Element Selected Spin-Dependent d-d Charge Transfer Transitions in Bi2FeMnO6 Film: An Ultrafast Pump-Probe Study
Round 1
Reviewer 1 Report
Authors investigated the temperature dependence of the transient reflectivity in a 150-nm-thick Bi2FeMnO6 film deposited on a silicon substrate. They found that the amplitude and decay times of the reflectivity change signal depend on both the temperature and pump photon energy. The results suggest a new aspect of the optical transition and spin-lattice coupling in Bi2FeMnO6 compounds. Accordingly, this manuscript is valuable for readers in related research fields.
On the other hand, I think that the manuscript needs some improvement in the following four points. After the author has improved the manuscript accordingly, the publication can be recommended.
1) In the last paragraph of Section 3, the authors discuss the temperature dependences of the decay times of the fast and slow components shown in Fig. 4. Authors explain that the electro-phonon interaction time does not vary with the temperature”. However, I can see the electro-phonon interaction time [Fig. 4(a)] obviously changes with temperature in addition to the pump photon energy. The authors should modify the explanation in related sentences.
2) In Section 3 on Page 3, the signal amplitude is discussed. The authors attribute the temperature dependence at 1.55 eV pump to “the blue-shift effect of d-d transition energy gap”. Does it mean the effect of the temperature-dependent shift of the absorption band? [Please refer e.g., the last section of the article: Appl. Sci. 2019, 9(4), 704] Some clarification is needed.
3) In Fig. 1(a), the authors describe the steady-state reflectivity spectrum in silicon. I wonder that the reflectivity is zero in an opaque wavelength range. Does the silicon sample have a surface structure [Please refer e.g., Appl. Phys. Lett. 2008, 93(9), 091106]? Or, have you measure the reflectivity spectrum at the Brewster angle?
4) (On page 4, line 128) What does it mean “signal size”? Does it mean the amplitude at zero time delay or the area of the reflectivity change signal? A longer decay time results in a larger signal area.
Author Response
Reviwer 1
Authors investigated the temperature dependence of the transient reflectivity in a 150-nm-thick Bi2FeMnO6 film deposited on a silicon substrate. They found that the amplitude and decay times of the reflectivity change signal depend on both the temperature and pump photon energy. The results suggest a new aspect of the optical transition and spin-lattice coupling in Bi2FeMnO6compounds. Accordingly, this manuscript is valuable for readers in related research fields.
On the other hand, I think that the manuscript needs some improvement in the following four points. After the author has improved the manuscript accordingly, the publication can be recommended.
1) In the last paragraph of Section 3, the authors discuss the temperature dependences of the decay times of the fast and slow components shown in Fig. 4. Authors explain that the electro-phonon interaction time does not vary with the temperature”. However, I can see the electro-phonon interaction time [Fig. 4(a)] obviously changes with temperature in addition to the pump photon energy. The authors should modify the explanation in related sentences.
Response: Thanks for the valuable comments. The explanation of the electron-phonon interaction times in Fig. 4(a) has been modified in our revised manuscript.
2) In Section 3 on 6Page 3, the signal amplitude is discussed. The authors attribute the temperature dependence at 1.55 eV pump to “the blue-shift effect of d-d transition energy gap”. Does it mean the effect of the temperature-dependent shift of the absorption band? [Please refer e.g., the last section of the article: Appl. Sci. 2019, 9(4), 704] Some clarification is needed.
Response: Thanks for the valuable comments. The blue-shift of the absorption band with the temperature is related to the emergence of the magnon and the optical phonon side bands which means the effect of the temperature-dependent shift of the absorption band in the article: Appl. Sci. 2019, 9(4), 704, which is clarified in our revised manuscript.
3) In Fig. 1(a), the authors describe the steady-state reflectivity spectrum in silicon. I wonder that the reflectivity is zero in an opaque wavelength range. Does the silicon sample have a surface structure [Please refer e.g., Appl. Phys. Lett. 2008, 93(9), 091106]? Or, have you measure the reflectivity spectrum at the Brewster angle?
