Exchange Bias and Exchange Spring Phenomena in NiFe/IrMn/Co Trilayers: The Influence of the Crystallographic Texture of the IrMn Layer.

Name: Isabel Liz Castro Merino
Type: PhD thesis
Publication date: 19/07/2013

Namesort ascending Role
Valberto Pedruzzi Nascimento Co-advisor *
Edson Passamani Caetano Advisor *

Examining board:

Namesort ascending Role
Valberto Pedruzzi Nascimento Co advisor *
Flávio Garcia External Examiner *
Elisa Maria Baggio-Saitovitch External Examiner *
Edson Passamani Caetano Advisor *
Carlos Larica Internal Examiner *
Antonio Alberto Ribeiro Fernandes Internal Examiner *

Summary: In the present work, a systematic investigation, through structural and magnetic characterizations, is discussed regarding the influence of the crystallographic texture on the Exchange Spring observed in the NiFe/IrMn/Co trilayer deposited at room temperature with Magnetron Sputtering technique. Using X-ray diffraction, it was shown that NiFe,
IrMn and Co layers have texture oriented fcc [111], when Ta seed layer deposited, and there was no of other types of orientations. For samples grown on Cu seed layer, beyond majority [111]-fcc, types of [200] and [220] orientations are also present in the diffraction patterns of the NiFe/IrMn/Co heterostructures. From X-ray reflectivity, it was determined that the Cu-system has much high Si/Cu interface roughness (Si/Cu, rms = 2.8 nm) than that of Ta (Si/Ta, rms = 0.2 nm). Combining results from the combination of DC magnetization and ferromagnetic resonance techniques, performed at room temperature, it is provided a planar magnetic anisotropy and an understanding of the Exchange
Bias (unidirectional exchange coupling at the NiFe/IrMn and IrMn/Co interfaces) and the Exchange Spring effects in NiFe/IrMn/Co heterostructures. DC magnetization data suggest that the Exchange Spring exists before the field cooling protocol (unidirectional anisotropy induction). The Exchange Spring phenomenon (consequently the IrMn
spins structure) was shown to be strongly dependent on the degree of [111] texture in the IrMn layer, which is, in turn, dependent on the seed layer (Cu or Ta) deposited on Si(100) substrate. It has also been demonstrated, a posteriori, that a field cooling of 10 Oe is sufficient to set the Exchange Bias effect on both interfaces (NiFe/IrMn and
IrMn/Co). In addition, it was observed that the highest Exchange Bias field occurs when an unstressed L12 IrMn structure is stabilized at 6-8 nm IrMn thickness interval. Due to different degree of texture (interface roughness may also be present), it can be shown that exchange bias effect is distinct for Ta and Cu systems. For the systems WHERE unidiviii
rectional anisotropy is set, it has been demonstrated that for increasing IrMn thickness (from 5 up to 30 nm), the magnetic coupling angle between NiFe and Co magnetizations increases for the Cu-system, while it is reduced in the Ta-system. These results were explained considering: (i) the measured different anisotropies of the Co and IrMn layers
induced by the [111] texture and (ii) distinct Exchange Bias field in NiFe/IrMn and IrMn/Co interfaces in both systems. It was shown that the coupling angle is strongly correlated by both Exchange Bias and Exchange Spring effects. Through resonant magnetic scattering measurements found a presence of reversible non-compensated under the action of magnetic field for both systems with unidirectional anisotropy (clearer effect for the Cu-system). Furthermore hysteresis loops through element selective obtained by this technique proved the presence of Exchange Spring effect in Cu system, even prior to applying cooling field.

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