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Research in nanotechnology, especially in nanoparticles, has been growing in several fields of study, from nanoelectronics to biotechnology. This wide range of applications requires different techniques for characterizing these nanoparticles in terms of properties such as structure, morphology, magnetic profile and chemical composition. The Scanning Magnetic Microscopy can be used for the magnetic characterization of these materials obtaining the magnetic maps and, with the use of the inversion technique, making it possible to recover the magnetic moment and then obtain the magnetization curve of these measurements. In this work, iron-aluminum spinel nanoparticles were produced by combustion reaction and characterized structurally by Transmission Electron Microscopy, and the magnetic properties by Scanning Magnetic Microscopy. For the inversion process, we model a set of magnetic maps using a cylindrical shape to obtain the magnetic moments, and then construct a magnetization curve. The inversion method is thereby validated by comparing the results with an in-line method already used in previous works.

The magnetic properties of core/shell nanoparticles can be finely tuned through the exchange coupling at the interface, enabling large heating powers under alternating magnetic fields. However, the...

We studied the influence of the superconducting state on the magnetic properties in a thick IrMn(100 nm)/NiFe(100 nm) bilayer deposited by magnetron sputtering onto an yttrium–barium–copper–oxide (YBCO) substrate that was previously synthesized by the acetate method. The results from magnetization experiments showed that the sign of the effective exchange fields switched from positive, in the as-prepared heterostructure, to negative values when the sample goes below the superconducting temperature of the YBCO substrate. We, thus, demonstrated that the YBCO substrate, in its superconducting state, strongly influences the magnetic anisotropy of the thick ferromagnetic NiFe layer due to the proximity effect that occurs at the YBCO–NiFe interface, where a non-collinear magnetic spin structure is formed during the in-field sample deposition.

Elemental tellurium is a small band-gap semiconductor, which is always p-doped due to the natural occurrence of vacancies. Its chiral non-centrosymmetric structure, characterized by helical chains arranged in a triangular lattice, and the presence of a spin-polarized Fermi surface, render tellurium a promising candidate for future applications. Here, we use a theoretical framework, appropriate for describing the corrections to conductivity from quantum interference effects, to show that a high-quality tellurium single crystal undergoes a quantum phase transition at low temperatures from an Anderson insulator to a correlated disordered metal at around 17 kbar. Such insulator-to-metal transition manifests itself in all measured physical quantities and their critical exponents are consistent with a scenario in which a pressure-induced Lifshitz transition shifts the Fermi level below the mobility edge, paving the way for a genuine Anderson-Mott transition. We conclude that previously puzzling quantum oscillation and transport measurements might be explained by a possible Anderson-Mott ground state and the observed phase transition. Elemental tellurium is a natural p-type semiconductor with a chiral structure and spin-polarized Fermi surface. Here, the authors show that the pressure-induced topological change of the Fermi surface at 17 kbar triggers an Anderson-Mott insulator-to-metal transition.

Magnetite (Fe3O4) nanoparticles are one of the most studied nanomaterials for different nanotechnological and biomedical applications. However, Fe3O4 nanomaterials gradually oxidize to maghemite (γ-Fe2O3) under conventional environmental conditions leading to changes in their functional properties that determine their performance in many applications. Here we propose a novel strategy to control the surface chemistry of monodisperse 12 nm magnetite nanoparticles by means of a 3 nm-thick Zn-ferrite epitaxial coating in core/shell nanostructures. We have carried out a combined Mössbauer spectroscopy, dc magnetometry, X-ray photoelectron spectroscopy and spatially resolved electron energy loss spectroscopy study on iron oxide and Fe3O4/Zn0.6Fe2.4O4 core/shell nanoparticles aged under ambient conditions for 6 months. Our results reveal that while the aged iron oxide nanoparticles consist of a mixture of γ-Fe2O3 and Fe3O4, the Zn-ferrite-coating preserves a highly stoichiometric Fe3O4 core. Therefore, the aged core/shell nanoparticles present a sharp Verwey transition, an increased saturation magnetization and the possibility of tuning the effective anisotropy through exchange-coupling at the core/shell interface. The inhibition of the oxidation of the Fe3O4 cores can be accounted for in terms of the chemical nature of the shell layer and an epitaxial crystal symmetry matching between the core and the shell. Fil: Lavorato, Gabriel Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina Fil: Rubert, Aldo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina Fil: Xing, Yutao. Universidade Federal Fluminense; Brasil Fil: Das, Raja. Phenikaa University; Vietnam. Phenikaa Research and Technology Institute; Vietnam Fil: Robles, Joshua. University of South Florida. Department of Physics; Estados Unidos Fil: Jochen Litterst, F.. Centro Brasileiro de Pesquisas Físicas; Brasil. Technische Universitat Carolo Wilhelmina Zu Braunschweig.; Alemania Fil: Baggio Saitovitch, Elisa. Centro Brasileiro de Pesquisas Físicas; Brasil Fil: Phan, Manh Huong. University of South Florida. Department of Physics; Estados Unidos Fil: Srikanth, Hariharan. University of South Florida. Department of Physics; Estados Unidos Fil: Vericat, Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina Fil: Fonticelli, Mariano Hernan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina

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.

The isostructural double perovskites Ba$_2$CuTeO$_6$ and Ba$_2$CuWO$_6$ are shown by theory and experiment to be frustrated square-lattice antiferromagnets with opposing dominant magnetic interactions. This is driven by differences in orbital hybridisation of Te$^{6+}$ and W$^{6+}$. A spin-liquid-like ground state is predicted for Ba$_2$Cu(Te$_{1-x}$W$_x$)O$_6$ solid solution similar to recent observations in Sr$_2$Cu(Te$_{1-x}$W$_x$)O$_6$., Comment: 4+4 pages, 3+2 figures

Bulk ceramic samples with chemical composition (Y 1-x Nd x )Ba 2 Cu 3 O 7-δ , where x = 0, 0.1, 0.2, 0.5, and 1 were sintered by a proposed acetate method. The samples were then submitted to structural characterization by X-ray diffraction with crystal structure refining using the Rietveld method. AC electrical resistivity measurements from ambient temperatures down to 4.2 K were performed, as well as magnetic measurements of magnetic moment as function of temperature and magnetic hysteresis loop. Results have shown superconductivity with critical temperatures of about 91 and 60 K for pure YBa 2 Cu 3 O 7-δ (x = 0) and pure NdBa 2 Cu 3 O 7-δ (x = 1) samples, respectively. Scanning electron microscopy images were also analyzed, revealing a regular parallel-piped grain shape. The proposed method showed as an advantage to reduce the grinding time without any considerable degradation on structural or superconducting properties of pure YBa 2 Cu 3 O 7-δ (x = 0) or pure NdBa 2 Cu 3 O 7-δ ( x = 1), as related to samples prepared by a conventional solid state route.

A quantum spin liquid state has long been predicted to arise in spin-1/2 Heisenberg square-lattice antiferromagnets at the boundary region between N\'eel (nearest-neighbor interaction dominates) and columnar (next-nearest-neighbor dominates) antiferromagnetic order. However, there are no known compounds in this region. Here we use $d^{10}$-$d^0$ cation mixing to tune the magnetic interactions on the square lattice while simultaneously introducing disorder. We find spin-liquid-like behavior in the double perovskite Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$, where the isostructural end phases Sr$_2$CuTeO$_6$ and Sr$_2$CuWO$_6$ are N\'eel and columnar type antiferromagnets, respectively. We show that magnetism in Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$ is entirely dynamic down to 19 mK. Additionally, we observe at low temperatures for Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$, similar to several spin liquid candidates, a plateau in muon spin relaxation rate and a strong $T$-linear dependence in specific heat. Our observations for Sr$_2$Cu(Te$_{0.5}$W$_{0.5}$)O$_6$ highlight the role of disorder in addition to magnetic frustration in spin liquid physics., Comment: 19+4 pages, 4+3 figures. Accepted for publication in Nature Communications

