Mumbai university > FE > SEM 1 > Applied Physics 1

Marks: 3M

Year: Dec 2014

Paramagnetism is the tendency of the atomic magnetic dipoles, due to quantum-mechanical spin angular momentum, in a material that is otherwise non-magnetic to align with an external magnetic field. This alignment of the atomic dipoles with the magnetic field tends to strengthen it, and is described by a relative magnetic permeability, μr greater than unity (or, equivalently, a small positive magnetic susceptibility greater than zero), i.e. $(i.e. μr = μ/μo = (1 + χm) \gt 1 \ \ and \ \ χm \gt 0)$. In pure Paramagnetism, the external magnetic field acts on each atomic dipole independently and there are no interactions between individual atomic dipoles. Such paramagnetic behavior can also be observed in ferromagnetic materials that are above their Curie temperature. Paramagnetic materials attract and repel like normal magnets when subject to a magnetic field. Under relatively low magnetic field saturation when the majority of the atomic dipoles are not aligned with the field, paramagnetic materials exhibit magnetization according to

$Curie's \ \ Law : M = C•B/T$, where

M is the resulting magnetization (magnetic dipole moment/unit volume), measured in Amps/meter.

B is the magnetic flux density of the applied field, measured in Tesla.

T is absolute temperature, measured in Kelvin.

C is a material-specific Curie constant

This law indicates that paramagnetic materials tend to become increasingly magnetic as the applied magnetic field is increased, but less magnetic as temperature is increased. Curie's law is incomplete because it fails to predict what will happen when most of the little magnets are aligned (after everything is aligned, increasing the external field will not increase the total magnetization) so Curie's

Constant really should be expressed as a function of how much ofthe material is already aligned. Paramagnetic materials in magnetic fields will act like magnets but when the field is removed, thermal motion will quickly disrupt the magnetic alignment. Ingeneral paramagnetic effects are small (magnetic susceptibility of the order of $χm~ 10^-3 \ \ to \ \ 10^-5$) e.g.Aluminum, barium.

Ferromagnetism- A phenomenon in some magnetically ordered materials in which there is a bulk magnetic moment and the magnetization is large. The electron spins of the atoms in the microscopic regions, domains, are aligned. In the presence of an external magnetic field the domains oriented favorably with respect to the field grow at the expense of the others and the magnetization of the domains tends to align with the field. Below a certain temperature, called the Curie point (or Curie temperature) an increasing magnetic field applied to a ferromagnetic substance will cause increasing magnetization to a high value called the saturation magnetization. This is because a ferromagnetic substance consists of small magnetized regions called domains. The total magnetic moment of a sample of the substance is the vector sum of the magnetic moments of the component domains.