Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique can be used to attain extremely low temperatures (well below 1 K), as well as the ranges used in common refrigerators, depending on the design of the system.
The effect was first observed by the German physicist Emil Warburg (1880) and the fundamental principle was suggested by Debye (1926) and Giauque (1927). The first working magnetic refrigerators were constructed by several groups beginning in 1933. Magnetic refrigeration was the first method developed for cooling below about 0.3 K (a temperature attainable by 3He refrigeration, that is pumping on the 3He vapors).
The magnetocaloric effect
The magnetocaloric effect (MCE, from magnet and calorie) is a magneto-thermodynamic phenomenon in which a reversible change in temperature of a suitable material is caused by exposing the material to a changing magnetic field. This is also known by low temperature physicists as adiabatic demagnetization, due to the application of the process specifically to create a temperature drop. In that part of the overall refrigeration process, a decrease in the strength of an externally applied magnetic field allows the magnetic domains of a chosen (magnetocaloric) material to become disoriented from the magnetic field by the agitating action of the thermal energy (phonons) present in the material. If the material is isolated so that no energy is allowed to (re)migrate into the material during this time, i.e., an adiabatic process, the temperature drops as the domains absorb the thermal energy to perform their reorientation. The randomization of the domains occurs in a similar fashion to the randomization at the curie temperature, except that magnetic dipoles overcome a decreasing external magnetic field while energy remains constant, instead of magnetic domains being disrupted from internal ferromagnetism as energy is added.
One of the most notable examples of the magnetocaloric effect is in the chemical element gadolinium and some of its alloys. Gadolinium's temperature is observed to increase when it enters certain magnetic fields. When it leaves the magnetic field, the temperature drops. The effect is considerably stronger for the gadolinium alloy Gd5(Si2Ge2). Praseodymium alloyed with nickel (PrNi5) has such a strong magnetocaloric effect that it has allowed scientists to approach within one thousandth of a degree of absolute zero.
The cycle is performed as a refrigeration cycle, analogous to the Carnot cycle, and can be described at a starting point whereby the chosen working substance is introduced into a magnetic field, i.e., the magnetic flux density is increased. The working material is the refrigerant, and starts in thermal equilibrium with the refrigerated environment.
- Adiabatic magnetization: A magnetocaloric substance is placed in an insulated environment. The increasing external magnetic field (+H) causes the magnetic dipoles of the atoms to align, thereby decreasing the material's magnetic entropy and heat capacity. Since overall energy is not lost (yet) and therefore total entropy is not reduced (according to thermodynamic laws), the net result is that the item heats up (T + ΔTad).
- Isomagnetic enthalpic transfer: This added heat can then be removed (-Q) by a fluid or gas — gaseous or liquid helium, for example. The magnetic field is held constant to prevent the dipoles from reabsorbing the heat. Once sufficiently cooled, the magnetocaloric substance and the coolant are separated (H=0).
- Adiabatic demagnetization: The substance is returned to another adiabatic (insulated) condition so the total entropy remains constant. However, this time the magnetic field is decreased, the thermal energy causes the magnetic moments to overcome the field, and thus the sample cools, i.e., an adiabatic temperature change. Energy (and entropy) transfers from thermal entropy to magnetic entropy (disorder of the magnetic dipoles).
- Isomagnetic entropic transfer: The magnetic field is held constant to prevent the material from heating back up. The material is placed in thermal contact with the environment being refrigerated. Because the working material is cooler than the refrigerated environment (by design), heat energy migrates into the working material (+Q).
Once the refrigerant and refrigerated environment are in thermal equilibrium, the cycle begins again.
