where now at any point and at any time there is the constraint m x 2 ( r , t ) + m y 2 ( r , t ) + m z 2 ( r , t ) ≡ ≡ M 0 2 . Here M 0 is constant, the constant magnitude of the non-constant magnetization vector m → → ( x , y , z , t ) . As a consequence the vector m → → in eqn. (*) has been modified to a more complex entity m → → e f f . This leads, among other points, to the following fact: