J.-Y. Guo

Anomalous exchange bias behavior in ion-beam bombarded NiCo/(Ni,Co)O bilayers

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.

Tuning in-plane and out-of-plane exchange biases in Ni80Fe20/Cr-oxide bilayers

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...

Correlating exchange bias with magnetic anisotropy in ion-beam bombarded NiFe/Mn-oxide bilayers

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.

Correlating antiferromagnetic spin structures with ion-beam bombardment in exchange-biased NiFe/Mn bilayers

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.

Characterization of Exchange-Biased CoFe/(Co,Fe)O Thin Films by Magnetometry and Ferromagnetic Resonance Techniques

Substrate and ion-bombardment effects on exchange bias were explored for CoFe/(Co,Fe)O bilayer films. The (Co,Fe)O component was sputtered onto different substrates and then ion-bombarded before a ferromagnetic CoFe layer was grown. Ferromagnetic resonance reveals that the CoFe is magnetically pinned by the (Co,Fe)O film. We find that substrate type heavily influences the magnitude of exchange bias and also the degree to which the exchange biased system is affected under ion-bombardment. In all cases there is a general decrease in the magnitude of exchange bias and coercivity, and for cases with high energy ion-bombardment unusual changes to the hysteresis loop are observed which may indicate the formation of an additional magnetic phase.

Magnetic and Magnetotransport Properties of Exchange-Biased NiFe/NiO Bilayers

The magnetism of a series of NiFe (20 nm)/NiO (10 nm) bilayers with different Ni oxides has been studied. Ni oxides were made using ratios of O2 to Ar ions ranging from 7 to 33% during ion-beam deposition. Transmission electron microscopy (TEM) has shown that with 7% O2/Ar used during bottom layer deposition, the film produced consisted of fcc Ni (a=3.52 A) and rock-salt NiO (a=4.21 A) phases. A bottom film layer prepared with 33% O2/Ar consisted of a pure NiO phase with an expanded lattice constant (a=4.32 A). A strong temperature dependence of the coercivity (Hc) and a 20 kOe field-cooled loop shift exchange bias field (Hex) were observed below 100 K. In addition, Hex increased with increasing O2/Ar ratio for the bottom layer antiferromagnetic (AF) component. At 10 K, a NiFe/NiO (33% O2/Ar) bilayer exhibited the higest Hc (110 Oe) and Hex (-60 Oe). This indicated that the more expanded the NiO lattice, the stronger the exchange coupling with the NiFe. The magnetotransport studies have shown that these NiFe/NiO bilayers exhibit anisotropic magnetoresistance (AMR) behavior. In addition, the total MR ratio of these NiFe/NiO bilayers increases with increasing O2/Ar ratio owing to strong anisotropic scattering at the NiFe/NiO interface.

Thermal Annealing Effects on the Structural, Magnetic, and Magnetotransport Properties of NiFe/(Ni,Fe)O Bilayers

In this study, the exchange bias and magnetoresistance of NiFe/(Ni,Fe)O bilayers were investigated. Transmission electron microscopy has shown that the (Ni,Fe)O single layer is a pure rock-salt solid solution (a= 4.24 A). After annealing at 500 °C for 2 h, the (Ni,Fe)O film undergoes a phase transformation from the pure antiferromagnetic (Ni,Fe)O into ferromagnetic Ni (a= 3.53 A) and ferrimagnetic NiFe2O4 (a= 8.29 A) phases. Furthermore, the NiFe/as-deposited (Ni,Fe)O bilayers exhibit a strong temperature dependence of exchange bias field [Hex(T)] that increases linearly with decreasing temperature (Hex= -30 Oe at 150 K). Surprisingly, no Hex was observed in the NiFe/annealed (Ni,Fe)O bilayers, even at 150 K. This indicates that the AF (Ni,Fe)O phase provides stronger exchange coupling than the ferrimagnetic NiFe2O4. Magnetotransport measurements have shown that these NiFe/(Ni,Fe)O bilayers exhibit anisotropic magnetoresistance (AMR). The NiFe/as-deposited (Ni,Fe)O bilayers exhibit a higher MR ratio than the NiFe/annealed (Ni,Fe)O bilayers, which suggests stronger anisotropic electron scattering between the NiFe and (Ni,Fe)O interfaces in the annealed system.

Tuning exchange bias effects by ion-beam bombardment in NiFe/NiO bilayers

We have studied the correlation between the ion-beam bombardment and exchange bias effects in NiFe/NiO bilayers. The dependence of the exchange bias field, H<inf>ex</inf>, with increasing V<inf>EH</inf> suggests strongly that the Ar ion-beam bombardment process may increase the spin misalignment in FM/AF interfaces (resulting in a decrease of H<inf>ex</inf>) or create uncompensated NiO spins (shown by a shift from negative to positive H<inf>ex</inf>), depending on the energy used. The polarity switch from −H<inf>ex</inf> to +H<inf>ex</inf> occurred with increasing ion-beam bombardment time (t<inf>bom.</inf>≫ 5 mins).