Magnetic Hyperfine Field Research Articles

Magnetic Phase Transitions in Ultrathin YFeO3 Films According to Synchrotron Mössbauer Reflectometry Data

The magnetic properties of ultrathin single-crystal Y57FeO3 orthoferrite films have been studied by Mössbauer reflectometry. Mössbauer spectra were measured using the ESRF synchrotron in the reflection geometry. As the temperature changes from 3.6 K to about 773 K, the splitting of the Zeeman sextet in the spectra successively decreases and, simultaneously, a quadrupole doublet appears in them, which indicates the development of a magnetic phase transition. From the temperature dependences of the magnetic hyperfine field Bhf for the main orthorhombic Y57FeO3 phase, Néel temperatures equal to TN ≈ 593 K, 562 K, and 567 K and the critical parameter values equal to β ≈ (0.28–0.3) ± 0.02 are determined in films with thicknesses of 28, 6.5, and 4 nm, respectively. An analysis of changing the ratio of the line intensities in the Zeeman sextet with temperature makes it possible to trace the successive rotation of the direction of the antiferromagnetic axes in Y57FeO3 toward the surface plane with an increase in the temperature and a decrease in the film thickness.

Crystal structure and hyperfine fields in Mn1-xCoFexGe magnetocaloric alloys

The phase analysis, site occupancy, and hyperfine fields of Mn1-xCoFexGe alloys (x = 0.01, 0.02, 0.05, 0.10) after annealing procedure are presented. Mössbauer spectroscopy was performed with and without an external magnetic field of B = 1.3 T parallel to the beam direction at room temperature. RT X-ray diffraction shows that Mn1-xCoFexGe compound is two phases of Ni2In type hexagonal structure and TiNiSi type orthorhombic structure. Samples with × = 0.01, 0.02 and 0.05 exhibit (pre)dominant contribution of the orthorhombic structure while for × = 0.10 the amount of hexagonal phase increases significantly. In the series of alloys, the lattice parameters and the unit cell volume remain weakly sensitive to Fe concentration at both phases. The measured Mössbauer spectra are superpositions of two doublets which come from the Fe atoms located in hexagonal structure and two broad sextets of Fe orthorhombic structure origin. The values of isomer shift and quadrupole splitting do not change with increasing iron content. Using information about the composition of local environment and measured values of isomer shift, we identify the origin of individual subspectra, and site occupation of Fe atoms in the alloy. The Mössbauer measurements carried out in external magnetic field shows that induced hyperfine magnetic field in the hexagonal structure is much higher for Fe at Mn sites than the observed for Fe at Co sites. Analysis of sextets shows that magnetic moments of Fe tend to arrange along the external field direction with almost the same values of about 0.87 µB.

Mössbauer and Structure-Magnetic Properties Analysis of AyB1-yCxFe2-xO4 (C=Ho,Gd,Al) Ferrite Nanoparticles Optimized by Doping.

AyB1-yCxFe2-xO4 (C=Ho,Gd,Al) ferrite powders have been synthesized by the sol-gel combustion route. The X-ray diffraction of the CoHoxFe2-xO4 (x = 0~0.08) results indicated the compositions of single-phase cubic ferrites. The saturation magnetisation of CoHoxFe2-xO4 decreased by the Ho3+ ions, and the coercivity increased initially and then decreased with the increase of the calcination temperature. The Mössbauer spectra indicated that CoHoxFe2-xO4 displays a ferrimagnetic behaviour with two normal split Zeeman sextets. The magnetic hyperfine field tends to decrease by Ho3+ substitution owing to the decrease of the A-B super-exchange by the paramagnetic rare earth Ho3+ ions. The value of the quadrupole shift was very small in the CoHoxFe2-xO4 specimens, indicating that the symmetry of the electric field around the nucleus is good in the cobalt ferrites. The absorption area of the Mössbauer spectra changed with increasing Ho3+ substitution, indicating that the substitution influences the fraction of iron ions at tetrahedral A and octahedral B sites. The X-ray diffraction of Mg0.5Zn0.5CxFe2-xO4(C=Gd,Al) results confirmed the compositions of single-phase cubic ferrites. The variation of the average crystalline size and lattice constant are related to the doping of gadolinium ions and aluminum ions. With increasing gadolinium ions and aluminum ions, the coercivity increased and the saturation magnetization underwent a significant change. The saturation magnetization of AlMg0.5Zn0.5FeO4 ferrite reached a minimum value (MS= 1.94 mu/g). The sample exhibited ferrimagnetic and paramagnetic character with the replacement with Gd3+ ions, that sample exhibited paramagnetic character with the replacement with Al3+ ions, and the isomer shift values indicated that iron is in the form of Fe3+ ions.

