Review of Helical Magnetic Structures in Magnetic Microwires

Round 1

Reviewer 1 Report

The manuscript titled “Review of helical magnetic structures in magnetic microwires” reviews the last findings in the field of helicoidal magnetic structures hosted in magnetic nanowires. The studies developed from the 90’s up to the more recent ones are reported, focusing on the different kinds of magnetic structures and on the fundamental parameters necessary for defining them, such as the spiral length and domain walls geometry. The review reports in detail the studies developed by the Magneto-optical Kerr effect (MOKE) technique that allows probing the topography of the magnetic structure, the specific magnetization signal along the nanowires and its dynamics. The review is complete and clearly written. 

 

I recommend it for publication after having improved the following aspects:

 

1.     Please define GMR and GMI terms when first mentioned.

2.     In line 108 the annealing temperature should be explicitly mentioned. Further, it should be made clear if it is the same or changes when mentioned in the text. 

3.     Please define “Hax” where it is first mentioned (line 122, if I read well).

4.     Caption Fig. 9: change ‘tree’ with ‘three’.

5.     There are plenty of typos in the sentence on lines 521-523. I also invite the authors to spell-check the whole manuscript in detail. 

6.     Please improve the resolution of Fig. 3.

7.     Finally, I invite the authors to consider the field of magnetic racetrack memories as a possible application for magnetic nanowires. In this case, magnetic domain walls trigger a new and efficient way of magnetic memory storage, please see the pioneering work of Stuart Parkin [DOI: 10.1126/science.1145799] and the following ones. I think that including this topic in the introduction could provide a broader vision to the reader of the possible and relevant application fields of magnetic nanowires. 

 

Author Response

Comments and Suggestions for Authors

The manuscript titled “Review of helical magnetic structures in magnetic microwires” reviews the last findings in the field of helicoidal magnetic structures hosted in magnetic nanowires. The studies developed from the 90’s up to the more recent ones are reported, focusing on the different kinds of magnetic structures and on the fundamental parameters necessary for defining them, such as the spiral length and domain walls geometry. The review reports in detail the studies developed by the Magneto-optical Kerr effect (MOKE) technique that allows probing the topography of the magnetic structure, the specific magnetization signal along the nanowires and its dynamics. The review is complete and clearly written. 

 

I recommend it for publication after having improved the following aspects:

 

1. Please define GMR and GMI terms when first mentioned.

The GMR and GMI terms have been defined.

2. In line 108 the annealing temperature should be explicitly mentioned. Further, it should be made clear if it is the same or changes when mentioned in the text. 

“The temperature, T_ann, inside the furnace was set to 350 °C.”

3. Please define “Hax” where it is first mentioned (line 122, if I read well).

“Hax” has been defined.

4. Caption Fig. 9: change ‘tree’ with ‘three’.

Caption Fig. 9 has been changed.

5. There are plenty of typos in the sentence on lines 521-523. I also invite the authors to spell-check the whole manuscript in detail. 

The manuscript has been spell-checked.

6. Please improve the resolution of Fig. 3.

The resolution of Fig. 3 has been improved.

7. Finally, I invite the authors to consider the field of magnetic racetrack memories as a possible application for magnetic nanowires. In this case, magnetic domain walls trigger a new and efficient way of magnetic memory storage, please see the pioneering work of Stuart Parkin [DOI: 10.1126/science.1145799] and the following ones. I think that including this topic in the introduction could provide a broader vision to the reader of the possible and relevant application fields of magnetic nanowires. 

The subject of magnetic racetrack memories has been added:

One of the important ways of using magnetic structures in micro and nanowires is the “magnetic domain-wall racetrack memory”. This use was conceptually shown in a famous work [47] and then was developed in other works.

The idea was to use the controlled motion of domain walls in magnetic nanowires. The control was supposed to be carried out by the method of short pulses of a spin-polarized electric current. This was to allow the creation of a storage device with high performance and reliability. At the same time, the low cost of these storages was also assumed. A series of magnetic nanowires had to be created on a silicon basis. In this case, there should be spintronic reading and writing nanodevices that store a series of bits. This is the so-called "hippodrome memory”.

