If the magnetizing field on a ferromagnetic material is increased, its permeability

The magnetic permeability of a substance is defined as 

$\mu =\frac{B}{H}$

where $B$ is the established magnetic field inside the material,

$H$ is the applied external magnetic field.

When the external applied magnetic field $H$ is increased, the established magnetic field for a ferromagnetic material remains the same. Thus the magnetic permeability decreases.