Electron paramagnetic resonance studies of GdMnO$_3$ single crystal and thin film deposited onto a LaAlO$_3$ substrate

Electronic paramagnetic resonance (EPR) spectra of a GdMnO$_3$ single crystal and GdMnO$_3$/LaAlO$_3$ thin film have been measured at $X$- and $Q$-band frequencies in the temperature range from $4.2$ to $300$ K. It is found that the EPR spectrum of a GdMnO3 single crystal consists of only one broad exchange-narrowed line. Unusual magnetism is observed at the interface between the GdMnO$_3$ thin film and LaAlO$_3$ substrate, where it is possible to see the fine structure of the EPR spectrum for a Gd$^{3+}$ ion. The parameters characterizing the fine structure related to the Gd$^{3+}$ ion in the GdMnO$_3$ film deposited onto the LaAlO$_3$ substrate are determined.