Abstract: In fields such as cosmic radiation detection, national security, counterterrorism, environmental protection, and nuclear medicine, the accurate identification of radionuclides requires high-resolution measurement of γ-ray spectra with closely spaced energy lines. The γ-rays of interest typically fall within the low-energy range of 50-200 keV, where traditional measurement methods often fail to resolve adjacent peaks due to limited resolution. The metallic magnetic calorimeter (MMC) technology, based on superconducting quantum interference devices (SQUID), leverages the significant change in the low-temperature magnetization of a paramagnetic thin film with temperature to achieve high-resolution determination of low-energy γ-ray spectra. For γ-rays around 100 keV, the energy resolution is better than 0.1%, which is ten times better than that of the current state-of-the-art high-purity germanium (HPGe) γ-spectrometers. The core component of the MMC is the paramagnetic metallic composite layer, which consists of an Au film material doped with Er atoms, with a thickness of approximately 2 000 nm. This layer generates a magnetic response from thermal stimuli. The concentration of Er is crucial for proper operation of MMC spectrometer; too low a concentration results in a magnetic signal that is too weak to detect, while too high a concentration not only increases signal noise proportionally but also causes the loss of paramagnetic properties due to magnetic interactions. The concentration of Er is optimally controlled at the level of hundreds to thousands of parts per million (ppm) for effective MMC signal detection. However, due to the low doping concentration of Er at the ppm level, precise control becomes a critical issue in the preparation of Au∶Er composite materials. This paper developed a doped sheet method based on magnetron sputtering to address the quantitative control of low-concentration Er doping in Au∶Er composite materials. The principle of the doped sheet method involved attaching an Er metal sheet to a gold target and adjusting the doping concentration of Er by controlling the size and position of the Er sheet. Based on the etching depth of the Er target during magnetron sputtering, a relationship between the position of the Er doped sheet and the doping concentration was established, enabling precise control of Er doping concentrations ranging from hundreds to thousands of ppm. Based on the doped sheet method, the Au∶Er composite material samples with different Er concentrations were prepared, and the magnetization of the samples was measured. The Au∶Er composite material samples show obvious paramagnetic properties at extremely low temperatures. The results were compared, and the performance optimization direction of the sensor layer was proposed. This method not only enhances the detection efficiency of MMC but also effectively reduces the material and time costs associated with experimental trials, significantly improving the efficiency of performance optimization. The magnetron sputtering based doped sheet method not only provides an efficient way for the performance optimization of metal magnetocaloric technology, but also provides a reliable technical reference for the preparation of other high-precision doped films.