Iron Sublattice Research Articles

Magnetocaloric, magnetostrictive and 57Fe Mössbauer studies of the multicomponent (Er,Y,Sm)Fe[formula omitted] compounds

The effect of substitutional 4f-elements on the magnetism of rare-earth compounds RFe2-type with the Laves phase structure is studied to find new multifunctional materials, as well as to define the macro- and microscopic parameters of multicomponent magnets. The crystal structure of (Er,Y,Sm)Fe2 compounds is investigated by X-ray powder diffraction. Detailed information on the magnetic properties of the iron sublattice for multicomponent compounds with three different rare-earth elements is obtained for the first time by means of the 57Fe Mössbauer spectroscopy. The main regularities in the magnitude variation of magnetocaloric effect and magnetostriction (linear, anisotropic and volume) in varying composition of the (Er,Y,Sm)Fe2 compounds are determined.

Rare Earth Based Iron Garnet – A Survey on Its Magnetic Properties

Garnet is a well-known material for a long-time by the scientific community but still today scientists are focusing on it due to the rapid application-based development of this material. When rare earth iron garnets (REIG) are formed by substituting the rare-earth ions with unfilled 4fn orbitals, the magnetic properties of the iron garnets exhibit an interesting characteristic. For rare earth elements, the 4f electrons are shielded from the crystal field as these are surrounded by 5s, 5p, or 5d orbitals. That is why the exchange field between rare earth ions is much smaller than that between iron-iron and rare earth-iron. The magnetic moment of REIG will be both due to the orbital and spin moment. The magnetization of REIG at different temperatures (T) is due to the dominant contribution of different sublattices. At high and low T, the dominant sublattices are iron and rare earth sublattices respectively. The magnetic and non-magnetic ion substitution in REIG also play a very important role in deciding their magnetic property. In this review, we have tried to figure out the basic underlying physics behind the origin of remarkable magnetic behavior in REIG.

Ion-Selective Scattering Studied Using the Variable-Energy Electron Irradiation in the Ba0.2K0.8Fe2As2 Superconductor.

Low-temperature variable-energy electron irradiation was used to induce non-magnetic disorder in a single crystal of a hole-doped iron-based superconductor, Ba1-xKxFe2As2, x = 0.80. To avoid systematic errors, the beam energy was adjusted non-consequently for five values between 1.0 and 2.5 MeV when sample resistance was measured in situ at 22 K. For all energies, the resistivity raises linearly with the irradiation fluence suggesting the creation of uncorrelated dilute point-like disorder (confirmed by simulations). The rate of the resistivity increase peaks at energies below 1.5 MeV. Comparison with calculated partial cross-sections points to the predominant creation of defects in the iron sublattice. Simultaneously, superconducting Tc, measured separately between the irradiation runs, is monotonically suppressed as expected, since it depends on the total scattering rate, hence on the total cross-section, which is a monotonically increasing function of the energy. Our work experimentally confirms an often-made assumption of the dominant role of the iron sub-lattice in iron-based superconductors.

Preparation, phase stability, and magnetization behavior of high entropy hexaferrites

The polycrystalline SrFe12O19 samples deeply substituted up to at.67% by Al3+, Ga3+, In3+, Co3+, and Cr3+ cations with a high configurational mixing entropy were prepared by solid-phase synthesis. Phase purity and unit cell parameters were obtained from XRD and analyzed versus the average ionic radius of the iron sublattice. The crystallite size varied around ∼4.5μm. A comprehensive study of the magnetization was realized in various fields and temperatures. The saturation magnetization was calculated using the Law of Approach to Saturation. The accompanying magnetic parameters were determined. The magnetic crystallographic anisotropy coefficient and the anisotropy field were calculated. All investigated magnetization curves turned out to be nonmonotonic. The magnetic ordering and freezing temperatures were extracted from the ZFC and FC curves. The average size of magnetic clusters varied around ∼350nm. The high values of the configurational mixing entropy and the phenomenon of magnetic dilution were taken into account.

