Remanent Squareness Research Articles

Angular dependence of the coercivity and remanence of ferromagnetic nanowire arrays

Magnetization properties of magnetic nanowire arrays are studied on various ferromagnetic materials grown in anodic alumina (alumite) and track etched polycarbonate (PCTE) membranes by pulsed electrodeposition. Magnetization curves were measured as functions of wire material, field orientation, and wire length. The coercivity (Hc) and remanent squareness (S) of the various wire arrays were derived from hysteresis loops as a function of angle (θ) between the field and wire axis. For PCTE membranes, Hc(θ) curves for CoNiFe, NiFe, and Co nanowire arrays all show an otherwise-bell-type variation, while they change shapes from the otherwise bell to bell type for Ni nanowire arrays as the wire diameter decreases to 30 nm. These characteristics can be understood based on different magnetization reversal mechanisms of small wires. The effect of magnetostatic interaction among wires on the magnetic properties was examined by changing the wire lengths in alumite membranes. It is found that the interaction reduces Hc and S values significantly and may cause the overall easy axis change from parallel to perpendicular to the wire axis. However, the interaction is much weaker than expected from an independent precession theory. The strong coupling among the wire may also induce a change of magnetization reversal mechanism.

Magnetic properties of magnetic nanowire arrays

The arrays of NiFe and CoNiFe nanowires were grown in anodic alumina (alumite) and track etched polycarbonate (PCTE) membranes with various pore sizes by pulsed electrodeposition. Magnetic properties of the array were studied as functions of wire material, length, and diameter as well as field orientation. In PCTE membranes, as high as 0.92 of remanent squareness (S) was obtained for the array with wire diameter of 30 nm. However, for the array of nanowires in anodic membranes, anisotropy with field applied in and out of plane is very low, and the obtained S is less than 10%. For nanowires in alumite with the same wire length, CoNiFe shows a higher anisotropic field (H/sub k/) than NiFe, leading to larger coercivity (H/sub c/) and S and a lower saturation field. Wire-length dependence was measured for alumite membranes. As wire length increases, H/sub k/, H/sub c/, and S decrease, but the saturation field increases. This magnetic behavior can be qualitatively explained by considering dipolar interactions among nanowires and shape anisotropy of an individual nanowire, but it remains difficult to give a quantitative explanation.

Superlattice magnetic recording media: experiment and simulation

Co/Pd superlattice perpendicular films are fabricated on 2 nm indium tin oxide (ITO) seed layers by DC magnetron sputtering. A film with perfect remanent squareness and highly linear demagnetization shear is obtained. Micromagnetic simulation suggests that the film has weak intergranular exchange coupling. Good SNR is obtained, but D 50 is reduced by the closely spaced soft underlayer and large fly height.

Co/Pd and Co/Pt multilayers with indium tin oxide seed layers and NiFe soft underlayers for perpendicular magnetic recording media

Co/Pd and Co/Pt multilayers have been fabricated on ultra-thin (2 nm) indium tin oxide (ITO) seed layers with soft underlayer for perpendicular magnetic recording. The coercivity of the Co/Pd multilayers was increased from 5.1 kOe to 6.3 kOe by increasing the thickness of the initial Pd layer from 1 nm to 2 nm. Hysteresis loops with an almost perfect remanent squareness, high nucleation field, and more sheared sides were obtained. TEM analysis shows that the films have well-segregated columnar structures which help to reduce exchange coupling and increase coercivity. Spin-stand testing shows that the Co/Pd multilayers have a D50 about 157 kfci without differentiating the output signals. Co/Pt multilayers with ITO seed layers deposited in 40 mTorr of Kr give a coercivity of 6.9 kOe and a D50 about 290 kfci when the output signal is differentiated.

