Link between Perpendicular Coupling and Exchange Biasing in ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{CoO}$Multilayers

In studying well-characterized, exchange-biased ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{CoO}$ superlattices, we demonstrate a causal link between the exchange bias effect and the perpendicular coupling of the ferrimagnetic and antiferromagnetic spins. Neutron diffraction studies reveal that for thin CoO layers the onset temperature for exchange biasing $T_B$ matches the onset of locked-in, preferential perpendicular coupling of the spins, rather than the antiferromagnetic ordering temperature $T_N$. The results are explained by considering the role of anisotropic exchange first proposed by Dzyaloshinsky and Moriya and developing a model based purely on information on structural defects and exchange for these oxides. The devised mechanism provides a general explanation of biasing in systems with perpendicular coupling.

Figure 1

Scan of the (111) reflection within the growth plane for a $[10\mathrm{nm}{\mathrm{Fe}}_3{\mathrm{O}}_4/3.0\mathrm{nm}\mathrm{CoO}{]}_{50}$ multilayer film at 78 K. The reflection has a two-component line shape: the broad F component is denoted by a dotted line. Inset illustrates the AF spin structure consisting of the four labeled $\{111\}$ domains with spins alternating in the directions indicated by the double-tipped arrows.

Figure 2

Changes in the CoO component of the (111) reflection intensity as a function of increasing temperature for a) $[10\mathrm{nm}{\mathrm{Fe}}_3{\mathrm{O}}_4/1.7\mathrm{nm}\mathrm{CoO}{]}_{50}$ and b) $[10\mathrm{nm}{\mathrm{Fe}}_3{\mathrm{O}}_4/3.0\mathrm{nm}\mathrm{CoO}{]}_{50}$ multilayer films. The filled circles indicate data taken after cooling from room temperature in zero field, while the open squares indicate data taken in zero field after cooling the sample in $4000\mathrm{kA}/\mathrm{m}$ field from room temperature to 78 K. The solid lines are guides to the eye; vertical lines indicate the blocking temperatures for the two samples as determined by magnetometry measurements.

Figure 3

Two possible atomic configurations of the ${\mathrm{Fe}}_3{\mathrm{O}}_4/\mathrm{CoO}$ interface, separated by an APB. The dashed lines represent mirror symmetry planes for each individual antiphase domain.