s21_magneticremanance

Introduction

The remanent magnetization and demagnetization of magnetite nanoparticles dispersed in an isoparaffin oil have been measured as a function of magnetite concentration. The magnetization and demagnetization remanences were measured for fields between 0 and 400 gauss, the field at which the remanence was saturated, and then plotted parametrically in magnetic field magnitude to produce Henkel plots.

For higher concentrations of magnetite the Henkel plot is nonlinear and consistent with antiferromagnetic interactions in the system. With decreased magnetite concentration the Henkel plots approach the linear relationship predicted by the Wohlfarth model for non-interacting particles.

Theory

The Wohlfarth model [1] describes the remanent magnetic behavior of a system of single-domain, non-interacting magnetic particles with uniaxial magnetic anisotropy. It predicts a linear relationship between the magnetization and demagnetization remanences of a thermally (or randomly) demagnetized system that is most easily seen when the magnetization and demagnetization remanences are plotted parametrically in what is known as a Henkel plot. This relationship between magnetization remanence, I_R , and demagnetization remanence, I_D , for a field magnitude H is given by the Wohlfarth relation,

I_D(H) = 1 - 2I_R(H) (Eq. 1)

Where we have normalized by the saturation, or maximum, remanence.

However, this linear relationship is only valid for a system with non-interacting particles. In the case of an interacting system, the effect on the Henkel plot is qualitatively shown in Figure 1. Note that ferromagnetic interactions are those that tend to magnetize the system, whereas antiferromagnetic tend to demagnetize.

Fig. 1 - Qualitative effect of interactions on the Wohlfarth model. The dashed line is Eq. 1.

Experimental Details

The samples used in this study contained isoparaffin-based ferrofluid [2] with varying concentrations of magnetite nanoparticles. These samples were prepared from the same batch of ferrofluid and diluted with additional isoparaffin oil. Room temperature was sufficient to thermally demagnetize this ferrofluid. The magnetization and demagnetization remanences of a thermally demagnetized sample were measured with a vibrating sample magnetometer (VSM) for applied fields between 0 Oe and 400 Oe while the sample was frozen at 77 K. Figure 2 shows the VSM used.

Fig. 2 - Vibrating Sample Magnetometer. The red circles are coils used to generate a magnetic field. A sample is attached to the vibrating rod coming form the top down to the center of the coils; as a magnetized sample vibrates, the changing flux of its magnetic field is measured using another set of measurement coils (not visible)

Results

Figure 3 shows the normalized Henkel plots for the samples with ferrofluid concentration between 100% and 10% by volume. The data show two key features:

1) The shift of the Henkel plot downwards and to the left, relative to the Wohlfarth relation, in Figure 3(a)-(c)

2) The convergence of the data to the Wohlfarth relation as the concentration of the ferrofluid is decreased