Cr Buffer Layer Research Articles

Ｃｏ２ＭｎＧｅホイスラー合金薄膜の構造と磁気特性

Co2MnGe is a full-Heusler alloy with L21 structure, theoretically predicted to be a half-metallic ferromagnet. In this paper the structural and magnetic properties of Co2MnGe films with a Cr buffer layer were investigated. The magnetization at 20 kOe of the prepared Co2MnGe film was 750 emu/cm3. The lattice parameter of the Co2MnGe film annealed at 400°C was 5.704 Å, which was not in agreement with the value for a bulk sample (5.743 Å), but in nearly agreement with the value calculated from first principles (5.711 Å). The magnetization at 20 kOe was 970 emu/cm3 which was in nearly agreement with the theoretical value (1002 emu/cm3).

Magnetic properties and microstructures of Mn−Ir/Ni−Fe exchange biased multilayers on Ta and Cr buffer layers

We have investigated the magnetic properties and the microstructures of Mn−Ir/Ni−Fe/(Ta, Cr)/Si exchange biased multilayers. The Mn−Ir/Ni−Fe on a Ta buffer layer exhibited a higher exchange biased field than the Mn−Ir/Ni−Fe on a Cr buffer layer. On the other hand, the Mn−Ir/Ni−Fe on a Cr buffer layer exhibited a higher blocking temperature and was composed of larger grains than Mn−Ir/Ni−Fe on a Ta buffer layer. Both samples had an epitaxial relationship at the interface between the Mn−r layer and the Ni−Fe layer due to the generation of misfit dislocation.

Effects of thickness of Ni layer deposited on glass substrate on the growth and emission properties of carbon nanotubes

Vertically aligned carbon nanotubes were grown on the Cr-coated glass substrate with different thickness of Ni catalyst layer at low temperatures (<600 °C) using plasma-enhanced hot filament chemical vapor deposition (PEHFCVD). The diameter of carbon nanotubes grown for 14 min increased from 32 to 113 nm as the thickness of Ni layer increased form 10 to 100 nm. However, the diameter of carbon nanotubes grown for 34 min was almost same irrespective of Ni layer thickness. The emission characteristics of carbon nanotubes (CNTs) were dependent on the diameter of carbon nanotubes and substrate. Turn on electric field, E to, increased from 6.9 to 9.6 V/μm as the diameter of carbon nanotubes increased from 44 to 64 nm. E to of carbon nanotubes without Cr buffer layer was lower than that of carbon nanotubes with Cr layer. The electron emission from carbon nanotubes grown on Si substrate was larger than that of carbon nanotubes on glass substrate with the same Cr and Ni layer.

Preisach analysis of epitaxial hard/soft bilayers

We used the moving Preisach model to study the magnetization-reversal process in epitaxial SmCo (1 1 ̄ 0 0) /Fe bilayers prepared by magnetron sputtering. The SmCo(20 nm)/Fe( t nm) ( t=0, 5, 10, 20) bilayers were grown onto single-crystal (1 1 0)MgO substrates with an epitaxial 20 nm Cr(2 1 1) buffer layer. The second-quadrant magnetization curves were strictly reversible up to a field H 0, close to the field H c at which the magnetization switched irreversibly. Henkel plots for these bilayers indicated very strong magnetizing interactions for all films. Magnetization reversal in these films reflects their epitaxial structure.

Giant magnetoresistance in Fe and Co based spin valve structures

Giant magnetoresistance (GMR) effects up to 7% have been observed in Fe/Cu/Co spin valve structures. The samples have been grown by ion beam sputtering at room temperature on a glass substrate covered by Cr buffer layer. The best results have been obtained for the sandwich in which a 0.5 nm Co thin layer has been deposited between Fe and Cu layer with the following structure: Cr 3nm/Fe 3nm/Co 0.5nm/Cu 3nm/Co 5nm/Cu 2nm/Cr 2nm. The role of the thin Co layer is to increase the spin-dependent scattering centers. Annealing treatments during 2 h have been performed on this sample at different temperature in order to study the thermal stability. A progressive increase of the GMR has been observed for the annealing at 200 and 250°C, while at 300°C, the GMR completely disappears. Rutherford back scattering (RBS) spectroscopy has been used to explain the physical origin of this drop, which is attributed to the total interdiffusion through grains boundaries between Fe, Co and Cu layers.

