Measuring the transverse magnetization of rotating ferrofluids

We report on measurements of the transverse magnetization of a ferrofluid rotating as a rigid body in a constant magnetic field, H-0, applied perpendicular to the axis of rotation. The rotation of the fluid leads to a nonequilibrium situation, where the ferrofluid magnetization M and the magnetic field within the sample, H, are no longer parallel to each other. The off-axis magnetization perpendicular to H-0 is measured as a function of both the applied magnetic field H-0 and the angular frequency Omega. The latter ranges from a few hertz to frequencies well above a characteristic inverse Brownian relaxation time. Our experimental results strongly indicate that the transverse magnetization is caused only by a small fraction of the colloidal ferromagnetic particles. The effect of the polydispersity of the ferrofluid is discussed. Experimental results are compared to predictions based on several theoretical models. A single-time relaxation approach for the so-called effective field and a field-dependent Debye relaxation of M yield reasonably good shapes of the curves of transverse magnetization vs Omega. However, like the other models, they overestimate their magnitudes.