[Paper Review] Metal-insulator transition in ultrathin LaNiO3 films

R. Scherwitzl, Stefano Gariglio+4 more|UCL Discovery (University College London)|Jan 26, 2011

Magnetic and transport properties of perovskites and related materials16 citations

TL;DR

This study investigates the metal-insulator (MI) transition in ultrathin LaNiO3 (LNO) films grown on SrTiO3 substrates, revealing a transition from metallic to strongly localized behavior as thickness decreases, with weak localization effects dominating in an intermediate regime. The key finding is isotropic negative magnetoresistance, indicating magnetic scattering linked to proximity to spin-glass or charge-ordered antiferromagnetic states, despite the absence of long-range order.

ABSTRACT

Transport in ultrathin films of LaNiO3 evolves from a metallic to a strongly localized character as the film's thickness is reduced and the sheet resistance reaches a value close to h/e2, the quantum of resistance in two dimensions. In the intermediate regime, quantum corrections to the Drude low- temperature conductivity are observed; they are accurately described by weak localization theory. Remarkably, the negative magnetoresistance in this regime is isotropic, which points to magnetic scattering associated with the proximity of the system to either a spin glass state or the charge ordered antiferromagnetic state observed in other rare earth nickelates.

Motivation & Objective

• To understand the transport evolution in ultrathin LaNiO3 films as thickness is reduced below 8 unit cells.
• To identify the origin of the metal-insulator transition in two-dimensional LaNiO3 films.
• To investigate the nature of quantum corrections and magnetoresistance anisotropy in the weak localization regime.
• To determine whether electron-electron correlations or magnetic scattering dominate transport near the localization transition.

Proposed method

• Epitaxial LaNiO3 films were grown on (001) SrTiO3 substrates via off-axis rf magnetron sputtering to achieve full strain and c-axis orientation.
• Transport measurements were performed from 1.5 K to 300 K in a He-flux cryostat with a superconducting magnet to probe resistivity and magnetoresistance.
• Sheet resistance and temperature-dependent resistivity were measured to identify metallic, weak localization, and insulating regimes.
• Weak localization theory was applied to fit low-temperature conductivity corrections, using the relation $ k_F l = h/e^2 / R_{\text{sheet}} $, with $ R_{\text{sheet}} \approx 25\,\text{k}\Omega $ as the quantum resistance.
• Magnetoresistance was measured for both in-plane and out-of-plane magnetic field orientations to assess anisotropy.
• The Maekawa-Fukuyama theory was used to interpret isotropic magnetoresistance by introducing magnetic scattering via spin fluctuations and energy relaxation times $ \tau_\varepsilon $ and $ \tau_s $.

Experimental results

Research questions

• RQ1 How does the transport behavior of ultrathin LaNiO3 films evolve with decreasing thickness, particularly near the 2D quantum resistance scale?
• RQ2 What causes the observed isotropic negative magnetoresistance in the weak localization regime, given that orbital contributions should be quenched in in-plane fields?
• RQ3 To what extent do magnetic scattering mechanisms, such as spin fluctuations or disorder-induced magnetic moments, explain the transport anomalies?
• RQ4 Is the metal-insulator transition driven by dimensionality crossover, disorder, or electron correlation effects in the 2D limit?
• RQ5 What is the role of oxygen vacancies and Ni2+ ions in inducing magnetic scattering and proximity to spin-glass or charge-ordered states?

Key findings

• The metal-insulator transition in LaNiO3 films occurs as thickness decreases below 8 unit cells, with the 5 unit cell film showing sheet resistance near $ h/e^2 \approx 25\,\text{k}\Omega $, indicating strong localization.
• In the intermediate regime (7 unit cell film), a low-temperature resistivity upturn is observed and quantitatively explained by weak localization theory with $ \rho = \rho_0 + A T^2 $, where $ A = 7.7 \times 10^{-3}\, \mu\Omega\text{cmK}^{-2} $.
• The magnetoresistance is isotropic for both in-plane and out-of-plane magnetic fields at 1.5 K, with no angular dependence within 0.01%, indicating dominant magnetic scattering.
• The isotropic magnetoresistance is explained by the Maekawa-Fukuyama theory, which accounts for magnetic scattering via spin fluctuations when $ \tau_s > \tau_\varepsilon $ and $ \tau_\varepsilon / \tau_s > \beta D^2 e^2 / \mu_B^2 $ with $ \beta \approx 0.06 $.
• Magnetic scattering is attributed to Ni2+ ions formed by oxygen vacancies or proximity to charge-ordered antiferromagnetic states in the RNiO3 family.
• No clear signatures of strong electron-electron interactions were found in the weak localization regime, but such effects may dominate in the most strongly localized films.