Temperature Dependence of the Hyperfine Magnetic Field at Fe Sites in Ba-Doped BiFeO3 Thin Films Studied by Emission Mössbauer Spectroscopy

Emission 57Fe Mössbauer spectroscopy (eMS), following the implantation of radioactive 57Mn+ ions, has been used to study the temperature dependence of the hyperfine magnetic field at Fe sites in Ba-doped BiFeO3 (BFO) thin films. 57Mn β decays (t1/2 = 90 s) to the 14.4 keV Mössbauer state of 57Fe, thus allowing online eMS measurements at a selection of sample temperatures during Mn implantation. The eMS measurements were performed on two thin film BFO samples, 88 nm and 300 nm thick, and doped to 15% with Ba ions. The samples were prepared by pulsed laser deposition on SrTiO3 substrates. X-ray diffraction analyses of the samples showed that the films grew in a tetragonal distorted structure. The Mössbauer spectra of the two films, measured at absorber temperatures in the range 301 K–700 K, comprised a central pair of paramagnetic doublets and a magnetic sextet feature in the wings. The magnetic component was resolved into (i) a component attributed to hyperfine interactions at Fe3+ ions located in octahedral sites (Bhf); and (ii) to Fe3+ ions in implantation induced lattice defects, which were characterized by a distribution of the magnetic field BDistr. The hyperfine magnetic field at the Fe probes in the octahedral site has a room temperature value of Bhf = 44.5(9) T. At higher sample temperatures, the Bhf becomes much weaker, with the Fe3+ hyperfine magnetic contribution disappearing above 700 K. Simultaneous analysis of the Ba–BFO eMS spectra shows that the variation of the hyperfine field with temperature follows the Brillouin curve for S = 5/2.

Novel Insights of Enhanced Magnetization and Magnetoelectric Coupling in the Nanocomposite of BiFeO3 with Lanthanum Strontium Manganite

AbstractNovel insights on the magnetic, ferroelectric and magnetoelectric coupling of the nano composites of BiFeO3 (BFO) and La0.7Sr0.3Mn0.95Fe0.05O3 (LSMFO) with the latter exhibiting colossal magnetoresistance (CMR) behavior are provided in this study for the first time. Appreciable magnetoelectric coupling is observed in the composite and is seen to be significantly varying at temperatures below the blocking temperature of LSMFO. The local structure and magnetic properties of both the BFO and LSMFO particles in the composite are investigated in a detailed manner using Mössbauer spectroscopy studies specially devised to address the problem concerned. Enhanced values of the effective magnetic fields are observed at the Fe sites associated with BFO particles which are in contact with LSMFO particles. Interestingly, an appreciable increase in the magnetic hyperfine fields at Fe sites corresponding to LSMFO is also observed in the nanocomposites. New insights are derived based on the results of Mössbauer studies toward comprehending the magnetic and magnetoelectric coupling effects as deduced in these composites.

