[Paper Review] Electronic structure and magnetic properties of 3d-4f double perovskite material

S. Kundu, A. Pal+10 more|arXiv (Cornell University)|May 22, 2022

Magnetic and transport properties of perovskites and related materials4 citations

TL;DR

This study investigates the electronic and magnetic properties of Ho2CoMnO6, a 3d-4f double perovskite, revealing long-range ferromagnetic ordering at T_C ≈ 76 K due to Co²⁺–O–Mn⁴⁺ superexchange, along with a re-entrant spin-glass state below T_f ≈ 30 K driven by anti-site disorder and competing interactions. First-principles calculations confirm a narrow band gap insulator with a gap formed by splitting of Co-3d t2g states into lower and upper Hubbard bands due to strong electron correlation.

ABSTRACT

Double perovskite-based magnets wherein frustration and competition between emergent degrees of freedom are at play can lead to novel electronic and magnetic phenomena. Herein, we report the electronic structure and magnetic properties of an ordered double perovskite material Ho2CoMnO6. In the double perovskite with general class A2BB'O6, the octahedral B and B'-site has a distinct crystallographic site. The Rietveld refinement of XRD data reveals that Ho2CoMnO6 crystallizes in the monoclinic P21/n space group. The X-ray photoelectron spectroscopy confirms the charge state of cations present in this material. The temperature dependence of magnetization and specific heat exhibit a long-range ferromagnetic ordering at Tc ~ 76 K owing to the presence of super exchange interaction between Co2+ and Mn4+ moments. Furthermore, the magnetization isotherm at 5 K shows a hysteresis curve that confirms ferromagnetic behavior of this double perovskite. We observed a re-entrant glassy state in the intermediate temperature regime, which is attributed to inherent anti-site disorder and competing interactions. A large magnetocaloric effect has been observed much below the ferromagnetic transition temperature. The temperature-dependent Raman spectroscopy studies support the presence of spin-phonon coupling and short-range order above Tc in this double perovskite. The stabilization of magnetic ordering and charge states is further analyzed through electronic structure calculations. The latter also infers the compound to be a narrow band gap insulator with the gap arising between the lower and upper Hubbard Co-d subbands. Our results demonstrate that anti-site disorder and complex 3d-4f exchange interactions in the spin-lattice account for the observed electronic and magnetic properties in this promising double perovskite material.

Motivation & Objective

• To understand the interplay between 3d-4f electronic interactions and magnetic ordering in double perovskite Ho2CoMnO6.
• To identify the origin of competing magnetic phases, including ferromagnetic ordering and re-entrant spin-glass behavior.
• To correlate experimental magnetic and spectroscopic data with first-principles electronic structure calculations.
• To evaluate the material's potential for magnetocaloric and spintronic applications.

Proposed method

• Synthesis of high-quality polycrystalline Ho2CoMnO6 via solid-state reaction method.
• X-ray diffraction (XRD) and Rietveld refinement to determine crystal structure and space group (P2₁/n).
• X-ray photoelectron spectroscopy (XPS) to confirm cationic charge states (Co²⁺, Mn⁴⁺, Ho³⁺).
• Magnetization measurements (ZFC/FC, M(H) isotherms) and specific heat to probe magnetic transitions and ordering.
• Temperature-dependent Raman spectroscopy to detect spin-phonon coupling and short-range magnetic correlations.
• Density functional theory (DFT) calculations with Hubbard U correction (DFT+U) to analyze electronic structure and band gap formation.

Experimental results

Research questions

• RQ1 What is the origin of ferromagnetic ordering in Ho2CoMnO6, and how does it relate to Co²⁺–O–Mn⁴⁺ superexchange?
• RQ2 Why does Ho2CoMnO6 exhibit a re-entrant spin-glass state below ~30 K, and what role does anti-site disorder play?
• RQ3 How do spin-phonon coupling and short-range magnetic correlations manifest in the temperature-dependent Raman spectra?
• RQ4 What is the nature of the electronic band gap in Ho2CoMnO6, and how does electron correlation influence its formation?
• RQ5 To what extent do first-principles calculations reproduce the experimentally observed charge states and magnetic ground state?

Key findings

• Ho2CoMnO6 crystallizes in the monoclinic P2₁/n space group with long-range ferromagnetic ordering at T_C ≈ 76 K.
• A re-entrant spin-glass state is stabilized below T_f ≈ 30 K, confirmed by bifurcation in ZFC-FC curves and frequency-dependent ac susceptibility.
• The magnetocaloric effect is significant, with ΔS_m ≈ 13.5 J/kg·K below T_C, indicating strong potential for magnetic refrigeration.
• XPS confirms the presence of Co²⁺, Mn⁴⁺, and Ho³⁺ ions with stable charge states consistent with DFT+U calculations.
• Raman spectroscopy reveals spin-phonon coupling, with spectral line shifts deviating from anharmonic behavior near 85 K, indicating short-range spin correlations.
• Electronic structure calculations show a narrow band gap insulator with a gap arising from splitting of Co-3d t2g states into lower and upper Hubbard bands in the spin-down channel.