Optimization of magnetic properties of magnetic microwires by Post-processing.

: The influence of post-processing conditions on the magnetic properties of amorphous and nanocrystalline microwires have been thoroughly analyzed, paying attention on the influence of magnetoelastic, induced and magnetocrystalline anisotropies on the hysteresis loops of Fe-, Ni- and Co-rich microwires. We showed that magnetic properties of glass-coated microwires can be tuned by the selection of appropriate chemical composition and geometry in as-prepared state or further considerably modified by appropriate post-processing, which consists of either annealing or glass-coated removal. Furthermore, stress-annealing or Joule heating can further effectively modify the magnetic properties of amorphous magnetic microwires owing to induced magnetic anisotropy. Devitrification of microwires can be useful for either magnetic softening or magnetic hardening of the microwires. Depending on the chemical composition of the metallic nucleus and on structural features (grain size, precipitating phases) nanocrystalline microwires can exhibit either soft magnetic properties or semi-hard magnetic properties. We demonstrated that the microwires with coercivities from 1 A/m to 40 kA/m can be prepared.


Introduction
Development of magnetic sensors is essentially influenced by the technological progress in the field of magnetic materials [1][2][3]. Indeed, the features of magnetic sensors are determined by selection of appropriate magnetic material. A significant portion of magnetic sensors use soft magnetic materials [3].
One of the most promising families of soft magnetic materials with a number of advantages, such as excellent magnetic softness, fast and inexpensive manufacturing process, dimensionality suitable for various sensor applications, and good mechanical properties, is a family of amorphous and nanocrystalline materials obtained using rapid quenching from the melt [4][5][6]. 3 of 27 Thus, if the quenching rate achieved during the quenching process from the melt is not high enough, metastable crystalline microwires with crystalline structure of metallic alloy nucleus can be prepared [36,40,41]. However, even magnetic properties of amorphous microwires are affected by the fabrication conditions (like quenching rate or glass-coating thickness) and chemical composition of the metallic alloy [18,43,44]. This compositional dependence is related to the magnetoelastic anisotropy affected by the magnetostriction coefficient as well as by the internal stresses values [18,[43][44][45][46][47]. However, appropriate post-processing is the other factor allowing either fine tuning or even drastic modification of magnetic properties [43,44,48].
In this review, we have analyzed the influence of various factors on the magnetic properties of glass-coated microwires and provide the guideline for selection of appropriate post-processing for optimization of properties of magnetic microwires.

Experimental Methods and Materials
We prepared and analyzed amorphous glass-coated microwires based on Fe-, Co-and Ni-alloys with minor metalloid additions (Si, B, C) necessary for preparation of amorphous alloys [6-8, 12, 36]. Employed Taylor-Ulitovsky technique is earlier described elsewhere [36].  38 The structure of the samples has been analyzed by the X-ray Diffraction (XRD) as well as by the Differential Scanning Calorimeter (DSC). The BRUKER (D8 Advance) X-ray diffractometer with Cu K (λ =1.54 Å) radiation has been used in the XRD studies. All amorphous (as-prepared or annealed) 4 of 27 microwires present a broad halo typical for completely amorphous materials. The DSC studies were performed using a 204 F1 Netzsch calorimeter.
The microwires were annealed at a temperature, Tann, in the range from 200 o C to 400 o C in a conventional furnace in order to avoid the crystallization typically reported for Tann ≥ 490 o C [49]. The advantage of amorphous microwires is their superior mechanical properties typically reported for amorphous materials [50,51]. Typically, we fixed annealing time, tann, of 60 min as commonly used for heat treatment of amorphous and nanocrystalline materials [48,49].
In the case of stress-annealing, the tensile stress was applied during the annealing, as well as during the sample cooling in the furnace. The stress value in the metallic nucleus, σm, was evaluated considering different Young's modulus of metal, E2, and glass, E1, as following [43,48,52]: where K=E2/E1, P is the applied mechanical load and Sm, and Sgl are the cross sections of the metallic nucleus and the glass coating, respectively.
Hysteresis loops have been recorded using the fluxmetric method adapted for studies of magnetic microwires [49]. Hysteresis loops can be represented as the normalized magnetization, M/Mo, versus the applied magnetic field, H, where Mo is the magnetic moment of the sample at the maximum magnetic field amplitude, Ho [49,53].
