Temperature dependent low-field measurements of the magnetocaloric ΔT with sub-mK resolution in small volume and thin film samples

We present temperature dependent ΔT measurements of the magnetocaloric effect in a thin film sample of Gd, employing magnetomodulation and detection of thermal radiation. A bulk sample of the metamagnetic material LaFe_11.05Co_0.91Si_1.04 shows a strong broadening of the ΔT peak for increasing field amplitudes between 4 and 45 mT. Bulk Gd in comparison shows only a weak broadening. All investigated samples exhibit a clear quadratic dependence of ΔT on the external field H_ext at the ΔT peak maximum, contrary to earlier predictions. An analytic expression is derived that interpolates between the $Hext2$-behavior at low and the well-known $Hext2/3$-behavior at high fields.