Tb is usually observed by ZFC magnetization curves, which are recorded at low (5-50 Oe) applied fields. The reason is that you don't want to disturb the energy barrier from which the Tb is extracted. When you increase the field, the zeeman term -(mu)H alters the shape and height of the anisotropy barrier by decreasing the energy at one direction while incresing the opposite. The results is a usual decrease of the energy barrier and thus you will need a lower kBT value to flip the spins over it. 

Just a link where simple but graphical ideas are depicted:

http://www.fgarciasanchez.es/thesisfelipe/node5.html 

or attached file.

Hope this helps.

thnq Professor

actually we are measure at different magnetic field ZFC curves it has shown Tb increases with increasing of magnetic field any reason for this behavior ? or if it is decreases (Tb) with increases of field

It is well established that the onset of superparamagnetic behavior occurs at a blocking temperature defined by the height of the barrier to magnetization rotation of the single domain particles (ideally at a barrier height of 25 kBTb). The barrier height, on the other hand, is field dependent. where an effective field component applied along the uni-axial anisotropy direction increases the barrier height, thus increasing the "apparent" blocking temperature. However, the blocking temperature of a superparamagnetic powder depends on intrinsic parameters (anisotropy constant and mean volume of the single-domain particles), and therefore, for a physically reliable determination of the blocking temperature, measurements of the magnetization vs temperature should be performed at low applied fields such that (mu)H/KV