Origin of Magnetic and Magnetoelastic Tweedlike Precursor Modulations in Ferroic Materials

Based on experimental observations of modulated magnetic patterns in a ${\mathrm{Co}}_{0.5}{\mathrm{Ni}}_{0.205}{\mathrm{Ga}}_{0.295}$ alloy, we propose a model to describe a (purely) magnetic tweed and a magnetoelastic tweed. The former arises above the Curie (or Néel) temperature due to magnetic disorder. The latter results from compositional fluctuations coupling to strain and then to magnetism through the magnetoelastic interaction above the structural transition temperature. We discuss the origin of purely magnetic and magnetoelastic precursor modulations and their experimental thermodynamic signatures.

Figure 1

Bright field image and corresponding electron diffraction pattern (inset) for the near [110] zone axis orientation of ${\mathrm{Co}}_{0.5}{\mathrm{Ni}}_{0.205}{\mathrm{Ga}}_{0.295}$. The tweed contrast (arrows) near the (004) bend contour is consistent with the diffuse streaks along apparent [112] directions (solid white lines in the diffraction pattern). $B2$ superlattice reflections are clearly present.

Figure 2

Zero-loss underfocus (a) and overfocus (b) images (Fresnel imaging mode, 400 kV) of a region near a bend contour. The in-focus image (c) shows tweed contrast, and the out-of-focus images reveal the presence of an additional modulated structure (arrowed) which is magnetic in nature. (d) shows the difference between (a) and (b). Contrast variations in (d) are indicative of magnetic tweed.