The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

Superconductivity in iron compounds

The tremendous interest in magnetic nanoparticles (MNPs) is reflected in published research that ranges from novel methods of synthesis of unique nanoparticle shapes and composite structures to a large number of MNP characterization techniques, and finally to their use in many biomedical and nanotechnology-based applications. The knowledge gained from this vast body of research can be made more useful if we organize the associated results to correlate key magnetic properties with the parameters that influence them. Tuning these properties of MNPs will allow us to tailor nanoparticles for specific applications, thus increasing their effectiveness. The complex magnetic behavior exhibited by MNPs is governed by many factors; these factors can either improve or adversely affect the desired magnetic properties. In this report, we have outlined a matrix of parameters that can be varied to tune the magnetic properties of nanoparticles. For practical utility, this review focuses on the effect of size, shape, composition, and shell-core structure on saturation magnetization, coercivity, blocking temperature, and relaxation time.

http://lee.chem.uh.edu/2013/Int.%20J.%20Mol.%20Sci.%202013,%2014,%2015977.pdf

Tuning the Magnetic Properties of Nanoparticles

The electronic structure of the perovskite LaCoO$_3$ for different spin states of Co ions was calculated in the LDA+U approach. The ground state was found to be a nonmagnetic insulator with Co ions in a low-spin state. Somewhat higher in energy we found two intermediate-spin states followed by a high-spin state at significantly higher energy. The calculation results show that Co 3$d$ states of $t_{2g}$ symmetry form narrow bands which could easily localize whilst $e_g$ orbitals, due to their strong hybridization with the oxygen 2$p$ states, form a broad $\sigma^*$ band. With the increase of temperature which is simulated by the corresponding increase of the lattice parameter, the transition from the low- to intermediate-spin states occurs. This intermediate-spin (occupation $t_{2g}^5e_g^1$) can develop an orbital ordering which can account for the nonmetallic nature of LaCoO$_3$ at 90 K$<$T$<$500 K. Possible explanations of the magnetic behavior and gradual insulating-metal transition are suggested.

https://arxiv.org/pdf/cond-mat/9709060.pdf

Intermediate-spin state and properties of LaCoO_3

Investigating the interaction patterns at nano-bio interface is a key challenge for safe use of nanoparticles (NPs) to any biological system. The study intends to explore the role of interaction pattern at the iron oxide nanoparticle (IONP)-bacteria interface affecting antimicrobial propensity of IONP. To this end, IONP with magnetite like atomic arrangement and negative surface potential (n-IONP) was synthesized by co-precipitation method. Positively charged chitosan molecule coating was used to reverse the surface potential of n-IONP, i.e. positive surface potential IONP (p-IONP). The comparative data from fourier transform infrared spectroscope, XRD, and zeta potential analyzer indicated the successful coating of IONP surface with chitosan molecule. Additionally, the nanocrystals obtained were found to have spherical size with 10-20 nm diameter. The BacLight fluorescence assay, bacterial growth kinetic and colony forming unit studies indicated that n-IONP (<50 μM) has insignificant antimicrobial activity against Bacillus subtilis and Escherichia coli. However, coating with chitosan molecule resulted significant increase in antimicrobial propensity of IONP. Additionally, the assay to study reactive oxygen species (ROS) indicated relatively higher ROS production upon p-IONP treatment of the bacteria. The data, altogether, indicated that the chitosan coating of IONP result in interface that enhances ROS production, hence the antimicrobial activity.

/pdf/antimicrobial-activity-of-iron-oxide-nanoparticle-upon-45am7jgqg0.pdf

Antimicrobial activity of iron oxide nanoparticle upon modulation of nanoparticle-bacteria interface

Magnetic properties of electron-doped La0.23Ca0.77MnO3 manganite nanoparticles, with average size of 12 and 60 nm, prepared by the glycine–nitrate method, have been investigated in the temperature range 5–300 K and magnetic fields up to 90 kOe. It is suggested that weak ferromagnetic moment results from ferromagnetic shells of the basically antiferromagnetic nanoparticles and from domains of frustrated disordered phase in the core. Assumption of two distinct sources of ferromagnetism is supported by the appearance of two independent ferromagnetic contributions in the fit of the T
 3/2 Bloch law to spontaneous magnetization. The ferromagnetic components, which are more pronounced in smaller particles, occupy only a small fraction of the nanoparticle volume and the antiferromagnetic ground state remains stable. It is found that the magnetic hysteresis loops following field cooled processes, display size-dependent horizontal and vertical shifts, namely, exhibiting exchange bias effect. Time-dependent magnetization dynamics demonstrating two relaxation rates were observed at constant magnetic fields upon cooling to T < 100 K.