We report on the experimentally observed variation of the superconducting critical temperature (T c ) of Ni/Bi systems as a function of the total deposited film thickness and on its explanation using a theoretical model. Two series of Ni/Bi systems have been analyzed which were obtained by depositions of Ni onto Bi in the proportions Ni3Bix (3 nm of Ni onto x nm of Bi) and NiyBi6y (y nm Ni onto 6 y nm of Bi). As shown recently, the formation of the superconducting compound NiBi3 at Ni/Bi interfaces in the resulting NiBi3-Bi films is thermodynamically favored by a volume contraction. Here we corroborate this result and estimate the thickness of the resulting NiBi3 and of the remaining Bi layers for the Ni3Bix and NiyBi6y series using the laws of mass and conservation of number of atoms. We consider the resulting film as being made up of two homogeneous and uniform layers of NiBi3 and Bi, respectively, and study this idealizing model using the Bogoliubov de Gennes (BdG) equations. It is assumed that superconductivity originates in the NiBi3 layer and penetrates the Bi layers via a potential barrier. Our theoretical calculations predict the dependence of T c with respect to the thicknesses of the NiBi3 and Bi layers, and also with the strength of the potential barrier that blocks the migration of electrons from the NiBi3 to the Bi layer. The calculations show that the superconducting gap also exists in Bi, although much weaker than in the NiBi3 layer. We compare the predicted T c values with the experimental data and find sufficient agreement to suggest that our model can explain the experimentally observed variation of T c with thickness. We interpret this dependence as shape resonance oscillations which are derived from the BdG theory applied to thin superconducting films.

In this work, we studied the cation occupancy of bimagnetic CoO/Co1−xZnxFe2O4 core/shell nanoparticles by means of X-ray absorption and Mossbauer spectroscopies, which provide element-sensitive information at the atomic scale. Our results indicate that, although the spinel ferrite forms a multi-grain shell, the Zn cations occupy solely tetrahedral sites, while the Co cations are mostly in the octahedral site. On the other hand, the Fe cations are distributed in both tetrahedral and octahedral sites for all concentrations. Also the results provide evidence for a Zn-deficient spinel with an excess of Co cations in the shell, whose origin is further rationalized in terms of the two-step synthesis process. In overall, this work gives a description of the cation occupancy in the core/shell nanoparticles and can serve as a guide to the interpretation of the magnetic properties of complex bimagnetic systems for future technological applications.

The magnetic profile of the Si(100)/NiO(35 nm)/NiFe(10 nm)/Ta(1 nm) sample has been obtained by X-ray absorption spectroscopy (XAS) and the X-ray magnetic circular dichroism (XMCD). Two experimental procedures were used. In the procedure 1, the magnetic depth profile has been determines using samples deposited with different NiFe thicknesses, Si(100)/NiO(35 nm)/NiFe(t)/Ta(1 nm), t = 1 , 3 , 5 , 7 , 10 nm . In procedure 2, the sample (NiFe=10 nm), was thinned by several in situ sputtering cycles with Ar+ ions, followed by XAS and XMCD analysis. In both procedures, the calculated magnetic moments values tend to decrease close to interface with the NiO antiferromagnetic (AF) layer, however, this decreasement is more evidenced in the sputtered sample. There is no charge transfer between Ni and Fe in the inner part of the NiFe layer, a reduction of the m orb / m spin eff has been found at the NiFe/NiO interface. Procedure 1 emerged as the most indicated to analyse the interface region.

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.

In this investigation, we synthesized monophasic and nanostructured (Zn1-x-yFexCoy)O samples, with x + y = 0.05. These nanophases were characterized by X-ray diffraction, magnetization techniques, and Mossbauer spectroscopy. The analyzes revealed that doping ZnO with iron or cobalt induced a morphological change for the crystallites. The presence of cobalt in the matrix showed to enhance clustering of iron atoms and two different groups of ferric cations were identifyed diluted in the wurtzite matrix. (Zn1-x-yFexCoy)O has no magnetic order at RT but makes a transition from the paramagnetic to the spin-glass state with decreasing temperature, passing through three different magnetic regimes. The unit cell volume, remanence and coercive field of the (Zn1-x-yFexCoy)O solid system reached a maximum for co-doped samples.

