Local probes

At present, to produce the strongest continuous field is necessary to built a superconducting magnet with enough space inside to house an electromagnet whose operation implies a large energetic cost and requires a complex cooling system. This makes the realization of experiments sensitive to mechanical vibrations more complicated. Nevertheless, constant improvements are being implemented in those magnets to reduce the mechanical vibrations.

Currently, high magnetic fields used  in Scanning Probe Microscopy experiments are produced by  superconducting magnets. The first superconducting solenoid was made by H. Kamerlingh  Onnes using lead 100 years ago. However, this first attempt did not succeed since lead losses its superconducting properties at relatively low field (0.05T). Since then, many scientists have tried to increase the magnetic field range in which the superconductors can carry an electrical current without losses. Most of the used materials are type II superconductors where the magnetic field penetrates in form of quantized flux lines or vortices. In these superconductors, vortex motion is responsible for the energy dissipation and great effort has been done to control the vortex pinning . At the early 60’s, several methods to efficiently pin the vortices were found making possible the development of the current thriving industry based on superconductors. Nowadays, the materials used by this industry are NbTi wires, that allow to fabricate magnets up to 9T, cables of Nb3Sn for magnets up to 20T as well as other alloys or high Tc superconductors for higher magnetic field magnets.  At present, superconducting magnets do not go beyond 22T or 23T although it is believed that 25T could be reached in a near future.

Figure 1: The images show several millestones in the construction of superconducting magnets since the 60’s. From the first superconducting magnet (left), built by Sir Martin Woods, until the 22.5T magnet made using high critical temperature superconductors (right), and going though magnetic resonance magnet (21.4T) and magnets for installations of neutron or synchrotron radiation (15 T split pair).

Low temperatures SPM set-ups working under high magnetic fields have been recently reviewed in Y.J. Son, et al. RSI 2010).

Some SPM Groups working at low temperatures and high fields: