Intrinsic versus extrinsic ferromagnetism in ${\mathrm{HfO}}_{2-x}$ and ${\mathrm{Ni}:\mathrm{HfO}}_{2-x}$ thin films

We have investigated the possible evolution of an intrinsic stable ferromagnetic moment in oxygen deficient undoped and magnetically doped ${\mathrm{HfO}}_{2-x}$ thin films grown by reactive molecular beam epitaxy. Neither oxygen vacancies nor substituted Ni in combination with such vacancies results in an observable magnetic moment for a broad range of oxygen vacancy concentrations. By combining integral and element specific magnetization measurements, we show that a fluctuating deposition rate of the magnetic dopant induces extrinsic ferromagnetism by promoting the formation of metallic clusters. We suggest the element specific measurement of an induced magnetic moment at the nonmagnetic site as a proof of intrinsic ferromagnetism in diluted magnetic semiconductors.

Figure 1

Normalized XANES (left-hand axis) and XMCD (right-hand axis) spectra at Hf $L_{2,3}$ edges of a typical ${\mathrm{HfO}}_{2-x}$ thin film recorded at 300 K and under an applied magnetic field of 6 T. The large peak in the XMCD signal is a diffraction peak.

Figure 2

X-ray diffraction pattern of a hafnium oxide thin film doped with nominally 8 at.% Ni on $c$-cut sapphire. (Inset) Variation of $d$($\overline{1}11$) with increasing nominal dopant concentration. The line is a guide to the eye.

Figure 3

Magnetic moment vs applied field with magnetic field applied parallel to the film plane for (a) a 27-nm hafnium oxide thin film doped with 8 at.% Ni grown with stable Ni rate (diamagnetic background of ${\mathrm{Al}}_2{\mathrm{O}}_3$ has not been substracted), and (b) a 50-nm hafnium oxide thin film doped with 8 at.% Ni grown under fluctuating rate (diamagnetic contribution of ${\mathrm{Al}}_2{\mathrm{O}}_3$ has been subtracted).

Figure 4

Ni $K$-edge XANES spectra (red and blue diamonds) for as-prepared 8 at.% Ni-doped hafnium oxide thin film grown under fluctuating rate. XMCD intensity (black curve) for the as-prepared sample measured at 300 K and 6 T.

Figure 5

XANES of the $L_{2,3}$ edges of Hf for hafnium oxide thin films grown with fluctuating Ni source, as-prepared (red) and annealed (blue) measured at 300 K and 6 T. The inset shows the crystal structure of doped ${\mathrm{HfO}}_2$.