The basic operating principle of an adiabatic demagnetization refrigerator (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 orientation of magnetic dipoles in the refrigerant. The stronger the magnetic field, the more aligned the dipoles are, and this corresponds to lower entropy and heat capacity because the material has (effectively) lost some of its internal degrees of freedom. If the refrigerant is kept at a constant temperature through thermal contact with a heat sink (usually liquid helium) while the magnetic field is switched on, the refrigerant must lose some energy because it is equilibrated with the heat sink. When the magnetic field is subsequently switched off, the heat capacity of the refrigerant rises again because the degrees of freedom associated with orientation of the dipoles are once again liberated, pulling their share of equipartitioned energy from the motion of the molecules, thereby lowering the overall temperature of a system with decreased energy. Since the system is now insulated when the magnetic field is switched off, the process is adiabatic, i.e., the system can no longer exchange energy with its surroundings (the heat sink), and its temperature decreases below its initial value, that of the heat sink.
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 these degrees of freedom of the refrigerant into a state of lowered entropy. The heat sink then absorbs the heat released by the refrigerant due to its loss of entropy. Thermal contact with the heat sink is then broken so that the system is insulated, and the magnetic field is switched off, increasing the heat capacity of the refrigerant, thus decreasing its temperature below the temperature of the helium heat sink. In practice, the magnetic field is decreased slowly in order to provide continuous cooling and keep the sample at an approximately constant low temperature. Once the field falls to zero or to some low limiting value determined by the properties of the refrigerant, the cooling power of the ADR vanishes, and heat leaks will cause the refrigerant to warm up.
The magnetocaloric effect is an intrinsic property of a magnetic solid. This thermal response of a solid to the application or removal of magnetic fields is maximized when the solid is near its magnetic ordering temperature.
The magnitudes of the magnetic entropy and the adiabatic temperature changes are strongly dependent upon the magnetic order process: the magnitude is generally small in antiferromagnets, ferrimagnets and spin glass systems; it can be substantial for normal ferromagnets which undergo a second order magnetic transition; and it is generally the largest for a ferromagnet which undergoes a first order magnetic transition.
Also, crystalline electric fields and pressure can have a substantial influence on magnetic entropy and adiabatic temperature changes.
Currently, alloys of gadolinium producing 3 to 4 K per tesla (K/T) of change in a magnetic field can be used for magnetic refrigeration.
Recent research on materials that exhibit a giant entropy change showed that Gd5(SixGe1−x)4, La(FexSi1−x)13Hx and MnFeP1−xAsx alloys, for example, are some of the most promising substitutes for gadolinium and its alloys — GdDy, GdTb, etc. These materials are called giant magnetocaloric effect materials (GMCE).
Gadolinium and its alloys are the best material available today for magnetic refrigeration near room temperature since they undergo second-order phase transitions which have no magnetic or thermal hysteresis involved.
In a paramagnetic salt ADR, the heat sink is usually provided by a pumped 4He (about 1.2 K) or 3He (about 0.3 K) cryostat. An easily attainable 1 T 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-ADRs has remained, due to the complexity and unreliability of the dilution refrigerator
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.
This refrigeration, once proven viable, could be used in any possible application where cooling, heating or power generation is used today. Since it is only at an early stage of development, there are several technical and efficiency issues that should be analyzed. The magnetocaloric refrigeration system is composed of pumps, electric motors, secondary fluids, heat exchangers of different types, magnets and magnetic materials. These processes are greatly affected by irreversibilities and should be adequately considered.
Appliances using this method could have a smaller environmental impact if the method is perfected and replaces hydrofluorocarbon (HFCs) refrigerators (some refrigerators still use HFCs which have considerable effect on the ozone layer. At present, however, the superconducting magnets that are used in the process have to themselves be cooled down to the temperature of liquid nitrogen, or with even colder, and relatively expensive, liquid helium. Considering these fluids have boiling points of 77.36 K and 4.22 K respectively, the technology is clearly not cost- and energy-efficient for home appliances, but for experimental, laboratory, and industrial use only.
Recent research on materials that exhibit a large entropy change showed that alloys are some of the most promising substitutes of gadolinium and its alloys — GdDy, GdTb, etc. Gadolinium and its alloys are the best material available today for magnetic refrigeration near room temperature. There are still some thermal and magnetic hysteresis problems to be solved for them to become truly useful [V. Provenzano, A.J. Shapiro, and R.D. Shull, Nature 429, 853 (2004)] and scientists are working hard to achieve this goal. Thermal hysteresis problems is solved therefore in adding ferrite (5:4).
