CoCrTa Films Research Articles

Low-temperature oriented growth of vanadium dioxide films on CoCrTa metal template on Si and vertical metal–insulator transition

The authors achieved oriented growth of vanadium dioxide (VO2) films on CoCrTa metal template grown on an Si substrate. Low-temperature (∼250 °C) deposition of VO2 films using inductively coupled-plasma-assisted sputtering technique realized an abrupt interface between VO2 and CoCrTa layers, suppressing the oxidation and diffusion of metal components. The films revealed a metal–insulator transition with resistance change of over 2 orders of magnitude. The CoCrTa film, in which Co hexagonal crystalline grains with c-axis orientation were surrounded by segregated Cr and Ta, serves for the oriented growth of VO2 crystalline film, enabling higher orders of transition in resistance and low voltage switching, even for the vertical (out-of-plane) direction.

Effect of Ti(Cr) underlayer on the magnetic properties and microstructure of CoCrTa film for perpendicular magnetic recording media

C/CoCrTa/X (X=Cr, Ti) films were fabricated with DC facing target magnetron sputtering apparatus. Their magnetic properties and microstructure were characterized by vibrating sample magnetometer (VSM), X-ray diffraction (XRD), and scan probe microscope (SPM), respectively. The experimental results indicate that the Ti underlayer can induce the c-axis orientation of Co grain in the direction perpendicular to the film surface. For samples with Ti underlayer, the grain size and the surface roughness are relatively finer, and magnetic domains can be observed obviously. These results show that the sample with Ti underlayer is more suitable for the perpendicular magnetic recording media.

Dynamic control of substrate bias for highly c-axis textured thin ferromagnetic CoCrTa film in inductively coupled plasma-assisted sputtering

This study shows highly c-axis textured thin ferromagnetic Co-based alloy (CoCrTa) film growth in inductively coupled plasma (ICP)-assisted sputtering with an internal coil with an insulated surface. Dynamic control of the substrate bias achieved highly c-axis textured CoCrTa film with a thickness of 70nm in 3min depositions on a Si substrate. The prepared film showed a smooth, dense surface consisting of small crystal grains. The film had a perpendicular magnetic coercivity of 1030Oe and coercive squareness of 0.36. ICP-assisted sputtering with an internal coil with an insulated surface enabled higher-density (⩾1.0×1011cm−3) plasma with lower space potential (⩽30V) compared to ICP-assisted sputtering with bare coil systems. Therefore, the proposed bias control is quite effective for textured growth of thinner Co layers via the effect of a high flux of ions with proper energies. This method can be a candidate for the deposition technique of c-axis textured films as perpendicular magnetic recording media.

Performance of inductively coupled plasma assisted sputtering with internal coil for ferromagnetic CoCrTa film deposition

The performance of inductively coupled plasma (ICP)-assisted sputtering with an internal coil was revealed for deposition of ferromagnetic Co-based alloy films. Three types of internal coil, a directly grounded bare coil, a capacitively coupled bare coil and an insulated coil, were investigated with regard to the crystalline structure of films and plasma characteristics. Highly c-axis textured hexagonal-close-packed (hcp) film growth was achieved in the bare coil system at a medium operating Ar pressure of 1.3 Pa. Pole figure measurements showed that the film grown on a low-temperature Si substrate contained small face-centered-cubic phase in the c-axis textured hcp crystal. Plasma diagnostics suggested that high plasma space potential beyond 80 V caused highly textured growth through the effect of ion bombardment with proper energies for crystallization. The superior capability of ICP-assisted sputtering with a capacitively coupled bare coil was revealed for textured growth in the thin layer on a substrate surface. A high deposition rate was achieved as a result of high electron density of the order of 1011 cm−3 in ICP-assisted sputtering with an insulated coil. However, the insulated coil system could not achieve textured growth.

Growth of Highly Oriented CoCrTa films by Inductively Coupled Plasma-Assisted Sputtering

Inductively coupled plasma (ICP)-assisted sputtering using an internal coil was applied to the deposition of ferromagnetic Co-based alloy (CoCrTa) films intended for magnetic recording media. The growth of highly oriented crystalline Co films on both low-temperature Si and glass substrates was achieved. Pole figure measurements indicated the growth of face-centered cubic Co with a (111) plane parallel to the substrate, while a hexagonal close-packed crystalline film with c-axis perpendicular to the glass substrate was obtained by adjusting the rf power applied to the inserted coil and the substrate temperature. In the ICP-assisted sputtering at a coil rf power of 120 W, the deposition rate at a pressure of 1.3 Pa was more than two times higher than that of conventional sputtering at the same target rf power of 150 W. The deposition performance responsible for highly oriented crystalline growth and higher deposition rate at low pressures was discussed in terms of target self-bias voltage and plasma parameters measured using a Langmuir probe. The film properties characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM) were also presented showing the superior capability of ICP-assisted sputtering for ferromagnetic targets.

