Structural and magnetic properties of stoichiometric epitaxialCoO∕Feexchange-bias bilayers

Abstract

We present a detailed study of the magnetic and structural properties of the CoO/ Fe bilayers by using a combination of x-ray diffraction ͑crystalline structure͒, Rutherford backscattering ͑chemical composition͒, and SQUID magnetometry ͑magnetic characterization͒ measurements. We prepared stoichiometric and single crystalline CoO thin films by a post-deposition annealing process in ultrahigh-vacuum conditions from sputter deposited hyperstoichiometric, polycrystalline CoO films. A simultaneous increase of the CoO crystalline quality and chemical phase purity has been achieved. Subsequently, the annealed CoO layers served as a template for the growth of epitaxial Fe films. The epitaxial relation between CoO and Fe was found to be of the Nishiyama-Wassermann type, where the Fe͓001͔ direction is parallel aligned to the CoO͓1-10͔ direction. The magnetic properties of CoO/ Fe bilayers are strongly influenced by the stoichiometry of the antiferromagnetic CoO layer. Especially the blocking temperature is sensitive to the oxygen concentration within the CoO layer. While the blocking temperature of stoichiometric, epitaxial CoO/ Fe exchange-bias bilayers almost equals the Néel temperature of bulk CoO, it is strongly reduced for superstoichiometric, polycrystalline CoO.

Tuning the ferromagnetic-antiferromagnetic interfaces of granular Co-CoO exchange bias systems by annealing

“…17,19 Specifically, while the annealed samples containing pristine O contents of 5%, 8%, and 15% exhibit T B values of around 225, 235, and 255 K, respectively, the rest of the samples show blocking temperatures close to that of the Neel temperature of bulk CoO (i.e., 290 K). 30 This is in concordance with the structural and magnetic characterization since annealing results in the formation (specially in samples with higher pristine O content) of thicker and more stoichiometric CoO counterparts with higher magnetic anisotropy, which are less prone to size effects. Since usually T B is highly affected (i.e., lowered) by deviations in stoichiometry of the AFM, 30 the high T B values of the different samples confirm that the formed CoO is rather well-defined from a structural point of view with minimized composition deficiencies.…”

“…30 This is in concordance with the structural and magnetic characterization since annealing results in the formation (specially in samples with higher pristine O content) of thicker and more stoichiometric CoO counterparts with higher magnetic anisotropy, which are less prone to size effects. Since usually T B is highly affected (i.e., lowered) by deviations in stoichiometry of the AFM, 30 the high T B values of the different samples confirm that the formed CoO is rather well-defined from a structural point of view with minimized composition deficiencies. This is further confirmed by the decreased positive exchange bias 31,32 close to the blocking temperature of samples implanted at low fluence (3 Â 10 16 and 5 Â 10 16 ions/cm 2 ) and annealed, indicating an increased AFM magnetic anisotropy of the formed CoO.…”

Rotatable anisotropy driven training effects in exchange biased Co/CoO films

“…Moreover, the blocking temperature of the samples of Gruyters is around 175 K, whereas the blocking temperature of our CoO layer is 125 K, suggesting that the formed CoO might be more hyperstoichiometric with a higher degree of polycrystallinity, i.e., more prone to nanostructuring effects. 22,37 The best fitting magnetization curves, simulated through the above model, are also plotted in Fig. 1.…”

Temperature-dependent exchange bias properties of polycrystalline PtxCo1− x/CoO bilayers

“…Although CoO is not used as an AF material in real devices, due to its strong anisotropy and convenient Néel temperature (T N = 291 K), which is very close to the room temperature, CoO becomes very preferable among other AF materials for laboratory measurements. In addition to the FM and AF material type, exchange bias field and blocking temperature (T B ) of an exchange bias system are also strongly affected by both FM and AF layer thicknesses [33][34][35], the stoichiometry of the AF material [35] and the number of exchange biased interfaces in multilayered systems [11].…”