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Iron oxide nanoparticles (FeONPs) prepared with plant extracts have been emerging as green and sustainable materials. FeONPs are usually amorphous due to the chelation of the tea polyphenols (TPs) to the iron, and the real nature of the iron compounds is not completely understood. The main goal of this study was to investigate the behavior of the green FeONPs synthesized from an Fe3+ salt and Cammelia sinensis (black tea) extract upon thermal treatment, in order to remove TPs and enable the formation of crystalline materials suitable for a thorough characterization and with the potential for diverse applications. The as-prepared FeONPs were assigned as mixed-valence Fe(III) oxyhydroxides and Fe(II)/Fe(III) ions bound to TPs. A detailed description of the phase transformation upon heating revealed the formation of the rare nano β-Fe2O3 phase at 400 °C, followed by a transformation to α-Fe2O3 as the temperature increased. Above 600 °C, the unprecedented formation of FePO4 and Fe3PO7 was observed, produced from the reaction of Fe2O3 and free phosphate ions present in the black tea leaves, Fe3PO7 being the major phase obtained at 900 °C. Finally, the catalytic potential of the FeONPs to treat the azo dye methyl orange through a heterogeneous Fenton-like system was investigated.

The magnetic properties of site-disordered ${\mathrm{Ni}}_{5}\mathrm{Sn}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$ ludwigite have carefully been studied using local probes and bulk experimental methods in temperatures down to 1.6 K and magnetic fields up to 9 T. Our results have clearly shown the role of the effect of order/disorder site occupancy in these complex compounds with spin moments $S=1$. Different from most ludwigites, site-disordered ${\mathrm{Ni}}_{5}\mathrm{Sn}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$ ludwigite has shown three magnetic transitions at 80, 50, and 5 K. Local probes ($^{119}\mathrm{Sn}$ M\"ossbauer and muon spin spectroscopy) have indicated a partial long-range magnetic order with onsets at 80 and 50 K with predominant ferromagnetic interactions at 80 K and antiferromagnetic ones at 50 K. At lower temperatures, a partial spin-glass-like freezing of the moments has been developed concomitantly with fraction of the moments ordered at higher temperatures, a spin-glass-like and long-range magnetic order coexist and a spontaneous exchange bias effect has been observed. Applied magnetic fields have gradually reduced magnetic frustration, leading to a magnetically ordered state of the complete structure at 51 K under 9 T. The absence of double-exchange interactions in order/disorder ${\mathrm{Ni}}_{5}\mathrm{Sn}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$ ludwigites shows how the magnetic interactions that compete as direct exchange and double exchange are modified by the site occupancy of nonmagnetic Sn ions. Features as a third magnetic transition, the appearance of spontaneous exchange bias, and the enhancement of the coercive field at low temperatures were not found in the site-ordered isomer compound.

The crystal structure and the magnetic properties of the ${\mathrm{Co}}_{2.5}{\mathrm{Cr}}_{0.5}{\mathrm{BO}}_{5}$ ludwigite have been investigated by x-ray diffraction, magnetization, and specific heat experiments. Cr ions mainly occupy sites 4 of the ludwigite lattice, substituting approximately half of the ${\mathrm{Co}}^{3+}$. It changes the interatomic distances and favors a high spin state for the remaining ${\mathrm{Co}}^{3+}$ ions. Doping the homometallic ${\mathrm{Co}}_{3}{\mathrm{BO}}_{5}$ with magnetic Cr drastically increases the magnetic transition temperature from 42 K, for the undoped compound, to 76 K for ${\mathrm{Co}}_{2.5}{\mathrm{Cr}}_{0.5}{\mathrm{BO}}_{5}$. The latter contains three different magnetic ions, ${\mathrm{Co}}^{2+}, {\mathrm{Co}}^{3+}$ and ${\mathrm{Cr}}^{3+}$ in high spin states, as evidenced by magnetization measurements. The specific position of Cr in the structure (sites 4) gives rise, according to calculations of the exchange interactions and the Goodenough-Kanamori rules, to a few frustrated magnetic interactions. This is responsible for the high ferrimagnetic ordering temperature of 76 K, when compared to that of the pure compound. The experimental results imply a magnetic structure for the system that we show is consistent with the known interactions and the Goodenough-Kanamori rules. Low temperature specific heat and magnetization measurements reveal that the two-dimensional nature of the magnetism ${\mathrm{Co}}_{3}{\mathrm{BO}}_{5}$ when doped with Cr acquires a three-dimensional character with a high temperature ferrimagnetic order.

