Antiferromagnetic ordering and structural phase transition inBa2Fe2As2with Sn incorporated from the growth flux

Abstract

Neutron diffraction experiments have been carried out on a Sn-flux grown BaFe 2 As 2 single crystal, the parent compound of the A-122 family of FeAs-based high-Tc superconductors. A tetragonal to orthorhombic structural phase transition and a three dimensional long-range antiferromagnetic ordering of the iron magnetic moment, with a unique magnetic propagation wavevector k = (1, 0, 1), have been found to take place at ~90 K. The magnetic moments of iron are aligned along the long a axis in the low temperature orthorhombic phase (Fmmm with b<a<c). Our results thus demonstrate that the magnetic structure of BaFe 2 As 2 single crystal is the same as those in other A-122 iron pnictides compounds. We argue that the tin incorporation in the lattice is responsible for a smaller orthorhombic splitting and lower Néel temperature T N observed in the experiment. Recent discovery of iron arsenic based superconductors LaFeAsO 1-x F x [1-2] (La-1111) has generated tremendous interests in searching for new high transition temperature (T C ) superconductors and the underlying mechanisms. Superconductivity with T C values up to 56 K has been subsequently discovered in other RE-1111 (RE=Sm, Ce, Nd, Pr) compounds [3]. The second family of FeAs-based superconductors, A 1-x K x Fe 2 As 2 (x ~ 0.4) (A=Ba, Ca, Sr) (A-122) with T C =38 K has been discovered more recently [4]. Two common aspects of both RE-1111 and A-122 families have been experimentally revealed. The first one is that their parent compounds likely display an antiferromagnetic (AF) ordering of iron spins at low temperatures and are accompanied by lattice distortions. The AF ordering and the corresponding structural phase transitions have been strongly supported by neutron powder diffraction experiments on LaFeAsO [5,6], NdFeAsO [7,8], CeFeAsO [9] and BaFe 2 As 2 [10,11]. Another aspect is that superconductivity could emerge upon electron-or hole-doping via chemical substitutions of the parent compounds. These two features are in strong reminiscence to the behaviors in the cuprates.Despite of the apparent similarities between the newly discovered iron pnictides and cuprates, the fundamental issues concerning the nature of magnetic ground states in the parent compounds and possible role of spin fluctuations in the pairing mechanism of superconductivity are far from clear. From nominal chemical composition (e.g. La

Anisotropic thermal expansion ofAEFe2As2(AE= Ba, Sr, Ca) single crystals

“…1). The average high temperature (between 300 K and 150 K) thermal expansion coefficients can be also estimated from the published scattering data [25]. For the c-axis, both sets of data are very close, whereas for the a-axis the scattering data give the value of the average thermal expansion coefficient factor of 2 -3 times lower than that presented here.…”

Lattice collapse and quenching of magnetism inCaFe2As2under pressure: A single-crystal neutron and x-ray diffraction investigation

“…What subtle, or striking, modifications in structure or magnetism occur with doping or pressure, and how are they related to superconductivity? Similar to other members of the AFe 2 As 2 (A=Ba, Sr) family, 5,6,7,8 at ambient pressure CaFe 2 As 2 undergoes a transition from a non-magnetically ordered tetragonal (T) phase (a = 3.879(3)Å, c = 11.740(3)Å) to an antiferromagnetic (AF) orthorhombic (O) phase (a = 5.5312(2)Å, b = 5.4576(2)Å, c = 11.683(1)Å) below approximately 170 K.9,10 In the O phase, Fe moments order in the so called AF2 structure 11 with moments directed along the aaxis of the orthorhombic structure.10 Neutron powder diffraction measurements 12 of CaFe 2 As 2 under hydrostatic pressure found that for p>0.35 GPa (at T=50 K), the antiferromagnetic O phase transforms to a new, non-magnetically ordered, collapsed tetragonal (cT) structure (a = 3.9792(1)Å, c = 10.6379(6)Å) with a dramatic decrease in both the unit cell volume (5%) and the c/a ratio (11%). The transition to the cT phase occurs in close proximity to the pressure at which superconductivity is first observed.…”

Muon spin rotation study of magnetism and superconductivity in Ba(Fe1−xCox)2Bernhard1, Wang2, Nuccio3 et al. 2012Phys. Rev. B5422871View full textAdd to dashboardCite

“…Magnetism in nonsuperconducting samples at 0 x 0.03 Figure 2 shows representative low-temperature ZF-μSR spectra of the Ba(Fe 1−x Co x ) 2 As 2 crystals that give an overview of how the AF order evolves as a function of the Co substitution. In the undoped parent compound, at x = 0, it has been established by magnetic neutron diffraction that a long-range AF order with an Fe moment of about 0.9-1 μ B develops below T N = 134 K. 13,[52][53][54][55] The corresponding ZF-μSR spectrum in Fig. 2(a) is also characteristic of a bulk, long-range ordered AF state.…”