Mössbauer and magnetization studies of A_0.8Fe _2-ySe₂ (A = K, Rb and K/Tl)

⁵⁷Fe Mössbauer spectroscopy and dc magnetization studies of the magneto-superconducting tetragonal A_0.8Fe_2-ySe ₂ (A = K, Rb and K/Tl, y = 0.2-0.3) materials at temperatures of 5-550K have been performed. Below the superconducting transition, magnetization studies performed under low applied fields (H), on both Rb_0.8Fe _2-ySe₂ and K_0.3Tl_0.5Fe _2-ySe₂ single crystals, show a positive field-cooled branch known as the paramagnetic-Meissner effect for but not for . This paramagnetic-Meissner effect is attributed to a peculiar isothermal hysteresis loop obtained for . Despite being superconducting, due to the antiferromagnetic nature of the materials, Mössbauer spectroscopy studies exhibit well-defined sextets at low temperatures. The full diagonalization of the hyperfine interaction spin Hamiltonian shows clearly that the iron moments are tilted by ∼ 40°-45° away from the c-axis of the low-temperature tetragonal type structure. Moreover, long-term Mössbauer spectroscopy measurements at elevated temperatures suppress the antiferromagnetic state, and the materials measured after cooling to ambient temperatures exhibit a paramagnetic doublet only; its origin is discussed.