X-ray diffraction, magnetization, ac susceptibility, and specific heat have been measured on high-quality single crystals of ${\mathrm{Co}}_{3}{\mathrm{Mn}}_{3}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$ ludwigite. Different from previously studied ludwigites, this compound is characterized by the existence two low-dimensional subunits each containing a unique ion. The subsystem formed by ions at sites 3-1-3, known as a three-legged ladder (3LL), contains only divalent Co ions, while the 3LL formed by ions at sites 4-2-4 contains both divalent and trivalent Mn ions. Although there is only a weak disorder caused by a very small amount of Mn at sites 1, the experimental results evidence the existence of a low-temperature spin-glass phase. A dynamic scaling analysis of ac susceptibility data according to conventional critical slowing down results in a spin-glass phase-transition temperature ${T}_{g}=31.9$ K and a dynamic exponent $z\ensuremath{\nu}=7.05$. The temperature dependence of heat capacity has a linear contribution with the coefficient $\ensuremath{\gamma}=21.9$ mJ/(mol ${\mathrm{K}}^{2}$), which shows that this compound has the highest degree of magnetic disorder among ludwigites, corroborating the spin-glass state at low temperatures. The origin of this state is discussed, taking into account the particular structure of each subunit and the competition between the different exchange interactions involved.

Photonics enables a multi-target experiment of coherent MIMO radar. It confirms that coherence introduces almost one order of magnitude improvement in the cross-range resolution. Simulations demonstrates the coherent bi-band operation benefits on the system performance.

The mixed-valent homometallic ludwigite $({\mathrm{Co}}_{2}^{2+}{\mathrm{Co}}^{3+}){\mathrm{O}}_{2}{\mathrm{BO}}_{3}$ is investigated above the ferrimagnetic ordering temperature ${T}_{C}=43\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ through structural, thermal, magnetic, electric, and spectroscopic probes. X-ray absorption at the Co ${L}_{2,3}$ edges is consistent with the coexistence of ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$ ions, as expected by the sample stoichiometry. Magnetic susceptibility shows a relatively large net paramagnetic moment per Co ion above room temperature, $p=4.87$, indicating that the ${\mathrm{Co}}^{3+}$ ions are not in a pure low-spin configuration at high temperatures, also showing a non-Curie-Weiss behavior below 300 K. Electrical conductivity and differential scanning calorimetry measurements on single crystals indicate two phase transitions at $\ensuremath{\sim}475$ and $\ensuremath{\sim}495\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. X-ray powder diffraction shows substantial lattice parameter anomalies below 500 K. These results indicate phase transitions associated with changes in the Co oxidation state in each of its four crystallographic sites. Such transitions are possibly dictated by a competition between (i) an ordered ground state with all ${\mathrm{Co}}^{3+}$ ions occupying the same crystallographic site and (ii) either partially or totally charge-disordered states that are favored at high temperatures due to their higher entropy.

This work presents a study of the oxyborate ${\mathrm{Ni}}_{5}\mathrm{Ti}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$ using single-crystal x-ray diffraction and magnetic and specific-heat measurements. This material has in its crystalline structure subunits in the form of three-legged ladders. Despite the magnetic ions occupying these low-dimensional structures, we find that this system has long-range magnetic order at low temperatures. The analysis of the magnetization and specific-heat data shows that the process of establishing this order is complex and occurs in several stages upon lowering the temperature. Initially at 92 K a partial magnetic ordering of the Ni moments in one of the two ladders sets in. As the system is further cooled, weakly coupled clusters of spins order, and, finally, at 78 K the whole system of spins in the two ladders are magnetically ordered. Since the Ti ions are found in the nonmagnetic valence state ${\mathrm{Ti}}^{4+}$, the magnetic order is exclusively due to the ${\mathrm{Ni}}^{2+}$ ions with spin $S=1$. We discuss the ferrimagnetic nature of the long-range order, which is consistent with our knowledge of the magnetic interactions, the atomic distances, and bond angles obtained from the x-ray data.

