Domain-wall structure in thin films with perpendicular anisotropy: Magnetic force microscopy and polarized neutron reflectometry study

Ferromagnetic domain patterns and three-dimensional domain-wall configurations in thin CoCrPt films with perpendicular magnetic anisotropy were studied in detail by combining magnetic force microscopy and polarized neutron reflectometry with micromagnetic simulations. With the first method, lateral dimension of domains with alternative magnetization directions normal to the surface and separated by domain walls in 20-nm-thick CoCrPt films were determined in good agreement with micromagnetic simulations. Quantitative analysis of data on reflectometry shows that domain walls consist of a Bloch wall in the center of the thin film, which is gradually transformed into a pair of Néel caps at the surfaces. The width and in-depth thickness of the Bloch wall element, transition region, and Néel caps are found consistent with micromagnetic calculations. A complex structure of domain walls serves to compromise a competition between exchange interactions, keeping spins parallel, magnetic anisotropy orienting magnetization normal to the surface, and demagnetizing fields, promoting in-plane magnetization. It is shown that the result of such competition strongly depends on the film thickness, and in the thinner CoCrPt film (10 nm thick), simple Bloch walls separate domains. Their lateral dimensions estimated from neutron scattering experiments agree with micromagnetic simulations.

Figure 1

In-plane () and out-of-plane (■) hysteresis loops of (a) 10- and (b) 20-nm-thick CoCrPt films. The inset in (b) shows the one-dimensional XRD pattern of the 20-nm-thick CoCrPt thin film (□).

Figure 2

MFM images of 20-nm-thick CoCrPt film on a smooth substrate after an out-of-plane ac demagnetization process (a) and at remanence after in-plane saturation with +10 kOe (b). Both images were obtained at the same magnification.

Figure 3

A CoCrPt rectangular crystalline grain of 10×10×20 ${\mathrm{nm}}^3$ made of smaller cells of 5×5×2 ${\mathrm{nm}}^3$. The $c$ crystallographic axis () of each grain deviated by $\approx 7$° from the film normal. Top view of the micromagnetic simulations of a 20-nm-thick CoCrPt film with image sizes of 1×1 μ${\mathrm{m}}^2$ (b, c) after out-of-plane ac demagnetization. (b) The out-of-plane component of the magnetic moment $\left(M_z\right)$; (c) the component of the magnetic moment parallel to the external magnetic field or $x$ axis $\left(M_x\right)$. Top view of the micromagnetic simulations of a 20-nm-thick CoCrPt film with image sizes of 1×1 μ${\mathrm{m}}^2$ at remanence (d) after in-plane saturation. (e) Top view of the micromagnetic simulations of a 10-nm-thick CoCrPt film with area of 1×1 μ${\mathrm{m}}^2$ at remanence after in-plane saturation. (f, g) The in-plane VSM hysteresis loops (), the micromagnetic simulations $(•)$, and the PNR data () of the 20- and 10-nm-thick CoCrPt films, respectively. The red color shows the magnetic moment along the $+z$ axis $\left(M_z\right)$, and blue shows the magnetic moment along the −$z$ axis $\left(M_z\right)$ (b, d, and e). On the other hand, (c) the red color shows the magnetic moment along the $+x$ axis $\left(M_x\right)$, and blue shows the magnetic moment along the −$x$ axis $\left(M_x\right)$.

Figure 4

The components (a) $M_x$, (b) $M_y$, and (c) $M_z$ of the magnetic moment in the $yz$ plane of a DW oriented along $x$ for a 20-nm-thick film. The red color shows the magnetic moment along the $+x$ axis in (a), $+y$ axis in (b), and $+z$ axis in (c), and blue shows the magnetic moment along the −$x$ axis in (a), −$y$ axis in (b), and −$z$ axis in (c). The inset in (a) shows the coordinate axes. A DW (d) and several DWs (e) formed by a Bloch wall in the center of the thin film with two Néel caps at the surfaces. (f) $z$ component of the magnetic moment $\left(M_z\right)$ of a DW in a 10-nm-thick CoCrPt film consisting of a Bloch wall. Red and blue colors show the magnetic moments along the $+z$ axis and −$z$ axis, respectively.

Figure 5

Polarized neutron experimental data (open symbols) and fit (lines) versus angle “th” from the (a, b) 20- and (c, d) 10-nm-thick CoCrPt films in (a, c) saturated and (b, d) low field states. $R^{++}$ (□ and black line), $R^{--}$ ( and red line), $R^{+-}$ ( and green line), and $R^{-+}$ ( and blue line).

Figure 6

Nuclear SLD (——), (nuclear + magnetic) SLDs in saturation () and at low field without () and with () gradients of magnetic SLD, and (nuclear-magnetic) SLDs in saturation () and at low field without () and with () gradients of magnetic SLD of the 20-nm-thick CoCrPt film. On the bottom, a magnification of the SLDs, indicated by a circle, is shown.

Figure 7

(a) Profile of the normalized in-plane magnetization in saturation (——) and at low field without () and with () gradients of magnetic SLD. (b) Components of the magnetic moment $(M_x,M_y$, and $M_z)$ obtained from the micromagnetic simulation of the 20-nm-thick CoCrPt film at 200 Oe after in-plane saturation. The image size was 1×1 μ${\mathrm{m}}^2$. The red color shows the magnetic moment along the $+z$ axis in $M_z,+y$ axis in $M_y$, and $+x$ axis in $M_x$, and blue shows the magnetic moment along −$z$ axis in $M_z$, −$y$ axis in $M_y$, and −$x$ axis in $M_x$.

Figure 8

(a) Nuclear SLD (——), (nuclear + magnetic) SLDs in saturation () and at low field (), and (nuclear − magnetic) SLDs in saturation () and at low field () of the 10-nm-thick CoCrPt film. (b) A magnification of the SLDs, indicated by a circle. (c) Profile of the normalized in-plane magnetization in saturation (——) and at low field ().