(a) This is because at lower temperature, the tendency to disrupt the alignment of dipoles (due to magnetising field) decreases on account of reduced random thermal motion.
(b) In a magnetic sample, each molecule is not a magnetic dipole in itself. Therefore, random thermal motion of molecules does not affect the magnetism of the specimen. That is why diamagnetism is independent of temperature.
(c) As bismuth is diamagnetic, therefore, the field in the core will be slightly less than when the core is empty.
(d) No, permeability of a ferromagnetic material is not independent of magnetic field. As is clear from the hysteresis curve, `mu` is greater for lower fields.
(e) Magnetic field lines are always nearly normal to the surface of a ferromagnet at every point. The proof of this important fact is based on the boundary conditions of magnetic fields (B and H) at the interface of two media. The magnetic permeability of a ferromanetic material `mugt gt1`. That is why the field lines meet this medium normally.
(f) Yes, maximum possible magnetisation of a paramagnetic sample will be the same order of magnitude as the magnetisation of a ferromagnet. The saturation however, requires very high magnetising fields, which are hard to achieve.