MAGNETIC CHARACTERIZATION OF 3D PRINTED HIGHPERFORMANCE POLYAMIDE MAGNETIC COMPOSITE

Polyamide 4.6 is classified as a high-temperature thermoplastic because of its service temperatures of up to 175°C, bringing it close to high-temperature plastics like PPS or PEEK. Due to its high-temperature capability and price/performance ratio, Polyamide 4.6 is considered viable in developing high-performance bonded magnets by serving as a binder/matrix to magnetic powders/fillers to form multifunctional magneto polymeric composites that offer superior properties to conventional materials. In this research, 10 wt.% and 30 wt.% strontium ferrite magnetic powders were compounded with polyamide 4.6 using a co-rotating twin screw extruder to fabricate monofilament bonded magnets for 3D printing. Morphological and magnetic characterization was performed on the 3D printing monofilament samples using Scanning Electron Microscopy, and Vibrating Sample Magnetometer, respectively. The morphological observations showed an even dispersion of the strontium ferrite fillers in the magnetic composite. The magnetic hysteresis results obtained via the VSM measurement at elevated temperatures indicate that the polyamide 4.6 binder was durable enough to keep the magnetic particles from moving in the bonded magnets. This suggests that the strontium ferrite bonded with Polyamide 4.6 binder is a much better composite and can be used in high-temperature applications up to 150 °C.Mechanical Engineerin

Thickness dependence of the magnetic hysteresis of NiFe-31% films as a function of an applied isotropic in-plane stress

The magnetic hysteresis of dc magnetron-sputtered Ni69Fe31 films that were sandwiched between titanium layers was investigated as a function of an externally applied isotropic in-plane strain. The hysteresis curves were measured with a Kerr magnetometer that monitored the longitudinal Kerr ellipticity as a function of the in-plane magnetic field. The strain was created by bending the samples in two dimensions using a pressure cell. Measurements were performed on films with different thicknesses. The magnetoelastic properties appeared to be much smaller for films with a thickness of 100 nm than for films with a thickness of 288 or 500 nm. This might be due to a change of the domain wall pinning, or a change of the domain wall density as a function of the film thickness. Measurements under compressive isotropic in-plane stress were shown to be possible by flipping the sample in the pressure holder and measuring through the glass substrate. The Faraday effect and stress-induced birefringence in the glass substrate did not hinder the accumulation of noiseless hysteresis data. The magnetoelastic effects appeared to be different for positive and negative values of the applied stress

Jones matrix description of Fabry-Perot interference in a single axis photo-elastic modulator and the consequences for the magneto-optical measurement method

When using a Photo-elastic modulator (PEM) in combination with a coherent light source, in addition to the modulation of the phase, Fabry-Perot interference in the PEM’s optical head induces large offsets in the 1ω and 2ω detector signals. A Jones matrix which describes both effects simultaneously, was derived for a single axis PEM and used to find an expression for the detector signal for two different MO Kerr setups. The effect of the PEM tilt angle, polarizer angle, analyzer angle, and retardation, on the detector signal offsets show that offsets can be zeroed by adjusting PEM tilt angle, polarizer angle, and retardation. This strategy will allow one to avoid large offset drifts due to the small retardation, intensity, and beam direction fluctuations caused by lab temperature fluctuations. In addition, it will enable one to measure in the most sensitive range of the lock-in amplifiers further improving the signal to noise ratio of the setup

The magnetic anisotropy of field-assisted 3D printed nylon strontium ferrite composites

Magnetic Field Assisted Additive Manufacturing (MFAAM), 3D printing in a magnetic field, has the potential to fabricate high magnetic strength anisotropic bonded magnets. Here, 10, 35, and 54 wt% strontium ferrite bonded magnets using polyamide 12 binder were developed by twin screw compounding process and then printed via MFAAM samples in zero, and in 0.5 Tesla (H parallel to the print direction and print bed). The hysteresis curves were measured using a MicroSense EZ9 Vibrating Sample Magnetometer (VSM) for 3 different mount orientations of the sample on the sample holder to explore the magnetic anisotropy. The samples printed in zero field exhibited a weak anisotropy with an easy axis perpendicular to the print direction. This anisotropy is caused by the effect of shear flow on the orientation of the magnetic platelets in the 3D printer head. For the MFAAM samples, the S values are largest along the print bed normal. This anisotropy is caused by the field. The alignment of the magnetic particles happens when the molten suspension is in the extruder. When the material is printed, it is folded over on the print bed and its easy axis rotates 90° parallel to the print bed normally. Little realignment of the particles happens after it is printed, suggesting a sharp drop in temperature once the composite touches the print bed, indicating that field-induced effects in the nozzle dominate the anisotropy of MFAAM deposited samples

Structural stability and electronic properties of XTO2 (X= Cu, Ag; T=Al, Cr): An ab initio study including X vacancies and Mg doping

