Section 1 - Jacob Nagel
Introduction
Heat exchangers are used in almost everything we use. Air conditioners, cars, electronics
etc. all use some sort of heat exchanger to operate. A heat exchanger is a tool used to transfer
energy from one “closed” system to another. For example, an air conditioner takes energy from
the air inside your house and transfers it outside into the atmosphere. Heat exchangers achieve
this through a system of using environmentally harmful coolants, noisy fans and coolant pumps.
Magnetocaloric metals could be a more efficient, environmentally safer, and more compact
replacement for typical heat exchangers.
Magnetocaloric effect
Magnetocaloric metals function through changes in magnetic and lattice entropy. Entropy
is a measure of disorder, randomness, or uncertainty in a physical system. There are many types
of entropy but the 2 we will focus on that are related to magnetocaloric metals are magnetic
entropy and lattice entropy. The total entropy of the magnetocaloric metal can be described as
∆𝑆 = ∆𝑆𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 + ∆Slattice
Magnetic entropy is defined as all possible combinations of magnetic spins, which are
directly related to the electron spin. Magnetic spin is the magnetic moment caused by the spin
state of an electron. Magnetic spins have direction so, when these magnetic spin directions are
pointing in random directions, magnetic entropy is high and when they are pointing in the same
direction magnetic entropy is low. Lattice entropy is the number of vibrational modes atoms in
the lattice can occupy. So, more vibrations, more lattice entropy. Exposing a magnetocaloric
metal to an outside magnetic field, it’s possible to align these magnetic spins, and decrease the
magnetic entropy. Because the magnetocaloric effect is adiabatic entropy must stay constant,
meaning the lattice entropy must increase to compensate for the loss of entropy from the
magnetic spins. The increase of entropy takes the form of increased atomic vibrations, or heat.
This is the basic function of the magnetocaloric effect and has many possible applications for
heat exchangers.
There are different variations of the magnetocaloric effect.
Giant magnetocaloric effect (GMCE)
Inverse magnetocaloric effect (IMCE)
Rotating magnetocaloric effect (RMCE)
All these effects function on the same basic thermodynamic principles but have different
effects. First the RMCE creates the MCE by rotating the metal inside of the magnetic field
instead of applying and removing the magnetic field. This can be useful to eliminate moving
parts in a cooling or heating system.
Secondly, the IMCE uses a different type of magnetocaloric metal that cools down when
exposed to a magnetic field. This is because the magnetic field increases the magnetic entropy,
lattice entropy compensates, making the atoms vibrate less/cool down to keep the total entropy
of the system. The IMCE can also be used in heat exchangers in conjunction with the
conventional MCE in desired applications.
Lastly, the (GMCE) creates a much larger cooling/heating effect than conventional MCE.
It uses a decrease in magnetic entropy as well as lattice entropy at the same time, meaning
temperature must rise to compensate for both decreases in entropy. The metal functions at a near
structural phase transition temperature and when a magnetic field is applied, it changes to a more
ordered lattice structure. This decreases lattice entropy as well as magnetic entropy
simultaneously, causing the temperature change to be larger.
To evaluate how effective a magnetocaloric metal is for heating/cooling there are several
key performance metrics to track. These metrics help engineers decide what type of
magnetocaloric metal to use.

Curie and transition temperature – Temperatures at which the metal is affected
the most by the magnetic field. A metal at near curie temperature will perform the
best.

Adiabatic Temperature – The change in temperature of the metal without
exchanging heat with its surroundings.

Entropy change – Measures the change in disorder when a magnetic field is
applied. Larger changes mean more heat can be absorbed/released.

