The Internal Field in a Ferromagnetic Crystal with Chiral Molecular Packing of Achiral Organic Radicals

Round 1

Reviewer 1 Report

The manuscript by Blundell et al. presents muon-spin rotation studies on chiral crystals of 3,5-dinitrophenyl nitronyl nitroxide, which enters a presumably ferromagnetically ordered state below 1.1 K. Perhaps, the most interesting finding of the present manuscript is the presence of two characteristic muon precession frequencies, observed in the measurements. The manuscript is nicely written and provides a very clear argumentation. I only have some minor remarks, which follow below.

The parameters alpha and beta that the authors get from the phenomenological critical exponents for the precession frequency and magnetization point toward the classification of the studied material as a three-dimensional Heisenberg ferromagnet. How is it consistent with the discussed one-dimensional ferromagnetic behaviour?

The authors discuss the temperature evolution of the frequencies for the two components they found in the µSR spectra, but do not mention how the partial contributions of these two components to the asymmetry function (i.e., their Ai) change with temperature. I believe this information may be helpful for understanding of the origin of the two precession frequencies.

Other remarks:

1. The representation of the crystal structure in Figure 1 is not optimal. Not only do the molecules overlap creating a crowded picture, but in addition, some molecules are “broken”, making an impression that isolated O atoms are present in the structure. Besides that, I believe that colour codes for the atoms would be helpful.
2. Both space groups P43 and P41 are of course tetragonal (hence the “4”), not orthorhombic.
3. The authors mention that they used an “array of very small crystals” for the measurements. Did these crystals have the same chirality and how was it checked? Also, I wonder if µSR studies could detect any differences between left-handed and right-handed crystals or their racemic mixture.
4. “This is close to the value expected to the three-dimensional Heisenberg model” – this wording sounds a bit off. Maybe “…the value expected in the three-dimensional…”?
5. “Monkhurst-Pack” should be “Monkhorst-Pack”.

Author Response

We thank the referee for their helpful comments.  We respond in detail below:

Question: The parameters alpha and beta that the authors get from the phenomenological critical exponents for the precession frequency and magnetization point toward the classification of the studied material as a three-dimensional Heisenberg ferromagnet. How is it consistent with the discussed one-dimensional ferromagnetic behaviour?

Answer: One-dimensional ferromagnets do not order, so as often happens in such systems the eventual ordering is three-dimensional.  Nevertheless, the interactions are predominantly one-dimensional (they are much stronger along one direction than the others) but to achieve long-range order the weaker interchain interactions must become important.

Question: The authors discuss the temperature evolution of the frequencies for the two components they found in the µSR spectra, but do not mention how the partial contributions of these two components to the asymmetry function (i.e., their Ai) change with temperature. I believe this information may be helpful for understanding of the origin of the two precession frequencies.

Answer: This information has now been added in the manuscript.

1. The representation of the crystal structure in Figure 1 is not optimal. Not only do the molecules overlap creating a crowded picture, but in addition, some molecules are “broken”, making an impression that isolated O atoms are present in the structure. Besides that, I believe that colour codes for the atoms would be helpful.

This figure has been altered in line with the referee's comments.

2. Both space groups P43 and P41 are of course tetragonal (hence the “4”), not orthorhombic.

Corrected.  Thank you.

3. The authors mention that they used an “array of very small crystals” for the measurements. Did these crystals have the same chirality and how was it checked? Also, I wonder if µSR studies could detect any differences between left-handed and right-handed crystals or their racemic mixture.

They were a racemic mixture, but µSR would not be able to detect the difference between the different handedness.  This has now been mentioned.

4. “This is close to the value expected to the three-dimensional Heisenberg model” – this wording sounds a bit off. Maybe “…the value expected in the three-dimensional…”?

Corrected to "expected for the..."

5. “Monkhurst-Pack” should be “Monkhorst-Pack” 

Corrected

 

 

Reviewer 2 Report

This paper described  results of a muon-spin rotation study of  3,5-dinitrophenyl nitronyl nitroxide (DNPNN).  By reviewing as a whole, in my opinion this manuscript will consider for publication in Magnetochemistry after the minor revisions. My detailed comments are given as follows:

1) There is no any information about preparation of sample.

2) More technical parameters about the muon-spin experiment informations should be give.

3) Some abbreviations used in equations are not explained.

4) Please consider show the Fourier Transform -mSR spectrum.

5) I don't see conclusion paragraph.

6) Almost all of the cited literature is older than 20 years and more, please consider update.

Author Response

We thank the referee for their comments.

1) There is no any information about preparation of sample.

