Wall Coercivity Research Articles

Magnetic parameters of exchange-coupled double layer films for magneto-optical recording

The authors have proposed a technique for determining magnetic parameters based on the bubble model modified for exchange-coupled double layer (ECDL) films. This technique was applied to direct overwritable ECDL media. It was found that wall coercivity of single layer films differs from that of the corresponding layers in ECDL films and interface wall energy changes with the shape of bubble domains. The experimental results indicate that the magnetic parameters obtained using this model are effective for understanding the phenomena concerning magnetooptical recording.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Micromagnetic computation for wall coercivity caused by magnetic nonuniformity

The wall coercivity caused by the magnetic nonuniformity has been studied numerically. The Landau-Lifshits-Gilbert (LLG) equation is integrated by an explicit scheme of the modified Dufort-Frankel method. The computation was carried out for a two-dimensional grid system representing the magnetization direction in a magnetic film plane. Typical magnetic parameters for magneto-optic recording media were assumed. Spatial variations of uniaxial anisotropy were treated as the magnetic nonuniformity. The validity of our numerical approach was demonstrated with preliminary one-dimensional computations, compared to analytical solutions. A wall coercive field of 2.5 kOe was observed for the wall coupling with a pinning site (30-Å width and 360-Å spacing along the wall) of 10 times larger anisotropy compared to the ordinary region. A two-dimensional anisotropy variation (K=106–107 erg/cm3) with a wavelength larger than 60 Å also caused a wall coercivity on the order of 1 kOe, compared to those in magneto-optic recording media. It was also found that a fine pinning site on the order of 100 Å caused a notable coercivity for the bubble domain wall surrounding it.

Wall and Bloch-line coercivity measurements in magnetic garnet films

The wall and Bloch-line motion at small magnetic fields in several garnet films has been investigated and coercive phenomena have been studied. Wall coercivity measurements have been carried out with various spatial resolutions. The coercivity curves show jumps and hysteresis effects at small observation windows. When increasing the observation window the measured curve represents the averaged response of all observed walls showing the mean value for the coercive field H c. The coercive field of vertical Bloch lines has also been measured by directly observing the Bloch lines. Both measurements, wall and Bloch line, refer to local material inhomogeneities of the garnet films.

Measurement of domain wall energy and simulation of overwrite process on TbFeCo films

In order to analyze the direct thermomagnetic overwrite process, the domain wall energy, saturation induction, and coercivity for TbFeCo films were measured at high temperatures at which the recorded domains are formed. The domain wall energy was determined from an observation of stripe domains after AC demagnetization at elevated temperatures. Using the obtained parameters, a simulation was performed on an overwrite process with a fixed bias magnetic field for 100- and 200-nm-thick films. The simulation indicates that significant shrinkage of a prerecorded domain occurs for the 200-nm-thick film, which is consistent with previous observations. >

The role of wall coercivity and energy on the read-out stability of GdTb-Fe based magneto-optical disks

Recording experiments in which the read-out stability of thermomagnetically written domains in GdTb-Fe disks was measured as a function of read laser power and the applied magnetic field are reported. It is shown that the domain stability increases significantly and the field dependence becomes much flatter with increasing Tb content. At low Tb content, compensation temperatures below room temperature result in a strong reduction of domain stability at higher applied magnetic fields. The results of a bubblelike model using the measured magnetic and thermal properties of the disks explain and predict the recording data with high precision. >

Magnetization reversal process and domain dynamics in thermomagnetic recording of TbFeCo amorphous films

The domain wall dynamics in thermomagnetic recording of TbFeCo amorphous films is interpreted in terms of a model comprising magnetization reversal nucleation followed by a wall displacement process. The nucleation of the reversed domain from the saturated magnetization state occurs with a field usually observed as a nominal coercivity in Kerr or magnetization hysteresis loops. The concept of wall coercivity, which is often smaller than the nucleation field, is explicitly incorporated in the stability condition of the cylindrical domain during the wall displacement process. The model predicts both the recorded domain size and its dependence on recording power and applied field.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical recording aspects of rf magnetron-sputtered iron-garnet films

