The performance characteristics of magnetic nanoparticles towards application, in medicine, imaging, or as sensors, is directly determined by their magnetization relaxation and total magnetic moment. In the commonly assumed picture, nanoparticles have a constant overall magnetic moment originating from the magnetization of the single-domain particle core surrounded by a surface region hosting spin disorder. In contrast, this work demonstrates the significant increase of the magnetic moment of ferrite nanoparticles with applied magnetic field. At low magnetic field, the homogeneously magnetized particle core initially coincides in size with the structurally coherent grain of 12.8(2) nm diameter, indicating a strong coupling between magnetic and structural disorder. Applied magnetic fields gradually polarize the uncorrelated, disordered surface spins, resulting in a magnetic volume more than 20% larger than the structurally coherent core. The intraparticle magnetic disorder energy increases sharply towards the defect-rich surface as established by the field-dependence of the magnetization distribution. In consequence, these findings illustrate how the nanoparticle magnetization overcomes structural surface disorder. This new concept of intraparticle magnetization is deployable to other magnetic nanoparticle systems, where the in-depth knowledge of spin disorder and associated magnetic anisotropies will be decisive for a rational nanomaterials design.

中文翻译：

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纳米粒子中磁异常的场相关性

磁性纳米粒子在医学，成像或传感器领域中的应用性能直接取决于其磁化弛豫和总磁矩。在通常假定的图片中，纳米粒子具有恒定的总磁矩，该磁矩源自被包含自旋无序的表面区域围绕的单畴粒子核的磁化。相反，这项工作证明了施加磁场后铁氧体纳米粒子的磁矩显着增加。在低磁场下，均匀磁化的颗粒核的尺寸最初与直径为12.8（2）nm的结构相干晶粒重合，表明磁性与结构无序之间存在强耦合。施加的磁场逐渐使不相关的极化，无序的表面自旋，导致磁体积比结构上相干的磁芯大20％以上。如通过磁化分布的场相关性所建立的那样，粒子内的磁性无序能朝着缺陷丰富的表面急剧增加。结果，这些发现说明了纳米粒子磁化如何克服结构表面紊乱。粒子内磁化的这一新概念可以部署到其他磁性纳米粒子系统，在该系统中，对自旋无序和相关磁各向异性的深入了解对于合理的纳米材料设计至关重要。如通过磁化分布的场相关性所建立的那样，粒子内的磁性无序能朝着缺陷丰富的表面急剧增加。结果，这些发现说明了纳米粒子磁化如何克服结构表面紊乱。粒子内磁化的这一新概念可以部署到其他磁性纳米粒子系统，在该系统中，对自旋无序和相关磁各向异性的深入了解对于合理的纳米材料设计至关重要。如通过磁化分布的场相关性所建立的那样，粒子内的磁性无序能朝着缺陷丰富的表面急剧增加。结果，这些发现说明了纳米粒子磁化如何克服结构表面紊乱。粒子内磁化的这一新概念可以部署到其他磁性纳米粒子系统，在该系统中，对自旋无序和相关磁各向异性的深入了解对于合理的纳米材料设计至关重要。