We report how the sequence of deposition of Ni and Bi affects the final morphology of Ni–Bi bilayers. Two samples have been prepared by sputtering with the same thickness of Ni (4 nm) and Bi (40 nm) but reversed sequence of deposition. The sample with Bi deposited first formed a smooth layer of NiBi3 intermetallic, while the sample Ni deposited first formed mainly NiBi3 nanoparticles with its out-of-plane dimensions bigger than the thickness of the total deposited bilayer. The NiBi3 phase in the two samples results from differing nucleation and growth processes leading to differing morphologies. The volume of NiBi3 is smaller than the pure Bi volume needed for the reaction and this volume contraction is a thermodynamic factor that favors the NiBi3 formation. Both samples are superconducting with the critical temperature Tc of ~4.0 K. The Ni–Bi system offers an example of the formation of a superconducting compound under a volume contraction of its constituents, thus being of interest for both researchers in physics and material sciences.

Two superconducting intermetallic compounds in the Ni-Bi system, namely NiBi and NiBi3, can be formed as result of interdiffusion, if Ni/Bi are produced as bilayers at room temperature by pulsed laser deposition (PLD). These two equilibrium phases are responsible for the observed superconductivity in this kind of bilayer material. Due to different behavior in external magnetic fields, there appear two-step superconducting transitions in the electrical transport measurements. In this work we studied the superconducting and magnetic properties of the Ni-Bi bilayers with the formation of the two intermetallic compounds, NiBi and NiBi3, which have similar Tc ~ 4.0 K but very different upper critical magnetic fields at zero temperature [Bc2(0)]. The magnetic hysteresis loops of the pure NiBi3 phase show the behavior of a weak type II superconductor without strong pinning effect. Upon formation of the NiBi phase caused by highly energetic atoms during pulsed laser deposition, strong pinning effects appeared in the hysteresis loops below Tc. The isotherm V(I) curves show that the residual Ni layer does not induce spontaneous vortices in the NiBi or NiBi3 phases.

The effect of mechanical milling in i-Al64Cu23Fe13 quasicrystalline and ω-Al70Cu20Fe10 crystalline phases is systematically investigated in the present work. The Al–Cu–Fe samples were obtained by arc furnace technique and then nanostructured by means of mechanical milling. The results indicate that the solid samples present a weak ferromagnetic behavior at 300 K, showing a saturation magnetization of 0.124 emu g−1 for the icosahedral phase (i-phase) and 0.449 emu g−1 for the tetragonal phase (ω-phase). These small values could be an indication that only a few percentage of Fe atoms carry magnetic moment. The magnetic response in the nanostructured ω-phase increases up to 3.5 times higher than its corresponding solid counterpart. Whereas for the i-phase this increment is about 16 times higher. Moreover, the speed of the variation of the studied physical parameters after reducing the average grain size has been obtained from the exponent (α) of a power law fit of the experimental data. The values of α, corresponding to the magnetic response, are slightly different in each phase, which should be related to the different chemical composition and/or the type of long range order. Additionally, we also search for a critical grain size. However, this critical value has not been observed in the studied samples.

We present a study of thermal conductivity, $\kappa$, in undoped and doped strontium titanate in a wide temperature range (2-400 K) and detecting different regimes of heat flow. In undoped SrTiO$_{3}$, $\kappa$ evolves faster than cubic with temperature below its peak and in a narrow temperature window. Such a behavior, previously observed in a handful of solids, has been attributed to a Poiseuille flow of phonons, expected to arise when momentum-conserving scattering events outweigh momentum-degrading ones. The effect disappears in presence of dopants. In SrTi$_{1-x}$Nb$_{x}$O$_{3}$, a significant reduction in lattice thermal conductivity starts below the temperature at which the average interdopant distance and the thermal wavelength of acoustic phonons become comparable. In the high-temperature regime, thermal diffusivity becomes proportional to the inverse of temperature, with a prefactor set by sound velocity and Planckian time ($\tau_{p}=\frac{\hbar}{k_{B}T}$)., Comment: 5 pages including 4 figures

