High‐Quality Magnetically Hard ε‐Fe₂O₃ Thin Films through Atomic Layer Deposition for Room‐Temperature Applications

The critical-element-free ε-Fe ₂O ₃ ferrimagnet exhibits giant magnetic coercivity even at room temperature. It is thus highly attractive material for advanced applications in fields such as spintronics, high-density data storage, and wireless communication. However, a serious obstacle to overcome is the notoriously challenging synthesis of ε-Fe ₂O ₃ due to its metastable nature. Atomic layer deposition (ALD) is the state-of-the-art thin-film technology in microelectronics. Herein, it is demonstrated that it has also true potential for the fabrication of amazingly stable in situ crystalline and high-performance ε-Fe ₂O ₃ thin films from simple (FeCl ₃ and H ₂O) chemical precursors at a moderately low deposition temperature (280 °C). Standard X-ray diffraction and Fourier transfer infrared spectroscopy characterization indicates that the films are of high level of phase purity. Most importantly, precise temperature-dependent ⁵⁷Fe Mössbauer spectroscopy measurements verify that the hematite (α-Fe ₂O ₃) trace in the films is below 2.5%, and reveal the characteristic low- and high-temperature transitions at 208–228 K and ≈480 K, respectively, while magnetization measurements confirm the symmetric hysteresis loops expected for essentially phase-pure ε-Fe ₂O ₃ films. Excitingly, the highly c-axis oriented film growth, the overall film quality, and the unique magnetic properties remain the same, independently of the substrate material used.