Surface magneto-optics in yttrium iron garnets

We report a study of surface reconstruction effects in yttrium iron garnets using density functional theory. This work responds to the need to explain the physical basis of recent experimental results showing a significant enhancement in Faraday rotation in iron garnets. These materials are extensively used in the telecom industry for nonreciprocal device applications. Understanding the physical basis of the heightened Faraday response at the surface is important for the development of ultrathin nonreciprocal devices. Our results show that the bandgap near the surface is significantly reduced compared to that of the bulk, and that spin-orbit coupling effects become more important near the surface. We find that the decrease in band gap results in an enhancement and change in direction in the Faraday rotation in the visible range all the way to the deep red. Electronic transition matrix elements for surface-sensitive ultra-thin layers are calculated and compared to those of bulk samples, leading to the conclusion that the octahedrally-oxygen-coordinated iron sublattice in these ferrimagnetic materials is more strongly affected by surface reconstruction than the other, anti-ferromagnetically coupled, tetrahedral sublattice. We explain how these changes contribute to the enhancement in magneto-optic response near the surface as compared to the bulk.

SIGN-REVERSING MAGNETOCALORIC EFFECT IN R2Fe10Al7 (R = Dy AND Ho) COMPOUNDS

The magnetic properties of rare-earth ferrimagnetic compounds Dy2Fe10Al7 and Ho2Fe10Al7 with a Th2Zn17 crystal structure have been studied. The temperature dependences of the magnetization and magnetocaloric effect have been examined in the temperature range of 4.2‒300 K in magnetic fields of up to 70 and 18 kOe, respectively. The temperatures of magnetic compensation of the magnetization of the rareearth and iron sublattices have been determined. The magnetocaloric effect has been determined by the direct method. It has been found that the sign of the effect changes near the magnetic compensation point. This phenomenon has an application potential.

Synthesis and characterization of Ba(Fe,Zr,Ni)O3−δ perovskites for potential electrochemical applications

Phase relationships in the BaFe1-x-yZryNixO3-δ system were evaluated in the search for Ba-rich electrode materials with potential application in solid electrolyte cells for electrochemical NOx reduction. Co-substitutions by zirconium and nickel into the iron sublattice of barium ferrite stabilize the cubic perovskite structure with a solid solution formation range limited to (x + y) ⪅ 0.3. Characterization of BaFe0.7Zr0.3-xNixO3-δ (x = 0.10–0.20) perovskites included the determination of oxygen nonstoichiometry variations by TGA, measurements of electrical conductivity as a function of temperature and oxygen partial pressure, studies of thermal expansion, and assessment of chemical compatibility with selected solid electrolytes. BaFe0.7Zr0.3-xNixO3-δ perovskites exhibit substantial oxygen losses from the lattice above 350 °C, reaching Δδ ∼ 0.2 on heating to 1000 °C in air, with effects on electrical properties and thermochemical expansion. Electrical conductivity is p-type under oxidizing conditions, increases with Ni content, and reaches 3.5 S/cm at 400 °C. The perovskite phase is stable down to p(O2) ∼ 10‐17 at 900 °C. The average thermal expansion coefficients in air increase from (15−18)× 10‐6 K‐1 at lower temperatures to ∼ 30 × 10‐6 K‐1 above 400 °C. The optimum co-doping level was found to be x = y = 0.15 as this composition exhibits the best chemical compatibility with yttria-stabilized zirconia and BaZr0.85Y0.15O3-δ solid electrolytes.

Activity and selectivity descriptors for iron carbides in CO2 hydrogenation

CO2 conversion into value-added products provides an attractive way to close carbon cycles and to minimize the dependence on oil-based feedstock. To understand how product distribution in CO2 hydrogenation can be controlled, as required for purposeful catalyst design, we prepared single-phase α-Fe2O3 materials differing in their reducibility due to the different size of crystallites and the presence of planar defects in the iron sublattice. Their structural evolution during reduction and under CO2 hydrogenation conditions as well as their catalytic and adsorptive properties towards CO, CO2, and H2 were elucidated through combining kinetic (spatially resolved), physicochemical and chemisorption analyses. The obtained results suggest that the reducibility of α-Fe2O3 is a key factor affecting in situ transformation of this oxide into Fe5C2 with planar defects, which seem to determine the generation of surface intermediates from CO, CO2, and H2, and accordingly CO2 hydrogenation to CO exclusively or also CO hydrogenation to hydrocarbons.

Tuning the terahertz electrodynamics in Ba-Pb hexaferrite single crystals

First detailed measurements of broad-band (2–8000 cm−1, i.e. 0.06-240 THz) temperature-dependent (5–300 K) spectra of complex permittivity of single-crystalline barium-lead hexaferrites Ba1-xPbxFe12O19 covering full interval of lead concentrations, x(Pb)=0…1, were performed to study the effect of Ba for Pb substitution on electrodynamic response. It is found that variation of Ba-Pb relative content allows “tuning” of dielectric losses ε"(0.3 THz; 300 K) from 0.1 to 45, and dielectric constant ε'(0.3 THz; 300 K) from 10 to 50. To account for the discovered concentration evolution of terahertz dielectric properties, a microscopic model is suggested that considers interaction of 6s2 lone pair of Pb2+ with nearest FeO5 clusters. Pronounced anomaly in concentration-dependent oscillator strength of terahertz bands is observed at x(Pb)=0.5–0.6 and interpreted by considering redistribution of Fe2+/Fe3+ ions over five iron sublattices. It is pointed out that the revealed “tunability” of terahertz characteristics of hexaferrites makes them promising materials for designing devices operating at terahertz frequencies.

Effect of configurational entropy on phase formation, structure, and magnetic properties of deeply substituted strontium hexaferrites

Solid solutions based on SrFe 12 O 19 strontium hexaferrite with deep (up to at.67%) co-substitution of triply charged Fe 3+ iron cations by triply charged diamagnetic Al 3+ aluminum, Ga 3+ gallium, and In 3+ indium cations, as well as by paramagnetic Co 3+ cobalt and Cr 3+ chromium cations, were obtained by conventional ceramic technology. This type of substitution leads to the formation of high-entropy phases. The phase composition of the obtained samples was studied by powder X-ray diffraction. The configurational mixing entropy was calculated for all the obtained samples, and its influence on the processes of phase formation was studied. The unit cell parameters are calculated and their dependences on the 〈r B 〉 average ionic radius of the iron sub-lattice are plotted. The average grain size varied within 2–5 μm. The field and temperature properties of the magnetization of the obtained samples were studied. The M s saturation magnetization is determined using the Law of Approach to Saturation. The M r residual magnetization, SQR loop squareness and H c coercivity are also determined. Based on the obtained data, the k magnetic crystallographic anisotropy coefficient and the H a anisotropy field are calculated. All the obtained and calculated values are plotted depending on the 〈r B 〉 average ionic radius of the iron sub-lattice. All plotted dependencies are non-linear. The T mo magnetic ordering and T f freezing temperatures are obtained from the ZFC and FC magnetization data. The cluster nature of the magnetic behavior for all the samples is revealed. The average cluster diameter reaches ∼ 400 nm. The interpretation of the magnetic properties is made taking into account the effect of high configurational mixing entropy and magnetic dilution of the iron sub-lattice.

The influence of strontium deficiency on thermodynamics of defect formation, structural stability and electrical transport of SrFe0.5Ta0.5O3-δ-based solid solutions with an excess tantalum content.

