Positive exchange bias in epitaxial permalloy/MgO integrated with Si (100)

S. S. Rao, J. T. Prater, Fan Wu, S. Nori, D. Kumar, L. Yue, Sy-Hwang Liou, J. Narayan

Research output: Contribution to journalArticle

11 Citations (Scopus)

Abstract

In magnetic random access memory (MRAM) devices, soft magnetic thin film elements such as permalloy (Py) are used as unit cells of information. The epitaxial integration of these elements with the technologically important substrate Si (1 0 0) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni82.5Fe17.5 (permalloy, Py) on Si (1 0 0) using a TiN/MgO buffer layer. Initial stages of growth are characterized by the formation of discrete islands that gradually merge into a continuous film as deposition times are extended. Interestingly, we find that the magnetic features of Py films in early stages of island coalescence are distinctly different from the films formed initially (discrete islands) and after extended deposition times (narrow distribution of equiaxed granular films). Isothermal in-plane and out-of-plane magnetic measurements performed on these transitional films show highly anisotropic magnetic behavior with an easy magnetization axis lying in the plane of the film. Importantly, when this sample is zero-field cooled, a positive exchange bias and vertical loop shift are observed, unusual for a soft ferromagnet like Py. Repeated field cycling and hysteresis loops up to the fields of 7T produced reproducible hysteresis loops indicating the existence of strongly pinned spin configurations. Classical interface related exchange bias models cannot explain the observed magnetic features of the transitional Py films. We believe that the anomalous magnetic behavior of such Py films may be explained by considering the highly irregular morphology that develops at intermediate growth times that are possibly also undergoing a transition from Bloch to Neel domain wall structures as a function of Py island size. This study broadens the current understanding of magnetic properties of Py thin layers for technological applications in magneto-electronic devices, integrated with Si (100).

Original languageEnglish (US)
Pages (from-to)140-146
Number of pages7
JournalCurrent Opinion in Solid State and Materials Science
Volume18
Issue number3
DOIs
StatePublished - Jun 2014

Fingerprint

Hysteresis loops
Magnetic properties
Magnetic devices
Magnetic thin films
Domain walls
Magnetic variables measurement
Buffer layers
Coalescence
Epitaxial growth
Magnetization
Data storage equipment
Substrates

Keywords

  • Domain matching epitaxy
  • Epitaxial integration
  • Permalloy islands
  • Positive exchange bias
  • Pulsed laser deposition

ASJC Scopus subject areas

  • Materials Science(all)

Cite this

Positive exchange bias in epitaxial permalloy/MgO integrated with Si (100). / Rao, S. S.; Prater, J. T.; Wu, Fan; Nori, S.; Kumar, D.; Yue, L.; Liou, Sy-Hwang; Narayan, J.

In: Current Opinion in Solid State and Materials Science, Vol. 18, No. 3, 06.2014, p. 140-146.

Research output: Contribution to journalArticle

Rao, S. S. ; Prater, J. T. ; Wu, Fan ; Nori, S. ; Kumar, D. ; Yue, L. ; Liou, Sy-Hwang ; Narayan, J. / Positive exchange bias in epitaxial permalloy/MgO integrated with Si (100). In: Current Opinion in Solid State and Materials Science. 2014 ; Vol. 18, No. 3. pp. 140-146.
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AU - Rao, S. S.

AU - Prater, J. T.

AU - Wu, Fan

AU - Nori, S.

AU - Kumar, D.

AU - Yue, L.

AU - Liou, Sy-Hwang

AU - Narayan, J.

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AB - In magnetic random access memory (MRAM) devices, soft magnetic thin film elements such as permalloy (Py) are used as unit cells of information. The epitaxial integration of these elements with the technologically important substrate Si (1 0 0) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni82.5Fe17.5 (permalloy, Py) on Si (1 0 0) using a TiN/MgO buffer layer. Initial stages of growth are characterized by the formation of discrete islands that gradually merge into a continuous film as deposition times are extended. Interestingly, we find that the magnetic features of Py films in early stages of island coalescence are distinctly different from the films formed initially (discrete islands) and after extended deposition times (narrow distribution of equiaxed granular films). Isothermal in-plane and out-of-plane magnetic measurements performed on these transitional films show highly anisotropic magnetic behavior with an easy magnetization axis lying in the plane of the film. Importantly, when this sample is zero-field cooled, a positive exchange bias and vertical loop shift are observed, unusual for a soft ferromagnet like Py. Repeated field cycling and hysteresis loops up to the fields of 7T produced reproducible hysteresis loops indicating the existence of strongly pinned spin configurations. Classical interface related exchange bias models cannot explain the observed magnetic features of the transitional Py films. We believe that the anomalous magnetic behavior of such Py films may be explained by considering the highly irregular morphology that develops at intermediate growth times that are possibly also undergoing a transition from Bloch to Neel domain wall structures as a function of Py island size. This study broadens the current understanding of magnetic properties of Py thin layers for technological applications in magneto-electronic devices, integrated with Si (100).

KW - Domain matching epitaxy

KW - Epitaxial integration

KW - Permalloy islands

KW - Positive exchange bias

KW - Pulsed laser deposition

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