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Influence of Ga+ ion irradiation on thermal relaxation of exchange bias field in exchange-coupled CoFe/IrMn bilayers

Qi Xian-Jin,Wang Yin-Gang,Miao Xue-Fei,Li Zi-Quan,Huang Yi-Zhong,

中国物理 B , 2011,
Abstract: This paper reports that the CoFe/IrMn bilayers are deposited by magnetron sputtering on the surfaces of thermally-oxidized Si substrates. It investigates the thermal relaxations of both non-irradiated and Ga+ ion irradiated CoFe/IrMn bilayers by means of holding the bilayers in a negative saturation field. The results show that exchange bias field decreases with the increase of holding time period for both non-irradiated and Ga+ ion irradiated CoFe/IrMn bilayers. Exchange bias field is also found to be smaller upon irradiation at higher ion dose. This reduction of exchange bias field is attributed to the change of energy barrier induced by ion-radiation.
Propagation of Exchange Bias in CoFe/FeMn/CoFe Trilayers  [PDF]
D. N. H. Nam,W. Chen,K. G. West,D. M. Kirkwood,J. Lu,S. A. Wolf
Physics , 2008, DOI: 10.1063/1.2999626
Abstract: CoFe/FeMn, FeMn/CoFe bilayers and CoFe/FeMn/CoFe trilayers were grown in magnetic field and at room temperature. The exchange bias field $H_{eb}$ depends strongly on the order of depositions and is much higher at CoFe/FeMn than at FeMn/CoFe interfaces. By combining the two bilayer structures into symmetric CoFe/FeMn($t_\mathrm{FeMn}$)/CoFe trilayers, $H_{eb}^t$ and $H_{eb}^b$ of the top and bottom CoFe layers, respectively, are both enhanced. Reducing $t_\mathrm{FeMn}$ of the trilayers also results in enhancements of both $H_{eb}^b$ and $H_{eb}^t$. These results evidence the propagation of exchange bias between the two CoFe/FeMn and FeMn/CoFe interfaces mediated by the FeMn antiferromagnetic order.
Quantitative description of the azimuthal dependence of the exchange bias effect  [PDF]
Florin Radu,Andreas Westphalen,Katharina Theis-Br?hl,Hartmut Zabel
Physics , 2005, DOI: 10.1088/0953-8984/18/3/L01
Abstract: While the principal features of the exchange bias between a ferromagnet and an antiferromagnet are believed to be understood, a quantitative description is still lacking. We show that interface spin disorder is the main reason for the discrepancy of model calculations versus experimental results. Taking into account spin disorder at the interface between the ferromagnet and the antiferromagnet by modifying the well known Meiklejohn and Bean model, an almost perfect agreement can be reached. As an example this is demonstrated for the CoFe/IrMn exchange biased bilayer by analyzing the azimuthal dependence of magnetic hysteresis loops from MOKE measurements. Both, exchange bias and coercive fields for the complete 360$^\circ$ angular range are reproduced by our model.
Training Induced Positive Exchange Bias in NiFe/IrMn Bilayers  [PDF]
S. K. Mishra,F. Radu,H. A. Dürr,W. Eberhardt
Physics , 2009, DOI: 10.1103/PhysRevLett.102.177208
Abstract: Positive exchange bias has been observed in the Ni$_{81}$Fe$_{19}$/Ir$_{20}$Mn$_{80}$ bilayer system via soft x-ray resonant magnetic scattering. After field cooling of the system through the blocking temperature of the antiferromagnet, an initial conventional negative exchange bias is removed after training i. e. successive magnetization reversals, resulting in a positive exchange bias for a temperature range down to 30 K below the blocking temperature (450 K). This new manifestation of magnetic training is discussed in terms of metastable magnetic disorder at the magnetically frustrated interface during magnetization reversal.
Training Effect and Hysteretic Behaviour of Angular Dependence of Exchange Bias in Co/IrMn Bilayers

