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


An international team of researchers disentangled the effect of spin-phonon coupling in PrNiO3, a promising spintronic material, by combining experiments and theoretical modelling. Temperature-dependent X-ray absorption measurements acquired at the beamline BM23 were crucial for identifying the weak anomalies of the Ni–O bond-distance variance associated to the spin-phonon coupling.

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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 (Å2) (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 (TN). 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 (Å2) below 130 K (Figure 1c), when compared to Einstein’s model.



Fig. 2: Temperature variation of the Einstein temperature θE + (ΔθE)sp−ph in PrNiO3 across TN ≈ 130 K, as extracted from a model for the Ni–O bond-distance variance of the first shell Ni–O. The variation (ΔθE)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 [θE + (ΔθE)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);
(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)

[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).


About the beamline: BM23
BM23 is the multi-purpose X-ray absorption spectroscopy (XAS) beamline of the ESRF. BM23 is optimised for high-quality EXAFS measurements in a large energy range (5 to 75 keV), in transmission or fluorescence mode, featuring stability, versatility, automation and online data analysis. The flexible beamline sample environments (viz., cryostats, ovens, gases, chemistry cells, high-pressure cell, DRIFTS/XAS/MS setup, user in-house setups, etc.) are in full operation, covering large scientific fields such chemistry, earth sciences, geoscience and hard condensed matter physics. In addition, a new micro-XAS station has been implemented, allowing a higher degree of mechanical precision and more flexibility in a sample environment requiring micro-sized beam spot-size (cryo-DAC, large-volume press, external or internal RH-DAC, etc.). The focal spot size is below 5×5 µm2 FWHM with 109 ph/s. The micro-XAS station can be operated between 5 and 45 keV. A Pilatus 1M detector is available for complementary XRD measurements.
About the beamline: BM25

The Spanish CRG BM25-SpLine beamline is dedicated to structural investigations using hard X-ray scattering mostly in materials science, specialised in the combination of diffraction and spectroscopy techniques. Grazing incidence X-ray diffraction can be combined with hard X-ray photoelectron spectroscopy (up to 15 keV) and X-ray absorption spectroscopy, to study the correlation between atomic structure and chemical and electronic properties, mostly in surfaces and buried interfaces.

The beam from a newly developed 2-Pole wiggler for EBS is focused by a toroidal mirror and monochromatised by a Si(111) Bragg monochromator. Additionally, a post-monochromator (220) can be placed in line with the main monochromator for high-resolution spectroscopy experiments. The working energy ranges from 4 to 35 keV with a photon flux of 1012 photons/s at 200 mA within the Si(111) bandwidth. The beam size on the sample can be changed from 2×2 mm2 to 80×80 µm2. Two experimental hutches allocate three different end-stations, equipped with Eiger2 CdTe pixel detectors from DECTRIS, a solid-state fluorescence detector with XIA electronics, and a high-energy electron analyser (from a few eV to 15 keV) from FOCUS GmbH.

The beamline is equipped with several sample environments for in-situ studies (LHe and LN2 cryostat, oven up to 1500 K, solid-liquid cell, solid-high pressure gas cell, capillary and flat plate spinner, ultra-high vacuum chamber, MBE evaporator).