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Detailed summary of responses and changes: Article ID: Title: ROP/136737/REV Studies of nanomagnetism using synchrotron-based x-ray photoemission electron microscopy (X-PEEM) First referee's report Studies of nanomagnetism using synchrotron-based x-ray photoemission electron microscopy (X-PEEM) By Cheng and Keavney This review article gives an excellent overview of the strengths of XMCD-PEEM. A broad range of experiments are discussed comprising a range of 0D, 1D, 2D and hybrid magnetic systems. The article is well written and of benefit to both the novice and more expert reader. Recommendations: - The term nanomagnetism is generally used in a broader sense than for the purpose of this article. It may include for example molecular magnets and various forms of magnetic particles. I suggest therefore specifying the area of nanomagnetism in the Introduction. Because X-PEEM is most suitable for imaging solid materials in a planar geometry, we naturally focus on work involving patterned thin films. We do not mean to exclude other forms of nanomagnetism, but due to the nature of the technique we’re reviewing, this is a necessary choice. We have added “especially to the research of artificially nanoscaled magnetic materials of various dimensions,” in the beginning of the last paragraph of the “introduction” section, to better specify the area we are covering. - The classification of quasi-0D, 1D and 2D in section 2 is very appealing and worth considering to repeat in the layout of section 4. The layout of section 4 has been changed to be parallel with that of section 2, using the categories of quasi0D, 1D and 2D. - The introduction to magnetism is rather brief for the non-specialist reader, and I’d suggest to expand the introduction to include concepts as domain walls, soft versus hard magnetism, exchange length. The following is added to page 4 about domain walls and the exchange length: “A magnetic domain wall is the boundary between two adjacent domains. It is the transition region in which spins gradually reorient from the direction in one domain to that in the next. The typical domain wall width, the thickness of the transition range, in an unpatterned magnetic film is called the exchange length. The term exchange length is also used for the typical size of a magnetization vortex core [Ultrathin Magnetic Structures IV: Applications of Nanomagnetism, Volume 4 By B. Heinrich, J. Anthony C. Bland].” The following is added to page 4 about soft and hard magnetism: “Along the preferred magnetization direction, the field needed to magnetize the magnetic material to saturation is smaller than any other directions, so this preferred direction is called the easy axis. Soft magnetic materials are those in which the magnetization processes (domain wall motion and domain magnetization rotation) occur in weak fields, while hard magnetic materials refer to the materials that require high magnetic field for magnetization reversal [30].” - Explain symbols in eq. 3. “where M is the vortex mass tensor, G is the gyrovector, and W(X) is the energy of the vortex deflected from the disk center.” is added - At times arguments are repeated in too much detail throughout the paper, like the benefit of element selectivity and spatial resolution. The authors feel it is worthwhile to emphasize some of the important features of PEEM by repeating from various perspectives. - The end of section 3.2 is a general argument rather than specifically about time resolved studies, and is much suited for introduction. The section number should be 3.4 and it was corrected. The end paragraph is not for section 3.4, but for the entire section 3(PEEM). It serves as a transition between sections 3 and 4. So we prefer to keep it at the end of section 3. Typos and minor suggestions: - Add square dot in Fig. 2 A square dot picture has been added to Fig. 2 - Page 4: qusi-0D --> quasi-0D We have fixed this error. - Page 5: Explain race track memory We have added the following text to the description of Figure 3: In this proposal, information is stored in the form of magnetic domains in a permalloy of 100 – 200 nm width. The domains are moved through the wire using the spin-torque effect by passing current pulses through the wire. This may be realized in a planar (Fig. 3(a)) or vertical (Fig. 3 (b)) geometry. As the domains pass over a magnetoresistive detector (most likely a magnetic tunnel junction), a domain-orientation-dependent signal is generated, allowing readout of the information (Fig 3 (c)). Writing is accomplished by pulsing a current line under the bit to be written, switching its orientation (Fig 3(d,e)). The result is a non-volatile shift register, with potentially high storage density. - Page 7: explain spin valve We have edited this discussion as follows: In the mid-1990s, an exchange bias layer was used in GMRbased spin valve read head. These heads relied on the magnetoresistance arising from two coupled ferromagnetic layers through a nonmagnetic