The magnetostriction coefficient, λs, of the studied microwire, was evaluated by the SAMR method recently adapted for microwire [54,55]. In this method λs -values are determined in the microwire saturated by an axial magnetic field, H. Simultaneously, low AC transverse field, Hc, generated by an AC electric current flowing along the microwire allows a reversible magnetization rotation. More detailed SAMR method description and of the set-up adapted for evaluation of magnetostriction coefficient in microwires are provided elsewhere [54,55].
The glass coating was removed from the microwires by chemical etching using diluted (10%) hydrofluoric (HF) acid.

Results and Discussion
Below we have summarized the highlight findings already published and also the obtained recently experimental results on tailoring of magnetic properties of glass-coated microwires, paying attention on amorphous and crystalline microwires.

Effect of magnetoelastic anisotropy on magnetic properties of amorphous glass-coated microwires
Magnetic properties of amorphous microwires are affected by the value and sign of the magnetostriction coefficient since the magnetoelastic interactions are the main source of the magnetic anisotropy of amorphous materials. The easiest way to tune the magnetostriction constant λs in amorphous alloys is the modification of its chemical composition [54][55][56][57].
The influence of the λs values and sign on hysteresis loops of amorphous microwires is shown in Figure 1.
As can be appreciated from Figure 1, the character of hysteresis loops for amorphous microwires with positive and negative λs -values is rather different: amorphous microwires with positive λs -values present rectangular hysteresis loops, while hysteresis loops of microwires with negative λs -values are almost non-hysteretic with low coercitivity, Hc, values.

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Such difference in hysteresis loops character is commonly attributed to different magnetic anisotropy of microwires with positive and negative λs -values: the rectangular hysteresis loop of microwires with positive λs -values was interpreted in terms of axial magnetic anisotropy [42]. Such axial magnetic anisotropy is intrinsically related to a peculiar domain structure consisting of inner axially magnetized single domain responsible for observation of the single and large Barkhausen jump and outer domain shell with radial magnetization orientation [9,59,60]. The remagnetization of such microwires is running by the depinning of the reversed domains inside the inner single domain and the consequent domain wall propagation [9,52,[59][60][61]. Perfectly, rectangular shape of the hysteresis loop is related to an extremely high velocity of such domain wall propagation.
On the other hand, the origin of quasi-linear hysteresis loops (see Figures 1b,c) is related to the quasi-reversible magnetization rotation from the circumferential to the axial upon application of an axial magnetic field [60].
There are several factors responsible for the internal stresses value and distribution: i) the difference in the thermal expansion coefficients of metallic alloy nucleus solidifying simultaneously with the glass coating surrounding it; ii) the quenching stresses itself related to the rapid solidification of the metallic alloy nucleus from the surface inside the wire axis; and iii) the drawing stresses [43][44][45][46][62][63][64].
Most theoretical evaluations of the internal stresses value and distribution show that the largest internal stresses are associated with the difference in the thermal expansion coefficients of the metallic alloy and the glass coating [62][63][64]. The quenching stresses are roughly an order of magnitude lower [42,43].
There are only several attempts of evaluation of internal stresses associated to the continuous mechanical drawing [63,64]. The value of such stresses has been estimated from the results on remanent magnetization measurements in glass-coated microwires with partially removed (by chemical etching) glass-coating under applied tensile stresses [63,64]. The value of this stress component depends on the microwire geometry and was estimated to be about 250-600 MPa, i.e., again about an order of magnitude below the internal stresses related to the difference in the thermal expansion coefficients of metallic alloy and the glass coating [63,64]. Furthermore, they further enhance the axial internal stresses arising from the difference in the thermal expansion coefficients of the metallic alloy and the glass coating. Provided description allows to predict that the internal stresses value inside the metallic nucleus can be tuned by the -ratio between the metallic nucleus diameter, d, and the total microwire diameter,  [42,43,[62][63][64]. In fact, this prediction is confirmed experimentally by correlation of magnetic properties, such as coercivity, Hc, or magnetic anisotropy field, Hk, in magnetic microwires of various chemical compositions and  -ratio [38,42,43,[62][63][64]. Below we provide several experimental evidences of such correlations. The influence of controllable glass-coating removal by etching in 10% HF on hysteresis loops of Co70.5Mn4.5Si10B15 microwire is shown in Figure 2. Gradual transformation of hysteresis loops from linear to almost perfectly rectangular must be attributed to relaxation of the internal stresses related to the presence of glass-coating. This evolution of hysteresis loops can be understood considering the low negative λs -values and the axial character of internal stresses in most part of the metallic nucleus [65]. Evident difference in hysteresis loops of as-prepared Co70.5Mn4.5Si10B15 microwires and microwires of the same composition with partially removed glass-coating experimentally confirms the aforementioned theoretical results on character of internal stresses. Low negative magnetostriction coefficient and preferentially axial character of internal stresses explain linear almost non-hysteretic character of hysteresis loops of as-prepared Co70.5Mn4.5Si10B15 microwires.