The crystalline and electronic band structures, thermodynamic stability, oxygen non-stoichiometry and high-temperature transport properties of perovskite-like solid solutions with a general formula Sr1-yFe0.5-xTa0.5+xO3-δ, where x, y ≥ 0, are thoroughly studied using a combination of experimental and theoretical methods. It is argued that the basic compound SrFe0.5Ta0.5O3-δ possesses an orthorhombic lattice symmetry, while its tantalum-doped derivatives belong to a tetragonal space group. Importantly, the purposeful addition of a certain deficiency in a strontium sublattice is shown to be a valid method for stabilizing the Sr1-yFe0.5-xTa0.5+xO3-δ oxides with an excess tantalum content. Detailed studies of charge states in an iron sublattice suggest the predominance of Fe3+ ions even in tantalum-enriched materials. Also, the band structure calculations support the semiconducting nature of electrical transport with localized n-type conductivity provided by small polarons represented by Fe2+ ions. The overall defect structure of Sr1-yFe0.5-xTa0.5+xO3-δ compounds is proved to heavily rely on oxygen vacancy (VO) formation processes; in turn, the presence of strontium vacancies is shown to be an important factor that can decrease the respective energy penalties to introduce VO defects in the lattice. As a result, the experimentally measured oxygen non-stoichiometry for Sr0.95Fe0.45Ta0.55O3-δ at elevated temperatures appears to be sufficiently enlarged as compared to pristine SrFe0.5Ta0.5O3-δ. Similar to that, the conductive properties of tantalum-enriched phase Sr0.95Fe0.45Ta0.55O3-δ are shown to be improved. On the basis of the obtained results, it is argued that cation non-stoichiometry is a valuable tool for enhancing thermodynamic and transport characteristics of perovskite-like compounds, which are currently viewed as promising materials for high-temperature applications.

Terahertz-driven magnetization dynamics of bismuth-substituted yttrium iron-gallium garnet thin film near a compensation point

Magnetization dynamics of bismuth-substituted yttrium iron-gallium garnet film $({\mathrm{Y}}_{3\ensuremath{-}z}{\mathrm{Bi}}_{z})[{\mathrm{Fe}}_{2\ensuremath{-}x}{\mathrm{Ga}}_{x}][{\mathrm{Fe}}_{3\ensuremath{-}y}{\mathrm{Ga}}_{y}]{\mathrm{O}}_{12}$ has been studied using a terahertz pump--optical probe spectroscopy. Two magnetic modes are observed whose frequencies intersect at a magnetization compensation point. The experimental dependence of the excited modes on terahertz pulse polarization, an external magnetic field and the temperature are analyzed. The theoretical description based on symmetry and the Lagrangian formalisms is proposed. Magnetic modes crossing is explained as interplay between exchange and anisotropy energies equally contributing near the compensation point. Simulations based on the Landau-Lifshitz-Gilbert equations show that a difference in magneto-optical susceptibilities of tetrahedral and octahedral iron sublattices can substantially enhance the sensitivity to the magnetic modes.

First-principles calculation of possible carbon positions over cementite lattice

The properties of iron-carbon alloys are largely determined by the formation of iron carbide. Therefore, knowledge about the structure of cementite plays an important role. The possible reason for changes in the nearest neighborhood of iron and carbon atoms in cementite during various heat treatments of steel could be the re-arrangement of carbon atoms over different interstitial sites of cementite iron sublattice. Crystal geometry analysis revealed four types of such sites:“normal” and “distorted” prismatic (NPS, DPS), and “normal” and “distorted” octahedral (NOS, DOS) sites. In this study, we tested this hypothesis using the density functional theory by full-potential method implemented in the WIEN2k program package. The simulated cementite supercells contained 16 and 32 atoms. We confirmed that carbon in NPS resulted in the system with the lowest energy and volume. Systems with all carbon atoms in DPS and one or all carbon atoms in DOS were mechanically unstable and transferred to conventional NPS structure during the system volume relaxation. In other cases, carbon atoms remained in NOS or DPS and yielded the increased structure total energy. The thermodynamic analysis showed that over 10 % of carbon atoms might move from NPS to other sites.

