Static and Dynamic Magnetic Behaviors in NiFe/IrMn (Ferromagnet/Antiferromagnet) Thin Film Bilayers

鎳鐵/銥錳(鐵磁/反鐵磁)雙層薄膜中的靜態和動態磁性行為研究

畢法瑞

指導教授 ： 李尚凡 ； 李志浩

國立清華大學/原子科學院/工程與系統科學系/博士(2014年)

摘要

無資料

關鍵字

鐵磁共振 ； 自旋電子學 ； 磁化動力學

並列摘要

Exchange bias is the manifestation of the exchange anisotropy of a ferromagnetic material when it is in contact with an antiferromagnetic one. This effect was first discovered by Meiklejohn and Bean in partially oxidized Co particles more than 50 years ago, then also verified in FM/AFM bilayers. Other concomitant effects to the exchange bias have been observed, such as an increase in the coercivity, a shift in the ferromagnetic resonance frequency, the training effect, and asymmetric magnetization reversal.
Since its discovery, exchange bias stands as one of the most interesting topics that has received much attention due to its intriguing physical origin, widespread applications in spintronics, and potential applications in high frequency devices based on magnetic thin films. Regarding high frequency applications, exchange bias is very effective in tailoring the ferromagnetic resonance frequency to a higher range through adding an additional unidirectional anisotropy.
From applications point of view, most of the devices based on EB, such as magnetoresistive sensors and magnetic random access memory devices are in thin film form. Consequently, FM/AFM bilayers have received more attention during the past decades. Recently, the thermal stability of exchange bias systems gained more attention due to its importance for the real applications. Most of the researches in this field were focused on studying the thermal stabilities of the static parameters for the exchange bias bilayers at or above the room temperature. Given the current trend toward decreasing the devices’ response time to increase their speeds, more understanding for the dynamical parameters and their thermal stability has become very necessary. However, the dynamic parameters still lack for a comprehensive study especially at low temperatures. Among the dynamic parameters that need more interests, is the rotatable anisotropy whish was found to have a considerable effect on the ferromagnetic resonance frequency.
In our research, a systematic experimental study on the exchange bias effect in ferromagnetic/antiferromagnetic bilayer system is performed both in the static (dc) and dynamic (high frequency) timescale to clarify the effects of temperature and antiferromagnetic layer thickness on the system’s stability and magnetic properties. Our system consists of NiFe/IrMn bilayers. First, the static behavior was studied confirming a low temperature exchange bias onset occurred at very low antiferromagnetic thickness that was increased to higher onset thickness at higher temperatures. Both parallel and perpendicular domain walls are suggested to explain the static exchange bias and coercivity behaviors. In the microwave region, peaks, which can only be suppressed at high temperatures with strong external field, were observed in the antiferromagnetic thickness dependence of the dynamic effective field and resonance frequency. The temperature dependence of both static and dynamic parameters suggests different values of the Néel temperature. The dynamic results show a rotatable anisotropy contribution, which has a peak value at the blocking temperature and vanishes at the dynamic Néel temperature. From the temperature dependence of the resonance field, a slight exponential decrease of the gyromagnetic ratio with 1/T was found.

並列關鍵字

Ferromagnetic Resonance ； Spintronics ； Dynamic Magnetization ； Exchange Bias

參考文獻

Chapter 1:

[1] H. W. Lee, K. C. Kim and J. Lee: Review of maglev train technologies. IEEE Trans. Magn. 42, 1917 (2006).

[3] M. H. Clark: Making magnetic recording commercial 1920-1955 (Invited paper). J. Magn. Magn. Mater. 193, 8 (1999).

[4] D. E. Speliotis: Magnetic recording beyond the first 100 years (Invited paper). J. Magn. Magn. Mater. 193, 29 (1999).

[5] D. Weller and A. Moser: Thermal Effect Limits in Ultrahigh Density Magnetic Recording. IEEE Trans. Magn. 35, 4423 (1999).