Charge doping effects on magnetic properties of single-crystal ${\mathrm{La}}_{1\text{−}x}{\mathrm{Sr}}_x\left({\mathrm{Cr}}_{0.2}{\mathrm{Mn}}_{0.2}{\mathrm{Fe}}_{0.2}{\mathrm{Co}}_{0.2}{\mathrm{Ni}}_{0.2}\right){\mathrm{O}}_3$ $(0\le x\le 0.5)$ high-entropy perovskite oxides

The influence of hole doping on magnetic properties is mapped for the compositionally complex high-entropy oxide ${AB\mathrm{O}}_3$ perovskite ${\mathrm{La}}_{1\text{−}x}{\mathrm{Sr}}_x\left({\mathrm{Cr}}_{0.2}{\mathrm{Mn}}_{0.2}{\mathrm{Fe}}_{0.2}{\mathrm{Co}}_{0.2}{\mathrm{Ni}}_{0.2}\right){\mathrm{O}}_3$ $(0\le x\le 0.5)$. It is found that aliovalent A-site substitution is a viable means to manipulate the magnetically active B-site sublattice. A series of single-crystal films are synthesized and show a general trend toward stronger ferromagnetic response and a shift in magnetic anisotropy as the Sr concentration increases. Magnetometry demonstrates complex and nonuniform responses similar to rigid and uncoupled composites at intermediate dopings. This behavior points to the presence of locally inhomogeneous magnetic phase competition, where ferromagnetic and antiferromagnetic magnetic contributions create a frustrated matrix containing uncompensated spins at the boundaries between these regions. The observations are discussed in the context of known responses to hole doping in the less complex ternary $\mathrm{La}{T}_M{\mathrm{O}}_3$ $(T_M=\mathrm{Cr},\mathrm{Mn},\mathrm{Fe},\mathrm{Co},\mathrm{Ni})$ oxides, and they are found to be different from a simple sum of the doped parents. The results are summarized in a preliminary magnetic phase diagram.

Figure 1

X-ray diffraction and reflectivity studies for all films. (a) XRD showing the (0 0 2) peak demonstrating high crystalline quality of films. (b) XRR data for each of the films demonstrating low surface roughness. (c) RSM data showing films are fully strained to the substrate. (d) Evolution of the $c$-axis lattice parameter and corresponding lattice strain.

Figure 2

Field-cooled and zero-field-cooled magnetization as a function of temperature on warming under 1000 Oe field for all compositions. In all cases, magnetic transitions are consistent with a macroscopically coherent ordering and not the result of small pockets that correlate to parent phase transitions.

Figure 3

Examples of field-dependent magnetic responses for different Sr compositions at 10, 25, 100, and 250 K for field applied in-plane (a)–(d) and out-of-plane (e)–(h).

Figure 4

A comparison of in-plane magnetic coercivity with temperature shows a drop in coercivity with temperature for low Sr concentrations and an increase in coercivity for higher Sr concentrations.

Figure 5

Evolution of out-of-plane magnetization for Sr 0% (top) and Sr 10% (bottom) films shows that low concentrations of Sr can induce an uncoupled magnetic response.

Figure 6

The addition of hard and soft loops leading to exchange spring behavior (top) with a micromagnetic cartoon (bottom) representation of behavior of local competing magnetic responses along the experimental loop as field is swept from zero field (1) to high positive field (2–3) and then to intermediate negative field (4).

Figure 7

Proposed magnetic phase diagram for the LS5BO system. The temperatures are chosen based on the blocking temperature ($T_B$) of the FM component and the change in coercivity ($\mathrm{\Delta }{H}_c$) at low temperature.