Crackling noise arises when a system responds to changing external conditions through discrete, impulsive events spanning a broad range of sizes A wide variety of physical systems exhibiting crackling noise have been studied, from earthquakes on faults to paper crumpling Because these systems exhibit regular behavior over many decades of sizes, their behavior is likely independent of microscopic and macroscopic details, and progress can be made by the use of very simple models The fact that simple models and real systems can share the same behavior on a wide range of scales is called universality We illustrate these ideas using results for our model of crackling noise in magnets, explaining the use of the renormalization group and scaling collapses This field is still developing: we describe a number of continuing challenges

Crackling Noise

The enhanced reversibility (stable transition temperature even at high strain under a solid-to-solid phase transition), low hysteresis and unusual riverine microstructure (ranging through thermal cycles) of the martensitic material Zn45Au30Cu25 makes it attractive for applications from eco-friendly fridges to medical sensors. Martensitic transformations are diffusionless, solid-to-solid phase transformations characterized by a change of crystal structure that can often be very useful. Applications include medical sensors, eco-friendly refrigerators and energy conversion devices. Repeated transformation cycles, however, can cause thermal hysteresis that modifies the material's properties and can cause permanent damage. Here Richard James and colleagues report the development of a martensitic alloy of zinc, gold and copper that maintains near-reproducible macroscopic properties despite drastic changes in its microstructure during each cycle. As well as providing a system that throws new light on the effects of hysteresis on reversible martensitic phase transformations, this work could help to extend applications for the materials in new areas — towards shape memory alloys for instance. Materials undergoing reversible solid-to-solid martensitic phase transformations are desirable for applications in medical sensors and actuators1, eco-friendly refrigerators2,3 and energy conversion devices4. The ability to pass back and forth through the phase transformation many times without degradation of properties (termed ‘reversibility’) is critical for these applications. Materials tuned to satisfy a certain geometric compatibility condition have been shown2,5,6,7,8,9,10,11,12,13,14 to exhibit high reversibility, measured by low hysteresis and small migration of transformation temperature under cycling6,9,12,15. Recently, stronger compatibility conditions called the ‘cofactor conditions’5,15 have been proposed theoretically to achieve even better reversibility. Here we report the enhanced reversibility and unusual microstructure of the first martensitic material, Zn45Au30Cu25, that closely satisfies the cofactor conditions. We observe four striking properties of this material. (1) Despite a transformation strain of 8%, the transformation temperature shifts less than 0.5 °C after more than 16,000 thermal cycles. For comparison, the transformation temperature of the ubiquitous NiTi alloy shifts up to 20 °C in the first 20 cycles9,16. (2) The hysteresis remains approximately 2 °C during this cycling. For comparison, the hysteresis of the NiTi alloy is up to 70 °C (refs 9, 12). (3) The alloy exhibits an unusual riverine microstructure of martensite not seen in other martensites. (4) Unlike that of typical polycrystal martensites, its microstructure changes drastically in consecutive transformation cycles, whereas macroscopic properties such as transformation temperature and latent heat are nearly reproducible. These results promise a concrete strategy for seeking ultra-reliable martensitic materials.

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Enhanced reversibility and unusual microstructure of a phase-transforming material

Shape memory in Cu-based alloys: phenomenological behavior at the mesoscale level and interaction of martensitic transformation with structural defects in Cu-Zn-Al

We experimentally demonstrate that arrays of interacting nanoscale ferromagnetic islands, known as artificial spin ice, develop reproducible microstates upon cycling an applied magnetic field. The onset of this memory effect is determined by the strength of the applied field relative to the array coercivity. Specifically, when the applied field strength is almost exactly equal to the array coercivity, several training cycles are required before the array achieves a nearly completely repeatable microstate, whereas when the applied field strength is stronger or weaker than the array coercivity, a repeatable microstate is achieved after the first minor loop. We show through experiment and simulation that this memory exhibited by artificial spin ice is due to a ratchet effect on interacting, magnetically charged defects in the island moment configuration and to the complexity of the network of strings of reversed moments that forms during magnetization reversal.

F. Marco

Papers

Systematic study of the martensitic transformation in a Cu-Zn-Al alloy. reversibility versus irreversibility via acoustic emission