CNR-Istituto di Struttura della Materia, via Salaria km 29,300, 00016 Monterotondo Stazione, Roma, Italy.
Phys Chem Chem Phys. 2012 Mar 7;14(9):3162-9. doi: 10.1039/c2cp22473a. Epub 2012 Jan 27.
The magnetic properties of ultra-small (~2 nm) δ-(Fe(0.67)Mn(0.33))OOH nanoparticles prepared by a microemulsion technique have been investigated by magnetization and ac susceptibility measurements at variable frequency. The results provide evidence of two different magnetic regimes whose onset is identified by two maxima in the zero-field-cooled susceptibility: a large one, centered at ~150 K (T(mh)), and a narrow one at ~30 K (T(ml)). The two temperatures exhibit a different frequency dependence: T(mh) follows a Vogel-Fulcher law τ = τ(0)exp[(E(a)/k(B))/(T-T(0))], indicating a blocking of weakly interacting nanoparticle moments, whereas T(ml) follows a power law τ = τ(0)(T(g)/T(mν)-T(g))(α), suggesting a collective freezing of nanoparticle moments (superspin-glass state). This picture is coherent with the field dependence of T(ml) and T(mh) and with the temperature dependence of the coercivity, strongly increasing below 30 K.
采用微乳液技术制备的超小（2nm）δ-(Fe(0.67)Mn(0.33))OOH 纳米粒子的磁性通过变温变场下的磁化和交流磁化率测量进行了研究。结果表明存在两种不同的磁状态，其起始点由零场冷却磁化率中的两个最大值确定：一个在150K（T(mh)）附近的大值，另一个在~30K（T(ml)）附近的窄值。这两个温度表现出不同的频率依赖性：T(mh)遵循 Vogel-Fulcher 定律 τ = τ(0)exp[(E(a)/k(B))/(T-T(0))]，表明弱相互作用的纳米颗粒磁矩被阻塞，而 T(ml)遵循幂律 τ = τ(0)(T(g)/T(mν)-T(g))(α)，表明纳米颗粒磁矩的集体冻结（超自旋玻璃态）。这种情况与 T(ml)和 T(mh)的场依赖性以及矫顽力的温度依赖性一致，矫顽力在 30K 以下强烈增加。