This paper presents an extensive study of the structural, magnetic, and thermodynamic properties of the hulsite ${\mathrm{Ni}}_{5.15}{\mathrm{Sn}}_{0.85}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$. The crystal structure of the hulsite has two planar substructures formed by Ni and Sn atoms: one with rectangular configuration and the other with a triangular arrangement. These substructures are linked by the boron ions and by Ni in another site closer to the rectangular arrangement, resulting in a quasi-two-dimensional character. Thus, this system literally adds a new dimension to the study of oxyborates. Our results point to a complex magnetic behavior consistent with these substructures. The planes with rectangular arrangement form a complex magnetic ordering at 180 K (one of the highest magnetic transitions among the oxyborates). The other subsystem, formed by Ni atoms located in a two-dimensional triangular lattice, does not order down to temperatures as low as 3 K. The experimental results suggest a spin-liquid behavior for this subsystem. The magnetic moments of the ions between these planes also freeze at low temperatures. The two magnetic planes coexist as independent subsystems down to the lowest temperatures of our experiments.

This work deals with the synthesis and characterization of two homogeneous and three heterogeneous iron(III) catalysts, which were evaluated in the oxidation of cyclohexane and cyclohexene, using H2O2 as oxidant. The homogeneous catalyst [Fe2(BPA)2(μ-OCH3)2(Cl)2], (1), where BPA is the deprotonated form of N-(2-hydroxybenzyl)-N-(pyridin-2-ylmethyl) amine, is a dinuclear iron(III) compound, as determined by X-ray diffraction studies. Compound [FeIII(HBPClNOL)(Cl)2], (2), where HBPClNOL is N-(2-hydroxybenzyl)-N-(2-pyridylmethyl)(3-chloro)(2-hydroxy) propylamine, is a mononuclear iron(III) compound. The reaction of the ligand HBPA with the 3-glycidoxypropyltrimethoxysilane molecule followed by the reaction with different inorganic matrices (silica-gel, NaY zeolite, MCM-41) resulted in the organo functionalized solids SiL, NaYL and MCM-41L, respectively. These materials were reacted with iron(III) salt, affording the heterogeneous catalysts SiLFe, NaYLFe and MCM-41LFe, respectively. They were characterized by elemental analyses, HR-CS AAS, solid state MAS NMR (29Si, 13C), IR, UV–vis, TGA, Mossbauer, and textural analyses. The major product formed in all the oxidation reactions was the hydroperoxide derivative. When cyclohexane was the substrate, the homogeneous catalysts were more efficient than the heterogeneous ones. In contrast, the heterogeneous systems showed better results with cyclohexene than with cyclohexane, reaching about 30% in the presence of NaYLFe.

This work describes the synthesis of green iron nanoparticles (FeNPs) supported on amino-functionalized silica, carried out in presence of the extract of the plants Yerba Mate (Ilex paraguariensis) or black tea (Camellia sinensis), or in presence of the biodiesel co-product glycerol, as green reducing and stabilizing agents. The materials were characterized by AAS, SEM, TEM, FTIR, XRD, ASAP, Mossbauer spectroscopy and thermogravimetric analysis, which revealed the formation of amorphous FeNPs of iron oxyhydroxides, such as akaganeite and ferrihydrite. FeNPs in the order of 5 to 20 nm were obtained from tea. Besides, the plant extracts were able to reduce the Fe3+ precursor ion to Fe2+ species bound to the plant polyphenols. The FeNPs were applied in the removal of the azo dye methyl orange by adsorption and heterogeneous Fenton degradation, at different reaction conditions, such as catalyst dosage, pH, hydrogen peroxide concentration, and temperature. All iron compounds prepared removed the dye completely after three hours of reaction, and could be successfully recycled. In particular the FeNPs prepared with glycerol were active up to the fifth cycle in the Fenton degradation tests, due to the lower iron and organic compounds leaching. Kinetics showed that the adsorption is a key step in the dye removal, and that the adsorption sites are composed by the FeNPs bound to polyphenols/glycerol. The superior activity of the supported FeNPs in relation to the free analogues showed the benefits of the functionalization of the silica by the amino groups to stabilize the green iron nanoparticles.

This study aims to prepare environmentally friendly iron catalysts supported on silica, using glycerol as green reducing and stabilizing agent, for application in heterogeneous Fenton degradation of the pollutant dye methyl orange (MO). The catalysts were characterized by X-ray powder diffraction, atomic absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, thermogravimetric analyses, Mössbauer and Fourier transform infrared spectroscopies, which revealed the formation of iron(II)/(III) oxalates from the oxidation of glycerol by the iron(III) nitrate precursor. Besides, iron oxihydroxide nanoparticles with superparamagnetic behavior were also formed. Iron catalysts prepared in the presence of nickel(II) or zinc(II) nitrates lead to the formation of the corresponding oxalates. The catalysts were able to degrade MO, efficiently in 180 min of reaction. Fe/SiO2 furnished higher reaction rates, followed by Zn4Fe2/SiO2, which presented higher iron content as well as the smallest nanoparticles. Reaction parameters such as catalyst dosage, hydrogen peroxide concentration, pH and reaction temperature were investigated.

There are two known ludwigites containing a single transition metal element, ${\text{Fe}}_{3}{\text{O}}_{2}{\text{BO}}_{3}$ and ${\text{Co}}_{3}{\text{O}}_{2}{\text{BO}}_{3}$. The structure of these materials has low-dimensional units in the form of three-legged ladders (3LL) that confer to each of them unique magnetic and electronic properties. ${\text{Fe}}_{3}{\text{O}}_{2}{\text{BO}}_{3}$ presents a staggered charge density wave (CDW) transition in the ladders near room temperature and two magnetic transitions. It has remained a mystery why the other compound ${\text{Co}}_{3}{\text{O}}_{2}{\text{BO}}_{3}$ behaves so conventionally, with a single magnetic transition and no CDW in spite of similar structural and electronic configurations. Neutron diffraction results presented here in this system finally unravel these differences. Far from a trivial explanation, we uncover a coexistence of low and high spin Co ions in well-defined octahedral sites. Our results allow one to solve the contrasting behavior of the Fe and Co ludwigites in terms of a subtle and unique charge ordering mechanism occurring at the microscopic level of the rungs of the 3LL.

Thick corrosion products of austenitic stainless steel obtained from a steam turbine structure exposed in offshore environment were studied by optical stereomicroscopy, X-ray diffraction, Mossbauer and energy-dispersive X-ray spectroscopy. The external surface of mechanical structure was exposed to a marine atmosphere and the internal one to a combustion gas for two decades. Characterization of corrosion products showed high content of akaganeite in the outer region and a mixture of akaganeite and hematite in the inner surface. Besides these characteristics, a distribution of nanosized superparamagnetic particles of akaganeite was found in the rusts.

Mössbauer spectroscopic measurements have been carried out for CeSn 3 and CeSn 3.1 single crystals in the temperature range 1.5 K to 300 K under applied magnetic field, with an aim to look into the local surroundings of the Sn site and the possibility of magnetization at low temperatures. The spectra indicate a paramagnetic ground state throughout the temperature range with two different doublets associated with two sites S1 and S2. This indicates two crystallographic sites of Sn with the presence of a distortion in the cubic crystal. However, the contribution of S2 site is very small (only 3% for the CeSn 3 but slightly higher, i.e., 4.3% for the CeSn 3.1) at room temperature. Upon cooling (below 4 K), the S2 contribution disappears for the stoichiometric sample ( CeSn 3) but continues to stay for the slightly nonstoichiometric compound ( CeSn 3.1). The isomer shift reveals a Sn 2+ valence state throughout. A weak hyperfine field has been observed only at low temperatures (4.2 and 1.5 K spectra) for both the compounds, but not for the 300 K spectra. This is indicative of some magnetization, i.e., an increase in magnetic moments of Ce atoms, that is felt by the neighbor Sn atoms through RKKY interactions. However, upon cooling the samples from 4.2 K to 1.5 K, no further enhancement in magnetization is observed. The magnetic field was also applied for the CeSn 3 sample at low temperatures to check if there is any enhancement in the magnetic properties that yielded negative results, i.e., the applied field is equal to the hyperfine field, indicating no enhancement of magnetic moment.

