FAST TRACK COMMUNICATION Origin of the dynamics of the spin state in undoped BaFe2As2: Mössbauer studies

Fe–As based superconductors provide a good system for understanding the relationship between magnetism and superconductivity. Considerable efforts have been expended in understanding the magnetic behavior of the parent compound, BaFe2As2. However, it had not been realized that traces of adsorbed O2 by the material bring about drastic changes in its magnetic behavior. O2 is known to trap electrons, forming O2− . Guided by this discovery, we observe in the absence of O2 a dynamic transition between intermediate and low spin states of Fe and hysteresis effects, and in the presence of O2 trapping of the magnetic state. These observations are likely to have a bearing on the role of magnetism in superconductivity. The Fe–As based superconductors are currently attracting a great deal of attention because of their unconventional behavior. In the 122 family, BaFe2As2 is the parent compound, where the iron atoms are arranged in a planar square with each one coordinated tetrahedrally to four As atoms resulting in layers of edge-sharing tetrahedra. Ba ions are sandwiched between the Fe–As layers. When Fe is substituted by a few per cent of a transition element like Co or Ni, it exhibits superconductivity [1–4]. It is generally believed that the electron(s), donated by the substituents, induce the superconductivity. The undoped compound has a tetragonal structure and exhibits paramagnetic behavior at room temperature. On lowering the temperature it changes to orthorhombic structure at ∼140 K with antiferromagnetic alignment of the Fe spins. It seems that there is considerable orbital-dependent reconstruction of the electronic configuration across the magnetostructural transition [4–6]. Therefore, unsurprisingly several groups carrying out Mössbauer measurements have observed a singlet at room temperature, characteristic of a paramagnetic compound, and a magnetically split sextet at low temperatures [5, 8–15]. The magnetism is centered on the Fe and not on the Ba or As. However, the origin of magnetism and its weakness have lead to considerable discussion and controversy [6, 7]. It is generally believed that the Fe–As based superconductors do not exhibit the phonon-mediated Bardeen–Cooper–Schrieffer pairing mechanism. In view of the above, the role of magnetism in mediating pair formation is currently being hotly debated [2, 3, 6]. In short, it is essential that we fully understand the nature of the magnetism in the parent compound, BaFe2As2. Our studies reported in this communication bear on the magnetic behavior in the orthorhombic phase of the parent compound. We find that a strong influence of adsorbed oxygen (air) on the material and other defects in the solid are responsible for masking the nature of the spin states of Fe, namely intermediate and low spins, and their dynamics. The reported investigations are based on observations made on two polycrystalline lots of BaFe2As2 prepared at FSU. They were characterized by x-ray diffraction and energy dispersive x-ray analysis (EDX) measurements. Mössbauer measurements were made on powdered materials using a 57Co(Rh) source. A closed cycle ARS (Advanced Research Systems) cryostat was used for recording the spectra at specified temperatures. At 90 K, for an atmospheric pressure of 1 mTorr or lower, one observes a singlet and it gradually transforms to a fully developed sextet at higher pressures (figure 1). Interestingly, 1 0953-8984/11/342201+03$33.00 c © 2011 IOP Publishing Ltd Printed in the UK & the USA J. Phys.: Condens. Matter 23 (2011) 342201 Fast Track Communication Figure 1. Mössbauer spectra of BaFe2As2 at T = 90 K and different atmospheric pressures: 1 mTorr (a), 35 mTorr (b), 40 mTorr (c), 160 mTorr (d), 750 Torr (e). the behavior at each step is completely reversible at 90 K. In the near absence of O2 the rate of fluctuation is very high and a singlet is observed (figure 1(a)); the rate decreases gradually and finally adsorbed O2, which is known to be an effective electron acceptor forming O2− (e.g. [16]), completely traps the magnetic state. At lower pressures, a limited amount of oxygen permeates the material forming small sized spin clusters (magnetic polarons) resulting in superparamagnetic like behavior. Spectra (a) and (b) in figure 1 arise when the thermal energy, kT, is sufficient to flip the magnetization vector of the small spin clusters among the two easy directions at a rate comparable to or faster than the Larmor frequency of the daughter 57Fe (∼107 s−1) which results in a zero or near zero field at the 57Fe nuclei. When a larger amount of O2 is available, the size of the spin clusters grows and they cannot fluctuate at a fast rate with the thermal energy available at 90 K. In addition, the magnitude of the hyperfine magnetic field increases with the size of the spin clusters from 41 kOe, through 48–50 kOe (figure 1(c)–(e)). Surprisingly if the O2 is not desorbed at a higher temperature like 90 K by pumping, one continues to observe a resolved sextet at lower temperatures. The literature is replete with observations of well resolved sextets for several other 122 compounds of the form REFe2As2 also, where RE = Eu, Sr or Ca, presumably because of traces of adsorbed O2 (e.g. [5, 8–15]). In a sample under high vacuum, we observed a singlet at 90 K depicting very rapid relaxation of spin states with a frequency of ≥107 s−1 and the rate slows down through 60 K with frequency 5.2 × 106 s−1 and to 6 K with frequency 1.5×106 s−1 (figure 2). Therefore (b) and (c) exhibit partially relaxed sextets. Figure 2. Mössbauer spectra of BaFe2As2 at low atmospheric pressure P < 1 mTorr. T = 90 K (a), T = 60 K (b), T = 6 K (c). We also observe a very interesting hysteresis behavior. For a sample cooled to 6 K we observe a singlet, which after 48 h converts into a resolved sextet (figure 3). The singlet arises because of the very rapid fluctuations between the intermediate and low spin states. After some duration, the intermediate spin state is trapped, presumably by some defects. This behavior is observed only for one of the preparations. It appears that the magnetic behavior at low temperatures is determined not only by adsorbed O2 but also by the nature of intrinsic defects in the material and ones created by grinding. Therefore the behavior can vary somewhat from preparation to preparation. Single crystals, which were shattered gently, also exhibited qualitatively similar behavior to the polycrystalline preparation. The role of defects is also obvious from observations made by Ran et al [17] for a crystal of CaFe2As2 quenched from a high temperature of 960 C: the Mössbauer spectrum at 5 K exhibits no change from that observed at 295 K and the sextet is conspicuously absent and the singlet survives. In summary, we observe trapping of spin states by adsorbed oxygen, which constitutes a very good electron trap [16], and other defects, pre-existing or formed by grinding, and strong hysteresis effects. All the observed features are typical for spin-crossover for Fe(II) complexes in the solid state [18–20]. Therefore we can safely conclude that we are observing the dynamics of two spin states of Fe(II) in BaFe2As2, namely the intermediate spin state and the low spin state. The singlet at 90 K in good vacuum should be interpreted as fast fluctuations between the two spin states; with increase in air (oxygen) pressure the rate of fluctuations decreases and finally at a higher partial pressure of O2 the intermediate spin state exhibiting a sextet is frozen (figure 1). The rate of fluctuations also decreases with temperature for a sample under relatively high vacuum at 90 K and going down to 6 K (figure 2). At lower temperatures, one would normally expect the low spin state to be stabilized. However, the situation is complicated by the role of defects and the low