Search

Your search keyword '"Sepehri-Amin, H."' showing total 378 results
Start Over Author "Sepehri-Amin, H."
378 results on '"Sepehri-Amin, H."'

1. Multi-level recording in dual-layer FePt-C granular film for heat-assisted magnetic recording

Author

Tozman, P., Isogami, S., Suzuki, I., Bolyachkin, A., Sepehri-Amin, H., Greaves, S. J., Suto, H., Sasaki, Y., Chang, H. T. Y., Kubota, Y, Steiner, P., Huang, P. -W., Hono, K., and Takahashi, Y. K.

Subjects
Condensed Matter - Materials Science, Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Multi-level magnetic recording is a new concept for increasing the data storage capacity of hard disk drives. However, its implementation has been limited by a lack of suitable media capable of storing information at multiple levels. Herein, we overcome this problem by developing dual FePt-C nanogranular films separated by a Ru-C breaking layer with a cubic crystal structure. The FePt grains in the bottom and top layers of the developed media exhibited different effective magnetocrystalline anisotropies and Curie temperatures. The former is realized by different degrees of ordering in the L10-FePt grains, whereas the latter was attributed to the diffusion of Ru, thereby enabling separate magnetic recordings at each layer under different magnetic fields and temperatures. Furthermore, the magnetic measurements and heat-assisted magnetic recording simulations showed that these media enabled 3-level recording and could potentially be extended to 4-level recording, as the up-down and down-up states exhibited non-zero magnetization.

Published
2023

8. Anisotropy induced spin re-orientation in chemically-modulated amorphous ferrimagnetic films

Author

Kirk, E., Bull, C., Finizio, S., Sepehri-Amin, H., Wintz, S., Suszka, A. K., Bingham, N. S., Warnicke, P., Hono, K., Nutter, P. W., Raabe, J., Hrkac, G., Thomson, T., and Heyderman, L. J.

Subjects
Condensed Matter - Materials Science
Abstract

The ability to tune the competition between the in-plane and out-of-plane orientation of magnetization provides a means to construct thermal sensors with a sharp spin re-orientation transition at specific temperatures. We have observed such a tuneable, temperature driven spin re-orientation in structurally amorphous, ferrimagnetic rare earth-transition metal (RE-TM) alloy thin films using scanning transmission X-ray microscopy (STXM) and magnetic measurements. The nature of the spin re-orientation transition in FeGd can be fully explained by a non-equilibrium, nanoscale modulation of the chemical composition of the films. This modulation leads to a magnetic domain pattern of nanoscale speckles superimposed on a background of in-plane domains that form Laudau configurations in micron-scale patterned elements. It is this speckle magnetic structure that gives rise to a sharp two step-reversal mechanism that is temperature dependent. The possibility to balance competing anisotropies through the temperature opens opportunities to create and manipulate topological spin textures.

Published
2020

13. Inducing out-of-plane precession of magnetization for microwave assisted magnetic recording using an oscillating polarizer in spin torque oscillator

Author

Zhou, W., Sepehri-Amin, H., Taniguchi, T., Tamaru, S., Sakuraba, Y., Kasai, S., Kubota, H., and Hono, K.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

We investigated the dynamics of a novel design of spin torque oscillator (STO) for microwave assisted magnetic recording. Using Ni$_{80}$Fe$_{20}$ (NiFe) as the polarizer and Fe$_{67}$Co$_{33}$ (FeCo) as the field generating layer, we experimentally observed the magnetization reversal of NiFe, followed by multiple signals in the power spectra as the bias voltage increased. The signals reflected the out-of-plane precession (OPP) mode oscillation of both FeCo and NiFe, as well as the magnetoresistance effect of the STO device, which had the frequency equal to the difference between the oscillation frequency of NiFe and FeCo. Such dynamics were reproduced by micromagnetic simulation. In addition to the merit of realizing the OPP mode oscillation with a simple and thin structure suitable for a narrow gap recording head, the experimental results using this design suggested that a large cone angle of $\sim$ 70$^{\circ}$ for the OPP mode oscillation of FeCo was achieved, which was estimated based on the macrospin model., Comment: 12 pages, 6 figures

Published
2019
Full Text
View/download PDF

19. Searching the weakest link: Demagnetizing fields and magnetization reversal in permanent magnets

Author

Fischbacher, J., Kovacs, A., Kühnel, L. Exl J., Mehofer, E., Sepehri-Amin, H., Ohkubo, T., Hono, K., and Schrefl, T.

Subjects
Condensed Matter - Materials Science
Abstract

Magnetization reversal in permanent magnets occurs by the nucleation and expansion of reversed domains. Micromagnetic theory offers the possibility to localize the spots within the complex structure of the magnet where magnetization reversal starts. We compute maps of the local nucleation field in a Nd2Fe14B permanent magnet using a model order reduction approach. Considering thermal fluctuations in numerical micromagnetics we can also quantify the reduction of the coercive field due to thermal activation. However, the major reduction of the coercive field is caused by the soft magnetic grain boundary phases and misorientation if there is no surface damage.

Published
2018
Full Text
View/download PDF

Catalog

Books, media, physical & digital resources
See catalog results