FM Phase Research Articles

Interaction-driven spin-orbit effects and Chern insulating phases in corundum-based 4d and 5d oxide honeycomb lattices

Using density functional theory calculations with a Hubbard U, we explore topologically nontrivial phases in X2O3 honeycomb layers with X=4d and 5d cation inserted in the band insulator α-Al2O3 along the [0001]-direction. Several promising candidates for quantum anomalous Hall insulators (QAHI) are identified. In particular, for X = Tc and Pt spin-orbit coupling (SOC) opens a gap of 54 and 59 meV, respectively, leading to Chern insulators (CI) with C = −2 and −1. The nature of different Chern numbers is related to the corresponding spin textures. The Chern insulating phase is sensitive to the Coulomb repulsion strength: X = Tc undergoes a transition from a CI to a trivial metallic state beyond a critical strength of Uc=2.5 eV. A comparison between the isoelectronic metastable FM phases of X = Pd and Pt emphasizes the intricate balance between electronic correlations and SOC: while the former is a trivial insulator, the latter is a Chern insulator. In addition, X = Os turns out to be a FM Mott insulator with an unpaired electron in the t2g manifold where SOC induces an unusually high orbital moment of 0.34 μB along the z-axis. Parallels to the 3d honeycomb corundum cases are discussed.

Effects of Mn-doping on the giant magnetocaloric effect of EuTiO3 compound

The magnetic properties and magnetocaloric effect of EuTi1−xMnxO3 (x = 0–0.1) compounds are investigated. When the Ti4+ was substituted by Mn2+, the lattice constants were changed, the Eu3+ state and the oxygen vacancy generated. The exchange mechanisms were more complex among the Mn2+ 3d, the Eu 5d and Eu2+ 4f. The FM phase was dominant between AFM and FM as Mn substitute for Ti, which improve the MCE under low magnetic field. The values of −ΔSMmax are evaluated to 11.7 and 11.1 J/kg K for EuTi0.975Mn0.025O3 and EuTi0.95Mn0.05O3 compounds, under a magnetic field change of 1 T. And, the values of RC were obviously enhanced under the magnetic field changes of 1 and 2 T. Therefore, the giant reversible MCE makes these compounds promising candidates for magnetic refrigeration.

The spin-1 Blume–Capel model on the Bethe lattice in ± J distribution with an adjustable parameter between FM and AFM phases

The spin-1 Blume–Capel (BC) model is studied on the Bethe lattice (BL) for the ± J distribution with a competing adjustable parameter α which alters the strength of bilinear exchange interaction parameter for the ferromagnetic phase (J > 0) with respect to antiferromagnetic phase (J < 0). The J > 0 and αJ < 0 values are also distributed throughout the BL with probabilities p and 1−p, respectively. The order-parameters are obtained on the BL in terms of exact recursion relations (ERR’s) and their temperature (T) variations are studied to calculate the phase diagrams on the (α, T) planes for given values of p, crystal field (D) and coordination number q=3 corresponding to honeycomb lattice. It is found that the model gives both first- and second-order phase transitions and also tricritical points. In addition to the well known ordinary phases and TCP’s, the spin glass phase and two more special points are also observed.

Polymorphous band structure model of gapping in the antiferromagnetic and paramagnetic phases of the Mott insulators MnO, FeO, CoO, and NiO

A band structure description of the observed large band gaps and moments in both the antiferromagnetic (AFM) and paramagnetic (PM) phases of the classic NaCl-structure Mott insulators MnO, FeO, CoO, and NiO is provided by ordinary, single-determinant density functional theory method. As noted by previous authors, the ordered AFM phases already show in band theory significant band gaps. However, for the disordered PM phases the commonly used band model has been to assume the macroscopically observed, averaged NaCl structure, where all transition metal sites are forced to be symmetry-equivalent (a monomorphous description); for the PM phase this forces zero moment on an atom by atom basis, thus producing a gapless PM state, in sharp conflict with experiment. Instead, we allow larger NaCl-type supercells where each TM site can have different local bonding and spin environments (a polymorphous description) and thus the geometric flexibility to acquire symmetry-lowering distortions that lower the total energy and can break the symmetry of the d orbitals. The existence of a distribution of different local spin and bonding environments allows large on-site magnetic moments to develop spontaneously in the DFT+U calculations leading to significant (1-3 eV) band gaps in the AFM, FM, and PM phases of the classic Mott insulators MnO, FeO, CoO, and NiO. We adapt to the spin disordered configurations in the PM phases the "special quasi-random structure" whereby supercell approximants which represent the best random configuration average for finite supercells of a given lattice symmetry are constructed. Thus, avoiding a monomorphous description of the disordered magnetic phases allows even ordinary DFT+U, which represents the N electron system with a single-determinant wavefunction, to describe the gapping not only in the AFM phases but also in the PM phases of MnO, FeO, CoO, and NiO.

