Ultracompact Binary Permanent Rare-Earth Magnet with 1.25-T Center Field and Fast-Decaying Stray Field

We present a very-compact bicomponent-permanent-magnet design capable of generating 1.25 T in a small volume, significantly above the 0.6 T available from a single uniformly magnetized permanent magnet. In addition to the enhanced maximum field, our design drastically limits the stray field present around a standard permanent magnet. These features make it suitable for retrofitting existing experiments with a substantial magnetic field, in particular, scanning probes and optical, Raman, and photoemission spectroscopy, in diverse environments, from ambient to ultrahigh vacuum and over a wide temperature range.

Figure 1

Basic idea behind the magnetic device. (a) Optimal cone separating the spatial regions with positive and negative values of the vertical component of the magnetic field generated by a vertically aligned point magnetic dipole in the cone center. (b) When the lower half of the space ($z<0$) is filled with magnetic dipoles along the same direction, this will generate a low magnetic field at the origin, as contributions from the region inside and outside the optimal cone will contribute with opposite signs, as indicated by $\pm$ signs next to the dipole. (c) When the spatial regions inside and outside the optimal cone are filled with magnetic dipoles aligned in opposite directions, the magnetic field at the origin will be maximized.

Figure 2

Prototype magnet device. (a),(b) Dimensions (in mm) of inner and outer parts, respectively. (c) Photograph of the assembled device on millimeter paper. (d) Computer-aided design drawing of the magnet mounting on the STM sample holder. Magnet, fixation piece, and STM holder are shown in yellow, blue, and black respectively.

Figure 3

$300\times 300{\mathrm{nm}}^2$ STM images of flux vortices on ${\mathrm{Nb}\mathrm{Se}}_2$ at $T=0.4$ K. Color scale corresponds to the conductance ratio $R=\sigma (V_g)/\sigma (0)$. (a) Vortex lattice generated by the field of the permanent magnet. (b) Vortex lattice in the same region after applying an additional magnetic field, $B_{\mathrm{ext}}$, of 0.5 T using the superconducting coil of the cryogenic setup. Slight misalignment of the lattice relative to the one in (a) is possibly due to vortex creep occurring throughout the measurement.

Figure 4

Simulated magnetic field distribution around the magnet. Magnetic field distribution around (a) bicomponent device and (b) single-component magnet of the same size ($y=0$ cross section is shown). Color denotes the absolute field intensity while arrows show the field direction. (c),(d) Three-dimensional (3D) surface plot of absolute field intensity from panels (a),(b), respectively, with the same color scale. Field inside the magnet (gray area) is not calculated.

Figure 5

Calculated magnetic field in the vicinity of the field maximum. (a) $B_z$ as a function of $z$ on the magnet’s axis ($x=0$). Solid lines correspond to the bicomponent magnet with the same dimensions as in Fig. 2, and dashed curves are obtained for a single-component magnet of the same size. Inset shows the dependence of $B_z$ on the cone angle for $z=0.1$ mm, while the circle denotes the actual angle in our device (${35}^{\circ }$). (b) $B_z$ and (c) $B_r$ as a function of $x$ for different values of $z$ (0.01, 0.03, 0.1, and 0.3 mm).