In this work, we investigated the effects of postdeposition heat treatment on structural and magnetic properties of CoFe2O4 nanoparticles produced by pulsed laser deposition. Structural analysis by X-ray diffraction, transmission electron microscopy (TEM) and Mössbauer spectroscopy indicate the formation of a single phase cobalt ferrite nanoparticles with the size ranging from 4.3 to 33.3 nm depending on the annealing temperature. The magnetic properties of the samples were investigated in a wide temperature range (50-400 K). Noticeable effects of the cubic magnetocrystalline anisotropy on the magnetization process of nanoparticles were observed for samples annealed at 450 °C and 600 °C, while for samples as-deposited and annealed at 300 °C the magnetization properties were dominated by a uniaxial effective anisotropy. ΔM technique was used to investigate the magnetic interaction among the nanoparticles. Only demagnetizing interactions were observed for the sample annealed up to 300 °C, while for the samples treated at 450 °C and 600 °C, both magnetizing and demagnetizing interactions were observed. The results are discussed considering the evolution of the nanoparticles' nanostructure with anneals and its effects on the magnetic properties. Fil: Fabris, Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidade Federal Fluminense; Brasil Fil: Xing, Y.T.. Universidade Federal Fluminense; Brasil Fil: Franceschini, D.F.. Universidade Federal Fluminense; Brasil Fil: Sanchez, D.R.. Universidade Federal Fluminense; Brasil Fil: Alzamora, M.. Universidade Federal do Rio de Janeiro; Brasil Fil: Nunes, W.C.. Universidade Federal Fluminense; Brasil

57Fe Mossbauer spectra of Fe3O2BO3 reveal a combined effect of charge ordering and electron delocalization between 112 and 450 K. On the basis of the temperature dependence of the isomer shifts and quadrupole interactions, together with the information from previously obtained transport data, we are able to discuss the arrangement of Fe2+ and Fe3+ in the structure and the dynamics of the electronic configurations. We found a charge-delocalization transition around 300 K. Below this temperature, formation of pairs of Fe ions with mixed valence takes place in part of the crystalline structure.

57 Fe Mossbauer spectra of RNiBC (R = Gd, Tb, Dy, Ho, and Er) compounds doped with 57 Fe (0.5 at.%) taken above their magnetic ordering temperatures reveal a quadrupole doublet with a splitting | EQ | which decreases on going from Gd to Er. The values of | EQ | depend linearly on the parameter c ´/a , with c ´ being the distance apart of the R-C layers between which the Ni2 -B2 layer is sandwiched and a being the lattice parameter in the basal plane. The | EQ | versus c ´/a relation is identical with that previously found for RNi2 B2 C compounds.

We present a study of the transport and magnetic properties of a series of $R\mathrm{NiBC}$ compounds $(R=\mathrm{Er},$ Ho, Dy, Tb, and Gd). All the materials investigated have long range magnetic order at sufficiently low temperatures. Magnetoresistance measurements are presented for a large range of temperatures (T) and magnetic fields (H). We show that below the critical temperature, the temperature dependence of the resistivity is determined by electron scattering due to the elementary excitations (spin waves) of the ordered magnetic phase and the values of the gap in the magnon spectra were derived. Finally we discuss the $H\ifmmode\times\else\texttimes\fi{}T$ phase diagram of these materials.

The magnetic order of ${\mathrm{TbNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ below ${T}_{N}=15\mathrm{K}$ has been studied via ${}^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy and muon spin relaxation (\ensuremath{\mu}SR). It is shown that only below 8 K all components of the Tb moment are locked. This is traced from an unusual increase and a turning of the transferred magnetic hyperfine field at the ${}^{57}\mathrm{Fe}$ dopants, and also from a decrease of the fluctuation rate of electronic moments detected from \ensuremath{\mu}SR. We relate this locking of the moment to the ferromagnetically ordered component which was concluded from neutron diffraction experiments.

Electron spin resonance (ESR) of Gd{sup 3+} in the normal state (T{gt}T{sub c}) of R{sub 1{minus}x}Gd{sub x}Ni{sub 2}B{sub 2}C (R=Y,Lu) is reported. The results show that the exchange coupling between the rare-earth localized magnetic moment and the conduction electrons depends on the conduction electrons momentum transfer ({vert_bar}{bold k}{sub F}{sup in}{minus}{bold k}{sub F}{sup out}{vert_bar}=q), i.e., J{sub fs}(q). The temperature dependence of the ESR linewidth yields a value for one of the exchange parameters, {l_angle}J{sub fs}{sup 2}(q){r_angle}{sub E{sub F}}{sup 1/2}, which is in agreement with that estimated from the slope of the initial linear decrease of T{sub c} by the Gd{sup 3+} impurities. These results indicate that the R{sub 1{minus}x}Gd{sub x}Ni{sub 2}B{sub 2}C (R=Y,Lu) compounds behave as conventional BCS superconductors, in agreement with previous reports. {copyright} {ital 1998} {ital The American Physical Society}

Mossbauer spectroscopy has been used to investigate the magnetic structure of doped with 1 at.% . Our experiments on polycrystalline reveal the known spin-rearrangement magnetic phase transition at , which until now has not been observed in polycrystalline samples. A spiral-like spin structure with a variable angle between the moment direction and the b- or c-axis seems to be the correct model for the spin structure of .

Static magnetic order of quasi-two-dimensional FeAs compounds Sr${}_{4}$$A$${}_{2}$O${}_{6\ensuremath{-}x}$Fe${}_{2}$As${}_{2}$, with $A=$ Sc and V, has been detected by $^{57}\mathrm{Fe}$ M\"ossbauer and muon spin relaxation ($\ensuremath{\mu}$SR) spectroscopies. The nonsuperconducting stoichiometric ($x=0$) $A=$ Sc system exhibits a static internal/hyperfine magnetic field both at the $^{57}\mathrm{Fe}$ and ${\ensuremath{\mu}}^{+}$ sites, indicating antiferromagnetic order of Fe moments below ${T}_{\mathrm{N}}$ $=$ 35 K with $~$0.1 bohr magneton per Fe at $T=$ 2 K. The superconducting and oxygen deficient ($x=0.4$) $A=$ V system exhibits a static internal field only at the ${\ensuremath{\mu}}^{+}$ site below ${T}_{\mathrm{N}}~40$ K, indicating static magnetic order of V moments coexisting with superconductivity without freezing of Fe moments. These results suggest that the 42622 FeAs systems belong to the same paradigm with the 1111 and 122 FeAs systems with respect to magnetic behavior of Fe moments.

(57)Fe Mössbauer spectroscopy has been used to investigate the magnetic order of non-superconducting NdFeAsO (T(N) = 140 K) and superconducting NdFeAsO(0.88)F(0.12) (T(c) = 45 K). A magnetic hyperfine field B(hf) was observed at the (57)Fe nucleus below T(N)∼140 K for NdFeAsO. Below ∼2 K an increase of B(hf) relative to the saturation value was attributed to the transferred B(hf) at the Fe site resulting from the collinear antiferromagnetic (AF) spin structure of the Nd moments. The analysis of the spectra is consistent with a commensurate AF order of Fe spins. No B(hf) is observed in superconducting NdFeAsO(0.88)F(0.12) down to 1.5 K.

We report muon spin rotation ($\mu$SR) measurements of single crystal Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$. From measurements of the magnetic field penetration depth $\lambda$ we find that for optimally- and over-doped samples, $1/\lambda(T\to 0)^2$ varies monotonically with the superconducting transition temperature T$_{\rm C}$. Within the superconducting state we observe a positive shift in the muon precession signal, likely indicating that the applied field induces an internal magnetic field. The size of the induced field decreases with increasing doping but is present for all Co concentrations studied.

We present an extensive study of the oxyborate material ${\text{Co}}_{5}\text{Ti}{({\text{O}}_{2}{\text{BO}}_{3})}_{2}$ using x-ray, magnetic, and thermodynamic measurements. This material belongs to a family of oxyborates known as ludwigites which presents low-dimensional subunits in the form of three leg ladders in its structure. Differently from previously investigated ludwigites the present material does not show long-range magnetic order although it goes into a spin-glass state at low temperatures. The different techniques employed in this paper allow for a characterization of the structure, the nature of the low-energy excitations and the magnetic anisotropy of this system. Its unique magnetic behavior is discussed and compared with those of other magnetic ludwigites.