Studies of the magnetic and reversal properties for thin CoCrTa films

The effects of magnetic layer thickness on film structural and magnetic properties were studied systematically with emphasis on the thermal effects on thin recording media films. X-ray diffraction measurements reveal structural changes as thickness decreases, and the existence of a “Cr enriched phase” associated with the interface. The saturation magnetization M s decreases with thickness and the thickness of the “dead layer” was found to be ∼23 Å. Systematic measurements of effective anisotropy, coercivity and saturation magnetization as functions of temperature have been carried out. Magnetic viscosity measurements reveal that thermal stability is affected not only by grain sizes but also by anisotropy reduction associated with nanostructure evolution, as the film thickness decreases.

New sputter-deposition processes for the formation of thin films with desired structure for magnetic recording

Two new sputter-deposition techniques, low voltage sputtering (LVS) and kinetic energy controlled particle deposition (KECD), are proposed to obtain a thin film with the desired microstructure. The LVS, RF–DC coupled magnetron sputtering with RF frequency above 30 MHz, is useful for sputtering at a sputtering voltage below 100 V and has a reasonable deposition rate. Indium tin oxide (ITO), Ti and CoCrTa films were deposited by using the LVS. Compared with conventional sputtering, LVS is effective in reducing high-energy particle bombardment of film surfaces during sputtering, which results in the formation of films with much smoother surfaces, although crystallization in the film is suppressed. On the other hand, KECD can control the kinetic energy and incidence angles of the depositing particles on the substrate. With KECD, it was confirmed that crystal growth is suppressed as the kinetic energy decreases in ITO film and the crystal orientation of Ti film depends remarkably on the kinetic energy of the deposition particles. Films with extremely smooth surface are obtained by KECD.

Analytic solution for magnetic orientation between grains

Transmission electron microscopy experiments have shown that CoCrTa films for hard disk drives experience chromium segregation at grain boundaries. The resultant changes in the ferromagnetic parameters of the material together with the variation in the anisotropy direction from grain to grain are determining factors governing its magnetic behavior. In this paper, the case of two grains, differing in the direction of their magnetic anisotropy and possessing a change in their material content at the interface which we model by a Gaussian distribution, is solved analytically. Effects due to the diffuse interface such as localized regional changes in the anisotropy, exchange energy, and magnetic moment are included using perturbation theory. One of the outputs is the profile of the magnetization orientation in the inhomogeneous medium-thus yielding the smoothness and extent of the change. Such knowledge, in addition to its intrinsic scientific interest, paints the direction, both for large scale computer calculations and for media improvement.

Magnetic properties and microstructure of CoCr and CoCrTa thin films sputtered at high pressure on a PET substrate

Thin magnetic films (CoCr/Cr, CoCrTa/Cr and CoCrTa/Cr/SiO) are deposited by DC-magnetron sputtering on a PET substrate. The optimum sputter pressure, giving rise to the highest coercive force, is found to be around 50 mTorr. However, the deposition parameters influence the value of this optimum pressure. The addition of Ta to CoCr leads to phase segregation, resulting in CoCrTa films in which the magnetic interaction between the grains is more dipolar than in CoCr films. The magnetic interactions in CoCrTa films are less dependent on the sputter parameters than those in CoCr films. The evaporation of a seedlayer of, e.g. SiO prior to the deposition of the Cr underlayer increases the coercive force of the CoCrTa thin films, especially when the CoCrTa layer is thinner than 50 nm. This makes these films more suitable for high-density magnetic recording.

In-plane magnetic anisotropy in CoCrPt and CoCrTa films deposited onto patterned silicon substrates

Topographically induced in-plane magnetic anisotropy has been observed in CoCrTa and CoCrPt films deposited onto oxidized silicon substrates which are lithographically patterned with fine grooves of period 200–320 nm and amplitude 20–50 nm. The coercivity and remanence are higher parallel to the grooves. Anisotropy has been observed in both rf- and dc-magnetron sputtered films with a (112̄0) preferred orientation, which is achieved by growth at elevated temperature on a (200)-oriented Cr underlayer at low base pressures. Anisotropy increases with the amplitude of the grooves in the silica substrate.