Insights into the Role of Defects in Fe2(MoO4)3 Catalysts

The catalytic activity and selectivity of Fe2(MoO4)3 obtained from solid-state synthesis protocols is investigated for the oxidative dehydrogenation (ODH) of ethanol to acetaldehyde. While Fe2(MoO4)3 annealed in a MoO3 atmosphere is found to be inactive, ball-milling of such solid-state synthesis precursors leads to an increase in activity, which cannot be attributed purely to the change in the specific surface area nor to a geometric effect of smaller particles. In a systematic study of the synthesis of Fe2O3-free Fe2(MoO4)3 samples, the correlation of ball-milling time, defect concentration, and catalytic activity is presented. In-depth X-ray diffraction studies, magnetic susceptibility, and 57Fe-Mössbauer spectroscopic measurements disclose characteristic signatures of the defect sites in the solid materials introduced by ball-milling and indicate: (i) MoO3-enriched amorphous layers of variable thickness (shell-like) and (ii) crystalline bulk defects corresponding to a reduction in the molar volume of the solid. The onset of recrystallization processes sets in around 300 °C and is accompanied by an enhanced MoO3 mobility in the particles, which adds to an increase in activity and a decreasing selectivity under catalytic conditions. Accordingly, the defects associated with the decreasing amorphous fraction are converted to crystalline bulk defects, which are monitored by the magnetic hyperfine field distribution of the respective Fe sites. Overall, this study shows the importance of the amorphous layer thickness with a sufficient defect concentration and of the bulk defects’ contribution to the chemical gradients, important for the MoO3 mobility and reduction of the MoO3 loss as a volatile species at the solid/gas phase boundary in ODH catalysis.

Hyperfine fields and site occupation of the Fe atoms in the Mn1−xCoFexGe giant magnetocaloric materials

The crystal properties, site occupation and hyperfine fields of Mn1−xCoFexGe (x = 0.01, 0.02, 0.05, 0.10) alloys after melting are presented. The X-ray diffraction and Mössbauer spectroscopy, without and with external magnetic field of B = 1.3 T parallel to beam direction, were carried out. X-ray diffraction shows that Mn1−xCoFexGe (x = 0.01 and 0.02) alloys are double phase with Ni2In-type (space group P63/mmc) hexagonal structure and TiNiSi-type (space group Pmna) orthorhombic structure. For higher Fe concentration (x = 0.05 and 0.10) the single hexagonal phase is observed. The Mössbauer spectra are superposition of two doublets which come from the Fe atoms located in hexagonal structure and additional broad sextet originated from Fe in orthorhombic structure for samples x = 0.01 and 0.02. The analysis of Mössbauer spectra shows that doped Fe atoms are located in both Mn and Co crystallographic positions in hexagonal phase with almost equal probabilities. The results show that values of isomer shift and quadrupole splitting do not change with increasing iron concentration. For hexagonal structure the isomer shift is equal to (0.30 ± 0.05) mm/s for Fe in Co and (0.49 ± 0.05) mm/s for Fe in Mn position. The quadrupole splitting for hexagonal structure are equal to (0.79 ± 0.05) mm/s and (0.87 ± 0.05) mm/s for Fe in Co and Mn site, respectively. The Mössbauer measurements carried out in external magnetic field for iron atoms located in hexagonal structure show, that observed induced hyperfine magnetic field on Fe in Mn position is much higher while on Fe in Co position smaller then applied external magnetic field.

Structural, morphological and magnetic properties of (Ni0.5Co0.5)[Ga Gd Fe2–2]O4 nanoparticles prepared via sonochemical approach

In this report, the impact of Ga3+ and Gd3+ co-substitution on different types of behavior of Ni-Co nanospinel ferrites (NSFs), (Ni0.5Co0.5)[GaxGdxFe2–2x]O4 (0.000 ≤ x ≤ 0.025), was investigated. NSFs were fabricated through ultrasound irradiation. The structure, composition, and spherical morphology of all products were verified by numerous characterization techniques such as X-ray diffraction (XRD), scanning and transmission electron microscopes (SEM & TEM), and selected area electron diffraction (SAED). Mossbauer spectra are composed of two ferromagnetic sextets and one paramagnetic doublet. These spectra were fitted to extract hyperfine parameters of the doped samples. Both sublattices' hyperfine magnetic field decreases with substitution. The isomer shift values show that the Mossbauer spectra are composed of magnetic Fe3+ sextets. The magnetization measurements at variable magnetic field (M-H) were investigated via a vibrating sample magnetometer (VSM) at a variable magnetic field (M-H) at temperatures (T) from 300 and 10 K. All NSFs disclose ferrimagnetic behavior at both 300 and 10 K where they are soft and hard, respectively. We determine that the remanence magnetization (Mr), saturation magnetization (Ms), and magneton number (nB) decrease with increasing Ga3+ and Gd3+ ions contents. The reduction in these values was predominantly recognized to the impact of cation redistribution and surface spins on tetrahedral (Td) and octahedral (Oh) sites. As the Ga3+ and Gd3+ contents increase, the coercivity (Hc) is also found to decrease.

