Simulation failed on 3D model with magnetic field, creeping flow and magnetophoretic force

Hi everyone,

I am doing some research about magnetophoresis using Comsol.
There are some questions about Comsol that simulation was successful on 2D model with magnetic field(,with or no currents), creeping flow and magnetophoretic force modulars, and it was successful on 3D model using magnetic field no current.

But when i created a 3D model, tried to use a current exciting magnetic field, the Comsol could not work out the particles magnetophoresis force, in that case, i found that the magnetic field gradient was 0 , the expressions: d(mf.Bx,x),d(mf.By,x),d(mf.Bx,x).......all were 0.

Has anyone done something like that? Or can anyone give some advice and help?

Thanks in anticipation