High gradient magnetic separation (HGMS) is a technique used in diverse fields, including cell separation, drug delivery, waste removal, and in various bio-diagnostic microsystems. The objective of this work is to study the equilibrium structure of magnetic nanoparticle solutions which are exposed to strong magnetic field gradients (exceeding 10 T/mm) as seen in (A). The authors have previously developed an analytic model that describes the local concentrations of magnetic nanoparticles in high magnetic field gradient. This analytic model is derived from the drift-diffusion equations and determines the local particle concentration through a non-dimensional ratio of thermal and magnetic energies. This model is reasonably well supported by experimental work using optical microscopy to probe the local concentration of magnetic nanoparticles through measurements of the local optical transmission as a function of externally applied field strength and bulk volume fraction of ferrofluid as seen in (B) and (C). However, the model does not incorporate the particle concentration's effect on the local magnetic field. In order for this problem to be solved accurately, the concentration dependence of the magnetic field has been by solved self-consistent numerical analysis using finite difference methods. Fig. (A) Experimental micrograph shows rectangular cobalt islands that are 8 by 4 um. Magnetic ferrofluid particles concentrate between the islands in the field maxima. (B) A calibration curve is shown comparing bulk concentration versus bulk light intensity. (C) Plots of local concentration of magnetic particles versus local field for different dilutions of ferrofluid.