High Nucleation Field Research Articles

Dependence of Nucleation Field on the Size of Soft Phase in Magnetic Hard–Soft Exchange Coupling Nanocomposites

Hard–soft exchange coupling nanocomposites have critical applications in various important materials. The magnetic properties of nanocomposite permanent magnetic films improve with a higher nucleation field (Hns) of the soft magnetic phase. Hns is sensitive to the thickness (ds) of the soft magnetic layer. Understanding the dependence of Hns and irreversible field (Hirr) on ds, especially at the nanometric scale, is crucial for comprehending the magnetic mechanism and facilitating the design and preparation of high-performance nanocomposite permanent magnets. However, during the high-temperature deposition process, diffusion between hard and soft magnetic phases occurs, leading to the generation of other phases. This makes it challenging to accurately reflect the relationship between Hns and ds. To address this issue, we successfully fabricated high-quality SmCo5/Fe nanocomposite bilayer films with different soft magnetic thicknesses and high textures by controlling the preparation process. We conducted a quantitative analysis of the relationship between Hns and ds within the range of 2–40 nm. Based on the experimental results, we propose a new theoretical simulation formula that enhances the understanding of the characteristics at the interface between the soft magnetic and hard magnetic phases. The theoretical simulation results show that a thin softened hard layer of about 4–6 nm thickness exists at the interfacial region, which concurrently reverses with the soft magnetic phase during the demagnetization process. Our results offer the generality and critical basis for the further study of hard–soft nanocomposite magnetic materials.

Switching Field Control by Utilizing Ferromagnetic Grain Boundary in Cap Layer for CoPt-Based Full Granular Media

Investigation of magnetic properties and nanostructure for full granular media with CoPt−Gd2O3 granular cap layers (CLs) of various Gd2O3 content were conducted. Media with low surface roughness of 0.19 nm is maintained even though Gd2O3 content is reduced from 30 to 5 vol%, which is lower than that of media with CoPtCrB continuous CL. The result is supported by surface morphology observed by transmission electron microscopy. According to cross section element analysis, homogeneous nanostructure in the thickness direction with one- text on-one columnar growth of magnetic grain is observed. Reduction of Gd2O3 content increases indirect intergranular exchange coupling through CL which results in media with higher nucleation field and lower saturation field than that of media with CoPtCrB continuous CL. Furthermore, at a CL thickness of 6 nm, when Gd2O3 content is reduced from 30 to 15 vol%, switching field distribution is reduced from 0.11 to 0.05. These results suggest that the reduction of Gd2O3 content of the CoPt−Gd2O3 granular CL is effective to reduce the switching field distribution of the full granular media to be lower than that of media with CoPtCrB continuous CL while retaining the extremely smooth media surface.

Magnetic domain structure and magnetization reversal in (Co/Ni) and (Co/Pd) multilayers

The temporal behavior of magnetization reversal mechanism and domain structures of (Co/Ni) and (Co/Pd) multilayers have been studied using magneto-optical Kerr effect microscopy and magnetometry measurements. The domain size and nucleation field of (Co/Ni) multilayers were found to be reduced as the number of bilayers increases. On the other hand, (Co/Pd)×10 multilayer has a much larger domain size, faster magnetization switching, and a higher nucleation field than that in (Co/Ni). From the imaging of magnetic domains, two different types were observed, namely, dendritic domains for (Co/Ni) and blocks type for (Co/Pd). From time dependence of magnetization reversal at a fixed applied magnetic field, it was revealed that the reversal mechanism in (Co/Ni) multilayer occurs by nucleation followed by domain propagation. Whereas, for (Co/Pd) multilayers, few large domains were observed, and they were found to expand at a faster rate until a complete saturation.

Effects of Different Crystal Structures of Ta Underlayer on Magnetic Properties and Microstructures of Pr-Fe-B Thin Films

Pr-Fe-B/Ta bilayer films were sputtered on glass substrates in a high vacuum magnetron sputtering system. Before sputtering the magnetic Pr-Fe-B thin films, Ta underlayers with two different crystal structures, hexagonal close-packed HCP(0002) and body-centered cubic BCC(110), were prepared at substrate temperatures of 300 and 600 °C, respectively. With the Ta HCP(0002) underlayer, the Pr-Fe-B film showed significant in-plane oriented Pr 2 Fe 14 B phase and a small amount of perpendicular magnetic anisotropy (PMA) phases. The magnetic properties hence exhibited high nucleation field H n , coercive field H c , and large area in-plane hysteresis loop. Further replacing the Ta HCP(0002) underlayer with a Ta BCC(110) underlayer promoted the growth of columnar Pr-Fe-B grains. The typical hard Pr 2 Fe 14 B phase with PMA was obtained. Low H n , H c , and small area in-plane hysteresis loop were consequently obtained. The PMA of the Pr-Fe-B/Ta bilayer film was also improved. The mechanisms to enhance the PMA of Pr-Fe-B/Ta bilayer films could be attributed to a lattice mismatch between the magnetic Pr-Fe-B layer and the Ta underlayers, where the smaller lattice mismatch at Pr 2 Fe 14 B (00l)/Ta(110) interface promoted the (00l)-oriented Pr 2 Fe 14 B planes to epitaxially grow on the Ta BCC(110) underlayer. Replacing the Ta BCC(110) underlayer with a Ta HCP(0002) underlayer just reduced the epitaxial effects and increased the pinning sites in the Pr-Fe-B magnetic films.