The intrinsic magneto-optical readout performance in reflection is calculated for bismuth and cobalt-substituted iron-garnet films on a multilayer interference mirror at 800-, 633-, 488-, and 420-nm wavelengths and is compared with that of a trilayer medium composed of an antireflection layer, a rare-earth transition-metal film, and a metallic mirror. It is found, when disregarding inhomogeneities, like irregular domain shape, ripple of the magnetic anisotropy, and surface roughness, that iron garnets are superior to rare-earth transition-metal films at blue to near-ultraviolet wavelengths if operated at thicknesses where optical interference occurs in the magnetic layer. Optical transmittance at these thicknesses is sufficiently high so that multilevel recording media can be conceived. In contrast, the optical absorption of rare-earth transition-metal alloys is much higher so that only thicknesses much above interference conditions are feasible, thus precluding them from multilevel recording. This comparative study is supplemented by calculating the magneto-optical performance in reflection of a recently reported multilayer medium composed of an antireflection coating and a periodically repeated sandwich of 4-Å Co and 9-Å Pt layers. In contrast to conventional rare-earth transition-metal films, the magneto-optical Kerr effects of this material do not degrade when decreasing the wavelength from 800 to 400 nm, but still do not reach the performance of bismuth-iron garnets in the green to ultraviolet spectrum. For the garnet system Y3−xBixFe5O12 the spectra of the real and imaginary parts of the diagonal and off-diagonal component of the dielectric tensor εij are reported in the range of photon energies between 1 and 5 eV, i.e., 1240- and 248-nm wavelengths and a bismuth concentration up x=1.4 Bi3+ atoms per garnet formula. In addition, the off-diagonal components ε′12 and ε■12 are parametrized in terms of paramagnetic optical transitions, taking the spectra for x=1.25 as a typical example. Furthermore, optical and magneto-optical spectra are presented for Co2+- and Co3+- substituted iron garnets and barium hexaferrite BaFe12O19. Finally, the spectral dependence of the magneto-optical figure of merit 2ΘF/α of (Y,Bi)3Fe5O12 and amorphous TbFe is compared. Furthermore, high-resolution transmission electron micrographs and x-ray double-crystal diffractograms are presented that elucidate the perfect epitaxial alignment of single-crystalline iron-garnet films and the columnar morphology of polycrystalline iron-garnet films prepared by rf magnetron sputtering. The initial nucleation period of polycrystalline garnet films can be influenced by low-energy ion bombardment for improving the film texture. Under favorable sputtering conditions single- and polycrystalline bismuth-iron garnet films develop a perpendicular magnetic anisotopy. It is not yet clear whether sputtered iron-garnet films can meet the critical requirements on magnetic wall coercivity and magnetic remanence.

Film-head pulse distortion due to microvariation of domain wall energy

Experiments show that the location of spurious peaks on the trailing edges of data pulses is strongly influenced by the application of small DC bias currents to the head coil during readback. A simple model of the head response which includes both magnetization rotation and domain wall motion for the case where domain wall coercivity is comparable to signal fields in the head reproduces many of the features of the observations. Calculations show that fields in pole tips during reading can exceed the wall-motion coercive force of permalloy. It is demonstrated that the resulting domain wall motion could give rise to the spurious signals sometimes seen in thin-film heads. It is proposed that such heads contain domain walls parallel to the signal flux path. The observed oscillations in the spurious response are primarily caused by filter dynamics, not domain wall dynamics. >

Dependence of coercivity on the measurement method in epitaxial magnetic garnet films

The coercivity of epitaxial magnetic garnet films is investigated by different measurement methods. On eliminating the influence of the neighbouring domains the domain wall oscillating method, measuring the start field of wall displacement, i.e., the domain will coercivity, was found to be the best means of characterizing the domain wall-defect interaction. Coencivities determined by domain expansion, bubble translation and from hysteresis measurements are significantly higher than the domain wall coercivity. Empirical correlation was found between the results of these latter methods, but no correlation was found with domain wall coercivity.

