Search Results (18)

In this study, the effect of aging heat treatment on the superelastic properties and microstructure of [001]-oriented Fe₄₁Ni₂₈Co₁₇Al_11.5Ti_1.25Nb_1.25 (at.%) single crystals was investigated using the cyclic superelastic strain test and a transmission electron microscope (TEM). The TEM results reveal that the average precipitate size is around 3–5 nm in the 600 °C/24 h samples, 6–8 nm in the 600 °C/48 h samples, and 10–12 nm in the 600 °C/72 h samples. The results indicate that precipitate size increases as aging time increases from 24 to 72 h. EDS analysis results show decreased Fe and increased Ni when the analyzed line crosses the precipitate region. The diffraction pattern results show that the precipitate has an L1₂ crystal structure. The thermo-magnetization curves of single crystals under the three aging conditions (600 °C/24 h, 600 °C/48 h, and 600 °C/72 h) show that the values of the transformation temperatures increased from 24 to 72 h. Magnetization was saturated at 140 emu/g under the magnetic field of 7 Tesla. When increasing the magnetic field from 0.05 to 7 Tesla, the transformation temperatures rose. The results indicate that magnetic fields can activate martensitic transformation. From the results of the superelasticity test at room temperature, [001]-oriented FeNiCoAlTiNb single crystals aged at 600 °C for 24, 48, and 72 h presented recoverable strains of 3%, 5.1%, and 2.6%, respectively. Digital image correlation (DIC) results of the aged samples show that two martensite variants were activated during the superelasticity test. The two variants form corresponding variant pairs (CVPs) and improve the recoverable strain of superelasticity. Although maximum recoverable strain was obtained for the 600 °C/48 h samples, the samples show poor cyclic stability at room temperature after applying the 6% strain. According to the DIC results, the retained martensite, which is pinned by dislocations, was observed after the test. The irrecoverable strain was attributed to the residual martensite. For the 600 °C/72 h samples, the large size of the precipitates poses an obstacle to dislocation transformation and formation. The dislocations increase the stress hysteresis width and stabilize the martensite, causing poor recoverability. Full article

We have prepared NiMnGa glass-coated microwires with different geometrical aspect ratios, ρ = d_metal/D_total (d_metal—diameter of metallic nucleus, and D_total—total diameter). The structure and magnetic properties are investigated in a wide range of temperatures and magnetic fields. The XRD analysis illustrates stable microstructure in the range of ρ from 0.25 to 0.60. The estimations of average grain size and crystalline phase content evidence a remarkable variation as the ρ-ratio sweeps from 0.25 to 0.60. Thus, the microwires with the lowest aspect ratio, i.e., ρ = 0.25, show the smallest average grain size and the highest crystalline phase content. This change in the microstructural properties correlates with dramatic changes in the magnetic properties. Hence, the sample with the lowest ρ-ratio exhibits an extremely high value of the coercivity, H_c, compared to the value for the sample with the largest ρ-ratio (2989 Oe and 10 Oe, respectively, i.e., almost 300 times higher). In addition, a similar trend is observed for the spontaneous exchange bias phenomena, with an exchange bias field, H_ex, of 120 Oe for the sample with ρ = 0.25 compared to a H_ex = 12.5 Oe for the sample with ρ = 0.60. However, the thermomagnetic curves (field-cooled—FC and field-heating—FH) show similar magnetic behavior for all the samples. Meanwhile, FC and FH curves measured at low magnetic fields show negative values for ρ = 0.25, whereas positive values are found for the other samples. The obtained results illustrate the substantial effect of the internal stresses on microstructure and magnetic properties, which leads to magnetic hardening of samples with low aspect ratio. Full article