Creation and Magnetic Study of Ferrites with Magnetoplumbite Structure Multisubstituted by Al3+, Cr3+, Ga3+, and In3+ Cations.

Multisubstituted barium ferrites with a magnetoplumbite structure were obtained by the method of solid-phase reactions with ferritization and pre-firing. Three-charged, mainly diamagnetic cations Al3+, Cr3+, Ga3+, and In3+ were chosen as substituents for the Fe3+ iron cations, the proportion of which in solid solutions did not exceed 50%. The values of the configurational mixing entropy were calculated for all the compositions. A slight deviation of the chemical composition of the obtained solid solutions from the nominal value was established by the energy-dispersive X-ray spectroscopy method. The phase purity and values of the unit cell parameters were refined from X-ray scattering data using full-profile analysis in the Rietveld method. A non-monotonic behavior of the unit cell parameters as a function of the B-sub-lattice average ionic radius of the magnetoplumbite structure was found. A minimum unit cell volume of ~667.15 Å3 was found for the composition BaFe6.11Al1.56Cr2.17Ga2.16O19 with a B-sub-lattice average ionic radius of ~7.449 Å. The average crystallite size varied within 5.5–6.5 μm. The temperature and field dependencies of the magnetization have been measured. The values of the saturation magnetization, residual magnetization, hysteresis loop squareness, and coercivity at 50 K and 300 K were extracted from the experimental data. Using the Law of Approach to Saturation, the magnetic crystallographic anisotropy coefficient and anisotropy field were calculated. Multisubstitution leads to a significant decrease in such magnetic parameters as the magnetic ordering temperature and spontaneous magnetization at both temperatures. The maximum magnetic ordering temperature of ~297.7 K was found for the composition BaFe5.84Ga6.19O19 with a B-sub-lattice average ionic radius of ~7.586 Å in a field of 500 Oe. A maximum saturation magnetization of ~24.7 emu/g was found for the composition BaFe5.84Ga6.19O19 with a B-sub-lattice average ionic radius of ~7.586 Å at 50 K. A maximum hysteresis loop squareness of ~0.72 was found for the composition BaFe6.11Al1.56Cr2.17Ga2.16O19 with an average ionic radius of ~7.449 Å at 50 K. A maximum magnetic crystallographic anisotropy coefficient of ~2.09 × 105 Erg/g was found for the composition BaFe6.19Al1.25Cr1.57Ga1.74In1.26O19 with a B-sub-lattice average ionic radius of ~7.706 Å at 50 K. The frustrated magnetic state including the nano-sized clusters with an average diameter in the range of 50–200 nm was established from the results of measuring the ZFC and FC temperature magnetizations. The interpretation of the obtained experimental data is carried out taking into account the increased stability of high-entropy phases and regular changes in the intensity of the Fe3+(Al3+, Cr3+, Ga3+, In3+)–O2−–Fe3+(Al3+, Cr3+, Ga3+, In3+) indirect superexchange interactions as a result of magnetic dilution of the iron sub-lattice in the magnetoplumbite structure.

