  Controlling multiferroic behavior in materials will enable the development of
a wide variety of technological applications. However, the exact mechanisms
driving multiferroic behavior are not well understood in most materials. Two
such materials are the spinels MnV2O4 and Mn3O4, where mechanical strain is
thought to play a role in determining magnetic behavior. Bulk studies of MnV2O4
have yielded conflicting and inconclusive results, due in part to the presence
of mesoscale magnetic inhomogeneity, which complicates the interpretation of
bulk measurements. To study the sub-micron-scale magnetic properties of
Mn-based spinel materials, we performed magnetic force microscopy (MFM) on
MnV2O4 samples subject to different levels of mechanical strain. We also used a
crystal grain mapping technique to perform spatially registered MFM on Mn3O4.
These local investigations revealed 100-nm-scale "stripe" modulations in the
magnetic structure of both materials. In MnV2O4, the magnetization of these
stripes is estimated to be Mz $\approx$ 105 A/m, which is on the order of the
saturation magnetization reported previously. Cooling in a strong magnetic
field eliminated the stripe patterning only in the low-strain sample of MnV2O4.
The discovery of nanoscale magnetostructural inhomogeneity that is highly
susceptible to magnetic field control in these materials necessitates both a
revision of theoretical proposals and a reinterpretation of experimental data
regarding the low-temperature phases and magnetic-field-tunable properties of
these Mn-based spinels.
