We present a comparative study on superparamagnetic uncoated and silica-coated NiFe₂O₄@SiO₂ nanoparticles with identical NiFe₂O₄ cores and two different silica coating thicknesses, focusing on the surface spin arrangement on NiFe₂O₄. For the investigation of surface spin dynamics, in addition to the SQUID magnetometry experiments which probe the macroscopic behaviour of particle magnetization, Mössbauer spectroscopy was utilized as being a local technique with superior characteristic measurement time. It was revealed that two common aspects observed in nanoscaled magnetic particles, namely the surface spin canting and interparticle dipolar interactions, are intertwined with respect to their contributions to the total magnetic anisotropy, where the latter is significantly decreased with silica coating thickness and allowed surface effects to manifest itself in Mössbauer spectra. The reduced saturation magnetization of uncoated NiFe₂O₄ nanoparticles with respect to their bulk counterparts indicated the existence of a “magnetically dead” layer on NiFe₂O₄ surface. In opposite to some assumptions in the literature, the Mössbauer spectra confirmed that such reduced magnetization arises from spin disorder, rather than a change in coordination from inverse spinel to the mixed spinel structure. On the other side, emergence of broad sextet peaks at room temperature Mössbauer spectra showed that for NiFe₂O₄@SiO₂ samples, silica coating further enhanced the spin canting on NiFe₂O₄ surface. Similarly, for NiFe₂O₄@SiO₂ sample, an unusual asymmetric hysteresis loop at spin freezing temperature was observed as a signature of the existence of disordered spins in the SiO₂/NiFe₂O₄ interface. It was shown that the sextet components in Mössbauer spectra can be reproduced with a continuous hyperfine field distribution due to the frustrated local spin arrangement at the surface. The thickness of the “magnetically dead” disordered surface spin layer was estimated for uncoated NiFe₂O₄, and average spin canting angle was calculated for NiFe₂O₄@SiO₂ nanoparticles.

Graphical abstract

中文翻译：

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二氧化硅包覆的镍铁氧体（NiFe 2 O 4 @SiO 2）纳米粒子中的自旋倾斜：Mössbauer光谱研究

我们针对具有相同NiFe ₂ O ₄核和两种不同二氧化硅涂层厚度的超顺磁性未涂层和二氧化硅涂层的NiFe ₂ O ₄ @SiO ₂纳米颗粒进行比较研究，重点是NiFe ₂ O ₄的表面自旋排列。为了研究表面自旋动力学，除了探究颗粒磁化的宏观行为的SQUID磁力测量实验外，穆斯堡尔光谱法还被用作具有优异特征测量时间的本地技术。揭示了在纳米级磁性粒子中观察到的两个常见方面，即表面自旋倾斜和粒子间偶极相互作用，在它们对总磁各向异性的贡献方面相互交织，其中，随着二氧化硅涂层厚度的增加和允许的表面效应，后者显着降低在穆斯堡尔光谱中表现出来。未涂覆的NiFe ₂ O _4的饱和磁化强度降低相对于其体积对应物，纳米颗粒表明在NiFe ₂ O ₄表面上存在“磁死”层。与文献中的某些假设相反，Mössbauer光谱证实这种降低的磁化强度是由于自旋紊乱引起的，而不是由反尖晶石向混合尖晶石结构的配位变化。另一方面，在室温下出现宽六重峰的Mössbauer光谱表明，对于NiFe ₂ O ₄ @SiO ₂样品，二氧化硅涂层进一步增强了NiFe ₂ O ₄表面的自旋倾斜。类似地，对于NiFe ₂ O ₄ @SiO ₂在样品中，在旋转冻结温度下观察到一个异常的不对称磁滞回线，这表明SiO ₂ / NiFe ₂ O ₄界面中存在无序自旋。结果表明，由于受挫的局部自旋排列，Mössbauer光谱中的六重组分可以连续的超精细场分布进行复制。对于未涂覆的NiFe ₂ O ₄，估计了“磁死”无序表面自旋层的厚度，并且对于NiFe ₂ O ₄ @SiO ₂纳米粒子，计算了平均自旋倾斜角。

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