Thermal conductivity – A more thermal conductivity metal will be more efficient at
transferring heat in or out of a system.
https://pmc.ncbi.nlm.nih.gov/articles/PMC10938420/
Section 2 – Connor Rosecrans
Magnetocaloric materials can be classified into a few main groups, mainly by
composition and magnetic phase behavior. Each group has its own advantages and
disadvantages. The first group is rare-earth metal-based materials; these were the first found
materials to have a highly effective magnetocaloric effect (MCE). Gadolinium (Gd) is a classic
benchmark because of its strong magnetocaloric effect near room temperature and useful
thermomagnetic behavior. There are also lots of Gd-based metallic compounds that are used for
the same reasons; these can have even larger entropy and temperature changes, meaning a larger
MCE. The disadvantages of this group include high cost, limited availability of the rare-earth
elements, and many rare-earth metal compounds are difficult to manufacture. The second group
is transition-metal-based compounds and alloys. In hopes of reducing the reliance on rare-earth
elements, lots of research has been going into this group specifically to find a better solution.
These compounds and alloys include combinations of Mn, Fe, P, Si, and many more. They are
favored due to their increased abundance and lower costs compared to rare-earth metals.
Although they suffer from a lower MCE, brittleness, and poor thermal conductivity. The third
group is Heusler alloys and intermetallic compounds. These are a class of compounds/alloys that
have a specific crystal structure (FCC or BCC) that has tunable properties. These metals often
include elements like Ni, Mn, Co, Ti, Ga, In, and Sn. A strong advantage to this class is the
possibility to tune the composition of the compounds/alloys to adjust the working temperature
range for the MCE. There are many disadvantages that come along with this, like structural
changes during phase transformations that cause cracks leading to eventual failure. This group,
like the transition metals, is still being researched in hopes of replacing the rare-earth metals.
The MCE is directly controlled by the crystal structure and how this structure changes in
a magnetic field and different temperatures. Materials often rely on a magnetic ordering
transition, which is a phase transition when a material changes its magnetic state from an ordered
(ferromagnetic) to a disordered state (paramagnetic) as temperature increases. These materials
can also undergo a structural transition under a changing magnetic field and temperature. This
transition results in an abrupt change in magnetic entropy and creates the giant magneto-caloric
effect (GMCE), which is just an even larger change in temperature. The order of the phase
transition is a significant factor that must be accounted for when deciding applications. Firstorder transitions involve both a magnetic and structural reconfiguration that produces a large
change in entropy and temperature. However, these often come with structural stress and
experience mechanical fatigue over many cycles. Second-order transitions are only magnetic
reconfiguration without a structural change. These have a more moderate MCE but are more
stable and resilient to fatigue, helping with long-term use. There are also microstructural factors
that play a role in how these materials behave. Grain size and boundaries are very important;
smaller grains help relieve stress from any structural transitions. Defects can affect the magnetic
coupling, which lowers the change in entropy and, in turn, lowers the MCE.
For application, magnetocaloric materials must be able to handle repeated thermal and
magnetic cycles and remain mechanically stable. As stated earlier, many of these materials with
high MCE often are brittle and have low fracture toughness, especially if they also have firstorder transitions. Repeated cycles can cause internal stress and cracks that can eventually lead to
failure. The most common application for these materials would be in a magnetic refrigeration
device. This means that these materials must have efficient heat exchange. According to an indepth review of magnetocaloric composites, many magnetocaloric materials have low thermal
conductivity for what would be needed for this application (Shao et al. Al). This can slow down
heat transfer, reducing the efficiency of the refrigeration systems.
Even with successful testing of magnetocaloric materials, many challenges come with
manufacturing. Key constraints include cost, raw material supply, manufacturability, and longterm reliability. Many of the reliable and best-performing materials include rare-earth metals
(Gd, Tb, Dy) that are expensive and are in limited availability. They are also very volatile in
price, which makes scalability a big risk. Producing these high-quality alloys/compounds also
comes with its own challenges. Specific methods such as arc melting are used to manufacture
these materials. These methods can be very energy-intensive and produce small batches,
meaning it can be very costly. There are also the challenges of the durability of these materials
under repeated cycles. Magnetocaloric devices require the material to cycle between magnetic
phases millions of times over their use. This can not only lead to a loss of MCE in the material
but also mechanical decline, which makes it hard for long-term use.
