P-78 Poster-II : Magnetism : mixed valence and perovskites Magnetic Ordering in EuNiO 3

The metal to insulator transition at TMIand related changes in structure and magnetic behavior in the rare-earth nickelates RNiO3 have been intensively studied in recent years. Evidence for charge disproportionation occurring in EuNiO3 has been reported using 1at% Fe substituted for Ni as nuclear probe for Mössbauer spectroscopy [1,2]. For EuNiO3 TMI = 460 K is much higher than the Néel temperature TN ≈ 210 K. The magnetic hyperfine patterns below TN in the insulating state reveal two high spin Fe sites with roughly equal spectral weights attributed to iron substituted into Ni sites with different charge state. Fe is expected to be magnetically frustrated in one of these sites as proven from the dynamic Mössbauer hyperfine spectra. In an earlier μSR study [3] on pure EuNiO3 only a partial loss of asymmetry was detected below TN . Neutron diffraction studies are precluded for studying the magnetism of EuNiO3 and the ordered state still is ill defined. We have performed μSR at the GPS facility of SμS between 5 K and 290 K, both for undoped and 1% Fe doped EuNiO3. TN of the Ni sublattice is only weakly indicated in susceptibility which reveals a temperature dependence which is far from typical for an antiferromagnet. The moment is further increasing below TN and saturating below ∼ 120 K. At low temperatures Eu is expected to be in the non-magnetic FJ=0 ground state. There are, however, contributions to susceptibility from induced moments due to mixing with the higher crystal electric field states. Our ZF μSR data clearly reveal several spontaneous muon spin rotation signals as expected in a magnetically ordered state with various sites occupied by μ. For the analysis we used a superposition of three rotating signals with exponential damping and a non-magnetic background signal. Above 160 K the three signals cannot be resolved. For the sample doped with 1% Fe only one rotating signal is detected. There appears however a considerable non-rotating magnetic contribution indicating a severe disturbance by doping. Magnetic long-range order occurs below TN= 210 K for both samples. The perturbation introduced by the iron, however, prevents the observation of two of the three muon sites detected for the undoped material. For the undoped material there is an indication for two magnetic regimes: Between 210 K and 160 K the muon magnetic response is the same for the three sites, below the signals can be well resolved. We propose that the antiferromagnetic spin structure of Ni just below TN is changing to a more complex one at lower temperatures as found e.g. for NdNiO3[4]. Reason is the increasing induced moment at Eu below TN . Saturation of this moment is only achieved below about 120 K where also the rotation frequencies saturate. Comparison will be made to YNiO3 where Y is carrying no moment at all.