Antiferromagnetism in AFe₂As₂ (A = Ba, Sr): the Parent Compounds of the New Iron Pnictide Superconductors

The discovery of high temperature superconductivity in the layered iron-pnictide compounds, first in RFeAsO_1-xF_x (R: La or other rare earth elements) [1] and subsequently in A_1-xB_xFe₂As₂ (A = Ba, Sr, Ca; B = K, Na), with the superconducting transition temperature exceeding 50 K has opened new opportunities to investigate the interplay between magnetism and superconductivity. The undoped parent compounds (RFeAsO and AFe₂As₂) are compensated metals and undergo an antiferromagnetic (AF) transition. The superconductivity appears upon elemental substitution or, in the case of AFe₂As₂, by applying pressure.


We have performed the nuclear magnetic resonance (NMR) experiments on ⁷⁵As nuclei using high quality single crystals of BaFe₂As₂ and SrFe₂As₂. The NMR spectra shown in Fig. 1 split symmetrically upon entering into the AF phase for the field along the c-axis but not for the field along the a- or b- axis. This result combined with the symmetry of the hyperfine interaction between As nuclei and Fe moments imposed by the crystal structure allowed us to select uniquely the stripe-type AF spin structure shown in the left inset of Fig. 1. Evidence for the orthorhombic crystal distortion in the AF phase was provided by the two sets of quadrupole satellite lines. The splitting of each set shows identical angular variation in the ab-plane with a phase shift of 90 degrees (Fig. 1 right inset). Similar results were obtained also for BaFe₂As₂.




Fig. 1. ⁷⁵As-NMR spectra in SrFe₂As₂ at 48.31 MHz obtained by sweeping the magnetic field along the c-axis (red) or along the a- or b- axis (blue). A single set of quadrupole-split three lines is observed in the paramagnetic state (205 K). Below T_t = 199 K symmetric line splitting occurs for H // c, while the lines shift without splitting for H �� c, indicating that the staggered fields due to AF order is parallel to the c-axis. This in turn determines the stripe-type spin structure shown in the left inset. The right inset shows the angular variation of the NMR spectra at T = 20 K with the field rotated in the ab-plane.

Figure 2 shows the T-dependence of the hyperfine field due to AF moments, which is proportional to the staggered moments. The discontinuous jump at the transition temperature indicates first order transitions for both materials. This is further confirmed by the hysteresis in the temperature dependence of the intensity of the central line at the paramagnetic resonance field (inset in Fig. 2).




Fig. 2. T-dependence of the hyperfine field �� due to AF order in BaFe₂As₂ and SrFe₂As₂. �� is determined from the splitting of the central line for H // c (crosshairs) or from the line shift for H �� c. The T-dependences of the intensity of the central line at the paramagnetic resonance fields shown in the insets provide clear evidence for the first order AF transitions with hysteresis.


Figure 3 shows the T-dependence of the nuclear spin-lattice relaxation rate divided by temperature 1/(T₁T). It shows substantial enhancement in the paramagnetic phase near the transition temperature, indicating development of the AF spin fluctuations. In particular, BaFe₂As₂ shows strong enhancement only for H // c. Such anisotropy may be due to orbital-selective spin fluctuations through the spin-orbit coupling. A finite value of 1/(T₁T) at low temperatures indicates that a part of the Fermi surface survives in the AF phase. Work is now in progress to investigate pressure induced superconductivity.




Fig. 3. T-dependence of the nuclear spin-lattice relaxation rate divided by temperature (T₁T)^-1 in BaFe₂As₂ and SrFe₂As₂ for different field orientations. In BaFe₂As₂ strong enhancement of (T₁T)^-1 near T_t is observed only for H // c, indicating development of anisotropic spin fluctuations at the stripe wave vector.


References
[1] Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am. Chem. Soc. 130 (2008) 3296.
[2] K. Kitagawa, N, Katayama, K. Ohgushi, M. Yoshida, and M. Takigawa, J. Phys. Soc. Jpn. 77 (2008) 114709.
[3] K. Kitagawa, N, Katayama, K. Ohgushi, and M. Takigawa, J. Phys. Soc. Jpn. 78 (2009) 063706.