By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity in the $A{\text{Fe}}_{2}{\text{As}}_{2}$ $(A=\text{Ca},\text{Ba},\text{Sr})$ systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of $({\text{Ba}}_{0.5}{\text{K}}_{0.5}){\text{Fe}}_{2}{\text{As}}_{2}$ exhibits a nearly linear variation in $T$ at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multigap effects.

Recently, high-transition-temperature (high-T(c)) superconductivity was discovered in the iron pnictide RFeAsO(1-x)F(x) (R, rare-earth metal) family of materials. We use neutron scattering to study the structural and magnetic phase transitions in CeFeAsO(1-x)F(x) as the system is tuned from a semimetal to a high-T(c) superconductor through fluorine (F) doping, x. In the undoped state, CeFeAsO develops a structural lattice distortion followed by a collinear antiferromagnetic order with decreasing temperature. With increasing fluorine doping, the structural phase transition decreases gradually and vanishes within the superconductivity dome near x D 0 : 10, whereas the antiferromagnetic order is suppressed before the appearance of superconductivity for x > 0.06, resulting in an electronic phase diagram remarkably similar to that of the high-T(c) copper oxides. Comparison of the structural evolution of CeFeAsO(1-x)F(x) with other Fe-based superconductors suggests that the structural perfection of the Fe-As tetrahedron is important for the high-T(c) superconductivity in these Fe pnictides.

Neutron diffraction and small angle neutron scattering measurements have been carried out on RNiBC (R=Er, Tb, Dy) to investigate the crystallography, magnetic correlations, and long range magnetic ordering. Crystallographic refinements show a strong boron-carbon and nickel-carbon bonding similar to RNi{sub 2}B{sub 2}C, but the rare-earth-carbon plane exhibits a buckling which does not exist in the RNi{sub 2}B{sub 2}C series. The magnetism in the RNiBC series is quite simple, with a magnetic moment direction going from the a-b plane to the c axis as the atomic number of the rare-earth increases. Er becomes ferromagnetic at T{sub C}=4.6 K, with a saturated moment of 7.2{mu}{sub B} along the c axis. Tb orders ferromagnetically at T{sub C}=15 K. Below 12.5 K an incommensurate modulation develops with a modulation wave vector of (0 0 1/8), as well as an antiferromagnetic component with k=(0 0 1/2). The moments, which vary in both magnitude and direction, are in the a-b plane with an average value of 7.7{mu}{sub B}. DyNiBC exhibits the same behavior, with a T{sub C} and T{sub N} of 17.5 and 13.5 K, respectively.

57Fe Mossbauer spectroscopy has been used to investigate the structural and magnetic phase transitions of CaFe2As2 (TN = 173 K) single crystals. For this compound we found that VZZ is positive and parallel to the c-axis of the tetragonal structure. For CaFe2As2 a magnetic hyperfine field Bhf was observed at the 57Fe nucleus below TN ~ 173 K. Analysis of the temperature dependence of Bhf data using the Bean–Rodbell model shows that the Fe spins undergo a first-order magnetic transition at ~ 173 K. A collinear antiferromagnetic structure is established below this temperature with the Fe spin lying in the (a, b) plane. Below TN the paramagnetic fraction of Fe decreases down to 150 K and for lower temperatures all the Fe spins are magnetically ordered.

The rare earth (RE) nickel borocarbides are intermetallic layered compounds which offer the possibility to study the interplay between superconductivity and magnetism. We have doped RENi2B2C (RE=Ho,Dy,Tb) with ∼1 at. % 57Fe in order to study the magnetic phase transitions occurring in these systems by a local probe, i.e., by 57Fe Mossbauer spectroscopy. Sample characterization by x‐ray diffraction as well as ac susceptibility measurements showed mainly single phase. The room temperature and 4.2 K 57Fe ME spectra essentially show one quadrupole doublet (Δ∼0.20 mm/s) for all compounds, except for RE=Tb. A magnetic hyperfine (hf) field appears in the case of RE=Tb and Ho at TN∼15 and 8 K, respectively. The magnetic hf field for Tb at 4.2 K is Bhf=0.86 T, while it is quite smaller for Ho [Bhf(5 K)∼0.30 T]. Since 57Fe at the Ni sites in RENi2B2C does not have its own magnetic moment, all magnetic hf fields are resulting from the neighboring RE moments. The absence of a hf field in DyNi2B2C at 4.2 K confirms th...