Cryogen-free Magnetic Refrigeration
Magnetic refrigeration is based on the magneto-caloric effect: When a medium is magnetized its magnetic moments get aligned and the heat of magnetization is released. Vice versa, if the medium is demagnetized its temperature drops.
Cooling systems by kiutra can make use of two different types of magnetic refrigeration methods:
Adiabatic Demagnetization Refrigeration (ADR)
Adiabatic Demagnetization Refrigeration (ADR) is a well-established technique for the generation of cryogenic temperatures. As the temperature is controlled directly by applying a magnetic field to a solid-state magneto-caloric medium, the ADR principle offers excellent temperature stability.
In a conventional configuration employing a single ADR unit, demagnetization cryostats provide sub-Kelvin cooling for a
limited amount of time. This so-called “hold time” is sufficiently long for various experiments and applications. However, permanent cooling of the sample is impossible because the refrigerant must be regenerated in order to restart the ADR process.
Starting from an initial base temperature provided by a closed-cycle cryocooler (Step 1), a suitable cooling medium is first magnetized (Step 2). The heat of magnetization is then dissipated by the cryocooler (Step 3). Subsequently, the cooling medium is thermally decoupled by means of a so-called heat switch (Step 4) and then demagnetized afterwards (Step 5). During the demagnetization process the temperature of the cooling medium drops. If the set-point temperature is reached before the magnetic field, B, is lowered to zero the cooling power can be regulated to provide constant temperature over time, e.g. for several hours or even some days (Step 6). When the magnetic field is finally decreased to zero, the cooling process stops (Step 7) and the medium warms up to base temperature again (Step 8). The process can be restarted after some waiting time.
Continuous Adiabatic Demagnetization Refrigeration (cADR)
kiutra now offers the first commercial magnetic refrigerators for continuous sub-Kelvin cooling. We combine multiple ADR units which are interconnected and carefully balanced to implement continuous Adiabatic Demagnetization Refrigeration (cADR). In this technique, the final ADR unit, which is connected to the sample stage, can be regenerated while the sample temperature is kept constant by another ADR unit.
Magnetic refrigeration systems by kiutra offer both single-shot and continuous ADR in a highly modular assembly. Depending on customer-specific demands single-shot ADR systems can be upgraded to multi-stage CADR refrigerators.