Effect of Milling Time on the Blocking Temperature of Nanoparticles of Magnetocaloric Gd$_{\mathrm{5}}$Si$_{\mathrm{4}}$.

Extensive research has been done on giant magnetocaloric material Gd$_{\mathrm{5}}$(Si$_{\mathrm{x}}$Ge$_{\mathrm{1-x}})_{\mathrm{4}}$ to improve adiabatic temperature/isothermal entropy change. However, there have been only a few reports on fabrication of nanostructure/nanoparticles that can be used to tune various properties by changing the length scale. Recently we have reported fabrication of room temperature ferromagnetic nanoparticles of Gd$_{\mathrm{5}}$Si$_{\mathrm{4}}$ using high energy ball milling. These nanoparticles have potential applications in biomedical engineering such as better T$_{\mathrm{2}}$ MRI contrast agents and in hypothermia. Here we report the effect of milling time on the blocking temperature, micro-structure, crystal structure, and magnetic properties of these nanoparticles. Magnetization vs. temperature at an applied field of 100 Oe is measured for all the ball milled samples. Bulk Gd$_{\mathrm{5}}$Si$_{\mathrm{4\thinspace }}$has a transition temperature of $\approx $340 K. There are two phase transitions observed in the nanoparticles, one near 300 K corresponding to the Gd$_{\mathrm{5}}$Si$_{\mathrm{4}}$ phase and another between 75-150 K corresponding to Gd$_{\mathrm{5}}$Si$_{\mathrm{3}}$. Zero Field Cooling (ZFC) and Field Cooling (FC) were measured. The blocking temperatures for the nanoparticles increase with decrease in milling time.