Adiabatic demagnetization is a technique for attaining temperatures well below 1 kelvin. The fundamental principle was suggested by Debye (1926) and Giaque (1927), and the first working adiabatic demagnetization refrigerators (ADRs) were constructed by several groups beginning in 1933. Adiabatic demagnetization was the first method developed for cooling below ~0.3 kelvin (the temperature attainable by pumping on liquid ³He). Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ...

Basic technique

The basic operating principle of an ADR is the use of a strong magnetic field to control the entropy of a sample of material, often called the "refrigerant". Magnetic field constrains the orientaition of magnetic dipoles in the refrigerant. The stronger the magnetic field, the more aligned the dipoles are. More aligned dipoles correspond to lower entropy and heat capacity. This means that if the refrigerant is kept at a constant temperature through thermal contact with a heat sink (usually liquid He) and the magnetic field is switched on, the refrigerant must lose some energy (or heat) to the heat sink. Conversely, when the magnetic field is switched off, the heat capacity of the refrigerant rises again because the degrees of freedom associated with orientation of the dipoles are liberated. If the refrigerant is held adiabatically when the magnetic field is switched off, i.e. it cannot exchange energy with its surroundings (no thermal contact with the heat sink), then its temperature decreases below the initial temperature. Current flowing through a wire produces a magnetic field (M) around the wire. ... The thermodynamic entropy S, often simply called the entropy in the context of thermodynamics, is a measure of the amount of energy in a physical system that cannot be used to do work. ... This article covers adiabatic processes in thermodynamics. ...

The operation of a standard ADR proceeds roughly as follows. First, a strong magnetic field is applied to the refrigerant, forcing its various magnetic dipoles to align and putting the refrigerant in a state of low entropy. The heat sink then absorbes the heat generated in the refrigerant due to the entropy loss. The thermal contact with the heat sink is then broken and the magnetic field is switched off, increasing the heat capacity of the refrigerant and thus decreasing its temperature below the temperature that could be achieved with the He heat sink alone. In practice the magnetic field is decreased slowly in order to provide continuous cooling and keep the sample at approximately constant low temperature. Once the field falls to zero (or to some low limiting value determined by the properties of the refrigerant material), the cooling power of the ADR vanishes and heat leaks will cause the refrigerant to warm up.

Implementations

Paramagnetic salts

The simplest choice of refrigerant is a sample of a paramagnetic salt, such as cerium magnesium nitrate. The active magnetic dipoles in this case are those of the electron shells of the paramagnetic atoms. Paramagnetism is the tendency of the atomic magnetic dipoles, due to quantum-mechanical spin, in a material that is otherwise non-magnetic to align with an external magnetic field. ... This article is about the electromagnetic phenomenon. ... In atomic physics, an electron shell is a group of atomic orbitals with the same value of the principal quantum number n. ...

In a paramagnetic salt ADR, the heat sink is usually provided by a pumped ⁴He (~1.2 K) or ³He (~0.3 K) cryostat. An easily-attainable 1 tesla magnetic field is generally required for the initial magnetization. The minimum temperature attainable is determined by the self-magnetization tendencies of the chosen refrigerant salt, but temperatures from 1 to 100 mK are accessible. Dilution refrigerators had for many years supplanted paramagnetic salt ADRs, but interest in space-based and simple to use lab-ADR's has recently revived the field (for example see http://www.cmr.uk.com/abcmrhis.html). Cryocoolers are refrigerators used to reach cryogenic temperatures. ... The tesla (symbol T) is the SI derived unit of magnetic flux density (or magnetic inductivity). ... A dilution refrigerator is a cryogenic device first proposed by Heinz London. ...

Nuclear demagnetization

One variant of adiabatic demagnetization that continues to find substantial research application is nuclear demagnetization refrigeration (NDR). NDR follows the same principle described above, but in this case the cooling power arises from the magnetic dipoles of the nuclei of the refrigerant atoms, rather than their electron configurations. Since these dipoles are of much smaller magnitude, they are less prone to self-alignment and have lower intrinsic minimum fields. This allows NDR to cool the nuclear spin system to very low temperatures, often 1 µK or below. Unfortunately, the small magnitudes of nuclear magnetic dipoles also makes them less inclined to align to external fields. Magnetic fields of 3 teslas or greater are often needed for the initial magnetization step of NDR.

In NDR systems, the initial heat sink must sit at very low temperatures (10–100 mK). This precooling is often provided by the mixing chamber of a dilution refrigerator or a paramagnetic salt ADR stage.

See also

• Dilution refrigerator

A dilution refrigerator is a cryogenic device first proposed by Heinz London. ...

References

Lounasmaa, Experimental Principles and Methods Below 1K, Academic Press (1974).

Richardson and Smith, Experimental Techniques in Condensed Matter Physics at Low Temperatures, Addison Wesley (1988).