Synthesis of GdFeO$_{3}$ nanoparticles via low-temperature reverse co-precipitation: the effect of strong agglomeration on the magnetic behavior

Gadolinium orthoferrite (GdFeO$_{3}$) seems to have potential as a dual-modal contrast agent for magnetic resonance imaging (MRI), thus its preparation in the form of ultrafine superparamagnetic nanoparticles is currently of great interest. In this work, nanocrystalline GdFeO$_{3}$ was successfully synthesized by the heat treatment (750$^{\circ}$C, 4 h) of gadolinium and iron(III) hydroxides reversely co-precipitated at low temperature (0$^{\circ}$C). Initial and resulting powders were analyzed by EDX, SEM, PXRD, Mössbauer spectroscopy, vibration magnetometry, etc. Gadolinium orthoferrite was formed as isometric nanocrystals with an average size of 23$\pm$3 nm, which were strongly agglomerated into clusters of about 200 nm in diameter. It was shown that the individual GdFeO$_{3}$ nanocrystals are superparamagnetic, but in the cluster form, they exhibit a collective weak ferromagnetic behavior. After ultrasonic-assisted disintegration of GdFeO$_{3}$ to a colloidal solution form, these clusters remained stable due to their strong agglomeration and low zeta potential value of 1 mV. Thus, it is concluded that the further use of the synthesized GdFeO$_{3}$ nanoparticles as a basis of MRI contrast agents will be possible only after the suppression of their clustering.