[Paper Review] YBa$_{1-x}$Sr$_{x}$CuFeO$_{5}$ layered perovskites: exploring the magnetic order beyond the paramagnetic-collinear-spiral triple point

Victor Porée, Dariusz Jakub Gawryluk+6 more|arXiv (Cornell University)|Feb 7, 2024

Magnetic and transport properties of perovskites and related materialsMaterials Science3 citations

TL;DR

This study investigates YBa₁₋ₓSrₓCuFeO₅ solid solutions to access the magnetic phase region beyond the paramagnetic-collinear-spiral triple point, where spiral order is expected to destabilize. By substituting Ba with Sr, the researchers enhance Fe-Fe defect concentration, experimentally stabilizing a non-frustrated, fully antiferromagnetic state with propagation vector k = (1/2, 1/2, 0) and ordering temperature T_coll2 ≥ T_spiral, demonstrating a disorder-driven mechanism that increases both T_spiral and T_coll2 at the same rate.

ABSTRACT

Layered perovskites of general formula AA'CuFeO$_5$ are one of the few examples of cycloidal spiral magnets where the ordering temperatures $T_{spiral}$ can be tuned far beyond room temperature by introducing modest amounts of Cu/Fe chemical disorder in the crystal structure. This rare property makes these materials prominent candidates to host multiferroicity and magnetoelectric coupling at room temperature. Moreover, it has been proposed that the highest $T_{spiral}$ value that can be reached in this structural family ($\sim$ 400 K) corresponds to a paramagnetic-collinear-spiral triple point with potential to show exotic physics. Since generating high amounts of Cu/Fe disorder is experimentally difficult, the phase diagram region beyond the triple point has been barely explored. To fill this gap we investigate here the YBa$_{1-x}$Sr$_{x}$CuFeO$_{5}$ solid solutions ($0 \leq x \leq 1$), where we replace Ba with Sr with the aim of enhancing the impact of the experimentally available Cu/Fe disorder. Using a combination of bulk magnetization, synchrotron X-ray and neutron powder diffraction we show that the spiral state is destabilized beyond a critical degree of Cu/Fe disorder, being replaced by a non-frustrated, fully antiferromagnetic state with propagation vector k$_{c2}$ = $(\frac{1}{2}, \frac{1}{2}, 0)$ and ordering temperature $T_{coll2}$ $\geq$ $T_{spiral}$, which is progressively stabilized beyond the triple point. Interestingly, $T_{spiral}$ and $T_{coll2}$ increase with $x$ at the same rate. This suggests a common, disorder-driven origin, consistent with theoretical predictions.

Motivation & Objective

• To explore the magnetic phase diagram beyond the paramagnetic-collinear-spiral triple point in Cu/Fe-based layered perovskites, a region experimentally inaccessible due to synthesis challenges.
• To test whether Sr doping in YBa₁₋ₓSrₓCuFeO₅ enhances Fe-Fe defect concentration, thereby stabilizing the non-frustrated antiferromagnetic state predicted beyond the triple point.
• To determine if the observed increase in T_spiral and T_coll2 with Sr content indicates a common, disorder-driven origin for both transition temperatures.
• To validate the random-exchange frustration model by experimentally probing the magnetic order evolution under controlled chemical disorder.

Proposed method

• Synthesis of YBa₁₋ₓSrₓCuFeO₅ solid solutions across the full x = 0 to 1 range to systematically vary Fe-Fe defect concentration.
• Use of bulk magnetization measurements to identify magnetic transitions and determine T_spiral and T_coll2.
• Employment of synchrotron X-ray and neutron powder diffraction to refine magnetic structures, including propagation vectors and magnetic moment orientations.
• Application of constrained Rietveld refinement with fixed moment ratios (Fe:Cu = 5:1) and circular spiral envelopes to ensure consistency with prior studies.
• Analysis of q_G values from neutron diffraction to correlate magnetic wavevector with T_spiral, comparing with literature data across different perovskite families.
• Use of restricted parameter spaces in refinement to avoid overfitting, including constraints on moment ratios and phase coexistence in mixed regions.

Experimental results

Research questions

• RQ1 Does increasing Sr doping in YBa₁₋ₓSrₓCuFeO₅ enhance Fe-Fe defect concentration sufficiently to access the phase beyond the paramagnetic-collinear-spiral triple point?
• RQ2 Is the spiral magnetic order destabilized beyond a critical Fe-Fe defect concentration, and if so, what is the nature of the resulting magnetic ground state?
• RQ3 Do T_spiral and T_coll2 increase at the same rate with increasing x, indicating a common disorder-driven origin?
• RQ4 Can the experimental observations be explained by the random-exchange frustration model, particularly in terms of the T_spiral(q_G) linear dependence?

Key findings

• The spiral magnetic order is destabilized beyond a critical Fe-Fe defect concentration, giving way to a non-frustrated, fully antiferromagnetic state with propagation vector k_c2 = (1/2, 1/2, 0).
• The ordering temperature T_coll2 of the new antiferromagnetic state is ≥ T_spiral, indicating stabilization beyond the triple point.
• Both T_spiral and T_coll2 increase with increasing Sr content (x) at the same rate, suggesting a common, Fe-Fe defect-driven origin.
• The T_spiral versus q_G relationship shows a nearly linear dependence with a slope consistent with previous studies, supporting the random-exchange frustration model.
• The observed T_spiral(q_G) trend across different Cu/Fe-based perovskites, including this study’s data, shows a common, low-dispersion linear law, indicating limited variation in exchange constants across materials.
• The results validate the theoretical prediction that chemical disorder—specifically Fe-Fe defects—can be used as a knob to stabilize high-T_c spiral magnets with potential for room-temperature multiferroic applications.