Ultrahigh-vacuum Sputtering System Research Articles

Hetero-interface effect on Gilbert damping in nonmagnetic metal/permalloy/nonmagnetic metal trilayers

Gilbert damping in nonmagnetic metal (NM)/permalloy (Py)/nonmagnetic metal (NM) trilayers was studied using ferromagnetic resonance (FMR) measurements for different constituent materials (NM = Pt, Ta, Cu, and Ti) of varying Py thickness (t = 2, 3, 4, 5, 6, 8, and 10 nm) fabricated using an ultrahigh vacuum sputtering system. We found that the Gilbert damping constant increases with decreasing Py thickness and varies for different constituents of nonmagnetic metals. For NM = Pt, the damping constant of Py increases with decreasing Py thickness more remarkably than for other nonmagnetic metals. According to quantitative estimations of the effective spin-mixing conductance Geffmix, it was found that the value of Geffmix = 1.17 × 1015 Ω−1·m−2 for Pt is one order of magnitude greater than Geffmix = 0.13 × 1015 Ω−1·m−2 for Cu and 0.11 × 1015 Ω−1·m−2 for Ti. The large Geffmix value for Pt/Py/Pt is known to be caused by the strong spin-orbit coupling (SOC) of Pt. The effective spin-mixing conductance for Pt is 3 times greater than that (0.27 × 1015 Ω−1·m−2) for Ta despite the SOC of both Pt and Ta is similar in magnitude. This work allows not only for an understanding of the interface effect of the Gilbert damping constant on NM/Py/NM trilayers but also for optimal design of low-power-consumption spintronic devices that are composed of a variety of hetero-interfaces between the different magnetic and nonmagnetic layers.

Hydrogen sensor based on palladium-yttrium alloy nanosheet

This paper presents a hydrogen sensor based on palladium-yttrium (Pd-Y) alloy nanosheet. Zigzag-shaped Pd-Y nanosheet with a thickness of 19.3 nm was deposited on a quartz substrate by using an ultrahigh-vacuum magnetron sputtering system and shadow mask. The atomic ratio of palladium to yttrium in the nanosheet was 0.92/0.08. The fabrication process was simple and low-cost, and the sensor can be mass-produced. The experimental results show the sensor has a superior sensitivity, reversibility, and reproducibility. The resistive-based hydrogen detection mechanism in this research is much simpler and more compact compared to the optical-based detection method.

Relationship Between Magnetostriction and Magnetic Domain Structure in Fe-Based Alloy Single-Crystal Films With bcc(001) Orientation

Fe, Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">94</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> , Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">98</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">95</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> (at. %) single-crystal thin films of bcc(001) orientation are prepared on MgO(001) substrates using an ultrahigh-vacuum magnetron sputtering system. The influence of magnetic anisotropy on the magnetostriction under rotating magnetic field is investigated. Fe, Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">94</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> , and Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">98</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films show four-fold symmetries in in-plane magnetic anisotropy, where easy and hard magnetization directions are, respectively, observed along bcc[100] and bcc[110]. Higher saturation fields measured along bcc[110] are recognized in the order of Fe (6 kOe) > Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">98</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (5 kOe) > Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">94</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> (4 kOe). Triangle output waveforms of magnetostriction appear when these films are measured under low-rotating fields. The behavior is related with the motion of 90° magnetic domain walls in magnetically unsaturated film. With increasing the rotating field, the films approach to magnetic saturations and the waveform gradually shifts to usual sinusoidal shape. Larger threshold rotating field is observed in the order of Fe > Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">98</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> > Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">94</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> , which is related to the saturation field. On the contrary, Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">95</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> film possesses an almost isotropic property in in-plane measurement and shows sinusoidal waveforms even when the film is magnetically unsaturated. The waveform is affected by the symmetry and the strength of magnetic anisotropy.