We studied Ni₂FeSi-, Co₂FeSi-, and Co₂MnSi-based full-Heusler alloy glass-coated microwires with the same geometric parameters, i.e., fixed nucleus and total diameters, prepared using the Taylor–Ulitovsky method. The fabrication of X₂YZ (X = Co and Ni, Y = Fe and Mn, and Z = Si)-based glass-coated microwires with fixed geometric parameters is quite challenging due to the different sample preparation conditions. The XRD analysis showed a nanocrystalline microstructure for all the samples. The space groups Fm3¯m (FCC) and Im3¯m (BCC) with disordered B2 and A2 types are observed for Ni₂FeSi and Co₂FeSi, respectively. Meanwhile, a well-defined, ordered L2₁ type was observed for Co₂MnSi GCMWs. The change in the positions of Ni, Co and Mn, Fe in X₂YSi resulted in a variation in the lattice cell parameters and average grain size of the sample. The room-temperature magnetic behavior showed a dramatic change depending on the chemical composition, where Ni₂FeSi MWs showed the highest coercivity (H_c) compared to Co₂FeSi and Co₂MnSi MWs. The H_c value of Ni₂FeSi MWs was 16 times higher than that of Co₂MnSi MWs and 3 times higher than that of Co₂FeSi MWs. Meanwhile, the highest reduced remanence was reported for Co₂FeSi MWs (Mr = 0.92), being about 0.82 and 0.22 for Ni₂FeSi and Co₂MnSi MWs, respectively. From the analysis of the temperature dependence of the magnetic properties (H_c and M_r) of X₂YZ MWs, we deduced that the H_c showed a stable tendency for Co₂MnSi and Co₂FeSi MWs. Meanwhile, two flipped points were observed for Ni₂FeSi MWs, where the behavior of H_c changed with temperature. For M_r, a monotonic increase on decreasing the temperature was observed for Co₂FeSi and Ni₂FeSi MWs, and it remained roughly stable for Co₂MnSi MWs. The thermomagnetic curves at low magnetic field showed irreversible magnetic behavior for Co₂MnSi and Co₂FeSi MWs and regular ferromagnetic behavior for Ni₂FeSi MWs. The current result illustrates the ability to tailor the structure and magnetic behavior of X₂YZ MWs at fixed geometric parameters. Additionally, a different behavior was revealed in X₂YZ MWs depending on the degree of ordering and element distribution. The tunability of the magnetic properties of X₂YZ MWs makes them suitable for sensing applications. Full article

In the present paper, the influence of partial substitution of Mn by Pd on structure, thermomagnetic properties, and phase transitions in the MnCoGe alloys was investigated. The studies of phase constitution revealed an occurrence of the orthorhombic TiNiSi-type and hexagonal Ni₂Ti- type phases. Deep analysis of the XRD pattern supported by the Rietveld analysis allowed us to notice the changes in lattice parameters and quantity of recognized phases depending on the Pd content. An increase of palladium in alloy composition at the expense of manganese induced a rise in the Curie temperature. The values of ΔS_M measured for the variation of external magnetic field ~5 T equaled 8.88, 23.99, 15.63, and 11.09 for Mn_0.97Pd_0.03CoGe, Mn_0.95Pd_0.05CoGe, Mn_0.93Pd_0.07CoGe, and Mn_0.9Pd_0.1CoGe alloy, respectively. The highest magnetic entropy change ΔS_M was observed for samples with Pd content x = 0.05 induced by magnetostructural transformation. The analysis of the n vs. T curves allowed confirmation of the XRD and DSC results of an occurrence of the first-order magnetostructural transition in Mn_0.95Pd_0.05CoGe and Mn_0.93Pd_0.07CoGe alloys samples. Full article