https://www.sciencedirect.com/science/article/abs/pii/S0304885322009428
Section 3 – Ultan Gaylord
Magnetic Refrigeration Systems
Over the past decade, innovations surrounding magnetic refrigeration systems
have been trending towards commercializing the technology, reaching the viability of it
being a household consumer product. Magnetic heating and cooling have
predominantly been seen in large scale specialty products. The rotary magnetic
refrigerator (RMR) was made in 2001 and is one of the earliest systems that could also
operate at room temperature. It uses an active magnetic regenerator (AMR) cycle. This
works by having a rotating wheel with a porous bed of magnetocaloric material
(Nd2Fe14B particularly), the wheel is rotating to create a magnetic field. The wheel is
then connected to a pump full of heat transfer fluid and heat exchangers. The fluid goes
through the bed, cooling down. A similar system was later used in a commercial
refrigeration device by Cooltech Applications. Whereas RMR use case is more
predominantly towards commercial application, ADR (adiabatic demagnetization
refrigerators), is used by NASA’s cryogenic group in extremely low temperature
conditions. ADR is used for cooling at different stages. Cooling can be allowed to be
done continuously this way, at 0.05 K, up until a heatsink at 4 K. ADR allows for 10
times greater cooling power than what was required by single-shot space systems.
More recently in 2016, GE and ORNL’s Building Equipment Research Group worked
with the Department of energy to create a prototype for a magnetic refrigerator
prototype using magnetocaloric materials. The design works in a way that combines
design ideas from the previously mentioned systems. The prototype rotates magnets
back and forth, changing their magnetic field, and then pumps cooling fluid between the
warm and cold sides of the object the device is cooling down. The magnets are then
also arranged into 50 cooling stages, making the first ever multi-stage cooling
refrigerator.
Advantages Over Traditional Refrigeration
The advantages of using magnetocaloric materials is that they cut back
significantly on energy usage, since most of the work is done by the generated
magnetic field. Magnets don’t make noises or have large vibrations. Therefore, when
compared to traditional refrigeration systems they are much quieter and have less
moving parts that could fail. Using magnetocaloric materials can also be safer overall
because traditional refrigerators emit greenhouse gasses, while magnetic refrigerators
don’t.
Current Limitations and Engineering Barriers
Although Magnetic Refrigeration Systems have a large amount of advantages
over other systems, the number one limitation is still how to manufacture them.
Producing these magnets in a manufacturing environment is significantly harder than
doing so in a laboratory setting. The magnets have complex assemblies and are also
much heavier than over refrigeration systems. Procurement of these magnetocaloric
materials is also difficult as only rare earth or toxic elements are feasible to use.
Future Research and Industry Outlook
Ultimately, the future of Magnetic Refrigeration Systems is going to be in
development of smaller units for consumer products. However much of the focus for
Magnetic Refrigeration Systems is on mass producing commercial applications such as
commercial display cabinets, beverage coolers, air conditioners, and heat pumps. A
particularly large commercial use case right now is with data centers, as larger data
centers can consume up to 5 million gallons of water per day, and overall, they account
for 4% of the entire energy grids usage. Magnetic Refrigeration Systems could
eventually be used to significantly reduce the resources of these data centers, solving a
major bottleneck in tech infrastructure.
https://magneticsmag.com/cooltech-applications-launches-the-first-magnetic-cooling-system-forcommercial-refrigeration/
https://www.coolingpost.com/world-news/magnetic-commercial-fridge-system-launches/
https://www.ge.com/news/reports/not-your-average-fridge-magnet
https://www.pewresearch.org/short-reads/2025/10/24/what-we-know-about-energy-use-at-usdata-centers-amid-the-ai-boom/
https://www.sciencedirect.com/science/article/abs/pii/S0140700706001526
https://www.eesi.org/articles/view/data-centers-and-water-consumption
https://ntrs.nasa.gov/api/citations/20230010022/downloads/2023.SCWpaper.pdf