NMR Spectroscopy of Solid Solutions of New Layered Arsenides Ba(Cr1–xCox)2As2
摘要
A series of new layered arsenides Ba(Cr1–xCox)2As2 with x = 0, 0.2, 0.3, 0.6, 0.8, and 1.0 has been studied by 59Co and 75As NMR spectroscopy. The hyperfine magnetic field on the 75As nuclei in the BaCr2As2 sample is absent due to the mutual compensation of hyperfine magnetic fields from neighboring magnetic Cr atoms. The mutual compensation of local magnetic fields on 75As nuclei in compounds with x = 0.2, 0.3, and 0.6 is disturbed, and a significant broadening of the line is observed, indicating the emergence of local magnetic fields of about 0.3–0.8 T on arsenic nuclei. With an increase in the Co content to 80%, the magnetic broadening of the 75As NMR line disappears completely, and the isotropic shift 75Kiso increases to 0.19%. This indicates the predominance of the Knight term due to conduction electrons in the total shift. The isotropic shift 75Kiso is maximal in the extreme BaCo2As2 compound, which indicates the maximum value of the density of states at the Fermi level. 59Co NMR in Ba(Cr0.7Co0.3)2As2 in zero external magnetic field measured for the first time has demonstrated a wide distribution of the intensity of the spin echo in the frequency range of 10‒55 MHz, corresponding to a local magnetic field of several Tesla on cobalt. This field is 5–7 times lower than the characteristic local fields detected in Co-based magnets, which indicates a significant reduction of the magnetic moment of cobalt in the Ba(Cr0.7Co0.3)2As2 compound. Starting from a cobalt content of 80%, the 59Co NMR line is sharply narrowed, which indicates the nonmagnetic state of Co in the Ba(C-r1‒xCox)2As2 compounds with x > 0.6.