In the current work, we illustrate the effect of adding a small amount of carbon to very common Co₂MnSi Heusler alloy-based glass-coated microwires. A significant change in the magnetic and structure structural properties was observed for the new alloy Co₂MnSiC compared to the Co₂MnSi alloy. Magneto-structural investigations were performed to clarify the main physical parameters, i.e., structural and magnetic parameters, at a wide range of measuring temperatures. The XRD analysis illustrated the well-defined crystalline structure with average grain size (D_g = 29.16 nm) and a uniform cubic structure with A2 type compared to the mixed L2₁ and B2 cubic structures for Co₂MnSi-based glass-coated microwires. The magnetic behavior was investigated at a temperature range of 5 to 300 K and under an applied external magnetic field (50 Oe to 20 kOe). The thermomagnetic behavior of Co₂MnSiC glass-coated microwires shows a perfectly stable behavior for a temperature range from 300 K to 5 K. By studying the field cooling (FC) and field heating (FH) magnetization curves at a wide range of applied external magnetic fields, we detected a critical magnetic field (H = 1 kOe) where FC and FH curves have a stable magnetic behavior for the Co₂MnSiC sample; such stability was not found in the Co₂MnSi sample. We proposed a phenomenal expression to estimate the magnetization thermal stability, ΔM (%), of FC and FH magnetization curves, and the maximum value was detected at the critical magnetic field where ΔM (%) ≈ 98%. The promising magnetic stability of Co₂MnSiC glass-coated microwires with temperature is due to the changing of the microstructure induced by the addition of carbon, as the A2-type structure shows a unique stability in response to variation in the temperature and the external magnetic field. In addition, a unique internal mechanical stress was induced during the fabrication process and played a role in controlling magnetic behavior with the temperature and external magnetic field. The obtained results make Co₂MnSiC a promising candidate for magnetic sensing devices based on Heusler glass-coated microwires. Full article

In the current study we have obtained Co₂FeSi glass-coated microwires with different geometrical aspect ratios, ρ = d/D_tot (diameter of metallic nucleus, d and total diameter, D_tot). The structure and magnetic properties are investigated at a wide range of temperatures. XRD analysis illustrates a notable change in the microstructure by increasing the aspect ratio of Co₂FeSi-glass-coated microwires. The amorphous structure is detected for the sample with the lowest aspect ratio (ρ = 0.23), whereas a growth of crystalline structure is observed in the other samples (aspect ratio ρ = 0.30 and 0.43). This change in the microstructure properties correlates with dramatic changing in magnetic properties. For the sample with the lowest ρ-ratio, non-perfect square loops are obtained with low normalized remanent magnetization. A notable enhancement in the squareness and coercivity are obtained by increasing ρ-ratio. Changing the internal stresses strongly affects the microstructure, resulting in a complex magnetic reversal process. The thermomagnetic curves show large irreversibility for the Co₂FeSi with low ρ-ratio. Meanwhile, if we increase the ρ-ratio, the sample shows perfect ferromagnetic behavior without irreversibility. The current result illustrates the ability to control the microstructure and magnetic properties of Co₂FeSi glass-coated microwires by changing only their geometric properties without performing any additional heat treatment. The modification of geometric parameters of Co₂FeSi glass-coated microwires allows to obtain microwires that exhibit an unusual magnetization behavior that offers opportunities to understand the phenomena of various types of magnetic domain structures, which is essentially helpful for designing sensing devices based on thermal magnetization switching. Full article

