1)-oriented MgO substrates exhibit homogenous biaxial compressive strain which increases as the film thickness is reduced. For that reason, in paramagnetic phase, the frequency of doubly-degenerate phonon increases with the strain, and splits below Néel temperature TN due to the magnetic-exchange interaction. The phonon splitting in the MnO (0 0 1) films is 20% larger than that of the bulk MnO. Films grown on (1 1 0)-oriented MgO substrates exhibit a huge phonon splitting already at room temperature due to the anisotropic in-plane compressive strain. Below TN, the lower-frequency phonon splits in the IR spectra and the higher-frequency phonon strongly hardens in AFM phase; these features are evidences for a spin-order-induced structural phase transition from tetragonal to a lower symmetry phase. Total phonon splitting is 55 cm−1 in (1 1 0)-oriented MnO film, which is more than twice the value in bulk MnO, but since the splitting is present already in paramagnetic phase, we cannot clearly correlate it with the value of exchange coupling constant. Nevertheless, at least observation of enhanced phonon splitting in strained MnO (0 0 1) films show that the exchange coupling could be enhanced by the compressive strain which supports recent theoretical predictions published by Wan et al (2016 Sci. Rep. 6 22743) and Fischer et al (2009 Phys. Rev. B 80 014408).

Long- and short-range magnetism in the frustrated double perovskite Ba2MnWO6

The structural and magnetic properties of the face-centered cubic double perovskite Ba2MnWO6 were investigated using neutron powder diffraction, DC-magnetometry, muon spin relaxation and inelastic neutron scattering. Ba2MnWO6 undergoes Type II long-range antiferromagnetic ordering at a Neel temperature of 8(1) K with a frustration index, f = 8. Inelastic neutron scattering was used to identify the magnetic coupling constants J1 and J2, which were found to equal -0.080 meV and -0.076 meV respectively. This indicated that both of the magnetic coupling constants were antiferromagnetic with similar magnitudes, which is in contrast to other known 3d metal double perovskites Ba2MWO6. Above the Neel temperature, muon spin relaxation measurements and inelastic neutron scattering techniques identify a short-range correlated magnetic state that is similar to that observed in the archetypical face-centered cubic lattice antiferromagnet MnO.

Lattice dynamical properties of antiferromagnetic MnO, CoO, and NiO, and the lattice thermal conductivity of NiO

Lattice dynamical properties of antiferromagnetic rocksalt oxides are often interpreted using the cubic space group $Fm\overline{3}m$, although below N\'eel temperature their magnetic substructure possesses a lower symmetry. For example, in the case of NiO, a rhombohedral structural distortion lowers the symmetry to trigonal space group $R\overline{3}m$ below 525 K. We performed hybrid density functional theory calculations on the phonon dispersion relations of MnO, CoO, and NiO, and the lattice thermal conductivity of NiO using both $Fm\overline{3}m$ and $R\overline{3}m$ space groups. The calculated acoustic phonon frequencies of all oxides agree well with the available experimental data, while the optical modes of MnO and CoO show somewhat larger discrepancies. Our calculations show the phonon density of states to be very similar with both studied space groups. The experimental thermal conductivity of antiferromagnetic NiO is reproduced well below the N\'eel temperature by solving the linearized phonon Boltzmann transport equation.

Assessing model-dielectric-dependent hybrid functionals on the antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO

Recently, two nonempirical hybrid functionals, dielectric-dependent range-separated hybrid functional based on the Coulomb-attenuating method (DD-RSH-CAM) and doubly screened hybrid functional (DSH), have been suggested by Chen et al (2018 Phys. Rev. Mater. 2 073803) and Cui et al (2018 J. Phys. Chem. Lett. 9 2338), respectively. These two hybrid functionals are both based on a common model dielectric function approach, but differ in the way how to non-empirically obtain the range-separation parameter. By retaining the full short-range Fock exchange and a fraction of the long-range Fock exchange that equals the inverse of the dielectric constant, both DD-RSH-CAM and DSH turn out to perform very well in predicting the band gaps for a large variety of semiconductors and insulators. Here, we assess how these two hybrid functionals perform on challenging antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO by comparing them to other conventional hybrid functionals and the GW method. We find that single-shot DD0-RSH-CAM and DSH0 improve the band gaps towards experiments as compared to conventional hybrid functionals. The magnetic moments are slightly increased, but the predicted dielectric constants are decreased. The valence band density of states (DOS) predicted by DD0-RSH-CAM and DSH0 are as satisfactory as HSE03 in comparison to experimental spectra, however, the conduction band DOS are shifted to higher energies by about 2 eV compared to HSE03. Self-consistent DD-RSH-CAM and DSH deteriorate the results with a significant overestimation of band gaps.