Magnetic properties of electron-doped La0.23Ca0.77MnO3 nanoparticles

Iron-chalcogenide single crystals with the nominal composition ${\text{FeSe}}_{0.5}{\text{Te}}_{0.5}$ and a transition temperature of ${T}_{c}\ensuremath{\simeq}14.6\text{ }\text{K}$ were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal x-ray and neutron powder diffraction, superconducting quantum interference device and torque magnetometry, and muon-spin rotation (\ensuremath{\mu}SR). Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of ${\text{Fe}}_{1.045}{\text{Se}}_{0.406}{\text{Te}}_{0.594}$. Additionally, a minor hexagonal ${\text{Fe}}_{7}{\text{Se}}_{8}$ impurity phase was identified. The magnetic penetration depth $\ensuremath{\lambda}$ at zero temperature obtained by means of \ensuremath{\mu}SR was found to be ${\ensuremath{\lambda}}_{ab}(0)=491(8)\text{ }\text{nm}$ in the $ab$ plane and ${\ensuremath{\lambda}}_{c}(0)=1320(14)\text{ }\text{nm}$ along the $c$ axis. The zero-temperature value of the superfluid density ${\ensuremath{\rho}}_{s}(0)\ensuremath{\propto}{\ensuremath{\lambda}}^{\ensuremath{-}2}(0)$ obeys the empirical Uemura relation observed for various unconventional superconductors, including cuprates and iron pnictides. The temperature dependences of both ${\ensuremath{\lambda}}_{ab}$ and ${\ensuremath{\lambda}}_{c}$ are well described by a two-gap $s+s$-wave model with the zero-temperature gap values of ${\ensuremath{\Delta}}_{S}(0)=0.51(3)\text{ }\text{meV}$ and ${\ensuremath{\Delta}}_{L}(0)=2.61(9)\text{ }\text{meV}$ for the small and the large gap, respectively. The magnetic penetration depth anisotropy parameter ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}(T)={\ensuremath{\lambda}}_{c}(T)/{\ensuremath{\lambda}}_{ab}(T)$ increases with decreasing temperature, in agreement with ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}(T)$ observed in the iron-pnictide superconductors.

/pdf/anisotropic-superconducting-properties-of-single-crystalline-2svx6od2h2.pdf

Anisotropic superconducting properties of single-crystalline FeSe 0.5 Te 0.5

Magnetic properties of compacted La0.8Ca0.2MnO3 manganite nanoparticles with average particle size of 18 and 70 nm and Curie temperatures TC 231 K and TC 261 K, respectively, have been investigated. The relative volume of the ferromagnetic phase has been estimated to be 52% for ensembles of 18 nm particles and 92% for 70 nm particles. It was found that applied hydrostatic pressure enhances TC of La0.8Ca0.2MnO3 nanoparticles at a rate dTC /dP 1.8– 1.9 K / kbar, independently on the average particle size. Pronounced irreversibility of magnetization below Tirr 208 K and strong frequency dependent ac susceptibility below TC for smaller 18 nm particles have been observed. 18 nm particles have also shown aging and memory effects in zero-field-cooled ZFC and field-cooled magnetization. These features indicate the appearance of spin-glasslike state, partially reminiscent the behavior of La1�xCaxMnO3 crystals, doped below the percolation threshold xxC = 0.225. In contrast, ensembles of larger 70 nm particles have shown insignificant irreversibility of magnetization only and no frequency dependence of ac susceptibility, similarly to the behavior of La1�xCaxMnO3 crystals with xxC. The temperature of the ZFC magnetization maximum for 18 nm particles decreases with increasing magnetic field and forms a critical line with an exponent 1.89 0.56. The results suggest that superspin-glass features in ensembles of interacting 18 nm particles appear along with superferromagnetic-like features.

Spin-glass-like properties ofLa0.8Ca0.2MnO3nanoparticles ensembles

Sm0.2Ca0.8Mn1‐xRuxO3(x=0‐0.08)の強磁と金属性 Ruドーピングと静水圧の相互関係

Measurements of electrical resistance, magnetization, ac magnetic susceptibility, and ferromagnetic resonance of ${\mathrm{La}}_{0.82}{\mathrm{Ca}}_{0.18}{\mathrm{MnO}}_{3}$ single crystal were carried out at temperatures below and above the Curie temperature, ${T}_{C}=180\ifmmode\pm\else\textpm\fi{}1\mathrm{K}.$ A state of mixed ferromagnetic phases, metallic and insulating, is clearly evident from the measurements of magnetization, ac susceptibility, and electron magnetic resonance, below ${T}_{C}.$ The effect of applied hydrostatic pressure P and magnetic field H on the magnetic and the resistive properties were also investigated. It was found that the pressure enhances the magnetization, ${T}_{C},$ and stabilizes the ferromagnetic metal-like phase. It is concluded that phase separation plays an important role in most magnetic and transport effects observed in ${\mathrm{La}}_{0.82}{\mathrm{Ca}}_{0.18}{\mathrm{MnO}}_{3}$ below and above ${T}_{C}.$

A. Wisniewski

Papers

Magnetic properties of electron-doped La0.23Ca0.77MnO3 nanoparticles

Anisotropic superconducting properties of single-crystalline FeSe 0.5 Te 0.5

Spin-glass-like properties ofLa0.8Ca0.2MnO3nanoparticles ensembles

Sm0.2Ca0.8Mn1‐xRuxO3(x=0‐0.08)の強磁と金属性 Ruドーピングと静水圧の相互関係

Magnetic, transport, and electron magnetic resonance properties of La 0.82 Ca 0.18 MnO 3 single crystals