Ru Thickness Research Articles

Perpendicular magnetic tunnel junctions with synthetic antiferromagnetic pinned layers based on [Co/Pd] multilayers

We fabricated MgO-based perpendicular magnetic tunnel junctions (p-MTJ) with Ta/CoFeB magnetic electrodes. Synthetic antiferromagnetic (SAF) pinned layers with perpendicular magnetic anisotropy (PMA) were included into the p-MTJs by using two Co/Pd multilayers (MLs) separated by a thin Ru spacer layer. The MTJs stack has the structure bottom contact/free layer CoFeB (1.0)/MgO (1)/pinned layer CoFeB (1.0)/Ta spacer layer/SAF/Ru cap layer/top contact (the units in parenthesis are in nanometers). The SAF was optimized by changing the repetition period n in one of the Co/Pd multilayers and the Ru thickness in order to obtain PMA with antiferromagnetic (AFM) coupling. The Ru spacer values were 0.7, 0.75, 0.8, 0.85, and 0.9 nm. The magnetic studies show that all magnetic films, including the Ta/CoFeB layers, are perpendicularly magnetized. The two Co/Pd MLs are AFM coupled for n > 2. Controlling the Ru thickness, the interlayer exchange coupling strength Jiec can be tailored. Jiec vs. Ru thickness exhibits a simple exponential decay. The electrical properties of the full p-MTJ with SAF show a low resistance-area (RA) product of 44.7 Ω μm2 and a tunnel magnetoresistance (TMR) ratio of 10.2%.

Influence of a Dy overlayer on the precessional dynamics of a ferromagnetic thin film

Precessional dynamics of a Co50Fe50(0.7)/Ni90Fe10(5)/Dy(1)/Ru(3) (thicknesses in nm) thin film have been explored by low temperature time-resolved magneto-optical Kerr effect and phase-resolved x-ray ferromagnetic resonance measurements. As the temperature was decreased from 300 to 140 K, the magnetic damping was found to increase rapidly while the resonance field was strongly reduced. Static x-ray magnetic circular dichroism measurements revealed increasing ferromagnetic order of the Dy moment antiparallel to that of Co50Fe50/Ni90Fe10. Increased coupling of the Dy orbital moment to the precessing spin magnetization leads to significantly increased damping and gyromagnetic ratio of the film while leaving its magnetic anisotropy effectively unchanged.

Interlayer exchange coupling between [Pd/Co] multilayers and CoFeB/MgO layers with perpendicular magnetic anisotropy

Interlayer exchange coupling between [Pd/Co] multilayers and CoFeB/MgO layers with perpendicular magnetic anisotropy (PMA) is investigated as functions of the thicknesses of the Ru spacer and CoFeB layer. The dependence of the coupling behavior on the Ru thickness is similar to that of in-plane anisotropy systems. However, one feature is that the PMA is strengthened through interlayer exchange coupling, as indicated by the fact that PMA of the interface-based CoFeB/MgO structure forms for a thick magnetic layer (1.4 nm). Another observation is the conversion from perpendicular to in-plane anisotropy with thick Ru spacers with almost zero exchange coupling strength.

Enhancement of magnetoresistance using CoFe/Ru/CoFe synthetic ferrimagnetic pinned layer in BiFeO3 based spin-valves

SrTiO3 (100) sub/BiFeO3/CoFe/Ru/CoFe/Cu/CoFe/Ta structure was prepared by a combination of chemical solution deposition and sputtering method, and followed by a systematical investigation for the structural, magnetic and magnetoresistance properties at room temperature (RT) as a function of CoFe and Ru thicknesses. It was revealed that introduction of synthetic CoFe/Ru/CoFe as a pinning layer increased the giant magentoresistance (MR) ratio to 8.3% at RT. This enhancement of MR ratio might be attributed to (i) the increase of pinning field, and (ii) suppression of the influence of the surface roughness of BiFeO3 by inserting the synthetic CoFe/Ru/CoFe layer.

Enhancement of perpendicular magnetic anisotropy thanks to Pt insertions in synthetic antiferromagnets

Synthetic antiferromagnets are of great interest as reference layers in magnetic tunnel junctions since they allow decreasing the dipolar coupling between the two magnetic electrodes and exhibit larger pinning fields than single reference layers. In this letter, we investigate the effect of the insertion of an ultrathin Pt layer in contact with the Ru spacer in synthetic antiferromagnets with perpendicular magnetic anisotropy. Surprisingly, for Ru thickness below 0.75 nm, the antiferromagnetic coupling amplitude through Ru first increases upon Pt insertion up to a critical Pt thickness (∼0.25 nm) above which coupling decreases. In addition, the corresponding increase of perpendicular magnetic anisotropy enhances the thermal stability of the structure.