Response: Thanks for the valuable comments. The reflectivity of our experiment is measured by a relatively absorption setup, the reflectivity is get from the comparison with a standard reflector, so the zero reflectivity is a relative one rather than a precise value. For our experimental setup, the light source is non-polarized light, so the reflectivity spectrum is not related to the Brewster angle in our results.
4) (On page 4, line 128) What does it mean “signal size”? Does it mean the amplitude at zero time delay or the area of the reflectivity change signal? A longer decay time results in a larger signal area.
Response: Thanks for the valuable comments. The amplitude at zero time delay is clarified in our revised manuscript.
Author Response File: 
Author Response.docx
Reviewer 2 Report
# Comments for the Authors In the manuscript "Element selected spin-dependent d-d charge transfer transitions in Bi2FeMnO6 film: An ultrafast pump probe study", Jia and report the experimental characterisation of the optical response of BFMO. By using a transient reflectivity spectroscopy at different temperatures, the authors investigate a range of optical stransitions associated with the magnetic phase of the sample. Overall, I found the article to be quite interesting and well written, and it provides useful hints in the properties of multiferrois. As such, I recommend publication after minor revision, once the authors have addressed the following remarks: * How did the authors verify that the deposited BFMO substrates are indeed single-phase crystalline? Does the lattice mismatch between BFMO and Si (especially at different doping ratios) affect the deposition process? * Have the authors performed a reference measurement of the Si substrate withouth BFMO? Does the interface between BFMO and Si affect the results? * The substrates are quite thin (150nm), I wonder if the surface field at the two interfaces (BFMO/air and BFMO/Si) could affect in any way the observed results. This could be verified, for example, by comparing substrates with different thicknesses or by providing an estimation of how the thickness can affect (or not affect) the measurements. Could the authors comment on this? * Minor comment: some of the labels in the figures seem too small and hard to read in a printed version of the manuscript. I would suggest the authors to improve readibility of axes and figure labels. Figures 4 and 5 seem also stretched along the horizontal direction.Author Response
Reviwer 2
# Comments for the Authors In the manuscript "Element selected spin-dependent d-d charge transfer transitions in Bi2FeMnO6 film: An ultrafast pump probe study", Jia and report the experimental characterisation of the optical response of BFMO. By using a transient reflectivity spectroscopy at different temperatures, the authors investigate a range of optical stransitions associated with the magnetic phase of the sample. Overall, I found the article to be quite interesting and well written, and it provides useful hints in the properties of multiferrois. As such, I recommend publication after minor revision, once the authors have addressed the following remarks:
* How did the authors verify that the deposited BFMO substrates are indeed single-phase crystalline? Does the lattice mismatch between BFMO and Si (especially at different doping ratios) affect the deposition process?
Response: Thanks for the valuable comments. The structure of the BFMO film is still interesting for the magnetic properties, the double perovskite or random BFO-BMO structure for a lattice are possible. For our experiment, the macroscopic signal is emphasized for the relatively large optical area than the phase boundary in the film. The lattice mismatch between BFMO and Si could affect the deposition process, it could be concerned that for different substrate the magnetic and the optical properties would be different.
* Have the authors performed a reference measurement of the Si substrate withouth BFMO? Does the interface between BFMO and Si affect the results?
Response: Thanks for the valuable comments. The ultrafast reflectivity signal of Si is common for a ultrafast experiment. A typically long process for a indirect band gap material could be obtained which could be referred to (PHYSICAL REVIEW B 66, 165217 2002) which indicates a long carrier dynamics about 100ps. The interface between BFMO and Si could affect the results for different substrate, however the interface could not affect the results in our experiment for the temperature dependence of the magnetic and optical properties is dominated by the BFMO films.
* The substrates are quite thin (150nm), I wonder if the surface field at the two interfaces (BFMO/air and BFMO/Si) could affect in any way the observed results. This could be verified, for example, by comparing substrates with different thicknesses or by providing an estimation of how the thickness can affect (or not affect) the measurements. Could the authors comment on this?
