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  • March 2012

Optics InfoBase > Spotlight on Optics > Magnetic field sensing based on V-shaped groove filled with magnetic fluids

Magnetic field sensing based on V-shaped groove filled with magnetic fluids

Published in Applied Optics, Vol. 51 Issue 8, pp.1010-1020 (2012)
by Hongzhu Ji, Shengli Pu, Xiang Wang, and Guojun Yu

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Spotlight summary: Magnetic colloids have an age-old bond with optics. More than a hundred years have elapsed since Quirino Majorana first published a detailed investigation on magnetic birefringence and dichroism in a special type of iron oxide colloids called “Bravais iron” which was aged for 10-20 years. He noted that “the wave that is the most retarded is also the one that is more absorbed”. This is known as the Majorana Law. The interest in optical effects in magnetic colloids has been revived by the development of fascinating ferrofluids (magnetic fluids). Such a fluid contains myriads of tiny magnets in host liquid carriers like water, hydrocarbons or oils. Close proximity of the magnetic particles promotes agglomeration due to van der Walls and dipole attractions. This is prevented either by coating each particle by an appropriate surfactant or by assigning positive or negative charge to the particle.

Magnetic fluids (MFs) are widely used in several engineering and physical devices like seals, sensors, switches etc. In sensor technology MFs are used in MEMs, location tracking, inclination sensors, optical current transformers, etc. Apart from being potentially useful, such fluids are also of basic scientific interest. Each tiny magnet has a macromolecular size ( ~10 nm ), so the fluid performs like a homogenous continuous medium and under zero magnetic field behaves like a nonmagnetic ordinary fluid obeying laws of ordinary hydrodynamics. When subjected to a uniform magnetic field, on the other hand, each nanomagnet becomes aligned in the direction of the applied field, and the medium manifest magnetization which increases with the field until reaching saturation. If the field presents a gradient, then a new body force called magnetic body force arises and the fluid exhibits several intriguing phenomena, like motion without mechanical means, defying of gravity, levitation of nonmagnetic objects, spontaneous generation of spikes, etc. Physical properties of the fluid like density, viscosity or dielectric permittivity can be modulated by subjecting it to an external magnetic field. The strength of the required field for such modulation is hardly of ~100 Oe.

The interest in magnetically modulated photonic effects in MFs is of recent origin. It has been shown that when a magnetic fluid is subjected to a magnetic field of proper value, it inhibits certain optical frequencies and acts like a photonic bandgap material akin to the forbidden bandgap in a semiconductor. With the help of a MF one can resonantly trap photons in magnetizable Mie scatterers. MFs are also used for domain detection in ferromagnetic materials, and the magnetic field of complex magnetic circuits can be mapped by it. Many of these effects are attributed to the magnetically tunable refractive index.

Ji et al have considered the tuning of the refractive and proposed a novel and remarkably simple technique to sense magnetic fields by placing a MF- filled V-shaped glass groove in the magnetic field. Such a groove serves like a hollow glass prism filled with the MF. The sensing field will then modify the refractive index of the MF. Accordingly, the emerging light ray will deviate from its original path and this deviation angle will depend on the refractive index and in turn on the applied field. Applying this simple reasoning, Ji et al have developed a magnetic field sensing system. From numerical simulation the authors have derived an expression which relates the change in height of the emerging light ray to the applied field. A parametric study of this expression is carried out and the results are compared with experimental findings. A detailed analysis of the sensing system is presented and a fairly successful matching between the numerical and experimental results is established. The sensor designed by Ji et al. may be useful to develop MF-based optical current transformers which will have an edge over other OCTs.

--R. V. Mehta

Technical Division: Light–Matter Interactions
ToC Category: Materials
OCIS Codes: (160.3820) Materials : Magneto-optical materials
(160.4236) Materials : Nanomaterials

Posted on March 14, 2012

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