The Fe K x-ray absorption near edge structure of BaFe(2-x)Co(x)As(2) superconductors was investigated. No appreciable alteration in shape or energy position of this edge was observed with Co substitution. This result provides experimental support to previous ab initio calculations in which the extra Co electron is concentrated at the substitute site and do not change the electronic occupation of the Fe ions. Superconductivity may emerge due to bonding modifications induced by the substitute atom that weakens the spin-density-wave ground state by reducing the Fe local moments and/or increasing the elastic energy penalty of the accompanying orthorhombic distortion.

Electron spin resonance ͑ESR͒ experiments at different fields or frequencies ͑4.1Յ Յ 34.4 GHz͒ in the Kondo lattice ͑T K Ӎ 25 K͒ YbRh 2 Si 2 single-crystal compounds confirmed the observation of a single anisotropic Dysonian resonance with g Ќc Х 3.55 and no hyperfine components for 4.2Շ T Շ 20 K. However, our studies differently reveal that ͑i͒ the ESR spectra for H Ќc show strong-field-dependent spin-lattice relaxation, ͑ii͒ a weak-field and temperature-dependent effective g value, ͑iii͒ a dramatic suppression of the ESR intensity beyond 15% of Lu doping, and ͑iv͒ a strong sample and Lu-doping ͑Յ15%͒ dependence of the ESR data. These results suggest a different scenario where the ESR signal may be associated to a coupled Yb 3+ -conduction electron resonant collective mode with a strong bottleneck and dynamiclike behavior.

In contrast with the simultaneous structural and magnetic first order phase transition T0 previously reported, our detailed investigation on an underdoped Ba(0.84)K(0.16)Fe2As2 single crystal unambiguously revealed that the transitions are not concomitant. The tetragonal (τ: I4/mmm)-orthorhombic (ϑ: Fmmm) structural transition occurs at T(S)≃110 K, followed by an adjacent long-range antiferromagnetic (AFM) transition at T(N)≃102 K. Hysteresis and coexistence of the τ and ϑ phases over a finite temperature range observed by NMR experiments confirm the first order character of the τ-ϑ transition and provide evidence that both T(S) and T(N) are strongly correlated. Our data also show that superconductivity develops in the ϑ phase below T(c)=20 K and coexists with AFM. This new observation, T(S)≠T(N), firmly establishes another similarity between the hole-doped BaFe2As2 and the electron-doped iron-arsenide superconductors.

The effects of K and Co substitutions and quasi-hydrostatic applied pressure (P < 9 GPa) in the local atomic structure of BaFe2As2, Ba(Fe0.937Co0.063)2As2 and Ba0.85K0.15Fe2As2 superconductors were investigated by extended x-ray absorption fine structure (EXAFS) measurements in the As K absorption edge. The As-Fe bond length is found to be slightly reduced ( 0.01Å) by both Co and K substitutions, without any observable increment in the corresponding Debye Waller factor. Also, this bond is shown to be compressible (κ = 3.3(3) × 10 −3 GPa −1 ). The observed contractions of As-Fe bond under pressure and chemical substitutions are likely related with a reduction of the local Fe magnetic moments, and should be an important tuning parameter in the phase diagrams of the Fe-based superconductors.

We present a systematic experimental and theoretical study of the first-order phase transition of epitaxially grown MnAs thin films under biaxial tensile stress. Our results give direct information on the dependence of the phase-transition temperature of MnAs films on the lattice parameters. We demonstrate that an increase of the lattice constant in the hexagonal plane raises the phase-transition temperature (T p ), while an increase of the perpendicular lattice constant lowers T p . The results of calculations based on density functional theory are in good agreement with the experimental ones. Our findings open exciting prospects for magneto-mechanical devices, where the critical temperature for ferromagnetism can be engineered by external stress. DOI: 10.1103/PhysRevLett.95.077203 PACS numbers: 75.70.Ak, 61.50.Ks, 68.60.2p MnAs presents a first-order phase transition at 40 C, changing from ferromagnetic/hexagonal ( -phase NiAs structure) to paramagnetic/orthorhombic ( -phase MnP structure) [1]. This magneto-structural phase transition has important implications for technological applications. The magneto-elastic effects are useful for transducers [2], while their magneto-caloric properties are interesting for developing refrigeration devices [3]. In recent years, the attention given to MnAs has been strongly amplified by the possibility of epitaxial growth on GaAs substrates [4]. The integration of ferromagnetic materials with semiconductors is a subject of great interest for spintronics, and MnAs grown on GaAs is a strong candidate for spin injection devices [5].From the theoretical point of view, the treatment of the first-order phase transition of materials with magnetoelastic properties is a rather complex issue. Early simple phenomenological thermodynamic treatments based on the localized Heisenberg model [1] have been used to explain the properties of MnAs under an external hydrostatic pressure and magnetic field. Sophisticated band structure calculations are required for a precise quantitative analysis, although in this case it is difficult to introduce a statistical treatment to describe a first-order phase transition [6].We present an experimental and theoretical investigation of the magneto-structural phase transition of MnAs films grown on GaAs. Those films present a nonabrupt phase transition with the coexistence [7] of the two phases in form of periodically alternating stripes [8,9] for a large temperature range ( 20 C) [7][8][9][10]. As a result of this phase coexistence a considerable fraction of the volume of the MnAs epitaxial films is usually in the paramagnetic phase at 30 C, which is a strong limitation for room temperature spintronic devices. The growth of MnAs films on different crystal orientations has been suggested as an alternative that can provide higher phase-transition temperatures [10]. The detailed mechanism that associates the crystal distortion (lattice parameter variation) with the phase-transition temperature is, however, still unclear, This issue was addressed in the ear...

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We present a systematic investigation of the effect of an external magnetic field on the first order structural ͑hexagonal/orthorhombic͒ and magnetic ͑ferromagnetic/paramagnetic͒ phase transition of MnAs films grown epitaxially on GaAs͑001͒. The experimental results obtained using x-ray diffraction, magneto-optical Kerr effect, and SQUID magnetometry clearly show two magnetic field regimes where the temperature evolution of the magnetic and structural macroscopic properties of the films exhibit different behavior near the phase transition. The different evolution of the magnetic and structural properties fades away when an external magnetic field larger than 1 kOe is applied along the easy magnetization axis. These results are attributed to two effects induced by the external magnetic field. The first one is the field-induced phase transition, similar to that reported in MnAs bulk. The second one, which dominates for fields below 1 kOe, is attributed to the effect of the geometry of the microstructured domains formed during the phase transition on the macroscopic magnetic properties of the film. As a result, the changes in the magnetic properties detected by macroscopic measurements take place at temperatures lower than those observed by structural measurements, even though structural and magnetic phase transitions must occur simultaneously.

The possible existence of a sign-changing gap symmetry in BaFe2As2-derived superconductors (SC) has been an exciting topic of research in the last few years. To further investigate this subject we combine Electron Spin Resonance (ESR) and pressure-dependent transport measurements to investigate magnetic pair-breaking effects on BaFe1.9M0.1As2 (M = Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence of localized magnetic moments, is observed only for M = Cu and Mn compounds, which display very low SC transition temperature (Tc) and no SC, respectively. From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of Tc cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function. Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.