  Coupled order parameters in phase-transition materials can be controlled
using various driving forces such as temperature, magnetic and electric field,
strain, spin-polarized currents and optical pulses. Tuning the material
properties to achieve efficient transitions would enable fast and low-power
electronic devices. Here we show that the first-order metamagnetic phase
transition in FeRh films becomes strongly asymmetric in mesoscale structures.
In patterned FeRh stripes we observed pronounced supercooling and an
avalanche-like abrupt transition from the ferromagnetic to the
antiferromagnetic phase while the reverse transition remains nearly continuous
over a broad temperature range. Although modest asymmetry signatures have been
found in FeRh films, the effect is dramatically enhanced at the mesoscale. The
asymmetry in the transitions is independent of applied magnetic fields and the
activation volume of the antiferromagnetic phase is more than two orders of
magnitude larger than typical magnetic heterogeneities observed in films. The
collective behavior upon cooling results from the role of long-range
ferromagnetic exchange correlations that become important at the mesoscale and
should be a general property of first-order magnetic phase transitions.
