Year: 2004
Type: article
Abstract: Epitaxial thin (110) films of the weak ferromagnet ${\text{NiF}}_{2}$ were deposited on single-crystal ${\text{MgF}}_{2}$ (110) substrates via molecular-beam epitaxy. Subsequently polycrystalline Co was grown on the ${\text{NiF}}_{2}$ film. The antiferromagnetic ordering of the ${\text{NiF}}_{2}$ was monitored as a function of temperature via neutron diffraction and the exchange bias ${H}_{E}$ of the Co layers was measured via standard magnetometry measurements. Because in ${\text{NiF}}_{2}$ the spins lie in the (001) plane, the maximum ${H}_{E}$ is observed after cooling the sample with a cooling field in the film plane perpendicular to the [001] direction of the ${\text{NiF}}_{2}$. In $60\phantom{\rule{0.3em}{0ex}}\text{nm}$, $49\phantom{\rule{0.3em}{0ex}}\text{nm}$, and $38\phantom{\rule{0.3em}{0ex}}\text{nm}$ thick ${\text{NiF}}_{2}$ samples, the N\'eel temperature is ${T}_{N}\ensuremath{\approx}81\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is significantly larger than the bulk value of ${T}_{N}=73.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This enhancement also occurs in films without Co overlayers and thus is not due to a proximity effect. For the $38\phantom{\rule{0.3em}{0ex}}\text{nm}$ sample with a Co overlayer cooled in a $50\phantom{\rule{0.3em}{0ex}}\text{kOe}$ field, ${H}_{E}>0$ and vanishes at a blocking temperature $({T}_{B})$ which coincides with the ${T}_{N}$ of the films. When the sample is cooled in $2.0\phantom{\rule{0.3em}{0ex}}\text{kOe}$, ${H}_{E}<0$, disappearing at $T=55\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, reappearing at $T=65\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and finally disappearing once again at $T=81\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. For the $12\phantom{\rule{0.3em}{0ex}}\text{nm}$ thick ${\text{NiF}}_{2}$ sample, ${T}_{B}⪡{T}_{N}$. Strain-induced enhancement of ferromagnetic exchange interactions between the nearest-neighbor ${\text{Ni}}^{2+}$ ions along the $c$ axis may be responsible for the ${T}_{N}$ enhancement. These results also demonstrate that in general, a diminished ${T}_{B}$ is not necessarily due to a lower ${T}_{N}$.
Source: Physical Review B
Language: Latvian
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funder: National Science Foundation
OpenAlex ID: W2019460109