Magnetic Phenomena Associated with Random Magnetic Anisotropy: A Monte Carlo Study

以蒙地卡羅法研究非等向隨機磁性物的性質

阮孟河(Nguyen Manh Ha)

指導教授 ： 蕭百沂

國立清華大學/原子科學院/工程與系統科學系/博士(2010年)

摘要

In this work, we conduct an extensive Monte Carlo (MC) study on magnetic phenomena associated with random magnetic anisotropy (RMA). Our MC study provides the following results.

Firstly, a second-order magnetic phase transition to magnetic quasi-long range order (QLRO) phase is possible for 3D Heisenberg RMA model with weak strength (D=J = 4) in both cases of isotropic and cubic istributions of RMA axes.Finite-size scaling analysis shows that the critical couplings for the former and latter are Kc = 0:70435(2) and Kc = 0:70998(4), respectively. While the critical exponent 1= = 1:40824(0) is the same for both cases.

Secondly, our MC study is carried out to investigate the magnetic ordering of RMA magnets and their behavior in ac field using this model. Our results show peculiar
similarities to recent experiments that the real part of ac susceptibility presents two peaks for weak RMA and only one for strong RMA regardless of glassy critical dynamics manifested for them. We demonstrate that the thermodynamic nature of the low-temperature peak is a ferromagnetic-like dynamic phase transition to QLRO for the former. Our simulation, therefore, is able to be incorporated with
the experiments to help clarify the existence of the QLRO theoretically predicted so far.

Finally, our MC study is performed to investigate novel bilayer and core/shell nanoparticle models of ferromagnet (FM)/amorphous magnet (AM). The RMA of the AM layer and shell is demonstrated to be a new source for exchange bias. Our simulated results show usual and unusual dependencies on cooling field, temperature, RMA strength, particle size, and FM (AM) thickness as well as training effect and time-dependent phenomenon. These FM/AM models open a new avenue, other than those of FM/antiferromagnet and FM/spin glass combinations, for magnetism of the exchange bias and for its applications.

並列摘要

無資料

並列關鍵字

Random magnetic anisotropy ； magnetic phase transition ； critical phenomena ； dynamic phase transition ； Monte Carlo simulation ； complex susceptibility ； hysteresis loop ； exchange bias ； magnetic bilayers ； random magnets ； classical spin models

參考文獻

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Exchange bias using a spin glass, Nature Materials 6 (2007) 70. vii, 49, 52, 56, 59,