[Paper Review] Intrinsic magnetism in superconducting infinite-layer nickelates

Jennifer Fowlie, Marios Hadjimichael+11 more|arXiv (Cornell University)|Jan 28, 2022

Iron-based superconductors research4 citations

TL;DR

This study uses low-energy muon spin rotation/relaxation (µSR) to demonstrate intrinsic, short-range antiferromagnetic order in superconducting infinite-layer nickelates, including in the superconducting state. The magnetism originates from the Ni²⁺ sublattice and persists across diverse rare earth dopants and Sr-doping levels, with magnetic fluctuations onset above 150 K, indicating a distinct magnetic ground state unlike that of cuprates.

ABSTRACT

The discovery of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ [1] introduced a new family of layered nickelate superconductors that has now been extended to include a range of Sr-doping [2, 3], Pr or La in place of Nd [4-6], and the 5-layer Nd$_6$Ni$_5$O$_{12}$ [7]. A number of studies indicate that electron correlations are strong in these materials [8-14], and hence a central question is whether or not magnetism is present as a consequence of these interactions. Here we report muon spin rotation/relaxation studies of a series of superconducting infinite-layer nickelates. In all cases we observe an intrinsic magnetic ground state, regardless of the rare earth ion or doping, arising from local moments on the nickel sublattice. The coexistence of magnetism - which is likely to be antiferromagnetic and short-range ordered - with superconductivity is reminiscent of some iron pnictides [15] and heavy fermion compounds [16], and qualitatively distinct from the doped cuprates [17].

Motivation & Objective

• To determine whether intrinsic magnetism exists in superconducting infinite-layer nickelates despite the absence of long-range magnetic order.
• To disentangle the contribution of rare earth ions from the Ni sublattice in generating magnetism.
• To investigate the nature of magnetic order (e.g., short-range, spin-glass-like) and its temperature evolution in these materials.
• To clarify the role of electron correlations and orbital degrees of freedom in the coexistence of superconductivity and magnetism.
• To compare the magnetic behavior of nickelates with that of cuprates and iron pnictides, particularly regarding the proximity of magnetic order to the superconducting dome.

Proposed method

• Low-energy muon spin rotation/relaxation (µSR) was employed to probe local magnetic fields with high spatial and temporal resolution.
• Muon implantation energy was tuned to 4.5 keV to maximize stopping depth within the 8 nm thick nickelate layer while minimizing backscattering.
• Monte Carlo simulations were used to model muon stopping profiles and optimize capping layer thickness (SrTiO₃) for depth-selective measurements.
• Zero-field (ZF) and weak transverse field (wTF) µSR experiments were performed across a range of temperatures to detect magnetic precession and relaxation.
• Data were analyzed using musrfit software, with stretched exponential fits used to extract the magnetic volume fraction and magnetic fluctuation rate (β parameter).
• The contribution of muonium formation in SrTiO₃ was corrected for in the asymmetry analysis to isolate the signal from the nickelate layer.

Experimental results

Research questions

• RQ1 Does intrinsic magnetism exist in superconducting infinite-layer nickelates, independent of external defects or interfacial effects?
• RQ2 What is the origin of the observed magnetism—specifically, is it localized on the Ni²⁺ sublattice or influenced by rare earth ions?
• RQ3 How does the magnetic state evolve with temperature and doping, and what is the nature of the magnetic order (long-range, short-range, spin-glass)?
• RQ4 How does the magnetic behavior of nickelates compare to that of cuprates and iron pnictides, particularly in terms of magnetic transition temperature and coexistence with superconductivity?
• RQ5 What is the role of electron correlations and orbital hybridization in stabilizing magnetism in these materials?

Key findings

• All studied superconducting infinite-layer nickelates—Nd₀.₈₂₅Sr₀.₁₇₅NiO₂, Pr₀.₈Sr₀.₂NiO₂, La₀.₈Sr₀.₂NiO₂, and LaNiO₂—exhibit intrinsic magnetic order, with no long-range magnetic order detected.
• The magnetism is localized on the Ni²⁺ sublattice, as confirmed by the identical µSR response across different rare earth elements despite the absence of magnetic moments on the rare earth ions.
• Magnetic fluctuations are present above 150 K, with the magnetic volume fraction increasing gradually with cooling, indicating short-range or spin-glass-like order.
• The ZF and wTF spectra show no evidence of long-range order, and the absence of precession in ZF confirms dynamic, fluctuating magnetic fields on the muon timescale.
• The magnetic response is consistent with antiferromagnetic coupling, as indicated by the absence of significant demagnetizing fields and the energy-dependent depth profile of muon stopping.
• The high-temperature onset of magnetism (above 150 K) contrasts with cuprates (typically <30 K), suggesting a distinct magnetic mechanism in nickelates, possibly linked to multi-orbital physics.