T. Shimatsu

Microstructure and magnetic properties of CoPtCr-SiO/sub 2/ perpendicular recording media

SiO/sub 2/ added CoPtCr magnetic layers are employed for perpendicular recording media. The microstructure, magnetic properties, and recording performance of these media are discussed. Very fine grains (size /spl sim/7 nm), surrounded by amorphous-like grain boundaries, are realized together with good c-axis orientation normal to the film plane. It is considered that the addition of SiO/sub 2/ to CoPtCr is very effective in realizing a well-isolated fine-grain structure without disturbing the epitaxial growth of CoPtCr grains on the Ru underlayer. The medium with a 12-nm-thick recording layer shows a large perpendicular anisotropy K/sub u/ of /spl sim/4 /spl times/ 10/sup 6/ erg/cm/sup 3/, and high coercivity H/sub c/ of /spl sim/4 kOe. Moreover, the medium shows excellent SNR performance together with high thermal stability, indicating great potential for high-density perpendicular recording media.

Optimization of the SiO/sub 2/ content in CoPtCr-SiO/sub 2/ perpendicular recording media for high-density recording

Optimization of the SiO/sub 2/ content for high-density recording was discussed for {(Co/sub 90/Cr/sub 10/)/sub 80/Pt/sub 20/}/sub 100-Z/-{SiO/sub 2/}/sub Z/ perpendicular recording media. The grain size D/sub grain/ decreases significantly from 8.8 to 5.4 nm as the SiO/sub 2/ content Z increases from 0 to 14.4; however, no significant difference in c-axis orientation was observed. Magnetocrystalline anisotropy of grains maintains a large value of nearly 8/spl times/10/sup 6/ erg/cm/sup 3/, even at 10 at % SiO/sub 2/ addition, indicating a high potential of perpendicular anisotropy K/sub u/ to resist thermal agitation of magnetization. The coercivity H/sub c/ increases significantly as the SiO/sub 2/ increases, which is coincident with the reduction of the slope of magnetization curve /spl alpha/=4/spl pi/(dM/dH) due to the enhancement of grain isolation. The H/sub c/ shows a maximum at around 11 at % SiO/sub 2/ (nearly 30 vol % SiO/sub 2/). A further increase of SiO/sub 2/ reduces the value of H/sub c/ significantly, although the /spl alpha/ maintains a constant value of nearly 2, due to thermal agitation of magnetization. The media noise reduces significantly as the SiO/sub 2/ increases; however, the addition of SiO/sub 2/ beyond about 13 at % leads to a slight increases in media noise probably due to a significant increase of thermal agitation. The recording resolution D/sub 50/ shows a broad maximum of /spl sim/420 kFCI at around 11 at % SiO/sub 2/, suggesting that the D/sub 50/ is related to the media noise performance. Experimental results indicate that there is a tradeoff to be made between the thermal stability and media noise performance as a function of SiO/sub 2/ composition.

Metal bonding during sputter film deposition

We studied the bonding between two flat Si substrates with thin metal films. The bonding was accomplished during thin film sputter deposition on contamination free surfaces of metal films. In this work we used Ti and Pt. Successful bonding of these metal films (each having a thickness of 10–20 nm) occurred at room temperature over the entire bonded area (12 mm × 12 mm). Self-diffusion, particularly at grain boundaries and film surface, was the mechanism for bonding. Suitable metal bonding only occurred if the film surface roughness is sufficiently smaller than the self-diffusion length of metals. Particularly in the bonding of Ti to Ti films, transmission electron microscope observation revealed that complete crystalline grains had been formed across the former interface between the single thin Ti films. The interfaceless bonding can be explained by recrystallization of the Ti lattice due to the high self-diffusion coefficient of Ti. This technique would be applied to bonding of wafers to fabricate thin film devices or microsystems. Moreover, this bonding technology can be used with many different thin film materials and various semiconductor substrates.

