The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mössbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Néel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1−xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57 Fe Mössbauer spectroscopy and muon spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5) µB at the iron site below TN = 138 K, well separated from a structural phase transition at TS = 156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below TN are reproduced. PACS numbers: 76.75.+i, 76.80.+y, 75.30.Fv, The recently discovered Fe-based superconductors LaFeAsO 1−x F x [1] and the related materials in which La is substituted by Sm, Ce, Nd, Pr, and Gd, respectively [2,3,4,5,6,7] has triggered an intense research in the oxypnictides. Besides the high critical temperature above 50 K there are further striking similarities to the properties of the high-T C cuprates. The oxypnictides have a layered crystal structure with alternating FeAs and LaO sheets, where the Fe atoms are arranged on a simple square lattice [1]. Theoretical studies reveal a two-dimensional electronic structure [8] and it is believed that conductivity takes place mainly in the FeAs layers while the LaO layers provide the charge reservoir when doped with F ions. Again similar as in the cuprates, superconductivity emerges when doping a magnetic mother compound with electrons or holes and thereby supressing the magnetic order [9]. This suggests an interesting interplay between magnetism and superconductivity and, indeed, a recent theoretical work suggests that magnetic fluctuations associated with quantum critical point are essential for superconductivity in the electron doped LaFeAsO 1−x F x superconductors [10].However, in contrast to the cuprates, the magnetic mother compound is not a Mott-Hubbard insulator but a poor metal. A large covalency in the FeAs layers was found [8,11], which in the case of tetragonal LaFePO, i.e. the compound where As is replaced by P, leads to a non-magnetic ground state [12,13]. In contrast, in LaFeAsO there is an additional structural distortion at elevated temperatures [14,15] and a long range spin density wave (SDW) antiferromagnetic order has been observed in neutron scattering experiments on powder samples below ∼150 K. [15]. First principle calculations yield antiferromagnetic order with Fe magnetic moments ranging from 1.5 µ B to 2.3 µ B [10,16,17,18], while the neutron scattering experiments indicate a much smaller value. Assuming that the full sample volume is contributing to the magnetic scattering an ordered moment of ∼ 0.35 µ B [15] is inferred from the weak superlattice reflections in powder neutron diffraction. A local probe measurement, which could verify...

The magnetic properties of monodisperse FeO-Fe3O4 nanoparticles with different mean sizes and volume fractions of FeO synthesized via decomposition of iron oleate were correlated to their crystallographic and phase compositional features by exploiting high resolution transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy and field and zero field cooled magnetization measurements. A model describing the phase transformation from a pure Fe3O4 phase to a mixture of Fe3O4, FeO and interfacial FeO-Fe3O4 phases as the particle size increases was established. The reduced magnetic moment in FeO-Fe3O4 nanoparticles was attributed to the presence of differently oriented Fe3O4 crystalline domains in the outer layers and paramagnetic FeO phase. The exchange bias energy, dominating magnetization reversal mechanism and superparamagnetic blocking temperature in FeO-Fe3O4 nanoparticles depend strongly on the relative volume fractions of FeO and the interfacial phase.

We have investigated single crystals and polycrystals from the series Nb1−yFe2+y, −0.004 ≤ y ≤ 0.018 by electron spin resonance, muon spin relaxation and Mössbauer spectroscopy. Our data establish that at lowest temperatures all samples exhibit bulk magnetic order. Slight Feexcess induces low-moment ferromagnetism, consistent with bulk magnetometry, while Nb-rich and stoichiometric NbFe2 display spin density wave order with small magnetic moment amplitudes of the order ∼ 0.001 − 0.01µB /Fe. This provides microscopic evidence for a modulated magnetic state on the border of ferromagnetism in NbFe2.

CitationHighly Uniformly sized and shaped iron oxide nanoparticles with a mean size of 25 nm were synthesized via decomposition of ironoleate. High resolution transmission electron microscopy and Mössbauer spectroscopy investigations revealed that the particles are spheres primarily composed of Fe 3 O 4 with a small fraction of FeO. From Mössbauer and static magnetization measurements, it was deduced that the particles are superparamagnetic at room temperature. The hydrophobic particles were successfully transferred into water via PEGylation using nitrodopamine as an anchoring group. IR spectroscopy and thermogravimetric analysis showed the success and efficiency of the phase transfer reaction. After the PEGylation, the particles retained monodisperse and their magnetic core remained intact as proven by photon cross-correlation spectrocopy, ac susceptibility, and transmission electron microscopy. The particle aqueous suspensions revealed an excellent water stability over a month of monitoring and also against temperature up to 40 • C. The particles exhibited a moderate cytotoxic effect on in vitro cultured bone marrow-derived macrophages and no release of inflammatory or anti-inflammatory cytokines. The PEGylated particles were functionalized with Herceptin antibodies via a conjugation chemistry, their response to a rotating magnetic field was studied using a fluxgate-based setup and was compared with the one recorded for hydrophobic and PEGylated particles. The particle phase lag rose after labeling with Herceptin, indicating the successful conjugation of Herceptin antibodies to the particles.