The influence of chemical etching on the hysteresis loops of Co 68.5 Si 14.5 B 14.5 Y 2.5 microwire with higher negative λs -values is even more remarkable (see Figure 3): as-prepared Co 68.5 Si 14.5 B 14.5 Y 2.5 microwire presents non-hysteretic loops with saturation at magnetic field, H, above 6 kA/m (see Figure 3a). However, upon chemical etching gradual transformation of hysteresis loops from linear to rectangular is observed (see Figures 3b,c). As previously reported [66], after etching in 10% HF for 50 min the glass-coating thickness decreases from 8.5 to 4 μm. Accordingly, the glass-coating thickness can be considered as one of the most relevant parameters that affect the hysteresis loops of glass-coated microwires. Gradual glass-coating removal by chemical etching must be associated to the gradual relaxation of the internal stresses related to different thermal expansion coefficients of metallic alloy and glass coating.
The commonly accepted model of domain structure of magnetic wires is the core-shell model experimentally proved several times by various methods [60,61,67,68]. In according to this model the domain structure of amorphous magnetic wires can be satisfactory described as consisting of inner axially magnetized core surrounded with the outer shell with transverse magnetization. In the case of Fe-rich wires the outer shell presents radial magnetization orientation, while in Co-rich microwires bamboo-like domain structure with circular magnetization orientation is reported [60,61,[67][68][69][70].
The origin for such domain structure is discussed considering the minimization of the energy though the counterbalance of the magnetoelastic energy related to the internal stresses distribution as well as to the exchange energy [69,70].
In the frame of this domain structure model the radius of the inner axially magnetized core, Rc, can be evaluated from the squareness ratio, Mr/Mo, as: where R is the metallic nucleus radius. From the evolution of the hysteresis loops upon chemical etching provided in Figures 2&3, the increase in the Mr/Mo upon chemical etching is evidenced. Consequently, we can assume that the radius of inner axially magnetized domain increase upon partial internal stresses relaxation associated to the glass removal. This assumption is evidenced from Figure 4 where evolution of Rc/R on time of chemical etching, t, for Co70.5Mn4.5Si10B15 and Co 68.5 Si 14.5 B 14.5 Y 2.5 microwires is shown. As can be observed, for the microwire with lower λs -value the increase in Rc/R with t is faster.
In order to prove the axial character of the internal stresses related to glass-coating (i.e., to the difference in thermal expansion coefficients of glass and metallic alloy) we evaluated the influence of applied tensile stresses, σ, on the hysteresis loops of Co-rich (Fe3.8Co65.4Ni1B13.8Si13Mo1.35C1.65) microwires with linear hysteresis loop.
As shown in Figure 5a, when tensile stress, σapp, is applied, magnetic anisotropy field, Hk, increases. At the same time, the hysteresis loops character (linear hysteresis loop with low coercivity) of the studied microwire remains almost the same.
Hk(σapp) dependence evaluated from Figure 5a shows a good linear tendency ( Figure 5b). Such linear Hk(σapp) dependence has been explained considering the relationship between the magnetoelastic anisotropy, Kme, and σapp given by eq. (2) [71]. Consequently, relation between the magnetostriction coefficient and magnetic anisotropy field, Hk, is given by [71]:  where μoMs is the saturation magnetization.
