Size Effects of Magneto-Structural Correlation in Mn₃O₄ Nanoparticles

We synthesized spinel manganese oxide Mn3O4 nanoparticles with sizes ranging from 7 to 25 nm and investigated their crystal structure and magnetic properties through X-ray diffraction, magnetization, and electron spin resonance (ESR) measurements. As the particle size decreased, the lattice constant a increased monotonically, while the lattice constant c and tetragonal distortion of the unit cell reached a minimum at 11 nm and then increased rapidly below 9 nm. The ESR spectra of the nanoparticles differed from those of the bulk crystal, and the nanoparticles exhibited a hyperfine structure of Mn2+ ions at the nanoparticle surface. The size dependence of the line width of the ESR spectrum of the nanoparticles was similar to that of the tetragonal distortion. The magnetic transition temperatures and coercive field of the nanoparticles decreased rapidly below 10 nm. The Mn3O4 nanoparticles maintained three magnetic transition temperatures similar to those of bulk crystals above 9 nm. However, the nanoparticles exhibited only two distinct transition temperatures below 9 nm. Distortion of the crystal structure leads to a change in the magnetic interactions and magnetic state. It is suggested that the change of a phase boundary appears to exist at approximately 9 nm in the phase diagram of the magnetic properties as a function of the particle size.