• Journal of Magnetics
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• v.16 no.4
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• pp.323-327
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• 2011

By means of Monte Carlo simulation, we investigate the effects of interface roughness and temperature on the exchange bias and coercivity in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers. Both exchange bias and coercivity are strongly dependent on interface roughness. For a perfect uncompensated interface a domain wall is formed in the AFM system during FM reversal, which results in a very small exchange bias. However, a finite interface roughness leads to a finite value of the exchange bias due to the existence of pinned spins at the AFM surface adjacent to the mixed interface. It is observed that the exchange bias decreases with increasing temperature, consistent with the experimental results. It is also observed that a bump in coercivity occurs around the blocking temperature.

• Journal of Magnetics
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• v.17 no.4
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• pp.245-250
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• 2012

Ferromagnetic resonance and X-ray specular reflectivity measurements were performed on $Ni_{81}Fe_{19}/Ir_{20}Mn_{80}/Co_{90}Fe_{10}$ exchange bias trilayers, which were grown using the pulsed-DC magnetron sputtering technique on Si(100)/$SiO_2$(1000 nm) substrates, to investigate the evolution of the interface roughness and exchange bias and their dependence on the NiFe layer thickness. The interface roughness values of the samples decrease with increasing NiFe thickness. The in-plane ferromagnetic resonance measurements indicate that the exchange bias field and the peak-to-peak line widths of the resonance curves are inversely proportional to the NiFe thickness. Furthermore, both the exchange bias field and the interface roughness show almost the same dependence on the NiFe layer thickness. The out-of plane angular dependent measurements indicate that the exchange bias arises predominantly from a variation of exchange anisotropy due to changes in interfacial structure. The correlation between the exchange bias and the interface roughness is discussed.

• Journal of Magnetics
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• v.14 no.4
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• pp.168-171
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• 2009

We propose a new type of spin torque nano-oscillator structure with an exchange- biased free rotating layer. The proposed spin torque nano-oscillator consists of a fixed layer and a free rotating layer with an additional anti-ferromagnetic layer, which leads to an exchange bias in the free rotating layer. The spin dynamics of the exchange-biased free rotating layer can be described as an additional exchange field because the exchange bias manifests itself by the existance of a finite exchange bias field. The exchange bias field plays a similar role to that of a finite external field. Hence, microwave generation can be achieved without an external field in the proposed structure.

• Journal of Magnetics
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• v.10 no.3
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• pp.99-102
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• 2005

Modification in exchange bias of a NiFe/FeMn/NiFe trilayer, on introduction of a nonmagnetic Al layer at the top FeMn/NiFe interface, is investigated in multilayers prepared by rf magnetron sputtering. The introduction of Al layer leads to vanishing of bias of the top NiFe layer. But the bias for the bottom NiFe layer increases steadily with increasing Al layer thickness and attains bias (230 Oe) which is greater than that of the trilayer without the Al layer (150 Oe). When the top NiFe layer thickness is varied, exchange bias has highest value at 12 nm thickness for 1 nm thicknes of Al layer. Ion beam etching of the top NiFe layer also leads to an enhancement in bias for the bottom NiFe layer.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.304-305
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• 2013

We report a positive exchange bias (HE) in thinmultilayered filmscontaining nano-oxide layer. The positive HE, obtained for our system results from an antiferromagnetic coupling between the ferromagnetic (FM) CoFe and the antiferromagnetic (AFM) CoO layers, which spontaneously form on top of the nano-oxide layer (NOL). The shift in the hysteresis loop along the direction of thecooling field and the change in the sign of exchange bias are evidence of antiferromagnetic interfacial exchange coupling between the CoO and CoFe layers. Our calculation indicates that uncompensated oxygen moments in the NOL results in antiferromagnetic interfacial exchange coupling between the CoO and CoFe layers. One of the interesting features observed with our system is that it displays the positive HE even above the bulk Neel temperature (TN) of CoO. Although the positive HEsystem has a different AFM/FM interfacial spin structure compare to that of the negative HE one, the results of the angular dependence measurements show that the magnetization reversal mechanism can be considered within the framework of the coherent rotation model.

