The evolution of the structure of glass-coated Fe 40 Ni 38 Mo 4 B 16 amorphous microwire with thermal treatments and its interplay with magnetism has been studied. As shown by x-ray diffraction, a primary crystallization process resulted into formation of ␥-͑Fe, Ni͒ nanocrystallites embedded in a residual amorphous matrix. The evolution of the saturation magnetization and the switching field after different thermal treatment was studied. Amorphous glass-coated microwires based on FeNi exhibit magnetic bistability even in the nanocrystalline state. This is explained by the high magnetoelastic anisotropy, which is also responsible for magnetic hardening after annealing at the temperatures above 670 K. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2969057͔Amorphous and nanocrystalline ferromagnetic microwires covered by a glass coating are very attractive material for applications, mainly as sensing elements in various sensors ͑i.e., magnetic field, coding, etc.͒, due to their outstanding magnetic characteristics, small dimensions and reliability on electrical, mechanical, and corrosive external effects ͑as a result of the Pyrex coating͒.1 These microwires, prepared by quenching and drawing technique, consist of a magnetic nucleus ͑with diameter from 1 -30 m͒ coated by a Pyrexlike glass ͑thickness of 2 -20 m͒. Due to their amorphous nature, magnetoelastic and shape anisotropy contributions govern their magnetic properties. As a result of abovementioned anisotropies, the domain structure of microwires with positive magnetostriction consists of single axial domain and their magnetization process is characteristic of the bistable behavior. 1,2The nanocrystalline microwires have been subject of different studies starting from Finemet based microwires, 3-7 as well as the Nanoperm based ones.8 Such nanocrystalline microwires are usually prepared by heat treatment. However, the microwires undergo strong stresses during the annealing due to the different thermal expansion coefficients of the metallic nucleus and the glass coating. As a result, a ␥-Fe phase appears in annealed glass-coated Finemet microwires instead of desired ␣-͑Fe, Si͒ phase.6,7 When the annealing is performed under high pressure, the formation of ␥-Fe phase is energetically more favorable due to its higher pack density, However, the ␥-Fe phase is nonmagnetic and deteriorates the good soft magnetic properties of nanocrystalline microwires. 6,7 It has been recently shown that partial crystallization of Fe 40 Ni 38 Mo 4 B 16 ribbon leads to the formation of nanocrystalline materials with ␥-͑Fe, Ni͒ nanocrystals having diameter of 10 nm.9 Annealing of such samples at T a = 700 K has been shown to be the optimum annealing temperature to obtain nanocrystalline Fe 40 Ni 38 Mo 4 B 16 alloy with optimized soft magnetic properties ͑lowest magnetostriction, lowest coercive field, and highest initial susceptibility͒. 10,11Since ␥-͑Fe, Ni͒ phase is ferromagnetic contrary to the paramagnetic ␥-Fe phase, it is a promising candidate for preparation of soft magne...

The frequency dependence of the switching field in glass-coated FeNiMoB microwires has been studied in the temperature range from 77 to 373 K. Two contributions to the domain wall switching mechanism were recognized: a magnetoelastic contribution coming from the magnetoelastic interaction of the magnetic moments with the stresses, and a relaxation contribution coming from the structural relaxation of the atomic level defects. The structural relaxation results in the unusual increase in the switching field at low frequencies, whereas the increase in the switching field at high frequencies was assigned to the frequency dependence of the magnetoelastic contribution, which obeys the power law H sw ∼ f 1/3 .

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Influence of the directional ordering on magnetization processes in the amorphous Fe44.5Co44.5Zr7B4 alloy was investigated in the temperature region of 27 to 300• C for stabilization times of up to 1000 min by means of the Perminvar effect. The time-temperature dependent Perminvar effect was found to be very sensitive on the stage of the domain structure stabilization. The Perminvar critical fields HCR were shifted to higher values with increasing stabilization temperatures and times. At lower temperatures, the susceptibility increases over wide range of the measuring fields. Stabilization at higher temperatures causes that the susceptibility abruptly increases in the narrow field range. The record critical field HCR = 246 Am −1 was obtained.

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