Exchange bias in a columnar nanocrystalline ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕\mathrm{Co}\mathrm{O}$ thin film

The effects of interfacial coupling at the boundary of ferromagnetic and antiferromagnetic components in a nanoscale columnar-structured thin film of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕\mathrm{Co}\mathrm{O}$ have been examined. Field-cooling the film results in very different temperature dependences of the enhanced coercivity and exchange-bias shift of the hysteresis loop. The exchange-bias temperature dependence is well described by thermal fluctuations of the interfacial spins while the coercivity temperature dependence indicates that single-domain-like columns are being coherently rotated by the thermal fluctuations of the interface spins. Furthermore, only a portion of the spins in the antiferromagnetic layer seem to be associated with the spin coupling that results in exchange bias. X-ray magnetic resonant scattering measurements show clearly the presence of canted Co interfacial moments that provide a local field which enables exchange bias at a significantly higher temperature than the onset of an enhanced coercivity.

Figure 1

(Color online) Measured grazing-angle x-ray diffraction patterns of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ (top) and CoO (bottom) films. The CoO film was deposited with a 8% ${\mathrm{O}}_2∕\mathrm{Ar}$ gas ratio (Ref. 11)

Figure 2

Plane-view transmission electron microscope images of the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ (a) and CoO (b) films with respective diffraction patterns in (b) and (d). (e) is a lower-resolution dark-field image of the CoO film showing the dispersion of nanocolumns.

Figure 3

(Color online) Cross-sectional TEM micrograph of the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕\mathrm{Co}\mathrm{O}$ bilayer grown on a Si(100) surface.

Figure 4

In-phase component of the temperature-dependent ac susceptibility $\left({\chi }_{\mathit{ac}}^{\prime }\right)$ of the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ thin film. The inset shows the out-of-phase component of ${\chi }_{\mathit{ac}}^{″}$ around the antiferromagnetic CoO Néel transition.

Figure 5

(Color online) (a) XRMS at the Fe, Ni, and Co $L_3$ edges at $50\mathrm{K}$. Nonzero XRMS provides a clear indication of the canted Co interfacial moment. (b) Ni and Co elemental hysteresis loops at $10\mathrm{K}$ after field cooling.

Figure 6

Exchange-bias field $H_{\mathit{ex}}=(H_{c1}+H_{c2})∕2$ as a function of temperature after field-cooling the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕\mathrm{Co}\mathrm{O}$ film in a $50\text{−}\mathrm{kOe}$ field. The solid line is to a fit described in the text. The inset shows a typical hysteresis loop at $5\mathrm{K}$ after being field cooled.

Figure 7

(Color online) Left $\left(H_{c1}\right)$ and right $\left(H_{c2}\right)$ coercivities measured by magnetometer (○) and XRMS (◇) after the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕\mathrm{Co}\mathrm{O}$ film was field cooled, with $H_{c1}$ and $H_{c2}$ values determined from field-cooled hysteresis loop measurements. The inset shows $H_c\left(T\right)=(H_{c1}-H_{c2})∕2$ from the magnetometry studies as a function of $\sqrt{T}$. The solid line is a guide to the eye.