In order to understand further the emplacement (i.e., volcanic growth) of 22 Icelandite and 3 Rhyodacite cooling units in one of the long volcanic sequences known as Mauna Kuwale of the Wai’anae volcano (ca. 3.3 Ma), Oahu Hawaii we have conducted appropriate rock magnetic experiments described below as well as anisotropy of magnetic susceptibility (AMS) studies of such 25 units. We have undertaken rock magnetic investigations such as continuous and partial thermo-magnetic cycles of low field magnetic susceptibility versus temperature dependence, (k-T) curves experiments. We classified the k-T heating-cooling dependence of susceptibility in three groups A, B and C. Type A: yielded two components of titano-magnetite with a predominat Ti rich phase and occasionally a relevant magnetite component phase. Type B: samples are characterized by Ti poor magnetites. Magnetite dominates as the main magnetic carrier. Type C: k-T curves show one single phase of titanomagnetite, and Ti poor magnetite. The coercivity or remanence, determined by back field magnetization is always <60 mT, which suggest the predominance of magnetic components of low coercivity, like magnetite. Usually, two coercivity components are identified in the specimens. In addition we also conducted magnetic granulometry analyses on 27 specimens to determine the domain state of the flows. The ratio of hysteresis parameters (Mr/Mrs versus Hcr/Hc) show that overall samples fall in the Pseudo-Single Domain (PSD) region with high values of Mr/Mrs and very low values of Hcr/Hc. Only two samples from cooling units 17 and specially 22 show a Single Domain (SD) magnetic behavior and a sample from one unit approaches the SD-MD mixture region. We measured the magnetic susceptibility of all cooling units and we found out that in all analyzed units the magnetic susceptibility is low 13.7 ± 8.8 (10⁻³ SI). Magnetic anisotropy/magnetic fabric is used as a tool in rock fabric analyses to investigate the preferred orientation of magnetic minerals in rocks. Magnetic anisotropy is low on all (measured) flows from the Icelandite cooling units from 1 to 17 (mean P’ = 1.010), but becomes noticeably distinct and high in rhyodacite cooling units 23, 24 and 25 (mean P’ = 1.074). Four units show a magnetic fabric with k₃ axes vertical to sub-vertical which may be denoted as normal for the horizontal to sub horizontal units. Two Icelandite cooling units display oblate shapes and two other cooling units triaxial shapes. K₁ axes are horizontal but point in different directions, i.e., NE and NW. Remaining cooling units show different magnetic fabric. Units 17, 23, 24 and 25, despite important variations in anisotropy (low for units 25 and high for units 23 and 24) and shape of ellipsoid (oblate in cooling unit 23, prolate in 24 and triaxial in 25) the k₃ axes show the same orientation, SW to SSW dipping around 45° and a very steady magnetic lineation azimuth NW nearly horizontal to sub horizontal. The magnetic mineralogy and magnetic fabric indicate that both the Icelandite and Rhyodacite cooling units the magmatic evolution during the shield stage of the entire Wai’anae volcano and that such growth was not affected by tectonic deformation. Full article

Tuning and controlling the magnetic properties of nanomaterials is crucial to implement new and reliable technologies based on magnetic hyperthermia, spintronics, or sensors, among others. Despite variations in the alloy composition as well as the realization of several post material fabrication treatments, magnetic heterostructures as ferromagnetic/antiferromagnetic coupled layers have been widely used to modify or generate unidirectional magnetic anisotropies. In this work, a pure electrochemical approach has been used to fabricate core (FM)/shell (AFM) Ni@(NiO,Ni(OH)₂) nanowire arrays, avoiding thermal oxidation procedures incompatible with integrative semiconductor technologies. Besides the morphology and compositional characterization of these core/shell nanowires, their peculiar magnetic properties have been studied by temperature dependent (isothermal) hysteresis loops, thermomagnetic curves and FORC analysis, revealing the existence of two different effects derived from Ni nanowires’ surface oxidation over the magnetic performance of the array. First of all, a magnetic hardening of the nanowires along the parallel direction of the applied magnetic field with respect their long axis (easy magnetization axis) has been found. The increase in coercivity, as an effect of surface oxidation, has been observed to be around 17% (43%) at 300 K (50 K). On the other hand, an increasing exchange bias effect on decreasing temperature has been encountered when field cooling (3T) the oxidized Ni@(NiO,Ni(OH)₂) nanowires below 100 K along their parallel lengths. Full article