Research and a demonstration proof of concept in 2001 succeeded in applying commercial-grade materials and permanent magnets at room temperatures to construct a magnetocaloric refrigerator which promises wide use.
This technique has been used for many years in cryogenic systems for producing further cooling in systems already cooled to temperatures of 4 K and lower. In England, a company called Cambridge Magnetic Refrigeration produces cryogenic systems based on the magnetocaloric effect.
On August 20, 2007, the Risø National Laboratory at the Technical University of Denmark, claimed to have reached a milestone in their magnetic cooling research when they reported a temperature span of 8.7 C. They hope to introduce the first commercial applications of the technology by 2010.
Current and future uses
There are still some thermal and magnetic hysteresis problems to be solved for these first-order phase transition materials that exhibit the GMCE to become really useful; this is a subject of current research. A useful review on magnetocaloric materials published in 2005 is entitled "Recent developments in magnetocaloric materials" by Dr. Karl A. Gschneidner, et al. This effect is currently being explored to produce better refrigeration techniques, especially for use in spacecraft. This technique is already used to achieve cryogenic temperatures in the laboratory setting (below 10K). As an object displaying MCE is moved into a magnetic field, the magnetic spins align, lowering the entropy. Moving that object out of the field allows the object to increase its entropy by absorbing heat from the environment and disordering the spins. In this way, heat can be taken from one area to another. Should materials be found to display this effect near room temperature, refrigeration without the need for compression may be possible, increasing energy efficiency.
The use of this technology for domestic refrigerators though is very remote due to the high efficiency of current Vapor-compression refrigeration cycles, which typically achieve performance coefficients of 60% of that of a theoretical ideal Carnot cycle.
This technology could eventually compete with other cryogenic heat pumps for gas liquefaction purposes.
Gschneidner stated in 1999 that: "large-scale applications using magnetic refrigeration, such as commercial air conditioning and supermarket refrigeration systems, could be available within 5–10 years. Within 10–15 years, the technology could be available in home refrigerators and air conditioners."
Major advances first appeared in the late 1920s when cooling via adiabatic demagnetization was independently proposed by two scientists, Peter Debye in 1926 and William Giauque in 1927.
This cooling technology was first demonstrated experimentally by chemist Nobel Laureate William F. Giauque and his colleague Dr. D.P. MacDougall in 1933 for cryogenic purposes when they reached 0.25 K. Between 1933 and 1997, a number of advances in utilization of the MCE for cooling occurred. See Reviews:
In 1997, the first near room temperature proof of concept magnetic refrigerator was demonstrated by Prof. Karl A. Gschneidner, Jr. by the Iowa State University at Ames Laboratory. This event attracted interest from scientists and companies worldwide who started developing new kinds of room temperature materials and magnetic refrigerator designs. A major breakthrough came 2002 when a group at the University of Amsterdam demonstrated the giant magnetocaloric effect in MnFe(P,As) alloys that are based on earth abundant materials.
Refrigerators based on the magnetocaloric effect have been demonstrated in laboratories, using magnetic fields starting at 0.6 T up to 10 T. Magnetic fields above 2 T are difficult to produce with permanent magnets and are produced by a superconducting magnet (1 T is about 20,000 times the Earth's magnetic field).
