Peer review of: Giant voltage-controlled modulation of spin Hall nano-oscillator damping (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

The manuscript by Fulara et al. describes an important experimental advance – control of spin-orbit 
torque oscillator dynamics by gate voltage. Gating of spin-orbit torque oscillators enables control of 
frequency and damping of individual oscillators in coupled oscillator arrays, which are used for 
neuromorphic signal processing. The manuscript reveals that voltage-controlled magnetic anisotropy 
(VCMA) can strongly modulate the effective damping parameter of the oscillator. Comparison of the 
data to micromagnetic simulations reveals that this giant modulation of damping is due to VCMA-
controlled mode confinement, which strongly affects the component of damping due to radiation of 
magnons into magnetic leads of the nano-constriction oscillator. The manuscript is well written and 
the conclusions of the paper are strongly supported by the experimental data and micromagnetic 
simulations. I recommend this paper for publication in Nature Communications if the authors address 
two comments below: 

- Figure 3d shows a convincing agreement between the gate voltage dependence of the measured and 
simulated values of the oscillator effective damping parameter. For better understanding of the 
underlying physics, the authors should also present a similar comparison of the measured (e.g. Fig. 2f) 
and simulated oscillator frequency shift versus gate voltage. 

- It is useful to benchmark the magnitude of VCMA (in e.g. fJ/Vm) in the oscillator system studied 
here to VCMA in other materials systems (e.g. T. Nozaki et al., APL Mater. 8, 011108 (2020); Y.J.Chen 
et al., Nano Lett. 17, 572 (2017)). Is there room for further enhancement of VCMA tunability of the 
oscillator frequency and damping if different material combinations are used? 




Reviewer #2 (Remarks to the Author): 

In this work, Fulara et al demonstrate the control of the properties of spin Hall nano-oscillator by 
voltage-controlled magnetic anisotropy. Spin Hall nano-oscillators have recently emerged as an 
excellent building block for neuromorphic computing, and this type of control can provide exciting 
prospects. In fact, the idea of the current submission was already mentioned by the authors in their 
recent Nat Nano paper, Ref 14 (“A more useful approach would be direct voltage control of the 
magnetic properties in the nano-constriction region, such as magnetic anisotropy and/or 
damping33,34, which could tune the SHNO frequency and possibly also turn them on or off at will.”). 
This paper, therefore, follows up on that. 

Overall, the paper is focused on understanding the physics of the mechanism – neuromorphic 
computing and applications are only a context. The paper introduces a clear hypothesis for the origin 
of the effects, which is consistent with multiple ST-FMR and micromagnetic simulations. This analysis 
is the core contribution of the paper. 

Overall, I liked how the paper is constructed. The results are exciting but overall, not oversold. The 
discussion section adds useful information, and I appreciated how the authors provide leads for 
enhancing the tunability further. I think that the paper can be inspirational to many. 

Speaking of “giant” control of damping seems reasonable to me based on the results (although it will 
be interesting to see the opinion of more physics-oriented reviewers). On the other hand, I would not 
call the frequency tunability “strong” (50MHz, whereas the mean frequency is around 11GHz), so I 
recommend toning down this particular point throughout the paper (the result itself remains very 
interesting). 

I feel that the paper should discuss more extensively the advantages, and also the drawbacks of this 
structure with regards to spin torque ones. 

So that the paper is more approachable by a diverse audience and that the story stands out, I would 
recommend moving some of the details of the body text to the Methods section. This would also allow 
adding a few more sentences so that the basic principle of the structure can be understood without 
having to study the previous work of the authors. 



Reviewer #3 (Remarks to the Author): 

The manuscript “Giant voltage control of spin Hall nano-oscillator damping” by H. Fulara, M. 
Zahedinejad, R. Khymyn, M. Dvornik, S. Fukami, S. Kanai, H. Ohno, & J. Åkerman should be accepted 
for publication after a minor revision. 

