Magneto-Optic Configuration & Pole Tip Options

Configuration options for the Magneto-Optic module are detailed below. The Magneto-Optic module allows the user to control and change the field strength with the use of exchangeable pole tips. A few of those options are explained in more detail in the following sections.

LENS POLE TIPS

The image below shows another view of the magnet pole in the sample space. Note the small lens at the center of the pole tip. This is at the end of a hole bored lengthwise through the magnet, and allows one to focus a laser beam to the center of the sample space.

The threaded pole tips also allow the use of a threaded optic that can focus the beam sent through the magnet onto the sample. For example, with a pole tip gap of 12mm, an optic with 3mm OD and 6mm focal length will focus a laser onto a sample in the center of the gap, and similarly a lens on the other side of the sample can collect the light accordingly.

Special pole tips may be used to allow larger samples or concentrate the field. Some of these options are explained in more detail below.

FIELD CONCENTRATOR POLE TIPS

Along the magnet axis, the user may insert small threaded extension tips that can reach through the side plate apertures (with the windows removed) to raise the field intensity very close to the sample in the center. These extension tips are at room temperature, so your sample temp may rise slightly, but the exposed area is small, so the rise might be only about 1 degree, for example. The field may be concentrated and strengthened by using screw-in pole tips. If the small windows in the side walls are removed, these extension tips can protrude into the central sample chamber area to get gaps smaller than the chamber width. In this manner, fields of 1T or more can be achieved for thin samples.

Figure 1: Screw in pole tips are used to get fields up to 1T

RECESSED OBJECTIVE & RAISED POLE TIPS

The recessed objective option for the Cryostation was originally designed for the Magneto-Optic option to allow a user to bring a microscope objective close to the top of the sample chamber. The recessed objective insert replaces the overhead 50mm outer window and is sealed with the o-ring and held in place by the retaining ring. The image on the left shows the recessed objective installed in the sample chamber lid with the recessed objective visible through the window. The image on the right shows how the insert approaches the sample space in the Magneto-Optic assembly. The recess has an ID of 39mm. In this configuration, the minimum working distance is 5mm.

Figure 2: Recessed Objective in the Magneto-Optic

The center image below shows how the recessed objective can be used with a modified pole piece, which brings the magnetic field up to the top of the sample space to get close optical access to a high magnetic field. The working distance with this “close pole” configuration is about 7.5mm, whereas the working distance to the centerline of the poles is 17mm. The raised poles may be used to bring the field nearer to the side window, but is not compatible with the radiation shield for the magnet option.

Figure 3: Raised Pole Tip (left), Cutaway View (center), Assembled View (right)

This configuration uses the “T” horizontal sample mount. The “T” sample mount can be used with the radiation shield, as shown below.

Figure 4: “T” sample mount in radiation shield

REACHING FULL FIELD STRENGTH

There are several configurations of the system when trying to optimize field strength.

  • Standard configuration with 12mm pole tip separation, radiation shield in place. This allows field strength of 0.7 Tesla and sample temp down to 3.2K. Uniformity is very good in this case.
  • Close pole tips configuration with 5mm separation, without our standard radiation shield. The field will be about 0.9 Tesla. If no radiation shield is in place, the sample temp may be about 10K. If the user can construct a thin radiation shield, typically with two thin windows on each side of their flat sample, then temperatures could be as low as 3.2K
  • Close pole tips configuration with 5mm separation, and with central screw in tips that can be adjusted to a central tip separation of about 3mm or less. There would probably be no room for a radiation shield. The field will be about 1.4 Tesla with a 3mm gap. Since no radiation shield is in place, the sample temp may be about 10K.
  • Other creative implementations of the field can be designed by the user, including using small coils or permanent magnets to alter the field.
  • In most cases, the Hall calibration device that is included with the system can be used to calibrate the relationship between supply current and field strength.