Negative-pressure-induced helimagnetism in ferromagnetic cubic perovskites ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x{\mathrm{CoO}}_3$

Helimagnetic materials are identified as promising for novel spintronic applications. Since helical spin order is manifested as a compromise of competing magnetic exchange interactions, its emergence is limited by unique constraints imposed by the crystalline lattice and the interaction geometries as exemplified by the multiferroic perovskite manganites with large orthorhombic distortion. Here we show that a simple cubic perovskite ${\mathrm{SrCoO}}_3$ with room-temperature ferromagnetism has the potential to host helimagnetic order upon isotropic lattice expansion. Increasing the Ba content $x$ in ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x{\mathrm{CoO}}_3$ continuously expands the cubic lattice, eventually suppressing the ferromagnetic order near $x=0.4$ where helimagnetic correlations are observed as incommensurate diffuse magnetic scattering by neutron-diffraction measurements. The emergence of helimagnetism is semiquantitatively reproduced by first-principles calculations, leading to the conjecture that a simple cubic lattice with strong $d\text{−}p$ hybridization can exhibit a variety of novel magnetic phases originating from competing exchange interactions.

Figure 1

$x$ dependence of the (a) cubic lattice constant $a$ at room temperature, (b) saturation magnetization $M_{\mathrm{S}}$ at the lowest temperature (2–4 K), and (c) electronic specific-heat coefficient $\gamma$ for ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x{\mathrm{CoO}}_3(0\le x\le 0.4)$ single crystals. (d) The magnetic phase diagram as functions of temperature $T$ and $x$ where the FM and HM transition temperatures are denoted by closed circles ($T_{\mathrm{C}}$) and closed triangles ($T_{\mathrm{M}}$), respectively. The Weiss temperatures $\theta$ are also plotted as open squares. The data for $x=0$ are from Refs. [20, 21].

Figure 2

(a) Temperature ($T$) profiles of magnetization ($M$) at 0.01 T for ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x{\mathrm{CoO}}_3(0.1\le x\le 0.4)$ single crystals. (b) Low-$T$ profiles of $M$ at 0.01 T for $x=0.4$ and 0.5 where the open circles denote the data in a warming run after zero-field cooling. (c) Field dependence of $M$ at the lowest temperature (2–4 K) for $x=0.1–0.4$. The profiles up to $\sim 40\mathrm{T}$ were measured with a pulsed magnet. The inset shows a schematic of the intermediate spin configuration of ${\mathrm{Co}}^{4+}$. The $d^6\underline{L}$ configuration ($\underline{L}$ denotes an oxygen ligand hole) is likely to be stabilized by strong hybridization between the Co $3d$ and the O $2p$ orbitals.

Figure 3

(a) Temperature profile of resistivity ($\rho$) at zero field for ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x{\mathrm{CoO}}_3(0.1\le x\le 0.4)$ single crystals. The closed triangles denote the ferromagnetic transition temperature $T_{\mathrm{C}}$. (b) Specific heat divided by temperature ($C/T$) versus $T^2$ for $0.1\le x\le 0.4$.

Figure 4

Reciprocal-space maps of neutron-diffraction intensities for $x=0.4$ around the (0 0 1) reflection on the ($hhl$) plane measured at (a) 50 K and at (b) and (c) 1.5 K. To clearly show the magnetic scattering intensity, the intensities in panels (a)–(c) are plotted after the subtraction of those at 150, 150, and 50 K, respectively. Line profiles of the intensity in panels (a)–(c) for cuts along the (d) $\langle 111\rangle$ and (e) $\langle 001\rangle$ directions. The cuts along the $\langle 111\rangle (\langle 001\rangle )$ directions were evaluated using the red (pink) boxes indicated in panels (a)–(c). Calculated total energy as a function of magnetic propagation vector (f) $\left[\delta \delta \delta \right]$ and (g) $\left[00\delta \right]$ for various lattice constants ($\AA$) where the energy is plotted with respect to the energy of the FM state $E(\delta =0)$.