[1]It may, well, be noted  that the shear strength depends upon the grain size. In most of the materials, the shear strength decreases with the increase in the grain size

Reference:                                                             

 ACTA MATERIALIA 57(10):2933-2943 · JUNE 2009 

[2]The shear strengths of FeO and Fe3O4 are predicted to be similar, ∼4.5MPa, while the shear strength of α-Fe2O3 is predicted to be the largest, ∼7MPa, for samples 5mm thick.

Reference:

 ACTA MATERIALIA 58(18):5912-5925 · OCTOBER 2010

Other information on mechanical properties of ferrites will be found in:

Measurement of Young's Modulus of Nanocrystalline Ferrites with Spinel Structures by Atomic Force Acoustic MicroscopyKes ter et al. Journal of physics and chemistry of solids 61 (2000), No.8, pp.1275-1284

Thank you for information.