We studied the magnetic properties of self-assembled aggregates of BiFeO3 nanoparticles (~ 20-40 nm). The aggregates formed two different structures - one with limited and another with massive cross-linking - via `drying-mediated self-assembly' process following dispersion of the nanoparticles within different organic solvents. They exhibit large coercivity H_C (>1000 Oe) and exchange bias field H_E (~ 350-900 Oe) in comparison to what is observed in isolated nanoparticles (H_C ~ 250 Oe; H_E ~ 0). The H_E turns out to be switching from negative to positive depending on the structure of the aggregates with |H_E| being larger. The magnetic force microscopy reveals the magnetic domains (extending across 7-10 nanoparticles) as well as the domain switching characteristics and corroborate the results of magnetic measurements. Numerical simulation of the `drying-mediated-self-assembly' process shows that the nanoparticle-solvent interaction plays an important role in forming the `nanoparticle aggregate structures' observed experimentally. Numerical simulation of the magnetic hysteresis loops, on the other hand, points out the importance of spin pinning at the surface of nanoparticles as a result of surface functionalization of the particles in different suspension media. Depending on the concentration of pinned spins at the surface pointing preferably along the easy-axis direction - from greater than 50\% to less than 50% - H_E switches from negative to positive. Quite aside from bulk sample and isolated nanoparticle, nanoparticle aggregates - resulting from surface functionalization - therefore, offer remarkable tunability of properties depending on structures.

中文翻译：

---

自组装 BiFeO3 纳米粒子中的大结构依赖室温交换偏差

我们研究了 BiFeO3 纳米颗粒（~ 20-40 nm）的自组装聚集体的磁性。聚集体形成了两种不同的结构 - 一种具有有限的交联，另一种具有大量的交联 - 通过将纳米粒子分散在不同的有机溶剂中后的“干燥介导的自组装”过程。与在孤立的纳米粒子中观察到的 (H_C ~ 250 Oe; H_E ~ 0) 相比，它们表现出较大的矫顽力 H_C (>1000 Oe) 和交换偏置场 H_E (~ 350-900 Oe)。事实证明，H_E 正从负值转换为正值，这取决于 |H_E| 的聚合结构。更大。磁力显微镜揭示了磁畴（跨越 7-10 个纳米粒子）以及磁畴切换特性，并证实了磁测量的结果。“干燥介导的自组装”过程的数值模拟表明，纳米颗粒-溶剂相互作用在形成实验观察到的“纳米颗粒聚集结构”中起着重要作用。另一方面，磁滞回线的数值模拟指出了由于粒子在不同悬浮介质中的表面功能化，在纳米粒子表面自旋固定的重要性。根据表面上固定自旋的浓度，优选沿易轴方向指向 - 从大于 50\% 到小于 50% - H_E 从负切换到正。除了大块样品和孤立的纳米颗粒之外，纳米颗粒聚集体 - 由表面功能化产生 - 因此，根据结构提供显着的特性可调性。