Permalloy Electrodes Research Articles

Spin injection and magnetoresistance in MoS2-based tunnel junctions using Fe3Si Heusler alloy electrodes

Recently magnetic tunnel junctions using two-dimensional MoS2 as nonmagnetic spacer have been fabricated, although their magnetoresistance has been reported to be quite low. This may be attributed to the use of permalloy electrodes, injecting current with a relatively small spin polarization. Here we evaluate the performance of MoS2-based tunnel junctions using Fe3Si Heusler alloy electrodes. Density functional theory and the non-equilibrium Green’s function method are used to investigate the spin injection efficiency (SIE) and the magnetoresistance (MR) ratio as a function of the MoS2 thickness. We find a maximum MR of ~300% with a SIE of about 80% for spacers comprising between 3 and 5 MoS2 monolayers. Most importantly, both the SIE and the MR remain robust at finite bias, namely MR > 100% and SIE > 50% at 0.7 V. Our proposed materials stack thus demonstrates the possibility of developing a new generation of performing magnetic tunnel junctions with layered two-dimensional compounds as spacers.

Introduction of Spin–Orbit Interaction into Graphene with Hydrogenation

Introduction of spin-orbit interaction (SOI) into graphene with weak hydrogenation ($\sim$0.1\%) by dissociation of hydrogen silsesquioxane resist has been confirmed through the appearance of inverse spin Hall effect. The spin current was produced by spin injection from permalloy electrodes excluding non-spin relating experimental artifact.

Magnetoresistance effect in Fe20Ni80/graphene/Fe20Ni80 vertical spin valves

Vertical spin valve devices with junctions of single- and bi-layer graphene interlayers sandwiched with Fe20Ni80 (Permalloy) electrodes were fabricated by exploiting the direct growth of graphene on the Permalloy. The linear current-voltage characteristics indicated that ohmic contacts were realized at the interfaces. The systematic characterization revealed the significant modification of the electronic state of the interfacial graphene layer on the Permalloy surface, which indicates the strong interactions at the interface. The ohmic transport was attributable to the strong interface-interaction. The vertical resistivity of the graphene interlayer and the spin asymmetry coefficient at the graphene/Permalloy interface were obtained to be 0.13 Ω cm and 0.06, respectively. It was found that the strong interface interaction modifies the electronic structure and metallic properties in the vertical spin valve devices with bi-layer graphene as well as single-layer graphene.

Role of hexagonal boron nitride in protecting ferromagnetic nanostructures from oxidation

Ferromagnetic contacts are widely used to inject spin polarized currents into non-magnetic materials such as semiconductors or 2-dimensional materials like graphene. In these systems, oxidation of the ferromagnetic materials poses an intrinsic limitation on device performance. Here we investigate the role of ex situ transferred chemical vapour deposited hexagonal boron nitride (hBN) as an oxidation barrier for nanostructured cobalt and permalloy electrodes. The chemical state of the ferromagnets was investigated using x-ray photoemission electron microscopy because of its high sensitivity and lateral resolution. We have compared the oxide thickness formed on ferromagnetic nanostructures covered by hBN to uncovered reference structures. Our results show that hBN reduces the oxidation rate of ferromagnetic nanostructures suggesting that it could be used as an ultra-thin protection layer in future spintronic devices.

Anisotropic magnetoresistance dominant in a three terminal Hanle measurement

Experiments are performed on mesoscopic nonlocal lateral spin valves with aluminum channels and Permalloy electrodes. Four-terminal magnetoresistance and Hanle measurements characterize the spin accumulation with results that compare well with published work. Three-terminal Hanle measurements of the Permalloy/aluminum (Py/Al) interfaces show bell-shaped curves that can be fit to Lorentzians. These curves are three orders of magnitude larger than the spin accumulation. Using anisotropic magnetoresistance measurements of individual Permalloy electrodes, we demonstrate that the three-terminal measurements are dominated by anisotropic magnetoresistance effects unrelated to spin accumulation.