Si/NiFe seed layers for Ru intermediate layer in perpendicular magnetic recording tape media

Si/NiFe seed layers prepared at room temperature is effective to attain better c-axis orientation of Ru intermediate layer in the FeCoB/Ru/CoPtCr-SiO2 granular type recording tape media. The crystallinity and c-axis orientation of Ru layer with Si/NiFe seed layers were improved than that without Si/NiFe seed layer deposited on the laminated FeCoB SULs. When the Ru is thicker than 8 nm, Δθ50 of the CoPtCr-SiO2 recording layer shows small value of about 6.5°. Furthermore, even though the Ru thickness was only 3 nm, the Δθ50 retained comparatively small value of 8.0°. Si/NiFe layer is effective as a seed layer for the Ru intermediate layer.

Ru employed as Counter Electrode for TCO-less Dye Sensitized Solar Cells

A TCO-less ruthenium (Ru) catalytic layer on glass substrate instead of conventional Ru/TCO/ glass substrate was assessed as counter electrode (CE) material in dye sensitized solar cells (DSSCs) by examining the effect of the Ru thickness on the DSSC performance. Ru films with different thicknesses (34, 46, 69, and 90 nm) were deposited by atomic layer deposition (ALD) on glass substrates to replace both existing catalyst and electrode layer. In order to make our comparison, we also prepared an Ru catalytic layer by a similar method on FTO/glass substrate. Finally, we prepared the 0.45 cm 2 DSSC device the properties of the DSSCs were examined by cyclic voltammetry (CV), impedance spectroscopy (EIS), and current-voltage (I-V) method. CV measurementsrevealed an increase in catalytic activity with increasing film thickness. The charge transfer resistance at the interface between the electrolyte and Rudecreased with increasing Ru thickness. I-V resultsshowed that the energy conversion efficiency increased up to 1.96%. Our results imply that TCO-less Ru/glass might perform as both catalyst and electrode layer when it is used in counter electrodes in DSSCs. (Received November 1, 2011)

Effect of the thickness of the Ru-coating on a counter electrode on the performance of a dye-sensitized solar cell

A ruthenium (Ru) catalytic layer was assessed as the counter electrode (CE) in dye sensitized solar cells (DSSCs) by examining the effect of the Ru thickness on the DSSC performance. Ru films with different thicknesses (34, 46, 69 and 90 nm) were deposited on glass/fluorine-doped tin oxide (FTO) substrates as the CE by atomic layer deposition (ALD) at 250 °C using RuDi as the precursor and O2 as the reaction gas. Finally, a 0.45 cm2 DSSC of glass/FTO/TiO2/dye(N719)/electrolyte(C6DMII, GSCN)/Ru CE structure was prepared. The properties of the DSSCs were examined by field emission scanning electron microscopy (FESEM), four-point-probe, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), current-voltage (I–V), incident photon-to-current conversion efficiency (IPCE), and dark current measurements. FESEM showed that the crystallized Ru films had been deposited quite uniformly and conformally on the glass/FTO surface. The sheet resistance of the Ru film decreased with increasing Ru thickness. CV profiling revealed an increase in catalytic activity with increasing film thickness. The charge transfer resistance at the interface between the Ru-coated CE and electrolyte decreased with increasing Ru thickness. I–V profiling showed that the energy conversion efficiency was increased up to 3.40 % by increasing the Ru thickness. Moreover, the IPCE and dark current results showed the efficiency of the Ru-coated CE was comparable to that of a conventional platinum (Pt) CE.

Non-collinear magnetization configuration in interlayer exchange coupled magnetic thin films

Element specific magnetic hysteresis loops of the interlayer exchange coupled CoFeB/Ru/[Co/Ni]4 structure were measured utilizing x-ray magnetic circular dichroism. It was found that the Co/Ni multilayer and the CoFeB layer have Ru thickness dependent oscillatory interlayer coupling. Due to its interlayer coupling with the perpendicularly magnetized Co/Ni multilayer, the CoFeB magnetization direction is slightly tilted out-of-plane from its in-plane magnetic easy axis. Quantitative measurements show that the tilting angle is small (<12°) and that a small in-plane magnetic field (∼50 Oe) applied to this structure will result in a completely in-plane CoFeB magnetization.

Magnetization precession and domain-wall structure in cobalt-ruthenium-cobalt trilayers

The magnetization dynamics of Co(5 nm)/Ru/Co(5 nm) trilayers with Ru thicknesses from 0.3–0.6 nm is experimentally and theoretically investigated. The coupling between the Co layers is antiferromagnetic (AFM) and yields a stable AFM domain structure with frozen domain walls. Comparing high-resolution magnetic force microscopy (MFM) and pump-probe measurements, we analyze the behavior of the films for different field-strength regimes. For moderate magnetic fields, pump-probe measurements provide dynamic characterization of the coupled precessional modes in the GHz range. The dynamics at small fields is realized by the pinning of AFM domain walls at inhomogeneities. The MFM images yield a domain-wall width that varies from about 150–60 nm. This behavior is explained in terms of a micromagnetic local-anisotropy model.

