Paramagnetic resonance of Fe$^{3+}$ and Cu$^{2+}$ centers in yttrium orthosilicate doped with iron

Two Fe$^{3+}$ centers, as well as the spectrum of an uncontrolled Cu$^{2+}$ impurity, were studied using paramagnetic resonance in a specially grown Y$_2$SiO$_5$ : Fe crystal. The parameters of spin Hamiltonians are determined both in laboratory and local (with diagonal second-rank fine structure tensor or hyperfine interaction tensor $\mathbf{A}$) coordinate systems. The localization of Fe$^{3+}$, Cr$^{3+}$, Gd$^{3+}$ and Cu$^{2+}$ centers in yttrium and scandium silicates is discussed.