EXAFS helps to unveil the spin-phonon coupling strength in a spintronic material

Nickelate perovskites, formula RNiO₃ (R = rare-earths), are promising spintronic materials that have been implemented recently in modern communication devices. In these magnetic systems, the spins of Ni cations (Ni³⁺) can interact with the lattice vibrations (phonons) [1], resulting in the spin-phonon coupling effect (Figure 1a). This interaction, if it occurs in the right proportion, allows the magnetic signal to be transferred with increased efficiency, reliability, and with limited energy dissipation, leading to a superior performance compared to conventional, electric current-based electronics. Although nickelates have been known about for 30 years, there are still open questions regarding the interplay between their structural, electronic, vibrational and magnetic properties: How does the spin interact with lattice vibrations? What is the strength of the interaction, and how does this influence the efficiency of the magnetic signal transfer? Such investigations are challenging because some key nickelates remain difficult to synthesise, preventing the disentanglement of the spin-phonon effect.

Fig. 1: a) Representation of [NiO₆] octahedron in PrNiO₃, together with the Ni spin (red arrow) and the stretching mode along the Ni–O bond (blue arrows), probed by EXAFS. b) Temperature-dependent moduli of the EXAFS oscillations at the Ni K-edge acquired from 300 K (red) down to 10 K (blue). The coloured and black lines denote the raw data and the corresponding fits, respectively. c) Evolution of the Ni–O bond-distance variance (Ų) (symbols) and Einstein's model fitted to the data above 130 K (black line).

Among the nickelates, PrNiO₃ represents a special class, because its electronic, magnetic and structural transitions coincide at 130 K (T_N). Below this temperature, a metallic orthorhombic phase (Pbnm) transforms into an insulating antiferromagnetic monoclinic phase (P2₁/n) [2]. Therefore, PrNiO₃ represents an ideal case to depict the interplay among the crystal lattice, vibrations and electronic and magnetic properties.

In this work, pure PrNiO₃ nickelate was synthesised using a high-pressure and high-temperature method, which allowed the Ni to be stabilised in the uncommon 3+ valence state. The purity of the metallic PrNiO₃ was validated by high-resolution powder X-ray diffraction data acquired at ambient temperature at beamline BM25. In a second step, high-quality Ni K-edge extended X-ray absorption fine structure (EXAFS) data were acquired down to 10 K at beamline BM23 (Figure 1b). EXAFS is an ideal probe because it provides information on the local atomic, vibrational and electronic structure around the Ni absorber atoms. Using this technique, a strong anomaly was observed in the thermal evolution of the Ni–O bond-distance variance (Ų) below 130 K (Figure 1c), when compared to Einstein’s model.
 

Fig. 2: Temperature variation of the Einstein temperature θ_E + (Δθ_E)_sp−ph in PrNiO₃ across T_N ≈ 130 K, as extracted from a model for the Ni–O bond-distance variance of the first shell Ni–O. The variation (Δθ_E)_sp−ph was obtained from a molecular field approximation which includes the strength of the spin-phonon coupling.

Based on the molecular field approximation, it was shown that the anomaly along the Ni–O bond-distance variance below 130 K was related to a continuous increase of the spin-phonon coupling in the monoclinic PrNiO₃ (Figure 2). The present results suggest that the insulator–metal transition in PrNiO₃ has an important contribution from the lattice vibrations, and that the coupling is revealed by an anomalous variation of the Einstein temperature [θ_E + (Δθ_E)_sp−ph]. These results demonstrate that the EXAFS technique is not only a powerful tool for depicting structural changes, but also for exploring the coupling behaviour between spin configuration and phonons.

Principal publication and authors
EXAFS evidence for the spin–phonon coupling in the monoclinic PrNiO₃ nickelate perovskite, J.E. Rodrigues (a), A.D. Rosa (a), J. López-Sánchez (b,c), E. Sebastiani-Tofano (b,c), N.M. Nemes (d), J.L. Martínez (b), J.A. Alonso (b), O. Mathon (a), J. Mater. Chem. C 11, 462-471 (2023); https://doi.org/10.1039/D2TC03063B
(a) ESRF
(b) Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Madrid (Spain)
(c) Spanish CRG BM25-SpLine, ESRF
(d) Departamento de Física de Materiales, Universidad Complutense de Madrid, Madrid (Spain)

References
[1] J.E. Rodrigues et al., J. Alloys Compd. 799, 563-572 (2019).
[2] M. Medarde et al., Phys. Rev. B: Condens. Matter Mater. Phys. 78, 2-5 (2008).