Peer review of: Giant magneto-birefringence effect and tuneable colouration of 2D crystal suspensions (2020)

REVIEWER COMMENTS 
Reviewer #1 (Remarks to the Author): 

 
The manuscript presents interesting results on magnetic birefrigence in magnetic 2D crystal 
suspensions. The authors demonstrate giant effect that leads to tunable colouration based on 
phase retardation. The results are presented rigorously and carefully, so they can be of interest to 
wide community of researchers, even including graduate students. The demonstrated effect has 
possible applications in the areas of medicine, communication devices, sensing, etc. 
 
However, there are some questions and comments. 
 
1) What are arbitrary units for transmittance in Fig.2 and how are they related to those in Fig. 3 
and in Supplementary figures? 
 
2) I recommend to include the definition of the generalized Langevin function to the main text of 
the article, since there are different definitions in literature. 
 
3) There are several misprints in Supplementary in Eqs. (1),(2),(3),(5) and (6): '≥' instead of 
'>=', '≻' instead of '>-', 'd' instead of 'dθ'. 
 
In general, I consider the manuscript deserves publishing. 
 
 

Reviewer #2 (Remarks to the Author): 
 
The manuscript reports a strong magnetic field tunable coloration in aqueous suspensions of 2D 
Co-TiOx. This coloration is the result of the birefringence of the suspension, which can be 
explained by the B-field tunable crystal orientation due to the magnetic anisotropy in the Co-TiOx. 
Such coloration covered the visible spectrum within a relatively small change in magnetic field. The 
authors further propose theoretical models and evaluate the Cotton-Mouton coefficient and cross 
compared with different systems, which is about several orders of magnitude larger than other 
transparent liquid. The manuscript is concluded by listing out the key requirement for the tunable 
coloration due to magneto-birefringence. 
Overall, I find these new results interesting and the interpretations are reasonable. And the 
discovery may provide different aspects of utilization of 2D material and may further expand the 
research in this field. Therefore, I recommend for publication in nature communication after some 
minor issues are addressed. 

Below are my specific comments: 
1. Regarding the magnetic alignment mechanism. The authors use both optical transmittance and 
magnetic susceptibility measurement to show that the crystal orientation aligned with magnetic 
field. However, some of these measurements need to be further discussed to give readers a more 
direct correlation between the measurements and the conclusion: 
a. In fig 2b, the authors present the transmittance along different axes. But only two axes are 
represented with ~15% total transmittance changes. It is recommended that the authors show 
that indeed the view from z is at the maximum and view from x is at the minimum of the 
transmittance. 
b. To support their claim, the authors also measured the magnetization along different axis in the 
bulk crystal. From supplementary fig.3, the material seems to be a weak ferromagnet. Even 
though magnetization indeed shows difference between H//surface and H_|_ surface, but this may 
not be a strong evidence to distinguish hard/easy axis. 
In order to further justify the claim, I would recommend that the author present some crystal 
orientation analysis with respect to different H direction (for example SAXS measurement) for give 
a direct and convincing evidence. If this is too difficult to conduct, the authors may need explain 
why it is challenging to do so, and introduce reader why magnetic field can align the crystal 
structure. 
2. In figure 3a and c, the authors compared the experiment data and the simulation. It would be 
more convincing if the author can compare directly the magnetic field dependence of the spectra. 
For example, overlapping the calculated result with the experiment one in fig 4a. 
3. For the greyscale maps in Fig. 3a, c and supplementary Fig. 4a, Fig.5 and Fig.6 please include 
scale bars. 
4. The author mentioned that the traditional diffraction induced coloration is usually non-uniform 
and unstable due to the highly sensitive to viewing angle. Is there any angle dependence of the 
coloration in this study? 
5. It would be nice if the authors can include some cycling test and see how reproducible and 
recyclable the suspension is.