Valley and spin splitting in monolayer TX2/antiferromagnetic MnO (T = Mo, W; X = S, Se) van der Waals heterostructures

The electronic structure of monolayer MoS2, WS2, MoSe2 and WSe2 on top of antiferromagnetic MnO(1 1 1) is investigated systematically by first-principles calculations. It is found that termination of the MnO substrate can switch the spin splitting of monolayer MoS2, WS2, MoSe2 and WSe2. The spin splitting of MoS2/MnO for six possible interface configurations is varied from 24 to 291 meV for the K point, and 18 to 253 meV for the Kʹ point. The pattern of stacking also induces p - or n-type doping of MoS2, revealing that the conductivity of the heterostructures could be tuned by stacking on MnO. In addition, we also calculate electronic structures of WS2/MnO, MoSe2/MnO and WSe2/MnO heterostructures in the Mn-terminated (III) configuration, and find that the spin splitting at the K point is 553, 324 and 481 meV, and 215, 9 and 284 meV for the Kʹ point, respectively. Furthermore, the time-reversal symmetry is broken by the stacking on MnO that leads to the valley polarization. The valley splitting of MoS2/MnO, WS2/MnO, MoSe2/MnO and WSe2/MnO is 161, 193, 171 and 125 meV in the Mn-terminated (III) configuration. The results present a new type of novel heterostructure that has potential applications in spintronic and valleytronic devices.

Laser Ablation Synthesis, Structure, and Exchange Bias of Mn4C/MnO Powders

Magnetic powders containing ferrimagnetic Mn4C phase and antiferromagnetic MnO phase were prepared from a Mn-C alloy by using the laser ablation method. The Mn-C alloys were composed of Mn4C and α-Mn(C) solid solution, which decomposed into Mn4C and Mn23C6 during laser ablation. The Mn clusters precipitated from the Mn(C) solid solution were spontaneously oxidized into MnO when exposed to air, forming an exchange bias system of Mn4C/MnO with ferrimagnetic/antiferromagnetic interfaces. The exchange bias field (22.3 mT at 5 K) of the Mn4C/MnO micro-powders, which show irregular shapes and a wide size distribution in the range of ~ 100 nm to ~ 20 μm, decreases with increasing temperature. Both the interface spins and the surface spins contribute to the exchange bias effect in these powders. The coercivity of the laser-ablated micro-powders reached up to ~ 150 mT at 5 K and ~ 40 mT at 400 K, respectively.

Electrochemical Magnetization Switching and Energy Storage in Manganese Oxide filled Carbon Nanotubes

The ferrimagnetic and high-capacity electrode material Mn3O4 is encapsulated inside multi-walled carbon nanotubes (CNT). We show that the rigid hollow cavities of the CNT enforce size-controlled nanoparticles which are electrochemically active inside the CNT. The ferrimagnetic Mn3O4 filling is switched by electrochemical conversion reaction to antiferromagnetic MnO. The conversion reaction is further exploited for electrochemical energy storage. Our studies confirm that the theoretical reversible capacity of the Mn3O4 filling is fully accessible. Upon reversible cycling, the Mn3O4@CNT nanocomposite reaches a maximum discharge capacity of 461 mA h g−1 at 100 mA g−1 with a capacity retention of 90% after 50 cycles. We attribute the good cycling stability to the hybrid nature of the nanocomposite: (1) Carbon encasements ensure electrical contact to the active material by forming a stable conductive network which is unaffected by potential cracks of the encapsulate. (2) The CNT shells resist strong volume changes of the encapsulate in response to electrochemical cycling, which in conventional (i.e., non-nanocomposite) Mn3O4 hinders the application in energy storage devices. Our results demonstrate that Mn3O4 nanostructures can be successfully grown inside CNT and the resulting nanocomposite can be reversibly converted and exploited for lithium-ion batteries.

Surfactant-Mediated Resistance to Surface Oxidation in MnO Nanostructures.