Indigenous development of ultra high vacuum (UHV) magnetron sputtering system for the preparation of Permalloy magnetic thin films

We have designed and developed an indigenous ultra high vacuum (UHV) sputtering system which can deposit magnetic thin films with high purity and good uniformity. The equipment consists of state-of the-art technologies and sophistication. Turbo-molecular pump combined with sputter ion pump is used to pump down the vacuum chamber up to 10−10 to 10−11 mbar of pressure. With this system it is possible to deposit coatings of various materials on a sample size of diameter 3". The Ni81Fe19 ferromagnetic thin films, with Tantalum (Ta) as a buffer and cap layers have been deposited on silicon substrates using this ultra high vacuum (UHV) sputtering system. The magneto transport measurement study indicated a significant variation in the AMR values of the films for varying thicknesses of tantalum and NiFe layers.

Vector control of induced magnetic anisotropy using an in situ quadrupole electromagnet in ultrahigh vacuum sputtering

We demonstrate the incorporation of a quadrupole electromagnet into an ultrahigh vacuum sputtering system for the vector control of induced magnetic anisotropy in magnetic thin-film heterostructures. A stationary quadrupole electromagnet is used to generate a magnetic field, which rotates synchronously with the physical axes of the substrate in situ during sputtering. An arbitrary anisotropy direction can be set for successive ferromagnetic layers by adjusting the phase difference of substrate and field rotation. The ability to rotate the substrate during deposition and change anisotropy without breaking vacuum enables the deposition of magnetically soft heterostructures with arbitrary in-plane anisotropy axes.

Si基板上に作製したNd-Fe-B/Nd-Cu薄膜の磁場中磁区構造観察

It is well known that Nd-Fe-B magnet shows high performance as permanent magnets. It has been applied for various products such as sensors, actuators, and the motors for hybrid and electric vehicles, because of its high magnetocrystalline anisotropy and high saturation magnetization. From environmental issues, further high performance permanent magnet is needed. However, the coercivity of Nd-Fe-B magnets has reached only about 20% of the theoretical values. In order to understand the magnetization process of Nd-Fe-B alloys, the control of the grain size and the grain boundary is of importance. In this work, Nd-Fe-B/Nd-Cu thin films and circular dot arrays with 2 μm in diameter have been prepared by using ultra high vacuum sputtering system, electron beam lithography and Ar ion etching system and their magnetic properties have been investigated. Remarkable difference of the magnetization process between the Nd-Fe-B thin films with and without Nd-Cu additional layer was observed.

Crystal Orientation of Epitaxial α-Ta(110) Thin Films Grown on Si(100) and Si(111) Substrates by Sputtering

We have prepared Ta thin films on Si(100) and Si(111) substrates by increasing the substrate temperature from room temperature (RT) to 500 °C, using an ultrahigh-vacuum dc magnetron sputtering system. The obtained film crystallinities were evaluated by X-ray diffraction and X-ray pole figure analyses. It was found that the crystal phase of the prepared Ta films transforms gradually from the β-Ta phase to the α-Ta phase with increasing substrate temperature, and the epitaxial growth of the α-Ta(110) plane was realized at substrate temperatures of 400 °C on Si(100) and 200 °C on Si(111). In addition, it was also clarified that the α-Ta(110) films are grown epitaxially in a twin state on Si(100), while they are grown epitaxially in a triplet state on Si(111).

Epitaxial Growth of (0001)Ru Thin Films on (111)ZrN/(111)Si by Low-Temperature Process and Their Surface Morphologies

We have investigated the epitaxial growth of (0001)Ru thin films on (111)ZrN/(111)Si epitaxial systems at room temperature using an ultrahigh-vacuum dc magnetron sputtering system. The film quality obtained and the epitaxial relationship were evaluated by X-ray diffraction, X-ray pole figure, grazing-incidence-angle X-ray reflectivity, and atomic force microscopy analyses. It was found that (0001)Ru films with a thickness of 100 nm were grown epitaxially on a (111)ZrN(40 nm)/(111)Si system even at room temperature with the directional relationship of Ru(0001)[1120]∥ZrN(111)[110]∥Si(111)[110]. In addition, it was also clarified that an epitaxial (0001)Ru film with a thickness of 3 nm on a (111)ZrN (6 nm)/(111)Si system can be regarded as a dense film with the same density as the bulk, and exhibits a fairly flat surface morphology with an average surface roughness of approximately 0.2–0.3 nm.