57Fe Mössbauer Effect Study of Y(Fe1 – xNix)2 Synthesized under High Pressure

The measurements of magnetic hyperfine fields (MHF), Hhf, and isomer shift, δ, in Y(Fe1 – xNix)2 intermetallic compounds (the MgCu2 structure type) synthesized at high pressure are performed. The MHF values that appear on 57Fe nuclei at a nickel concentration x below 20 at % practically do not change and are approximately equal to 22 T, and in the range from x = 0.4 to 0.98 they decrease linearly with an increase in the Ni concentration. However, linear extrapolation of the hyperfine field as a function of Ni concentration does not lead to its disappearance in YNi2. For YFe2, the rotation of the easy axis from the [101] direction to the [111] direction with increasing temperature is found. As the Ni concentration increases to x = 0.3 at a temperature of 5 K, the easy magnetization axis [101] is observed, and at x = 0.4 the axis changes direction to [100]. Based on the shape of the concentration dependence of the hyperfine field, it is assumed that during the crystallization of Y(Fe1 – xNix)2 under high pressure conditions, a magnetic moment exists on Ni ions. First-principles calculations of magnetic properties and hyperfine interactions are performed, which are consistent with experiment.

Antiferromagnetism and Structure of Sr1−xBaxFeO2F Oxyfluoride Perovskites

Recently, a series of oxyfluorides, Sr1−xBaxFeO2F with x = 0, 0.25, 0.50, and 0.75 obtained through a novel synthesis route, were characterized by X-ray and neutron powder diffraction (NPD), magnetization measurements, and 57Fe Mössbauer spectroscopy (MS). The diffraction data revealed random occupancy of Sr and Ba atoms at the A-cation site, and a statistical distribution of O and F at the anionic sublattice of the perovskite-like structure specified in space group Pm-3m. MS spectra analysis consistently indicated the presence of Fe3+ ions at B-site, confirming the Sr1−xBaxFeO2F stoichiometry. Magnetic structure determination from the NPD data at room temperature established G-type antiferromagnetic arrangement in all compositions with Fe3+ moments of about 3.5 μB oriented along the c axis. In this study, we present and analyze additional NPD data concerning the low-temperature chemical and magnetic structure of Sr0.5Ba0.5FeO2F (x = 0.5) and SrFeO2F (x = 0). Basically, the three-dimensional G-type magnetic structure is maintained down to 2 K, where it is fully developed with an ordered magnetic moment of 4.25(5) μB/Fe at this temperature for x = 0.5 and 4.14(3) μB/Fe for x = 0. The data processing is complemented with a new approach to analyze the temperature dependence of the magnetic order TN on the lattice parameters, based on the magnetic hyperfine fields extracted from the temperature-dependent MS data.

The Creation of Complex Multi-Component Models of Mössbauer Spectra Based on the Example of a Study of Hyperfine Interactions in Quasi-Binary Alloys with the Laves Phase Structure

In this paper, we describe a method for creating new custom models of partial Mössbauer spectra based on the existing ones built into the SpectrRelax program without changing the program itself by writing code fragments in the built-in Lua programming language. The extension of the capabilities of the SpectrRelax program is demonstrated using the example of studying hyperfine interactions of 57Fe nuclei in quasibinary Tb(Fe0.8Al0.2)2 and Ho(Fe0.8Mn0.2)2 alloys with the C15 Laves phase structure. A model for processing the spectra, which simultaneously takes the composite and magnetic nonequivalence of the positions of Fe atoms in these alloys, has been created and used, which makes it possible to obtain new data about the orientation of the axis of the easiest magnetization, the anisotropy of the hyperfine magnetic field, and the effect of substitution of Al and Mn atoms for Fe atoms on hyperfine parameters of Mössbauer spectra.

