Optical and magnetic properties in epitaxial GdN thin films

We studied the optical and magnetic properties in epitaxial AlN/GdN/AlN double heterostructures grown using reactive sputtering under ultrapure conditions. The indirect and direct optical transitions in a 95-nm-thick GdN film were found to be 0.95 and 1.18 eV, respectively. The considerable size effects of the optical band gap were observed with a decrease in the GdN thickness. The AlN/GdN/AlN double heterostructures clearly exhibited ferromagnetic states at low temperature. The short-range correlation of spins began to develop below ∼60 K, and long-range ordering that obeyed the Arrott relation was confirmed below ∼30 K. Because of the film-size independence for the ferromagnetic ordering, the cooperative correlation length in GdN was considered to be shorter than 30 nm. Furthermore, we found that the band gap is dramatically reduced with the ferromagnetic spin ordering.

Figure 1

(a) SEM cross-sectional image of an AlN/GdN (30 nm)/AlN double heterostructure. (b) AFM image in 5 × 5 μm${}^2$ of the film.

Figure 2

(a) X-ray diffraction spectra for AlN/GdN/AlN double heterostructures with 30- and 95-nm-thick GdN. (b) X-ray pole figures of in and out planes of AlN and GdN.

Figure 3

Radial structure functions around Gd ions in 95-nm-thick GdN.

Figure 4

A typical depth profile of the secondary-ion intensity obtained for an AlN/GdN multilayered structure. The dashed line indicates a level of 1.0 × 10${}^{20}$ cm${}^{-3}$ of the oxygen content in AlN.

Figure 5

(a) Absorption spectra of double heterostructures with 95-, 59-, and 30-nm-thick GdN. The inset is a transmitted optical photomicrograph of the AlN/GdN (95 nm)/AlN heterostructure. (b) Comparison of an absorption spectrum of 30-nm-thick GdN and a calculated spectrum.

Figure 6

(a) Tauc plots (αE)${}^{1/2}$ for heterostructures with 95- and 30-nm-thick GdN. Dashed lines indicate linear dependences for (αE)${}^{1/2}$. The inset is an analysis of the indirect band edge for 95-nm-thick GdN, and the dashed line indicates a linear dependence of α${}^{1/2}$. (b) Direct transition energies estimated by the Tauc plot as a function of the GdN thickness. The dashed line indicates a fitting curve representing a $d^{-2}$ dependence.

Figure 7

Magnetization of 95-nm-thick GdN at $B$ $=$2.0 mT is shown as a function of temperature. Magnetic field was applied in the plane. A broken line represents a fitting curve. The inset shows the magnetization of 95-nm-thick GdN at 4.2 K.

Figure 8

Arrott plot of $M^2$ versus $B$/$M$ for 95-nm-thick GdN at various temperatures. Magnetic field was applied in the plane. Broken lines indicate extrapolations of experimental data.

Figure 9

Temperature dependence of the direct band-gap energy of 95-nm-thick GdN. Δ$E_{\mathrm{g}}$ corresponds to the change in the band-gap energy versus the room-temperature value.