Superparamagnetic and Perfect-Paramagnetic Zinc Ferrite Quantum Dots from Microwave-Assisted Tunable Synthesis

The properties of quantum dot (QD)-size material depend directly upon its unit cell structure. Spinel zinc ferrite QD powder is produced via a one-pot microwave-assisted hydrothermal synthesis for just 5 min. Varying initial pH values of the preparation sol from 6 to 12 enlarges the Zn/Fe atomic ratio (by ca. 10%), unit cell volume (by ca. 0.5%), particle size (3.5-4.5 nm), and degree of inversion. This leads to a change in the magnetic behavior of the QD-size zinc ferrite from a superparamagnetic to a perfect-paramagnetic type. This novel finding points that the significant changes in the inherent structural parameters of spinel ZnFe₂O₄ QDs (Zn/Fe ratio and degree of inversion) induced by the systematic pH change of the preparation sol are exclusively responsible for the observed unique magnetic behavior instead of mere QD (single domain) nanosizes.