The magnetostriction coefficient is affected by the stresses, σ, as described elsewhere [71,72]: where s is the magnetostriction coefficient under stress, s,0 is the zero-stress magnetostriction constant and B is a positive coefficient of order 10 −10 MPa and − stresses. Therefore, a decrease in s reported for Co-rich microwires (s <0) upon applied stresses [54,55] can be associated with the development of circumferential magnetic anisotropy in the outer shell [71]. Consequently, experimentally observed linear Hk(σapp) dependence can be explained considering eqs. (2,4,5).
The influence of chemical etching on the hysteresis loops and on Hk observed in Figures 2, 3 is exactly the opposite to the effect of tensile stresses. Therefore, the theoretically predicted character of internal stresses with a dominant axial tensile character and its dependence on the -ratio look reasonable. Accordingly, it is expected that the internal stresses value can be tuned by the glass-coating thickness through the −ratio.
The correlation of the −ratio and the hysteresis loops of Co-rich microwires with a vanishing magnetostriction coefficient is reported elsewhere [12,[73][74][75]. For the case of Co67Fe3.85Ni1.45B11.5Si14.5Mo1.7 microwires with vanishing magnetostriction coefficient (see Figure 6) linear almost non-hysteretic loops with extremely low coercivities (up to 4 A/m) are observed. If we plot Hk obtained from the hysteresis loops of Co67Fe3.85Ni1.45B11.5Si14.5Mo1.7 microwires with various d and D-values versus the −ratio, we can find out that there is a correlation between these parameters. Magnetic anisotropy field, Hk, increases with decreasing the −ratio ( Figure 6b).
As mentioned above, glass-coated microwires with positive λs -value generally present perfectly rectangular hysteresis loops related to spontaneous magnetic bistability. Typical hysteresis loop of microwires with positive λs -value is shown in Figure 1a. Fast magnetization switching and related single DW propagation reported in magnetic micro-and nano-wires are proposed for various technical applications, like magnetic sensors, electronic surveillance, magnetic memories and logics [7,[75][76][77]. Thus, the method for magnetic codification using magnetic tags [75] is based on sharp voltage signals induced by fast magnetization switching of magnetically bistable microwires. In this application, each tag contains several microwires with well-defined coercivities, each of them characterized by a rectangular hysteresis loop. Once the magnetic tag submitted to the AC magnetic field, each particular microwire is remagnetized at different magnetic field giving rise to an electrical signal on a detecting system. Variety of coercivities allows to extend the number of combinations for magnetic codification. Therefore, tunability of coercivities, Hc, of magnetically bistabile microwires is essentially relevant for this application and has been extensively studied [46,[78][79][80].   Figure 7e).
Considering aforementioned results, one can expect that stresses relaxation by heat treatment can efficiently affect the magnetic properties of microwires.
One of the examples of annealing influence on hysteresis loops of Fe-rich microwires is shown in Figure 8.
For the Fe75B9Si12C4 microwires, annealing does not affect the hysteresis loop character. However, a slight Hc decrease is observed (see Figure 8b).
More complex behavior has been reported for Fe-Ni based microwires with positive magnetostriction and hence presenting spontaneous magnetic bistability [81,82].
As-prepared Fe62Ni15.5Si7.5B15 microwires present rectangular hysteresis loops (see  After annealing, an increase in coercivity, Hc, is generally observed (see Figure 9 b-g and Figure 10). The hysteresis loop character remains unchanged: all hysteresis loops present rectangular shape.
Although generally higher Hc -values are observed in annealed samples, Hc(tann) dependence is not monotonic: for tann, =128 min some Hc decrease is observed (see Figure   10).
Similarly, the second Fe-Ni based (Fe49.6Ni27.9Si7.5B15) microwire presents rectangular hysteresis loops in as-prepared state and after annealing (see Figure 11).  conditions, i.e., Tann and tann) first a slight decrease in Hc followed by Hc rising is observed (see Figure   11e).
Considering that the annealing is the common route for the internal stresses relaxation,  expansion coefficient [91]. The Invar anomaly is intrinsically related to the local atomic structure of Fe-Ni alloys. Considering similarity of short range order of amorphous and crystalline materials [92] one can expect lower internal stresses influence in Fe49.6Ni27.9Si7.5B15 microwire. In spite of observed magnetic hardening of Fe-Ni based microwires upon annealing, observed experimental dependencies allows to tune the coercivity value by annealing.
