K.-W. Lin

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...

[Pt/Co]4/NiO thin film perpendicular magnetic anisotropy dependence on Co layer thickness

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.

Anisotropy engineering using exchange bias on antidot templates

We explore an emerging device concept based on exchange bias used in conjunction with an antidot geometry to fine tune ferromagnetic resonances. Planar cavity ferromagnetic resonance is used to study the microwave response of NiO/NiFe bilayers with antidot structuring. A large frequency asymmetry with respect to an applied magnetic field is found across a broad field range whose underlying cause is linked to the distribution of magnetic poles at the antidot surfaces. This distribution is found to be particularly sensitive to the effects of exchange bias, and robust in regards to the quality of the antidot geometry. The template based antidot geometry we study offers advantages for practical device construction, and we show that it is suitable for broadband absorption and filtering applications, allowing tunable anisotropies via interface engineering.

Exchange bias in a nanocolumnar Ni80Fe20/α-Fe2O3 thin film

A Ni80Fe20∕α-Fe2O3 thin film has been deposited using a dual ion-beam deposition technique where the α-Fe2O3 is formed in situ by O2 bombardment of pure Fe. AC susceptibility results indicate moment fluctuations at temperatures above 150K. This behavior combined with the thermal fluctuation driven temperature dependence of the negative field coercivity indicates that the α-Fe2O3 nanocolumnar crystallite magnetism evolves from static single domain moments to superparamagnetic moments with warming. 25kOe field cooling establishes exchange bias with an onset temperature of 50K. The positive (Hc2) and negative (Hc1) field coercivities exhibit very different temperature dependencies, with Hc1(T) well described by a thermally activated coherent domain wall reversal model, while Hc2(T) is essentially constant except for the lowest temperatures. These results point towards different domain reversal mechanisms in positive and negative fields.

Anomalous positive exchange bias in Ni80Fe20/NixFe1-xO thin-film bilayers induced by ion-beam deposition effects

We present results on a Ni80Fe20∕NixFe1−xO thin-film bilayer that shows a positive exchange bias loop shift of ∼90Oe at 10 K under zero-field-cooled conditions. Zero-field-cooled and field-cooled hysteresis loops were double shifted at temperatures below 200 K. This behavior is due to the presence of a range of antiferromagnetic crystallite sizes in addition to multiple magnetic phases (e.g., FeO, Fe2O3, and NiO). Furthermore, the positive exchange bias loop shift decreases linearly with increasing temperature, with a compensation temperature Tcomp∼220K, after which negative exchange bias is measured. This temperature dependence of the exchange bias reflects the competition between the Ni80Fe20 ferromagnet and antiferromagnetic Fe oxide and NiO phases as well as a range of local blocking temperatures. We attribute the appearance of a positive exchange bias loop shift at low temperatures to temperature-dependent changes in the interfacial pinning and exchange coupling due to a complex NixFe1−xO structure f...

Enhancement of the exchange-bias onset temperature in a columnar nanocrystalline Ni80Fe20∕Co3O4 thin film

Typically, exchange coupling can only occur when ferromagnetic spins couple to antiferromagnetic spins below the Neel temperature. We present results on a Ni80Fe20∕Co3O4 thin film that is composed of ∼10nm diameter nanocrystalline columns. Field cooling the film reveals low-temperature exchange-bias hysteresis loop shifts that are the same magnitude as those measured in a similar Ni80Fe20∕CoO system, although the bulk magnetocrystalline anisotropy is a factor of 5 smaller. The coercivity and exchange-bias loop shift exhibit the same temperature dependence with individual “blocking” temperatures. Most surprising is that the onset temperature of the exchange-bias is nearly four times higher than the bulk Neel temperature.

The Effect of Single Crystalline Substrates and Ion-Beam Bombardment on Exchange Bias in Nanocrystalline

Methods to modify the magnetic coercivity and exchange bias field of nanocrystalline antiferromagnetic/ferromagnetic NiO/Ni80Fe20 thin films were investigated for bilayers grown using ion-assisted deposition onto different single crystalline substrates. An enhanced coercivity was found at 298 K for the films deposited on single crystalline MgO (100) and Al2O3 (11-20) substrates. After field cooling the films to 50 K, the NiO/NiFe bilayer grown on Al2O3 (11-20) exhibited the largest exchange bias ( - 25 Oe). The second part of the study investigated ion-beam modification of the ferromagnetic surface prior to the deposition of the NiO layer. A range of ion-beam bombardment energies (VEH) were used to modify in situ the NiFe surface during the deposition of NiO/NiFe/SiO2 films. Cross-sectional transmission electron microscopy showed a systematic reduction in the thickness of the NiFe layers with increasing Ar+ bombardment energies attributed to etching of the surface. In addition, the bombardment procedure modified the magnetic exchange bias of the composite structure in both the as-prepared and field-cooled state.

{\hbox{NiO}}/{\hbox{Ni}}_{80}{\hbox{Fe}}_{20}

We investigate the angular dependence of the ferromagnetic resonance (FMR) frequency in a Ni80Fe20/NiO bilayer for varied external magnetic fields by means of broadband microwave absorption spectroscopy. The dispersion curves of the FMR frequencies exhibit an angular-dependent low-field behaviour not predicted by current ferromagnetic/antiferromagnetic interface models. Our experimental data can be modelled using a single domain approach assuming that NiO introduces anisotropies at the interface.

Bilayers

Exchange bias and interlayer exchange coupling are interface driven phenomena. Since an ideal interface is very challenging to achieve, a clear understanding of the chemical and magnetic natures of interfaces is pivotal to identify their influence on the magnetism. We have chosen Ni80Fe20/CoO(t CoO)/Co trilayers as a model system, and identified non-stoichiometric Ni-ferrite and Co-ferrite at the surface and interface, respectively. These ferrites, being ferrimagnets typically, should influence the exchange coupling. However, in our trilayers the interface ferrites were found not to be ferro- or ferri-magnetic; thus having no observable influence on the exchange coupling. Our analysis also revealed that (i) interlayer exchange coupling was present between Ni80Fe20 and Co even though the interlayer thickness was significantly larger than expected for this phenomenon to happen, and (ii) the majority of the CoO layer (except some portion near the interface) did not contribute to the observed exchange bias. We also identified that the interlayer exchange coupling and the exchange bias properties were not interdependent.