Experimental evidence is presented to support predictions made by an analytical model describing the temperature-dependent behavior of an assembly of noninteracting magnetic nanoparticles with uniaxial anisotropy under an external field. When the applied field is smaller than the anisotropy field of the particles and is oriented perpendicular to the easy axis, a maximum of the magnetization occurs at finite temperature. The theory shows good agreement with measurements of an array of CoCrPt nanoislands with uniaxial anisotropy. Deviations are discussed taking into account the thermal dependencies of the saturation magnetization and the anisotropy of the magnetic material.