Magnetic Microstructure Research Articles

Microstructure, magnetic properties and magnetic hardening in 2 : 17 Sm–Co magnets

Abstract A comprehensive and systematic study has been made on Sm(Co, Fe, M, L)z magnets (M = Cu or Ni, and L = Zr or Ti) to completely understand the effects of composition and processing on the microstructure and magnetic properties of magnets. Ti-containing magnets do not have a lamellar phase but exhibit only a cellular microstructure, resulting in a much lower coercivity (below 10 kOe). Ni-containing magnets exhibit a perfect cellular/lamellar microstructure, but without a large domain wall energy gradient at the interface of the 2 : 17 and 1 : 5 phases, leading to a low coercivity. Only in the magnets containing both Cu and Zr, a uniform and stable cellular/lamellar microstructure with a high domain wall energy gradient across the 1 : 5 phase is formed, resulting in high coercivity. These results indicate that the conditions for effective magnetic hardening are: (1) Formation of a cellular/lamellar microstructure, and (2) establishment of a domain wall energy gradient at the cell boundaries. Based on all of these experimental results, the magnetization reversal mechanism of 2 : 17 Sm–Co magnets can be explained by both the domain wall pinning and nucleation models. The nucleation mechanism holds at any temperature in the Cu-rich magnets, and only above the Curie temperature of the 1 : 5 phase in the alloys with the lower Cu content. In these cases, the 2 : 17 cells become magnetically decoupled.

Tailoring the magnetic properties and microstructure of Alnico 8 magnets by various Ti contents and processing conditions

Shape anisotropy, developed as a result of spinodal decomposition (SD) is the basis for the hysteresis behavior of Alnico magnets. The SD process is sensitive to processing conditions and constituent elements. In this paper, the development of spinodal structure and its effects on magnetic properties and serviceability of Alnico 8 alloys are investigated. The Ti-content is varied between 2.5 and 7.5 wt%. The magnets are fabricated by conventional casting, homogenization, magnetic field annealing and tempering treatments. The magnetic properties of intrinsic coercivity, Hcj = 30 kA/m, remanence, Br = 0.53 T and maximum energy density, (BH)max = 6.03 kJ/m3 for 2.5 wt% Ti are increased to Hcj = 150 kA/m, Br = 0.0.88 T and (BH)max = 29.3 kJ/m3 for 6.5 wt% Ti alloy. These magnetic properties are 100% reversible up to 800 K. Furthermore, an alloy with 5.5 wt% Ti is directionally solidified and magnetic properties of Br = 1.01 T and (BH)max = 58.9 kJ/m3 are obtained. The evolution of mosaic nano structure, the mechanism of magnetic hardening and the interactions of various phases have been investigated.

Magnetic Hysteresis Properties and Microstructure of High-Coercivity (Nd,Dy)–Fe–B Magnets with Dy less than 10 wt % and Low Oxygen

Magnetic Hysteresis Properties and Microstructure of High-Coercivity (Nd,Dy)–Fe–B Magnets with Dy less than 10 wt % and Low Oxygen

Fabrication of Textured Porous Ti<sub>3</sub>SiC<sub>2</sub> by Slip Casting under High Magnetic Field and Microstructural Evolution through High Temperature Deformation

To clarify the effect of constraint conditions on the kink formation, fabrication process of the texture and porosity controlled Ti3SiC2 polycrystals was investigated and microstructural evolution during high temperature deformation was examined in it under high temperature uniaxial compression tests at 1200°C. Dense textured Ti3SiC2 sintered body was fabricated by slip casting in the high magnetic field of 12 T and following pressureless sintering at 1400°C for 1 h. The porosity of the textured Ti3SiC2 was controlled by dispersing polymethyl methacrylate (PMMA) particles into the textured Ti3SiC2 as a spacer media. The highly textured Ti3SiC2 polycrystals with porosity of 8.4 vol% and 16.7 vol%, respectively, were successfully fabricated by the slip casting in the high magnetic field. After the high temperature uniaxial compression perpendicular to the c-axis of the textured structure, both the porous and dense Ti3SiC2 showed kink formation, which is a common deformation mode for anisotropic layered materials. However, the average rotation angles of the kink boundaries were higher in the porous specimen than in the dense specimen. Since the crystal rotation is necessary for the kink formation, kink bands would be preferably developed in the porous area due to its weaker constraint than in the dense area. It can be concluded from the microstructural analysis that the constrain factor caused by the neighbor grains affects the crystalline rotation, resulting in the kink boundary formation with different rotation angles.

