Departamento de Fisica, Universidad Pública de Navarra, Campus de Arrosadía, 31006 Pamplona, Spain.
J Phys Condens Matter. 2012 Jul 4;24(26):266007. doi: 10.1088/0953-8984/24/26/266007.
In this work the effect of a SiO2 coating on the magnetic properties of Fe3O4 nanoparticles obtained by the sol-gel method is analyzed. Two sets of samples were prepared: Fe3O4 nanoparticles and Fe3O4@SiO2 core-shell composites. The samples display the characteristic spinel structure associated with the magnetite Fe3O4 phase, with the majority of grain sizes around 5-10 nm. At room temperature the nanoparticles show the characteristic superparamagnetic behavior with mean blocking temperatures around 160 and 120 K for Fe3O4 and Fe3O4@SiO2, respectively. The main effect of the SiO2 coating is reflected in the temperature dependence of the high field magnetization (μ(0)H = 6 T), i.e. deviations from the Bloch law at low temperatures (T < 20 K). Such deviations, enhanced by the introduction of the SiO2 coating, are associated with the occurrence of surface spin disordered effects. The induction heating effects (magnetic hyperthermia) are analyzed under the application of an AC magnetic field. Maximum specific absorption rate (SAR) values around 1.5 W g(-1) were achieved for the Fe3O4 nanoparticles. A significant decrease (around 26%) is found in the SAR values of the SiO2 coated nanocomposite. The different heating response is analyzed in terms of the decrease of the effective nanoparticle magnetization in the Fe3O4@SiO2 core-shell composites at room temperature.
本工作分析了 SiO2 涂层对溶胶-凝胶法制备的 Fe3O4 纳米粒子磁性能的影响。制备了两组样品：Fe3O4 纳米粒子和 Fe3O4@SiO2 核壳复合材料。样品显示出与磁铁矿 Fe3O4 相相关的特征尖晶石结构，大多数晶粒尺寸约为 5-10nm。在室温下，纳米粒子表现出典型的超顺磁行为，Fe3O4 和 Fe3O4@SiO2 的平均阻塞温度分别约为 160 和 120 K。SiO2 涂层的主要影响反映在高场磁化率（μ(0)H = 6 T）的温度依赖性上，即在低温（T < 20 K）下偏离 Bloch 定律。这种偏离，由于引入了 SiO2 涂层而增强，与表面自旋无序效应的发生有关。在施加交流磁场的情况下分析感应加热效应（磁热疗）。Fe3O4 纳米粒子的最大比吸收率（SAR）值约为 1.5 W g(-1)。在 SiO2 涂层的纳米复合材料中，SAR 值显著降低（约 26%）。从 Fe3O4@SiO2 核壳复合材料中室温下有效纳米粒子磁化强度的降低来分析不同的加热响应。