Observed modification of the hysteresis loops shape and obtained Rc(Tann) dependence are consistent with the evolution of the hysteresis loop shape upon glass-coating removal by chemical etching (see Figures 3,4). Accordingly, Rc(Tann) dependence and observed evolution of the hysteresis loops upon annealing must be related to the relaxation of internal stresses as discussed elsewhere [74,[93][94][95] Such evolution of hysteresis loops upon annealing is confirmed in various Co-based microwires with low and negative λs -values [93][94][95][96]. One more example for another Co-rich (Co69.2Fe4.1B11.8Si13.8C1.1) microwire with low negative λs -values is shown in Figure 14. In this case the annealing temperature was fixed (Tann =250 o C) and the hysteresis loops have been recorded at different annealing time, tann. Similarly to the case of Fe3.6Co69.2Ni1B12.5Si11Mo1.5C1.2 microwires, we can observe a remarkable Hc rising and gradual Mr/Mo, increase with an increase in tann (see Figure 15).
Accordingly, from Rc(tann) evaluated from Mr/Mo we can again observe gradual increase of the inner axially magnetized domain radius upon annealing.
Consequently, similarly to glass-coating removal, annealing of Co-rich microwires allows to obtain magnetically bistable Co-rich microwires. Such Co-rich microwires with magnetic bistability induced by annealing present fast magnetization switching by propagation of single domain wall,

Effect of induced magnetic anisotropy on hysteretic magnetic properties of amorphous glass-coated microwires
Considering new functional properties provided by insulating and flexible glass-coating, most attention has been paid to tuning of the hysteresis loops of Fe-Co-Ni-based microwires by appropriate annealing. As shown above, conventional annealing provides limited possibilities for tuning the hysteresis loops. Accordingly, several attempts have been performed recently to search more efficient post-processing allowing tuning the magnetic properties [43,52,97].  One of the most promising and effective methods for tuning of the magnetic properties of magnetic microwires is stress-annealing. In the case of magnetic microwires with positive magnetostriction coefficient, this post-processing allows remarkable magnetic softness improvement [43,52,86,87,94,95].
From previous knowledge on the origin of induced magnetic anisotropy, it is known that the magnetic anisotropy of amorphous materials can be effectively tailored by either stress or magnetic field annealing [83,84].
Recently, stress-annealing has been successfully employed for tailoring of magnetic properties in glass-coated microwires. Thus, according to several publications on the origin of the induced anisotropy in glass-coated microwires [74,86,87,98] the presence of the glass-coating can be even beneficial for tuning the magnetic anisotropy.
Several examples on the influence of stress-annealing on magnetic properties of glass-coated microwires are provided below.
From Figure 16 we can clearly see that stress-annealing performed at the same conditions (Tann and tann) allows better magnetic softening and transverse anisotropy induction in Fe-based (Fe75B9Si12C4) microwire. As reported elsewhere [87,98], such transverse anisotropy depends on various parameters, like Tann, tann, and stress, σm, applied during the annealing [87]. Clear example is shown in Figure 17. As can be appreciated from Figure 17, for high enough Tann or σm a remarkable transverse magnetic anisotropy can be induced. However, for low enough Tann, tann or σm the hysteresis loops maintain rectangular shape (see Figure 18a) and can present all the features typical for magnetically bistable microwires, i.e., fast and single domain wall propagation [98]. Lower coercivity is generally observed in stress-annealed Fe-rich microwires (see Figure 18a,b). Consequently, stress annealing allows more effectively tune Hc and Mr/Mo -values (see Figure 18c). However, for sufficiently high Tann, tann or σm the hysteresis loops of Fe-rich microwires become inclined (similar to that of as-prepared Co-rich microwires) with clear transverse magnetic anisotropy (see Figures 17d, 18b). As recently reported [42,43,86,87], such Fe-rich microwires with stress-induced transverse magnetic anisotropy present better GMI response Fe75B9Si12C4 microwire. Adapted from [98].