A new insight on evolution of texture and cellular structure of P/M processed Sm2Co17 permanent magnets and their effect on magnetic properties

Sm2Co17 type permanent magnets were prepared by a novel processing technique involving pulsed magnetization followed by cold isostatic pressing (CIP). The magnetic properties and microstructure of the CIPed magnetswere compared with those of the magnets processed through conventional uniaxial compaction in a transverse field press (TP)and heat treated under identical conditions. Enhanced values of remanence (Br ~ 10.7 kG) and energy product [(BH)max ~ 29 MGOe] were obtained in the CIPed magnetsas compared to those of the TP magnets[Br ~ 10 kG and (BH)max ~ 25 MGOe]. However, the values of intrinsic coercivity (iHc) and knee field (Hk) of CIPed magnet were found to be lower than those of the TP magnet.The results were explained on the basis of the difference in texture, microtexture and microstructure obtained in these magnets due to different processing methods adopted.

Magnetic Fabrics and Petrography of Rocksalts Reveal Preferred Orientation of Anhydrites within a Halite Matrix

We investigate the magnetic fabrics and microstructures of diamagnetic rocksalt samples from the Sedom salt wall (diapir), Dead Sea Basin, as possible strain markers. A comprehensive study of anisotropy of magnetic susceptibility (AMS), combined with magnetic, microtextural, geochemical and mineralogical analyses allows us to depict the deformation mechanisms and to reveal the mineral sources of the AMS. The rocksalts are composed of halite as the major mineral phase (>80%) and anhydrite as a minor phase (5–20%), and have an average magnetic susceptibility value of −13.4 ± 0.7 × 10−6 SI. Ferromagnetic and paramagnetic minerals make a negligible contribution to the bulk magnetic properties of the samples. The AMS indicates and reveals significant anisotropy with the maximum susceptibility axis (K1) subparallel to the bedding strike, although the cubic halite crystals are isotropic. Polarizing microscope and SEM images show preferred alignment of needle-like anhydrite crystals parallel to the direction of the K1 axis. Petrographic investigation of gamma irradiated thin sections reveals the deformation recorded in the microstructures of the rocksalts and points to a dominant contribution by dislocation creep, although both dislocation creep and pressure solution were active deformation mechanisms. We infer that during dislocation creep, the thin bands of anhydrite crystals deform along with the surrounding halite grains. We suggest that although the shape preferred orientation of halite grains is not indicative of finite strain because of resetting by grain boundary migration, the preferred orientation of the anhydrite crystals may be. These results suggest that the AMS of the rocksalts provides a textural proxy that reflects deformation processes of the rocksalts, despite their very low magnetic susceptibility.

Magnetic properties and microstructure of FePt (Ag, C) films with MgTiOBN/CrRu underlayer

The (001) textured L10 FePt film is the promising candidate material for coming heat assisted magnetic recording technology. The material of epitaxial intermediate layer for FePt based granular film was considered by the surface energy and lattice mismatch between FePt and MgO-based materials. The (00L) textured FePt (Ag, C)/MgTiOBN/CrRu film was prepared and the MgTiOBN intermediate layer was prior explored in this study. This layer was sputtered by MgTiOBN target at 435 °C and formed the (002) texture on CrRu (002)/glass. The 6–12 nm thick FePt films were finally deposited on MgTiOBN at 470 °C. The FePt (Ag, C) films illustrate high perpendicular magneto- crystalline anisotropy (Ku) (3.85–3.97 × 107 erg/cm3) and out-of-plane coercivity (17.3–20.1 kOe). The lower in-plane magnetization is evidenced by the lower remanence ratio, [Mr(in-plane)/Mr(out-of-plane) ~0.08–0.14]. The saturation magnetization (Ms) of FePt films are around 752–775 emu/cm3 after co-sputtering with 40 vol% (Ag, C) segregants. From cross-sectional transmission electron microscopy image, the FePt shows island-like grains with contact angle between 35 and 67° and the average FePt grains size is around 20 nm estimated from plane view microstructural images. The reason for nonuniform trapezoidal islands is that diffusive C can penetrate and separated the FePt grains but agglomerated again during thin film formation and the lower grains contact angles was due to close surface energy between MgTiOBN and FePt layer. The release of lattice strain energy was formed the misfit dislocation in between FePt/MgTiOBN interface and epitaxial domain match was evidenced in the (111) plane system.

Determination of small magnitudes of magnetostriction in amorphous microwires with an arbitrary type of magnetic anisotropy

