PARAMAGNETISM, ANTIFERROMAGNETISM, AND SUPERCONDUCTIVITY IN LA2NIO4

ESR and energy loss measurements at 9 GHz have been carried out on gra nular samples of La2NiO4.00 in the range from 3.8 K to room temperatur e. The observed phenomena are dependent on the temperature and O2 Samp le treatment. For exactly 4.00 oxygen stoichiometry, the ESR spectra i ndicate that the material undergoes a series of phase transitions whic h are of some importance for understanding the antiferromagnetic and/o r superconducting behavior observed in La2NiO4. A low temperature regi me (3.8 < T < 70 K) is characterized by a zero field microwave absorpt ion signal and by the noise generated in fields above 0.3 T; this is c onventionally interpreted as a very sensitive signature for the presen ce of superconducting domains. There also is a broad, temperature-inde pendent ESR absorption arising from antiferromagnetic domains. Phase t ransitions are identified by changes in the ESR spectra above 70 and 1 60 K. Above 160 K (162-298 K), the ESR spectra remain unchanged, as wo uld 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.