ZHANG Jing,DU Jun,BAI Xiao-Jun,YOU Biao,ZHANG Wei,HU An,

中国物理快报 , 2009,
Abstract: The training effect and the hysteresis behaviour of the angular dependence of exchange bias are extensively investigated upon the variation of the IrMn layer thickness tIrMn in a series of Co/IrMn bilayers. When tIrMn is very small, both of them are negligible. Then they increase very sharply with increasing tIrMn and then reach maxima at almost the same value of tIrMn. Finally they both decrease when tIrMn is further increased. The similar variation trends suggest that these phenomena arise from irreversible change of antiferromagnet spin orientations, according to the thermal activation model.
Anomalous training effect of perpendicular exchange bias in Pt/Co/Pt/IrMn multilayers  [PDF]
Z. Shi,S. M. Zhou
Physics , 2008, DOI: 10.1063/1.3039059
Abstract: A new characteristic is observed in the training effect of perpendicular exchange bias. For Pt/Co/Pt/IrMn multilayers with perpendicular magnetic anisotropy, the magnetization reversal process is accompanied by pinned domain wall motion and the asymmetry of hysteresis loop is always equal to zero during subsequent measurements. It is interesting to find that the exchange field decreases greatly as a function of the cycling number while the coercivity almost does not change. It is clearly demonstrated that the training effect of perpendicular exchange bias strongly depends on the magnetization reversal mechanism of the ferromagnetic layer.
Large exchange bias in polycrystalline MnN/CoFe bilayers at room temperature  [PDF]
Markus Meinert,Bj?rn Büker,Dominik Graulich,Mareike Dunz
Physics , 2015, DOI: 10.1103/PhysRevB.92.144408
Abstract: We report on the new polycrystalline exchange bias system MnN/CoFe, which shows exchange bias of up to 1800Oe at room temperature with a coercive field around 600Oe. The room temperature values of the interfacial exchange energy and the effective uniaxial anisotropy are estimated to be $J_\mathrm{eff} = 0.41\,\mathrm{mJ}/\mathrm{m}^2$ and $K_\mathrm{eff} = 37\,\mathrm{kJ}\,/\,\mathrm{m}^3$. The thermal stability was found to be tunable by controlling the nitrogen content of the MnN. The maximum blocking temperature exceeds $325^\circ$C, however the median blocking temperature in the limit of thick MnN is $160^\circ$C. Good oxidation stability through self-passivation was observed, enabling the use of MnN in lithographically defined microstructures. As a proof-of-principle we demonstrate a simple GMR stack exchange biased with MnN, which shows clear separation between parallel and antiparallel magnetic states. These properties come along with a surprisingly simple manufacturing process for the MnN films.
Thermal Decay and Reversal of Exchange Bias Field of CoFe/PtMn Bilayer after Ga+ Irradiation

ZHOU Guang-Hong,ZHU Yu-Fu,LIN Yue-Bin,

中国物理快报 , 2011,
Exchange Bias and Vertical Shift in CoFe2O4 nanoparticles  [PDF]
A. Mumtaz,K. Maaz,B. Janjua,S. K. Hasanain
Physics , 2006, DOI: 10.1016/j.jmmm.2007.01.007
Abstract: Magnetic properties of core-shell cobalt ferrite nanoparticles 15 to 48nm prepared by a sol-gel route have been studied. It is shown that the coercivity follows non-monotonic size dependence varying as 1/d above the maximum (d is the particle size). Field cooled magnetization exhibited both horizontal (exchange bias) and vertical shifts. The exchange bias is understood as originating at the interface between a surface region with structural and spin disorder and a core ferrimagnetic region. The dependence of the exchange bias and vertical shifts on the particle sizes and cooling fields are found to have significant differences and the differences are explained in the light of recent results which suggest that both weakly and strongly pinned spins are present at the interface. It is suggested that the exchange bias is dominated by the weakly pinned spins while the vertical shift is affected by the strongly pinned ones.
Origin of the reduced exchange bias in epitaxial FeNi(111)/CoO(111) bilayer  [PDF]
F. Radu,S. K. Mishra,I. Zizak,A. I. Erko,H. A. Durr,W. Eberhardt,G. Nowak,S. Buschhorn,K. Zhernenkov,M. Wolff,H. Zabel,D. Schmitz,E. Schierle,E. Dudzik,R. Feyerherm
Physics , 2008, DOI: 10.1103/PhysRevB.79.184425
Abstract: We have employed Soft and Hard X-ray Resonant Magnetic Scattering and Polarised Neutron Diffraction to study the magnetic interface and the bulk antiferromagnetic domain state of the archetypal epitaxial Ni$_{81}$Fe$_{19}$(111)/CoO(111) exchange biased bilayer. The combination of these scattering tools provides unprecedented detailed insights into the still incomplete understanding of some key manifestations of the exchange bias effect. We show that the several orders of magnitude difference between the expected and measured value of exchange bias field is caused by an almost anisotropic in-plane orientation of antiferromagnetic domains. Irreversible changes of their configuration lead to a training effect. This is directly seen as a change in the magnetic half order Bragg peaks after magnetization reversal. A 30 nm size of antiferromagnetic domains is extracted from the width the (1/2 1/2 1/2) antiferromagnetic magnetic peak measured both by neutron and x-ray scattering. A reduced blocking temperature as compared to the measured antiferromagnetic ordering temperature clearly corresponds to the blocking of antiferromagnetic domains. Moreover, an excellent correlation between the size of the antiferromagnetic domains, exchange bias field and frozen-in spin ratio is found, providing a comprehensive understanding of the origin of exchange bias in epitaxial systems.
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