Fe${}_3$Co${}_{67}$Cr${}_3$Si${}_{15}$B${}_{12}$ ribbons with a high degree of flexibility and excellent corrosion stability were produced by rapid quenching technique. Their structural, magnetic, and thermomagnetic (Anomalous Nernst Effect) properties were studied both in an as-quenched (NR) state and after stress annealing during 1 h at the temperature of 350 °C and a specific load of 230 MPa (AR). X-ray diffraction was used to verify the structural characteristics of our ribbons. Static magnetic properties were explored by inductive technique and vibrating sample magnetometry. The thermomagnetic curves investigated through the Anomalous Nernst Effect are consistent with the obtained magnetization results, presenting a linear response in the thermomagnetic signal, an interesting feature for sensor applications. Additionally, Anomalous Nernst Effect coefficient $S_{ANE}$ values of $2.66\mathsf{\mu }$V/K and $1.93\mathsf{\mu }$V/K were estimated for the as-quenched and annealed ribbons, respectively. The interplay of the low magnetostrictive properties, soft magnetic behavior, linearity of the thermomagnetic response, and flexibility of these ribbons place them as promising systems to probe curved surfaces and propose multifunctional devices, including magnetic field-specialized sensors. Full article

The temperature behavior of saturation magnetization and the temperature behavior of the integral signal intensity in electron magnetic resonance spectra is experimentally studied comprehensively using a low-dimensional Al₂O₃/Ge/Al₂O₃/Co (aluminum oxide–cobalt–aluminum oxide–germanium) tunnel junction with different deposition velocities of a ferromagnetic metal (Co) thin layer and non-magnetic layers (Al₂O₃/Ge/Al₂O₃). The cobalt ferromagnetic layer was deposited on aluminum oxide in two ways: in one cycle of creating the structure and with atmospheric injection before deposition of the cobalt layer. The thermomagnetic curves revealed the appearance of minima observed at low temperatures on both sides of the cobalt layer. Possible sources of precession perturbations at low temperatures can be explained by: the influence of the Al₂O₃ layer structure on the Al₂O₃/Co interface; residual gases in the working chamber atmosphere and finely dispersed cobalt pellets distributed over the cobalt film thickness. The work offers information of great significance in terms of practical application, for both fundamental physics and potential applications of ultrathin films. Full article

We present initial rock magnetic results for both lava flows and lapilli produced by the 2021 eruption of the Cumbre Vieja, La Palma (Canary Islands). Samples were taken during the eruption to minimize early alteration and weathering of the rocks and tephra. Standard procedures included progressive alternating field and thermal demagnetization, hysteresis curves, thermomagnetic experiments, progressive acquisition of isothermal remanent magnetization (IRM), and First-Order Reversal Curves (FORCs). Overall, our observations, including low to medium unblocking temperatures, isothermal remanent magnetization to 1 Tesla, and the abundance of wasp-waist hysteresis loops, strongly suggest the presence of Ti-rich titanomagnetites as the main remanence carriers in both lava flows and lapilli, in addition to some hematite as well. Whereas the former has been directly seen (SEM), hematite is elusive with nonmagnetic-based methods. Rock magnetic data, on a Day plot, also reveal that the magnetic grain size tends to be larger in the lava flows than in the lapilli. Full article

The shape memory properties and microstructure of Fe₄₁Ni₂₈Co₁₇Al_11.5(Ti+Nb)_2.5 (at.%) cold-rolled alloys were studied at the first time using the values reported in constant stress thermal cycling experiments in a three-point bending test. Thermo-magnetization curves of 97% cold-rolled and solution-treated sample aged at 600 °C for 24, 48 and 72 h showed evidence of the martensitic transformation, and the transformation temperatures increased their values from 24 to 72 h. The alloy cold-rolled to 97% and then solution-treated at 1277 °C for 1 h showed that most grains were aligned near <100> in the rolling direction in the recrystallization texture. The intensity of texture was 13.54, and an average grain size was around 400 μm. The sample aged at 600 °C for 48 h showed fully recoverable strain up to 1.6% at 200 MPa stress level in the three-point bending test. However, the experimental recoverable strain values were lower than the theoretical values, possibly due to the small volume fraction of low angle grain boundary, the formation of brittle grain boundary precipitates, and a grain boundary constraint lower than the expected intensity of texture in the samples. Full article

