PARAMAGNETISM, ANTIFERROMAGNETISM, AND SUPERCONDUCTIVITY IN LA2NIO4

ESR and energy loss measurements at 9 GHz have been carried out on granular samples of La2NiO4.00 in the range from 3.8 K to room temperature. The observed phenomena are dependent on the temperature and O2 Sample treatment. For exactly 4.00 oxygen stoichiometry, the ESR spectra indicate that the material undergoes a series of phase transitions which are of some importance for understanding the antiferromagnetic and/or superconducting behavior observed in La2NiO4. A low temperature regime (3.8 < T < 70 K) is characterized by a zero field microwave absorption signal and by the noise generated in fields above 0.3 T; this is conventionally interpreted as a very sensitive signature for the presence of superconducting domains. There also is a broad, temperature-independent ESR absorption arising from antiferromagnetic domains. Phase transitions are identified by changes in the ESR spectra above 70 and 160 K. Above 160 K (162-298 K), the ESR spectra remain unchanged, as would be expected for the ESR absorption from antiferromagnetic domains (the Neel temperature for La2NiO4 has been reported as high as 650 K). The transition near 160 K is interpreted as being due to the onset of motional narrowing of the spin-spin interactions in the crystal. (C) 1994 Academic Press, Inc.