Nanoscale magnetic bubbles in ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ at room temperature

The increasing demand for computer data storage with a higher recording density can be addressed by using smaller magnetic objects, such as bubble domains. Small bubbles predominantly require a strong saturation magnetization combined with a large magnetocrystalline anisotropy to resist self-demagnetization. These conditions are well satisfied for highly anisotropic materials. Here, we study the domain structure of thin ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ lamellae. Magnetic bubbles with a minimum diameter of 74 nm were observed at room temperature, approaching even the range of magnetic skyrmions. The stripe domain width and the bubble size are both thickness dependent. Furthermore, a kind of bubble was observed below the spin-reorientation transition temperature that combine bubbles with opposite helicity. In this paper, we reveal ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ to be a good candidate for a high-density magnetic bubble-based memory.

Figure 1

(a) Kerr microscopy images: domain structure of a thin lamella of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ (2 μm thick), observed by Kerr microscopy on of the (001) surface with a magnetic field applied parallel to the $c$ axis (perpendicular to the observed plane) at room temperature. The magnetic fields corresponding to the domain patterns in A–I can be taken from the plot in (b). (b) Magnetization curve in a thin lamella of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$.

Figure 2

Domain structure of stripe and magnetic bubbles observed by Lorentz transmission electron microscopy (L-TEM) at room temperature. (a) ${\mu }_{0}H=0\mathrm{T}$. (b) ${\mu }_{0}H=0.8\mathrm{T}$. (c) Enlargement of the box in (b). (d) Individual bubbles at 0.95 and 1.04 T in a sample that is 250 nm thick. (e) Bubble at 0.63 T in a 90-nm-thick sample. (f) Direction of the projected in-plane magnetic induction reconstructed by the transport-of-intensity equation (TIE).

Figure 3

Thickness-dependent domain structure. (a) Wedge-shaped thin lamella viewed from the side. (b) Domain structure as seen from the top of the wedge from [001]. (c) Domain structure from the area demarcated by the box in (b) in an applied magnetic field of 0.9 T. Some of the bubbles are marked by white circles to show their size. (d) Dependence of the stripe domain width on thickness. (e) Dependence of the bubble diameter at 0.9 T on thickness.

Figure 4

Domain structure below the spin-reorientation transition temperature at 110 K. (a) Domain structure in the absence of a magnetic field. (b) Domain structure at 0.48 T without 90° domain wall. (c) Schematic view of the domain structure above (left) and below (right) $T_{\mathrm{SR}}$ at zero magnetic field. Additional 90° domain wall appears below $T_{\mathrm{SR}}$. (d) Domain structure at 0.59 T with individual bubbles starting to form. (e) Bubbles with two cores inside and a tail extending in the easy direction. (f) Direction of the projected in-plane magnetic induction from the area inside the red box in (e).

Figure 5

Phase diagram for (a) bulk and (b) thin lamella ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$. (c) Minimum observed diameter of skyrmions or bubbles at room temperature compiled from previous reports [9, 10, 27, 29, 30, 31, 32, 33].