Effect of Substrate Temperature on Epitaxial Orientation of Rh Thin Films Sputtered on A-Plane Sapphire

We have examined the control of the epitaxial orientation of Rh films with a thickness of 100 nm on (1120) sapphire by changing substrate temperature (Ts) under the low-deposition-rate condition (deposition rate: Rd=1.0 nm/min) using an ultrahigh-vacuum sputtering system, because the lattice mismatches between the two-dimensional superlattice cells of (111)Rh/(1120) sapphire and (001)Rh/(1120) sapphire systems are approximately the same. As a result, it was revealed that the epitaxial orientation of Rh film can be changed from a mixed state of the epitaxially grown (001) and (111)Rh planes to a single state of the epitaxially grown (001)Rh plane on (1120) sapphire by adjusting only Ts.

Control of Epitaxial Growth Plane of Rh Thin Films on A-Plane Sapphire by Sputter Deposition

We have expected that the epitaxial growth of (111)Rh and (001)Rh films with a thickness of 100 nm can be controlled on (11 2 0) sapphire by changing sputtering parameters using an ultrahigh-vacuum sputtering system, because the lattice mismatches between the two-dimensional superlattice cells of (111)Rh/(11 2 0) sapphire and (001)Rh/(11 2 0) sapphire systems are the same. Thus, the effect of rf sputtering power (Prf) on each epitaxial growth was examined, and the crystal quality, surface morphology, resistivity and temperature coefficient of resistivity (TCR) of the prepared Rh films were evaluated. As a result, it is revealed that both the (111)Rh and (001)Rh films can be grown epitaxially on (11 2 0) sapphire by adjusting only the values of Prf (deposition rate: Rd) under the condition of the substrate temperature (Ts) of 500 °C. In addition, it is confirmed that the average surface roughness of these epitaxially grown Rh films is below approximately 1.2 nm, and resistivity of approximately 5.6 ×10-6 Ω cm is obtained.

Developing SmCo5 perpendicular magnetization thin films for magnetic recording media

A series of our studies on sputter-deposited SmCo5 thin films exhibiting very high perpendicular magnetic anisotropy is reviewed. Introduction of a Cu underlayer is a key to imparting perpendicular magnetic anisotropy to SmCo5 thin films. Perpendicular magnetic anisotropy was enhanced by depositing the Sm-Co layer with the alternate lamination of Sm and Co sublayers and by using the Cu/Ti dual underlayer for producing a smooth film surface. As a result, under an optimized condition, the magnetic anisotropy constant was as large as 4.0 x 107 erg/cm3, which is greater by a factor of ten than that of material used for conventional magnetic recording media. Furthermore, we found that high perpendicular magnetic anisotropy was obtained even in very thin films and that the coercivity was controllable in a wide range when the films were prepared using an ultra-high vacuum sputtering system. Microstructural analysis suggested that the Cu-rich region in the initial growth stage of the Sm-Co layer played an important role in generating perpendicular magnetic anisotropy. In addition, we tested SmCo5 thin films for use in conventional and heat-assisted magnetic recording media.

Large tunnel magnetoresistance at room temperature with a Co2FeAl full-Heusler alloy electrode

Magnetic tunnel junctions (MTJs) with a Co2FeAl Heusler alloy electrode are fabricated by the deposition of the film using an ultrahigh vacuum sputtering system followed by photolithography and Ar ion etching. A tunnel magnetoresistance (TMR) of 47% at room temperature (RT) are obtained in a stack of Co2FeAl∕Al–Ox∕Co75Fe25 magnetic tunnel junction (MTJ) fabricated on a thermally oxidized Si substrate despite the A2 type atomic site disorder for Co2FeAl. There is no increase of TMR in MTJs with the B2 type Co2FeAl, which is prepared by the deposition on a heated substrate. X-ray photoelectron spectroscopy (XPS) depth profiles in Co2FeAl single layer films reveal that Al atoms in Co2FeAl are oxidized preferentially at the surfaces. On the other hand, at the interfaces in Co2FeAl∕Al–Ox∕Co75Fe25 MTJs, the ferromagnetic layers are hardly oxidized during plasma oxidation for a formation of Al oxide barriers.