The design of novel nanostructured magnetic materials requires a good understanding of the variation in the magnetic properties due to different synthesis conditions. In this work, four different procedures for fabricating Co-ferrite nanoparticles with similar sizes between 7 and 10 nm are compared by studying their structural and magnetic properties. Non-aqueous methods based on the thermal decomposition of metal acetylacetonates at high temperatures, either with or without surfactants, provide highly crystalline nanoparticles with large saturation magnetization values and a coherent reversal of the magnetic moment. However, variations in the density of defects and in the shape of the nanocrystals determine the distribution of switching fields and the effective magnetic anisotropy, which reaches up to ≈1 × 107 erg cm−3 for oleic acid-capped 9 nm nanoparticles. It is shown that the saturation magnetization values for nanoparticles produced by different methods are in the range between 49 and 95 emu g−1 due to differences in the stoichiometry, in the cation occupancy, in the magnetic disorder and in the spin canting of the magnetic sub-lattices, the latter evaluated by in-field Mössbauer spectroscopy Fil: Lavorato, Gabriel Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Centro Brasileiro de Pesquisas Físicas; Brasil Fil: Alzamora, Mariella. Universidade Federal do Rio de Janeiro; Brasil Fil: Contreras, Cynthia. Centro Brasileiro de Pesquisas Físicas; Brasil Fil: Burlandy, Gabriel. Centro Brasileiro de Pesquisas Físicas; Brasil Fil: Litterst, F. Jochen. Centro Brasileiro de Pesquisas Físicas; Brasil. Technische Universitat Carolo Wilhelmina Zu Braunschweig.; Alemania Fil: Baggio Saitovitch, Elisa. Centro Brasileiro de Pesquisas Físicas; Brasil

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.

In this work we report on the preparation and structural characteristics of good quality superconducting samples of La 2− x Sr x Cu 1− y Zn y O 4− δ ( x = 0.05, 0.08, 0.10, 0.115, 0.15, 0.18, 0.21, 0.25 and y = 0, 0.02) using a wet-chemical method. Previous studies in optimally doped samples prepared with this method demonstrated that the cation distribution inside these samples is improved compared to similar samples obtained by the traditional solid-state method. Since this fact is very important for the study of the superconducting cuprates because their physical properties are strongly correlated with the microstructure and chemical homogeneity, we have extended this wet-chemical method to produce polycrystalline samples with strontium concentrations of interest to superconductivity. In particular, compounds with strontium doping as low as x = 0.05 and 0.08, located at the onset of superconductivity (underdoped region), up to dopant concentrations situated in the highly overdoped region ( x = 0.25) were prepared and characterized. X-ray diffraction measurements and subsequent Rietveld analysis were used to identify the principal lattice parameters of the samples and their doping dependence. Zero field cooling magnetization and resistivity measurements allowed following the critical temperature as a function of the charge carrier concentration. Our experimental results obtained here are compared to others reported on similar La 2− x Sr x CuO 4− δ samples showing that good quality single-phase polycrystalline samples, with several carrier concentrations, can be obtained using the wet-chemical method explored here.

Phase separation and suppression of critical dynamics at quantum phase transitions of MnSi and (Sr 1− x Ca x )RuO 3

The effect of Sn-doping on (La 1− x Sr x ) 2 Cu 1− x Sn x O 4 ( x =0.075, 0.090 and 0.110) has been studied. Lattice parameters monotonically decrease with Sn dopant increasing. Sn atoms dominantly occupy Cu sites and form Sn–O clusters as point-like defects. Flux pinning properties of ceramic samples of (La 1− x Sr x ) 2 Cu 1− x Sn x O 4 have been investigated by magnetic measurements from 6 to 30 K up to 5 T. The current carrying scale-length analysis has been determined from the reverse leg of the magnetization hysteresis loops and compared to the actual dimensions of the sample. Magnetization hysteresis loops of the more richly Sn-doped sample exhibit stronger pinning behavior, especially in the higher temperature range. The intra-granular critical current J c in the rich Sn-doping sample has a significant increase. We discuss the experimental results within a pinning model of point-like defects. From our data we infer that the Sn–O cluster near Cu–O layers provides an effective pinning center.