The Creation of Complex Multi-Component Models of Mössbauer Spectra Based on the Example of a Study of Hyperfine Interactions in Quasi-Binary Alloys with the Laves Phase Structure

In this paper, we describe a method for creating new custom models of partial Mössbauer spectrabased on the existing ones built into the SpectrRelax program without changing the program itself by writing code fragments in the built-in Lua programming language. The extension of the capabilities of the SpectrRelaxprogram is demonstrated using the example of studying hyperfine interactions of 57Fe nuclei in quasibinary Tb(Fe0.8Al0.2)2 and Ho(Fe0.8Mn0.2)2 alloys with the C15 Laves phase structure. A model for processingthe spectra, which simultaneously takes the composite and magnetic nonequivalence of the positions of Fe atoms in these alloys, has been created and used, which makes it possible to obtain new data about the orientationof the axis of the easiest magnetization, the anisotropy of the hyperfine magnetic field, and the effect of substitution of Al and Mn atoms for Fe atoms on hyperfine parameters of Mössbauer spectra.

Cluster-layered [Fe/Cr]30 structure exhibited Kondo-like effect studied by GISAXS and Mössbauer spectroscopy

Structure and morphology of epitaxial [Fe/Cr]30 multilayers with ultrathin Fe layers (nominally 0.12 nm and 0.08 nm) have been investigated by X-ray reflectivity, synchrotron Mössbauer spectroscopy at low temperature and grazing-incidence small angle X-ray scattering (GISAXS). The films demonstrate Kondo-like behavior of electrical resistivity. The GISAXS patterns reveal their cluster-layered structure. The observed side maxima give the information about the sizes and distances between lateral inhomogeneities. Mössbauer reflectivity spectra measured below the critical angle of the total reflection support the existence of the cluster-layered structure of the samples. Magnetic hyperfine field distributions show that largest number of 57Fe atoms is situated in interfaces of iron clusters and their number increases in the thinnest films.

Interface-resolved study of magnetism in MgO/FeCoB/MgO trilayers using x-ray standing wave techniques

Interfaces in the MgO-FeCoB-MgO trilayer have been studied with grazing incident nuclear resonance scattering (GINRS) using the x-ray standing waves (XSW) technique. High depth selectivity of the present method allows one to measure magnetism and structure at the two interfaces of FeCoB, namely, FeCoB-on-MgO and MgO-on-FeCoB, independently, yielding an intriguing result that both interfaces are not symmetric. A high-density layer with an increased magnetic hyperfine field at the FeCoB-on-MgO interface suggests different growth mechanisms at the two interfaces. The azimuthal angle-dependent magneto-optic Kerr effect measurements reveal the presence of unusual uniaxial magnetic anisotropy (UMA) in the trilayer. An in-situ temperature-dependent study discovered that this UMA systematically reduces with temperature. The trilayer becomes isotropic at 450C with an order-of-magnitude increase in coercivity. The asymmetry at the interfaces is, in turn, explained by boron diffusion from the FeCoB interface layer into the nearby MgO layer. Stress-induced UMA is observed in the boron-deficient FeCoB layer, superimposed with the bulk FeCoB layer, and found to be responsible for unusual UMA. The temperature-dependent variation in the UMA and coercivity can be understood in terms of variations in the internal stresses and coupling between FeCoB bulk and the interface layer.

Elastic, electronic, thermal and magnetic investigations of PrX2(X = Fe,Ru) superconductors materials

The structural, elastic, electronic properties, thermal properties and magnetic hyperfine field and thermoelectric response of Laves phase PrFe2 and PrRu2 compounds are researched by using density functional theory via full-potential linear augmented plane waves (FP-LAPW) method within the local orbitals joining the generalized gradient approximation (GGA-PBESol) of exchange-correlation functional as applied in WIEN2k software package. Additionally, the semi-classical Boltzmann theory is utilized to investigate the stress behavior and superconductors applications on magnetic moments of each atom from 0 to 25 GPa for both materials are researched. The GGA-PBESol+U approximation is employed to address the f states of Pr atoms and d states of Fe and Ru atoms. The geometrical analysis of structural parameters is applied. The structural parameters calculated by two approximations are in a good agreement with other results. The calculated elastic constants, Young’s modulus, shear modulus, Poisson’s ratio, sound velocities and Debye temperature are investigated. The thermodynamic properties are calculated by a semi-harmonic Debye model in the pressure range, 0–25 GPa and temperature, 0–1000 K. The partial and total density of states (DOS) are investigated for PrFe2 and PrRu2 compounds. The partial DOS illustrates a hybridized strong at Fermi level. While, the hyperfine magnetic field is determined using two GGA-PBESol and GGA-PBESol+U approximations. Both PrFe2 and PrRu2 have superconducting critical temperatures; 550 K and 580 K, respectively. The comprehensive experimental characterization of PrRu2 is still absent in the literature and superconducting critical-temperature values are favorable with a similar experimental value; 530 K for PrFe2.