Effects of Sputtering Parameters on the Formation of Single-Oriented (002) Ti Film on Si

We investigated the preparation conditions of a single-oriented (002) Ti film on a Si substrate, as functions of sputtering parameters, using an ultrahigh-vacuum dc magnetron sputtering system. We found that a single-oriented (002) Ti film with an excellent electrical property grew on (001) and (111) Si under an optimum sputtering power of 100 W and an optimum substrate temperature of 150°C. It was revealed that this is due to the formation of an amorphous silicide interlayer at the Si interface as a buffer layer for relaxing the misfit.

Spin-dependent quantum oscillations in magnetic tunnel junctions with Ru quantum wells

The influence of quantum-well (QW) states formed in a Ru spacer layer on tunneling magnetoresistance (TMR) was investigated. A magnetic tunnel junction (MTJ) with a Ru spacer inserted between an $\mathrm{Al}\mathrm{O}x$ tunnel barrier and one of the ferromagnetic ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}$ layers was deposited on a single crystal MgO(110) substrate using an ultrahigh vacuum sputtering system. We clearly observe an oscillation of TMR ratios as a function of Ru thickness up to $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ with the same phase previously observed in an interlayer exchange coupling. The TMR ratios also oscillate and change their signs under both positive and negative bias voltage applications for the Ru thickness more than $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. These periodical oscillations are caused by the oscillation of spin polarization derived from the contribution of QW states of up spin electrons confined in the Ru spacer layer to the spin-dependent electron transport.

Magnetic switching properties of magnetic tunnel junctions using a synthetic ferrimagnet free layer

Magnetic tunnel junctions (MTJs) using a synthetic ferrimagnet (SyF) free layer consisting of Co90Fe10/Ru/Co90Fe10 were deposited on a thermally oxidized Si substrate using an ultrahigh-vacuum sputtering system, and were patterned into micron to submicron sizes using electron-beam lithography and Ar ion milling. Magnetic switching properties and tunneling magnetoresistance (TMR) were investigated. A SyF free layer can maintain high remanence even for the aspect ratio of 1, and exhibits a size-independent switching field in all element widths from 0.25 to 16 μm investigated. Additionally, these MTJs show large TMR ratio up to 40% after annealing at 250 °C for 60 min. These results demonstrate that a SyF free layer can be applied to future spintronics nanodevices.

Magnetization reversal and domain structure of antiferromagnetically coupled submicron elements

Switching characteristics and magnetic domain structures in submicron size synthetic antiferromagnets (SyAF), Co90Fe10/Ru/Co90Fe10 have been studied. Submicron size elements with well defined geometry were prepared by electron beam lithography and argon ion milling for SyAFs and Co90Fe10 monolayers deposited by an ultrahigh vacuum sputtering system. Hysteresis loops were obtained by a focused magneto-optic Kerr effect (MOKE) system and magnetic images in the remanent state were observed by magnetic force microscopy (MFM). We found that the MFM images of SyAF exhibit a single domain structure even in the case of aspect ratio of 1, and there is an optimum ferromagnetic film thickness at which SyAF can obtain a single domain structure with such a low aspect ratio. The MOKE results show that the switching field is dependent on the element width and aspect ratio.

Magnetic switching field and giant magnetoresistance effect of multilayers with synthetic antiferromagnet free layers

Synthetic antiferromagnets (SyAF) made of Co90Fe10(t1)/Ru(d)/Co90Fe10(t2) trilayers and spin-valve type multilayers using a SyAF free layer were deposited on a buffer layer of Cu(2.5 nm)/Ru(10 nm) on a SiO2/Si substrate by using an ultrahigh vacuum sputtering system and a metal mask. The magnetization switching field of the SyAF in both trilayers and multilayers was independent of the Ru thickness, suggesting that the switching field is independent of the magnitude of the antiferromagnetic interlayer exchange coupling, and was proportional to t1/Δt (Δt=|t1−t2|), which is in good agreement with the model of coherent rotation with perfect antiparallel alignment during magnetization reversal of the SyAF. Giant magnetoresistance of the multilayers exhibited two types of curves depending on t1&amp;gt;t2 or t1&amp;lt;t2.