Temperature dependent Mössbauer spectroscopy on 57Fe doped (1 at % of Ni) RNi2B2C and RNiBC provided clear evidence of a pair-breaking field at the Ni site for non-superconducting compounds. This field is not present in the superconducting collinear AF DyNi2B2C, however it appears when this compound is diluted with non-magnetic Lu (Dy1-xLu xNi2B2C). Important local information on the spin structure of the R magnetic moments is obtained for both systems. The local symmetries of RNi2B2C and RNiBC compounds, measured through the deltaE Q, were found to be linearly correlated with the c¢/a ratio (c´ is the distance of the R-C layers between which the Ni2-B2 layers are sandwiched). A scaling of Tc with the local symmetry (c¢/a) was found to be the same for all the RNi2B2C and RNiBC (except for Y) compounds.

57Fe Mossbauer spectroscopy has been used to study rare earth intermetallic compounds having strongly correlated electronic properties, where the transition metal carries no magnetic moment. In this case 57Fe is a local probe to detect very small transferred hyperfine fields (Bthf) from the rare earth sites.

In this work, we report on the physical properties of good-quality polycrystalline superconducting samples of La 2-x Sr x Cu 1-y Zn y O 4-δ (y = 0, 0.02) prepared by a wet-chemical method, focusing on the temperature dependence of the critical current. Using the wet-chemical method, we were able to produce samples with improved homogeneity compared to the solid-state method. A complete set of samples with several carrier concentrations, ranging from the underdoped (strontium concentration x≈0.05) to the highly overdoped (x≈0.25) region, were prepared and investigated. The X-ray diffraction analysis, zero-field cooling magnetization and electrical resistivity measurements were reported on earlier. The structural parameters of the prepared samples seem to be slightly modified by the preparation method and their critical temperatures were lower than reported in the literature. The temperature dependence of the critical current was explained by a theoretical model which took the granular structure of the samples into account.

In this work we present a magnetoresistance study on the CeRu2Ge2 compound. We analyze the ρ(T) curves for several applied magnetic fields using the electron–magnon scattering model for a ferromagnetic spin arrangement. From this analysis, the field dependence of the energy gap of the magnon spectrum is obtained. The magnetoresistance ρ(H) at various temperatures arises from a normal metal contribution with an additional scattering mechanism due to electron–magnon interaction.

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 Fe-doping effects on superconductivity, transport and microstructure are investigated by means of Hall coefficient measurements, thermogravimetric analysis, Mossbauer spectroscopy and the calculation of the point-charge-effective-valence model (PCEV model) in Fe-doped Tl 0.5 Pb 0.5 (Sr 0.8 Ba 0.2 ) 2 Ca 2 (Cu 1− x Fe x ) 3 O y (Tl-1223) superconductors. In the doping level of Fe ( x =0–0.05), both zero-resistance temperature T co and carrier concentration n H decrease linearly with Fe dopants increasing. The striking linear behavior of T co and n H suggests the potential effects of charge localization on the transport properties and also shows that Fe dopants occupy different Cu sites in a stable manner with Fe content increasing. The thermogravimetric analysis shows the Fe 3+ ions substituting for Cu 2+ ions can bring excess oxygen atoms into lattice and form some Fe–O defect clusters. Mossbauer measurements are performed to investigate the microstructure of Fe–O defects. In addition, the atomic position of Fe dopant and the electric field gradient (EFG) on Fe sites are investigated under the PCEV model. We found that the signs of the EFG on Fe sites are positive. According to quadrupole splittings deduced from Mossbauer spectroscopies and results of model calculation, it is further confirmed that there is a small shift of Fe ion relative to the original center of the CuO 2 network, and in some different kinds of Fe–O defect clusters, Fe dopants can lead to excess oxygen atoms entering lattice. These results provide a further support for the interpretation about the displacement of Fe dopant and the assignment of Fe site. Our discussion suggests that the microstructural distortion and the excess oxygen defect induced by Fe doping encourage in the localization of cruising carrier on CuO 2 planes, which is one of the reasons to explain the decrease of T c and carrier concentration.