More than ten years of active research have allowed to get closer to the creation of model prototypes. For example, the article [48] mentions “chiral domain wall (DW) motions based racetrack memory”. Read-write heads are created with the help of tunnel junctions. A single layer (CoFeB) or multilayer (Co/Ni) magnetic nanowire can store information because the magnetic domains have different magnetizations (the "up" direction is "0" and the "down" direction represents "1"). To separate the data, direct currents, or artificial potentials, are used. An essential condition is the control of domain nucleation. Nucleation did not interfere with propagation if the nucleation process is determined mainly by the switching mechanism. The charge current directed in the plane of the magnetic nanowires is generated by the DW propagation circuit.

At a certain stage of research, scientists came to significant technical solutions that allow the creation of the real prototypes. Initially it was proposed [47] that the reading is performed by a magnetoresistive sensor. It was located near the track. This was done in order to be able to to use the emanating fringing fields to distinguish magnetic states. Another method involved placing the magnetic tunnel junction (MTJ) sensor directly on the racetrack. The use of MTJ [49] is promising. The size, compactness allows ccomplementary metal–oxide–semiconductor (CMOS) be compatible with racetrack memory (RTM) applications. In addition, the practical values of the tunneling magnetoresistance (TMR) are much higher than those of the giant magnetoresistance (GMR).

It is important to note such a property of tunneling currents as spin polarization. It is this property that allows the magnetic bit to be reoriented by spin transfer torques (STT). Also, the polarity of the current allows determining the orientation of the bit being written, which is also an essential practical step.

In addition to the technical details, conceptual modeling is also essential. In the paper [50] it is shown by means of simulation how a domain wall can pass through an inhomogeneity. In particular, it was demonstrated the passage through such in-homogeneity as the corner of the intersection of an L-shaped three-dimensional nanowire. The passage of the inhomogeneity was accompanied by the transformation of the domain wall. Namely, the domain wall was transformed from a head-to head type into a Neel-type domain wall or vice versa. The direction of transformation depended on the direction of movement. An important way out for practical application is to consider the threshold current density required to push the domain wall through the corner.It is this parameter that carries information about the technical details of recording information.

 

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors provide an overview of helical magnetic structures in magnetic microwires, and analyze the experimental data describing the magnetic behavior of magnetic microwires. However, the manuscript focuses on the investigation of their own work only. For a comprehensive review, it should cover more work in this field as many as possible, including the work of other groups. Furthermore, the review should also pay attention on an introduction of basic concepts, such as what are Helical, spiral, elliptical, vortexes, etc. My recommendation is major revision. Minors: GMR and GMI should be interpreted.

Author Response

The authors provide an overview of helical magnetic structures in magnetic microwires, and analyze the experimental data describing the magnetic behavior of magnetic microwires. However, the manuscript focuses on the investigation of their own work only. For a comprehensive review, it should cover more work in this field as many as possible, including the work of other groups. Furthermore, the review should also pay attention on an introduction of basic concepts, such as what are Helical, spiral, elliptical, vortexes, etc.

The works of other groups have been added. Basic concepts: Helical, spiral, elliptical, vortexes have been considered:

3. Background. Helical, elliptical, spiral and vortex magnetic structures.

           The main characteristic feature of magnetic vortexes is the presence of an axis around which the magnetization smoothly rotates [58-61]. The simplest version of a vortex is a flat vortex with a centrally located axis. It is the ordinary vortex that is mentioned in article [62]. There, the experimentally observed difference between a domain wall in the form of a plane vortex and a transversal domain wall was shown. The domain wall dynamics was measured by the classical Sixtus-Tonks experiments [51]. The energy difference between two formations led to the difference in the mobile properties of domain walls observed in Fe-rich microwires. It was found that the transversal domain wall has no time to de-pin. Instead, the vortex-type domain wall nucleates at the end of the microwire.

The energy of the vortex-type domain wall is much higher than transversal ones, since the exchange is higher. Therefore, it does not appear at low applied field. However, the vortex-type domain wall does not create the free poles at its surface and it can propagate without interaction with the radial domain structure below the surface. Therefore, its domain wall mobility is higher and it can reach very high velocities. A more complex version of the vortex is shown in [63]. Magnetic configuration consists of a periodic series of anti-parallel transverse domain-like regions separated by the transverse vortex states with opposite alternating chirality.

As for the helical magnetic structure, the main feature is the inclination effect with a fixed inclination angle. In turn, the helical structure is divided into elliptical and spiral structures. The length of the domain wall in the form of an ellipse is limited only by the length of the ellipse and is determined precisely by the angle of the ellipse inclination. The limiting case of an elliptic domain structure is the classical circular bamboo structure. In this case the angle of the inclination from the circular direction is equal 0.

