Iron Sublattice Research Articles

Lifting the geometric frustration through a monoclinic distortion in “114” YBaFe4O7.0: Magnetism and transport

The possibility to lift the geometric frustration in the “114” stoichiomeric tetragonal oxide YBaFe 4 O 7.0 by decreasing the temperature has been investigated using neutron and synchrotron powder diffraction techniques. Besides the structural transition from tetragonal to monoclinic symmetry that appears at T S =180 K, a magnetic transition is observed below T N =95 K. The latter corresponds to a lifting of the 3D geometric frustration toward an antiferromagnetic long range ordering, never observed to date in a cubic based “114’” oxide. The magnetic structure, characterized by the propagation vector k 1 =(0,0,½), shows that one iron Fe2 exhibits a larger magnetic moment than the three others, suggesting a possible charge ordering according to the formula YBaFe 3 + Fe 3 2 + O 7.0 . The magnetic M ( T ) and χ ′( T ) curves, in agreement with neutron data, confirm the structural and magnetic transitions and evidence the coexistence of residual magnetic frustration. Moreover, the transport measurements show a resistive transition from a thermally activated conduction mechanism to a variable range hopping mechanism at T S =180 K, with a significant increase of the dependence of the resistivity vs. temperature. Mössbauer spectroscopy clearly evidences a change in the electronic configuration of the iron framework at the structural transition as well as coexistence of several oxidation states. The role of barium underbonding in these transitions is discussed. Atomic displacements at the tetragonal-monoclinic transition in YBaFe 4 O 7 . • The structural and magnetic phase transitions of YBaFe 4 O 7 were studied below room temperature. • The tetragonal to monoclinic transition, characterized by NPD and SXRD, was studied using mode crystallography approach. • Monoclinic distortion allows the lifting of the geometrical frustration on the iron sublattice, leading to AF order at T =95 K.

Magnetic structure of Sr2Fe2O5 brownmillerite by single-crystal Mössbauer spectroscopy

Abstract In order to determine orientation of the Fe3+ magnetic moments and electric field gradient (efg) axes in the brownmillerite-type strontium ferrite structure for both iron sublattices where the efg tensor is not axially symmetric, the Mossbauer spectra of powdered and oriented single-crystal Sr2Fe2O5 were analyzed by solving the complete Hamiltonian for hyperfine interactions in the excited and ground states of the 57Fe nuclei. The magnetic moments of both octahedrally and tetrahedrally coordinated iron cations lie on the ac-plane of the orthorhombic unit cell and are parallel to the shortest c-axis, whilst the main efg axes are parallel to the longest crystallographic axis, b. This orientation is similar to that in Ca2Fe2O5, in spite of the structural differences of strontium and calcium ferrite brownmillerites at low temperatures.

Hyperfine magnetic interactions of 57Fe nuclei in NaFeAs arsenide

The results of the Mossbauer effect studies of layered NaFeAs arsenide in a wide temperature range are presented. The measurements at T > TN demonstrate that the main part (∼90%) of iron atoms are in the low-spin state Fe2+. The other atoms can be attributed to the impurity NaFe2As2 phase or to the extended defects in NaFeAs. The structural phase transition (at TS ≈ 55 K) does not produce any effect on hyperfine parameters (δ, Δ) of iron atoms. At T < TN, the spectra exhibit the existence of a certain distribution of the hyperfine magnetic field (HFe) at 57Fe nuclei, indicating the inhomogeneity of the magnetic environment around iron cations. The analysis of the temperature behavior of the distribution function p(HFe) allows us to determine the temperature of the magnetic phase transition (TN = 46 ± 2 K). It has been found that the magnetic ordering in the iron sublattice has a two-dimensional type. The analysis of the HFe(T) dependence in the framework of the Bean-Rodbell model reveals a first-order magnetic phase transition accompanied by a drastic change in the electron contributions to the main component (VZZ) and the asymmetry parameter (η) of the tensor describing the electric field gradient at 57Fe nuclei.

Sublattice Magnetic Relaxation in Rare Earth Iron Garnets

The magnetic properties of rare earth garnets make them attractive materials for applications ranging from optical communications to magnetic refrigeration. The purpose of this research was to determine the AC magnetic properties of several rare earth garnets, in order to ascertain the contributions of various sublattices. Gd3Fe5.O12, Gd2Ga5O12, Tb3Fe5O12, Tb3Ga5O12, and Y3Fe5O12 were synthesized by a solid state reaction of their oxides and verified by x-ray diffraction. Frequency-dependent AC susceptibility and DC magnetization were measured versus temperature (10-340 K). Field cooling had little effect on AC susceptibility but had a large effect on DC magnetization, increasing magnetization at the lowest temperature and shifting the compensation point to lower temperatures. Data suggest that interaction between the two iron sublattices result in the two frequency dependent magnetic relaxations in the iron garnets, which were modeled using the Vogel-Fulcher law.

Structural, Transport and Electrochemical Properties of LiFePO₄ Substituted in Lithium and Iron Sublattices (Al, Zr, W, Mn, Co and Ni).

LiFePO4 is considered to be one of the most promising cathode materials for lithium ion batteries for electric vehicle (EV) application. However, there are still a number of unsolved issues regarding the influence of Li and Fe-site substitution on the physicochemical properties of LiFePO4. This is a review-type article, presenting results of our group, related to the possibility of the chemical modification of phosphoolivine by introduction of cation dopants in Li and Fe sublattices. Along with a synthetic review of previous papers, a large number of new results are included. The possibility of substitution of Li+ by Al3+, Zr4+, W6+ and its influence on the physicochemical properties of LiFePO4 was investigated by means of XRD, SEM/EDS, electrical conductivity and Seebeck coefficient measurements. The range of solid solution formation in Li1−3xAlxFePO4, Li1−4xZrxFePO4 and Li1−6xWxFePO4 materials was found to be very narrow. Transport properties of the synthesized materials were found to be rather weakly dependent on the chemical composition. The battery performance of selected olivines was tested by cyclic voltammetry (CV). In the case of LiFe1−yMyPO4 (M = Mn, Co and Ni), solid solution formation was observed over a large range of y (0 < y ≤ 1). An increase of electrical conductivity for the substitution level y = 0.25 was observed. Electrons of 3d metals other than iron do not contribute to the electrical properties of LiFe1−yMyPO4, and substitution level y > 0.25 leads to considerably lower values of σ. The activated character of electrical conductivity with a rather weak temperature dependence of the Seebeck coefficient suggests a small polaron-type conduction mechanism. The electrochemical properties of LiFe1−yMyPO4 strongly depend on the Fe substitution level.

Magnetoresonance properties of antiferromagnetic TbFe3(BO3)4 at low temperatures

The magnetic resonance and field dependence of magnetization were studied in a single crystal of TbFe3(BO3)4 at temperatures from 2 to 13 K and frequencies from 18 to 142 GHz. Two pairs of lines with different intensities were found in the EPR spectrum. The found lines can be assigned to two types of centers: the Tb3+ ions which neighbor with the Bi and Mo growth impurities. The initial splitting of the lowest quasi-doublet of such Tb3+ ions by the crystal field and exchange field acting on the rare-earth ions from the iron sublattices were determined. The amount of these centers was estimated.

Effects of magnetic anisotropy and exchange in Tm2Fe17

Neutron diffraction experiments have been carried out to study the magnetocrystalline anisotropy of two (2b and 2d) Tm sublattices and four (4f, 6g, 12j, and 12k) Fe sublattices in ferrimagnetic compound Tm2Fe17 (space group P63/mmc). We have determined the temperature dependence of the magnitude and orientation of magnetization for each of the thulium and iron sublattices in the range (10–300) K. A spontaneous rotation (at about 90 K) of the Tm and Fe sublattice magnetizations from the c-axis to the basal plane is accompanied by a drastic change in the magnetization magnitude, signifying a large magnetization anisotropy. Both Tm sublattices exhibit an easy-axis type of the magnetocrystalline anisotropy. The Fe sublattices manifest both the uniaxial and planar anisotropy types. The sublattice formed by Fe atoms at the 4f position reveals the largest planar anisotropy constant. The Fe atoms at the 12j position show a uniaxial anisotropy. We find that the inelastic neutron scattering spectra measured below and above the spin-reorientation transition are remarkably different.

High-Temperature Magnetic Properties of Dysprosium Iron Garnet in Strong Magnetic Fields

The high-temperature (T ≥ 20 K) magnetic properties of dysprosium iron garnet (DyIG) are theoretically presented in strong magnetic fields based on three-sublattice model. Under the consideration of the molecular fields among the different magnetic sublattices, our theory confirms that the magnetization in DyIG shows obvious isotropy with the increase of the temperatures. Also, at higher temperatures, the variations of the magnetization in DyIG with the magnetic fields (He) and the temperatures (T) are successfully fitted. Similar with the conclusion at low temperatures, it is also proved that the contribution of the magnetization of the rare-earth magnetic sublattice to the total magnetization in DyIG is greater than that of iron sublattices at higher temperatures. In addition, theoretical calculations reveal that the temperature and field dependence of the coefficients αi (i=a, c and d) associated with the molecular field parameters λ and magnetic susceptibility χ, and at higher temperatures, it can be described as αi=P1+P2/He where P1 and P2 are the coefficients.

Magnetic properties of a GdFe5Al7single crystal

Magnetic properties of a tetragonal intermetallic compound GdFe${}_{5}$Al${}_{7}$ have been studied on a single crystal in static magnetic fields up to 14 T. The system is a ferrimagnet (${T}_{\mathrm{C}}$ $=$ 265 K) and on this premises we succeeded in describing its low-temperature magnetization curves as well as the temperature dependence of the spontaneous magnetization. The compound is interesting in two ways: (i) the iron sublattice is diluted so heavily that it is close to the stability limit and cannot be regarded as saturated, (ii) at low temperatures the system is in a state of near compensation, that is, the sublattice moments, albeit distinct, are close in magnitude. The latter has several consequences. (i) The net spontaneous magnetization is small, ${M}_{\mathrm{s}}$ $=$ 0.58 ${\ensuremath{\mu}}_{\mathrm{B}}$/f.u. at $T$ $=$ 2 K. (ii) The system is in the strong-anisotropy regime despite the weakness of its anisotropy (attributed entirely to the iron sublattice) in energy terms. (iii) A moderate field (\ensuremath{\sim}7 T) applied in the easy direction [110] suffices to induce a phase transition into a noncollinear magnetic state.

Negative thermal expansion and antiferromagnetism in the actinide oxypnictide NpFeAsO

A neptunium analogue of the LaFeAsO tetragonal layered compound has been synthesized and characterized by a variety of experimental techniques. The occurrence of long-range magnetic order below a critical temperature T_N = 57 K is suggested by anomalies in the temperature-dependent magnetic susceptibility, electrical resistivity, Hall coefficient, and specific heat curves. Below T_N, powder neutron diffraction measurements reveal an antiferromagnetic structure of the Np sublattice, with an ordered magnetic moment of 1.70(0.07) \mu_B aligned along the crystallographic c-axis. No magnetic order has been observed on the Fe sublattice, setting an upper limit of about 0.3 \mu_B for the ordered magnetic moment on the iron. High resolution x-ray powder diffraction measurements exclude the occurrence of lattice transformations down to 5 K, in sharp contrast to the observation of a tetragonal-to-orthorhombic distortion in the rare-earth analogues, which has been associated with the stabilization of a spin density wave on the iron sublattice. Instead, a significant expansion of the NpFeAsO lattice parameters is observed with decreasing temperature below T_N, corresponding to a relative volume change of about 0.2% and to an invar behavior between 5 and 20 K. First-principle electronic structure calculations based on the local-spin density plus Coulomb interaction and the local density plus Hubbard-I approximations provide results in good agreement with the experimental findings.

Effect of alloying by gadolinium on the magnetic properties of irradiation-amorphized oxides of the Y3 − x Gd x Fe5O12 system

Magnetometry and X-ray diffraction have been used to study the structural state and magnetic properties of the system of oxides with an iron garnet structure, Y3 − xGdxFe5O12 (x = 0, 0.75, 1.5), in the course of amorphization via irradiation by fast neutrons. It has been established that the partial replacement of diamagnetic yttrium ions by paramagnetic gadolinium dramatically changes the behavior of the magnetic moment of the garnets during their gradual amorphization. The magnetic moment of the yttrium iron garnet decreases significantly with increasing fluence, whereas the magnetic moments of gadolinium-alloyed garnets, on the contrary, increase noticeably along with the simultaneous increase in the compensation temperatures of the sublattice magnetizations. The observed differences in the radiation behavior of the garnets are explained by the different behavior of the iron and gadolinium sublattices under irradiation. It is shown that the irradiation to a fluence of 3 × 1020 cm−2 leads to a complete amorphization of all samples investigated. It has been established that the amorphous samples were in a spin-glass state.

Nuclear inelastic scattering studies of lattice dynamics in magnetite with a first- and second-order Verwey transition

Nuclear inelastic-scattering studies were performed to infer temperature evolution of iron atom dynamics in magnetite samples exhibiting Verwey transition of first- and second-order (type-I and type-II materials). The possible difference in this evolution could rationalize the distinct properties of these classes of materials observed in heat capacity and diffuse scattering below the Verwey transition temperature ${T}_{V}$ and could explain the change in transition order triggered by a minute (below 0.3$%$) altering of the iron sublattice. Although we have found the apparent stiffening of the phonon iron spectrum in the low-temperature phase, at the same time, we have shown that these spectra are rather similar for type-I and type-II materials, rendering the lattice vibration-based explanation of the distinct behavior of heat capacities very improbable. The calculation of phonon spectra, aimed at tracing the origin of various features in the phonon density of states (DOS), has shown that the local Coulomb interaction $U$ may have a large effect on phonon DOS. However, the change in the $U$ parameter cannot explain the difference in heat-capacity results for both classes of materials. Thus, an additional factor that differentiates these materials and possibly is responsible for the discontinuous character of the Verwey transition in stoichiometric magnetite still must be found.

Optical properties and electronic structure of multiferroic hexagonal orthoferrites RFeO3 (R = Ho, Er, Lu)

We report on optical studies of the thin films of multiferroic hexagonal (P.G. 6mm) rare-earth orthoferrites RFeO3 (R = Ho, Er, Lu) grown epitaxially on a (111)-surface of ZrO2(Y2O3) substrate. The optical absorption study in the range of 0.6–5.6 eV shows that the films are transparent below 1.9 eV; above this energy four broad intense absorption bands are distinguished. The absorption spectra are analyzed taking into account the unusual fivefold coordination of the Fe3+ ion. Temperature dependence of the optical absorption at 4.9 eV shows anomaly at 124 K, which we attribute to magnetic ordering of iron sublattices.

Magnetic structure of iron borate DyFe3(BO3)4: A neutron diffraction study

Neutron diffraction measurements were performed on the iron borate DyFe3(BO3)4 to investigate details of the crystallographic structure, the low temperature magnetic structure and its magnetic properties. DyFe3(BO3)4 adopts at room temperature the P3121 symmetry and becomes antiferromagnetic below TN = 39 K. Both, the rare earth and the iron sublattice, follow the same magnetic propagation vector τ = [0, 0, ½] which leads to a doubling of the crystallographic unit cell in the c-direction. The easy axis anisotropy of the rare earth determines the moment orientation to be mainly along c. No spin reorientation is found between TN and 1.5 K, however, a small anomaly in the thermal dependence of the unit cell a-parameter is found at about 27 K which could be connected to repopulation of low lying Kramers doublets of Dy3+. The magnetic moment value of the Fe-moment is at 1.5 K with μFe = 4.5 μB only slightly smaller than expected for an S = 5/2 ion while the Dy moment is strongly reduced and amounts only to μDy = 6.4 μB.

Mechanisms of the Magneto-Optical Activity of Rare-Earth Ions in Rare-Earth Iron Garnet Single Crystals

The magnetic and magneto-optical properties of the typical representatives of three rare-earth iron garnets (RIG) groups: with heavy rare-earth elements Yb, Er, Dy, Tb; with elements from the middle of lanthanide series Gd, Sm, and with light rare-earth element Nd are presented. In contradistinction to other work on the Faraday rotation, which were done only at 1152 nm (8696 cm−1), here we present FR spectra obtained in the energy region 5500–20000 cm−1 with high optical resolution. The investigations have been done at temperatures of 5, 82, 130, 295 K using magnetic field up to 25 kOe applied parallel to the [111] crystallographic axis of the crystals. It has been shown that the contribution proportional to the magnetic field and independent of temperature to the mixing of the ground state multiplets exceeds the paramagnetic contribution in YbIG, ErIG, GdIG, SmIG. In Tb and Dy iron garnets contributions from the two mechanisms have opposite signs, and the paramagnetic mechanism gives the greatest contribution to the Faraday rotation. Nevertheless, the contribution of the diamagnetic mechanism, caused by the influence of the exchange field in the iron sublattices on rare-earth ions, is significant, and it is necessary to take it into account. Anomalously large magneto-optical activity is observed in NdYIG. This is the result of contributions of the same sign and approximately equal in magnitude from the paramagnetic and diamagnetic mechanisms.

Magnetic properties and 57Fe NMR studies of U-type hexaferrites

U-type hexaferrites with compositions Ba4Me2Fe36O60 (Me=Cu, Fe, Co, Mn and Mg) have been synthesized and characterized by magnetization measurements and zero field 57Fe nuclear magnetic resonance (NMR) spectra. The NMR spectra show resonance peaks corresponding to different crystallographic sites of iron (tetrahedral, octahedral and trigonal bipyramidal) with integral intensities according to their site multiplicity. In comparison with the NMR spectrum of M-type barium hexaferrite the intensities of some of the peaks arising due to iron sublattices (fIV, a and b sites) are different, though there is no difference in peak positions indicating the same local hyperfine field strength at Fe nuclei. The maximum saturation magnetization at room temperature was found in Cu2U (70emu/g) and Fe2U (67emu/g). The Curie temperatures, saturation magnetizations and coercitivities for the U-type hexaferrites are given.

Fe57Mössbauer study of magnetic ordering in superconductingK0.80Fe1.76Se2.00single crystals

The magnetic ordering of superconducting single crystals of ${\mathrm{K}}_{0.80}{\mathrm{Fe}}_{1.76}{\mathrm{Se}}_{2.00}$ has been studied between 10 and 550 K using $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy. Despite being superconducting below $T$${}_{\mathrm{sc}}~30$ K, the iron sublattice in ${\mathrm{K}}_{0.80}{\mathrm{Fe}}_{1.76}{\mathrm{Se}}_{2.00}$ clearly exhibits magnetic order from well below $T$${}_{\mathrm{sc}}$ to its N\'eel temperature of $T$${}_{N}=532\ifmmode\pm\else\textpm\fi{}2$ K. The iron moments are ordered almost parallel to the crystal $c$ axis. The order collapses rapidly above 500 K and the accompanying growth of a paramagnetic component suggests that the magnetic transition may be first order, which may explain the unusual temperature dependence reported in recent neutron diffraction studies.