Unicrystal Co–alloy media on Si(110)

Unicrystal Co–alloy/Cr/Ag films, which exhibit a single, in-plane easy axis orientation, were epitaxially grown on hydrofluoric acid etched Si(110) single crystal substrates by sputter deposition for the purpose of the systematic study of Co–alloy magnetocrystalline anisotropy. The orientation relationship was studied by x-ray θ/2θ diffraction, pole figure φ scan, and transmission electron microscopy, and it was determined to be Co(101̄0)[0001]‖Cr(112)[11̄0]‖Ag(110)[001]‖Si(110)[001]. The φ scan also showed two twin-related orientations of Cr grains. The easy axis hysteresis loops had a square shape, while the hard axis loops showed zero openness. The uniaxial anisotropy constants K1 and K2 of the unicrystal Co and CoCrTa films were determined from torque and hard axis hysteresis loop measurements.

Chromium segregation in CoCrTa/Cr and CoCrPt/Cr thin films for longitudinal recording media

Analytical electron microscopy is employed to correlate Cr segregation in Co/sub 34/Cr/sub 12/Ta/sub 4//Cr and Co/sub 76/Cr/sub 12/Pt/sub 12//Cr films with specific microstructural features such as grain boundary misorientation. Energy-filtered (EFTEM) chemical maps show that Cr segregation occurs independently of the Cr underlayer, and is highly alloy dependent. The CoCrTa film contained extensive grain boundary Cr enrichment whereas EFTEM images from the CoCrPt media show homogeneous Cr distribution. No statistically significant Ta or Pt segregation was observed. EFTEM elemental maps and energy dispersive spectroscopy (EDS) indicate that grain boundary Cr segregation depends on the type of boundary. Quantitative analysis of the Cr levels using nanoprobe EDS shows that the random angle grain boundaries contain more Cr (23/spl plusmn/4 at%) than 90/spl deg/ boundaries (17/spl plusmn/4 at%). EDS and EFTEM composition profiles show Cr enriched grain boundaries surrounded by regions of Cr depletion.

Microstructures and basic magnetic properties of Co-based epitaxial magneticthin films

To investigate the basic magnetic and recording characteristics of Co and Co-based alloy thin films, single crystalline and bicrystalline magnetic thin films are prepared by employing an epitaxial thin film growth technique using MgO(110) and (100) single crystal substrates. Uniaxial magneto-crystalline constants ( K u) and Gilbert's damping constants ( α) of Co, CoCrTa, and CoCrPt thin films are determined by Torque and FMR measurements. (11̄00) oriented single crystalline and (112̄0) oriented bicrystalline Co-based thin films are prepared. K u values of Co, CoCr 16Ta 4, CoCr 15Pt 12 thin films are determined to be 4.5×10 6, 1.7×10 6, 3.3×10 6 erg/cc, respectively. The effect of lattice mismatch between the magnetic layer and the Cr alloy underlayer on K u value of CoCr 15Pt 12 is systematically investigated. The α value of the Co and the Co alloy thin films are determined to be ranging between 0.01–0.04. Recording resolution and noise characteristics of the bicrystalline CoCr 15Pt 20 disk medium are also examined as a function of angle from the effective easy axis of the disk medium.

Orientation ratio reduction through pre-deposition substrate annealing

The orientation ratio (OR) of magnetic hard disks was modified through pre-deposition substrate annealing. This heat treatment leads to a modulation of the NiP surface chemistry which brought about a different CoCrTa film texture. Our studies verified the essential factors to obtaining OR>1, namely a non-oxidized NiP surface which established a (1120) CoCrTa film texture, substrate heating, and mechanical texturing. Parametric tests showed that disks with near unity OR have worse signal-to-noise ratio but possess better off track capability when tested under squeezed conditions.

A Simulation of Rotational Hysteresis Energy Loss in Longitudinal Thin-Film Media

AbstractThe field dependence of the rotational hysteresis energy loss was simulated for longitudinal thin-film media with the purpose of examining the validity of this method for measuring the anisotropy field. The field at which the rotational hysteresis vanishes, which is usually taken as the experimentally measured anisotropy field, was found to be smaller than the real anisotropy field when intergranular magnetostatic and exchange interactions are included in the simulations. Hence the rotational hysteresis method may result in an underestimation of the anisotropy field for the films with non-negligible grain interactions. To confirm this experimentally, two CoCrTa films, one with strong in-plane easy axis texture and the other with uniaxially aligned grain easy axes, were prepared under the same conditions. The anisotropy field determined by the rotational hysteresis of the first sample was found to be smaller than the more accurate value obtained from the hard axis hysteresis loop of the second sample.