The simultaneous measurements of Doppler-broadened radiation spectra and lifetime spectra have been performed on (Y 1− x Eu x )Ba 2 Cu 3 O 7− y ( x =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1) superconductors. The mean lifetime and S -parameter, respectively show a characteristic significantly symmetric to Eu-doping concentration. Since lifetime and Doppler broadening spectroscopies are rather sensitive to the variation of the lattice distortion and defects induced by the stress-field of lattice mismatch, such symmetric behaviors of the S -parameter and the mean lifetime suggest a striking characteristic of stress in Y 1− x Eu x -123 superconductor. When the doping concentration x reaches 30%, the density of defects corresponding to stress-field appears the maximum, and there is an optimum distribution of the stress-field so as to increase critical current J c in Eu-doped Y-123 superconductors. Our results of the critical current density obtained by means of measurements of susceptibility χ ′( T , H AC ) and χ ″( T , H AC ) also suggest that the enhancement of the critical current density to reach an optimum when Eu doping x is 30%. We also qualitatively discuss experimental results in the framework of the model of stress-field pinning. The model calculating results also show the stress-field induced by lattice mismatch is an effective pinning center, and further conform that the stress pinning mechanism is dominant in RE-doped Y-123 superconductors.

For La 2−2 x Sr 2 x Cu 1− x Sn x O 4 ( x =0.075, 0.090 and 0.110) superconductors, the effects of Sn-doping on local microstructure is studied by means of Mossbauer spectroscopy. The results show that Sn ions in the 4+ valence state dominantly occupy Cu sites rather than La(Sr) site. In the view of the effective oxygen content (or excess oxygen), the dual role of Sn dopant on the carrier concentration is discussed. We attempt to explain the high transition temperature T c in (Sr,Sn)-doped La 2−2 x Sr 2 x Cu 1− x Sn x O 4 samples under the model of amphoteric effect of Sn-doping on the concentration.

Evolution of the structural, magnetic, transport and magneto-transport properties of the series is studied by powder x-ray diffraction, Mossbauer spectroscopy, magnetization and resistivity in zero and applied magnetic field. Changes from concentrated Kondo system-like to single Kondo ion-like behaviour are observed. Magneto-resistivity behaviour in both regimes is discussed within the existing theoretical views which include non-negligible `normal' metal contributions.

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.

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 .

The effect of hydrostatic pressure and alloying on the magnetic ordering temperature in the series was studied. The observed behaviour was related to changes in the density of states at the Fermi level caused by pressure or Ni substitution. In addition, the elastic constants of , which is a representative member of the family, were measured.

The effect of hydrostatic pressure on the resistivity of Ce(Ru 1− x Fe x ) 2 Ge 2 was studied. Chemical pressure (alloying) and hydrostatic pressure produce similar changes in the physical properties of the series driving it from the ferromagnetic (CeRu 2 Ge 2 ) to the heavy fermion (CeFe 2 Ge 2 ) ground state with (decreasing) unit cell volume as a unique parameter; f-p(CeGe) hybridization is, probably, also giving some contribution to the changes.

The first-principles electronic structure calculations are performed for the nonoxide perovskite MgCNi3−xFex compounds by FP-LAPW approach. A small local magnetic moment for Fe in MgCNi3−xFex is always obtained in the cases condsidered. The consequence on spin moment of a distorted structure (Pnma) as well as bond length variation is discussed. The spin moment of Fe decreases significantly when the bond length of Fe–C reduces 5%.

We prepared samples with starting composition Ba 8 –Ge 46− x –Co x ( x =0, 4 and 6) by direct melting. These Ge-based compounds were characterized by X-ray diffraction and WDS, and we found two superconducting transitions at T C =10 and 4 K in the Co-free sample. Co-doping results in the suppression of T C to 7 K. The superconducting volume fraction also decreases with increasing Co-doping. For Co-doped samples, there is no 4 K superconducting transition. X-ray refinement shows that the compounds are mixtures of several phases. The dominant phase is diamond Ge, and we found no Ge-clathrate phases. Besides diamond Ge, there are also several weak diffractions from an unknown Ba–Ge phase, and most of them were indexed on the basis of a monoclinic unit cell. Diffraction peaks for Ba 2 Ge, BaGe, BaGe 2 , BaGe 2 O 5 , BaGeO 3 , Ba 3 GeO 5 , α-BaGeO 3 , BaGe 2 O 5 , Ba 2 Ge 5 O 12 , β-BaGeO 3 , Ba 2 GeO 4 and BaGe 4 O 9 were carefully searched for but not seen in the samples. For the Co-doped sample, besides the main diamond Ge phase, there is also a semiconducting phase CoGe 2 . With increasing Co content, the CoGe 2 content increases. The WDS results agree with this result. The main phase composition for the Co-free sample is Ba 0.01 Ge 99.9 . We also discuss the origin of two superconducting transitions in Ge-based compounds.