Room temperature devices
Some recent research has focused on the use of the process to perform refrigeration near "room temperature". Constructed examples of room temperature magnetic refrigerators are listed in the table below:
Room temperature magnetic refrigerators Institute/Company Location Announcement date Type Max. cooling power (W) Max ΔT (K) Magnetic field (T) Solid refrigerant Quantity (kg) Ames Laboratory/Astronautics Ames, Iowa/Madison, Wisconsin, USA February 20, 1997 Reciprocating 600 10 5 (S) Gd spheres Mater. Science Institute Barcelona Barcelona, Spain May 2000 Rotary ? 5 0.95 (P) Gd foil Chubu Electric/Toshiba Yokohama, Japan Summer 2000 Reciprocating 100 21 4 (S) Gd spheres University of Victoria Victoria, British Columbia Canada July 2001 Reciprocating 2 14 2 (S) Gd & Gd1−xTbx L.B. Astronautics Madison, Wisconsin, USA September 18, 2001 Rotary 95 25 1.5 (P) Gd spheres Sichuan Inst. Tech./Nanjing University Nanjing, China 23 April 2002 Reciprocating ? 23 1.4 (P) Gd spheres and Gd5Si1.985Ge1.985Ga0.03 powder Chubu Electric/Toshiba Yokohama, Japan October 5, 2002 Reciprocating 40 27 0.6 (P) Gd1−xDyx L.B. Chubu Electric/Toshiba Yokohama, Japan March 4, 2003 Rotary 60 10 0.76 (P) Gd 1−xDyx L.B. 1 Lab. d’Electrotechnique Grenoble Grenoble, France April 2003 Reciprocating 8.8 4 0.8 (P) Gd foil George Washington University  USA July 2004 Reciprocating ? 5 2 (P) Gd foil Astronautics Madison, Wisconsin, USA 2004 Rotary 95 25 1.5 (P) Gd and GdEr spheres / La(Fe0.88Si0.12)13H1.0 University of Victoria Victoria, British Columbia Canada 2006 Reciprocating 15 50 2 (S) Gd, Gd0.74Tb0.26 and Gd0.85Er0.15 pucks 0.12 1maximum cooling power at zero temperature difference (ΔT=0); 2maximum temperature span at zero cooling capacity (W=0); L.B. = layered bed; P = permanent magnet; S = superconducting magnet
In one example, Prof. Karl A. Gschneidner, Jr. unveiled a proof of concept magnetic refrigerator near room temperature on February 20, 1997. He also announced the discovery of the giant MCE (GMCE) in Gd5Si2Ge2 on June 9, 1997  (see below). Since then, hundreds of peer-reviewed articles have been written describing materials exhibiting magnetocaloric effects.
- Lounasmaa, Experimental Principles and Methods Below 1 K, Academic Press (1974).
- Richardson and Smith, Experimental Techniques in Condensed Matter Physics at Low Temperatures, Addison Wesley (1988).
- Lucia, U. General approach to obtain the magnetic refrigeretion ideal Coefficient of Performance COP, Physica A: Statistical Mechanics and its Applications, 387/14 (2008) 3477–3479, doi: 10.1016/j.physa.2008.02.026; see also http://arxiv.org/abs/1011.1684
- ^ Zemansky, Mark W. (1981). Temperatures very low and very high. New York: Dover. p. 50. ISBN 0-486-24072-X.
- ^ a b Karl Gschneidner, Jr. and Kerry Gibson (December 7, 2001). "Magnetic Refrigerator Successfully Tested". Ames Laboratory News Release. Ames Laboratory. http://www.external.ameslab.gov/news/release/01magneticrefrig.htm. Retrieved 2006-12-17.
- ^ Emsley, John (2001). Nature's Building Blocks. Oxford University Press. p. 342. ISBN 0-1985-0341-5.
- ^ Gibson, Kerry (November 2001). "Magnetic Refrigerator Successfully Tested: Ames Laboratory develoments help push boundaries of new refrigeration technology". INSIDER Newsletter for employees of Ames Laboratory. http://www.ameslab.gov/news/ins01-11Magnetic.htm. (Vol. 112, No.10 )
- ^ Milestone in magnetic cooling, Risø News, August 20, 2007. Retrieved August 28, 2007.
- ^ Gschneidner, Karl A., Jr.; Pecharsky, V. K. and Tsokol1, A.O. Recent developments in magnetocaloric materials Report on Progress in Physics. (2005) Volume 68, pages 1479–1539.