The Interrelation between Iron Valence and Oxygen Vacancies in Substituted Orthoferrite La0.67Sr0.33FeO3– during Heat Treatment

In this paper, we studied substituted lanthanum orthoferrite La0.67Sr0.33FeO3– γ by Mössbauer spectroscopy using X-ray diffraction data. A series of vacuum annealings was performed in the temperature range of tann = 200–650°C, after which no significant changes in the structure of the samples were detected. The Mössbauer measurements at room temperature show that the Fe ions are in an average valence statebetween Fe3+ and Fe4+. Upon vacuum annealing, as the temperature tann increased, the average hyperfine magnetic field on the 57Fe nuclei and the isomer shift of the spectrum increased, which is associated with an increase in the number of vacancies and, accordingly, a decrease in the amount of Fe4+. Mössbauer measure-ments at 85 K showed that the average valence state of iron does not manifest itself. The hyperfine parameters of the low-temperature Mössbauer subspectra obtained from the model interpretation indicate that one of them belongs to Fe4+ ions, and the rest belong to Fe3+. The presence in the spectra of several sextets related to Fe3+ ions is due to the appearance of oxygen vacancies (breaking of the Fe3+–O2––Fe exchange bond) and Fe4+ ions (weakening of the Fe3+–O2––Fe exchange bond) in the nearest ionic neighborhood of Fe atoms. Both factors cause a decrease in the hyperfine magnetic field and a change in the isomer shift of the spectrum. As a result of model interpretation of the Mössbauer spectra, the numbers of oxygen vacancies and Fe4+ ions per formula unit depending on vacuum annealing temperature tann were determined for all samples. It wasshown that at tann above 450°C, the process of oxygen leaving the lattice ends and only Fe3+ ions are detected.

Investigation of Mn-Fe magnetic interaction in mechanically alloyed Fe doped-MnAlC by 57Fe Mössbauer spectrometry

Four samples γ2-Mn40Fe0.5Al59.5, β-Mn60Fe0.5Al39.5, and ε-Mn50.5Fe0.5Al47C2, τ-Mn50.5Fe0.5Al47C2 were synthesized by mechanical alloying method. X-ray diffraction results showed high purity of the γ2, β, ε, and τ phases in each sample. Mössbauer measurement at room temperature showed non-ferromagnetic characteristic of the γ2-Mn40Fe0.5Al59.5, β-Mn60Fe0.5Al39.5, and ε-Mn50.5Fe0.5Al47C2. Interestingly, τ-Mn50.5Fe0.5Al47C2 showed a complex magnetic hyperfine splitting, indicating magnetic ordered structure. The hyperfine interaction in τ-Mn50.5Fe0.5Al47C2 was found dependent with temperature, in which isomer shift (IS) and magnetic hyperfine field (Bhf) decreased with increasing temperature. From the IS temperature dependence curve, the Debye temperature (θD) is determined to be 422 ± 80 K. The temperature dependence of Bhf values and the thermomagnetic measurement yielded a Curie temperature (Tc) values of 560 ± 13 K and 565 ± 2 K respectively. In order to investigate the magnetic interaction between Fe and Mn atoms in τ-Mn50.5Fe0.5Al47C2, local magnetic order of τ-Mn50.5Fe0.5Al47C2 was probed by using in-field 57Fe Mössbauer spectrometry. The obtained results showed that Fe and Mn formed a non-collinear magnetic structure with magnetic moments of Fe at different sites showing different canting angles with neighboring Mn atoms.

