On the Cubic Phase Stability and Magnetic Properties of Cu-doped ZrO$_{2}$

Magnetic properties and room temperature cubic-phase stability of Cu-doped ZrO$_{2}$ nano-crystallites (16 nm size) with various compositions of Zr$_{1-x}$Cu$_{x}$O$_{2-x}$ (0.01$\le $x$\le $0.25) are reported. The samples were synthesized at a constant pH $=$ 8 using zirconyl nitrate hydrate and copper acetate monohydrate as precursors and ethanol as a solvent. Thermal analysis shows that the cubic phase is not stable beyond calcination temperature of 500$^{\circ}$C for 8 hours in air and a critical composition x$_{c} \approx $ 0.10. For x \textgreater x$_{c}$ , monoclinic ZrO$_{2}$ and CuO emerge as secondary phases with a shrinking unit-cell volume for increasing Cu content. Against expectations [1], the temperature and magnetic field dependence of magnetization exhibit no signatures of ferromagnetism down to 2 K. Instead, temperature dependence of magnetic susceptibility for all compositions yields Curie-law variation with a magnetic moment $\mu \approx 1.3\mu_{B} $ per Cu$^{2+}$, which is somewhat smaller than the expected value of $\mu \approx 1.9\mu_{B} $ per Cu$^{2+}$. Electron magnetic resonance studies show a signal near g $\approx $ 2.1 due to Cu$^{2+}$ substituting at Zr$^{4+}$ sites with four-line hyperfine splitting from the nuclear spin I $=$ 3/2 of Cu.\\[4pt] [1]. S. Ostanin et al, Phys. Rev. Lett. 98, 016101,(2007)