This study aims at detailed characteristics and comparison between dusts from various iron and non-ferrous metal production processes in order to identify individual mineral phases, chemical composition, and their influence on the values of magnetic susceptibility. Various analytical methods used include inductively coupled plasma optical emission spectroscopy, X-ray diffraction, scanning electron microscopy, and Mössbauer spectroscopy integrated with magnetic susceptibility measurements and thermomagnetic analysis. Metallurgical wastes that have arisen at different production stages of iron and non-ferrous steel are subjected to investigation. The analyzed dust samples from the iron and non-ferrous metallurgy differ in terms of magnetic susceptibility as well as their mineral and chemical composition. The research confirmed the presence of many very different mineral phases. In particular, interesting phases have been observed in non-ferrous dust, for example challacolloite, which was found for the first time in the dusts of non-ferrous metallurgy. Other characteristic minerals found in non-ferrous metallurgy dusts are zincite, anglesite, and lanarkite, while dusts of iron metallurgy contain mostly metallic iron and iron-bearing minerals (magnetite, hematite, franklinite, jacobsite, and wüstite), but also significant amounts of zincite and calcite. Full article

Industrial and urban dusts were characterized by investigating their magnetic properties. Topsoil composed of technogenic magnetic particles (TMP) originating from areas affected by three ironworks, street dust mainly composed of traffic-related pollution, and particulate matter (PM) from urban agglomeration in Warsaw, Poland were investigated. Several magnetic methods, namely magnetic susceptibility, thermomagnetic curves, hysteresis loops, decomposition of isothermal remanent magnetization acquisition curves, and first-order reversal curves, were performed to evaluate the magnetic fraction of dust. Magnetite was the main magnetic phase in all types of samples, with a small amount of high-coercive hematite within ironworks and street dust samples. Significant differences were observed in the domain structure (grain size) of industrial and traffic-related magnetic particles. The grain size of TMP obtained from steel production was in the range of 5–20 µm and was predominated by a mixture of single-domain (SD) and multidomain (MD) grains, with the prevalence of SD grains in the topsoil affected by Třinec ironwork. The traffic-related dust contained finer grains with a size of about 0.1 µm, which is characteristic of the pseudo-single-domain (PSD)/SD threshold. Street dusts were composed of a slightly higher proportion of MD grains, while PM also revealed the typical behavior of superparamagnetic particles. Full article

Ba₂Mg_0.4Co_1.6Fe₁₂O₂₂ was prepared in powder form by sonochemical co-precipitation and examined by X-ray diffraction, Mössbauer spectroscopy and magnetization measurements. Careful XRD data analyses revealed the Y-type hexaferrite structure as an almost pure phase with a very small amount of CoFe₂O₄ as an impurity phase (about 1.4%). No substantial changes were observed in the unit cell parameters of Ba₂Mg_0.4Co_1.6Fe₁₂O₂₂ in comparison with the unsubstituted compound. The Mössbauer parameters for Ba₂Mg_0.4Co_1.6Fe₁₂O₂₂ were close to those previously found (within the limits of uncertainty) for undoped Ba₂Mg₂Fe₁₂O₂₂. Isomer shifts (0.27–0.38 mm/s) typical for high-spin Fe³⁺ in various environments were evaluated and no ferrous Fe²⁺ form was observed. However, despite the indicated lack of changes in the iron oxidation state, the cationic substitution resulted in a significant increase in the magnetization and in a modification of the thermomagnetic curves. The magnetization values at 50 kOe were 34.5 emu/g at 4.2 K and 30.5 emu/g at 300 K. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves were measured in magnetic fields of 50 Oe, 100 Oe, 500 Oe and 1000 Oe, and revealed the presence of two magnetic phase transitions. Both transitions are shifted to higher temperatures compared to the undoped compound, while the ferrimagnetic arrangement at room temperature is transformed to a helical spin order at about 195 K, which is considered to be a prerequisite for the material to exhibit multiferroic properties. Full article