The main feature of the spiral magnetic structure is its "infinity". In other words, its length is determined only by the length of the real sample. The limiting version of the spiral structure is the longitudinal structure, when the angle of deviation from the axial axis becomes equal to zero. In this case, the domain wall is directed strictly along the microwire axis. It is this exotic structure that was observed in the work [64].

When the vortex becomes non-flat, it merges with the helical structure, and the distinctions between them are blurred. If the plane cross-section of this structure continues to be a vortex, then in the general distributed form, the helicity of the structure is more pronounced [63, 65, 66].

A wide range of different structures have been demonstrated in the works [66, 67], both experimentally (CoNi nanowires) and as a result of simulations. It was shown experimentally and theoretically the existence of such structures as longitudinaly distributed vortexes of positive and negative chirality, chiral domain wall, and two divided vortexes, smoothly passing one into another. The transformation of a multi-domain cylindrical structure into a compact series of transverse-vortex chain is shown in detail. Particular attention is paid to longitudinal vortex domains.

In work [67], a pure helical structure was shown. A complex and inhomogeneous magnetic configuration is revealed, consisting of a periodic configuration of exotic antiparallel transverse domain-like regions separated by transverse-vortex states of alternating chirality and polarity in a region rich in hexagonal close packed (hcp) crystal structures. In turn, axial domains predominate in some areas. A transition between these two regions has also been identified. The experimental results were compared with micromagnetic simulations showing that a vortex chain is formed inside the cobalt-rich CoNi alloy. Correlated local changes in composition and crystal structure were identified as the source of different magnetic configurations: the vortex chain is the result of the hcp phase.

 

58. Vock, S.; Hengst, C.; Wolf, M.; Tschulik, K.; Uhlemann, M.; Sasvari, Z.; Makarov, D.; Schmidt, O.G.; Schultz, L.; Neu, V. Magnetic vortex observation in FeCo nanowires by quantitative magnetic force microscopy. Appl. Phys. Lett. 2014, 105, 172409.
59. Moreno, J.; Bran, C.; Vazquez, M.; Kosel, J. Cylindrical magnetic nanowires applications, IEEE Trans. Magn. 2021, 57, 800317.
60. Geng, L.D.; Jin, Y.M. Magnetic vortex racetrack memory. Magn. Magn. Mater. 2017, 423, 84.
61. Janutka, A.; Brzuszek, K. Giant magnetoreactance in magnetic nanowires. Magn. Magn. Mater. 2020, 515, 167297.
62. Richter, K.; Kostyk, Y.; Varga, R.; Zhukov, A.; Larin, V. Domain Wall Dynamics in Amorphous Microwires. Acta Physica Polonica A 2008, 113, 7.
63. Andersen, I.M.; Rodríguez, L.A.; Bran,; Marcelot, C.; Joulie, S.; Hungria, T.; Vazquez, M.; Gatel, C.; Snoeck E. Exotic Transverse-Vortex Magnetic Configurations in CoNi Nanowires. ACS Nano 2020, 14, 1399.
64. Richter, K.; Thiaville, A.; Varga, R.; McCord The role of uniaxial magnetic anisotropy distribution on domain wall tilting in amorphous glass-coated microwires. Journal of Applied Physics 2020, 127, 193905.
65. Fernandez-Roldan, J.A.; del Real, R.P.; Bran, C.; Vazquez, M.; Chubykalo-Fesenko Electric current and field control of vortex structures in cylindrical magnetic nanowires. Phys.Rev. B 2020, 102, 024421.
66. Andersen, I.M.; Wolf D.; Rodriguez, L.A.; Lubk, A.; Oliveros, D.; Bran, C.; Niermann, T.; Rößler, U.K.; Vazquez , M.; Gatel, C.; Snoeck, E. Field tunable three-dimensional magnetic nanotextures in cobalt-nickel nanowires. Rev. Res. 2021, 3, 033085.
67. Fernandez-Roldan A.; Ivanov Yu. P.; Chubykalo-Fesenko O. 4 - Micromagnetic modeling of magnetic domain walls and domains in cylindrical nanowires. Magnetic Nano- and Microwires. (Second Edition) Design, Synthesis, Properties and Applications Woodhead Publishing Series in Electronic and Optical Materials 2020, Pages 403-426.

 

Minors: GMR and GMI should be interpreted.

GMR and GMI have been interpreted.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

accept as is.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.