Magnetotransport properties of a few-layer graphene-ferromagnetic metal junctions in vertical spin valve devices

Magnetotransport properties were studied for the vertical spin valve devices with two junctions of permalloy electrodes and a few-layer graphene interlayer. The graphene layer was directly grown on the bottom electrode by chemical vapor deposition. X-ray photoelectron spectroscopy showed that the permalloy surface fully covered with a few-layer graphene is kept free from oxidation and contamination even after dispensing and removing photoresist. This enabled fabrication of the current perpendicular to plane spin valve devices with a well-defined interface between graphene and permalloy. Spin-dependent electron transport measurements revealed a distinct spin valve effect in the devices. The magnetotransport ratio was 0.8% at room temperature and increased to 1.75% at 50 K. Linear current-voltage characteristics and resistance increase with temperature indicated that ohmic contacts are realized at the relevant interfaces.

Spin precession in lateral all-metal spin valves: Experimental observation and theoretical description

Mesoscopic all-metal spin-valve devices containing two ferromagnetic permalloy electrodes and an interconnecting aluminum strip have been studied experimentally and theoretically in nonlocal geometry. Spin-valve devices have been fabricated by thermal evaporation of permalloy electrodes, formation of tunnel barriers by natural oxidation in air or in pure oxygen, and subsequent deposition of an aluminum strip on top of the electrodes. Transport measurements at temperatures of liquid helium have been performed. Spin-dependent phenomena, namely the nonlocal spin-valve effect and spin precession, are observed. A theoretical description of spin-dependent transport is presented including spin diffusion, spin relaxation, spin precession, and tunnel barriers at the interfaces between the electrodes and the aluminum strip. From the comparison of the experimentally observed spin precession to the theoretical description, we obtain a spin-relaxation time of 111 ps and a spin-relaxation length of 1034 nm in aluminum.

Gate-tunable graphene spin valve

The authors perform nonlocal four-probe spin-valve experiments on graphene contacted by ferromagnetic Permalloy electrodes. They observe sharp switching and often sign reversal of the nonlocal resistance at the coercive field of the electrodes, indicating the presence of a spin current between injector and detector. The nonlocal spin-valve signal changes magnitude and sign with back-gate voltage, and is observed up to T=300K. The gate voltage variation of the spin-valve signal may result from quantum-coherent transport, as evidenced by Fabry-Pérot-like oscillations of the current.

Anisotropic magnetoresistance and spin-valve effect in all-metal mesoscopic spin-valve devices

We have fabricated all-metal lateral spin-valve devices consisting of two permalloy electrodes and an interconnecting aluminum strip. The micromagnetic behavior of the device has been imaged with a magnetic-force microscope in external magnetic fields at room temperature. During a single cooling cycle at temperatures between 2 and 120K we have measured the anisotropic magnetoresistance of both electrodes and the magnetoresistance of the entire device. In the latter, we can clearly identify the contributions of the anisotropic magnetoresistance and the mesoscopic spin-valve effect.

Spin-polarized transport in all-metal mesoscopic spin-valve devices

In spintronic devices the spin of the electron as well as its charge is utilized. We have fabricated a spin-valve device consisting of two permalloy electrodes which are connected by a normal-metal strip made of aluminum. By changing the relative alignment of the magnetizations of the electrodes the resistance of the device can be controlled. The domain configurations of the electrodes are imaged by magnetic-force microscopy in external fields at room temperature. In transport measurements at temperatures between 1.8 and 150 K we identify the observed magnetoresistance as a spin-valve effect.

Optimization of nanopatterned permalloy electrodes for a lateral hybrid spin-valve structure

Ferromagnetic electrodes of a lateral semiconductor-based spin-valve structure are designed to provide a maximum of spin-polarized injection current. A single-domain state in remanence is a prerequisite obtained by nanopatterning permalloy thin film electrodes. Three regimes of aspect ratios m are identified by room temperature magnetic force microscopy: (i) high-aspect ratios of m⩾20 provide the favored remanent single-domain magnetization states, (ii) medium-aspect ratios m∼3–m∼20 yield highly remanent states with closure domains, and (iii) low-aspect ratios of m⩽3 lead to multidomain structures. Lateral kinks, introduced to bridge the gap between microscale and macroscale, disturb the uniform magnetization of electrodes with high- and medium-aspect ratios. However, sloped regions (vertical flanks) help to maintain a uniformly magnetized state at the ferromagnet-semiconductor contact by domain wall pinning.

