| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系所 === 98 === Exchange bias in NiFe(9nm)/NiO(16nm) bilayers were investigated as a function of bottom NiO layers with dual ion beam bombarded by (1) different End-Hall voltage (0~150 V) for 5 minutes and (2) different periods (1~20 minutes) with VEH=150 V. And used different substrates (3) MgO(110) single crystal, (4) nanostripe. X-ray diffractometry results have shown NiFe/NiO bilayers consisted of a f.c.c. NiFe (a~ 3.55 Å) and a rock-salt f.c.c. NiO (a~ 4.25 Å). The grain sizes of NiO single layer range from 5 nm to 15 nm, as revealed by TEM. From AFM the more End-Hall voltage bombarded NiO surface would get smoother roughness and rised the periods cause the roughness to drop then increase.
Different ion-beam energy(VEH=0V、70V、100V、130V、150V)was used to change the bottom layer of NiO structure in NiFe. Magnetometry results at 5K under a 20 kOe field-cooling, only the NiFe/NiO (VEH=150 V) bilayer deposited on a SiO2 substrate exhibited an unusual positive exchange bias field, Hex~ +200 Oe with an asymmetric hysteresis loop among all the films.
The results of different periods with VEH=150 V would found exchange bias field irregularity, when tbomb.> 5 minutes all were show positive Hex. The drastic change from conventional negative to positive Hex is likely attributed to the AF NiO spin reorientation due to ion-beam bombardment, as revealed by the increased peak ratio of (200) to (111) determined by XRD.
when replacing the SiO2 substrate with a MgO (110) substrate, a much smaller Hex (~ -20 Oe) was found in NiFe/NiO (VEH= 0 V and 130 V) bilayers. This indicates that the compensated AF NiO spins (thus smaller Hex) may be formed by using a MgO (110) substrate. Our results suggest that the strength and polarity of exchange bias field are affected by the substrate type (single crystal vs. amorphous) as well as by ion-beam bombardment. Discuss of NiO crystalline and surface roughness may effect exchange bias field and coervice field. The exchange bias field has different sign because influence of FM/AF interface has different exchang coupling. In other words, surface spin reorientation that is due to moderate ion-beam bombardment effects on the surface of the NiO layer.
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