In the case of Co-based microwires higher annealing temperature, time or stresses are required to prevent magnetic hardening associated to relaxation of internal stresses related to glass-coating [42,43,101]. As can be observed in Figures 19 and 20, for extended range of Tann, tann or σm, stress-annealed Co-rich microwires present rectangular hysteresis loops. Therefore, stress-annealing of Co-rich microwires allows to tune their coercivity in quite extended range. Thus, at certain stress-annealing conditions Co68.7Fe4Ni1B13Si11Mo2.3 microwires with rectangular hysteresis loops and extremely low coercivity of about 1A/m can be obtained (see Figure 21) [102].
For sufficiently high Tann, tann or σm-values linear hysteresis loop of Fe3.6Co69.2Ni1B12.5Si11Mo1.5C1.2 microwires can be recovered (see Figure 19). However, Tann, tann or σm-values at which transverse  Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 7 July 2020 doi:10.20944/preprints202007.0107.v1 16 of 27 magnetic anisotropy in Co-rich microwires can be induced are considerably higher than those for Fe-rich microwires. One more efficient method for tuning of hysteresis loops of Co-rich microwires, preventing excessive magnetic hardening, is the Joule heating (see Figure 22) [29]. In this case, the current flowing through the microwires produce heating itself as well as the circumferential magnetic field, Hcirc, (associated to the current I flowing through the sample) [29,103].
Aforementioned circumferential magnetic field, Hcirc, produced by the current (Oersted field) in the surface of the metallic nucleus can be evaluated from the formula [29,86]: Hcirc=I/2πr (6) where I is the current value, r-radial distance.
Magnetic softening obtained at certain conditions of Joule heating is evidenced by Figure 22 where effect of Joule heating on hysteresis loops of Co67Fe3.9Ni1.5B11.5Si14.5Mo1.6 microwire is shown. In this case the microwires were heated by a DC current, I, of 30 and 40 mA. These conditions were selected in order to avoid the crystallization and related deterioration of the magnetic properties. In the case of Joule heating the current density is one of the main parameters determining the sample heating [104]. Although the thickness of the glass coating and the metallic nucleus diameter also affect the heat transfer rate [40]. In the given case, the current densities (58.3 and 77.7 A/mm 2 for 30 and 40 mA, respectively) are well-below the value that can produce magnetic hardening related to the crystallization [104].
Observed magnetic softening is related to the presence of magnetic field that can considerably affect the magnetic anisotropy of amorphous materials [105]. It was reported that the macroscopic magnetic anisotropy of amorphous materials is originated by a preferred magnetization direction during the annealing and was discussed in terms of either the directional ordering of atomic pairs or compositional and topological short-range ordering [83][84][85][93][94][95]105]. Adapted from ref [102].

Tuning of hysteretic magnetic properties in crystalline and devitrified glass-coated microwires
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 7 July 2020 doi:10.20944/preprints202007.0107.v1 17 of 27 The devitrification of amorphous nucleus reached by post annealing process is another useful tool allowing considerable modification of the magnetic properties and even magnetic softening in some Fe-rich microwires [48].
In the case of FeSiBNbCu (so-called Finemet) alloys low magnetostriction values and better magnetic softness can be achieved by the devitrification of amorphous precursor [57]. Magnetic softening of Finemet alloys is commonly attributed to vanishing magnetocrystalline anisotropy, as well as to nearly-zero magnetostriction value of the material obtained by nanocrystallization of their amorphous precursors, and consisting of nano-sized grains, with average size about 10 nm, embedded in an amorphous matrix [57].