The magnetic anisotropy and magnetic structure of amorphous ferromagnets are largely determined by magnetoelastic interactions due to the absence of magnetocrystalline anisotropy, e.g., in amorphous microwires with a glass sheath, the source of anisotropy is the mechanical stresses that arise in the ferromagnetic core upon manufacturing. Hence, to control the magnetic structure and magnetization reversal processes occurred in amorphous magnetics, it is necessary to know the magnetostriction coefficient of the material. We propose an improved approach to measuring extremely small values of the magnetostriction coefficient of ferromagnetic microwires with an arbitrary type of magnetic anisotropy and magnetic microstructure. The samples of amorphous wires in a glass sheath made of Co67Fe5B12Si14Cr3 alloys were studied. The type of magnetic anisotropy of the samples (from axial to circular) was changed using current annealing. The developed method is based on small-angle precession of magnetization around the wire axis, resulted from the effect of the axial magnetic field induced by an alternating current passed through the wire. A voltage signal generated in a detection coil wound around the sample at a frequency doubled with respect to the frequency of the alternating current was recorded using a lock-in amplifier. When exposed to external mechanical loads, the voltage signal changes, and an additional axial magnetic field (bias field) is required to maintain a constant level of this signal. The value of magnetostriction is determined from the dependence of the displacement field on mechanical loads. The maximum sensitivity of measurements in the range of 10–8 – 10–7 is achieved at a uniform magnetization, increased frequency of the alternating current, and high value of the ratio between the length and diameter of tested wire samples. The sign and magnitude of the magnetostriction constant change upon current annealing which correlates with modification of the magnetization curves. The results obtained can be used to determine and adjust the parameters of the actuators developed on the basis of the considered microwires (in particular, microsensors of mechanical stresses and microactuators).

Tailoring magnetic property and corrosion resistance of FeCoNiCuAl high-entropy alloy with Ce additive

The FeCoNiCuAl high-entropy alloy (HEA) has good magnetic and –mechanical properties which can be finely tuned by the fraction, composition, and distribution of the BCC and FCC phases. In this paper, we report the magnetic properties, corrosion resistance, and microstructure of the FeCoNiCuAlCex (0 ≤ x ≤ 0.09) HEAs. The results show that Ce addition is beneficial to the remanence (Br), coercivity (Hc), and hysteresis losses (Pu), but detrimental to the maximum permeability (μm) and maximum flux density (Bm). When x = 0.09, the Br, Hc, and Pu of the HEAs are improved by about 91%, 64% and 91%, respectively. The FeCoNiCuAl HEA has good corrosion resistance, comparable to 304 stainless steels. The addition of Ce improved the Icorr of the HEAs. X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM) results show that the FeCoNiCuAl HEAs are composed of a face-centered cubic (FCC) phase and a body-centered cubic (BCC) phase, with the Cu-rich nano-precipitates embedded in the BCC phase. With the addition of Ce, the nano-precipitates become even smaller and more uniform. Meanwhile, the FCC phase becomes Cu-rich and its magnetism changes from ferromagnetic to non-ferromagnetic. The electronic density of state (DOS) for the alloys calculated by density functional theory (DFT) is discussed.

Local Characterization of Ferromagnetic Resonance in Bulk and Patterned Magnetic Materials Using Scanning Microwave Microscopy

We have demonstrated the capabilities of the scanning microwave microscopy (SMM) technique for measuring ferromagnetic resonance (FMR) spectra in nanometric areas of magnetic samples. The technique is evaluated using three different samples, including a yttrium iron garnet (YIG) polycrystalline bulk sample and a thick YIG film grown by liquid phase epitaxy (LPE). Patterned permalloy (Py) micromagnetic dots have been characterized to assess the performance for imaging applications of the technique, measuring the variation of the magnetic properties of the sample along its surface. The proposed technique may pave the way for the development of high spatially resolved mapping of magnetostatic modes in different nanomagnetic and micromagnetic structures.

Enhancement of Magnetic Properties and Microstructural Changes in Tbcu7-Type Sm-Fe-Co-Nb-B Melt-Spun Ribbons

Enhancement of Magnetic Properties and Microstructural Changes in Tbcu7-Type Sm-Fe-Co-Nb-B Melt-Spun Ribbons

Effects of Nb Microalloying on the Magnetic Properties and Microstructure of Fe-Si-B-P-Cu-Nb Nanocrystalline Alloys Prepared with Pure and Industrial Raw Materials

Effects of Nb Microalloying on the Magnetic Properties and Microstructure of Fe-Si-B-P-Cu-Nb Nanocrystalline Alloys Prepared with Pure and Industrial Raw Materials

Magnetic Properties and Microstructure of Ce-Cu-Al Low Melting Alloy Bonding Sm2fe17n3 Magnet Fabricated by the Hot-Pressing Method

Magnetic Properties and Microstructure of Ce-Cu-Al Low Melting Alloy Bonding Sm2fe17n3 Magnet Fabricated by the Hot-Pressing Method

Spatial magnetic imaging of non-axially symmetric vortex domains in cylindrical nanowire by transmission X-ray microscopy.