- ^ http://www.ameslab.gov/final/News/1999rel/99crada.html
- ^ http://prola.aps.org/abstract/PR/v43/i9/p768_1
- ^ Gschneidner K.A. Jr. and Pecharsky V.K. 1997 Rare Earths: Science, Technology and Applications III ed R.G. Bautista et al. (Warrendale, PA: The Minerals, Metals and Materials Society) p 209
- ^ Pecharsky V.K. and Gschneidner K.A. Jr. 1999 J. Magn. Magn. Mater. 200 44
- ^ Gschneidner K.A. Jr. and Pecharsky V.K. 2000 Annu. Rev. Mater. Sci. 30 387
- ^ Gschneidner K.A. Jr. and Pecharsky V.K. 2002 Fundamentals of Advanced Materials for Energy Conversion ed D. Chandra and R.G. Bautista (Warrendale, PA: The Minerals, Metals and Materials Society) p 9
- ^ Tegus O., Brück E., de Boer F.R., Buschow K.H.J, Transition-metal-based magnetic refrigerants for room-temperature applications. Nature (London) 415, 150–152 (2002).
- ^ Zimm C, Jastrab A., Sternberg A., Pecharsky V.K., Gschneidner K.A. Jr., Osborne M. and Anderson I., Adv. Cryog. Eng. 43, 1759 (1998).
- ^ Bohigas X., Molins E., Roig A., Tejada J. and Zhang X.X., IEEE Trans. Magn. 36 538 (2000).
- ^ Hirano N., Nagaya S., Takahashi M., Kuriyama T., Ito K. and Nomura S. 2002 Adv. Cryog. Eng. 47 1027
- ^ Rowe A.M. and Barclay J.A., Adv. Cryog. Eng. 47 995 (2002).
- ^ Rowe A.M. and Barclay J.A., Adv. Cryog. Eng. 47 1003 (2002).
- ^ Richard M.A., Rowe A.M. and Chahine R., J. Appl. Phys. 95 2146 (2004).
- ^ Zimm C, Paper No K7.003 Am. Phys. Soc. Meeting, March 4, Austin, Texas (2003) 
- ^ Wu W., Paper No. K7.004 Am. Phys. Soc. Meeting, March 4, Austin, Texas (2003) 
- ^ a b Hirano N., Paper No. K7.002 Am. Phys. Soc. Meeting March 4, Austin, Texas, 
- ^ Clot P., Viallet D., Allab F., Kedous-LeBouc A., Fournier J.M. and Yonnet J.P., IEEE Trans. Magn. 30 3349 (2003).
- ^ F. Shir, C. Mavriplis, L.H. Bennett, E. Della Torre, “Analysis of room temperature magnetic regenerative refrigeration,” International Journal of Refrigeration, 28, 4 (2005) 616.
- ^ Zimm C, Paper No. K7.003 Am. Phys. Soc. Meeting, March 4, Austin, Texas (2003) 
- ^ Rowe A.M. and Tura A., International Journal of Refrigeration 29 1286–1293 (2006).
- ^ http://prola.aps.org/abstract/PRL/v78/i23/p4494_1
- NASA – How does an Adiabatic Demagnetization Refrigerator Work ?
- What is magnetocaloric effect and what materials exhibit this effect the most?
- Magnetocaloric materials keep fridges cool by C. Wu
- Ames Laboratory news release, May 25, 1999, Work begins on prototype magnetic-refrigeration unit.
- Magnetic refrigerator successfully tested
- Refrigeration Systems Terry Heppenstall's notes, University of Newcastle Upon Tyne (November 2000)
- XRS Adiabatic Demagnetization Refrigerator
- Executive Summary: A Continuous Adiabatic Demagnetization Refrigerator (.doc format) (Google cache)
- Origin and tuning of the magnetocaloric effect in the magnetic refrigerant Mn1.1Fe0.9(P0.8Ge0.2)
- Magnetic technology revolutionizes refrigeration
- Evaluation of thermodynamic quantities in magnetic refrigeration
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