spacer. Exchange biased layer were used to pin one layer, while the free layer flipped in response to the stray field from the disk medium. - Page 7: short-rang --> short-range We have fixed this error. - Page 14: Fig 10 (c)-(e) --> Fig 10 b. We have fixed this error. - Page 16: explain cross-tie domain A reference for cross-tie domain wall to [29 O’Handley R C Modern Magnetic Materials Principles and Applications (Wiley-Interscience Publication, New York, 2000).] is added. - Figure 16 and discussion is unclear The following explanation added to page 19: “Before the current pulse, the magnetization is aligned along the wire and no domain walls are present, resulting in an almost homogeneous XMCD intensity (figure 16 (a)). The x-ray beam direction is parallel to the magnetization direction before the current pulses in the bending parts of the nanowire. During the pulses, the NiFe magnetization tilts away from the wire direction. The tilt is counter-clockwise (figures 16 (b)-(f)) for the positive part of the bipolar current pulse and clockwise (figures 16 (g)-(i)) for the negative part, as can be inferred from the magnetic contrast in the differently oriented sections of the wire. The tilt angle jt extracted from the time-dependent XMCD intensity in the bending parts of the nanowire is plotted in figure 16 (j). This is expected because the Oersted field generated by the current pulse is in opposite directions transverse to the wire for opposite current directions. The oscillations in jt at the beginning of the positive and negative parts of the pulse indicate magnetization precession about the effective Oersted field.” - Page 28: repeat of FEL, Hamburg These are not actually redundant, one is the name of facility (FLASH), one is the location (Hamburg). For a little beter readability, we reworded this to: “Two such facilities currently exist, the soft x-ray Free Electron Laser in Hamburg (FLASH) at Hasylab, Hamburg, Germany,…” Article ID: Title: ROP/136737/REV Studies of nanomagnetism using synchrotron-based x-ray photoemission electron microscopy (X-PEEM) Second referee's report Studies of nanomagnetism using synchrotron-based x-ray photoemission electron microscopy (X-PEEM) by X M Cheng1 and D J Keavney2* The manuscript represents a well though out summary of the work in the field of nanomagnetism as studied by PEEM. The only general comment is that I would prefer more extended discussion on some of the topics covered. On number of places the statements are given without description of how they are obtained or what they are based on. In number of topics one is forced to go to the original manuscript to fully understand the conclusions. Clearly, many of the studies summarized are complex and require more explanation to be fully comprehended. Overall, the manuscript represent a comprehensive summary of the state of the field that will be useful reference and should be published in the Reports on Progress in Physics after some minor issues listed below are addressed. page. 9 in the sentence below the “figure 7” should be Fig. 1 (d): “One example is a multilayer stack of alternating hard and soft ferromagnetic (FM) rings separated by a nonmagnetic Cu ring [77, 78] (as shown in figure 7),” We have fixed this error. page11. “may be measure” should read may be measured We have fixed this error. figure 8 needs to be more clear, what are the Auger electrons and what are the secondary electrons mentioned in the text, needs better description We have added the following to the discussion of Fig 8: These Auger electrons typically scatter before escaping from the surface of the sample, resulting in secondary electrons. the difficulty with K-edge PEEM magnetic imaging is not due to low flux but rather to very small XMCD effect We do not make the claim that K edge PEEM is difficult due to low incident flux alone, rather that oxygen K, transition metal K, and semiconductor L edges are difficult due to low XMCD signal. At TM K edges in particular, the absorption cross sections are smaller, resulting in a smaller interaction volume, and the electron yield relative to fluorescence yield is smaller. However, in principle these low signals could be overcome with greater flux, so there is some validity to the statement that PEEM at these edges is flux limited. We have reworded this sentence slightly to reflect this point: “For PEEM imaging, however, the small XMCD signals inherent at these transitions makes it difficult to obtain sufficient signal to noise with the photon flux density presently available at typical sources.” page 12 obtaining magnetic PEEM images by dividing L3 with L2 image contains other complexities that need to be mentioned We have added the following to the discussion of image generation: “However, obtaining magnetic PEEM images by dividing L3 with L2 image doers not completely remove the charge contrast, therefore adding complexity to the normalization of the images.” need better description of XMLD, what is “average electric field vector” ? “XMLD effect is only sensitive