Structure and magnetic properties of L1 CoPt(Ag/MgO,MgO) thin films

Disordered fcc (face-centered-cubic) [100] fiber-textured CoPt thin films with thicknesses of 20–40 nm were deposited on a nonmagnetic sputtered MgO seed layer with and without a Ag intermediate layer. These were subsequently annealed at 600–750 °C for 5–10 min using rapid thermal annealing (RTA). The structural variants were determined to coexist in the in-plane and normal to the plane directions after the RTA process. The MgO underlayer without the intermediate layer revealed strong in-plane anisotropy by magnetic hysteresis measurement. Selected area diffraction (SAD) by TEM and XRD measurements for these samples confirmed the preferential growth of in-plane variants of c axis compared to the perpendicular direction. High remanence and coercive squareness (S*) were obtained due to strong in-plane texture and especially due to exchange coupling among the grains. Evidence for this was provided by ΔM curves. Initial magnetization curves for samples annealed above 600 °C, showed the possibility of assembled single domain or domain pinning mechanism but with negligible free wall motion inside of the grains.

Co alloy longitudinal thin film media with ultrahigh coercivity

The effect of CoCrTa intermediate layer and postannealing on the magnetic properties and microstructure of CoCrPt/Cr thin film media has been investigated. Through a postdeposition annealing in a vacuum oven, a tremendous increase of coercivity up to 6000 Oe was observed for these CoCrPt/CoCrTa/Cr films, in which the thickness of CoCrTa intermediate layer is about 2.6 mm. X-ray diffraction results showed that the hexagonal-close-packed crystallographic structure for the cobalt alloy did not show any change for the sample after postannealing. In-plane transmission electron microscope images for the CoCrPt/CoCrTa/Cr films showed no change of the morphology before and after annealing and about 15% increase of the grain size for the films after annealing. Delta M curves and remanent squareness results showed that intergrain diffusion may result in this improvement of magnetic properties. Atomic force microscope images showed that the surface roughness for the sample after annealing was almost the same as that before annealing. After tailoring the thickness of the magnetic layer to 10 nm, the coercivity as high as 5200 Oe and Mrt value as low as 0.24 memu/cm2 were obtained, which may support ultrahigh bit density than current media.

The effect of Pt interlayers on the magnetic and structural properties of perpendicularly oriented barium ferrite media

Perpendicular barium ferrite (BaM) films with Pt interlayers were successfully fabricated. The magnetic and structural properties of BaM films were compared with BaM films without Pt interlayers. It was found that perpendicular c-axis orientation was greatly improved for films with Pt interlayers. Perpendicular remanent squareness is increased, and the c-axis dispersion angle Δθ50 is decreased with Pt interlayers in BaM films. Also the coercivity was found to increase for films with Pt interlayers. A relative increase in perpendicular nucleation sites over in-plane and/or random nucleation sites contributes to the improvement in perpendicular c-axis orientation. The increase in coercivity may be explained by a weakening of magnetostatic interactions among BaM grains, due to magnetostatic isolation by the nonmagnetic Pt interlayers and the interdiffusion between Pt layers and BaM layers during high temperature ex situ annealing.

C-Axis Perpendicularly Oriented Barium Ferrite Thin Film Media On Silicon Substrate

ABSTRACTBarium ferrite thin films with excellent perpendicular c-axis orientation were successfully fabricated on Si substrate without any buffer layer. To compensate for possible barium deficiency due to the inter-diffusion between films and substrate, a barium-rich target was used. For a 900 Å-thick film, the perpendicular remanent squareness is about 0.9, while the in-plane remanent squareness is about 0.3. The saturation magnetization (Ms) is about 220 emu/cc, while the coercivity is around 3500 Oe. X-ray diffraction (XRD) results show the (001) perpendicular c-axis texture in the films. It was also found that the rapid thermal annealing conditions greatly affect the magnetic properties of barium ferrite films. With a certain flow rate of oxygen gas in the rapid thermal annealer (RTA), barium ferrite films generally crystallize with good perpendicular c-axis texture. Without oxygen gas, the hexagonal barium ferrite phase fails to develop; instead spinel Fe 30 4 phase forms. The reason for the collapsing of hexagonal barium ferrite texture is thought to be an oxygen deficiency in the barium ferrite films due to the reduction of oxygen in the films during the high temperature annealing.

