Above-room-temperature strain-stable half-metallic phase owing to a high thermoelectric response in CaCu3Cr2Re2O12

Abstract

We present the electronic, magnetic, and thermoelectric properties of the CaCu₃Cr₂Re₂O₁₂ quadruple double perovskite oxide (QDPO) using ab initio calculations. Strong antiferromagnetic interactions between Cu²⁺↑Cr³⁺↑Re⁵⁺↓ result in a ferrimagnetic (FiM) ground state. A half-metallic (HM) state is evident with a finite energy gap (E _g) of 1.62 eV in the spin-majority channel (N ^↑), which is large enough to ensure a long mean free path for spin and prevent spin-flipping with a colossal spin-filtering ability. Due to a significant E _g in N ^↑, the system displays a high figure of merit (0.76) at 300 K. The calculated spin moments of +0.48/+2.55/-0.92 μ _B and spin-magnetization densities on the Cu/Cr/Re further verify the FiM state. The magnetic phase transition yields a Curie temperature (T _C) of 340 K, which is below the experimental value (360 K). Because thermal energy near T _C disrupts the magnetic ordering, magnetization is consequently reduced, which is also reflected in the susceptibility curve. Additionally, the FiM state of the structure was confirmed under modulated applied magnetic fields. Finally, the HM FiM state is also verified under a moderate biaxial ([110]) strain of ±5%. Hence, this work provides deep insights into this QDPO, highlighting its stable HM FiM behavior, above-room-temperature T _C and high thermoelectric response, which promises potential applications in spintronics.

Fig. 1. Crystal structures of the CaCu₃Cr₂Re₂O₁₂ quadruple structure in a (a) ferromagnetic (FM), (b) ferrimagnetic (FiM)-I, (c) FiM-II, and (d) FiM-III spin ordering.

Fig. 2. Calculated spin-polarized density of states (DOS) in the CaCu₃Cr₂Re₂O₁₂ quadruple structure: (a/a′) total/partial DOS for the GGA+U and (b) total DOS for the GGA+U+SOC method,.

Fig. 3. Calculated spin-polarized band structures in the CaCu₃Cr₂Re₂O₁₂ quadruple structure, for the (a/a′) spin-majority/spin-minority channel within the GGA+U (1st-row) and (b) GGA+U+SOC (2nd-row) methods.

Fig. 4. Calculated three-dimensional spin-magnetization density iso-surfaces with an iso-value of ±0.005 e Å⁻³ in the CaCu₃Cr₂Re₂O₁₂ quadruple structure. For simplicity, Ca and oxygen ions are omitted.

Fig. 5. Schematic representation of the antiferromagnetic superexchange coupling between Cu²⁺ (3d⁹)/Cr³⁺ (3d³) and Re²⁺ (5d²) orbitals via O⁻² (2p²) in the CaCu₃Cr₂Re₂O₁₂ quadruple structure.

Fig. 6. Calculated susceptibility and magnetization curves versus Curie temperature of the CaCu₃Cr₂Re₂O₁₂ quadruple structure.

Fig. 7. Variation of the saturated magnetization (M) with applied magnetic field (H) for (a) 2 T and (b) 5 T at a fixed temperature of 2/100/200/300 K in the CaCu₃Cr₂Re₂O₁₂ quadruple structure.

Fig. 8. Calculated (a) electrical conductivity per unit relaxation time , (b) Seebeck coefficient (S), (c) thermal conductivity per unit relaxation time , (d) molar magnetic susceptibility (χ), (e) Hall-coefficient (R_H), and (f) figure of merit (ZT) of the CaCu₃Cr₂Re₂O₁₂ quadruple structure.

Fig. 9. Calculated (a) formation/cohesive energy (E_for./E_coh.) in blue/red color and (b) energy gap (E_g) in the spin-majority channel with respect to ±5% biaxial ([110]) strain in the CaCu₃Cr₂Re₂O₁₂ quadruple structure.

Fig. 10. GGA+U calculated total density of states (TDOS) for (a/a′) −/+1%, (b/b′) −/+3%, and (c/c′) −/+5% compressive/tensile biaxial ([110]) strains in the stable ferrimagnetic-I spin ordering of the CaCu₃Cr₂Re₂O₁₂ quadruple structure.

Fig. 11. Calculated partial spin magnetic moments (m_s) on the Cu, Cr, and Re ions for ±5% biaxial ([110]) strain in the stable ferrimagnetic-I ordering of the CaCu₃Cr₂Re₂O₁₂ quadruple structure.