[Paper Review] Absence of zero-field-cooled exchange bias effect in single crystalline La2-xAxCoMnO6 (A = Ca, Sr) compounds

C. Macchiutti, J. R. Jesus+5 more|Warwick Research Archive Portal (University of Warwick)|Jun 2, 2021

Magnetic and transport properties of perovskites and related materials4 citations

TL;DR

This study investigates the absence of zero-field-cooled exchange bias (ZEB) in single-crystalline La2-xAxCoMnO6 (A = Ca, Sr) compounds, synthesized via the floating zone method. Despite exhibiting ferromagnetic behavior and higher Curie temperatures than polycrystalline counterparts, no ZEB or conventional exchange bias is observed, indicating that grain boundaries and spin glass-like phases—absent in single crystals—are essential for the ZEB effect in these double perovskites.

ABSTRACT

Magnetic properties of A2BB'O6 (A = rare or alkaline earth ions; B,B' = transition metal ions) double perovskites are of great interest due to their potential spintronic applications. Particularly fascinating is the zero field cooled exchange bias (ZEB) effect observed for the hole doped La2-xAxCoMnO6 polycrystalline samples. In this work we synthesize La2CoMnO6, La1.5Ca0.5CoMnO6, and La1.5Sr0.5CoMnO6 single crystals by the floating zone method and study their magnetic behavior. The three materials are ferromagnetic. Surprisingly, we observe no zero or even conventional exchange bias effect for the Ca and Sr doped single crystals, in sharp contrast to polycrystalline samples. This absence indicates that the lack of grain boundaries and spin glass-like behavior, not observed in our samples, might be key ingredients for the spontaneous exchange bias phenomena seen in polycrystalline samples.

Motivation & Objective

• To investigate the magnetic properties of single-crystalline La2-xAxCoMnO6 (A = Ca, Sr) compounds.
• To determine whether the zero-field-cooled exchange bias (ZEB) effect, observed in polycrystalline samples, persists in single crystals.
• To identify the structural and magnetic factors responsible for the absence of ZEB in single crystals.
• To clarify the role of grain boundaries and spin glass-like behavior in enabling the ZEB effect in double perovskite oxides.

Proposed method

• Synthesized single crystals of La2CoMnO6, La1.5Ca0.5CoMnO6, and La1.5Sr0.5CoMnO6 using the floating zone method.
• Performed X-ray diffraction to confirm crystal structure and phase purity, identifying orthorhombic (Pnma) for LCMO and LCCMO, and rhombohedral (R3̄c) for LSCMO.
• Measured magnetization as a function of temperature and applied magnetic field to identify ferromagnetic transitions and Curie temperatures.
• Conducted zero-field-cooled and field-cooled magnetization measurements to assess magnetic irreversibility and spin glass-like behavior.
• Analyzed hysteresis loops under zero-field and field-cooled conditions to detect exchange bias shifts.
• Calculated tolerance factors using ionic radii to correlate structural distortion with magnetic behavior.

Experimental results

Research questions

• RQ1 Does the zero-field-cooled exchange bias (ZEB) effect persist in single-crystalline La2-xAxCoMnO6 (A = Ca, Sr) compounds?
• RQ2 What structural and magnetic differences between single crystals and polycrystalline samples account for the absence of ZEB in single crystals?
• RQ3 Is the presence of grain boundaries or spin glass-like phases necessary for the emergence of the ZEB effect in these double perovskites?
• RQ4 How do changes in A-site doping (Ca, Sr) influence the Curie temperature and magnetic phase transitions in single-crystalline samples?
• RQ5 To what extent do structural distortions, such as bond angles and lattice symmetry, correlate with the suppression of exchange bias in single crystals?

Key findings

• Single-crystalline La2-xAxCoMnO6 (A = Ca, Sr) compounds exhibit no zero-field-cooled exchange bias (ZEB) effect, despite being ferromagnetic.
• The Curie temperature increases to 165 K for La1.5Ca0.5CoMnO6 and 205 K for La1.5Sr0.5CoMnO6, higher than the 150 K observed in undoped La2CoMnO6.
• No spin glass-like behavior or magnetic irreversibility was detected in the single crystals, unlike in polycrystalline samples.
• The absence of exchange bias is attributed to the lack of grain boundaries and the absence of a reentrant spin glass-like phase in single crystals.
• The tolerance factor calculations indicate structural trends: t ≈ 0.96 for LCMO (orthorhombic), t ≈ 0.98 for LCCMO, and t ≈ 0.99 for LSCMO, correlating with increasing structural symmetry and higher T_C.
• No conventional exchange bias shift was observed even after cooling in a 9 T magnetic field, confirming the absence of interface-driven exchange bias effects.