Ruthenium based metals using atomic vapor deposition for gate electrode applications

The impacts of ruthenium-based metal gate electrodes (Ru,RuOx,RuSiOx) with atomic vapor deposition (AVD) on flatband voltage (VFB) and equivalent oxide thickness (EOT) are demonstrated using a low temperature (<400 °C) process. Increasing thickness of Ru and RuOx exhibits higher VFB, attributed to filling oxygen vacancies [Vo] in high-k gate dielectric with oxygen supplied from AVD metal gate electrodes upon annealing. Ru is efficient to attain a higher work-function and thinner EOT compared to RuOx and RuSiOx. Subsequent physical-vapor-deposition (PVD) TiN capping on AVD metals blocks oxygen out-diffusion, leading to higher VFB than PVD W or AVD TiN capping.

Perpendicular L1 0-FePt/Fe and L1 0-FePt/Ru/Fe graded media obtained by post-annealing

L1 0-FePt/Fe and L1 0-FePt/Ru/Fe films with perpendicular orientations were fabricated by magnetron sputtering and post-annealing. In comparison to films with sharp interfaces, the graded L1 0-FePt/Fe interface was more favorable for coercivity reduction. Inserting a ruthenium (Ru) interlayer between the hard L1 0-FePt and the soft iron (Fe) layers yielded a continuous L1 0-FePt/Fe graded film, which contributed to a large reduction in coercivity. The coercivity of L1 0-FePt/Ru/Fe films can be manipulated via the Ru thickness when the soft Fe layer is thin and via the Fe thickness when the soft Fe layer is thick.

Co/Ni]N-Based Synthetic Antiferromagnet with Perpendicular Anisotropy and Its Application in Pseudo Spin Valves

The dependence of magnetic properties on layer repetition number and Ru thicknesses have been studied for perpendicularly magnetized synthetic antiferromagnet (SAF) in a structure of [Co/Ni] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> /Ru/[Co/Ni] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . The optimum SAF with strong antiferromangetic coupling field and large switching field of the net magnetization have been determined and utilized as the reference layer in the pseudo spin valves. Compared with the rapid drop of GMR signal for the normal [Co/Ni]-based pseudo spin valves after annealing at low temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) of 150°C, the spin valve with SAF reference layer exhibits much stable thermal stability due to the large switching field difference between the free and reference layers which avoids the simultaneous magnetization rotation. The GMR signal of the SAF spin valve sample is 6.0% at room temperature, it decreases very gradually with the increase of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> . We attribute the slow GMR reduction observed in the SAF spin valve to the effects of domain formation and perpendicular anisotropy deterioration caused by high temperature anneals.

Magnetic antiphase domains in Co/Ru/Co trilayers

Ultrathin Co/Ru/Co trilayers are investigated experimentally by magnetization curves and magnetic-force microscopy (MFM). Emphasis is on the domain-wall fine structure of antiphase domain walls in the films. The trilayers are produced by sputtering and consist of two Co layers of equal thickness (5 nm), exchange-coupled through a Ru layer of variable thickness. The sign and magnitude of the interlayer exchange are tuned by the thickness of the Ru interlayer. The exchange and its distribution are investigated by measurements of the static magnetization curves. For a Ru thickness of 0.4 nm, the exchange is predominantly antiferromagnetic and the MFM images show fairly immobile domain walls. Micromagnetic model calculations yield immobile antiphase domain walls whose thickness decreases with increasing magnetic field but is typically of the order of 100 nm in agreement with experiment.

Synthetic antiferromagnet with Heusler alloy Co2FeAl ferromagnetic layers

Heusler alloy Co2FeAl was employed as ferromagnetic layers in Co2FeAl (3nm)∕Ru (xnm)∕Co2FeAl (5nm) synthetic antiferromagnet structures. The experimental results show that the structure with a Ru thickness of 0.45nm is strongly antiferromagnetic coupled, which is maintained after annealing at 150°C for 1h. The structure has a very low saturation magnetization Ms of 425emu∕cm3, a low switching field Hsw of 4.3Oe, and a high saturation field Hs of 5257Oe at room temperature, which are favorable for application in ultrahigh density magnetic read heads or other magnetic memory devices. Crystal structure study testifies that the as-deposited Co2FeAl film is in the B2 phase. Therefore, Heusler alloys can be used to fabricate synthetic antiferromagnetic and it is possible to make “all-Heusler” spin valves or magnetic tunneling junctions with better magnetic switching properties and high magnetoresistance.

