This work is important and relevant to the field of magnetometry. The new copper alloy is novel and builds upon work done many decades ago. The direct comparison with the research standard 1” ring core permalloy sensors is useful in evaluation of the materials. The construction and evaluation of the material is well described in this paper. That is unique in that work done several decades ago on this topic was either not well described or not published in unclassified documentation. This is a good reference for fluxgate materials going forward.

The paper is clear and well written. The plots and diagrams are appropriate and well done. The references are good.

One small comment: On line 126, we read “e quasi-toroidal drive windings are time-consuming to apply, but the closed flux path of the racetrack should reduce stray fields and offsets error compared to traditional parallel rod sensors"However, there are neither measurements nor references used to support this statement. Additionally, the racetrack is compared to parallel rods here, but in the rest of the paper, the racetrack topology is compared to ring cores. This section is potentially improved by citing offset measurements, providing reference(s), and/or comparing the racetrack versus ring core topologies, as is done in the remainder of the document, in place of the parallel rods.

This is a good work and I look forward to following this research in the future. It is both important and relevant to the field of fluxgate magnetometry.

Excellent work, which demonstrates, including technological details of core production for fluxgate magnetometers. The noise of fluxgate sensors made using cores of fundamentally different alloys (6-81 Mo and 28-62 Cu) and with different geometries (ring and racetrack) is studied. The authors describe in detail the subtleties of heat treatment of alloys, which significantly improves the value of magnetic noise for both used alloys. It has been shown that fluxgate sensors with racetrack core have lower noise values ​​and lower power consumption compared to ring cores. The authors showed that the alloy 28-62 Cu has slightly higher noise values ​​compared to the classical alloy 6-81 Mo. The advantage of the 28-62 Cu alloy is lower power consumption. The authors claim that this alloy is promising and in the future, after its improvement, it may compete with 6-81 Mo permalloy.

In general, the paper presents a large number of important results. This work will be useful for technologists and scientists working in the field of magnetic sensor development.

I have a few comments and suggestions.

In paragraph 2.6 Racetrack Cores (line 110 et seq.) It would be desirable to indicate what was the width of the foil in the ring core (1.57 mm, as in https://doi.org/10.5194/gi-2019-15)? Was it the same or comparable to the width of the straight foil segments in the racetrack core (6.45 mm minus the width of the hole (cut oval))?

In line 116 the phrase "A plastic lid closed the core and supports a quasi-toroidal drive of AWG 32 magnet wire" it would be desirable not to use the combination "magnet wire". Better: "A plastic lid closed the core and supports a quasi-toroidal drive of AWG 32 wire, which induces a magnetic flux."

Line 305 has a mistake: All cores were three layers of 50 μm 28-62 Cu ("μm" -please, remove)  foil heat-treated with a dwell temperature of 1150 °C.

Figure 13 shows a dark brown bar in the range of 15÷16 pT/√Hz. However, there is no explanation for this color in the figure.

It would be desirable to supplement the work with studies of the integral magnetic properties of cores, in particular, as was done for permalloy ring cores in the work of David M Miles and co-authors (https://doi.org/10.5194/gi-2019-15). For example, to build the dependence of the sensor noise on the maximum magnetic permeability (or/end core loss) of cores made of different alloys.

My remarks and wishes, of course, do not reduce the value and novelty of the results of the research presented by the authors.