C-Axis Perpendicularly Oriented Barium Ferrite Thin Film Media on Silicon Substrate

ABSTRACTBarium ferrite thin films with excellent perpendicular c-axis orientation were successfully fabricated on Si substrate without any buffer layer. To compensate for possible barium deficiency due to the inter-diffusion between films and substrate, a barium-rich target was used. For a 900 Å-thick film, the perpendicular remanent squareness is about 0.9, while the in-plane remanent squareness is about 0.3. The saturation magnetization (Ms) is about 220 emu/cc, while the coercivity is around 3500 Oe. X-ray diffraction (XRD) results show the (001) perpendicular c-axis texture in the films. It was also found that the rapid thermal annealing conditions greatly affect the magnetic properties of barium ferrite films. With a certain flow rate of oxygen gas in the rapid thermal annealer (RTA), barium ferrite films generally crystallize with good perpendicular c-axis texture. Without oxygen gas, the hexagonal barium ferrite phase fails to develop; instead spinel Fe3O4 phase forms. The reason for the collapsing of hexagonal barium ferrite texture is thought to be an oxygen deficiency in the barium ferrite films due to the reduction of oxygen in the films during the high temperature annealing.

High remanence (Sm, Zr)Fe7Nx+α-Fe nanocomposite magnets through exchange coupling

We have investigated the influence of α-Fe content on the magnetic properties of (Sm, Zr)Fe7Nx+α-Fe nanocomposite permanent magnet materials. As α-Fe content increases, the saturation magnetization and the remanence increase while the coercivity decreases monotonously. Also we confirmed the influence of microstructure on magnetic properties. Uniform and fine grain sizes of around 20 nm realized high remanence and high squareness of demagnetizing curve such as Br=9.9 kG, HcJ=9.1 kOe, Hk/HcJ=29.9%, and (BH)max=17.8 MGOe. These experimental results agree well with micromagnetic calculations by other researchers. Compaction-molded bonded magnet was prepared by magnetic powders with magnetic properties of Br=9.9 kG, HcJ=8.2 kOe, (BH)max=17.5 MGOe and 2 wt% of epoxy resin. Bonded magnet with a density of 5.91 g/cm3 showed high magnetic properties as follows: Br=7.6 kG, HcJ=8.4 kOe, and (BH)max=12.0 MGOe.

MR recording media: microstructure and magnetic properties

To achieve high recording areal density, it is required that future MR media have high coercivity (H/sub c/), high coercive and remanent squareness (S/sup */ and S), low media noise and narrow track recording performance. The microstructure, which strongly affects magnetic properties, should be fully controlled in media fabrication process. The microstructural characteristics like grain size, grain separation, orientation, lattice mismatch, second phases, and stacking faults can affect intergranular magnetic coupling and media noise. The composition of ternary and quaternary cobalt alloy systems, e.g. CoCrTa, CoCrPt, and CoCrPtTa, has significant effects on magnetic performances. The underlayer design, e.g. Cr, CrV/sub x/, and CrTi/sub x/, can affect lattice mismatch, stacking fault density, and other microstructural features. Stress, shadowing, preferred orientation, and c-axis alignment are four mechanisms which control orientation ratio (OR). Oriented medium has more stress-induced noise than isotropic medium has. Thin film medium with bicrystal structure has 5 dB better SNR than non-bicrystal medium. Ultra clean sputtering process (UC-process) can enhance coercivity and reduce noise. For GMR recording media, it requires Hc>3000 Oe and M/sub r/t <0.5 memu/cm/sup 2/. Media with CoCrPtTa, and CoSm/Cr design will be compared for areal density upto 10 Gb/in/sup 2/. Mechanisms of high coercive force and low noise in MR media are reviewed in connection with microstructure and magnetic properties.

Magnetic properties of FeAlN films at elevated temperatures

FeAlN films were fabricated by reactive sputtering from an Fe alloy target in a nitrogen containing plasma. By varying the nitrogen content in the films, magnetic moment as high as 20 kG and coercivities less than 1 Oe can be achieved at room temperature. However, it has been observed that the soft properties of these films experience a reversible degradation at elevated measuring temperatures. The critical temperature, T0c, defined as the temperature at which the coercivity doubles, generally decreases as more nitrogen is incorporated into the film. T0c agrees well with the temperature where remanent squareness (Br/Bs) equals 0.83, indicating a reduced intergranular exchange coupling in films. However, T0c is far below either the reported Curie temperature of the γ′-Fe4N phase or that of the α-Fe phase, which are found in these films. It is suggested that an interfacial granular phase between the grain boundaries of α-Fe or γ′-Fe4N crystallites is responsible for the low T0c. When the temperature increases above T0c, the interfacial phase becomes paramagnetic, and the exchange interaction between crystalline grains becomes weak, thereby causing the coercivity increases.

Parameter identification for a micromagnetic recording medium model (abstract)

Many recent studies of recording and playback in magnetic recording systems have been based on the computer simulation of micromagnetic models of magnetic recording media. Medium models are capable of representing a large class of recording media by selection of appropriate values of model parameters representing the intrinisic properties of the recording medium material. Such parameters include grain size, anisotropy energy density, and exchange energy constant. Largely missing in these studies, however, has been a discussion of how to identify a set of parameter values to represent a particular medium of interest. In this presentation, we describe an identification procedure for selecting the values of the five intrinsic parameters of a micromagnetic medium model to represent a recording medium for which the following hysteresis properties are known: remanent magnetization, coercivity, remanence squareness, and coercivity squareness. Calculation of hysteresis loops verifies that the particular instance of the medium model identified by this procedure reproduces the hysteresis behavior of the medium of interest. Comparison of computer simulations of recording operations to measured recording performance of a medium on a laboratory tester may then be meaningfully made.

Magnetic and magneto-optical properties of Mn/sub x/CuBi(x=0.75-3.5) films

Magnetic and magneto-optical properties of Mn/sub x/CuBi(x=0.75-3.5) thin films are presented. With increasing Mn concentration x, the perpendicular anisotropy constant (K/sub u/), the remanence squareness (S=M/sub r//M/sub s/) and the coercivity (H/sub c/), of the perpendicular hysteresis loop increase. Remanence squareness as high as 0.95 has been obtained. Kerr rotation (/spl theta//sub k/) and ellipticity (/spl eta//sub k/) spectra from all samples measured from the substrate side are similar, with /spl theta//sub k/ exhibiting a broad peak in the blue wavelength region with maximum Kerr rotation up to 0.7 degrees.

Reactively evaporated Co thin film for tape media

The dependence of the magnetic properties of reactively evaporated Co on film thickness and oxygen pressure have been studied. The coercivity was found to be very sensitive to oxygen pressure, but the coercive squareness and coercivity peak at different values of oxygen pressure for the same thickness. An optimized 500 /spl Aring/ Co-O film deposited at an oxygen pressure of 1.6 /spl mu/torr, exhibits a coercivity of 2000 Oe, a coercive squareness S* of 0.7, an M/sub r/t of 2.2 memu/cm/sup 2/, a remanence squareness of 0.9 and an anisotropy constant of 3.5/spl times/10/sup 6/ erg/cc. Auger spectroscopy reveals an oxygen content of 30 at.% throughout the film thickness.

Two- and three-dimensional calculation of remanence enhancement of rare-earth based composite magnets (invited)

Micromagnetic calculations using a finite element technique rigorously describe the magnetic properties of novel, isotropic rare-earth-based composite magnets. Numerical results obtained for a composite material of Nd2Fe14B, SmCo5 or Sm2(Fe0.8Co0.2)17N2.8 and α-Fe particles show that remanence, coercivity, and coercive squareness sensitively depend on microstructural features. Interparticle exchange interactions enhance the remanence by about 60% with respect to noninteracting particles for a mean-grain size approaching the exchange length of the soft magnetic phase and a significant percentage of α-Fe. On the other hand, exchange interactions between the phases suppress the nucleation of reversed domains and thus preserve a high coercive field. Therefore, optimally structured, isotropic composite magnets show remarkably high energy products exceeding 400 kJ/m3.