The manuscript “Giant voltage control of spin Hall nano-oscillator damping” is devoted to the 
experimental and numerical/ theoretical study of the damping control in a nano-constriction spin Hall 
nano-oscillator (SHNO) due to the voltage-controlled magnetic anisotropy effect. The results obtained 
by the authors evidently show that DC gate voltage (from –2 V to +2 V) applied to the nano-oscillator 
can substantially change its threshold DC current required for the excitation of magnetization 
dynamics. The authors propose using the discovered effect to control the state of an individual SHNO 
in the oscillator arrays used for neuromorphic computing, which makes the obtained results to be 
relevant for the electronics of the future. 

The paper is technically sound and clearly written. All the main claims of the manuscript are basically 
understandably formulated and rather clearly explained. 
The paper conclusions are based on evident experimental and numerical results, simple but good 
theoretical description. 
The obtained results are novel; they pave a new way for the implementation of systems for 
neuromorphic computing. 
In general, the article complies with the standards used by the scientific community and is important 
for the field of applied physics and spintronics. 

However, I think several minor corrections should be made: 

1. Recently several review papers discussing neuromorphic computing have been published, namely: 
Sangwan, V.K., Hersam, M.C. Neuromorphic nanoelectronic materials. Nat. Nanotechnol. (2020). 
https://doi.org/10.1038/s41565-020-0647-z 
Grollier, J., Querlioz, D., Camsari, K.Y. et al. Neuromorphic spintronics. Nat. Electron. (2020). 
https://doi.org/10.1038/s41928-019-0360-9 
I recommend the authors to cite these papers in the manuscript. 

3. In my opinion, there is a small imperfection in the article: several similar abbreviations “DC”, “dc” 
and “RF”, “rf” are widely used in the manuscript. I suggest the authors to use any but single spelling 
of such abbreviations in the whole manuscript, for instance “DC” and “RF” or “dc” and “rf”. 

Despite of made remarks the reviewed paper has a good scientific quality. Its publication in the Nature 
Communications is advisable, if the corrections listed above will be made. 



Reviewer #4 (Remarks to the Author): 

The manuscript reports a large effect of electrostatic gating on the effective damping and the 
oscillation frequency in spin-Hall nano-oscillators (SHNO) based on ultrathin CoFeB ferromagnets, 
enabling efficient voltage-controlled tuning and on-off switching of SHNO's oscillation. Surprisingly, the 
large effect on damping can be explained by a very modest ~1% voltage-induced variation of 
magnetic anisotropy, which controls the spatial characteristics of the excited dynamical mode, and 
thus its relaxation due to the coupling to the propagating spin wave modes in the magnetic system. 
These are nice and somewhat unexpected results that are well-explained in the manuscript and 
reproduced by the simulations presented by the authors. In my opinion, these results represent a 
substantial contribution to our understanding of SHNO and the development of efficient and useful 
spin Hall nanodevices, and are likely to generate a significant interest in the broader spintronics 
community. Therefore, I recommend the manuscript for publication in Nature Communications, 
provided that the authors address the following minor suggestions: 
i) I would recommend that the wording of the title is changed. One could say that the voltage control 
is efficient, or perhaps that the effect of voltage on damping is giant, but in the present form the title 
is confusing. More generally, I recommend that the authors carefully proof-read the manuscript for 
grammatical errors and confusing phrases, to improve its readability and increase its impact on the 
research community. 
ii) The first sentence of the main text, “High-density and low-power neuromorphic computing requires 
a large number of mutually synchronized nanoscale spintronic oscillators…” implies that neuromorphic 
computing can be only implemented with spintronic oscillators, which I do not believe was the 
meaning of the sentence intended by the authors. It should probably be rephrased. 
iii) The initial discussion of the effects of voltage in terms of damping may be somewhat confusing to 
some readers, because it creates the impression that Gilbert damping is significantly varied, which is 
not the case. The later descriptions in the manuscript clarify this possible confusion, but I still believe 
that the relevant terminology needs to be somewhat modified to avoid confusion. As a possible 
alternative to the term “damping” used by the authors, one can use the term “relaxation rate of the 
excited dynamical mode”. Alternatively, this issue can be addressed by adding a sentence, early on, 
explicitly specifying that Gilbert damping is not significantly affected. 
iv) I do not believe that the use of the term “eigenmode” by the authors in the discussion of the 
excited dynamical mode is justified, because this dynamical mode is produced by the balance of 
strongly dissipative processes, and is not really an eigenmode of some Hamiltonian.