Fabrication of (Hg 1− xRe x)Ba 2Ca 2Cu 3O y tapes

Fabrication of c-axis oriented (Hg 0.75Re 0.25)Ba 2Ca 2Cu 3O y [Hg 0.75Re 0.25)1223] thick film tapes has been attempted using Ni substrates with the buffer layer of Cr or NiO. Coexistence of Hg 0.75Re 0.25Ba 2Ca 3Cu 4O y or Hg 0.6Re 0.4Ba 2CaCu 2O y pellets as Hg vapor source was found to stabilize (Hg 0.75Re 0.25)1223 phase of the tape. The c-axis oriented tapes were reproducibly obtained on the NiO/Ni substrate and they recorded high B irr at 77K, > 5T.

Giant magnetoresistance and structural properties in Co/Cu/Co sandwiches with Si and Cr buffer layers

Cobalt 5.5 nm/Cu 3 nm/Co 5.5 nm sandwiches with Si and Cr buffer layers were prepared by ultra-high vacuum electron beam evaporation. A large in-plane anisotropy of the giant magnetoresistance (GMR) effect was found in Si buffered sandwiches when the buffer layer thickness was equal to or larger than 0.9 nm. In the easy axis, the GMR effect reached a value of 5.5% with a high field sensitivity of approximately 0.7%/Oe, while in Cr-buffered Co/Cu/Co sandwiches, the GMR effect showed only in-plane isotropic properties with a maximum GMR value of 6%. The XRD spectrum and HRTEM image revealed that there exists a Co 2Si compound between the Si buffer and the lower Co magnetic layer. All these results indicate that the anisotropic GMR effect in Si-buffered sandwiches results from the cobalt silicide between the Si buffer and the lower Co magnetic layer.

Fabrication of pseudo-spin-valves and 100 nm sized periodic elements for magnetic memory application

Co–Cu–Co pseudo spin-valves (PSV’s) and Co–Cu multilayers have been fabricated and studied for the giant magnetoresistance (GMR) effect. Attention is focused on optimizing the low field response. They are deposited on Si (001) wafers without removing the natural silicon-dioxide layer, using S-research sputtering guns in high vacuum. A buffer layer of Cr–Cu was used before depositing the PSVs. Realization magnetic switching at low field for the soft layer and high field for the hard-layer makes the structure suited for MRAM application. The low and high switching fields are typically 100 and 2200 Oe, respectively. The magnetic measurement using SQUID magnetometry and VSM confirms the switching fields in the hysteresis loops of the magnetization. The GMR multilayers have a low switching field of 60 Oe at low temperatures. They are patterned into two-dimensional 100 nm sized periodic dot-array using ultraviolet interferometric lithography. The purpose is to develop processes of fabricating GMR structures over large areas with nanometer feature sizes for high-density memory devices. The coercive field for the patterned structures is enhanced by a factor of four at low temperatures and by a small fraction at higher temperatures. The magnetization remains to be in plane as in un-patterned case.

Coercitivity and its temperature dependence in NdFeB thin films with Cr, Mo, Ti, or Ta buffer layers

NdFeB thin films of the form A(20 nm)/NdFeB(d nm)/A(20 nm) were prepared by magnetron sputtering on a Si(100) substrate. The hard Nd2Fe14B phase is formed by a postanneal in vacuum. The buffer layers A used are Cr, Mo, Ta, Ti, and NdFeB layer thickness used are 540, 180, 90, and 54 nm. We have found several different combinations of anneal temperature, buffer layer, and NdFeB film thickness d that lead to large coercivity, as high as 17.0 kOe, and energy product, as high as 10.3 MG Oe. The temperature dependence of coercivity was analyzed using a modified Brown’s equation and we find that the microstructural parameter αK and the effective demagnetization factor Ne change with the buffer material and anneal temperature. Possible magnetic reversal mechanisms are discussed in light of these results.

