Article

# Magnetic excitations of Fe(1+y)Se(x)Te(1-x) in magnetic and superconductive phases.

Department of Physics, Clarendon Laboratory, Oxford University, Oxford OX1 3PU, UK.

Journal of Physics Condensed Matter (Impact Factor: 2.22). 04/2010; 22(14):142202. DOI: 10.1088/0953-8984/22/14/142202 Source: PubMed

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**ABSTRACT:**The evolution of magnetic order in Fe1+ySexTe1-x crystals as a function of Se content was investigated by means of ac/dc magnetometry and muon-spin spectroscopy. Experimental results and self-consistent density functional theory calculations both indicate that muons are implanted in vacant iron-excess sites, where they probe a local field mainly of dipolar origin, resulting from an antiferromagnetic (AFM) bicollinear arrangement of iron spins. This long-range AFM phase becomes progressively disordered with increasing Se content. At the same time all the tested samples manifest a marked glassy character that vanishes for high Se contents. The presence of local electronic/compositional inhomogeneities most likely favours the growth of clusters whose magnetic moment 'freezes' at low temperature. This glassy magnetic phase justifies both the coherent muon precession seen at short times in the asymmetry data, as well as the glassy behaviour evidenced by both dc and ac magnetometry.Journal of Physics Condensed Matter 03/2013; 25(15):156004. · 2.22 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Through neutron diffraction experiments, including spin-polarized measurements, we find a collinear incommensurate spin-density wave with propagation vector k= [0.4481(4) 0 1/2] at base temperature in the superconducting parent compound Fe1+xTe. This critical concentration of interstitial iron corresponds to x≈12% and leads to crystallographic phase separation at base temperature. The spin-density wave is short-range ordered with a correlation length of 22(3) Å, and as the ordering temperature is approached its propagation vector decreases linearly in the H direction and becomes long-range ordered. Upon further populating the interstitial iron site, the spin-density wave gives way to an incommensurate helical ordering with propagation vector k= [0.3855(2) 0 1/2] at base temperature. For a sample with x≈9(1)%, we also find an incommensurate spin-density wave that competes with the bicollinear commensurate ordering close to the Néel point. The shifting of spectral weight between competing magnetic orderings observed in several samples is supporting evidence for the phase separation being electronic in nature, and hence leads to crystallographic phase separation around the critical interstitial iron concentration of 12%. With results from both powder and single crystal samples, we construct a magnetic-crystallographic phase diagram of Fe1+xTe for 5%<x<17%.Physical Review B 01/2011; 84(6):064403/1-13. · 3.66 Impact Factor -
##### Article: Interstitial iron tuning of the spin fluctuations in the nonsuperconducting parent phase Fe1+xTe

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**ABSTRACT:**Using neutron inelastic scattering, we investigate the low-energy spin fluctuations in Fe1+xTe as a function of both temperature and interstitial iron concentration. For Fe1.057(7)Te, the magnetic structure is defined by a commensurate wave vector of (1/2,0,1/2). The spin fluctuations are gapped with a sharp onset at 7 meV and are three dimensional in momentum transfer, becoming two dimensional at higher-energy transfers. On doping with interstitial iron, we find, in Fe1.141(5)Te, the ordering wave vector is located at the (0.38,0,1/2) position and the fluctuations are gapless with the intensity peaked at an energy transfer of 4 meV. These results show that the spin fluctuations in the Fe1+xTe system can be tuned not only through selenium doping, but also with interstitial iron. We also compare these results with superconducting concentrations and, in particular, the resonance mode in the Fe1+xTe1−ySey system.Physical Review B 01/2011; 84(4):045124/1-6. · 3.66 Impact Factor

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