Mini Cryogen-Free Magnet System - Cryogenic Limited

Jiří Liška, Adam Dubroka

The Mini Cryogen-Free Magnet System from Cryogenic Limited company is a physical device used for production of strong magnetic field created by a superconducting coil. The system combines the latest cryogen-free technology with sophisticated measurement techniques. It provides a versatile, powerful investigative device achieving low temperatures and high magnetic fields without the direct use of liquid helium or nitrogen. Closed loop system uses helium gas for refrigeration of the superconducting magnet and the sample. The instrument is designed for the measurement of magnetic properties using Vibrating Sample Magnetometer - VSM (magnetic moment and AC susceptibility) and electrical properties (resistivity and Hall effect). Measurement range is for magnetic field from –9 T to 9 T and for temperature from 1.6 K to 400 K.

Measurement options


VSM measurements

The Cryogenic VSM (Vibrating Sample Magnetometer) module is designed to measure DC magnetic moment. It allows the measurements of magnetic hysteresis loops or temperature dependencies of magnetic moment (B = –9 T – +9 T, T = 1.6 K – 400 K). In this system, materials in various form can be analysed (bulk, thin films, powders or liquids). Orientation of the magnetic field can be parallel or perpendicular to the sample. VSM probe can be used also for AC susceptibility measurements. Since June 2017 our system contains upgraded VSM setup which has more stable adjustment.

Electrical and magnetic transport measurements

The Cryogenic Resistivity module provides the capability to perform DC resistance and Hall voltage measurements. Samples with resistance in the range from 1 μΩ to 1 MΩ can be measured with variable temperature (T = 1.6 K – 400 K) and magnetic field (B = –9 T – +9 T). Orientation of the magnetic field can be parallel or perpendicular to the sample.

Instrument is comprised of the following components:

  • A cryostat incorporating a cryocooler, superconducting magnet and a variable temperature sample space.
  • Rack incorporating electronics for control and monitoring of the cryostat and any measurement options.
  • Measurement system software.
  • Sample probes (two types - VSM, Resistivity).

Cryocooler system and cryostat

Cooling materials to cryogenic temperatures has traditionally used liquid cryogens (usually helium and nitrogen). The same results may be achieved more simply by mechanical means, using a cryocooler. Cryocoolers operate using a helium compressor, which requires just mains power and a source of cooling water. The cryocooler high pressure helium circuit is completely independent to the rest of the measurement system. However, it provides the cooling to both the magnet and the variable temperature insert (VTI). The cryostat is a vacuum insulated chamber whose primary function is to support and thermally shield the superconducting magnet and VTI.

Superconducting magnet

The magnet in our system is a vertically oriented solenoid wound from copper stabilised filamentary niobium titanium (NbTi) superconducting wire. The coil is cooled by the cryocooler to an operating temperature of 3–4 K.


Main parameters


Magnet

Magnet typeNbTi solenoid with persistent mode switch
Central field homogeneity0.1% over 10mm diameter x 10mm long cylinder
Decay rate in persistent mode (ppm/hr)
10
Field directionVertical
Magnet inductance (Henries)5.3 H
Maximum Operating Field (Tesla)9 T
Magnet constant0.08309T/A
Nominal operating current (amps)108.3 A
Radial stray field 5 gauss contour (m)
1.3 (from centre of magnet)
Vertical stray field 5 gauss contour (m)
1.6 (from centre of magnet)
Initial magnet energisation rate0.3 T/min
Maximum subsequent energisation rate
1.0 T/min
Magnet power supplyCryogenic SMS120C (120A, +/-5V)
Low field option +/=300mA (24 mT)

Cryocooler (Water cooled Compressor)

Type
Sumitomo 1 W Pulse Tube Cryocooler
Compressor mains requirements410 V, 3 phase, 16A or 32A
Compressor cooling water minimum flow rate (l/min)
7
Static compressor gas pressure (bar)17.0
Working compressor gas pressure (bar)21
Typical cooldown time (hrs)11

VTI

Temperature range (K)1.6–400
Temperature stability (K)+/- 0.05
Internal diameter (mm)25
Normal operating pressure (mbar)5–15
Dump vessel pressure with system at room temperatureAtmosphere plus 0.25 bar (approx)
Dump vessel pressure with VTI at base temperatureAtmosphere minus 0.5 bar (approx)

Sample chamber

Variable temperature insert without VSM coils inserted
Circulating helium gas 25 mm inner diameter
With VSM coils insertedStatic exchange gas 14 mm inner diameter