钴铁合金基磁相互作用纳米体系的设计、制备及磁性质研究

Co-Fe alloy has the largest saturation magnetization among all known alloy magnets, and is a kind of typical soft magnetic material with a small coercivity. Its application fields can be expanded if its magnetic performance is improved by compounding with CoFe2O4 with the higher coercivity. In the previously reported results of Co-Fe/CoFe2O4 nanocomposites, the remanence to saturation magnetization ratio is very small and the interpretation to the underlying mechanism is still a matter of academic discussion. Key issues in previous reports are as follows: 1) Particles, obtained through the routes of preparation in liquid phase followed by reduction in solid phase, conglomerate seriously, causing that the interparticle dipolar interaction suppresses the exchange coupling interaction and consequently leading to the small remanence to saturation magnetization ratio. 2) The dipolar interaction in Co-Fe alloys or in Co-Fe/CoFe2O4 nanocomposites has not been systematically studied so far, even though the dipolar interaction widely consists in nano-scaled system and significantly affects magnetic properties. This project carefully designs the fabrication stage, utilizing techniques of preparing, reducing (oxidizing) and separating in liquid phase, and systematically investigates the effects of interparticle dipolar interaction on the magnetic properties in single-phase Co-Fe alloys or in Co-Fe/CoFe2O4 nanocomposites, for the purposes of (1) obtaining the phase diagram of magnetic states (superferromagnetism, superparamagnetism and superspin glass, etc.) versus distances and moments of particles and (2) revealing inherent correlation between the coercivity, remanence to saturation magnetization ratio and the dipolar interaction.

Co-Fe合金在所有已知合金磁体中饱和磁化强度最大，且是典型的软磁材料，矫顽力很小。将其与矫顽力较大的硬磁材料CoFe2O4复合，调控磁性能，可以拓宽其应用领域。在已报道的Co-Fe/CoFe2O4纳米复合体系的结果中，剩磁比很小，且对机理的解释也莫衷一是，关键问题在于：1）以往采用液相制备→固相还原途径所制备的粒子聚集严重，致使样品中粒子间偶极相互作用抑制了晶粒间交换耦合作用，导致小的剩磁比。2）以往未系统研究Co-Fe/CoFe2O4中的偶极相互作用，尽管偶极作用普遍存在于纳米体系中且对磁性有显著影响。本项目精心设计制备环节，拟采用液相制备→液相还原（氧化）→液相隔离等技术合成样品，系统研究单相Co-Fe合金、Co-Fe/CoFe2O4复合体系中偶极相互作用对磁性质的影响：获得磁状态（超铁磁、超顺磁、超自旋玻璃）与粒子间距、磁矩之间关系的相图；揭示矫顽力、剩磁比与偶极作用间的内在关联。

该项目采用Co-Fe合金基复合物Co-Fe/CoFe2O4为研究对象。根据偶极相互作用决定于磁粒子的磁矩和间距（或浓度）这一基本原理，通过对CoFe2O4纳米粒子稀释和还原以改变粒子间距和磁矩，设计并合成Co-Fe/CoFe2O4复合体系，揭示粒子间偶极相互作用对复合物宏观磁性的影响。认识矫顽力、剩磁比与偶极作用强弱的内在关联。主要结果如下：. 1、单相CoFe2O4纳米颗粒体系中磁相互作用研究. 1.1 获得纯偶极相互作用（无交换耦合作用）体系粒子，用δm测试手段证实偶极相互作用强弱与剩磁比成反比。. 1.2适当稀释可以获得较高的室温矫顽力（3056 Oe）和剩磁比（0.63）。剩磁比大于以往报道的值（<0.5）。. 1.3表面自旋间相互作用使得10 K时的各向异性常数K达到9.90×106 erg/cm3，比块体CoFe2O4的值大三倍以上。. 1.4 表面自旋与芯自旋之间的交换耦合作用，使10 K时剩磁比（0.85）高于理论值0.832。. 1.5 低温磁性主要由表面各向异性决定，而200 K以上的磁性主要有偶极相互作用决定。. 2、CoFe2O4/CoFe2复合体系中磁相互作用研究. 2.1 将CoFe2O4纳米粒子在氢气环境300 ℃还原1.5小时合成的CoFe2O4/CoFe2，在室温时具有比文献报道更高的矫顽力和剩磁比。. 2.2在系列粒径大小/磁矩/浓度的CoFe2O4@CoFe2复合体系中观察到偶极相互作用与粒子体系的剩磁比之间满足经验公式Mr/Ms∝1/lgHdip。. 3、CoFe2芯@Fe氧化物壳层复合体系中磁相互作用研究. 3.1 各向异性能垒分布曲线呈现两个峰值，分别位于低温T1 (< 30 K)和高温T2 (120 K)位置。随外场增加，T1不变，而T2向低温偏移。. 3.2 在T1温度以下，10 K时测量的起始磁化曲线呈现异常的现象，在高场时，磁化强度随磁场的增加反而降低。