Compositional Microstructure and Micromagnetics of Co-Based Thin Film Media

AbstractElemental segregation of CoCrTa and CoCrPt thin films for longitudinal and perpendicular media is investigated using high spacial resolution transmission electron microscopes equipped with compositional analysis facilities. Strong Cr segregation exceeding 20 at% within 1.5-2 nm width is observed along the grain boundaries for both types of CoCrTa films prepared at elevated substrate temperatures. Weaker Cr segregation is observed along the grain boundaries of the longitudinal and the perpendicular CoCrPt films. The strong Cr segregation at grain boundaries is related with the small magnetic cluster size and the low media noise characteristics of CoCrTa thin film media. The Cr content inside the grain is several % lower than the average composition of the CoCrTa films. The magnetocrystalline anisotropy constants(Ku) for different Cr compositions are determined using single crystalline thin film technology to discuss the thermal stability of recorded information.

Epitaxial Ag templates on Si(001) for bicrystal CoCrTa media

Ag thin films were sputter deposited on hydrofluoric acid-etched Si(001) single crystal substrates and employed as templates for the epitaxial growth of Cr(001) films and consequently bicrystal CoCrTa(112̄0) films. The orientation relationship was determined to be bicrystal CoCrTa on Cr(001)[100]‖Ag(001)[110]‖Si(001)[110]. X-ray diffraction analysis showed only strong Ag(002) peaks throughout the Ag film thickness range of 50–1500 Å. Atomic force microscopy showed that almost continuous Ag films can be achieved in the thickness range of 500–1000 Å, which result in the highest coercivities in CoCrTa films whose magnetic properties were found to vary with the Ag template thickness and surface morphology. The periodic in-plane angular variation of magnetic properties and torque curves with a four-fold symmetry were confirmed in the bicrystal CoCrTa films.

Effects of film thickness on the structural and magnetic properties of CoCrTa/Co3O4/Pt perpendicular thin-film media

This paper reports the effects of film thickness of room temperature RF sputtered trilayer CoCrTa/Co3O4/Pt films on the structural and perpendicular magnetic properties of CoCrTa magnetic layer. Using a 100 Å Pt film with a weak (111) texture as the buffer layer improves the nucleation and initial growth of the (111) oriented Co3O4 seed layer, which in turn induces a significantly strong (00.2) texture in CoCrTa films. If the Pt is too thin, there are not enough (111) oriented sites created for the nucleation of Co3O4. If the Pt is too thick, the large grain size with significant size variation in the magnetic layer (originating from the thick buffer layer) leads to poor perpendicular magnetic properties. A 50 Å Co3O4 seed layer sputtered on 100 Å Pt is most effective in promoting a strong (00.2) texture in CoCrTa films by providing (111) oriented sites for (00.2) growth of CoCrTa without creating significant lattice mismatch. For CoCrTa films sputtered on this composite underlayer, the in-plane coercivity is not sensitive to the film thickness whereas the perpendicular coercivity is. The maximum Hc⊥ (perpendicular coercivity) ∼1650 Oe with an S∥ (in-plane remanant squareness) ∼13% is obtained when the film thickness is ∼2000 Å. The effect of the size of domain clusters on perpendicular coercivity Hc⊥ has also been studied by magnetic force microscopy.

Magnetic and magneto-optical investigation of CoCrTa and CoCrPt films

Magnetic and magneto-optic measurements (polar Kerr rotation and ellipticity) have been taken for a series of CoCrTa and CoCrPt single layer films with different thicknesses. The magnetic layers were rf sputter deposited onto constant thickness titanium layers on glass substrates at room temperature. Experimental data were obtained at two different wavelengths, i.e., 825 and 670 nm. The perpendicular coercivity Hc measured at the two wavelengths increased with film thickness. Conversely, the lower thickness showed the highest θk and this decreased as the thickness increases. The opposite trend has been noticed for CoCrPt films. There is no difference in the grain size from transmission electron microscopy examination. The results of the investigation are presented in the context of the need for magneto-optic material to be optimized for short wavelength applications in the future.

Correlation of switching volume with magnetic properties, microstructure, and media noise in CoCr(Pt)Ta thin films

The technique of “field sweep-rate dependence of coercivity” together with the delta-M curves, transmission electron microscopy, and media noise measurement have been employed to reveal the correlation among the magnetic switching volume, physical grain size, intergrain interaction, and noise properties. The switching volume for both CoCrTa and CoCrPtTa films was found to be ∼15 nm in diameter, and weakly dependent on grain size and coercivity for the systems studied. In films with physical grain size larger than the switching volume, strong intergrain interaction and high media noise is observed. In films with the physical grain size close to the switching volume, smaller interactions and lower media noise is found. It is argued that a change of the grain size results in a change in the nature of the magnetic interactions.