Nonuniform wall motion in magnetic stripe domains and its effect on vertical Bloch line propagation

Nonuniform transient motion of magnetic stripe domain walls was observed in bubble garnet material. A number of films with 2–8 μm stripe width were used for investigation. The nonuniformity of motion varies from place to place on a submicron scale in the ‘‘perfect’’ single-crystal garnet film. Observing the development through a time sequence shows that the nonuniform motion begins within the first wall width of displacement. The degree of the nonuniformity increases as the effective drive field decreases. This nonuniform wall motion is due to the existence of local fluctuations of wall coercivity within the garnet crystal film. The response of the stripe domain wall due to dc bias field variation shows behavior similar to Barkhausen jumps indicating the intrinsic nonuniformity of the garnet material. The nonuniform wall motion may seriously affect the reliability of the propagation of vertical Bloch lines (VBLs) in a VBL memory.

Sample size effect in coercivity measurement of epitaxial magnetic garnet films

The full set of characteristic parameters of epitaxial garnet films was measured as a function of the sample size from about 2000 to 5 mm2. The type of domain structure, width of the stripe domain period, characteristic material length, domain wall energy, and anisotropy did not change within the error of measurement on reducing the sample size. The bubble domain collapse field decreased by about 2.5%, and the domain wall coercivity was measured and found to have decreased—almost by an order of magnitude. The experiments showed the lack of dependence of coercivity on the domain wall length. The coercivity and collapse field are suggested as being related to the sample dimensions via the sample-size-dependent derivatives of the total free energy with respect to the domain wall positions. The assumption of the direct effect of the free energy derivatives on the coercivity is supported by a model experiment.

Magnetoelastic and optoelastic coupling in (111)- and (110)-oriented bismuth-iron garnet films prepared by sputter epitaxy

High-quality iron-garnet films of composition Gd3−vBivFe5−w−x−yGdw GaxAlyO12−zArz with 1.02≤v≤1.45, 0.01≤w≤0.36, 0.22≤x≤0.24, 0.26≤y≤0.38, and 0.002≤z≤0.014 have been grown epitaxially on (111)- and (110)-oriented gallium garnet substrates with the lattice constant in the range 1.2475≤a0(nm)≤1.2553 by rf magnetron sputtering in argon plasma. For the study of the optoelastic and magnetoelastic properties of these sputter-epitaxial films the lattice mismatch Δa⊥=as−a⊥f has been varied in the range −1.7&amp;lt;Δa⊥(10−2 nm)&amp;lt;0.5 by using substrates with various lattice constants. The near-infrared optical anisotropy normal to (Δnxy) and in the plane (Δnyz) of (110)-oriented films turn out to depend linearly on Δa⊥ over this large range of lattice misfits. No growth-induced contribution has been observed. The same behavior is found for the induced uniaxial (K⊥) and in-plane (K∥) magnetic anisotropy constants at room temperature. The optoelastic coefficients P44 and ΔP, as well as the magnetostriction constants λ111 and λ100 inferred from the measured optical and magnetic anisotropies, respectively, compare well with literature values for the considered film compositions if excess gadolinum is taken as diamagnetic substitution on octahedral lattice sites. The epitaxial structure of the films has been confirmed by the narrow x-ray diffraction linewidth from x-ray double-crystal rocking curves and is supported by the extremely low waveguide losses at 1.33-μm wavelength and the low magnetic wall coercivity, i.e., α=0.15 cm−1 and 60 A m−1, respectively. The structural quality of (110)-oriented films is the same as that of (111)-oriented films.

Initial permeability of amorphous multilayered films

Permeability of amorphous single and multilayered films was studied. A very small anisotropy was obtained by annealing the samples above the Curie temperature followed by rapid cooling. Initial permeability higher then 10000 was obtained. As the multilayered structure becomes finer the domain structure becomes simpler with higher permeability and better frequency characteristics. In the finest structure where thickness of a magnetic layer is 1000 Å wall coercivity increased and permeability became lower. But the finest structure exhibited little deterioration of permeability at high frequencies.

Preparation of High Remanence TbFe Films

The remanence of amorphous TbFe films with perpendicular anisotropy deposited by several deposition methods was investigated. The uniaxial anisotropy was enhanced in films deposited onto rotating substrates. The coercivity increased with dual gun sputtering compared to single gun sputtering. The remanence did not depend on the anisotropy directly but was mainly related to the wall coercivity. In dual gun rf magnetron sputter-deposited films onto rotating substrates, the wall coercivity was so large that magnetization rotation dominated the magnetization reversal, resulting in high remanence. The wall coercivity also increased in laminated films. A high remanence of 300 Gs with a residual ratio of 1 has been obtained in Tb31Fe69 films produced with a repetition of the deposition of a 0.5 µm thick lamina and the time intervals between the sputtering of each lamina.

