We used ultrafast optical spectroscopy to study photoinduced spin relaxation in 10-100 nm thick La 0.7 Ca 0.3 MnO 3 films. The spin-lattice relaxation time displays a strong dependence on thickness below the Curie temperature. Our simulations show that the observed thickness-dependent relaxation results from much faster thermal decay through the substrate in thinner films that leads to artificially faster demagnetization. Furthermore, we provide an analytical approach to gain insight into the spin-lattice relaxation time for highly thermal dissipative films. Our study strongly suggests that careful consideration of the influence of transient thermal variations on photoinduced demagnetization is mandatory when incorporating absorbing thin magnetic films into heterostructures and devices.