High-entropy oxides: An emerging prospect for magnetic rare-earth transition metal perovskites

It has been shown that oxide ceramics containing multiple transition and/or rare-earth elements in equimolar ratios have a strong tendency to crystallize in simple single-phase structures, stabilized by the high configurational entropy. In analogy to the metallic alloy systems, these oxides are denoted high-entropy oxides (HEOs). The HEO concept allows to access hitherto uncharted areas in the multielement phase diagram. Among the already realized structures there is the highly complex class of rare-earth transition element perovskites. This fascinating class of materials generated by applying the innovative concept of high-entropy stabilization provides a new and manyfold research space with promise of discoveries of unprecedented properties and phenomena. The present study provides a first investigation of the magnetic properties of selected compounds of this novel class of materials. Comprehensive studies by DC and AC magnetometry are combined with element specific spectroscopy in order to understand the interplay between magnetic exchange and the high degree of chemical disorder in the systems. We observe a predominant antiferromagnetic behavior in the single-phase materials, combined with a small ferromagnetic contribution. The latter can be attributed to either small ferromagnetic clusters or configurations in the antiferromagnetic matrix or a possible spin canting. In the long term perspective it is proposed to screen the properties of this family of compounds with high throughput methods, including combined experimental and theoretical approaches.

Figure 1

Upper part: Crystal structure of a representative orthorhombic ($Pbnm$) PE-HEO, $\left(5A_{0.2}\right)\left(5B_{0.2}\right){\mathrm{O}}_3$. Lower part: Illustration of the increasing magnitude of tilting of the $B{\mathrm{O}}_6$ polyhedra observed in PE-HEOs along the [001] axis with decreasing tolerance factor (larger deviation from a an ideal lattice with tolerance $\mathrm{factor}=1$) for $\mathrm{La}\left(5B_{0.2}\right){\mathrm{O}}_3$, $\left(5A_{0.2}\right)\left(5B_{0.2}\right){\mathrm{O}}_3$, and $\mathrm{Gd}\left(5B_{0.2}\right){\mathrm{O}}_3$.

Figure 2

(a) Temperature dependent magnetization after ZFC and in FC mode; the right-hand ordinate refers to the inverse magnetization. (b) Magnetization as function of the magnetic field ${\mu }_{0}H$ of $\mathrm{La}\left(5B_{0.2}\right){\mathrm{O}}_3$ at $T=10\mathrm{K}$, after FC in ${\mu }_0{H}_{\mathrm{FC}}=\pm 5\mathrm{T}$. The inset shows the region around the center of the coordinate system; the same axis labels apply.

Figure 3

In comparison to the AC magnetizations $m^{\prime }$ and $m^{″}$, the normalized DC magnetization as function of temperature is shown.

Figure 4

Left column: Mössbauer spectra in zero magnetic field as function of temperature, represented with two sextets: one broad spectrum (green) representing dynamic fluctuating spins (on the characteristic timescale of the measurement) and one well defined subspectrum (blue) from static magnetic order. Right column: A tentative sketch of the evolution of the proposed magnetic structure. At high temperatures spins are dynamically fluctuating (PM, indicated by circles), with decreasing temperature spins start to couple FM and AFM (blue areas), followed by more and more AFM coupling areas. At low temperatures five different kinds of spins are coupled predominantly antiferromagnetically, but one pair of mixed spins couples ferromagnetically (center). This leads to a local FM cluster which is coupled to the surrounding AFM matrix. Naturally not all exchange interactions can be satisfied, resulting in magnetic frustration and spin canting.

Figure 5

Magnetization of $A\left(5B_{0.2}\right){\mathrm{O}}_3$ as a function of temperature, measured after zero-field cooling and field cooling in ${\mu }_{0}H=10\mathrm{mT}$. The curves are grouped with respect to their magnitude.

Figure 6

Magnetic transition temperature of the samples as function of the tolerance factor of the structures and the RE element. The line serves as a guide to the eye.