Interactions and thermal effects in systems of fine particles: A preisach analysis of CrU2 audio tape and magnetoferritin

P. D. Mitchler, R. M. Roshko, E. Dan Dahlberg, B. M. Moskowitz

Research output: Contribution to journalArticlepeer-review

6 Scopus citations

Abstract

We present an analysis and comparison of magnetization and remanence data from two particulate systems: commercial CrC>2 audiotape and a frozen suspension of magnetoferritin particles. The measurements were performed over a range of temperatures, 25 °C < T < 120 °C for CrO2 and 5 K < T < 10 K for magnetoferritin, and over a range of fields sufficient to saturate the remanence at each temperature. The CrU2 tape was prepared by both ac demagnetization and by thermal demagnetization from above the Curie temperature, while the magnetoferritin was thermally demagnetized from above its highest blocking temperature. The data were analyzed within the framework of a scalar Preisach model that included thermally activated relaxation and a reversible nonlinearity. In the case of CrU2, our fits showed that changes in hysteresis with temperature were due primarily to thermal variations in the spontaneous moment and the anisotropy of the particles, and in their magnetostatic interaction fields, and hence to changes in the free-energy landscape. In magnetoferritin, the temperature dependence of the magnetic response was due almost exclusively to changes in the relative populations of blocked and unblocked particles which relax by thermal activation over a fixed distribution of energy barriers. Our analysis of interaction effects was based on the Wohlfarth-Henkel parametric plot of the principal magnetizing and demagnetizing remanences. These plots showed curvature indicative of demagnetizing-like interactions in both thermally demagnetized magnetoferritin and ac-demagnetized CrC>2, but essentially no curvature in thermally demagnetized CrO2. Our analysis revealed that there were no long-range, mean field interactions in either system, but that thermal demagnetization of CrO2 produced a random initial state that was insensitive to fluctuations in the interaction field due to disorder, provided that mean field effects were absent, while thermal demagnetization of magnetoferritin produced a highly asymmetric initial state resembling that induced by ac demagnetization, which was sensitive to these fluctuations.

Original languageEnglish (US)
Pages (from-to)2029-2042
Number of pages14
JournalIEEE Transactions on Magnetics
Volume35
Issue number3 PART 2
DOIs
StatePublished - 1999

Keywords

  • Fine particles
  • Hysteresis
  • Interactions
  • Preisach model
  • Thermal relaxation

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