The intrinsic physical properties of nanostructures of metals and their oxides are altered when they are prone to surface oxidation in ambient atmosphere. To overcome this limitation, novel synthesis methodologies are required. In this study, solid octahedral shapes of MnO limit the inward oxygen diffusion compared to that of the MnO-nanoparticle-assembled octahedra. In addition to morphology control, which restricts the thickness of the Mn3O4 surface layer, the binding chemistry of the surfactants plays an essential role. For example, the Mn3O4 surface layer is 0.4 nm thinner with trioctylphosphine oxide than with trioctylamine as the surfactant. The nanostructures were prepared by varying the surfactants, surfactant-to-precursor molar ratio, accelerating agent, and reaction heating rate. The surface oxidation of MnO nano-octahedra was probed by Rietveld analysis of X-ray diffraction patterns and X-ray photoelectron spectroscopy and characterized by magnetic measurements, as the presence of ferrimagnetic Mn3O4 shell on the antiferromagnetic MnO core provides an exchange coupling at the core–shell interface. Thicker the Mn3O4 shell, higher is the exchange-biased hysteresis loop shift.

Magnetism of monomer MnO and heterodimer FePt@MnO nanoparticles

We report about the magnetic properties of antiferromagnetic (AF) MnO nanoparticles (NPs) with different sizes (6-19nm). Using a combination of polarized neutron scattering and magnetometry we were able to resolve previously observed peculiarities. Magnetometry, on the one hand, reveals a peak in the zero field cooled (ZFC) magnetization curves at low temperatures (~25K) but no feature around the N\'eel temperature at 118K. On the other hand, polarized neutron scattering shows the expected behavior of the AF order parameter vanishing around 118K. Moreover, hysteresis curves measured at various temperatures reveal an exchange bias effect indicating a coupling of an AF core to a ferromagnetic (FM)-like shell. ZFC data measured at various fields exclude a purely superparamagnetic (SPM) scenario. We conclude that the magnetic behavior of MnO particles can be explained by a superposition of SPM-like thermal fluctuations of the AF-N\'eel vector inside the AF core and a strong magnetic coupling to a ferrimagnetic Mn$_2$O$_3$ or Mn$_3$O$_4$ shell. In addition, we have studied heterodimer ('Janus') particles, where a FM FePt particle is attached to the AF MnO particle. Via the exchange bias effect, the magnetic moment of the FePt subunit is stabilized by the MnO.

Verification of Anderson Superexchange in MnO via Magnetic Pair Distribution Function Analysis and abinitio Theory.

We present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a ∼1 nm length scale that differ substantially from the low-temperature long-range spin arrangement. Abinitio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominated by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. The Anderson superexchange model for MnO is thus verified by experimentation and confirmed by abinitio theory.

Magnetic structure determination from the magnetic pair distribution function (mPDF): ground state of MnO.

An experimental determination of the magnetic pair distribution function (mPDF) defined in an earlier paper [Frandsen et al. (2014). Acta Cryst. A70, 3-11] is presented for the first time. The mPDF was determined from neutron powder diffraction data from a reactor and a neutron time-of-flight total scattering source on a powder sample of the antiferromagnetic oxide MnO. A description of the data treatment that allowed the measured mPDF to be extracted and then modelled is provided and utilized to investigate the low-temperature structure of MnO. Atomic and magnetic co-refinements support the scenario of a locally monoclinic ground-state atomic structure, despite the average structure being rhombohedral, with the mPDF analysis successfully recovering the known antiferromagnetic spin configuration. The total scattering data suggest a preference for the spin axis to lie along the pseudocubic [10{\overline 1}] direction. Finally, r-dependent PDF refinements indicate that the local monoclinic structure tends toward the average rhombohedral R{\overline 3}m symmetry over a length scale of approximately 100 Å.

Empirical Magnetic Structure Solution of Frustrated Spin Systems

Frustrated spin systems can exhibit a macroscopic degeneracy of magnetic ground states which suppresses periodic magnetic order [1]. The importance of understanding these correlations lies primarily in determining the origin of exotic phenomena that emerge from frustrated spin systems, e.g. the evolution of high-temperature superconductivity from spin liquids and the ability of spin ices to support emergent magnetic charges [2]. The magnetic structures of ordered magnets can usually be determined by crystallographic analysis of the magnetic Bragg scattering observed in neutron diffraction experiments. However, the suppression of periodic magnetic order in frustrated magnets means that only magnetic diffuse scattering is observed. This has led to a consensus that single-crystal data are needed for a detailed understanding of spin correlations in frustrated systems. In my presentation, I will explore how the powder magnetic diffuse scattering pattern I(Q) can be converted robustly into a magnetic-structure model. Our approach is to consider simulated I(Q) data for a number of test cases. These data are then fitted using the reverse Monte Carlo (RMC) method. Finally, the quality of the models obtained is assessed by calculating the 3D scattering function I(Q). I will show that the extent of information loss during spherical averaging of I(Q) is surprisingly minimal, and that the full I(Q) is recoverable from powder diffraction data for each frustrated system that we explore [3]. I will go on to discuss real examples where we have used the RMC approach to obtain insight into frustrated materials. First, I will examine the interplay between geometrical frustration and low dimensionality in the paramagnetic phase of the spin-chain compound Ca3Co2O6. Finally, I will show how the RMC method can be extended for the analysis of large single-crystal diffuse scattering datasets, using as an example the canonical antiferromagnet MnO.

