Zero Field Cooling (ZFC) and Field Cooling (FC) protocols are commonly used to investigate the properties of magnetic nanoparticle systems. For non-interacting conditions the particle properties are fairly well correlated with the shape of the ZFC/FC curves. However, that is not the case when significant dipolar interparticle interactions (DII) are present, what frequently occurs in experimental samples (e.g. aggregates in biological systems; or the dried powder often used for the ZFC/FC measurements). The purpose of this work is to show how the influence of the DII on the ZFC/FC curves, computed by the volume sample concentration c, can be described in a general way if scaled by the dimensionless parameter c_0 = 2K/M_S^2; where K and M_S are the anisotropy and saturation magnetization constants of the particles, respectively. This scaling parameter, which is straightforwardly derived from the energy equation governing the system, has an analogous meaning to the normalization of the external magnetic field H by the anisotropy field of the particles, H_A = 2K/M_S. We use a Monte Carlo technique to show how apparently different T_B vs. c curves of various particles types (where T_B is the blocking temperature), follow the same trend if scaling c/c_0.
Magnetic properties of Fe3O4 nanoparticles coated with oleic and dodecanoic acids
合成了油酸（OA）和十二烷酸（DA）包覆的Fe 3 O 4磁性纳米粒子（NP），并通过透射电镜（TEM）、磁化强度M和交流磁化率测量对其进行了研究。OA涂覆的样品用不同的磁性浓度（78、76和65%）生产，DA样品用63%的Fe 3 O 4生产。来自TEM的图像表明NP具有近球形几何形状和平均直径约5.5nm。在不同的外部磁场H下，在零场冷却（ZFC）和场冷却（FC）工艺中进行的磁化测量在ZFC曲线中的给定温度TB处表现出最大值，其取决于NP涂层（OA或DA）、磁铁矿浓度和H。温度TB随着H的增加而单调降低，并且对于给定的H，磁铁矿浓度的增加导致TB的增加。所观察到的行为与NP之间的偶极相互作用（DI）有关，这似乎是所有研究样品中的一个重要机制。这一点得到了交流磁化率Xac测量结果的支持，其中不同频率的X'峰值温度遵循Vogel-Fulcher模型，这是偶极相互作用系统中常见的特征。具有不同涂层和磁铁矿浓度的样品的H与TB/TB（H=0）的曲线塌陷成通用曲线，表明样品的定性磁性行为可以由NP本身描述，而不是涂层或偶极相互作用的强度。在TB以下，M与H的曲线示出了随着温度降低而单调增加的矫顽场（HC）。饱和磁化强度（MS）遵循布洛赫定律和MS值在室温下高达78 emu/g的估计，结果对应于约80%的体值。给定样品的M/MS与H/T曲线的重叠和高温下的低HC表明研究的所有样品中存在超顺…
