Epitaxial strain effect on transport properties in Ca${}_{2-x}$Sr${}_x$RuO${}_4$ thin films

We have grown Ca${}_{2-x}$Sr${}_x$RuO${}_4$ ($x$ $=$ 0, 0.1, 0.5, and 2) epitaxial thin films using a pulsed laser deposition method and characterized their structures and magnetotransport properties. We find that the $x$ $=$ 0, 0.1, and 0.5 films grown on LaAlO${}_3$ substrates exhibit coherent strain with tetragonal structure. The nature of strain is dependent on Sr content: the Ca${}_2$RuO${}_4$ ($x$ $=$ 0) film features biaxial compressive strain, while the $x$ $=$ 0.5 film shows biaxial tensile strain. The strain in the $x$ $=$ 0.1 film is relatively weak and strongly anisotropic, with compressive strain along the $a$ axis and tensile strain along the $b$ axis. In contrast, the Sr${}_2$RuO${}_4$ films show strain relaxation. The epitaxial strain effect leads the properties of the $x=0$, 0.1, and 0.5 films to be distinct from those of bulk materials. The bulk material shows antiferromagnetic Mott-insulating properties for $x$ < 0.2 and a nearly ferromagnetic state for $x$ ∼ 0.5 [Nakatsuji and Maeno, Phys. Rev. Lett. 84, 2666 (2000)], whereas the film displays itinerant ferromagnetism for $x$ $=$ 0 and 0.1 and paramagnetic metal for $x$ $=$ 0.5. Furthermore, in the $x$ $=$ 0 and 0.1 films, we observed distinct fourfold ferromagnetic anisotropy, with the minimum magnetoresistivity along the diagonal directions for $x$ $=$ 0 and $a$ and $b$ directions for $x$ $=$ 0.1. Such evolution of magnetic anisotropy may be associated with the tuning of the spin-orbit coupling by the epitaxial strain.

Figure 1

(a) HR-XRD 2θ-θ scan of 50-nm CSRO films with $x$ $=$ 0, 0.1, 0.5, and 2 on (001) LAO substrates. The stars indicate LAO reflection peaks. RSMs of HR-XRD around the LAO (103) peaks and the (119) peaks of (b) CRO with $x$ $=$ 0, (c) CSRO#1 with $x$ $=$ 0.1, and (d) CSRO#2 with $x$ $=$ 0.5. (e) Rocking curves around the (002) reflections of the films with $x$ $=$ 0, 0.1, 0.5, and 2 (the data has been shifted for clarity).

Figure 2

HR-XRD RSMs around (119) reflections of SRO films on (a) LAO, (b) LSAT, and (c) STO substrates.

Figure 3

FWHMs of the SRO (002) rocking curve, Δ${\theta }_{\mathrm{FWHM}}$, as a function of the lattice mismatch for SRO films on various substrates.

Figure 4

(a) In-plane resistivity ${\rho }_{ab}$ vs temperature for CRO, CSRO#1 ($x$ $=$ 0.1), and CSRO#2 ($x$ $=$ 0.5) films. The arrows indicate warming and cooling. (b) In-plane resistivity ${\rho }_{ab}$ vs temperature for SRO films grown on LAO, LSGO, NGO, LSAT, and STO substrates.

Figure 5

(a) Susceptibility vs temperature for CSRO#2, measured with a magnetic field of 1 kOe, along [110]${}_{\mathrm{CSRO}}$. These data are acquired by subtracting the susceptibility of the bare LAO substrate obtained by polishing off the CSRO film from the overall susceptibility data of the film and substrate. (b) In-plane resistivity ${\rho }_{ab}$ vs $T^2$ for CSRO#2.

Figure 6

In-plane angular dependence of normalized intraplanar magnetoresistance $\mathrm{AMR}=[{\rho }_{\mathrm{ab}}(T,H)-{\rho }_{\mathrm{ab}}(T,0)]/{\rho }_{\mathrm{ab}}(T,0)$ of (a) CRO, (b) CSRO#1 with $x$ $=$ 0.1, and (c) CSRO#2 with $x$ $=$ 0.5 thin films at various temperatures and at a fixed field of ${\mu }_{0}H$ $=$ 8 T.

Figure 7

Twofold squared weight $W_2$ and fourfold squared weight $W_4$ defined as $W_n\left(T\right)={\left|A_n\left(T\right)\right|}^2/{\sum }_n|A_n{\left(T\right)|}^2$, as a function of temperature for (a) CRO, (b) CSRO#1 with $x$ $=$ 0.1, and (c) CSRO#2 with $x$ $=$ 0.5.