Spectroscopic signature of the superparamagnetic transition and surface spin disorder in CoFe 2O 4 nanoparticles

Qi C. Sun, Christina S. Birkel, Jinbo Cao, Wolfgang Tremel, Janice L. Musfeldt

Research output: Contribution to journalArticle

24 Scopus citations

Abstract

Figure Persented: Phonons are exquisitely sensitive to finite length scale effects in a wide variety of materials. To investigate confinement in combination with strong magnetoelastic interactions, we measured the infrared vibrational properties of CoFe 2O 4 nanoparticles and compared our results to trends in the coercivity over the same size range and to the response of the bulk material. Remarkably, the spectroscopic response is sensitive to the size-induced crossover to the superparamagnetic state, which occurs between 7 and 10 nm. A spin-phonon coupling analysis supports the core-shell model. Moreover, it provides an estimate of the magnetically disordered shell thickness, which increases from 0.4 nm in the 14 nm particles to 0.8 nm in the 5 nm particles, demonstrating that the associated local lattice distortions take place on the length scale of the unit cell. These findings are important for understanding finite length scale effects in this and other magnetic oxides where magnetoelastic interactions are important.

Original languageEnglish (US)
Pages (from-to)4876-4883
Number of pages8
JournalACS nano
Volume6
Issue number6
DOIs
StatePublished - Jun 26 2012
Externally publishedYes

Keywords

  • CoFe O
  • nanoparticles
  • spin-phonon coupling
  • spinel
  • superparamagnetism
  • surface spin disorder

ASJC Scopus subject areas

  • Materials Science(all)
  • Engineering(all)
  • Physics and Astronomy(all)

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