Magnetization studies in IrMn/Co/Ru/NiFe spin valves with weak interlayer coupling

The magnetic interactions in the Ir20Mn80/Co/Ru/Py (Py=Ni81Fe19) spin valve structures have been studied by magnetization measurements. The Ru thickness was varied between 24 and 58 Å, and the thicknesses of the other layers were keep constant, resulting in multilayers with a strong exchange bias field (Heb) and weak oscillatory interlayer coupling. The bilinear and biquadratic coupling constants between Co and Py and the exchange anisotropy at the IrMn/Co interface were considered for modeling the magnetic hysteresis loops of the whole system. The interplay between these interactions gives rise to an increase (decrease) in the Heb value of the Co pinned layer for antiferromagnetic (ferromagnetic) interlayer coupling, if compared to the system with no interlayer coupling, and in a small exchange anisotropy field, proportional to the bilinear coupling energy, induced to the Py free layer.

Inverted magnetoresistance in dual spin valve structures with a synthetic antiferromagnetic free layer

We report an oscillation of the giant magnetoresistance (GMR) ratio as a function of Ru layer thickness in the CoFe/Cu/[CoFe/Ru/CoFe]SAF/Cu/CoFe/IrMn dual spin valve (SV) structure. A normal GMR with a positive sign is observed for the thickness of Ru providing a ferromagnetic interlayer exchange coupling (IEC). The inverted GMR is observed for the thickness of Ru providing an antiferromagnetic IEC, which is consistent with IEC period across the Ru spacer as well as the electrical separation of the overall structure into two SVs connected in parallel.

Negative Magnetoresistance in Dual Spin Valve Structures With a Synthetic Antiferromagnetic Free Layer

Giant magnetoresistance (GMR) in spin valves is due to spin-dependent scattering occurring at ferromagnet/normal metal (F/N) interfaces and/or in the ferromagnetic layers. In a spin valve with a typical F/N/F structure where the spin scattering asymmetry factor (alpha) of both F/N interfaces is the same (more or less than 1), the GMR is expected to be positive. If alpha is greater than one at one F/N interface and less than one at the other F/N interface, however, the GMR is expected to be negative. Here, we show that the F1/Cu/SAF/Cu/F2/IrMn dual spin valve structure exhibits negative GMR, where F1 and F2 are CoFe and SAF = CoFe/Ru t/CoFe, due to both opposite electron spin scattering asymmetry factor at the CoFe/Ru/CoFe interfaces as well as the electrical separation of the overall structure into two GMR spin valves connected in parallel. A GMR of 6% is observed in the structure without the Ru spacer layer, insertion of a 0.6 nm thick Ru in the SAF results in a negative GMR ratio of -3% , which becomes positive again at the Ru thickness of 0.8 nm, the oscillation from positive to negative MR is consistent with interlayer exchange coupling period across the Ru spacer.

Magnetic phases and structural properties in Co/Ru superlattices

We report studies by x-ray diffraction, ferromagnetic resonance (FMR), and x-ray magnetic circular dichroism (XMCD) in Co/Ru superlattices grown by magnetron sputtering. We studied the [Co(50 Å)/Ru(tRu)]20 samples, which were deposited at room temperature on Si substrates with the Ru thicknesses, tRu, varying between 9 and 33 Å. The main and secondary uniform absorption modes, observed in the FMR spectra, are associated with the Co/Ru interfaces and the bulk Co regions, respectively. The main mode becomes more intense than the secondary one for increasing tRu. This is attributed to the roughness and/or atomic interdiffusion, which leads, with increasing tRu, to an increasing volume of Co/Ru interfacial regions and a decreasing volume of pure Co regions. The XMCD measurements provide Co spin magnetic moments lower than the bulk Co value, confirming the presence of a Co magnetic region with a lower local effective magnetization attributed to the Co/Ru interfaces.

MFM studies of interlayer exchange coupling in Co/Ru/Co films: Effect of Ru layer thickness

Antiferromagnetically coupled magnetic thin films are promising candidates for the design of new magnetic storage and logic devices. The ability to control the interlayer thickness, therefore the magnetic reversal response, of exchange-coupled magnetic layers is of paramount importance in nanotechnology, especially in magnetic sensing element design and applications. In this work, magnetic force microscopy (MFM) with improved sensitivity and high spatial resolution probes was used to obtain a more detailed view of magnetization reversal behavior and domain evolution in the indirect exchange-coupled trilayer system: Co/Ru/Co. The effect of the variable Ru interlayer thickness on the exchange coupling and thus the magnetic domain structure during the ferromagnetic (FM)/antiferromagnetic (AF) coupling transition in Co/Ru/Co films is well demonstrated. The MFM images display a distinct signature of AF coupling for the films with Ru thickness of 0.4 nm. MFM has proven to be an effective tool for detecting FM/AF interlayer coupling and exploring magnetic domain structures in exchange-coupled layered thin films.