NbN/Nb/AlOx/Nb/NbN junctions fabricated using an ultra-highvacuum dc-magnetron sputtering system

This paper presents the electrical characteristics of NbN/Nb/AlOx/(Nb)/NbN junctionsfabricated using an ultra-high vacuum (UHV) dc-magnetron sputtering system. TheNbN/Nb/AlOx/(Nb)/NbN junctions show overdamped characteristics at or over 10 K, although theyshow small- or medium-hysteretic characteristics at 4.2 K. The UHV sputtering system with aload-lock chamber enables us to prepare high-quality NbN films with high reproducibility and verythin Al2O3 films with high homogeneity. We have obtained the overdamped junctions with thecritical current density Jc up to 10 kA cm-2 and the characteristic voltage Vc up to 0.50 mV over10 K. When Al oxidation conditions were changed, Jc and Vc were obtained in the range of 0.63-10 kA cm-2and 0.22-0.44 mV, respectively. These Jc and Vc values were much larger than those, typicalvalues of 1.1 kA cm-2 and 0.20 mV, of junctions fabricated using a conventional batch-type sputteringsystem. When the upper-Nb film thickness was changed, Jc and the degree of hysteresis, Ir/Ic, wereobtained in the range of 0.31-0.64 kA cm-2 and 0.87-1.0, respectively. The spread of Ic at 10 K was astandard deviation, 1σ, of 6.3% for a series array of 1000 junctions and 4.4% for 100 junctions.

Overdamped NbN junctions with Nb/AlO/sub x//Nb multilayered barriers

This paper describes electrical characteristics of NbN junctions with Nb/Al/sub 2/O/sub 3/-Al/(Nb) multilayered barriers. The junctions show overdamped behavior at 10 K because of Nb working as a normal layer, which enables us to use them as basic elements in single flux quantum (SFQ) circuits. The junctions, with critical current densities J/sub c/'s up to 3.4 kA/cm/sup 2/ and characteristic voltages V/sub c/'s up to 0.20 mV, were obtained by changing the film thicknesses of lower and upper Nb layers and the Al oxidation conditions when using a conventional batch-type sputtering system. The run-to-run variations of J/sub c/ and V/sub c/ were the maximum-to-minimum spreads of /spl plusmn/32% and /spl plusmn/4.3%, respectively. On the other hand, a V/sub c/ of 0.70 mV and a J/sub c/ of 13 kA/cm/sup 2/ were obtained for the junction fabricated by the ultrahigh-vacuum sputtering system. These results show that NbN/Nb/AlO/sub x//Nb/NbN junctions have high potential and can be applicable to high performance 10 K SFQ circuits.

Initial silicide formation process of Mo/(100) Si system prepared using an ultrahigh-vacuum sputtering system

In order to investigate the effect of the vacuum conditions on the process of initial formation of Mo silicides, Mo/Si systems were subjected to heat treatment under high vacuum and clean ultrahigh vacuum, after which we compared and analyzed the behavior of interfacial solid-phase reactions by means of X-ray analysis, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. Results of investigation suggest that the temperature at which the initial silicide formation takes place in the ultrahigh vacuum is lower than that in the high vacuum. However, no differences in the formation phase were found under the different vacuum conditions. It was established that the initial formation processes under both high vacuum and ultrahigh vacuum conditions involve multiphase silicides consisting of Mo3Si, Mo5Si3, and MoSi2, and the film growth continues also as the multiphase silicides. During this time, the main phase is MoSi2, but since the dominant diffusing species are different for various vacuum conditions, during the sputtering under high vacuum the dominant diffusing species is Si, while under the ultrahigh vacuum, Si diffusion begins at a lower temperature simultaneously with Mo. © 2001 Scripta Technica, Electron Comm Jpn Pt 2, 84(3): 71–78, 2001