Temperature dependence of static and dynamic magnetic properties in NiFe/IrMn bilayer system
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Dung-Shing Hung Department of Information and Telecommunications Engineering, Ming Chuan University, Taipei 111, Taiwan
Shang-Fan Leea) Institute of Physics, Academia Sinica, Taipei 11529, Taiwan; and Nano Science and Technology Program, TIGP, Academia Sinica, Taipei 11529, Taiwan (Received 27 December 2013; accepted 2 May 2014)
A systematic experimental study on the exchange bias (EB) effect in a ferromagnet/ antiferromagnet bilayer system is performed both in the static (dc) and dynamic (high frequency) timescale to clarify the effects of temperature and antiferromagnetic (AFM) layer thickness on the system’s stability and magnetic properties. Our system consists of NiFe/IrMn. Both parallel and perpendicular domain walls are suggested to explain the static EB and coercivity behaviors. In the microwave region, peaks, which can only be suppressed at high temperatures with strong external fields, were observed in the AFM thickness dependencies of the dynamic effective field and resonance frequency. The temperature dependence of both static and dynamic parameters suggests different values of Néel temperatures. The dynamic results show a rotatable anisotropy contribution, which has a peak value at the blocking temperature and vanishes at the dynamic Néel temperature.
I. INTRODUCTION
When a ferromagnetic (FM) material is placed in intimate contact with an antiferromagnetic (AFM) material, the response of the FM to externally applied magnetic fields can be significantly modified. This change is expressed in terms of the well-known offset of the hysteresis loop of the FM layer, known as exchange bias (EB) field (HEB), which was first discovered by Meiklejohn and Bean1,2 in partially oxidized Co particles more than 50 years ago, and later, also verified in FM/AFM bilayers.3,4 Other concomitant effects to the EB have been observed, such as an increase in the coercivity,5 a shift in the ferromagnetic resonance (FMR) frequency,6–9 the training effect,10,11 and asymmetric magnetization reversal.12 EB stands as one of the interesting topics that has received much attention3–13 due to its intriguing physical origin, widespread applications in spintronics,11,13–16 and potential applications in high-frequency devices based on magnetic thin films.17–22 For spintronics device applications, the AFM serves as a pinning layer to keep the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.108 J. Mater. Res., Vol. 29, No. 11, Jun 14, 2014
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magnetization of the adjacent FM layer aligned in a given direction.11,13–16,23 Regarding high frequency applications, EB is effective in tailoring the FMR frequency (fres) to a higher range through an additional unidirectional anisotropy which is tunable in many ways17–22 by controlling the layer thicknesses24,25 and recently, by the exploitation of the rotatable anisotropy.26 The system of EB is very complicated because there are a number of parameters which influence HEB, such as the anisotropy,24,2
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