Exchange Bias in Layered GdBaCo2O5.5 Cobaltite

It is established that excess oxygen content δ influences the exchange bias (EB) in layered GdBa-Co2O5 + δ cobaltite. The EB effect arises in p-type (δ > 0.5) cobaltite and disappears in n-type (δ < 0.5) cobaltite. The main parameters of EB in GdBaCo2O5.52(2) polycrystals are determined, including the field and temperature dependences of EB field H EB , blocking temperature T B , exchange coupling energy J i of antiferromagnet–ferromagnet (AFM–FM) interface, and dimensions of FM clusters. The training effect inherent in systems with EB has been studied. The results are explained in terms of exchange interaction between the FM and AFM phases. It is assumed that the EB originates from the coexistence of Co3+ and Co4+ ions that leads to the formation of monodomain FM clusters in the AFM matrix of cobaltite.

Hall detection of anisotropic domain walls during magnetic phase transition

We investigate the electrical detection of anisotropic antiferromagnetic/ferromagnetic (AFM/FM) domain walls in (2 at%) Pd-doped FeRh films. The Hall signals keep almost at zero at both AFM and FM states, while a sizeable Hall conductivity up to 300 (Ω cm)−1 emerges during the AFM–FM phase transition of Pd-doped FeRh films around room temperature. Such Hall signals could be ascribed to the transverse current scattered by the anisotropic AFM/FM domain walls, which are mainly aligned around the [1 0 0] axis of the Pd-doped FeRh films. The highly reproducible Hall signals in both the temperature and magnetic field-induced magnetic phase transition are supported by the observation of a nonrandom formation of AFM/FM domain walls. Our findings not only advance the understanding of the magnetic phase transitions, but also propose a considerable way for the detection of AFM/FM domain walls by an electrical means.

Structural stability and electronic behaviors of Co1-xOsxSi and macroscopic magnetic susceptibilities of CoSi and OsSi: GGA-PBEsol, GW-approximation and QTAIM investigations

The present work represents a theoretical investigation based on FP-(L)APW + lo method of structural properties, mechanical stability and electronic properties of Co1-xOsxSi as well as the macroscopic magnetic susceptibilities of CoSi and OsSi. The structural properties such as cell parameter, bulk modulus, internal parameters and total energy of non-magnetic NM, ferromagnetic FM and antiferromagnetic AFM phases were predicted by GGA-PBEsol semilocal functional. The obtained results for CoSi and OsSi are in good agreement with those found previously. The spin, orbital and total macroscopic magnetic susceptibilities of CoSi and OsSi have been estimated and confirmed that these compounds are diamagnetic. The total energy of the ferromagnetic phase of Co1-xOsxSi (with x = 0.25, 0.5 and 0.75) is the lowest indicating that they are ferromagnetic materials. The generalized stability criteria indicate that Co1-xOsxSi maintain their mechanical stabilities under a hydrostatic pressure less than 10 GPa. The electronic properties calculated by GW-approximation indicate that CoSi and Co1-xOsxSi (with x = 0.25, 0.50 and 0.75) are semimetals whereas OsSi is a semiconductor with a pseudo-direct band-gap. The topological analysis by QTAIM and the charge density plots indicate that the strong covalent character is predominant for CoSi, OsSi and CoOs bonds.

Magnetic behavior study of samarium nitride using density functional theory

In this work, the state-of-art density functional theory is employed to study the structural, electronic and magnetic properties of samarium nitride (SmN). We have performed calculation for both ferromagnetic and antiferromagnetic states in rock-salt phase. The calculated results of optimized lattice parameter and magnetic moment agree well with the available experimental and theoretical values. From energy band diagram and electronic density of states, we observe a half-metallic behaviour in FM phase of rock salt SmN in while metallicity in AFM I and AFM III phases. We present and discuss our current understanding of the possible half-metallicity together with the magnetic ordering in SmN. The calculated phonon dispersion curves shows dynamical stability of the considered structures. The phonon density of states and Eliashberg functional have also been analysed to understand the superconductivity in SmN.

