[Paper Review] Negative Spin Valve effects in manganite/organic based devices

Alberto Riminucci, I. Bergenti+7 more|arXiv (Cornell University)|Jan 24, 2007

Magnetic and transport properties of perovskites and related materials3 citations

TL;DR

This study demonstrates a negative spin valve effect in manganite/organic vertical heterostructures, where resistance is lower in the antiparallel magnetic configuration below 210 K. The effect arises from electron transport dominated by resonant tunneling through the narrow LUMO level of Alq3, with strong voltage asymmetry explained by interface dipole-induced Fermi level alignment and a semi-quantitative model based on energy-level matching at the manganite/Alq3 interface.

ABSTRACT

We report detailed investigations of hybrid organic-inorganic vertical spin valves. Spin polarized injection in tris(8-hydroxyquinoline) aluminum (Alq3) organic semiconductor (OS) was performed using La0.7Sr0.3MnO3 manganite as the bottom electrode and Co as the top electrode. While manganite was directly connected to the organic semiconductor layer, a thin tunnel barrier was placed between the OS and the Co electrode. A clear negative spin valve effect - low resistance for antiparallel electrodes configuration - was observed below 210 K in various devices using two different tunnel barriers: LiF and Al2O3. The magnetoresistance effect was found to be strongly asymmetric with respect to the bias voltage. Photoelectron Spectroscopy (PES) investigation of the interface between manganite and Alq3 revealed a strong interface dipole, which leads to a better matching of the metal Fermi level with Alq3 LUMO (1.1 eV) rather than with HOMO level (1.7 eV). This unequivocally indicates that the current in these devices is dominated by the electron channel, and not by holes as previously suggested. The knowledge of the energy diagram at the bottom interface allowed us to work out a semi- quantitative model explaining both negative spin valve effect and strong voltage asymmetry. This model involves a sharp energy selection of the moving charges by the very narrow LUMO level of the organic material leading to peculiar resonant effects.

Motivation & Objective

• To investigate spin-dependent transport in hybrid manganite/organic vertical spin valves.
• To understand the origin of the negative spin valve effect, where resistance is lower in the antiparallel configuration.
• To clarify whether charge transport is dominated by electrons or holes in manganite/organic heterostructures.
• To determine the role of interfacial electronic structure in governing magnetoresistance behavior.
• To develop a semi-quantitative model explaining the observed negative spin valve effect and strong voltage asymmetry.

Proposed method

• Fabricated vertical spin valve devices with La0.7Sr0.3MnO3 (LSMO) as the bottom electrode, Alq3 as the organic semiconductor, and Co as the top electrode.
• Inserted a thin tunnel barrier (LiF or Al2O3) between the Co electrode and Alq3 to control injection and measure magnetoresistance.
• Performed photoelectron spectroscopy (PES) to measure the energy level alignment at the LSMO/Alq3 interface.
• Analyzed the magnetoresistance as a function of temperature and applied bias voltage to identify asymmetries and negative spin valve behavior.
• Constructed a semi-quantitative model based on the LUMO energy level (1.1 eV) and Fermi level alignment to explain resonant electron transport.

Experimental results

Research questions

• RQ1 What causes the negative spin valve effect in manganite/organic vertical heterostructures below 210 K?
• RQ2 Why is the magnetoresistance strongly asymmetric with respect to bias voltage in these devices?
• RQ3 Which charge carrier channel—electrons or holes—dominates transport in LSMO/Alq3-based spin valves?
• RQ4 How does the interfacial dipole at the LSMO/Alq3 junction affect Fermi level alignment and charge injection?
• RQ5 Can a semi-quantitative model explain both the negative spin valve effect and the observed voltage asymmetry?

Key findings

• A clear negative spin valve effect was observed below 210 K, with lower resistance in the antiparallel magnetic configuration.
• The magnetoresistance effect exhibited strong asymmetry with respect to applied bias voltage, indicating non-symmetric injection or transport.
• Photoelectron spectroscopy revealed a strong interfacial dipole at the LSMO/Alq3 junction, aligning the metal Fermi level with the Alq3 LUMO (1.1 eV) rather than the HOMO (1.7 eV).
• This interface dipole confirms that electron transport dominates, contradicting previous suggestions of hole-dominated conduction.
• The observed behavior was explained by a semi-quantitative model based on resonant tunneling through the narrow LUMO level of Alq3.
• The model accounts for both the negative spin valve effect and the strong voltage asymmetry through sharp energy selection of electrons at the interface.