• Proceedings of the Korean Magnestics Society Conference
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• 2003.06a
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• pp.132-133
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• 2003

• Journal of Magnetics
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• v.13 no.3
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• pp.97-101
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• 2008

We investigated the exchange bias fields at the NiFe/FeMn and FeMn/CoFe interfaces in 18.9-nm NiFe/15.0-nm FeMn/17.6-nm CoFe trilayer thin films as the annealing temperature was varied from room temperature to $250^{\circ}C$ in a vacuum for 1 hour in a magnetic field of 150 Oe. Interestingly, magnetic hysteresis (M-H) measurements showed that NiFe/FeMn/CoFe trilayer thin films exhibited a completely contrasting variation of the exchange bias fields at both the NiFe/FeMn and FeMn/CoFe interfaces with annealing temperatures. High-angle X-ray diffraction (XRD) measurements indicated the absence of any discernible effect of thermal treatment on the NiFe(111) and FeMn(111) peaks. The compositional depth profile obtained from X-ray photoelectron spectroscopy (XPS) results presented the asymmetric compositional depth profiles of the Mn and Fe atoms throughout the FeMn layer. We contend that this asymmetric compositional depth profile and the preferential Mn diffusion into the NiFe layer, compared to that into the CoFe layer, are conclusive experimental evidence of the contrasting variation of the exchange bias fields at two interfaces having a common polycrystalline FeMn(111) layer.

• Journal of Magnetics
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• v.13 no.1
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• pp.1-6
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• 2008

MacDonald and co-workers recently predicted that high current densities could affect the magnetic order of antiferromagnetic (AFM) multilayers, in ways similar to those that occur in ferromagnetic (F) multilayers, and that changes in AFM magnetic order can produce an antiferromagnetic Giant Magnetoresistance (AGMR). Four groups have now studied current-driven effects on exchange bias at F/AFM interfaces. In this paper, we first briefly review the main predictions by MacDonald and co-workers, and then the results of experiments on exchange bias that these predictions stimulated.

• Journal of Magnetics
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• v.10 no.4
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• pp.133-136
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• 2005

The NiMn/NiFe bilayer structure which was short time annealed in order to induce unidirectional anisotropy were studied as a function of annealing temperature. The maximum exchange bias field of NiMn/NiFe bilayer was presented at $250^{\circ}C$ after short time annealing process with no external field. The appearance of exchange bias was due to phase transformation of NiMn layer. In plane angular dependence of a resonance field distribution which measured by FMR was analysed as a combined effect of unidirectional anisotropy and uniaxial anisotropy. The resonance field and the line width from FMR measurement were also analysed with annealing temperature.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.436-436
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• 2011

Recently, spintronic devices with submicron structures are widely investigated to take advantage of their unique micromagnetic properties. In this work, we study the temperature dependence of exchange bias in bilayer anti-dot arrays made by depositing Co (40 nm)/Ni (5 nm) ferromagnetic bilayer on Si substrate to form anti-dot arrays with a diameter $1{\mu}m$. The anti-dot patterning was done only for the upper Co layer, while the Ni underlayer was kept unperforated. The temperature dependences of magnetoresistance (MR) and exchange bias were studied along magnetic easy and hard axes. The in-plane MR measurements were performed using a physical-property measurement system (PPMS ; Quantum Design Inc.) at various temperatures. The standard in-line four-point probe configuration was used for the electrical contacts. As temperature was varied, the MR data were obtained in which in-plane field (H=3 kOe) was applied in the directions along the hard and the easy axes with respect to the lattice plane. The temperature dependences of magnetic anisotropy and exchange bias were also studied along the magnetic easy and hard axes. As temperature decreases, the single peak splits into two peaks. While no exchange bias was observed along the magnetic easy axis, the exchange bias field steadily increased with decreasing temperature along the magnetic hard axis. These results were interpreted in connection with the magnetic anisotropy and the effect of the anti-dots in pinning domain wall motion along the respective direction.