Spectroscopic Signature of the Superparamagnetic Transition and Surface Spin Disorder in CoFe2O4 Nanoparticles

详细信息    查看全文

• 作者：Qi -C. Sun ; Christina S. Birkel ; Jinbo Cao ; Wolfgang Tremel ; Janice L. Musfeldt
• 刊名：ACS Nano
• 出版年：2012
• 出版时间：June 26, 2012
• 年：2012
• 卷：6
• 期：6
• 页码：4876-4883
• 全文大小：833K
• 年卷期：v.6,no.6(June 26, 2012)
• ISSN：1936-086X

文摘

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 CoFe₂O₄ 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鈥損honon coupling analysis supports the core鈥搒hell 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.

Keywords:

spinel; CoFe₂O₄; nanoparticles; superparamagnetism; spin鈭抪honon coupling; surface spin disorder