Photoemission electron microscopy of three-dimensional magnetization configurations in core-shell nanostructures

We present a photoemission electron microscopy method that combines magnetic imaging of the surface and of the inner magnetization in three-dimensional core-shell nanostructures. The structure investigated consists of a cylindrical nickel core that is completely surrounded by a shell of iron oxide and silicon oxide layers. The method enables one to image the magnetization configuration of the nickel core even though the shell is thicker than the mean-free path of the photoelectrons. Characteristic $L_3$ and $L_2$ edges can be observed not only in the yield of the photoelectrons emitted from the surface of the nanostructure but also in its shadow. X-ray magnetic circular dichroism in the electron yield of the x rays absorbed and transmitted by the multilayered nanowire allows for the individual imaging of the magnetization configurations of the iron oxide tube and the nickel core. The method suggests novel approaches for the characterization of the magnetic and material properties of complex three-dimensional nanostructures.

Figure 1

(a) Schematic of a core-shell wire during PEEM imaging. (b) Scanning electron micrograph of an iron oxide tube partially filled with a nickel core. The blow up shows the transition from the empty tube (left) to the nickel core (right) that appears bright. (c) Cutout of a scanning electron micrograph combined with a PEEM image recorded at the nickel edge. The absorption of the nickel core can be distinguished in the shadow of the wire.

Figure 2

(a) Photoemission spectra recorded on the wire (A, blue), in the shadow of the empty tube (B, green), and in the shadow of the nickel core (C, red). The respective regions are marked in the PEEM image shown in the middle. The red curves are shifted for clarity by $-0.3$. Note, that no absorption edges of the silicon substrate contribute in the energy range shown. (b) Schematic of a partially filled core-shell wire exposed to x rays of different photon energies. At 707.8 eV (Fe $L_3$ edge) XMCD contrast revealing the magnetic configuration of the iron oxide tube is obtained in the entire shadow region (green), while at 851.8 eV (Ni $L_3$ edge) magnetic contrast is only obtained in the shadow of the nickel core (red).

Figure 3

(a) PEEM images of the core-shell wire recorded at the iron $L_3$ edge and the nickel $L_3$ edge (background extracted). (b)–(e) Differential XMCD images of the iron oxide tube (left) and the nickel core (right) recorded at the respective absorption edges. The color bar indicates the direction of the magnetic contrast in arbitrary units. The magnetic configurations are imaged in remanence after saturating at (b) $-123$ mT and (c)–(e) $+123$ mT and applying the displayed field values. (The values of the magnetic field are estimated from the position of the wire relative to the center of the pole shoes of the sample holder and the coil currents applied. Due to the remanence of the pole shoes we assume an error of approximately 1.4 mT in the displayed field values.) The arrows indicate the magnetization components along the wire axis. The green dotted lines mark the ends of the nickel core.

Figure 4

(a) PEEM image of the core-shell wire taken at an angle of $0^{\circ }$ between wire axis and incident x rays. (b) Differential XMCD image recorded at the iron $L_3$ edge in zero magnetic field after annealing (as-prepared) and (c) after the first field sweep 120 mT $\to$ $-120$ mT $\to$ 0 mT. The blue and red XMCD contrasts represent magnetizations along and antiparallel to the wire axis, respectively. The green lines mark the wire end.