[Paper Review] Long-range magnetic ordering in rocksalt-type high-entropy oxides

Marco Polo Jimenez-Segura, T. Takayama+5 more|arXiv (Cornell University)|Feb 5, 2019

High Entropy Alloys Studies4 citations

TL;DR

This study demonstrates long-range magnetic ordering in rocksalt-type high-entropy oxides, specifically (MgCoNiCuZn)O, despite significant cationic disorder. Using magnetic susceptibility and neutron diffraction, the authors identify a transition temperature of 120 K, revealing that magnetic order can emerge in highly disordered systems, with magnetic ground states tunable via chemical substitution between antiferromagnetic order and spin-glass behavior.

ABSTRACT

We report the magnetic properties of Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$O, a high-entropy oxide with rocksalt structure, and the influence of substitutions on these properties. From the magnetic susceptibility and neutron diffraction measurements, we found that this compound exhibits long-range magnetic order below 120 K despite the substantial structuraldisorder. The other rocksalt-type high-entropy oxides with various chemical substitutions were found to host either an antiferromagnetic order or spin-glass state depending on the amount of magnetic ions. The presence of magnetic order for such a disordered material potentially provide a route to explore novel magnetic properties and functions.

Motivation & Objective

• To investigate the magnetic properties of rocksalt-type high-entropy oxides (HEOx) with high configurational disorder.
• To determine whether long-range magnetic order can persist in such disordered systems, contrary to expectations of spin-glass behavior.
• To explore the influence of varying magnetic ion content and substitution on magnetic ground states.
• To establish a framework for tuning magnetic functionality in entropy-stabilized oxides for potential applications.

Proposed method

• Synthesis of a series of rocksalt-structured HEOx via solid-state reaction of binary oxides and carbonates, followed by high-temperature annealing and rapid quenching.
• X-ray diffraction to confirm single-phase rocksalt structure (Fm3̄m) and lattice parameter determination.
• Magnetic susceptibility measurements (ZFC/FC) to detect phase transitions and Curie-Weiss analysis to extract effective moments and Weiss temperatures.
• AC susceptibility measurements at frequencies from 100 Hz to 8 kHz to probe dynamics and identify spin-glass-like freezing transitions.
• Neutron diffraction to confirm long-range magnetic order and rule out short-range or spin-glass character in the prototype (MgCoNiCuZn)O.
• Systematic variation of cationic composition, including substitution of divalent ions with Li/Ga or Fe, to tune magnetic ion concentration and valence states.

Experimental results

Research questions

• RQ1 Can long-range magnetic order emerge in rocksalt-type high-entropy oxides with strong cationic disorder?
• RQ2 How does the concentration of magnetic ions (Co²⁺, Ni²⁺, Cu²⁺) influence the magnetic ground state in these disordered oxides?
• RQ3 What is the nature of the magnetic transition in (MgCoNiCuZn)O, and how does it compare to binary rocksalt oxides?
• RQ4 Can magnetic properties in HEOx be tuned via chemical substitution, and what are the resulting magnetic phases?
• RQ5 What role does cationic valence state variation (e.g., Fe³⁺) play in modifying magnetic exchange pathways and ground states?

Key findings

• The prototype high-entropy oxide (MgCoNiCuZn)O exhibits long-range antiferromagnetic order below 120 K, confirmed by neutron diffraction and a clear cusp in magnetic susceptibility.
• The magnetic transition temperature (Tmag) of 120 K is substantially reduced compared to binary oxides, attributed to dilution of magnetic ions in a disordered lattice.
• Magnetic ground states are tunable: compounds with three magnetic ions (Co, Ni, Cu) show long-range antiferromagnetic order, while those with fewer magnetic ions (e.g., MgNiCuZn) exhibit spin-glass-like freezing at 10–60 K.
• AC susceptibility shows frequency-dependent cusp at Tmag in low-magnetic-ion systems, indicating spin-glass behavior with glassy dynamics.
• The effective magnetic moment (μeff) of (MgCoNiCuZn)O is 2.95 μB/f.u., close to the calculated value of 2.66 μB/f.u., supporting the presence of localized magnetic moments.
• Substitution with Fe³⁺ (in (MgCoNiCuZn)₀.₈(LiFe)₀.₂O) leads to a Tmag of ~100 K with weak frequency dependence, suggesting a complex magnetic state possibly intermediate between long-range order and glassiness.