Regensburg 2010 – scientific programme

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MA: Fachverband Magnetismus

MA 9: Magnetic Particles, Clusters

MA 9.4: Talk

Monday, March 22, 2010, 16:00–16:15, H23

Cross-over between spin-glass freezing and blocking in NiFe₂O₄ nanoparticles — •Kashif Nadeem and Heinz Krenn — Institute of Physics, Karl-Franzens University Graz, Universitätsplatz 5, A-8010 Graz, Austria

Single-phase NiFe₂O₄ nanoparticles (8-27nm) dispersed in SiO₂ matrix have been prepared by sol-gel method. Disorder and core-shell interaction induces surface spin-glass like freezing which is manifested by a low temperature peak in the AC susceptibility well separated from magnetic blocking peak. The spin-glass freezing peak vanishes as the particle size grows (>18nm ). Exchange bias is present down to 8nm particle sample which indicates the existence of core-shell interactions. For 8nm sample, Arrhenius model is fitted to the frequency dependent AC susceptibility data and it gives spin-flip time τ_o~2.10⁻¹² sec and activation energy E_A/k_B = 370 K. The dynamic scaling law is also fitted to the same data and it yields: T_o = 11 K, τ_o~10⁻⁰⁵ sec and zv = 7.5 (which lies in spin-glass regime zv = 4-12). In the presence of DC field (10-500 Oe) in AC susceptibility, the freezing peak (T_f = 13 K) does not shift but the blocking peak (T_b = 233 K) moves towards lower temperature and finally collapses into the freezing peak as the DC field is increased. Time dependent TRM shows a non-monotonic behaviour in the vicinity of freezing peak which is consistent with the sharp increase of coercivity in the same temperature regime. Our system also exhibits memory effects. All these measurements demonstrate the presence of blocking/unblocking at high temperatures and core-shell mediated spin-glass like freezing at low temperatures.