Magnetization

Magnetization, M, is the magnetic moment per unit volume, V of a material, defined in terms of the magnetic moments of its constituents by:

$\mathbf {M} ={\frac {1}{V}}\sum _{j=1}^{N}\mathbf {m_{j}}$

where the magnetic moment m_j of the j-th constituent is a vector property that determines the torque the object experiences in a magnetic field tending to align its moment with the field. Here, N is the number of magnetic moments in the volume V. The M-field is measured in amperes per meter (A/m) in SI units. Usually the magnetization is referred to a particular location r by imagining the volume V to be a microscopic region enclosing point r, and is anticipated to change with time t in the general case (perhaps because the moments are moving), defining a magnetization field, M(r, t).

At a microscopic level, the origin of the magnetic moments responsible for magnetization is traced to angular momentum, such as due to motion of electrons in atoms, or to spin, such as the intrinsic spin of electrons or atomic nuclei.

Magnetic moments

As magnetization is related to magnetic moments, its understanding requires a notion of where magnetic moments originate. As a general statement, magnetic moments are related to either angular momentum or to spin, both of which at a microscopic level are related to rotational phenomena. The connection is made via the gyromagnetic ratio, the proportionality factor between magnetic moment and spin or angular momentum for a given object.

Although these ideas apply to nucleii and other particles, here attention is focused on electrons and electrons in atoms. The magnetic moment m_S of a system of electrons with spin S is:

$\mathbf {m_{S}} =2m_{B}\mathbf {S} \ ,$

and the magnetic moment m_L of an electronic orbital momentum L is:

$\mathbf {m_{L}} =m_{B}\mathbf {L} \ .$

Here the factor m_B refers to the Bohr magneton, defined by:

$m_{B}={\frac {e\hbar }{2m_{e}}}\ ,$

with e = the electron charge, ℏ = Planck's constant divided by 2π, and m_e = the electron mass. These relations are combined using the g-factor:

$\mathbf {m_{I}} =g_{I}m_{B}\ \mathbf {I} \ ,$

with g=2 for spin and g=1 for orbital motion. The resultant total spin S of an ensemble of electrons in an atom is the vector sum of the constituent spins s_j:

$\mathbf {S} =\sum _{j=1}^{N}\ \mathbf {s_{j}} \ .$

Likewise, the orbital momenta of an ensemble of electrons in an atom add as vectors.

Where both spin and orbital motion are present, they combine by vector addition:

$\mathbf {J=L+S} \ ,$

and the magnetic moment is

$\mathbf {m} =gm_{B}\mathbf {J} \ ,$

with g the Lande g-factor or spectroscopic splitting factor:

$g={\frac {3}{2}}+{\frac {S(S+1)-L(L+1)}{2J(J+1)}}\ .$