Magnetization of thin Fe films on V(1 1 0) and Cr(1 1 0)

The magnetization of thin Fe(1 1 0) films was measured by magneto-optical Kerr effect (MOKE) magnetometry and polarized neutron reflectometry (PNR). In order to further understand the influence of strain and roughness on the Fe magnetization different surfaces were prepared by evaporating Cr and V on V(1 1 0) single crystals by molecular beam epitaxy. By varying the growth temperature both smooth and faceted Cr and V surfaces were created. For the facets we obtained an up and down staircase structure with the ridges along the [0 0 1] direction that consisted approximately of subsequent (1 0 0) and (0 1 0) surfaces. For the thin Fe films that were evaporated on the smooth V(1 1 0) surface a relatively high reduction of the magnetization was obtained, independently of the film thickness. In terms of thickness the observed reduction is equivalent to an offset of Δ t=−(4.0±1.0) Å. A V layer evaporated on this structure does not further reduce the magnetization of the Fe. For comparison Fe evaporated on a smooth Cr film was investigated, and the reduction amounted to Δ t=−(1.4±0.4) Å. For thin Fe films on faceted V and Cr substrates Δ t=−(3.3±0.4) and −(2.6±0.3) Å were measured, respectively. For samples with a 2 Å Cr buffer layer between the V(1 1 0) single crystal and the Fe film we obtained Δ t=−(4.08±0.10) Å, a value which is close to the result obtained for the smooth V(1 1 0) substrate.

Magneto-optic Kerr effect at the interface of Co/Gd bilayers

We have grown a set of crystalline Co/Gd(500 Å) bilayers on glass substrates with a Cr buffer layer by magnetron sputtering. The interface between Co and Gd has been investigated by Auger spectroscopy depth profiles, vibrating sample magnetometry (VSM) and magneto-optic Kerr effect (MOKE) on the air and glass sides of the sample at room temperature. The use of a Cr buffer with different thicknesses allowed us to probe the interface in MOKE measurements on the glass side. These show a small and reversed Kerr loop whereas those of the air interface are normal. Depth profiles indicate a wide interface ( ∼ 3–4 nm thick) in the Gd side that is partly oxidized. The results are discussed on the basis of polarization of interfacial spins or the effect of the oxygen contamination.

Coercivity analysis in sputtered Sm–Co thin films

Hard nanocrystalline Sm–Co thin films having a thickness of 500 nm have been prepared by heat-treating magnetron sputtered amorphous samples deposited over a 300 nm Cr buffer layer onto room temperature water cooled Si substrates. The thermal treatments resulted in the nanocrystallization of the precursor films and in the development of room temperature coercivities covering a range going from a few kOe, measured in the samples treated at low temperature, up to some tens of kOe, observed in the case of the films annealed at high temperatures (the room temperature coercivity of the sample treated at 825 K was 26 kOe). Our analysis of the influence of the thermal treatments on the particularities of the reversal process was based on the measurement of the temperature dependence of the coercive force. It was carried out in the framework of the micromagnetic model, which allowed us to evaluate two parameters accounting for the local anisotropy reduction and the magnitude of the local dipolar fields, respectively. Our results show that the high temperature treatments result in a better degree of crystallization (lower local anisotropy deterioration). That crystallinity improvement is associated to an increase of the local dipolar interactions.

Nanocrystalline Sm-Co-Cu(Ni) thin films with giant coercivity

Nanostructured Srn-Co, Sm-Co-C;u, Sm-Co-Ni thin films have been obtained by crystallizing the as-deposited amorphous films which were sputtered onto Cr buffer layers. Room temperature coercivity increases from less than 100 Oe in the amorphous state up to 42 kOe in a SmCo 2Cu 3 Sample annealed 30 min. at 550 °C. Initial magnetization curves showed a behavior characteristic of uniform domain wall pining. Magnetic viscosity measurements indicated that the switching volume is of the same order as the Sm(Co,Cu) 5 crystallites size. Remanence Measuremcnts showed that interparticulate interactions were magnetizing and rather independent of the crystallization static. These data pointed to domain wall pinning at the high anisotropy Sm-C′.o precipitatcs as the origin of the huge coercivity in the annealed films.

