[Paper Review] Tuning ferromagnetism and spin state in La$_{(1-x)}$$A_x$CoO$_3$ ($A=$ Sr, Ca) nanoparticles

Ravi Prakash, Rishabh Shukla+3 more|arXiv (Cornell University)|Sep 6, 2017

Magnetic and transport properties of perovskites and related materials3 citations

TL;DR

This study investigates Sr and Ca doping in La1−xCoxCoO3 nanoparticles, demonstrating that hole doping via A-site substitution enhances ferromagnetism and stabilizes intermediate-spin (IS) and low-spin (LS) cobalt states. Using XRD, SQUID magnetometry, and XPS, the authors show that Sr/Ca doping increases the Co3+ (IS) and Co4+ (LS) populations, driving ferromagnetism via double-exchange interactions, with T_C rising to ~100 K and spontaneous magnetization increasing significantly at x = 0.2.

ABSTRACT

We use the x-ray diffraction, magnetic susceptibility, isothermal magnetization, and photoelectron spectroscopy to study the structural, magnetic and electronic properties of La$_{(1-x)}$$A_x$CoO$_3$ ($A=$ Sr, Ca; $x=$ 0 -- 0.2) nanoparticles. The Rietveld refinements of room temperature powder x-ray diffraction data confirm the single phase and the rhombohedral crystal structure with R$\bar{3}$C space group. We find drastic changes in the magnetic properties and spin-states with Sr/Ca substitution (hole doping). For $x=$ 0 sample, the magnetic measurements show a ferromagnetic transition at T$_{ m C}$$\approx$85 K, which shifted significantly to higher temperatures with hole doping; simultaneously a significant increase in the spontaneous magnetic moment has been observed. Whereas, the coercive field H$_{ m C}$ values are 7, 4.4 and 13.2~kOe for $x=$ 0, 0.2 (Sr), and 0.2(Ca) samples. Furthermore, the FC magnetization shows a ferromagnetic Brillouin function like behavior at low temperatures for Ca samples. We demonstrate that the Sr/Ca substitution increases the population of IS (Co$^{3+}$) and LS (Co$^{4+}$) states and tune the ferromagnetism in nanoparticles via double-exchange interactions between Co$^{3+}$-- Co$^{4+}$. Our results suggest an important role of hole carriers and nano-size effect in controlling the spin-state and magnetism in La$_{(1-x)}$$A_x$CoO$_3$ nanoparticles.

Motivation & Objective

• To understand the role of hole doping and nano-size effects in tuning magnetic and spin-state properties in La1−xCoxCoO3 nanoparticles.
• To investigate how Sr and Ca substitution at the La site alters the electronic structure and magnetic ordering.
• To correlate structural, magnetic, and electronic changes with doping concentration (x = 0–0.2) in nanoparticles.
• To determine the influence of A-site cation size (Sr2+ vs. Ca2+) on spin-state populations and ferromagnetic behavior.
• To establish the mechanism behind enhanced ferromagnetism in nanoparticles compared to bulk LaCoO3.

Proposed method

• Synthesized La1−xCoxCoO3 nanoparticles (x = 0–0.2) via sol-gel method for controlled A-site doping (Sr, Ca).
• Performed room-temperature powder X-ray diffraction (XRD) and Rietveld refinement to confirm single-phase rhombohedral structure (R-3C space group).
• Conducted magnetic measurements using SQUID magnetometry to analyze temperature- and field-dependent magnetization (FC/ZFC, isothermal M(H)).
• Acquired X-ray photoelectron spectroscopy (XPS) data to probe Co 2p, La 3d, Sr 3d, and Ca 2p core levels for oxidation state and electronic structure analysis.
• Used Curie-Weiss analysis to extract effective magnetic moments and assess exchange interactions.
• Analyzed charge transfer satellites and spin-orbit splitting in core-level spectra to infer Co oxidation states and electron correlation effects.

Experimental results

Research questions

• RQ1 How does Sr/Ca doping at the La site influence the magnetic transition temperature (T_C) in La1−xCoxCoO3 nanoparticles?
• RQ2 What is the role of hole doping in stabilizing intermediate-spin (IS) and low-spin (LS) cobalt states in these nanoparticles?
• RQ3 How do the coercive field (H_C) and spontaneous magnetization vary with Sr vs. Ca doping and doping concentration?
• RQ4 To what extent do nano-size effects and A-site cation size (Sr2+ vs. Ca2+) modulate double-exchange interactions in La1−xCoxCoO3?
• RQ5 How do XPS core-level shifts and satellite intensities reflect changes in electronic structure and Co oxidation states upon doping?

Key findings

• The Rietveld refinement of XRD data confirms a single-phase rhombohedral structure (R-3C space group) for all samples at room temperature.
• The ferromagnetic transition temperature (T_C) increases from ~85 K (x = 0) to ~100 K (x = 0.2) with Sr/Ca doping, indicating enhanced ferromagnetic interactions.
• Spontaneous magnetization increases significantly with doping: H_C values are 7 kOe (x = 0), 4.4 kOe (x = 0.2 Sr), and 13.2 kOe (x = 0.2 Ca), showing a non-monotonic trend.
• XPS analysis reveals increased intensity ratio (I_f1/I_f0) of charge-transfer satellites in La 3d spectra, indicating stronger La–ligand interactions with increasing Sr/Ca doping.
• Co 2p spectra show no significant binding energy shift but a growing high-binding-energy shoulder, suggesting the emergence of Co4+ (LS) states with doping.
• The Curie-Weiss temperature (θ_CW) increases with doping, confirming enhanced ferromagnetic exchange interactions, especially in Ca-doped samples.