Nitrogen-Vacancy-Mediated Magnetism in Sputtered GdN Thin Films

Abstract

Among rare-earth nitrides (RENs), gadolinium nitride (GdN) stands out as a promising material for spintronics owing to its distinctive combination of semiconducting behavior, strong exchange interactions, and intrinsically soft ferromagnetism. Its relatively high Curie temperature and large saturation magnetization make it an attractive candidate for device concepts such as non-volatile memory elements and spin-based transistors, motivating efforts toward low-cost, uniform, and compositionally controlled thin-film growth. In this work, we deposited GdN thin films on SiO2/AlN substrates using DC sputtering under reactive nitridation conditions, with thicknesses varying from 18 to 180 nm, and systematically investigated their structural and magnetic properties. The films exhibit soft ferromagnetic ordering, characterized by a coercive field of approximately 200 Oe and a Curie temperature (Tc) near 70 K. Structural analysis reveals lattice distortions and local strain associated with nitrogen-vacancy defects, whose concentration varies with film thickness. Our theoretical studies establish a direct correlation between the observed Raman modes of the GdN lattice and the reduced magnetization induced by nitrogen vacancies. These vacancies give rise to defect-mediated ferromagnetism, leading to a measurable enhancement of Tc from 68 K to 82 K across the studied thickness range. The observed magnetic behavior is well described by the bound magnetic polaron (BMP) model, confirming that nitrogen vacancies are key contributors to ferromagnetic ordering while preserving the soft-magnetic character intrinsic to GdN. This study underscores the pivotal role of defect engineering in optimizing GdN thin films for spintronics applications.

Figures (6)

• Figure 1: (a) The 2 theta ($\theta)$- omega ($\omega)$ scan of the X-ray diffraction (XRD) pattern of the G24 and G43 sample, where G24 shows non-growth and G43 shows growth of GdN thin of (111) plane film deposited on an AlN/SiO$_2$ substrate (indicates the effect of nitridation during thin film growth). (b) The $\omega$-scan of the GdN (111) reflection, fitted with a Gaussian function having a full width at half maximum (FWHM) of 0.12, confirmed the crystalline, textured thin films with an incursion of dislocation density of 2.84 $\times$ 109 cm2. (c) The cubic crystal lattice of the grown GdN thin film, analyzed using Crystal Diffract software and modeled by Crystal Maker software, exhibits a NaCl-type cubic structure, where Gd and N atoms are arranged in an ABAB... pattern, having (111) reflection, with a lattice spacing of 3.529 Å. (d) The X-ray photoelectron (XPS) depicts the valence band region, revealing the energy positions of the Gd 5p orbitals at 21 eV and 4f orbitals at 7.5 eV correspond to the multiplet final state of 4$f^6$ Gd$^{3+}$ ions, which are situated below the Fermi level, thereby confirming the successful growth of GdN thin films. (e) Cross-sectional transmission electron microscopy energy dispersive spectroscopy (TEM-EDS) mapping images shows the GdN growth in AlN/SiO$_2$ substrate (f) Atomic force microscopy (AFM) image of the deposited GdN thin film of 2 $\times$ 2 µm2 confirms smooth and homogeneous growth, with a roughness (R$_q$) of approximately 0.9 nm.
• Figure 2: (a) Raman spectra of a grown cubic GdN thin film at room temperature, exhibiting two distinct frequency modes at 235 and 435 cm$^{-1}$, corresponding to the TO (Γ) and LO (X) modes of GdN, respectively. (b) In the absence of a magnetic field, it is observed that the decrease in temperature causes the frequency mode of the GdN shoulder to broaden, resulting in the disappearance of the Raman mode at 12 K, attributed to lattice strain in the grown thin films. Conversely, with an applied magnetic field of 100 mT, the cubic symmetry of GdN is further broken, leading to an increased shoulder; however, it follows the same pattern of broadening with a reduction in temperature.
• Figure 3: (a) The computed phonon dispersion of GdN lattice within Brillouin zone (b) The Gd and N atom projected vibrational density of states per cm$^{-1}$.
• Figure 4: Temperature-dependent magnetization (MT) of the grown GdN thin films, measured under an applied magnetic field of 200 mT, exhibits a Curie temperature (T$_c$) of approximately 70 K. Inset depicts the total magnetization contribution, comprising of various magnetic contributions including paramagnetic (PM) and ferromagnetic (FM) phases, fitted by the combination of Curie $\left(\frac{C}{T} \right)$ and Curie-Weiss equation $\left(\frac{C}{T - \theta_P}\right)$, resulting in two GdN phases, I and II. These phases originated due to nitrogen vacancies that have similar lattice parameters and distinctive magnetic properties.
• Figure 5: Field-dependent magnetization (MH) of grown GdN thin films at 52 K, depicting a coercivity of 200 Oe and a magnetic moment of 151 emu/cc, confirms the soft magnetic characteristics of GdN. Inset illustrates the virgin MH curve with the influence of nitrogen vacancies with Gd$^{3+}$ ion interaction on the magnetic characteristics of GdN by forming bound magnetic polarons (BMP), resulting in high magnetizations value due to Paramagnetic (PM) contribution. The BMP function was fitted to forward magnetic hysteresis (MH) sweeps using the equation $M = M_0 L(x) + \chi_M H$, where the first term contributes to the effect of BMP density, and the second term represents the PM contribution.