Concentration patterns of the magnetic suspension in flat layers under
the action of the normal magnetic field are considered theoretically.
From the balance of the demagnetized field energy and surface energy
critical magnetic Bond numbers for instability of the magnetized strip
e with respect to its branching and division are obtained. Energy of t
he stripe with multiple branchings is calculated and on that basis cro
ssover behaviour of the concentration structure period in dependence o
n the layer thickness is described.