Stability and temperature hysteresis of magnetic stripe domain structures

A phenomenological model is outlined that connects the effects of the domain structure stability with respect to an external magnetic field with those of the domain structure temperature stability i.e. with the temperature hysteresis of the geometry of stripe domain structure. The previously observed fact that the magnitude of the applied field, H s, needed for irreversibly moving domain walls depends on the domain patterns geometry and that H s is maximum and equals to the domain wall coercivity H c only when the domain structure corresponds to the total energy minimum is described theoretically. Formulae obtained from modelling the effective pressures on the domain walls were computed using the magnetic parameters of an epitaxial garnet layer and good agreement of the theory with experiment was found.

Domain Wall and Domain Structure of Amorphous Multilayered Films

Studies on the effects of nonmagnetic (SiO 2 ) intermediate layers of various thicknesses in amorphous Co-Zr-Nb films are described. Both SiO 2 and amorphous Co-Zr-Nb layers were formed by ion beam sputtering, and samples were annealed in rotating and dc magnetic fields to induce a uniaxial anisotropy. Domain wall and structure observations and coercivity measurements both indicated that as the nonmagnetic layer thickness b is increased, a transition from a multilayered to a single-layer film structure occurs at between 10 and 20 A, while for b ≫ 1000 A the film loses its multilayered character.

Remanent Magnetization and Wall Coercivity of Continuous Films with Perpendicular Anisotropy

Remanent Magnetization and Wall Coercivity of Continuous Films with Perpendicular Anisotropy

Magneto-optic recording technology (invited)

Magneto-optic recording has great potential as an erasable/rewritable storage technology. Bit densities of 108/cm2 are being achieved and improvements can be expected. This paper reviews the status and discusses the most critical issues of the technology. It is shown that the fundamental broadband signal-to-noise ratio in existing rare-earth transition-metal media is well above 20 dB and therefore adequate to maintain error rates less than 10−12. Substrate imperfections in optical media lead to error rates of 10−5–10−6, but even these relatively high raw bit error rates are corrected to 10−13 by error correction codes and hardware being used in write-once optical disk storage systems. The stability of magnetic domains recorded into magneto-optical materials may be explained by a model involving wall energy, demagnetizing fields, applied fields and wall coercivity, and there exist materials which fulfill the requirements necessary to have recorded data remain stable against fields up to 100 Oe and temperatures from 0 to 100 °C. Of more concern is the stability of the amorphous thin films against long-term annealing. Passivation with optical dielectrics is essential, and preliminary data on some rare-earth transition-metal alloys indicate that recorded data would be stable for five years when stored at 50 °C and that the media could withstand in excess of 1010 write/erase cycles with less that 10% change in minimum stable domain size. It is concluded that magneto-optic recording technology is a promising high-density recording technology with a place in the digital storage hierarchies of the future.

Domain wall motion in perpendicular recording films

The wall motion in films for perpendlcular recording is investigated in terms of the atomic layer model, One-dimensional inhomogeneities of magnetic parameters which may be caused by the film structure can be introduced in the model. As an example fluctuations of the Cr content in Co-Cr films are described by a sine-shaped profile of inhomogeneity. This yields a calculated dependence of the wall coercivity as a function of the profile period which is in good agreement with experimental results of other authors. Taking into account the known formula for demagnetizing fields of stripe domains perpendicular hysteresis loops can be calculated, One example is shown and compared with an experimental loop.

Stability of domain patterns and domain wall coercivity

The stability of demagnetized stripe domain patterns of epitaxial magnetic garnets with respect to applied field pulses has been observed to be strongly dependent on the magnetic history of the sample. The highest stability with regard to irreversible changes of the wall positions has been measured after the demagnetization by a slowly decreasing ac field normal to the specimen which is commonly considered to yield the lowest total energy domain structure with the zero field stripe period. In such a case the stability field has been found just equal to the sample's coercivity. The measurement is believed to make it possible to decide experimentally which of the alternative domain structures of a certain type is the lowest energy one.