Magnetic exchange forces andd-state filling: Antiferromagnetic MnO(001) and NiO(001) surfaces

Magnetic exchange force microscopy (MExFM) is one of the most important methods to investigate the spin structure of magnetic surfaces. However, a deep understanding of the measured spin contrast and the spin-dependent forces is missing. For the prototypical antiferromagnetic MnO(001) and NiO(001) surfaces probed by ferromagnetic Fe tips we demonstrate by means of spin-polarized density functional theory that the differences in the tip-surface interaction are due to the distance-dependent occupation of the transition-metal $3d$ states. While agreement is achieved with recent MExFM measurements of NiO(001), we predict opposite spin-contrast for MnO(001).

Terahertz radiation from antiferromagnetic MnO excited by optical laser pulses

Terahertz (THz) radiation from antiferromagnetic (AFM) MnO excited by femtosecond laser pulses has been investigated. The radiated THz pulse consists of a single-cycle broadband pulse and an oscillating component with a frequency of 0.83 THz at 10 K. The oscillations are magnetic dipole radiation from optically excited AFM magnons. We found that the peak amplitude of the broadband pulse from MnO increases as the temperature decreases and is significantly enhanced below AFM order temperature. The origin of the broadband pulse radiation observed below AFM order temperature can be attributed to second-order nonlinear optical effects depending on the AFM order.

Exchange bias effect in single crystalline phase MnO nanoparticles

The MnO nanoparticles with uniform size (～15 nm) and shape have been synthesized in an inert atmosphers by high-temperature oil phase method. XRD patterns and HRTEM reveals that the as-synthesized MnO nanoparticles are of pure crystalline phase. TEM image shows that the MnO nanoparticles are in spherical shape with a narrow size distribution. Both oil phase and the nanoparticles protected with inert gas in the synthesis process can avoid the oxidization of Mn2+. The obvious exchange bias effect can be observed from the M-H loop of MnO nanoparticles under zero field cooling (ZFC) and FC measurements. Furthermore, the temperature-dependent AC susceptibility of the MnO nanoparticles reveals that the exchange bias effect originates from the antiferromagnetic MnO core and the spin-glass surface, rather than the superparamagnetic phase.

Optical and energy-loss spectra of the antiferromagnetic transition metal oxides MnO, FeO, CoO, and NiO including quasiparticle and excitonic effects

We calculate the frequency-dependent dielectric function for the series of antiferromagnetic transition metal oxides (TMOs) from MnO to NiO using many-body perturbation theory. Quasiparticle, excitonic, and local-field effects are taken into account by solving the Bethe-Salpeter equation in the framework of collinear spin polarization. The optical spectra are based on electronic structures which have been obtained using density-functional theory with a hybrid functional containing screened exchange (HSE03) and a subsequent quasiparticle calculation in the $GW$ approximation to describe exchange and correlation effects adequately. These sophisticated quasiparticle band structures are mapped to electronic structures resulting from the computationally less expensive $\mathrm{GGA}+U+\ensuremath{\Delta}$ scheme that includes an on-site interaction $U$ and a scissors shift $\ensuremath{\Delta}$ and allows us to calculate the large number of electronic states that is necessary to construct the Bethe-Salpeter Hamiltonian. For an accurate description of the optical spectra, an appropriate treatment of the strong electron-hole attraction is mandatory to obtain agreement with the experimentally observed absorption-peak positions. The itinerant $s$ and $p$ states as well as the localized transition metal $3d$ states have to be considered on an equal footing. We find that a purely atomic picture is not suitable to understand the optical absorption spectra of the TMOs. Reflectivity spectra, absorption coefficients, and loss functions at vanishing momentum transfer are computed in a wide spectral range and discussed in light of the available experimental data.