Magnetic properties of annealed core-shell CoPt nanoparticles

Jean Yves Bigot, Hasan Kesserwan, Valérie Halté, Ovidiu Ersen, M. Simona Moldovan, Tae Hee Kim, Jung Tak Jang, Jinwoo Cheon

Research output: Contribution to journalArticlepeer-review

45 Scopus citations


A precise control and understanding of the magnetization dynamics of nanostructures is an important topic in applied nanosciences. Herein, we perform such control by annealing crystalline (Co/core)-(Pt/shell) nanoparticles. Using electron tomography, temperature dependent electron microscopy and time-resolved magneto-optics, we establish a clear correlation between the magnetization dynamics and the crystalline structure of the nanoparticles. For a mild laser annealing (370 K) the Co-Pt nanoparticles keep their core-shell structure and remain superparamagnetic with a blocking temperature T B = 66 K. Their time-resolved reflectivity shows that they are locally organized into a supra-crystalline ordered layer in the region of the laser spot. In contrast, a thermal annealing at higher temperatures (up to 700 K) modifies the structure of the individual nanoparticles into a CoPt crystalline ferromagnetic phase, with T B,anneal = 347 K. Correspondingly, the magneto-crystalline anisotropy of the annealed CoPt nanoparticles increases and their magnetization dynamics displays a motion of precession, characteristic of ferromagnetic nanostructures and which is absent in the superparamagnetic Co-Pt core-shells.

Original languageEnglish
Pages (from-to)1189-1197
Number of pages9
JournalNano Letters
Issue number3
StatePublished - 14 Mar 2012


  • Magnetic nanoparticles
  • electron microscopy
  • electron tomography
  • ultrafast magnetization dynamics


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