TEM observation of the microstructure in sputtered NiFe/Au multilayer films

The cross-sectional nanostructure and plan-view structure of NiFe/Au multilayer films (MLFs) were investigated using high-resolution electron microscopy (HREM). MLF series with different Au layer thicknesses, unseeded or with Cr or Cu buffer layers, were deposited on Si (100) wafers by magnetron sputtering. The HREM images show that the Au layers have a strong (111) texture along the film normal direction and dominate the crystallographic growth of the MLFs. Although the layer structure looks wavy near the grain boundaries, the interfaces between the Au and NiFe layers are very flat - some atomic steps and twins are visible. The difference in lattice parameters between the NiFe and Au is taken up by misfit dislocations at the interfaces. Plan-view images show the fcc (111) lattice planes of NiFe and Au lying in the plane of the layers. Thin buffer layers of Cr and Cu do not affect the crystal growth, but a thick Cu buffer (15 nm) disorders the (111) texture and the interface structure of the MLFs.

Magnetic properties of Fe films epitaxially grown on Cr/GaAs(100) by dc magnetron sputtering

We report on the growth of single-crystal Fe films by magnetron sputtering onto GaAs substrates. In order to establish the epitaxial orientation with the substrate a 100 Å Cr buffer layer was rf sputtered. The crystalline and magnetic properties were studied by x-ray diffraction, ferromagnetic resonance, and Kerr effect magnetometry techniques. The θ-2θ x-ray measurements show that only the Fe (200) peak is present, and the rocking curve shows a full width half maximum of 2°. Ferromagnetic resonance lines exhibit a line width of about 30 Oe, and the in-plane resonance field as a function of the azimuth angle presents a fourfold symmetry with no induced anisotropy. The in-plane hard- and easy-axis magneto-optical hysteresis loop traces are consistent with the ferromagnetic resonance results.

Magnetic properties and growth texture of high-coercive Nd–Fe–B thin films

We have investigated the growth texture and magnetic properties of sputtered Nd–Fe–B thin films with thicknesses from 5 to 350 nm. Films deposited directly onto a quartz substrate grow with a pronounced c-axis texture perpendicular to the film plane and have a coercivity of about 160 kA/m. If the films thickness is reduced below 150 nm, a significant part of the Nd is oxidized by substrate oxygen and the coercivity decreases to values below 20 kA/m. The deposition of a 80 nm Cr buffer layer between substrate and Nd–Fe–B film prevents oxidation of the film and decreases the growth texture of the films. This enables us to fabricate Nd–Fe–B films as thin as 20 nm with good hard magnetic properties [Hcj=800 kA/m, Jr=1.1 T, and a maximum energy product (BH)max=190 kJ/m3 perpendicular to film plane]. An inverse relationship between growth texture and coercivity is found which can be understood in terms of domain wall motion. The deposition of a Cr buffer leads to an island type of growth and a high surface roughness, while films without Cr buffer are very smooth. By depositing a protective Cr layer on top of the film, the corrosion resistance can be enhanced.

Dependence of structural features on substrates in Co/Cu multilayers

Metallic multilayers with giant magnetoresistance (GMR) effect is an active research field. GMR is governed by several physical parameters. In this paper, one of the most important parameters, surface and interface roughness is investigated. The Co/Cu multilayers on Si(111) and Si(100) with a Cr buffer layer were grown by electron beam evaporation in one run. Auger depth profiling, grazing incidence X-ray reflectivity and Atomic Force Microscopy clearly demonstrate the structural difference in the two Co/Cu multilayer samples. The multilayers on Si(111) have smoother surface and interfaces, while the multilayers on Si(100) have rougher surface and interfaces, which originates from the different surface roughness of the Si(111) and Si(100) substrates, 0.36 and 0.68 nm respectively. The surface evolution of the multilayers during deposition is also discussed.