Crystal shapes, triglyphs, and twins in minerals: The case of pyrite

Abstract The euhedral shapes of pyrite FeS2 are usually exposing three main surfaces: striated (001), smooth (111), and striated (210), leading to the cubical, octahedral, and pyritohedral morphology, respectively. The macroscopic striations, sometimes called triglyphs on cubic crystals, are parallel on specific surfaces and aligned to the <100> directions. Other types of striated and unstriated (hkl) surfaces can be observed on pyrite crystals from Peru, a country offering a rich diversity of pyrite shapes. A rare specimen from Elba Island (Italy) is a pyritohedron with uncommon directions of striations (so-called “negative” striations, first described in Japanese minerals). The Wulff kinetic growth and the periodic bond chain (PBC) theories were not relevant enough to explain crystal shapes, the texture of the surfaces, and twinning. To bring some new insights on crystal growth, twinning, and anisotropy, pyrite samples are analyzed using XRD, SEM, and EDS techniques coupled with atomistic simulations. A first analysis points out that sulfur terminations play a key role in the growth of striations in distinguishing the six <001> directions. The negative striated pyritohedral pyrite would be, in fact, a special case that has stabilized the {120} surfaces, which are structurally different from the {210} facets. The {120} surface has a slightly higher surface energy than the {210} surface (surface energies of 1.68 and 1.65 J/m2, respectively, calculated with force field methods). {120} pyritohedra from Elba, Italy, are growing next to micaceous iron oxides (a type of hematite), which are also peculiar specimens with magnetic properties. Another specificity is that some rare earth elements have been identified in the pyrite sample from Elba, which leads to a hypothesis that geothermal conditions favor “negative” striations (e.g., discernible in Akita prefecture-Japan, Boyacá-Colombia, and Cassandra-Greece). The striation directions become useful to distinguish (hkl) surfaces and to identify twinning as they follow the same patterns on each interpenetrated crystal. The most common twinning is the “iron cross,” a penetration twin of two crystals defined by a rotation of 90° along an [001] axis with a coincidence in the iron sub-lattice (e.g., twinning by merohedry) and with a twin center. The sulfur network also plays a fundamental role in stabilizing the (001) interface and in keeping the chemical bulk properties at the boundary, as confirmed by additional ab initio simulations. The grain boundary is a 2D defect in which the (001) twinning is relatively stable as it is common. The calculated formation energy of the rotation twinning is 0.8 J/m2. The rotation twinning is associated with an apparent reflection on (110) planes. The formation energy of the (110) mirror grain boundary is 1.7 J/m2, and the interface at the atomic scale is relatively uniform in agreement with experimental observations.

4f-3d interaction dominated field tailoring spin switching in rare earth doped Dy0.5Er0.5FeO3 single crystal

A large single crystal Dy0.5Er0.5FeO3 of 5.2 mm in diameter and 55 mm in length is prepared using the optical floating zone furnace system. The good quality of the single crystal without impurities is confirmed by powder and Laue X-ray diffraction experiments. Magnetic measurements are conducted using 1~2 millimeter cubic sample of Dy0.5Er0.5FeO3 single crystal, which experiences spin switching in two measurement modes of zero-field-cooling (ZFC) and field-cooling (FC) magnetization vs temperature (M-T) curves under magnetic fields less than 48 Oe. The co-occurrence of spin switching (i.e. magnetization drop in FC mode and magnetization jump in ZFC mode) has not been reported in the parent materials of DyFeO3 and ErFeO3 single crystals. More interestingly this kind of spin switching can be easily tailored by using low fields in a large temperature range from below 50 K to nearly 400 K. Spin reorientation transition occurs near 35.9-48 K, where the spin configuration of the iron sublattice changes from Γ4 and Γ2. The magnetic interaction between (Dy, Er)-4f and Fe-3d electrons in the two different sublattices of Dy0.5Er0.5FeO3 single crystal which governs the spin reorientation transition and spin switching under different temperature and magnetic field are discussed.

Features of magnetostriction behavior of the intermetallic compound Sm0.2Y0.8Fe2 near the spin reorientation transitions

The structural and magnetic properties of Sm0.2Y0.8Fe2 intermetallic compound are investigated using X-ray diffraction and magnetic measurements and compared with the properties of the SmFe2 compound. It is established that the partial substitution of Y for Sm in SmFe2 leads to the decrease in both the Curie temperature TC and the two spin-reorientation transition temperatures TSR1 and TSR2. The complex behavior of the field and temperature dependencies of the longitudinal, transversal, anisotropic and volume magnetostriction in a wide temperature range (including the spin-reorientation region) is discussed in terms of the single-ion anisotropy and magnetostriction theory. The influence of other factors, such as peculiarities of the state of Sm3+ ions and competition between contributions to magnetostriction from the iron and samarium sublattices, is also considered.

