–PublishedJanuary5,2015Ferromagnetic materials like cobalt and iron producemagnetic fields whose magnitudes are measured by realnumbers. Imaginary or complex fields are, however, es-sentialinthefundamentaltheorythatunderliesthesta-tistical physics of phase transitions, such as those asso-ciated with the onset of magnetization. Long thoughtto be merely mathematical constructs, a realization ofthese imaginary fields has now been observed in mag-netic resonance experiments performed on the spins ofamolecule[1]. Followinganearliertheoreticalproposal,XinhuaPengattheUniversityofScienceandTechnologyofChinaandhercolleagueshaveshownthatimaginarymagneticfieldsassociatedwithaspinbath—inthiscase,themolecule—approachingaphasetransitioncanbere-latedtothequantumcoherenceofaprobespincoupledtothebath[1]. Thedemonstrationthatthisconnectionexistsinaphysicalsystemisimportantatafundamen-tal level, and could lead to new ways of studying phasetransitionsincomplex,many-bodiedmaterials.One of the simplest systems to exhibit a continuous(second-order)phasetransitionistheferromagneticIsingmodel. In this system, which can describe magnets, bi-naryalloysandlatticegasses,atomicspinsarrangedonalatticecanbeinoneoftwodiscretestates—upordown.Interactionsbetweenneighboringspinsfavortheircollec-tivealignmentintoamagneticstatebelowacriticaltem-perature