The temperature dependence of the surface magnetization $M_S\mathrm{}\left(T\right)\mathrm{}$ in the spin-wave regime is obtained from measurement of the spin polarization of the low-energy cascade electrons. With the $T^{\frac{3}{2}}$ law, the ratio $k$ of the mean spin deviation in surface and bulk is determined, where $k$ reflects the perpendicular exchange interaction $J\perp$. In the ferromagnetic glass FeNi${\mathrm{B}}_{0.5}$, $J\perp$ depends critically on the chemistry of the first layer and is weakened even with a bulklike surface composition.

• Received 21 April 1988

DOI:https://doi.org/10.1103/PhysRevLett.61.758