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This paper was the first to present the results of studying the influence of structural defects on the physical properties of pre-synthesized ytterbium iron garnet (YbIG). The concentration and type of structural defects were controlled by mechanically activating the powders using Bridgman anvils. Using scanning electron microscopy and X-ray diffraction (XRD), it was revealed that the particle morphology, the unit cell parameters, mean size of the coherent scattering regions (D ) and microstrains ( Δ d / d ) vary in a wide range. Using optical spectroscopy, XRD and FTIR to study the dynamic properties of the crystal lattice, showing that the bandgap and force constants of dodecahedral-coordinated ytterbium (Yb–O)d and octahedral-coordinated iron (Fe–O)o change non-monotonically with decreasing crystallite size. By analyzing the dependence of the magnetic hysteresis loop M (H) on the crystallite size D , it was concluded that there was a critical crystallite size ( D crit = 75 nm) of YbIG, at which the coercive force H c has a maximum value. For the first time, changes in g-factor and the half-width Δ H of the electron paramagnetic resonance (EPR) spectra of nanoscale crystallites were found to be correlated with the parameters of the magnetic hysteresis loop M (H) . [ABSTRACT FROM AUTHOR]