Response: Thanks for the valuable comments. The BFMO/air and BFMO/Si interfaces could affect the photoconductivity of the sample, which is due to the emerge of the depletion layer in the interfaces, which have an similar situation in our previous paper (APPLIED PHYSICS LETTERS 111, 152906 (2017)) . However, the 800nm probe signal could be dominated by the d-d transfer of the sample rather than the depletion layer signal which is sensitive to the terahertz probe. The thicknesses of the film can affect the measurements, however, for a BFO film thicker than 100nm, the strain induced by the mismatch of the substrate could rarely affect the structural of the BFO films which is similar to our sample. (Physical Review B 92 (7), 075310)
* Minor comment: some of the labels in the figures seem too small and hard to read in a printed version of the manuscript. I would suggest the authors to improve readibility of axes and figure labels. Figures 4 and 5 seem also stretched along the horizontal direction.
Response: Thanks for the valuable comments. The figures is optimized in our revised manuscript.
Author Response File: Author Response.docx
Reviewer 3 Report
I read the manuscript "Element selected spin-dependent d-d charge transfer transitions in Bi2FeMnO6 film: An ultrafast pump-probe study". I found that the experimental data shown here are often below the level of contemporary condensed-matter physics. The followings are the main points. I do not recommend publication in the present form.
The paper PHYSICAL REVIEW B 91, 054421 (2015) "Synthesis and magnetic properties of double-perovskite oxide La2MnFeO6 thin films" should be cited.
The XRD and SQUID data of the thin film should be shown. In the present form, it is hard to see the sample properties.The XMCD study will be also quite useful.
To support the energy diagram in Fig. 1 (b), band-structure calculation and/or photoemission spectroscopy should be performed.
By using the time-resolved MOKE, the authors will be able to study magnetization dynamics, which will be more interesting than just time-resolved reflectivity.
Author Response
Reviwer 3
I read the manuscript "Element selected spin-dependent d-d charge transfer transitions in Bi2FeMnO6 film: An ultrafast pump-probe study". I found that the experimental data shown here are often below the level of contemporary condensed-matter physics. The followings are the main points. I do not recommend publication in the present form.
The paper PHYSICAL REVIEW B 91, 054421 (2015) "Synthesis and magnetic properties of double-perovskite oxide La2MnFeO6 thin films" should be cited.
Response: Thanks for the valuable comments. The magnetic properties of LMFO is cited in our revised manuscript.
The XRD and SQUID data of the thin film should be shown. In the present form, it is hard to see the sample properties. The XMCD study will be also quite useful.
Response: Thanks for the valuable comments. The XRD and SQUID data of the thin film is indeed very visualized for the phase transition during the temperature changes, while the key point in our results is the ultrafast electron-lattice-spin coupling process in the film and the optically selective transition process for Fe and Mn ions during the magnetic phase transition process.
To support the energy diagram in Fig. 1 (b), band-structure calculation and/or photoemission spectroscopy should be performed.
Response: Thanks for the valuable comments. The band-structure calculation is useful to our deduction of the optically selective transition process while our experimental results for the statistic and ultrafast 800nm pump-probe spectroscopy is reasonable to inference our element selected transition for different optical wavelength pump.
By using the time-resolved MOKE, the authors will be able to study magnetization dynamics, which will be more interesting than just time-resolved reflectivity.
Response: Thanks for the valuable comments. Time-resolved MOKE is more related to the ultrafast spin control process and must be more interesting. The MOKE experiment have been published in the other paper which have little relation to our results presented in our manuscript.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
I read the revised manuscript "Element selected spin-dependent d-d charge
transfer transitions in Bi2FeMnO6 film: An ultrafast pump-probe study".
I suggest that magnetic information should be at least mentioned by citing suitable papers.
The authors write "The
MOKE experiment have been published in the other paper".
What is "the other paper"? This should be cited.
After such a revision, I can recommend publication.
Author Response
Thanks for the valuable comments. The MOKE experiment of the BFMO film, has been published for the paper "Ultrafast spin polarization in a multiferroic manganite BiFe0.5Mn0.5O3 thin film", EPL (Europhysics Letters) 112.3 (2015): 37007.