Synthesis and Characterization of Gadolinium Oxide-Hematite Magnetic Ceramic Nanostructures

Mixed-oxide nanostructures of the type xGd2O3-(1-x)α-Fe2O3 (x=0.1, 0.3, 0.5 and 0.7) were synthesized by mechanochemical activation for ball milling times of 0, 2, 4, 8 and 12 hours. The systems were subsequently analyzed by Mӧssbauer spectroscopy, X-ray powder diffraction (XRPD), magnetic measurements and optical diffuse reflectance spectroscopy. The magnetic hyperfine field was studied as function of ball milling time for all sextets involved and found to be consistent with the formation of a limited solid solution in the systems investigated. The end-product was the gadolinium perovskite, represented by a doublet whose abundance was derived as function of the milling time. The XRPD patterns recorded for the equimolar composition were dominated by the diffraction peaks of GdFeO3 after 12 hours of milling. The hysteresis loops were recorded at 300 and 5 K in an applied magnetic field of 5 T and were interpreted as a superposition of paramagnetic behavior of gadolinium oxide and weak ferromagnetic behavior of hematite and gadolinium perovskite. The Morin transition of hematite was inferred from zero-field-cooling-field-cooling (ZFC-FC) curves measured with a magnetic field of 200 Oe in the 5-300 K temperature range and was found to depend on the ball milling time. Optical diffuse reflectance spectra showed that the compounds were semiconductors with an optical band gap of 2.1 eV.

Synthesis and structural, magnetic and spectroscopic characterization of iron(III) complexes with in situ formed ligands from methyl-2-pyridyl ketone transformations.

Reactions of methyl-2-pyridyl ketone, pyCOMe, with FeCl3·6H2O in various solvents gave complexes [Fe4Cl6(OMe)2(L1)2]·0.7MeCN·0.4MeOH (1·0.7MeCN·0.4MeOH) and [Fe3Cl4(bicine)(L2)]·Me2CO·0.2H2O (2·Me2CO·0.2H2O). The ligands (L1)2- = pyCO(Me)CHCOpy (in 1) and (L2)2- = pyCO(Me)CH2CO(OMe)py (in 2) are formed in situ, through an aldol reaction-type mechanism between the carbanion pyC(O)CH2- (formed by the nucleophilic attack of the MeO- in pyCOMe) and pyCOMe which results in the formation of a new C-C bond. The intermediate compound undergoes attack in the -CH2- or -CO- group by a MeO- group, and the new ligands (L1)2- and (L2)2-, respectively, are formed. The molecular structure of 1 consists of three corner-sharing [Fe2O2] rhombic units in cis-arrangement. The two terminal FeIII ions display distorted square pyramidal geometry and the two central FeIII ions are distorted octahedral. The molecular structure of 2 consists of two corner-sharing [Fe2O2] rhombic units, with the two terminal FeIII ions in distorted square pyramidal geometry and the central FeIII in distorted octahedral. The differentiation in the coordination environment of the FeIII ions in 1-2 is reflected in the values of the Mössbauer hyperfine parameters. In agreement with theoretical calculations, the square pyramidal sites exhibit a smaller isomer shift value in comparison to the octahedral sites. Magnetic studies indicate antiferromagnetic interactions leading to an S = 0 ground state in 1 and to an S = 5/2 ground state in 2, consistent with Electron Paramagnetic Resonance spectroscopy. Mössbauer spectra of 2 indicate the onset of relaxation effects below 80 K. At 1.5 K the spectrum of 2 consists of magnetic sextets. The determined hyperfine magnetic fields are consistent with the exchange coupling scheme imposed by the crystal structure of 2. Theoretical calculations shed light on the differences in the electronic structure between the square pyramidal and the octahedral sites.

Local Probing ErCrO3

Local distortions in perovskite-like orthochromites are of extreme importance for the properties they exhibit. Here, we present the results of structural and DC magnetisation measurements combined with local probe studies in polycrystalline ErCrO3. The electric field gradient (EFG) parameters’ evolution with temperature shows two clear signals of local environment changes, one at the ferroelectric phase transition (TFE) and the other below 250 K. At the claimed TFE, the EFG changed from a slightly distorted axial symmetric to an EFG with axial symmetry (evidence that the local point-symmetry of the crystal might have changed). At a temperature around 250 K, we observed the development of a magnetic hyperfine field (MHF) and a change in the EFG to an axial slightly distorted one. These observations are rather in line with our magnetisation measurements, as a relatively strong coercive field was observed well above the Cr sub-lattice ordering temperature.