The crystal Pr₄Ni₃O₁₀ is like a three-layer sandwich: nickel oxide layers separated by praseodymium spacers. As it cools, three magnetic regimes switch on in sequence, as if, inside the sandwich, the nickel spins first align, then slightly rearrange, and finally the praseodymium spins join in.
Muons, heavy cousins of electrons, penetrate the crystal and sense internal magnetic fields. This allowed researchers to capture that the first transition at 158 K is very sharp: the transition width is only 0.65 K. Upon heating and cooling, the transition temperature differs slightly (by 0.27 K), revealing a weakly first-order character.
Under pressure, the main magnetic transition shifts downward at a rate of 4.9 K per gigapascal, and the nickel magnetic moment decreases by 2% per gigapascal. The magnetic order does not vanish but becomes noticeably weaker. This means that the magnetic properties of layered nickelates can be smoothly tuned by pressure, like adjusting volume with a knob.
Muons live only 2.2 microseconds, but in that time they manage to tell us about the magnetic fields inside matter.