A pot of water on the stove: the hotter the burner, the faster ice turns to water, and water to steam. Heat destroys order. But sometimes nature pulls a trick, and heating makes a liquid freeze, like a puddle turning to ice on a frosty morning. The most famous trickster is helium-3: at low temperatures it's a liquid, but if you warm it to 0.3 degrees above absolute zero, it suddenly becomes solid. This is known as the Pomeranchuk effect.
It turns out that a similar 'reverse' transition hides in the world of magnets. Scientists studied a crystal of erbium heptatantalate — a material where erbium atoms act like tiny magnetic needles arranged in a triangular lattice. When cooled to 0.4 kelvin (which is -272.75 °C), the needles align into a strict three-color pattern — a magnetic 'solid' order. But if you remove a bit more heat, dropping to 0.2 kelvin, the pattern falls apart: the needles lose long-range order and form a striped ripple — a 'liquid' state of spins. The magnetic 'solid' melted from cold.
The key lies in competition. On the triangular lattice, magnetic needles can't satisfy all neighbors at once: this is called frustration. At the 'high' temperature (0.4 K), one interaction wins, building a three-sublattice order. When the temperature drops, weaker forces come into play, and the system settles on a compromise — a striped structure without rigid order. Thermal kicks, which usually break everything, here actually help maintain the crystalline order, and their disappearance destroys it.
The Pomeranchuk effect in helium-3 was used to achieve ultra-low temperatures: compressing solid helium drew heat away from the liquid, acting like a refrigerator.