to the angle between the atomic moments and the average electric field vector” We have edited this discussion and changed “average electric field vector” to “photon electric field vector”, and added θ to the definition of the angle between the magnetization and the electric field vector. This should make more clear the quantities being discussed. page 13 explain why ~100ps and ~100nm or less are of interest: “Typically, it is desired to study the influence of fast field transitions that at least approach the time resolution available (~100ps), and at the spatial resolution available from PEEM (~100nm).” We have reworded this to read: “To make the best use of the available time resolution, it is desirable to match the speed of the pump to that of the probe (~100ps).” “because time-resolved X-PEEM has the advantages of both the high spatial resolution and the high temporal resolution.” What are the disadvantages of X-PEEM, are there other time-and-space resoving imaging techniques and how do they compare to X-PEEM ? It is unclear where the referee is taking this quote from, however have added a short discussion of PEEM vs Ker microscopyr and Transmission X-ray Microscopy to the previous section (3.1): An additional characteristic of X-PEEM is the high surface sensitivity that is inherent with the detection of secondary electrons. In most materials, the low kinetic energy of the secondaries (typically <10 eV) limits the electron escape depth to 2 – 5 nm. This can be an advantage in some cases, however it can also limit the range of samples that can be studied and introduce uncertainties due to incomplete knowledge of the surface condition. If thick capping layers are present, it may be impossible to obtain a usable signal from the later of interest. Conversely, if the sample is not capped but a thick oxidation layer has formed, this can reduce the magnetic contrast dramatically. Other magnetic imaging techniques can get around some of these problems, even in time-resolved imaging, although with some drawbacks. Kerr microscopy offers very high sensitivity and probing depths of ~50 nm or greater. However, it lacks element selectivity and the spatial resolution is limited by the diffraction in the visible light optics, so imaging in the sub-micron regime is very difficult. Transmission x-ray microscopy can be accomplished in the soft x-ray regime, offering high sensitivity to the entire thickness of the sample and element selectivity. In addition, the spatial resolution is superior to non-aberrationcorrected PEEM, with a current state of the art of 15 nm as of this writing [add ref to H. Jung et al, APL 97, 222502 (2010).]. The principle disadvantage here is that the entire sample must be thin enough to allow the soft x-ray beam through, or typically in the range of only 100 nm. This requires that the samples be grown on special membranes, which are fragile and may preclude samples that require high deposition temperatures or other processing. Page 19 “demonstrated that the exchange bias occurred on a domain-by-domain basis, confirming its microscopic origins” Explain how did they conclude this, explain what is domain-by-domain exchange bias ? We have replaced the above sentence by “As shown in figure 17, the magnetic contrast in figure 17 (a) arises from antiferromagnetic domains in LaFeO3 with an in-plane projection of the antiferromagnetic axis oriented horizontally (light) and vertically (dark). The three distinct grey scales in figure 17 (b) corresponds to ferromagnetic domains in Co aligned vertically up (black) and down (white), and horizontally left or right (grey). Comparison of the in-plane projections of the antiferromagnetic axis and the ferromagnetic spin direction in each individual domain shows that the ferromagnetic Co spins are aligned parallel or anti-parallel to the in-plane projection of the antiferromagnetic axis.” page. 20 the XMLD signal is anisotropic by definition, it would be better to say “the sign of the XMLD signal” instead of “the anisotropy of XMLD signal”. Preferably, it requires more detailed statement on what is actually meant here as indicated at the end of the paragraph but not related to term “the anisotropy of XMLD” We have reworded “the anisotropy of XMLD signal” to “the dependence of the XMLD signal on the relative orientation of polarization, magnetic moments, and crystallographic axes”, “the angular dependence of the XMLD signal”, or “the dependence of the XMLD signal on the experiment geometry” remove “the” in : “uncompensated Mn spins and the at the CoFeB/MnIr interface” We have fixed this error. Page. 22 Section on “Studies of spin reorientation transitions in coupled magnetic multilayers” is confusing, especially the Figure 20, the text sates that the Ni moment does not change in magnitude but figures shows different shades of Ni spin moment ? The contrast of the PEEM image reveals the component of the magnetization projected to the x-ray propagation direction, which is determine by the magnetic moment and the angle between the