Precision transport measurements uncover a hidden four-fold pattern in nickelate thin films, offering fresh evidence that magnetic correlations may shape their superconducting state.
Paper: Universal four-fold symmetry in infinite-layer nickelates. Image credit: AI-generated image created using ChatGPT/OpenAI
A recent article-in-press study in the journal Communications Materials provides new insight into the long-standing question of whether nickelates host magnetic-order signatures related to those implicated in cuprate superconductivity. Researchers tried to address that question by investigating the angular magnetoresistance (AMR) of hole-doped Nd1−xSrxNiO2 thin films. They observed a universal four-fold rotational symmetry that persists across the studied doping range, spanning both weakly insulating and superconducting phases.
Connecting Magnetism and Superconductivity in Infinite-Layer Nickelates
Despite decades of research, the mechanism that explains unconventional superconductivity remains poorly understood. High-temperature cuprate superconductors offer an important clue. In these materials, superconductivity emerges from an insulating antiferromagnetic parent phase, suggesting that magnetic interactions play a central role in electron pairing. Infinite-layer nickelates closely resemble cuprates in both their crystal structures and their formal electronic configurations.
Addressing this question has proven difficult because superconducting nickelates are currently available only as thin films under ambient-pressure conditions. Most experimental techniques used to directly probe magnetic order require large bulk crystals and, therefore, cannot be applied to these materials. Although previous studies have reported magnetic excitations and short-range antiferromagnetic correlations, they have not established the precise nature of magnetic order in infinite-layer nickelates or how it depends on hole doping.
The researchers employed angular magnetoresistance (AMR) measurements to investigate the electronic ground state of Nd1−xSrxNiO2 thin films. AMR measures how electrical resistance changes as an applied magnetic field rotates within the crystal plane. By examining samples spanning both weakly insulating and superconducting compositions, the researchers investigated whether a common magnetic anisotropy underpins the observed portion of the nickelate phase diagram.
Probing Magnetic Order Through Angular Magnetoresistance
The researchers prepared high-quality Nd1−xSrxNiO2 thin films by pulsed laser deposition, followed by topochemical reduction, to yield the infinite-layer crystal structure. They grew approximately 10 nm thick precursor films on TiO2-terminated SrTiO3 substrates and varied the strontium concentration to obtain samples spanning weakly insulating and superconducting states.
The team first verified the films' structural and electronic properties. X-ray diffraction confirmed the formation of the infinite-layer phase, while electrical resistivity measurements tracked the evolution from weakly insulating behavior to superconductivity with increasing strontium content. These characterization results established a reliable platform for investigating changes in the electronic ground state.
The researchers then employed AMR measurements to probe magnetic anisotropy. They rotated an in-plane magnetic field of up to 14 T while recording changes in electrical resistance over a range of temperatures, magnetic fields, and doping levels. Because electron transport can reflect spin-related electronic anisotropy through electron-spin interactions, AMR provides a sensitive probe of subtle changes in electronic and magnetic symmetry.
The team developed a theoretical model based on the Hubbard framework for strongly correlated electron systems. The model examined how antiferromagnetic ordering, spin-flop transitions, and magnetic anisotropy influence electron transport under different magnetic field conditions. Comparing the simulations with the experimental data supported an antiferromagnetic interpretation of the observed symmetry changes.
A Universal Four-Fold Symmetry Across the Studied Doping Range
AMR measurements revealed a distinctive four-fold rotational symmetry across the measured doping range. In the superconducting samples, electrical resistance reached its minimum along the Ni-O-Ni crystal directions and its maximum along the diagonal directions, producing a characteristic C4 symmetry. This anisotropy gradually disappeared as the temperature approached the superconducting transition. The observation suggests a close connection between superconductivity and the underlying electronic order.
The weakly insulating samples exhibited behavior that was different but closely related. Instead of disappearing, the four-fold pattern rotated by 45°, producing a C~4 symmetry. Underdoped superconducting samples displayed both symmetry states. At low magnetic fields, they retained the C4 pattern, whereas at higher fields beyond the critical field, they shifted toward the C~4 pattern. This characteristic π/4 phase shift suggests that the insulating and superconducting phases may share a common origin for AMR symmetry breaking.
Theoretical modeling was consistent with antiferromagnetic ordering. The model predicts that electron spins preferentially align along the diagonal crystal directions. As the magnetic field increases, the spins undergo a spin-flop transition. During this process, the staggered antiferromagnetic moments reorient to minimize the system's magnetic energy. This spin reorientation naturally explains the rotation of the magnetoresistance pattern and its evolution with magnetic field and doping.
The researchers also compared their results with electron-doped cuprate superconductors, where similar four-fold magnetoresistance patterns indicate antiferromagnetic order. The findings point to AMR signatures consistent with antiferromagnetic-related correlations across the studied nickelate phase diagram. They also indicate that these correlations may coexist with superconductivity in the measured superconducting films.
Advancing the Understanding of Nickelate Superconductivity
The study suggests that magnetic correlations may remain a defining feature of infinite-layer nickelates as they evolve from weakly insulating materials to superconductors. The discovery of a universal four-fold symmetry and its characteristic π/4 rotation demonstrates that angular magnetoresistance (AMR) is a powerful tool for probing hidden magnetic order-related features in thin-film quantum materials.
The results also strengthen the growing connection between nickelates and cuprate superconductors. Both material families exhibit similar transport features consistent with antiferromagnetic ordering, suggesting that magnetic interactions may play a comparable role in unconventional superconductivity. Understanding this relationship could help researchers uncover the pairing mechanism responsible for high-temperature superconductivity and refine theoretical models of strongly correlated materials.
AMR provides a practical way to investigate magnetic-order-related anisotropy in ultrathin materials that cannot be studied with conventional neutron scattering techniques. It also provides a sensitive probe of electronic symmetry-breaking in quantum materials.
Future research could extend these measurements to a broader range of doping levels, to different rare-earth nickelates, and to stronger magnetic fields. Overall, the study demonstrates how precision transport measurements can reveal hidden electronic order and accelerate the search for the microscopic origin of superconductivity in nickelates and other strongly correlated quantum materials.
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