The spatial magnetization texture of a cylindrical nanowire has been determined by Transmission X-ray Microscopy (TXM) and X-ray magnetic circular dichroism (XMCD). For this purpose, nanowires with designed geometry, consisting of CoNi/Ni periodic segments, have been grown by designed electrodeposition into alumina templates. Experimental data allow one to conclude the presence of mono- and trivortex magnetic domains in CoNi segments but, unusually, these states are characterized by an asymmetric XMCD contrast across the nanowire's section. Micromagnetic modelling shows non-trivial three-dimensional structures with ellipsoidal vortex cores and non-axially symmetric magnetization along the nanowire direction. The modelled TXM contrast of micromagnetic structures allows to correlate the experimental asymmetric XMCD contrast to the easy axis direction of the uniaxial magnetocrystalline anisotropy.

Формирование скирмионных состояний в ионно-облученных тонких пленках CoPt

The possibilities of controlled exposure to ion irradiation (He+ with an energy of 20 keV and a fluence in the range from 3×1014 to 3×1015 cm–2) as a method for modifying the magnetic properties and domain structure of Co0.35Pt0.65 thin ferromagnetic films have been studied. It was found that the ion irradiation causes a change in the Dzyaloshinsky-Moriya interaction constant and a change in the skyrmion density that correlates with it. This result shows the possibility of homogeneous ion irradiation as a way to control the micromagnetic structure, namely the process of formation of skyrmion states.

Manipulation of Micromagnetic Structure of Thin Co/Pt Multilayer Films by Precise Variation of Co and Pt Thicknesses

Manipulation of Micromagnetic Structure of Thin Co/Pt Multilayer Films by Precise Variation of Co and Pt Thicknesses

Formation of skyrmion states in ion-irradiated CoPt thin films

The possibilities of controlled exposure to ion irradiation (He+ with an energy of 20 keV and a fluence in the range from 3·1014 to 3·1015 cm-2) as a method for modifying the magnetic properties and domain structure of Co0.35Pt0.65 thin ferromagnetic films have been studied. It was found that the ion irradiation causes a change in the Dzyaloshinskii--Moriya interaction constant and a change in the skyrmion density that correlates with it. This result shows the possibility of homogeneous ion irradiation as a way to control the micromagnetic structure, namely the process of formation of skyrmion states. Keywords: ferromagnetic thin films, ion irradiation, domain structure, magnetic force microscopy, Dzyaloshinskii-Moriya interaction, skyrmions.

Isotropic Ferromagnetic Resonances Induced by Suppressed Anisotropy in Soft Magnetic Microstructures

Isotropic Ferromagnetic Resonances Induced by Suppressed Anisotropy in Soft Magnetic Microstructures

Synthesis and magnetic properties of Sb added MnBi magnets for applications at sub-zero temperatures

MnBi-LTP bulk magnets containing substitutional Sb element were fabricated using gas-atomizing process, surfactant-assisted wet magnetic compaction process, and thermal compaction process. Magnetic properties and microstructures were investigated according to the processing parameters. It was found that substitutional Sb element effectively refined the grain size of gas-atomized powders. The conventional ball milling process was able to pulverize these powders into fine particles with a single crystalline structure and minimize their phase decomposition, resulting in a (BH)max of 11.0 MGOe. By analyzing the effects of the compaction pressure on the density and magnetic alignment during the surfactant-assisted wet compaction process, Mn54Bi45.9Sb0.1 bulk magnet with Hc of 10.0 kOe at room temperature and temperature coefficient Hc of 0.68%/K was successfully fabricated. Furthermore, effects of Sb element on the temperature dependency of Hc in the bulk magnet were discussed.

Touchless Sensing Interface Based on the Magneto-Piezoresistive Effect of Magnetic Microstructures with Stacked Conductive Coating.

Robotics capable of human-like operations need to have electronic skin (e-skin) with not only tactile sensing functions but also proximity perception abilities. Especially, under the current widespread of COVID-19 pandemic, touchless interfaces are highly desirable. Magnetoreception, with inherent specificity for magnetic objects, is an effective approach to construct a non-contact sensing e-skin. In this work, we propose a new touchless sensing mechanism based on the magneto-piezoresistive effect. The substrate of the sensor is made of hierarchically microstructured ferromagnetic polydimethylsiloxane, coated with a three-dimensional (3D) piezoresistive network. The 3D network is constructed by stacked layers of reduced graphene oxide and carbon nanotubes through layer-by-layer deposition. With this integrated design, a magnetic force induced on the ferromagnetic substrate can seamlessly be applied to the piezoresistive layer of the sensor. Because the magnetic force relates strongly to the approaching distance, the position information can be transduced into the resistance change of the piezoresistive network. The flexible proximity sensor exhibits an ultrahigh spatial resolution of 60 μm, a sensitivity of 50.47 cm-1, a wide working range of 6 cm, and a fast response of 10 ms. The repeatable performance of the sensor is shown by over 5000 cycles of approaching-separation test. We also demonstrate successful application of the sensor in 3D positioning and motion tracking settings, which is critical for touchless tactile perception-based human-machine interactions.