magnetization in each domain and the x-ray direction. To make it clearer,we have reworded the Ni spin moment part as follows: “Taking into account the different grey scales in figure 20 (a), representing projections of the different in-plane and perpendicular spin moments in different domains, and the spin orientations in these domains, the authors calculated a constant effective Ni spin moment of about 0.65 μB, similar to the Ni bulk magnetic moment.” Page. 23 What is the meaning of white and black horizontal/vertical arrows at the bottom of the Fig.22 ? “The grey scales of the PEEM images indicate different magnetization directions: parallel (white), antiparallel (black), or perpendicular (grey) to the x-ray propagation direction.” is added to the caption of figure 22. Pg.25 It would be useful to state what is the “low-energy states” configuration in these structures Also the label of “out-of-plane” and “in-plane” axis is confusing as H appears to be allied in plane in both cases. “The low energy state for the one- ring kagome structure is the vortex state; the low energy states for the two-ring kagome structure include the double-vortex state and the external-flux-closure state; and the low energy states for the three-ring kagome structure include the ground state with two vortices and the external-flux-closure state [86].” is added to explain the “low-energy states” The “out-of-plane” and “in-plane” axis refer to the sample rotation axis in the demagnetization process, not the applied field direction, as specified in the figure caption for figure 25 “(a) Geometries for demagnetization with in-plane and out - of- plane rotation axes” Pg.26 When using brighter synchrotron sources (4th generation) are there any concerns of space charge limitation, there have been some discussion on the topic in the recent literature. Space charge effects should not be a concern for the storage ring based short pulse generating schemes discussed in this section, as they do not involve higher peak flux than a standard 3 rd generation source. For FELs, this may be a concern, so we discuss that in section 5.3, as indicated below. The statement “In most cases, the effective limit for the resolution is 2-5 nm.” should be supported with a reference or/and argument on which this is based. It is based on the mean free path of electrons in the material. We have edited last sentence to make more this clear: “In most cases, this limit for the resolution will therefore be 2-5 nm.” It would be informative to a reader what is the state of SMART and PEEM-3 project and what their target resolution. Added the following text to the end of section 5.1: The performance goal of the SMART PEEM/LEEM instrument is 2 nm spatial resolution, with 2.6 nm recently being demonstrated in LEEM mode [add ref to Th. Schmidt, et al., Ultramicroscopy 110, 1358 (2010)]. The PEEM-3 design goal is ~5 nm, currently it operates at 50 nm or better without aberration correction, with the addition of the correcting mirror planned for the future. Pg. 29 What about space charge limitation on PEEM experiments with FEL sources ? Since there have been no PEEM experiments at FELs so far, there is no work to reference here. However, there has been work using ultrafast UV laser pumped PEEM, which is relevant to potential FEL experiments. We have added the following discussion on this: “An additional complication that can potentially arise from the very high instantaneous flux densities present at FEL sources is image degradation due to space charge effects. Coulomb repulsion within the cloud of emitted photoelectrons will broaden the cloud in real space and in energy distribution. This leads to greater chromatic aberrations and general image blurring. The degree of spatial resolution degradation will depend on the peak flux, and naturally the threshold where these effects become important will depend on the baseline spectral resolution. In aberration corrected PEEMs, these effects will be of much more concern. Studies of space charge effects in PEEM using 4th generation soft x-ray sources have not been conducted as of this writing, however these phenomena have been explored using ultrafast UV lasers. Recently, Buckanie, et al. [Buckanie N M, Göhre J, Zhou P, von der Linde D, Horn-von Hoegen M, Meyer zu Heringdorf F-J 2009 Space charge effects in photoemission electron microscopy using amplified femtosecond laser pulses J. Phys.: Condens. Matter 21 314003] used 200 and 800-nm femtosecond laser pulses to examine both the threshold of image degradation of a Ag island and the location in the electron microscope column where space charge effects are most critical. They found that for 200 nm photons, which are above the work function of the Ag island, significant energy broadening begins to occur at ~3μW average laser fluence, corresponding to ~3 × 107 photons/pulse. Therefore, given the 1012 – 1013 photons/pulse available at an FEL, it is likely that space charge would present an even greater problem for PEEM experiments than sample damage.