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We report a controllable method for fabricating hexagonal Sn doped ZnO microdisks. The photoluminescence mechanism of the Sn doped ZnO microdisks is investigated, the defect emission is attributed to the singly charged oxygen vacancy. Under the excitation of a femtosecond pulsed laser with a wavelength of 325 nm, exciton-exciton collision process is clearly demonstrated, and amplified spontaneous emission is further realized under strong excitation. Using the perfect hexagonal symmetric structure of the Sn doped ZnO microdisks, the whispering-gallery mode lasing with high quality factor and fine mode structure is obtained from a single microdisk.

We have examined the dependence of the magnetism from perpendicular magnetic anisotropy with Co thickness in [Pt/Co] multilayers coupled to a NiO layer. We measure a perpendicular exchange bias field that changes polarity with Co thickness and find that with 2 nm of Co the perpendicular magnetic anisotropy is no longer present. Further evidence of the change in the Co layers preferred orientation is offered by zero field cooled susceptometry, where the low temperature susceptibility is negative for thin Co layers (0.9–1.2 nm) and becomes positive with increasing Co thickness (1.5–2.0 nm). Thin Co layered films also exhibit a compensation point around 100 K. These results indicate that the Co and NiO interface moments are coupled strongly and that there is competition between Pt/Co and Co/Ni at the film layer interfaces that sets the overall anisotropy from perpendicular and parallel configurations.

The correlation between the ion-beam bombardment and the exchange bias magnetism in NiFe (10 nm)/Mn (25 nm) bilayers was studied. While the bottom Mn layers bombarded by different Ar ion-beam energies (VEH from 70 to 150 V) retained the same structure, significant differences in exchange bias were observed when in contact with a top NiFe layer. The dependence of the exchange bias field, Hex, with increasing VEH suggests strongly that the Ar ion-beam bombardment process may create uncompensated Mn spins (Hex enhancement) or increase the spin misalignment in ferromagnet (FM)/antiferromagnet (AF) interfaces (Hex decrease), depending on the energy used. A schematic FM/AF spin structure was proposed to explain this unusual exchange bias behavior.

The exchange bias effects of NiFe/Cr-oxide bilayers were studied. Results have shown that NiFe/Cr-oxide bilayers exhibited an exchange bias loop shift when field cooled to 5 K. A strong linear dependence of ferromagnetic NiFe and antiferromagnetic Cr2O3 thicknesses on the exchange bias field Hex was observed. The largest interfacial exchange energy Eint∼5.4×10−2 erg/cm2 was found in bilayers with the thickest Cr2O3 layer indicating that stronger interface exchange coupling is enabled by thicker Cr2O3 layers. In addition, Hex decreased linearly with increasing %O2/Ar ratio, reflecting that ion-beam bombardment tends to degrade the Cr2O3 surface spin structures. We also find that annealing the Cr-oxide layer yields both a structural phase transformation and improved crystallinity, giving rise to stronger exchange bias behavior. Further, the coexistence of in-plane as well as out-of-plane exchange biases was observed in a NiFe/annealed Cr2O3/Al2O3(0001) bilayer. This clearly indicates that by using the singl...

The exchange bias field dependence on the Mn-oxide and its microstructure in NiFe/Mn-oxide bilayers was investigated. Transmission electron microscopy results have shown that the bilayer bottom consisted of either α-Mn, rocksalt MnO, or a composite of tetragonal Mn3O4+MnO, depending on the ratio of O2/Ar used during dual ion-beam deposition. Magnetometry results at 5 K indicate that the exchange bias field (Hex∼−300 Oe) is largest in a NiFe/Mn (0%O2/Ar) bilayer. The MnO formation by in situ Mn oxidation results in a decrease in Hex in a NiFe/Mn-oxide (21%O2/Ar) bilayer. In contrast, a further increase in the O2/Ar ratio during deposition results in larger Hex and Hc. This is attributed to the oxidation of MnO into a harder ferrimagnet, Mn3O4. Our results indicate that the antiferromagnetic Mn enabled stronger coupling with NiFe than MnO. In addition, we find that the MnO–Mn3O4 coupling dominates the exchange bias effects at high oxygen concentrations.

The structural and magnetic properties of NiCo∕(Ni,Co)O bilayers were investigated. X-ray diffractometry results have shown that the top NiCo layer consisted of a fcc NiCo phase. The bilayer bottom was either a pure (Ni,Co)O or a composite [NiCo+(Ni,Co)O] phase, depending on the percent of O2∕Ar ratio used during deposition. A double-shifted hysteresis loop exhibiting components that were from positive or negative coupling was observed in the NiCo∕(Ni,Co)O (8%O2∕Ar) bilayers. The microstructural changes, which result from a combination of deposition oxygen content and the ion-beam bombardment, will result in the unusual exchange bias behavior.