The magnetic features of GdNi2B2C (14/mmm, a=3.578 A, c=10.361 A) and GdNiBC (P4/nmm, a=3.631 A, c=7.546 A) have been studied for 1.2 K or=125 kOe at 1.7 K) is approximately three times that of GdNiBC implying that the average interlayer interactions in the former are much stronger than in the latter. The overall magnetic features of both compounds are interpreted in terms of the picture that assumes the Gd moments to have strong ferromagnetic intralayer couplings and averaged antiferromagnetic interlayer interactions.

The magnetic and transport properties of the quaternary intermetallic HoNiBC compound (space group P4/nmm, a = 3.563(1) A , c = 7.546(1) A ) were studied for 1.2 K ⩽ T ⩽ 300 K and H ⩽ 80 kOe. The compound orders antiferromagnetically at 9.8(3) K and, in the ordered state, the Ho moment saturates to 8.5(1) μB. In contrast with the structurally-related reentrant superconductor HoNi2B2C, superconductivity is not found nor are there indications of a helical ground-state in the whole temperature range. These features are attributed, respectively, to the positioning of the Fermi level at a DOS minimum and the presence of the HoC double layers that inhibits the establishment of a helical spin arrangement.

Alloying influences on the superconducting and magnetic properties of the (Gd x Y 1-x Ni 2 B 2 C series are studied. For x 0.25 and the coexistence of superconductivity with long-range antiferromagnetic order for 0.1 < x < 0.25 are discussed.

Magnetic properties of Y 1− x Gd x Co 2 B 2 C series of compounds were studied. Two magnetically ordered phases were observed for 0.7 ≤ x ≤ 1. Change of the magnetic ordering temperatures T 1 and T 2 with the Gd concentration is argued to be related to the dilution of magnetic Gd by nonmagnetic Y and to the reduction of the distance between ( Y 1− x Gd x ) C planes. However, significant contribution to the changes in T 1 between pure GdNi 2 B 2 C and GdCo 2 B 2 C samples is suggested to be due to the different 3 d band filling in these compounds.

The superconducting and magnetic phase diagram (characteristic temperatures versus effective ionic radii) of the RNi 2 B 2 C (R Lu, Tm, Er, Ho, Dy, Tb, Gd) compounds are considered. Although the gradual degradation of superconductivity can be scaled to the de Gennes factor, ( g −1) 2 J ( J + 1), the unique reentrant behavior of the HoNi 2 B 2 C compound and the abrupt quenching of superconductivity for R lighter than Ho are most probably unaccountable within this scheme. Rather, it is argued that their low- T magnetic and transport properties as well as the main features of the interplay between magnetism and superconductivity can be accounted for if the low- T magnetism of HoNi 2 B 2 C, as reported by Grigereit et al., is generalized to the other isomorphous R members. Thus the onset of the 4f moments antiferromagnetic state at T 1 is accompanied by an oscillatory component, which transforms to a commensurate antiferromagnetic state at T 2 . For HoNi 2 B 2 C, the pressure and magnetic-field influence on T c , T 1 and T 2 will be discussed.

Structural and superconducting properties of Y(Ni1−xMx)2B2C compounds with MCo, Fe and Ru have been studied. A fast decrease of Tc with the concentration of the dopants was found. This effect can be attributed to the shift of the Fermi level induced by the dopant concentration with the additional contributions from the chemical-pressure effects and from the decrease of the electronic mean free path.

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.