Bicrystal advanced thin-film media for high density recording

A new type of thin-film recording media with advanced microstructure: the bicrystal film, has been studied experimentally. The easy axes of the bicrystal films are oriented along two specified orthogonal directions in the film plane. In this study, an LiF single crystal substrate with (100) orientation is utilized to obtain the Cr monocrystal underlayer with (100) orientation, and on top of the (100)Cr underlayer the CoCrTa with (112̄0) texture was grown epitaxially. X-ray and transmission electron microscopy (TEM) diffraction studies confirm the fabricated thin-film media has the bicrystal microstructure. The magnetic properties of the bicrystal films have demonstrated very high coercivity (HC∼3000 Oe) and coercive and remanent squareness (S*∼0.9, S∼0.9), all desired for high areal density magnetic recording. The in-plane hysteresis loops of the fabricated films show fourfold symmetry which is the characteristic of the bicrystal microstructure. The effects of Cr underlayer thickness on the magnetic properties of the bicrystal film has also been studied.

Grain Boundary Diffusion in Co/Cu and Co/Cr Magnetic Thin Films

ABSTRACTX-ray photoelectron spectroscopy (XPS) has been used to investigate grain boundary diffusion of Cu and Cr through 1000 Å thick Co films in the temperature range of 325°C to 400°C. Grain boundary diffusivities were determined by modeling the accumulation of Cu or Cr on Co surfaces as a function of time at fixed annealing temperature. The grain boundary diffusivity of Cu through Co is characterized by a diffusion coefficient, D0gb, of 2 × 104cm2/sec and an activation energy, Ea,gb, of 2.4 eV. Similarly, Cr grain boundary diffusion through Co thin films occurs with a diffusion coefficient, Do,gb, of 6 × 10-2cm2/sec and an activation energy, Ea,gbof 1.8 eV. The Co film microstructure has been investigated before and after annealing by x-ray diffraction and transmission electron Microscopy. Extensive grain growth and texturing of the film occurred during annealing for Co deposited on a Cu underlayer. In contrast, the microstructure of Co deposited on a Cr underlayer remained relatively unchanged upon annealing. Magnetometer Measurements have shown that increased in-plane coercivity Hc, reduced remanence squareness S, and reduced coercive squareness S*result from grain boundary diffusion of Cu and Cr into the Co films.

High density recording on longitudinal thin film media

High density recording measurements have been carried out on CoP thin film tapes produced by electroless deposition. Media have been produced with coercivities in the range 1260-1860 Oe. In general D50 is found to be proportional to Hc. However, it is also found that the initial slope of the demagnetization remanence curve is a further factor in determining D50, presumably because this characterizes the resistance of the tape to demagnetization, which is an important factor at high recording densities. Reasons for the high remanence squareness of the tapes are discussed.

Magnetoelastic properties of very thin Co-alloy films

The magnetoelastic properties of thin longitudinal Co-alloy films were investigated by measuring the changes in the hysteresis loop during and after the application of a mechanical strain. Co, Co/sub 90/Pt/sub 10/, Co/sub 88/Cr/sub 12/, and Co/sub 78/Pt/sub 10/Cr/sub 12/ thin films, ranging in thickness and from 250 to 3000 AA, were deposited on Mylar substrates and subjected to bending strains in the 10/sup -3/ range. For all samples the coercivity, coercive squareness, and remanent squareness decreased along the tensile axis, consistent with a negative magnetostriction constant. The magnetoelastic effects were found to scale approximately as H/sub c//sup -1/. The consequences of these observations for magnetic recording-in particular, stress-induced signal loss and transition broadening-are discussed. It is pointed out that temporary application of stress on a recorded medium can have a twofold effect: a partial demagnetization of the recorded cells and a broadening of the transitions. It is estimated that, for the type of alloys investigated, both these effects should be on the order of a few percent per 1% strain for coercivities of 1000 Oe. >