Metal insulator transition in FeWN2: DFT and DFT+U study

We have studied the electronic and magnetic properties of layered ternary transition metal nitrides, FeWN2. We found that the density functional theory (DFT) based approaches within the generalized gradient approximation frameworks fail to predict the correct ground state configuration. While the DFT plus on-site repulsion U method (DFT+U) predict the AF ordering which it’s in good agreement with experiment. We found that the metal-insulator transition is accompanied by a magnetic phase transition which occurred at the same time, a ferromagnetic-metallic to antiferromagnetic-insulating transition where Ueff=3eV. It is shown also that magnetism and correlation effects are important in band-gap formation in this compound. Our calculated results by DFT-mBJ show the same trend of the band gap. The fully spin polarized DOS at the Fermi level in FM phase makes from FeWN2 a promising candidate material for spintronic applications.

Reentrant behavior in Cr doped bilayer manganite LaSr2Mn2O7

We have studied the effect of replacing Mn3+ by Cr3+ on the structure, transport and magnetism in the bilayered manganite LaSr2Mn2O7. Although no structural transition was observed in LaSr2Mn2−yCryO7 (0.1≤y≤0.6), the electrical transport and the magnetic properties were found to be affected significantly by this substitution. Substitution of Cr3+ reduces the conductivity by restricting the hopping of small polarons. Magnetization increases with increasing Cr3+ concentration suggesting that Cr3+-ions induce ferromagnetic moments. The ferromagnetic and an antiferromagnetic phase observed above ∼60K merge into an inhomogeneous phase below this temperature. Thermopower yields an essentially concentration independent charge density nearly equal to its value for chromium free composition inspite of its expected decrease with this substitution suggesting that the small charge density of the insulating AFM phase is supplemented by the free carriers in the FM phase. The inhomogeneous phase shows a relaxor type behavior which contrasts with the spin glass behavior seen in La0.46Sr0.54Mn0.98Cr0.02O3 having an identical AFM magnetic state. The difference is attributed to the non-JT character of Cr-ions which reduce the distortion of the MnO octahedra located within the FM domains. With a higher lattice strain in the surrounding AFM matrix the carriers remain confined within the FM domains leading to the relaxor type behavior.

Theoretical Study of Electronic and Magnetic Properties of Newly Derived Iron–Pnictide Compound

A theoretical study based on DFT-LDA of the structural, electronic, and magnetic properties of the new substitution CaFe2B2 compound derived from BaFe2As2 is presented. Through a relaxation calculation, we found that this compound crystallizes in tetragonal and orthorhombic phases. Formation energies, lattice parameters, density of states, and magnetic moments are calculated. The ground state for this compound is nonmagnetic (NM). As for the anti-ferromagnetic state, more formation energy is characterized by large magnetic moment on each Fe atoms for AFM spin configuration. The results are compared with previous calculations and experimental data. The results of electronic and magnetic properties show that the partial and total density of states for NM, FM, and AFM phases are characterized by strong hybridization between Fe and B atoms. The energy band structure indicates the presence of overlapping valence and conduction bands at the Fermi level. The analyses of charge densities show that the type of bonding in the CaFe2B2 compound is metallic. An important resemblance with the original compound is observed which leads us to think that this compound is maybe a superconducting material.

The structural, electronic and magnetic properties of a novel quaternary Heusler alloy TiZrCoSn

The structural, electronic and magnetic properties of the quaternary Heusler alloy TiZrCoSn have been investigated firstly by using the first-principles calculations within GGA and GGA+U methods. The structural study shows the Y-type (I) is the most stable configuration among three possible configurations of the TiZrCoSn alloy in FM phase. The GGA calculation shows the TiZrCoSn alloy at its equilibrium lattice constant 6.536Å is HM ferromagnet with an indirect band gap of 1.043eV and a HM gap of 0.353eV in the spin-down channel. The formation energy of −1.041eV and the cohesion energy of 24.402eV indicate the stability of the TiZrCoSn alloy. The Curie temperature of the TiZrCoSn alloy is higher than room temperature shows the TiZrCoSn alloy is suitable for spintronic applications. The band gap in the spin-down channel is formed by the bonding (t2g) states and nonbonding (tu) states created from the d states hybridisation of transition metal atoms Ti, Zr and Co. The TiZrCoSn alloy has an integer total magnetic moment of 3 μB/f.u., satisfying the Slater-Pauling rule μt=Zt−18. In addition, the HM character is kept as hydrostatic strain ranges from −10.3% to 8.4% and tetragonal strain ranges from −16.3% to 20.8%. The similar results are also obtained by GGA+U calculation.

