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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 H-ex was observed. The largest interfacial exchange energy E-int similar to 5.4x10(-2) erg/cm(2) was found in bilayers with the thickest Cr2O3 layer indicating that stronger interface exchange coupling is enabled by thicker Cr2O3 layers. In addition, H-ex decreased linearly with increasing %O-2/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 single crystal Al2O3(0001) substrate together with a rapid thermal annealing process, the antiferromagnet Cr2O3 spins are tilted toward the out-of-plane direction and thus exhibit this unusual exchange bias behavior

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.

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 alpha-Mn, rocksalt MnO, or a composite of tetragonal Mn(3)O(4)+MnO, depending on the ratio of O(2)/Ar used during dual ion-beam deposition. Magnetometry results at 5 K indicate that the exchange bias field (H(ex)similar to-300 Oe) is largest in a NiFe/Mn (0%O(2)/Ar) bilayer. The MnO formation by in situ Mn oxidation results in a decrease in H(ex) in a NiFe/Mn-oxide (21%O(2)/Ar) bilayer. In contrast, a further increase in the O(2)/Ar ratio during deposition results in larger H(ex) and H(c). This is attributed to the oxidation of MnO into a harder ferrimagnet, Mn(3)O(4). Our results indicate that the antiferromagnetic Mn enabled stronger coupling with NiFe than MnO. In addition, we find that the MnO-Mn(3)O(4) coupling dominates the exchange bias effects at high oxygen concentrations

In this study, the structural and magnetic properties of exchange coupled nanocomposite FePt‐FePt3 films were investigated. X‐ray diffraction and transmission electron microscopy results showed that after rapid thermal annealing (600 °C for 6 minutes), a phase transformation from [Pt+Fe] into composite FePt‐FePt3 phases was obtained. Annealing created an ordered FePt phase that resulted in a large room temperature coercivity (Hc ∼ 3200 Oe) compared to the as‐deposited film (Hc ∼ 20 Oe). While no exchange bias field (Hex ∼ 0 Oe) was measured at room temperature, the annealed sample showed significant Hex ∼ ‐500 Oe at lower temperatures (150 K ≤ T ≤ 250 K), indicative of exchange coupling between FePt and FePt3. The high blocking temperature (∼250 K) indicates FePt3 exhibits an enhanced Néel temperature. Further increasing the annealing temperature to 700 °C resulted in a reduction of Hex which is attributed to the transformation of FePt3 into the FePt phase at this higher annealing temperature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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

Exchange bias in [Pt/Co]/NiO multilayers were studied as a function of film thickness and [Pt/Co] layer repetition. A strong temperature dependence of the coercivity, Hc, and exchange bias field, Hex, was observed for the thick and thinnest [Pt/Co]/NiO multilayers. While the thinnest [Pt(3 nm)/Co(1.25 nm)]4/NiO multilayers exhibits no in‐plane exchange bias field, a perpendicular Hex⊥ ∼ ‐150 Oe at 80 K was measured. By contrast, the thickest [Pt(12 nm)/Co(10 nm)]1/NiO multilayers exhibited an in‐plane Hex// ∼ ‐600 Oe (with Hex// ∼ ‐1300 Oe at 5 K) with no measurable perpendicular exchange bias field. The estimated interfacial exchange coupling energy implies the effective Co layer thickness contributing to the exchange bias is effective only in Co layer in contact with NiO bottom layer. AC susceptibility and the temperature dependence of Hex show that the a 1.25 nm thick Co component enables perpendicular exchange bias with a reduced blocking temperature TB∼200 K, compared to that (TB∼250 K) for the thick [Pt/Co]/NiO multilayers. This is attributed to disordered CoPt phases that formed due to intermixing between Co and Pt during deposition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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