From Ferromagnet to Antiferromagnet: Dimensional Crossover in (111) SrRuO3 Ultrathin Films

Abstract

SrRuO3 is a canonical itinerant ferromagnet, yet its properties in the extreme two-dimensional limit on a (111) crystal plane remain largely unexplored. Here, we demonstrate a complete transformation of its ground state driven by dimensional reduction. As the thickness of (111)-oriented SrRuO3 films is reduced to a few unit cells, the system transitions from a metallic ferromagnet to a semiconducting antiferromagnet. This emergent antiferromagnetism is evidenced by a vanishing magnetic remanence and most strikingly, by the appearance of an unconventional twelve-fold anisotropic magnetoresistance. First-principles calculations confirm that an A-type antiferromagnetic order is the stable ground state in the ultrathin limit. Our findings establish (111) dimensional engineering as a powerful route to manipulate correlated electron states and uncover novel functionalities for antiferromagnetic spintronics.

Figures (4)

• Figure 1: Growth and structural characterization of (111) SrRuO$_3$ ultrathin films. (a) Special crystallographic directions within the (111) plane denoted in angle positions, with 0$^{\circ}$ defined as the [$\bar{1}$10] direction. (b) In-situ RHEED patterns of SrTiO$_3$ substrate and 6 u.c. SrRuO$_3$ ultrathin films. The electron beam is incident along the [1$\bar{1}$0] direction. (c) AFM graph on the surface morphology of 6 u.c. SrRuO$_3$ on Ti4+-terminated SrTiO$_3$(111) substrate. The height of each step identified by line profile analysis is about 2.27 Å. (d) Cross-sectional TEM image of the 6 u.c. SrRuO$_3$ projected along the [11$\bar{2}$] crystallographic direction. The inset shows the corresponding diffractogram of the SrTiO$_3$ substrate.
• Figure 2: Electrical and magnetic properties of (111) SrRuO$_3$ films. (a) Temperature-dependent resistivity curves of SrRuO$_3$ films (6 - 75 u.c.). Inset: First derivative $d\rho / dT$ with the kink temperatures marked by red triangles. (b) Temperature-dependent magnetization curves of both 6 and 75 u.c. SrRuO$_3$ after field cooling (FC) under 2000 Oe magnetic field for both IP and OOP orientations. (c) Isothermal magnetization hysteresis loops at 4 K for IP and OOP field orientations.
• Figure 3: In-plane AMR of both 75 and 6 u.c. SrRuO$_3$ with Fourier analysis and Stoner-Wohlfarth modelling. (a) AMR data of the 75 u.c. SrRuO$_3$ at 4 K and 5 T magnetic field. The peak positions are indicated in red. (b) AMR data of the 6 u.c. SrRuO$_3$ at 4 K and 5 T magnetic field. The peak positions are indicated in red. (c) Amplitude of each harmonics $C_{2n}$ normalized to $C_2$ extracted from Fourier analysis. (d) Amplitude of each harmonics $C_{2n}$ normalized to $C_2$ extracted from Fourier analysis. The enhanced magnitude of the twelve-fold $C_{12}$ is highlighted in orange. (e) Stoner Wohlfarth modeling on buckled honeycomb lattice for collinear $\mathbf{H}\parallel\mathbf{M}$ configuration. (f) Stoner Wohlfarth modeling on buckled honeycomb lattice for nonparallel $\mathbf{H}$--$\mathbf{M}$ configuration with a small misalignment angle $\Delta\theta\neq 0$.
• Figure 4: The structural and electronic properties of SrTiO$_3$-SrRuO$_3$ (111) directional superlattice based on GGA+U method. (a) The (111) directional superlattice of STO-SRO with layer ratio equal to 5:1. (b) The calculated energies of AFM-A type and FM magnetic configuration along with U value from 3 to 4 eV. The turning point for AFM-A as the ground state appears around 3.7 eV. The band structures of (c) AFM-A type with spin along the IP and (d) FM with spin along c axis. With the varied magnetic state, the conductivity changes from the metallic state of FM to the insulating result of AFM. The corresponding projected density of states for (e) AFM-A type and (f) FM configuration.