properties. The setup should be electronically isolated from ground loops. Electromagnetic stray fields should be shielded and filtered from the leads. In this section the general setup is discussed briefly. The STM is based on the design developed for scan heads used at higher temperatures [6, 7] but with a modified tip approach in order to allow for measurements in magnetic fields. The most important features of the design are its versatility and compactness which allows to fit it into the cryostat tail within a superconducting solenoid. The setup allows cycle times of about two to three days combined with a high mechanical stability. During refill with liquid helium, mechanical distortions occure, nevertheless the tip does not need to be retracted completely from its position close to the sample surface. This advantage compensates for the long warming and cooling time necessary for tip and sample changes. A standard $^{3}$He/$^{4}$He dilution refrigerator (GVL Leiden Cryogenics, Leiden, Netherlands) with a continuous 1 K plate and a base temperature well below T min ≈ 30 mK is used to host the home-built STM. The cryostat is equipped with a superconducting solenoid installed outside the vacuum chamber and designed to reach magnetic fields up to B max = 13 T. The general setup is sketched in figure 1. The STM body is made of highly resistive stainless steel (fig. 2 A) in order to reduce eddy currents, when high magnetic fields are applied. Furthermore it is supposed to be nonmagnetic, although a small magneto mechanical effect, e. g., due to magnetostriction can not be excluded, when high magnetic fields are applied. The STM sample position is placed in the center of the magnet bore for having a homoge- neous field. A heater made of a twisted-pair manganin wire allows heating of the whole scan head. It is wound around the stainless steel body and thermally anchored but electrically isolated with stycast epoxy (Stycast 2850FT Polymer). To avoid local hot spots, the heater is glued in a meander-like fashion to cover most of the surface. The temperature of the STM body is controlled by a Cernox bare chip thermometer calibrated in the temperature range from T = 30 mK to T = 1 . 5 K. The sensor is mounted inside an incised Cu screw into the STM body to allow good thermal contact between thermometer and STM body especially at low temperatures. The STM body itself is thermally anchored to the mixing chamber via an oxygen-free copper tube. In addition, a heat shield thermal anchored to the 3 A. General Assembly