Fabrication of L11-type (Co–Ni)–Pt ordered alloy films by sputter deposition

L11 type Co-Pt ordered alloy films with a large uniaxial magnetic anisotropy, Ku, of the order of 107erg∕cm3 were successfully fabricated at relatively low substrate temperatures of 270–390°C using ultrahigh vacuum sputter film deposition. L11 type ordered Co-Pt films, with the ⟨111⟩ direction (easy axis of magnetization) perpendicular to the film plane, were fabricated on MgO(111) single crystal substrates and glass disks. The ordered structure was formed in a wide Pt content region of 40–75at.%, and Ku showed a maximum at around 50at.% Pt content. The values of the order parameter S and Ku for L11 type Co50Pt50 films increased as the substrate temperature Ts, increased. Ku reached about 3.7×107erg∕cm3 (S=0.54) at Ts=360°C for the single crystal films deposited on MgO(111) substrates, indicating a potential increase in Ku by enhancing the ordering. The values of Ku for polycrystalline films deposited on glass disks were smaller than those for the single crystal films on MgO(111) substrates, however, Ku r...

High perpendicular magnetic anisotropy of CoPtCr/Ru films for granular-type perpendicular media

The perpendicular magnetic anisotropy K/sub u/ of CoPtCr films deposited on Ru seed layers is discussed as a function of film composition. Moreover, the change in K/sub u/ by the addition of SiO/sub 2/ is examined in light of thermal stability of CoPtCr-SiO/sub 2/ media. The K/sub u/ of Co-10 at % Cr shows a high value of /spl sim/5/spl times/10/sup 6/ erg/cm/sup 3/, which is much higher than that reported by Bolzoni et al. The addition of Pt to the Co-10 at % Cr film results in a further enhancement of K/sub u/. A maximum K/sub u/ of nearly 10/spl times/10/sup 6/ erg/cm/sup 3/ is observed at 25/spl sim/30 at % Pt. All series of films with various Cr contents show maximum values of K/sub u/ at 25/spl sim/30 at % Pt. The maximum value for CoPt (Cr=0) films reaches /spl sim/15/spl times/10/sup 6/ erg/cm/sup 3/. The value of magnetocrystalline anisotropy of grains K/sub u//sup g/, calculated by taking account of the volume fraction of CoPtCr grains, decreases as the SiO/sub 2/ content increases. However, the K/sub u//sup g/ maintains a large value of more than 7/spl times/10/sup 6/ erg/cm/sup 3/, even at 10 at % SiO/sub 2/ in addition to (Co/sub 90/Cr/sub 10/)/sub 80/Pt/sub 20/, which indicates a high potential of CoPtCr-SiO/sub 2/ media to resist thermal agitation of magnetization.

High-potential magnetic anisotropy of CoPtCr-SiO/sub 2/ perpendicular recording media

The magnetic anisotropy of CoPtCr-SiO/sub 2/ perpendicular recording media, including higher energy terms, was studied as a function of film composition and seed layer materials. All series of CoPtCr films with various Cr content, deposited on Ru seed layers, show maximum values of total anisotropy K/sub u/ at 25-30 at%Pt. The maximum value for CoPt(Cr=0) films reaches /spl sim/15/spl times/10/sup 6/ erg/cm/sup 3/. The addition of SiO/sub 2/ to the CoPtCr films reduces the grain K/sub u/, however the grain K/sub u/ maintains a large value of 8/spl times/10/sup 6/ erg/cm/sup 3/ even when 10at%SiO/sub 2/ is added to (Co/sub 90/Cr/sub 10/)/sub 80/Pt/sub 20/, for instance, which indicates the high-potential thermal stability. Theoretical calculations for media designs of 400 Gbits/in/sup 2/ revealed that the ratio of the high-energy anisotropy term K/sub u2/ to K/sub u1/(K/sub u/=K/sub u1/+K/sub u2/) is required to be 0.2-0.35 to enhance the energy barrier for the remanent state, without a notable change in switching field. The films deposited on Ru seed layers were found to show negligibly small K/sub u2/ values, however, the values of K/sub u1/ and K/sub u2/ vary significantly with the seed layer material used. K/sub u1/ decreases almost linearly as the K/sub u2/ value increases. It is concluded that CoPtCr films have a sufficient potential in the values of K/sub u1/ and K/sub u2/ for high-density perpendicular media.