Strengthening the Intersublattice Exchange Interaction in R(Fe,Ti)$${}_{12}$$N (R = Ho and Er) Nitrides

The field dependences of the magnetization of nitrogen-containing compounds R(Fe,Ti) $${}_{12}$$ N (R = Ho and Er) are analyzed. It was found that the initial HoFe $${}_{11}$$ Ti sample at $$T=4.2$$ K exhibits the eaxy-axis anisotropy; however, as a result of nitriding, the anisotropy type changes to the easy cone of magnetization. As in the original samples of HoFe $${}_{11}$$ Ti and ErFe $${}_{11}$$ Ti, in HoFe $${}_{11}$$ TiN and ErFe $${}_{11}$$ TiN nitrides the cascade of spin-reorientation phase transitions is observed which is induced by external magnetic field. In the framework of the molecular field theory, the effect of nitriding on the interlattice exchange interactions is estimated. It is established that the molecular field that acts from the iron sublattice on rare-earth ions (holmium and erbium) is enhanced. The effect of nitriding and hydrogenation on the intersublattice exchange is compared.

Magnetism in NdMn0.1Fe0.9O3 compound

The low temperature crystal and magnetic structures of NdMn0.1Fe0.9O3 were determined on the basis of X-ray and neutron powder diffraction experiments as well as from specific heat and magnetization measurements. The compound crystallizes in the orthorhombic crystal structure, space group Pnma, in the entire temperature interval 20 mK < T < 300 K. The lattice parameters exhibit standard thermal expansion effects for T > 20 K and at lower temperatures the anomalies due to magnetostriction effect were observed. The iron sublattice orders magnetically into 3 different magnetic structures, namely into Γ5 = (Ax, Fy, Gz) for 130 K < T < TN (Néel temperature); Γ1 = (Gx, Cy, Az) for 15 K < T < 130 K and Γ3 = (Cx, Gy, Fz) for 20 mK < T < 15 K. The Nd ions order at temperatures below 1.6–1.75 K into the same Γ3 phase as the iron sublattice at these temperatures. The obtained magnetic structures fit perfectly in between the magnetic phases of closely related NdFeO3 and NdMn0.5Fe0.5O3 compounds. Our study, together with all previously published data, completes the entire magnetic phase diagram of NdMn1−xFexO3 (0 ≤ x ≤ 1) solid solution substitutional system.

Bismuth iron garnet: Ab initio study of electronic properties

Bismuth iron garnet (BIG), i.e. Bi3Fe5O12, is a strong ferrimagnet that also possess outstanding magneto-optical properties such as the largest known Faraday rotation. These properties are related with the distribution of magnetic moments on octahedral and tetrahedral sites, the presence of spin gaps in the density of state and a strong spin-orbit coupling. In this work, first-principles ab initio calculations are performed to study the structural, electronic and magnetic properties of BIG using Density Functional Theory with Hubbard+U (DFT+U) correction including spin-orbit coupling and HSE06 hybrid functional. We found that the presence of spin gaps in the electronic structure results from the interplay between exchange and correlation effects and the crystal field strengths for tetrahedral and octahedral iron sublattices. The DFT+U treatment tends to close the spin-gaps for larger U due to over-localization effects, notably in the octahedral site. On the other hand, the hybrid functional confirms the occurrences of three spin gaps in the iron states of the conduction band as expected from optical measurements. A strong exchange splitting at the top of the valence bands associated with a lone-pair type mixture of O p and Bi s,p states is also obtained. Similar exchange splitting was not previously observed for other iron based garnets, such as for yttrium iron garnet. It follows that hole doping, as obtained by Ca substitution at Bi sites, results in a full spin polarized density at the Fermi energy. This work helps to shed more light on the theoretical comprehension of the properties of BIG and opens the route towards the use of advanced Many Body calculations to predict the magneto-optical coupling effects in BIG in a direct comparison with the experimental measurements.