Interfacial coupling between ferromagnets and random and dilute antiferromagnets

Depth profiles for pinned and unpinned magnetizations were determined across the interface between a ferromagnet (F) and random and dilute antiferromagnets (RAF and DAF) exemplified by Fe${}_{0.45}$Ni${}_{0.55}$F${}_2$/Co and Fe${}_{0.34}$Zn${}_{0.66}$F${}_2$/Co bilayers, respectively, using polarized neutron reflectivity (PNR). PNR measurements were complemented by magnetometry using applied fields as large as 160 kOe to assure saturation of the entire sample, including magnetic moments that are normally pinned at lower fields. The locations of pinned and unpinned magnetization in the ferro- and antiferromagnets were identified. The origin of exchange bias in the RAF system is noticeably different than that of the DAF system. In the RAF system, a domain wall is formed at the RAF/F interface when the ferromagnet's magnetization is reversed. In the DAF system, some domains within the bulk of the DAF are reversed upon reversal of the ferromagnet while others remain pinned. In this case, the interface magnetization is entirely reversed.

Figure 1

Sketch of (a) RAF Fe${}_{0.45}$Ni${}_{0.55}$F${}_2$/Co and (b) DAF Fe${}_{0.34}$Zn${}_{0.66}$F${}_2$/FeF${}_2$/Co samples used in XRR model. The location of the interface roughness $\sigma$ and the layers used in the fitting model are indicated.

Figure 2

(a) X-ray reflectivity measurements (black dots) and fit (red curve) of RAF Fe${}_{0.45}$Ni${}_{0.55}$F${}_2$/Co sample and (b) \the x-ray charge scattering length density profile (red curve) and its derivative (blue dashed curve) obtained from the fit. The vertical dotted lines indicate the positions of the interfaces, with the substrate-film interface set at $z=0$.

Figure 3

Hysteresis loop of the RAF/F Fe${}_{0.45}$Ni${}_{0.55}$F${}_2$/Co sample at $T=5$ K after cooling at different fields. The hysteresis loop were measured in the range of $H=\pm 10$ kOe applied along the direction of the $c$ axis of the Fe${}_{0.45}$Ni${}_{0.55}$F${}_2$ layer.

Figure 4

(a) Hysteresis loop of Fe${}_{0.45}$Ni${}_{0.55}$F${}_2$/Co at $T=30$ K measured between $H=\pm 160$ kOe after field cooling from room temperature in $H_{\mathrm{CF}}=2.0$ kOe. (b) Magnification of the same data for small fields. The red arrows indicate the data with decreasing field while the blue arrow indicate the data with increasing field. Magnification of data at large fields is shown in (c) and (d).

Figure 5

(a) Hysteresis loop of the RAF/F Fe${}_{0.45}$Ni${}_{0.55}$F${}_2$/Co sample at $T=30$ K measured between $H=\pm 110$ \kOe after field cooling at $H_{\mathrm{CF}}=2$ kOe. Magnification of the same data at small fields (b) and negative (c) and positive (d) fields are also shown.

Figure 6

PNR data measured at $T=30$ K at fields corresponding to points (i)–(v) indicated in Fig. \5 for the DAF/F Fe${}_{0.45}$Ni${}_{0.55}$F${}_2$/Co sample. The $R^{--}$ data are the blue upright triangles and the $R^{++}$ are the black inverted triangles. The solid orange and red lines are the fits to the $R^{--}$ and $R^{++}$ data, respectively.

Figure 7

(a) Depth profile of magnetization obtained from the PNR data at $T=30$ K for the RAF/F Fe${}_{0.45}$Ni${}_{0.55}$F${}_2$/Co sample. Panels (i)–(v) correspond to data obtained at the fields labeled (i)–(v) in Fig. \5. The vertical dotted lines indicate the position of the layer interfaces obtained from PNR. Panel (a) is a close-up of the data near the interface layer between the DAF and the F, with the line colors corresponding to the data in panels (i)–(v). Error bars indicate the uncertainty in the magnetization obtained from fits of PNR data. The region between vertical dotted lines denotes the position of the interface layer.

Figure 8

Summary of magnetic configuration of the layers for the RAF/F Fe${}_{0.45}$Ni${}_{0.55}$F${}_2$/Co sample obtained from the PNR data. The red arrow in the top layer is the magnetization at the center of the Co ferromagnetic layer. The three brown arrows in the middle represent the magnetization profile in the interface layer near the Co layer, the center, and the RAF layer. The dark blue arrow at the bottom is the magnetization in the bulk of the RAF. The arrows are drawn to scale but magnified as indicated on the left hand side of the diagram and provide a visual representation of the magnetization as deduced from Fig. \7.

Figure 9

(a) X-ray reflectivity measurements (black dots) and fit (red curve) of the DAF/F Fe${}_{0.34}$Zn${}_{0.66}$F${}_2$/FeF${}_2$/Co sample. (b) Nuclear scattering length density profile (red curve) and the derivative of the scattering length density profile (blue dashed curve) obtained from the x-ray reflectivity fit.

Figure 10

Hysteresis loop at $T=5$ K after cooling down from $T=95$ K with $H_{\mathrm{CF}}=2.0$ kOe along the $\left[001\right]$ direction. Polarized neutron reflectivity measurements were performed at $H=\pm 6.5$ kOe, indicated by the red circles. Inset: close-up of hysteresis loop for small magnetic fields showing the negative EB.

Figure 11

PNR Profiles measured at $T=5$ K for (i) $H=+6.5$ and (ii) $-6.5$ kOe for the DAF/F Fe${}_{0.34}$Zn${}_{0.66}$F${}_2$/Co sample. The $R^{++}$ data are the blue upright triangles and the $R^{--}$ are the black inverted triangles. The solid orange and red curves are the fits to the $R^{++}$ and $R^{--}$ data, respectively.

Figure 12

(a) Depth profile of the magnetization from the PNR data at $T=5$ K. The magenta dashed curve and solid green curve correspond to $H=+6.5$ and $-6.5$ kOe, respectively. (b) Depth profile of the pinned and unpinned magnetizations (solid blue and dashed red curves, respectively) calculated for $H=-6.5$ \kOe with respect to $H=+6.5$ kOe. The vertical dotted lines indicate the position of the interfaces, with the substrate-film interface set to $z=0$.

Figure 13

Summary of pinned and unpinned magnetizations in each layer for the DAF/F Fe${}_{0.45}$Ni${}_{0.55}$F${}_2$/Co sample obtained from the PNR data. Red arrows indicate unpinned magnetization components and white arrows in DAF layer signify pinned magnetization components. The arrows are drawn to scale as indicated.