Ab initio density functional theory-based calculations are used to study the structural and electronic properties of CuAlO2, AgAlO2, CuCrO2, and AgCrO2 transparent conducting oxides (TCOs). The hexagonal 2H delafossite polymorph is determined here to be one of the most stable polymorphs, by comparing total energies for different structural phases. The simple antiferromagnetic configuration is chosen to model magnetic effects in CuCrO2 and AgCrO2 due to it having one of the lowest ground state total energies and containing the most semiconductor like behavior of the magnetic configurations considered. Electronic structures of 2H CuAlO2 and AgAlO2 obtained from different approximations for the exchange-correlation functional, GGA (PBE), PBE + U, PBE + mBJ, PBE + mBJ + U, and the Hybrid HSE06 are compared. Supercells are employed to model 6.25% Cu and Ag vacancies, 3.13% O vacancies, and 6.25% Mg doping replacing Al and Cr, from which structural and electronic properties are obtained and used to predict on the effectiveness of these native defects and dopant on increasing the conductivity in all TCOs studied in this work. The obtained partial density of states for the pristine systems supports a model of hole conduction in the a – b plane, perpendicular to the O – X – O dumbbells of the delafossite structure. Additionally, the partial density of states of the defective and doped systems suggest a growth environment deficient in X and saturated in O may increase conductivity in these materials

Computational modelling of a triaxial vibrating sample magnetometer

Magnetic Field Assisted Additive Manufacturing (MFAAM) enables 3D printing of magnetic materials of various shapes which exhibit a complex anisotropy energy surface containing contributions generated from different origins such as sample, particle, and agglomerate shape anisotropy, flow and field induced anisotropy, and particle crystal anisotropy. These novel magnet shapes require the need to measure the x, y, and z components of the magnetic dipole moment simultaneously to fully understand the magnetic reversal mechanism and unravel the complex magnetic anisotropy energy surface of 3D printed magnetic composites. This work aims to develop a triaxial vibrating sample magnetometer (VSM) by adding a z-coil set to a pre-existing biaxial VSM employing a modified Mallison coil set. The optimum size and location of the sensing coils were determined by modeling the sensitivity matrix of the z-coil set. The designed coil set was implemented using 3D printed spools, a manual coil winder, and gauge 38 copper wire. A 3D printed strontium ferrite nylon composite sample was used to estimate the sensitivity of the z-coils (50 mV/emu). The results herein are applicable for any VSM using a modified Mallison biaxial coil configuration allowing for a quick implementation on pre-existing systems

Symmetry considerations on band filling and first optical transition in NiO

Recent theoretical works on NiO have not agreed upon the nature of the first optical transition. By altering band filling – with highly concentrated O vacancies and Fe impurities – here, the orbital density of states is changed near the Fermi energy. The variation in optical properties, relative to the changes in orbital character, along with group theory analysis of hybridized orbitals, provides new insight when evaluating the first optical transition of NiO. Here, based on density functional theory, the first optical transition is found to have two possibilities – either superexchange site-hopping or a transition from the hybridized eg state to the hybridized a1u state, rather than the intra-atomic transitions which are causing disagreement in the recent literature

Wire texture C-axis distribution of strontium ferrite/PA-12 extruded filament

The magnetic anisotropy of strontium ferrite (SF)/PA12 filament, a popular hard magnetic ferrimagnetic composites that is used for 3D-printing of permanent magnets, is studied by vibrating sample magnetometry. The studied filaments have a composition of SF/PA-12 thermoplastic composite with a 40% wt.  ratio of SF. SF particles are non-spherical platelets with an average diameter of 1.3 um and a diameter to thickness ratio of 3. Filaments are produced by a twin-screw extruder and have a diameter of 1.5 mm. SEM images show that the SF particles are homogeneously distributed through the filament. VSM measurements on different parts of the filaments show that the outer part of the cylindrical filament has a higher anisotropy, and the core is mostly isotropic. This conclusion is consistent with computational work by others which suggest that particle alignment predominantly takes place near the walls of the extruder die where shear flow is maximum. Additional hysteresis curve measurement of the outer cylindrical part of the filament parallel to the r and ϕ directions indicates that the squareness of the hysteresis curve (S) is larger in the r-direction. This indicates that the outer surface of the filament has a strong easy axis in the r-direction. We conclude that the SF platelets line up parallel to the walls of the extrusion die

Electronic and optical properties of antiferromagnetic iron doped NiO – A first principles study

Antiferromagnetic NiO is a candidate for next generation high-speed and scaled RRAM devices. Here, electronic and optical properties of antiferromagnetic NiO: Fe 25% in the rock salt structure are studied and compared to intrinsic NiO. From density of states and complex dielectric function analysis, the first optical transition is found to be at lower frequency than intrinsic NiO due to an Fe impurity level being the valence band maximum. The resulting effects on refractive index, reflectivity, absorption, optical conductivity and loss function for Fe-doped NiO are compared to those of intrinsic NiO, and notable differences are analyzed. The electronic component of the static dielectric constant of NiO: Fe 25% is calculated to be about 2% less than that of intrinsic NiO