Positron annihilation characterization of free volume in micro- and macro-modified Cu0.4Co0.4Ni0.4Mn1.8O4 ceramics - H. Klym; A. Ingram; O. Shpotyuk; I. Hadzaman; V. Solntsev; O. Hotra; A. I. Popov (Low Temperature Physics, 2016)