As discussed elsewhere, after nanocrystallization the average magnetostriction constant takes nearly -zero values [57,106], thanks to the control of the crystalline volume fraction: the existence of two phases (amorphous and crystalline) provides a good balance of a negative magnetostriction of α-Fe-Si nanocrystallites of about ( ≈ -6 x 10 -6 ) [48] and a positive one for the amorphous matrix of about ( ≈ 20 x 10 -6 ) [48] resulting finally in vanishing net magnetostriction values [105]: being λs eff the saturation magnetostriction coefficient, and Vcr denotes the crystalline volume fraction. This nanocrystallization of FeSiBNbCu alloys is usually observed after annealing in the range of 500-600 o C for 1 hour (i.e., at temperatures between the first and second crystallization stages). One of the examples of the evolution of the hysteresis loops of Finemet-type microwires upon nanocrystallization is shown in Figure 23. As can observed from Figure 23, in the case of the Fe70.8Cu1Nb3.1Si14.5B10.6 microwire, annealing at Tann up to 550 o C allows considerable decrease of coercivity. For these annealing conditions the character of hysteresis loops does not change: all the hysteresis loops present rectangular shape. In some cases rectangular hysteresis loops are reported not only upon devitrification of Finemet-type, but even after second crystallization process when values up to 2400 A/m are observed [107]. One of the examples is shown in Figure 24a, where hysteresis loop of Fe71,8Cu1Nb3,1Si15B9,1 microwire (=0.282) annealed at Tann =700 o C is shown. However, Fe71,8Cu1Nb3,1Si15B9,1 microwire (=0.467) present rather different step-wise hysteresis loops (see Figure 24b) that can be attributed to partially crystalline (bi-phase) structure. Such partially crystalline magnetic microwires, with step-wise hysteresis loops related to magnetic interaction between crystals or mixed amorphous-crystalline structure, can be interesting for applications in electronic surveillance systems [108].
The microwires obtained by devitrification exhibit higher saturation magnetization and at certain annealing conditions can present better magnetic softness and GMI response than as-prepared Fe-rich microwires and therefore they are useful for GMI sensors and metacomposites applications [47]. In fact, microwires with nanocrystalline structure can be obtained even directly in as-prepared state without annealing [109,110]. The advantage of such microwires is that they can present better mechanical properties [109,110].
It is worth mentioning, that the use of specially designed compositions allows further increase of saturation magnetization, μoMs, [111] and also obtain extremely magnetically soft nanocrystalline materials. In the case of microwires, the use of a similar chemical composition allows preparation of nanocrystalline microwires with improved DW mobility without any post processing [109]. The partially crystalline (Fe0.7Co0.3)83.7Si4B8P3.6Cu0.7 microwire presents elevated values of Hc (about 480 A/m) and rather high saturation magnetization of about 1.6 T (see Figure  25).
On the other hand, magnetically hard and semi-hard wires are potentially quite interesting for various applications related to the design of smart markers for the electronic article surveillance, motors, compass needles and tachometers, magnetic microelectromechanical systems (magnetic MEMS), dentistry and for magnetic tips for magnetic force microscopy [112,113].
Recently a few successful attempts to achieve hard magnetic properties in microwires have been reported. Among others, various approaches are being developed to enhance hard magnetic behavior by employing novel alloys, i.e., Fe-Pt based alloys [114] or Heusler-type (Ni-Mn-Ga) alloys [115]. Magnetic hardening in Fe50Pt40Si10 microwires has been observed after annealing upon the formation of L10-type superstructure after crystallization of as-prepared amorphous precursor (see Figure 27). In this case after devitrification of amorphous Fe50Pt40Si10 microwire Hc ≈40 kA/m is observed.
Magnetic hardening is also reported in Co-rich microwires annealed by Joule heating [116].
Consequently, magnetic properties of crystalline microwires depend on the chemical composition of the metallic nucleus and on structural features (grain size, precipitating phases) of either as-prepared or annealed microwire: crystalline microwire can exhibit either soft magnetic properties or semi-hard magnetic properties.

Conclusions
We showed that the magnetic properties of glass-coated microwires prepared by the Taylor-Ulitovsky method can be tuned in as-prepared state or further modified by appropriate post-processing.
Magnetic properties of amorphous magnetic microwires can be tuned either in as-prepared state or by controlling the magnetoelastic anisotropy through the magnetostriction coefficient value and by the internal stresses values related to the fabrication conditions and geometry of microwires. Furthermore, appropriate post-processing (including either conventional heat treatment, heat treatment in the presence of applied stress or magnetic field or glass-coating removal) allows further tuning of magnetic properties of magnetic microwires.
We showed that the microwires with coercivities from 1 A/m to 40 kA/m can be prepared. Depending on the chemical composition of metallic nucleus as well as structural features (grain size, precipitating phases) prepared microwire can exhibit soft magnetic properties or semi-hard magnetic properties. temperatures. Adapted from ref. [114].