Giant magnetoresistance in NiFeCo/Cu/Co/Cu/NiFeCo multilayers prepared by electron-beam evaporation

NiFe/Co/Cu/Co/Cu/NiFeCo multilayers grown on silicon substrates by electron-beam evaporation at ultra-high vacuum were studied. It was found that buffer layer plays a very important role in the giant magnetoresistance effect in the simple symmetric structure. Without buffer layer, the magnetoresistance ratio was only 0.4% in the structure. By using a Cr buffer layer 50 Å thick, a largest MR ratio of 8.2% with a maximum MR slope of 0.5%/Oe was obtained for NiFeCo(35 Å)/Cu(30 Å/Co(35 Å)/Cu(30 Å)/NiFeCo(35 Å) at room temperature. Decreasing temperature to 80 K, the same sample presented a MR ratio of 15.5% with a maximum MR slope 0.55%/Oe. After annealing at 400°C for 10 min, MR ratio was reduced to 1.8% at room temperature. Atomic force microscope images showed that the grain size at the surface was greatly enlarged.

Role of the buffer layers in determining the antiferromagnetic coupling and magnetoresistance of NiFeCo/Cu superlattices

The antiferromagnetic (AF) coupling and magnetoresistance in magnetron-sputtered polycrystalline NiFeCo/Cu superlattices grown on Fe, Cr, or Zr buffer layers have been studied. The results show that the buffer layers significantly affect AF coupling and magnetoresistance through modulating the growth of specifically oriented crystalline multilayered structures. Two types of buffer layer effects on AF coupling and magnetoresistance are considered: modifying interfacial roughness and producing layer-thickness fluctuations. Interfacial roughness is formulated in the first Born approximation using data obtained from specular and off-specular (diffuse) x-ray scattering measurements. For the roughness at interfaces, correlated interfacial roughness is found to predominate and estimated to be as large as 5–7 Å with a moderately large lateral correlation length scale ξ that ranges from 120 to 200 Å. Upon approximately the same AF coupling, for Fe or Cr buffer layered superlattices characteristic of well-defined, comparatively flat layer structures throughout the entire specimen, magnetoresistance turns out to increase as the correlated interfacial roughness increases. For the superlattices grown on Zr buffer layers in which the presence of columnar structures is confirmed, a constant, weak AF coupling and moderate magnetoresistance concur and are both almost independent of buffer layer thickness. These features are explained in terms of the averaging of AF coupling and magnetoresistance through parallel layer-thickness fluctuation structures approximating the columnar structures.

Temperature-dependent study of ion-channeling in [formula omitted] superlattices

Giant- Magneto- Resistance (GMR), as large as 150% at 4 K, occurs in Fe Cr superlattices as a result of antiferromagnetic interlayer coupling. We have successfully grown epitaxial single-crystal Fe Cr multilayers using magnetron sputtering. Ion channeling was employed to study the structural and vibrational properties of the sputter-deposited Fe Cr superlattices, and of a Cr thin film, between temperatures of 100 and 330 K. Channeling in the latter specimen was used to investigate the importance of depositing a Cr-buffer layer in order to obtain superlattices with large GMR values. Once the buffer layer exceeded a critical thickness, a high quality Cr film was observed. The epitaxial quality of the superlattices grown on such buffer layers by sputtering was found to be excellent. Minimum yields nearly equal to theoretical predictions were found for channeling along the 〈001〉 growth direction; slightly higher values were found along the 〈111〉 axis. Because of the high structural quality of the sputter-deposited films, it was possible to investigate changes in thermal vibration amplitudes, even though their magnitude is only of the order of a few pm (10 −12 m). No unusual structural changes of this magnitude were observed in angular channeling scans obtained while cooling the Fe Cr superlattice from 330 down to 100 K.