Magnetization reversal mechanism evaluated by rotational hysteresis loss analysis for the thin film media

Rotational hysteresis loss analysis has been carried out for CoCrTa, CoNiPt, and CoCrPt thin-film media to evaluate the influence of microstructure on coercive force through the measurement of magnetic anisotropy. In each medium, the magnitudes of H/sub k//sup grain/ defined as a magnetic field where rotational hysteresis loss W/sub r/ diminishes remain almost a constant value independently of the values of coercive force. With increasing coercive force, the magnitude of the coercive force gradually becomes smaller than that of the switching field of magnetization H/sub p/ evaluated by torque analysis. Through the analysis of H/sub k//sup grain/ and H/sub p/, it is suggested that the coercive force in each examined medium is strongly dependent on the degree of intergranular exchange coupling and/or magnetostatic interactions. While on CoNiPt and CoCrPt media coercive force depends on the rotational hysteresis integral, no correlation between them was observed in CoCrTa films. >

Thermal fluctuation of magnetization in nanocrystalline FePt thin films with high coercivity

The effect of thermal fluctuations of magnetization on static and dynamic properties is discussed for nanocrystalline FePt films. The large magnetocrystalline anisotropy K/sub u/ of L1/sub 0/ type FePt results in large K/sub u/V/(kT) values of more than 70 even for very fine grain sizes of 7-8 nm, indicating the potential of this alloy film to resist thermal fluctuation of magnetization. It is successfully demonstrated that larger K/sub u/V/(kT) values of these films lead to lower magnetic viscosity. Annealing at higher temperature results in larger K/sub u/V/kT values and smaller V/sub act/. The remanent coercivity measured at high sweep rate by using pulsed magnetic fields indicates the high thermal stability of these alloy films at high frequencies. However, the results indicated that care should be taken to induce an adequate magnitude of K/sub u/ for the FePt alloy to be used as ultrahigh density recording media.

Thermally induced switching field distribution of a single CoPt dot in a large array

Magnetic dot arrays with perpendicular magnetic anisotropy were fabricated by patterning Co(80)Pt(20)-alloy continuous films by means of laser interference lithography. As commonly seen in large dot arrays, there is a large difference in the switching field between dots. Here we investigate the origin of this large switching field distribution, by using the anomalous Hall effect (AHE). The high sensitivity of the AHE permits us to measure the magnetic reversal of individual dots in an array of 80 dots with a diameter of 180 nm. By taking 1000 hysteresis loops we reveal the thermally induced switching field distribution SFD(T) of individual dots inside the array. The SFD(T) of the first and last switching dots were fitted to an Arrhenius model, and a clear difference in switching volume and magnetic anisotropy was observed between dots switching at low and high fields.

Microwave assisted magnetization switching in Co/Pt multilayer

In this study, we have experimentally investigated the microwave assisted magnetization by switching (MAS) on the microstructured Co/Pt multilayer. The sample exhibits the typical magnetization curve peculiar to perpendicular anisotropy films, that is, a steep reversal initiated by nucleation of a reversed domain followed by its subsequent gradual expansion by the domain wall displacement. By applying microwaves with the frequency of GHz order, the nucleation field Hn is significantly reduced at three frequencies. Taking into account the effective anisotropy field of our sample, the first dip of Hn at the lowest frequency probably corresponds to the Kittel mode excitation, and the other two dips at higher frequencies correspond to unidentified excitation modes other than the Kittel mode. Among them, the last dip of Hn at the highest frequency reaches about 1/3 of that without microwave application. These results suggest the existence of more effective excitation modes for MAS than the Kittel mode.