Magnetic cylindrical nanowires with single modulated diameter

Nucleation and propagation of a domain wall along a modulated wire as a function of its geometry has been investigated. In all the cases, nucleation began at the thicker section and propagated toward the thinner section, regardless of the reversal mode. At the interface, the wall was pinned and a higher nucleation field was required to continue the propagation of the wall. This behavior led to the existence of two nucleation fields along each branch of the hysteresis curve.

Epitaxially Grown FCC/BCC/HCP Multi-Underlayers for High Performance Perpendicular Media

In this study, a new underlayer structure consisting of FCC NiW, BCC Cr and HCP Ru layers is investigated as a promising underlayer that offers good perpendicular magnetic and recording properties. The FCC (111) planes of NiW layer enhances hetero-epitaxial growth of the subsequent BCC Cr (110), HCP Ru and Co (0002) layers in the direction perpendicular to the film plane. After the NiW-Cr-Ru layer structure is optimized, the Co (0002) <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> axis dispersion is greatly reduced and ~ 3 degree of XRD rocking width is achieved. As a result, excellent magnetic properties such as high coercivity, high nucleation field and narrow switching field distribution (SFD) are obtained. In addition, low media jitter and high signal-to-media noise (SNR <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> ) can be achieved. The SNR <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> is also highly dependent on head-to-SUL spacing which is altered by varying NiW or Cr layer thickness without affecting crystallography. Optimized head-to-SUL spacing is needed to gain head writeability as well as resolution.

Perpendicular Recording Technology: From Research to Commercialization

Perpendicular magnetic recording was a long-awaited technology to break the areal density limit of the longitudinal recording system. Through the research on high squareness CoPtCrO/Ru recording media, we have established the intrinsic design concept of stable perpendicular magnetic recording in which the high nucleation field of perpendicular magnetic media supports the very stable magnetization against disturbance fields in recording processes. By intensive study of the perpendicular magnetic recording integration into the 2.5-in 50-80 GB/platter test hard disk drives (HDDs), we have successfully developed the first 1.8-in 40 GB/platter HDD product featuring the perpendicular magnetic recording technology at 133 Gbit/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> recording density. Thermal stability and spacing robustness in the writing process of the perpendicular recording system were also found to be great advantages of the future portable HDD systems.

Asymmetric initial magnetization process of elongated particles in nucleation-type L1 0 FePt films

Following the sequence of applied magnetic field H=0→+6 kOe→−6 kOe along easy axis, an asymmetric initial magnetization process of multi-domain particles against the direction of H was observed by using an in-field magnetic force microscopy in nucleation-type L1 0 FePt films with nominal thickness t N⩾25 nm. The initial magnetizations increased quickly due to domain wall movement. At H=+6 kOe, a fraction of particles kept multi-domain configurations though the majority became single domain successively with the increase of H. However, almost all these multi-domain particles became single domain at lower opposite field H=−2 kOe. The saturation magnetizations of particles are hard to reverse because of high nucleation field. This asymmetric behavior is likely to be caused by the intrinsic asymmetry of energy barrier against the movement/wiping out of domain walls due to the geometry of particle, and can be degenerated by the irregularity of particle shape.

Anisotropy enhanced dual magnetic layer media design for high-density perpendicular recording

An anisotropy enhanced dual magnetic layer structure is proposed to improve the recording performance of perpendicular media. The initial layer is made of low noise Co-Cr-Pt-O layer and the top layer is made of higher anisotropy Co-Cr-Pt-B layer. A higher nucleation field than the single layer structures is obtained for the dual magnetic layer media. By optimizing the layer structure and properties, better signal-to-noise and lower adjacent track erasure can be achieved for the dual magnetic layer design over single magnetic layer media.

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.

A new design to suppress recording demagnetization for perpendicular recording

Perpendicular recording is expected to be a high areal density recording system because of strong thermal stability potential and highly efficient writing process. However large demagnetizing fields in uniformly magnetized areas cause recording demagnetization and signal decrease with time. We propose a new recording design to suppress recording demagnetization for perpendicular magnetic recording, using high nucleation field media. A CoPtCrO perpendicular medium with high nucleation field successfully demonstrated the low noise performance. It shows less recording demagnetization and strong thermal stability. CoPtCrO perpendicular medium exhibited good BER performance with ring head recording.