Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA.
ACS Nano. 2012 Jun 26;6(6):4876-83. doi: 10.1021/nn301276q. Epub 2012 May 4.
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(2)O(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.
声子对各种材料中有限长度尺度效应非常敏感。为了研究限制与强磁弹相互作用的组合，我们测量了 CoFe(2)O(4)纳米粒子的红外振动特性，并将我们的结果与同一尺寸范围内矫顽力的趋势以及对大块材料的响应进行了比较。值得注意的是，光谱响应对大小诱导的超顺磁状态交叉敏感，该交叉发生在 7 至 10nm 之间。自旋-声子耦合分析支持核壳模型。此外，它还提供了对磁无序壳层厚度的估计，该厚度从 14nm 颗粒中的 0.4nm 增加到 5nm 颗粒中的 0.8nm，表明相关的局部晶格畸变发生在单元晶格的长度尺度上。这些发现对于理解这种和其他磁氧化物中的有限长度尺度效应很重要，因为在这些磁氧化物中磁弹相互作用很重要。
