Magnetocaloric Materials for Freezing, Cooling, and Heat-Pump Applications - Magnetocaloric Energy Conversion: From Theory to Applications - page 23

Weiss and Piccard were published in 1917 and 1918 [ 1 , 2 ], where they discovered a
reversible heating of a nickel sample near its Curie temperature (354 ° C) when a

magnetic eld was applied. They found that the nickel sample increased its tem-

perature by 0.7 K when a magnetic eld of 1.5 T was applied. Furthermore, they

also stated that the reversibility of the effect and also its larger order of magnitude
could distinguish it from the heat that emerges from the hysteresis. Finally, they
called their discovery a “ novel magnetocaloric phenomenon ” , thereby coining the

word .

The discovery of Weiss and Piccard was undoubtedly acknowledged and well
known in the scienti

“ magnetocaloric ”

c community until the end of twentieth century, when the
sudden misconception arose, attributing the discovery of the MCE to Warburg. The
reasons for this misconception will not be discussed here. However, we encourage
the reader of this topic to investigate the paper of Smith [ 4 ], where this is explained
in detail.
The rst ideas that ferromagnetic materials could be usefully applied in power

generation, refrigeration or heat pumping emerged with the works of the Slovenian
physicist Stefan in the last quarter of the nineteenth century [ 8 , 9 ]. Stefan explained
how a thermomagnetic motor should work by exploiting the transition from the
ferromagnetic to the paramagnetic state of the material by heating it above its Curie
temperature. Edison [ 10 , 11 ] and Tesla [ 12 , 13 ] then patented their versions of
thermomagnetic generators at the end of nineteenth century. In 1926 Debye [ 14 ] and
in 1927 Giauque [ 15 ] independently discussed that if paramagnetic salts are adia-
batically demagnetized, extremely low temperatures (under 1 K) could be achieved.
This was experimentally proven in 1933 by Giauque and MacDougall [ 16 ]. In 1935,
Urbain et al. [ 17 ] discovered ferromagnetism in gadolinium. This was the rst fer-

romagnetic material discovered that has a Curie temperature near room temperature.
However, it was not until the middle of the 1960s that the MCE of gadolinium was
investigated [ 18 , 19 ] by researchers from West Virginia University. This opened up
the possibility of magnetic refrigeration devices operating near room temperature. In
this manner Brown showed in his paper from 1976 [ 20 ] that gadolinium could be a
possible MCM to be used in magnetic refrigeration. He built and experimentally
tested the

rst-ever magnetic refrigeration prototype working near room temperature.
From that point on the amount of research in magnetic refrigeration near room
temperature started to increase. For example, Barclay and Steyert presented and
patented the idea of an active magnetic regenerator in 1982 [ 21 ]. Active magnetic
regeneration is an important invention in magnetic refrigeration. Active magnetic
regeneration also implies that not only are the magnetocaloric properties of a material
important, but also its thermal properties, as well as the manufacturability and pro-
cessing properties to enhance the heat-transfer characteristics.
Another important milestone in magnetic refrigeration happened in 1997 with
the discovery of the so-called giant MCE close to room temperature in a

rst-order
transition material Gd 5 Si 2 Ge 2 by Pecharsky and Gschneidner [ 22 ]. The giant MCE
observed at a transition temperature of 276 K was much higher (in terms of
magnetic entropy change) than that of other known MCMs at that time. This
discovery further increased the research on magnetic refrigeration near room