Magnetotransport study of nanoscale Permalloy–Si tunnelling structures in lateral spin-valve geometry

Magnetotransport measurements were performed on a Permalloy/Mg/SiO2/n+-Si (100) spin-valve tunnelling device. Magnetic force microscope imaging proves the single-domain state of the 1 µm elongated and 200 nm wide parallel Permalloy (Py) electrodes. Anisotropic magnetoresistance (AMR) measurements reveal precisely the magnetic field interval of antiparallel magnetization configuration essential for spin-valve operation, in between the switching fields of 15 and 40 mT. Unlike the negative AMR of the Py wires, the tunnelling device shows a small positive magnetoresistance for antiparallel magnetization configuration, as expected for the spin-blockade mechanism.

Hybrid semiconductor/metal nanostructures with two-dimensional electron systems

A brief account of our recent work on InAs/metal hybrids is given. In particular we discuss Josephson field-effect transistors with niobium source and drain contacts, first spin transistor structures with permalloy electrodes, and devices with gold shunts exhibiting the extraordinary magnetoresistance effect.

Magnetic tunneling junctions with permalloy electrodes: a study of barrier, thermal annealing, and interlayer coupling

Magnetic properties of Ni 81Fe 19/Al 2O 3/Ni 81Fe 19 tunneling junctions are studied for different Al thicknesses and plasma oxidation times. A maximal magnetoresistance of 34% is obtained with Al thickness of 20 Å. Magnetometry reveals large exchange bias fields (∼400 Oe) over a wide range of barrier thicknesses, indicating junctions of high quality. Transport measurements conducted on junctions before and after thermal annealing show a dramatic improvement in barrier quality after annealing. Interlayer coupling fields have been measured as a function of barrier thickness for different oxidation times.

Low frequency noise in Co/Al2O3⟨δ(Fe)⟩/Ni80Fe20 magnetic tunnel junctions

The time dependences, up to 200 s, and the noise power spectrum (0.005-10 Hz) in the electron transport response at bias up to 300 mV of Co/Al2O3/Ni80Fe20 magnetic tunnel junctions (MTJs) and of Co/Al2O3⟨δ(Fe)⟩/Ni80Fe20 (with Fe δ dopants of thickness1.8 Å inside the barrier) were investigated. The magnetic field waschanged between +100 and -100 G in steps of 1 G. The measurements werecarried out at different temperatures between 77 and 300 K for the sampleswith large tunnel magnetoresistance (exceeding 14% at 300 K). We foundthat the magnetization reversal of the Co and permalloy electrodes, asdetected from the time response near the coercive field, occurs viarelaxation on the timescale of about 102 s with sudden jumps in theresistance (ΔR/R~10-2-10-3). We link this noise tothe depinning of the domain walls. In addition to the magnetic noise, insome of the studied MTJs with Fe δ dopants, we observed atwo-level-system telegraph-type noise, which was independent of themagnetic field, indicating its relation to the trapped charges inside theinsulating barrier. For MTJs, the noise power spectrum has 1/fαcharacter for a wide frequency range below a few Hz. At low bias and parallel state the exponent α is close to 1-1.5, but at higher bias or inthe antiparallel state the exponent increases to 2. We link these effects tonon-equilibrium noise in magnetic structure of the electrodes in the antiparallelstate and to non-equilibrium transport inside the barrier at high bias.

Investigation of ferromagnetic microstructures as electrodes for spin-polarized transport experiments

Patterned ferromagnetic structures are intended as electrodes to drive a spin-polarized current through the quasi two-dimensional electron system of a semiconductor. To utilize such structures in transport experiments it is important to know their micromagnetic behavior. We have prepared Permalloy electrodes for the ballistic transport regime by electron-beam lithography on InAs single crystals. Using magnetic-force microscopy we measure the magnetization configurations at zero external magnetic fields. Micromagnetic simulations verify that the observed magnetization states are the energetically most favorable ones. In addition, the micromagnetic behavior in external magnetic fields is simulated.

Magnetic tunnel junctions fabricated at tenth-micron dimensions by electron beam lithography

Magnetic tunnel junctions consisting of permalloy and cobalt thin film electrodes, separated by a thin aluminum oxide tunnel barrier, have been fabricated by e-beam lithography at dimensions down to 120 nanometers. The devices are fabricated by sputter deposition and ion milling. They exhibit magnetoresistances of up to 22% at room temperature. Evidence of individual domain switching is observed. The smaller junctions have resistances in the kilohm range, which are easily measured, leading to the possibility of sensing and microelectronic applications.