Large magnetovolume effect induced by ferromagnetic-antiferromagnetic competition in a cobaltite perovskite*

Materials that show negative thermal expansion (NTE) have significant industrial merit because they can be used to fabricate composites whose dimensions remain invariant upon heating. In some materials, NTE is concomitant with the spontaneous magnetization, known as the magnetovolume effect (MVE). Here we report a new class of MVE material; namely, a layered perovskite PrBaCo$_{2}$O$_{5.5+}_{x} $(0 $\le \quad x \le $ 0.41)$_{,\, }$in which strong NTE ($\beta \quad \approx $ -3.3$\times$ 10$^{-5}$ K$^{-1}$ at $x =$ 0.24) is triggered by embedding ferromagnetic (F) clusters into the antiferromagnetic (AF) matrix. The strongest MVE is found near the boundary between F and AF phases in the phase diagram, indicating the essential role of competing interaction between the F-clusters and the AF-matrix. Furthermore, the MVE is not limited to the PrBaCo$_{2}$O$_{5.5+}_{x}$ but is also observed in the NdBaCo$_{2}$O$_{5.5+}_{x}$. The present study provides a new approach to obtaining MVE and offers a path to the design of NTE materials.