Multi-frame acquisition scheme for efficient energy-dispersive X-ray magnetic circular dichroism in pulsed high magnetic fields at the FeK-edge

Using a fast silicon strip detector, a multi-frame acquisition scheme was implemented to perform energy-dispersive X-ray magnetic circular dichroism at the iron K-edge in pulsed high magnetic fields. The acquisition scheme makes use of the entire field pulse. The quality of the signal obtained from samples of ferrimagnetic erbium iron garnet allows for quantitative evaluation of the signal amplitude. Below the compensation point, two successive field-induced phase transitions and the reversal of the net magnetization of the iron sublattices in the intermediate phase were observed.

Effect of hydrogen as interstitial element on the magnetic properties of some iron rich intermetallic compounds

Some structural and magnetic properties of RFe 11Ti and RFe 11TiH compounds, both of which crystallize with the I4/mmm tetragonal ThMn 12-type structure, are presented. The effect of hydrogen insertion on the ordering temperature and the iron sublattice contribution to the ordering of the RFe 11Ti phase is investigated. Then the effects of hydrogen on the magnetic properties is discussed and compared to what has been reported for other series of iron rich intermetallic compounds such as R 3Fe 29− x M x, R 2Fe 17, RFe 13− x M x and R 2Fe 14B, RFe 2... It is shown that if similar effects including increase of the ordering temperature and of the saturation magnetization are observed for the R 2Fe 17, R(Fe,M) 12, R 3Fe 29− x M x and R 2Fe 17 series, very different behaviour can also be observed for other series like RFe 2 where hydrogen insertion can be detrimental to the magnetic properties.

Structural, Electronic, and Magnetic Properties of UFeS3 and UFeSe3

Black prisms of UFeS(3) and UFeSe(3) have been synthesized by solid-state reactions of U, Fe, and S or Se with CsCl as a flux at 1173 K. The structure of these isostructural compounds consists of layers of edge- and corner-sharing FeS(6) or FeSe(6) octahedra that are separated by layers of face- and edge-sharing US(8) or USe(8) bicapped trigonal prisms. The isomer shifts in the iron-57 Mössbauer spectra of both UFeS(3) and UFeSe(3) are consistent with the presence of high-spin iron(II) ions octahedrally coordinated to S or Se. The XANES spectra of UFeS(3) and UFeSe(3) are consistent with uranium(IV). Single-crystal magnetic susceptibility measurements along the three crystallographic axes of UFeSe(3) reveal a substantial magnetic anisotropy with a change of easy axis from the a-axis above 40 K to the b-axis below 40 K, a change that results from competition between the iron(II) and uranium(IV) anisotropies. The temperature dependence of the magnetic susceptibility along the three axes is characteristic of two-dimensional magnetism. A small shoulder-like anomaly is observed in the magnetic susceptibilities along the a- and b-axes at 96 and 107 K, respectively. Below 107 K, the iron-57 Mössbauer spectra of UFeS(3) and UFeSe(3) show that the iron nuclei experience a magnetic hyperfine field that results from long-range magnetic ordering of at least the iron(II) magnetic moments because the field exhibits Brillouin-like behavior. Below 40 K there is no significant change in the Mössbauer spectra as a result of change in magnetic anisotropy. The complexity of the iron-57 Mössbauer spectra and the temperature and field dependencies of the magnetic properties point toward a complex long-range magnetic structure of two independent iron(II) and uranium(IV) two-dimensional sublattices. The temperature dependence of the single-crystal resistivity of UFeSe(3) measured along the a-axis reveals semiconducting behavior between 30 and 300 K with an energy gap of about 0.03 eV below the 53 K maximum in susceptibility, of about 0.05 eV between 50 and 107 K, and of 0.03 eV above 107 K; a negative magnetoresistance was observed below 60 K.