Non-Joulian Magnetostriction and Non-Joulian Magnetism

Complementing volume-conserving Joule magnetostriction (JM), the non-Joulian magnetostriction (NJM) phenomenon has recently been discovered whose hallmark is change in volume of the crystals in magnetic fields. This article reviews the unique set of non-Joulian properties that are so non-conforming relative to conventional magnets that they merit the designation of a distinctly new class of (functional) magnets. This review distils key aspects of non-Joulian magneto-elasticity and magnetism, including the sign of volume change (contracting or expanding); the orientation dependence of volume change that is key to realizing large volume changes (remarkably, this volume-anisotropy coexists with a seemingly isotropic magnetization response for all crystal axes); and a digital coercivity landscape unlike that seen in conventional magnets. NJM lay undiscovered for nearly two decades because prior literature assumed, without experimental foundation, that iron-based alloys obeyed volume-conserving JM. Measurements now show that volume change is inherent across wide composition ranges, occurring in both quenched, metastable, and inexorably slow-cooled, equilibrated crystals. While not essential, quenching promotes long-range periodicity of magneto-elastic gradients with enhanced NJM. Another objective is to "frame" Joulian-to-non-Joulian behavior within a single framework of magnetically responsive, wave-like elastic medium in which approach to homogeneity yields the limiting case of JM; in fact, gradient magnet-elasticity is indispensable to describing non-Joulian magnetism. A consequential auxiliary finding is the large differences in intrinsic magneto-elastic constants for seemingly equivalent crystal axes, consistent with gradient magneto-elasticity. Consequentially, a sizeable literature that previously characterized these crystals using volume-conserving assumption and framework of phenomenological magnetostriction theory of homogeneous cubic crystals is rendered moot, and unreliable for high-fidelity applications.