The sample preparation information is given in reference 21.  We now explicitly refer to this.  

2) More technical parameters about the muon-spin experiment informations should be give.

We do not which technical parameters the referee suggests we include here, but further information on the technique is available in references 22-25.

3) Some abbreviations used in equations are not explained.

We have improved this.

4) Please consider show the Fourier Transform -mSR spectrum.

The Fourier transform of the data is not so informative and so we did not include it.  

5) I don't see conclusion paragraph.

We have included one now.

6) Almost all of the cited literature is older than 20 years and more, please consider update.

A lot of the foundational work on nitronyl nitroxides is from the 1990s and so this is not surprising.  However, 20% of our references are from the last 10 years.

Reviewer 3 Report

The authors report on the results of muon-spin rotation study of magnetic properties in achiral organic radical dinitrophenyl nitronyl nitroxide (DNPNN). Confirming long-range ordering is proposed as the main result. In addition, two oscillatory components with different temperature dependences were found and their origin was discussed.

The authors studied an attractive object, indeed, compounds representing purely organic ferromagnets are rare and tuning magnetic interactions in them is a nontrivial task. Authors used sound experimental technique and the obtained data enable analysis as presented in the manuscript.

Generally, the manuscript is well organized and written. Nevertheless, the significance of the presented results still might be questioned. The studied compound is not new, previous studies of thermodynamic properties (Ref. 21) already unambiguously proved the existence of long-range ordering at T_C = 1.1 K, suggested ferromagnetic order, the existence of S = ½ degrees of freedom and estimated the magnitudes of intrachain and interchain exchange interactions, respectively. Thus, as the authors state, the demonstration of long-range order in the studied system below 1.1 K in DNPNN as the main result of their study, does not bring new physical information. On the other hand, the authors found very interesting behavior of the two oscillatory components. Attributing their different temperature dependences to the temperature dependence of magnetic structure is plausible. Although physical mechanism(s) responsible for the proposed temperature dependence remain unclear the authors very systematically considered potential reasons of the observed behavior. In my opinion, the main value of the presented results is just in the new features which could not be deduced from the study reported in Ref. 21. The authors could consider tuning the corresponding sentences in   manuscript accordingly.

The authors compare the value of the critical exponent β with that for O(4)model (Ref. 34). However, n>=4 vector models require doubling of the unit cell in one or more directions at the critical temperature. Are there reasons to assume such a behavior in the studied system?

Dzyaloshinskii-Moryia interaction was considered as one of the potential sources of the observed temperature dependences. Does the symmetry of the structure enable to anticipate Dzyaloshinskii-Moryia interaction?

In summary, the presented results and the related discussion ultimately deserve publishing, after devoting attention to the aforementioned remarks the manuscript may be recommended for publication in Magnetochemistry.  

Author Response

We thank the referee for a thoughtful reading of our paper.

The referee comments that, in their opinion, "the main value of the presented results is just in the new features which could not be deduced from the study reported in Ref. 21. The authors could consider tuning the corresponding sentences in   manuscript accordingly."  While it is true that the present study confirms the conclusions of Ref. 21, the use of the muon as a local probe confirms that the sample orders throughout its bulk (something that is hard to be sure of with thermodynamic probes).  We have stressed this point more strongly in the manuscript.

The referee notes our comparison of the value of the critical exponent β with that for O(4)model but points out that "n>=4 vector models require doubling of the unit cell in one or more directions at the critical temperature" and asks whether "there reasons to assume such a behavior in the studied system?"  The answer is no, but it is important to exclude this possibility if at all possible.

The referee also asks whether "the symmetry of the structure enable to anticipate Dzyaloshinskii-Moryia interaction?"  The structure would allow this but it depends on which route the exchange pathway would go.  There would need to be an intermediate atom or group through which a DM interaction could be mediated.  Moreover, as we already stated in the manuscript, the low atomic number of all the atoms in this material seem to imply that spin-orbit interactions (which would underpin a DM mechanism) are probably too weak for this to be important.

Reviewer 4 Report

The authors of manuscript entitled: "The internal field in a ferromagnetic crystal with chiral molecular packing of achiral organic radicals" presented very interesting studies of a muon-spin rotation studies for chiral crystals consisting of achiral DNPNN organic radicals. Analysis of these data demonstrated that DNPNN shows long-range magnetic ordering with critical temperature below 1.10(1) K. Moreover, this material also reveals the two precession frequencies with two different temperature dependences. These extensive studies are a valuable contribution for scientists working on molecular magnetism. Therefore, this paper is suitable for publication in Magnetochemistry journal.

Author Response

We thank the reviewer for these very generous comments.