Ferrimagnetism and spontaneous ordering of transition metals in double perovskite La${}_2$CrFeO${}_6$ films

We report on the atomic ordering of B-site transition metals and magnetic properties of epitaxial La${}_2$CrFeO${}_6$ double-perovskite films grown by pulsed-laser deposition under various conditions. The highest-ordered sample exhibited a fraction of antisite disorder of only 0.05 and a saturation magnetization of ∼2 ${\mu }_B$ per formula unit at 5 K. The result is consistent with the antiferromagnetic ordering of local spin moment (3$d^3$${}_{\downarrow }$3$d^5$${}_{\uparrow }$; $S$ = −3/2 + 5/2 = 1). Therefore, the magnetic ground state of La${}_2$CrFeO${}_6$ double perovskite that has been long debated is unambiguously revealed to be ferrimagnetic. Our results present a wide opportunity to explore novel magnetic properties of binary transition-metal perovskites upon epitaxial stabilization of the ordered phase.

Figure 1

(a) Epitaxial growth conditions of LCFO films mapped in a $T_g$-$P_{\mathrm{O}2}$ diagram. Circles with different sizes and colors represent the growth conditions, where degree of order determined by XRD are >80% (large and red), 60 ∼ 80% (middle and orange), 40 ∼ 60% (small and green) and <40% (the smallest and blue). Solid lines and shaded regions indicate phase equilibrium of Fe-O binary system and stable conditions for coexistence of Cr${}_2$O${}_3$ and Fe, FeO, Fe${}_3$O${}_4$, or Fe${}_2$O${}_3$ (ordered from lower left to upper right), respectively. (b) Contour mapping of the FWHM of rocking curves for the LCFO (1 1 1) reflection. (c) and (d) RHEED intensity oscillations for the specular beam during the initial growth of LCFO films on (111) STO substrates under conditions A and D, respectively. The insets depict AFM images of each film. The range of color code corresponds to ∼3 nm in height.

Figure 2

Schematic crystal-structure model of LCFO. In this model, the atomic displacement of La and O are indicated by ${\delta }_{\mathrm{La}}$ and ${\delta }_{\mathrm{O}}$. $AS$ is the degree of antisite disorder.

Figure 3

(a)–(c) X-ray diffraction profiles of the samples grown under conditions A, B, and D, respectively. The experimentally observed intensity and calculated one are indicated by circles and bars, respectively. (d) X-ray rocking curves of LCFO (1 1 1) and $\left(\frac{1}{2}\frac{1}{2}\frac{1}{2}\right)$ reflections taken for a sample grown under condition A. (e) X-ray reciprocal space mapping around STO (3 3 0) reflection taken for a sample grown under condition A. Double reflections are due to CuK${\alpha }_1$ and CuK${\alpha }_2$ radiations. (f) Distance between the nearest-neighbor Fe and Cr ions as a function of antisite-disorder ($AS$) fraction. SLs and bulk are referred as to values reported for artificial superlattices[10, 13] and a powder sample with $AS$ = 0.5.[16]

Figure 4

(a) HAADF-STEM image observed along a substrate [110] zone axis for a sample grown under condition B. Circles highlight positions of La (bright spots), Fe/Cr (weak spots), and O (background), respectively. Horizontal arrow indicates position of the interface between LCFO film and STO substrate. Selected area diffraction patterns of (b) STO and (c) LCFO. The presence of (h h h) and (h h k) diffraction spots with half-integer $h$ and $k$ in the latter indicates the NaCl-type ordering of B-site cations.

Figure 5

(a) Magnetization hysteresis curves taken at 5 K for samples grown under conditions A to D and annealed at 400 °C. (b) The temperature dependence of field-cooled magnetization for the same samples taken during warming under 0.1 T. Inset shows the temperature dependence of inverse magnetization. Solid lines are linear fits to the plots above $T_C$. (c) Saturation magnetization ($M_s$ = $M$ at 5 K under 1 T) for as-grown and annealed (400 °C) samples as a function of the antisite-disorder fraction. Inset depicts the annealing temperature ($T_{\mathrm{A}}$) dependence of $M_s$ for sample A. The error bars reflect inaccuracy of the sample volume.