Hematite Thin Films Grown on Z-Cut and Y-Cut Lithium Niobate Piezoelectric Substrates by Pulsed Laser Deposition

Abstract

Altermagnets are a newly identified class of materials that combine advantageous characteristics of both ferro- and antiferromagnets, making them highly promising for spintronic applications. Hematite has recently been identified as an altermagnetic material and exhibits several noteworthy properties, including a high Néel temperature, a temperature dependent spin reorientation transition (SRT) at the Morin temperature ($T_\mathrm{M}$), and low magnetic damping. In this work, we demonstrate the epitaxial growth of hematite thin films on y- and z-cut lithium niobate (LiNbO$_3$) substrates using pulsed laser deposition (PLD). LiNbO$_3$ as piezoelectric substrate is of particular interest as it enables the efficient excitation of surface acoustic waves (SAWs) with interdigital transducers. The different substrate cuts allow for different orientations of the Néel vector. Films grown on y-cut LiNbO3 are single-crystalline and single-phase, while those deposited on z-cut LiNbO$_3$ exhibit two distinct in-plane (ip) domains rotated 60° relative to each other. On both substrates, the hematite thin films exhibit a temperature dependent SRT which allows the antiferromagnetic Néel vector to be controlled. This study paves the way for the development of high-quality piezoelectric/altermagnetic hyprids for magnonics and spintronics.

Figures (13)

• Figure 1: XRD pattern of hematite films deposited at different temperatures on z-cut LiNbO$_3$ with an oxygen partial pressure of 2×$10^{-4}\,$mbar. a) Overview and b) enlargement around the (0006) substrate peak.
• Figure 2: XRD pattern of hematite films deposited at different temperatures on y-cut LiNbO$_3$ with an oxygen partial pressure of 2×$10^{-4}\,$mbar. a) Overview and b) enlargement of the (3$\bar{3}$00) peak.
• Figure 3: $\phi$-scans for hematite grown on a) z-cut and b) y-cut LiNbO$_3$ at $575\,$°C, 2×$10^{-4}\,$mbar. For the z-cut LiNbO$_3$, three substrate reflections, corresponding to the (20$\bar{2}$10) reflection are observed. For hematite six peaks are observed, spaced $60\,$° apart. For the thin film on y-cut LiNbO$_3$$\phi$-scans around the (30$\bar{3}$0) reflection were performed. Both, film and substrate, showing only two reflections that are perfectly aligned.
• Figure 4: Images of the microstructure of hematite thin films, grown at $475\,$°C and an O$_2$ pressure of 2×$10^{-4}\,$mbar, on a) z-cut and c) y-cut LiNbO$_3$ using a FSD. The orange rectangles in a) and b) are guidance for the eye to see the correlation between FSD and EBSD image. Panels b) and d) show the corresponding IPF coloring along an ip direction for hematite on z-cut and y-cut LiNbO$_3$, respectively. Gray pixels correspond to areas where no hematite was identified either no Kikuchi patterns were detected or they could not be assigned to hematite.
• Figure 5: $M$ vs $T$ measurements along the ip and oop directions of hematite films grown at $575\,$°C with an oxygen partial pressure of 2×$10^{-3}\,$mbar on a) z-cut and b) y-cut LiNbO$_3$ substrates. Hematite films on y-cut and z-cut LiNbO$_3$ were 62 and $61\,$nm thick, respectively. Samples were magnetized before the measurement and the magnetization versus temperature was measured with no guiding field during cooling. c) Schematic spin alignment of hematite grown on z-cut and y-cut LiNbO$_3$ substrates above and below the Morin temperature. The arrows in the hexagonal unit cell indicating directions of the spins for the different substrate cuts and measurements configurations above and below $T_\mathrm{M}$. $H$ is the applied saturation magnetic field before the measurement, aligned either ip or oop, and $m$ is the small net moment arising from spin canting.