[Paper Review] Large room-temperature magnetoresistance in lateral organic spin valves fabricated by in-situ shadow evaporation

Martin Grunewald, Johannes Kleinlein+4 more|arXiv (Cornell University)|Apr 10, 2013

Organic Light-Emitting Diodes Research3 citations

TL;DR

This study demonstrates lateral organic spin valves with sub-100 nm channel lengths fabricated via in-situ shadow evaporation under UHV conditions, achieving a record 50% room-temperature magnetoresistance. The method enables clean, oxygen-free interfaces between Ni/CoFe electrodes and the n-type organic semiconductor PTCDI-C4F7, with magnetoresistance arising from a nanoscale tunneling contact formed by a ferromagnetic protrusion, not uniform lateral spin transport.

ABSTRACT

We report the successful fabrication of lateral organic spin valves with a channel length in the sub $100\,nm$ regime. The fabication process is based on in-situ shadow evaporation under UHV conditions and therefore yields clean and oxygen-free interfaces between the ferromagnetic metallic electrodes and the organic semiconductor. The spin valve devices consist of Nickel and Cobalt-iron electrodes and the high mobility \emph{n}-type organic semiconductor $N,N'$-bis(heptafluorobutyl)-$3,4:9,10$-perylene diimide. Our studies comprise fundamental investigations of the process' and materials' suitability for the fabrication of lateral spin valve devices as well as magnetotransport measurements at room temperature. The best devices exhibit a magnetoresistance of up to $50\,%$, the largest value for room temperature reported so far.

Motivation & Objective

• To develop a fabrication method for lateral organic spin valves with sub-100 nm channel lengths and contamination-free interfaces.
• To achieve large room-temperature magnetoresistance in organic spin valves by minimizing interfacial oxidation and contamination.
• To investigate the origin of unexpectedly high magnetoresistance values in lateral devices with non-uniform contact geometry.
• To explore the role of ferromagnetic electrode coercivity differences and spin diffusion length in lateral spin transport.

Proposed method

• In-situ shadow evaporation under ultra-high vacuum (UHV) conditions to prevent oxidation and contamination of ferromagnetic electrodes.
• Use of optical lithography and lift-off to pattern a Ni stripe (80 nm thick) on SiO2/Si substrate as the first electrode.
• Evaporation of CoFe (10 nm) at 45° angle to create a sub-100 nm gap via shadowing, forming a bilayer Ni/CoFe contact.
• Sequential in-situ evaporation of the n-type organic semiconductor PTCDI-C4F7 at a controlled angle to fill the gap.
• Equation: l_channel = t_Ni × tan(Θ_evaporation), where t_Ni = 80 nm and Θ_evaporation = 45° yields l_channel ≈ 80 nm.
• Use of a large-area shadow mask to define multiple devices without additional lithography steps.

Experimental results

Research questions

• RQ1 Can in-situ shadow evaporation under UHV produce lateral organic spin valves with sub-100 nm channel lengths and clean interfaces?
• RQ2 Why do some devices exhibit magnetoresistance values as high as 50% at room temperature, exceeding previous reports?
• RQ3 What physical mechanism underlies the hysteretic, non-symmetric magnetoresistance traces observed in these devices?
• RQ4 How do local magnetic inhomogeneities, such as ferromagnetic protrusions, influence the observed magnetoresistance?

Key findings

• The best devices achieved a magnetoresistance of 50%, the highest reported value for room-temperature organic spin valves.
• The magnetoresistance effect is attributed to a nanoscale tunneling junction formed by a small ferromagnetic protrusion on one electrode, not uniform lateral spin transport.
• Device b exhibited a baseline resistance of approximately 500 GΩ and a magnetoresistance of 50%, with hysteretic MR traces showing lateral shifts consistent with exchange bias from NiO.
• The observed MR behavior deviates from standard spin valve models, indicating strong dipolar coupling between the small protrusion and the mm-sized opposing contact.
• The resistance variations across devices (from few GΩ to hundreds of GΩ) correlate with statistical size variations of accidental protrusions, not uniform channel geometry.
• Magnetotransport measurements were limited to room temperature due to current dropping below detection limits at lower temperatures.