Giant Controllable Magnetization Changes Induced by Structural Phase Transitions in a Metamagnetic Artificial Multiferroic.

The realization of a controllable metamagnetic transition from AFM to FM ordering would open the door to a plethora of new spintronics based devices that, rather than reorienting spins in a ferromagnet, harness direct control of a materials intrinsic magnetic ordering. In this study FeRh films with drastically reduced transition temperatures and a large magneto-thermal hysteresis were produced for magnetocaloric and spintronics applications. Remarkably, giant controllable magnetization changes (measured to be as high has ~25%) are realized by manipulating the strain transfer from the external lattice when subjected to two structural phase transitions of BaTiO3 (001) single crystal substrate. These magnetization changes are the largest seen to date to be controllably induced in the FeRh system. Using polarized neutron reflectometry we reveal how just a slight in plane surface strain change at ~290C results in a massive magnetic transformation in the bottom half of the film clearly demonstrating a strong lattice-spin coupling in FeRh. By means of these substrate induced strain changes we show a way to reproducibly explore the effects of temperature and strain on the relative stabilities of the FM and AFM phases in multi-domain metamagnetic systems. This study also demonstrates for the first time the depth dependent nature of a controllable magnetic order using strain in an artificial multiferroic heterostructure.

V3DM+: BIM interactive collaboration system for facility management

AbstractBackgroundFacility management (FM) is an important phase in the building lifecycle. Although building information modeling (BIM) technology has been widely used in the design and construction phase, efforts to transfer information to the FM phase are still in the nascent stage. A good BIM-to-FM tool should be able to inherit and integrate the building information built in the design and construction phase as well as be able to provide a platform for managers to browse the information, discuss potential problems, and arrange maintenance work.MethodsIn this research, a BIM interactive collaboration system for FM is designed and implemented. Two modules were developed for optimizing the BIM system in the FM phase: the data arrangement module, which has functionalities of data importing and processing, and the data presentation module, which provides functionalities of data revealing and collaboration.ResultFrom the results of an accessibility analysis, we verified that the developed system, V3DM+, can be implemented on real projects. Managers can use V3DM+ to support operation and maintenance of buildings. Moreover, a web-based platform allows users to manage facilities intuitively without investing on professionals or specialized software.ConcludingThe proposed system, V3DM+, provides a data arrangement module to integrate building information constructed by various types of commercial software and present them in a visual manner. The user can find the required information, initiate a discussion on an issue, or arrange operation and maintenance tasks directly on this system.

A pathway to optimize the properties of magnetocaloric Mn2-xFexP1-yGey for magnetic refrigeration

Magnetocaloric materials can be useful in magnetic refrigeration applications, but to be practical the magneto-refrigerant needs to have a very large magnetocaloric effect (MCE) near room temperature for modest applied fields (<2 T) with small hysteresis and magnetostriction, and should have a complete magnetic transition, and environmentally friendly. One system that may fulfill these requirements is Mn2-xFexP1-yGey, where a combined first-order structural and magnetic transition occurs between the high temperature paramagnetic and low temperature ferromagnetic phase. We have used neutron diffraction, differential scanning calorimetry, and magnetization measurements to study the effects of Mn and Ge location in the structure on the ordered magnetic moment, MCE, and hysteresis for a series of compositions of the system near optimal doping. The diffraction results indicate that the Mn ions located on the 3f site enhance the desirable properties, while those located on the 3 g sites are detrimental. The phase fraction that transforms, hysteresis of the transition, and entropy change can be affected greatly by both the compositional homogeneity and the particle size, and an annealing procedure has been developed that substantially improves the performance of all three properties of the material. We also establish a correlation between applied magnetic field to complete the transition and the temperature range of coexistence of the PM and FM phase. On the basis of these results we have identified a pathway to understand the nature and to optimize the MCE properties of this system for magnetic refrigeration applications.

Al3+ doping effects and high-field phase diagram of La0.5Sr0.5Mn1−xAlxO3

Magnetization measurements of La0.5Sr0.5Mn1−xAlxO3 (0 ⩽ x ⩽ 0.25) under pulsed high magnetic fields up to 50 T have been carried out, in which the Al3+ ions doping and magnetic field effects on the charge-ordering/antiferromagnetic to ferromagnetic phase transitions have been discussed. A triple-phase diagram with the critical field, doping level and temperature has been determined, in which the antiferromagntic and ferromagnetic phase boundaries were clearly defined. The change from long-range charge-ordered/antiferromagnetic phases to the robust short-range ones upon the Al3+-doping was observed. According to the experimental results, we assume that Al3+ ion doping at the Mn sites dilutes the Mn3+-O-Mn4+ network, weakens the double-exchange interaction and further suppresses the FM phase (metallic conduction), which leads to the critical magnetic fields destroying the antiferromagnetic order increase with the increase of the doping level.

