Impact of Controlled Magneto-Structural Coupling in MnCoGe-Based Compounds on the Design of Multifunctional Materials...

Tailoring the magneto-structural coupling in magnetic martensitic materials is pivotal for optimizing multifunctional properties such as magnetocaloric effect (MCE) and negative thermal expansion (NTE). This study demonstrates how Ni substitution in Mn_{1- x}Ni_xCoGe (x = 0.03 to 0.07) modulates the magneto-structural transitions as investigated by in-situ X-ray diffraction, magnetization measurements, and geometric compatibility analysis. Ni doping is shown to stabilize the hexagonal phase, lower the martensitic transformation temperature, and introduce intermediate ferromagnetic hexagonal (FM-Hex) states, thereby altering the transition pathways from a single paramagnetic hexagonal (PM-Hex) ↔ ferromagnetic orthorhombic (FM-Orth), to a two-step PM-Hex ↔ FM-Hex ↔ FM-Orth sequence. This modification in the magneto-structural coupling alleviates lattice incompatibility, broadens phase transition temperature window, and enhances magnetization changes during the phase transition. The magnetocaloric refrigeration capacity increases from 213(14) J kg^-1 for x = 0.04 to 308(18) J kg^-1 for x = 0.07 under a 5 T driving field, while the NTE coefficient is tuned from -375(1) × 10^-3 K^-1 for x = 0.05 to -143(1) × 10^-3 K^-1 for x = 0.07. These findings provide mechanistic insights into the interplay between magnetization states and lattice compatibility, thereby advancing the design of energy-efficient solid-state cooling and precision actuators through controllable magneto-structural coupling.