Phase relationship, structural and magnetic properties of Nd-deficient Nd0.95−xCaxMnO2.93±δ

Phase relationships in Nd-deficient solid solutions Nd0.95−xCaxMnO2.93±δ (x=0.00, 0.05, 0.10, 0.15, 0.20) were investigated in the temperature range T=800–1100°C in air and under reduced oxygen partial pressure (at T=1000°C). Phase diagrams in the coordinates “Temperature-calcium content” and “Po2-calcium content” were constructed. The boundaries of two-phase regions on the phase diagrams were defined.The oxidation mechanism and the stability conditions for the perovskite structure were determined.The magnetic properties of single phase samples with x=0, 0.1, 0.2 have been studied as a function of temperature and magnetic field. The observed magnetic anomalies: magnetization sign reversal and spin-reorientation transitions are compatible with the coexistence of AFM and FM phases, clusters of different nature, quasi heterogeneity (presence of clusters enriched/depleted with neodymium or manganese oxide). Quadrupole centers are considered as a source of spin inversion phenomena.

Critical behavior and magnetic entropy change in Nd0.7Sr0.1Ba0.1Ca0.1MnO3−δ perovskite manganite

The aim of our study was to investigate the critical exponents of Nd0.7Sr0.1Ba0.1Ca0.1MnO3−δ material around its paramagnetic to ferromagnetic phase transition. Reliable critical exponents i.e. β=0.355, γ=1.329, and δ=4.867 at the critical temperature TC=117K were refined using modified Arrott plot and Kouvel–Fisher methods. The critical exponent δ=4.760±0.129 was further separately determined from the isothermal magnetization data. Finally these critical exponents fulfill the Widom scaling relation δ=1+γ/β. Based on these analysis, the magnetization–field–temperature (M–H–T) data around TC collapses into two independent curves obeying the single scaling equation M(H,ε)=|ε|βf±(H/|ε|β+γ), with ε=(T−TC)/TC is the reduced temperature. Once again, the critical exponents are confirmed by the field dependence of the magnetic entropy change relation ∆SM|T=TC ∞Hn with n=1+(β−1)/(β+1). The obtained critical exponents for Nd0.7Sr0.1Ba0.1Ca0.1MnO3−δ manganite are in accordance with the prediction of the 3D-Heisenberg model with short-range magnetic interactions. The temperature dependence of specific heat and magnetic entropy change ΔSM further supports second order PM–FM phase transition and shed light the A-site-disordered perovskite effect on the magnetocaloric properties.

Magneto-Optical Properties of Multilayer Nanostructures with Composite Magnetic Layers near Percolation Threshold

Spectra and field dependencies of Transversal Kerr Effect (TKE) for [(Co45Fe45Zr10)46(Al2O3)54(X)/Si (Y)]nmultilayers (ML) with thin layers (X &lt; 2,5 nm; Y &lt; 3,5 nm) have been studied in the energy range 0,5 – 4,0 eV. It was found that TKE field dependencies in the nearest infrared (IR) and visible energy ranges exhibit anomalous behavior. TKE spectra measured in weak (~30 Oe) and strong (~2 kOe) magnetic fields were essentially different. It was shown that magneto-optical response of multilayered structure composite-silicone is the sum of magnetic composite layers and composites formed on interface contributions. And these contributions may have an opposite sign of TKE. Interface magnetic states depend on composite and semiconductor layers’ thicknesses and FM phase concentration in composite layers.

High pressure effects on the crystal and magnetic structure of nanostructured manganites La0.63Sr0.37MnO3 and La0.72Sr0.28MnO3

The crystal and magnetic structure of nanostructured manganites La0.63Sr0.37MnO3 and La0.72Sr0.28MnO3 has been studied by means of a neutron diffraction method in the pressure range 0–4.5 GPa and the temperature range 5–300 K. In both samples the FM ordering is formed close above the room temperature and at cooling below T < 270 K the ferromagnetic FM phase coexists with an A-type antiferromagnetic AFM phase. At high pressures the volume fraction of AFM phase increases, while FM is gradually suppressed. The structural aspects of the magnetic phase separation and pressure effects on the studied nanostructured manganites are discussed.