Small Saturation Magnetization Research Articles

Optimizing preparation conditions and characterizing for CoxPt1-x alloy cylindrical nanowires fabricated by electrodeposition on nanoporous polycarbonate membranes

This study investigated the influence of the preparation conditions on the Co composition, crystal structure, and magnetic and electric conduction properties of cylindrical CoxPt1-x alloy nanowires having 0.08 ≤ x ≤ 0.90, which were electrodeposited on nanoporous templates. When prepared with an optimal concentration ratio of Co and Pt aqueous solutions and electrodeposition potentials, highly oriented crystal structures of fcc Co-Pt (111) was obtained in CoxPt1-x nanowires (0.58 ≤ x ≤ 0.80). Nanowires with (111)-oriented fcc Co-Pt crystals exhibited average lengths and diameters of 5.8–10.8 μm and 130 nm, respectively, and showed an easy axis of magnetocrystalline anisotropy in the longitudinal direction of the nanowire. These results indicate a direct correlation between the crystal structure and magnetic properties. The concentration ratio of Co and Pt in the electrolytes and the electrodeposition potentials for fabricating CoxPt1-x alloy nanowires using the diluted Co composition (0.08 ≤ x ≤ 0.25) were determined. These nanowires were polycrystalline and had a small saturation magnetization. Furthermore, the anisotropic magnetoresistance (AMR) for a single CoxPt1-x nanowire was measured to estimate the domain wall (DW) width in the single CoxPt1-x nanowire. The value of the magnetoresistance (MR) ratio for the single nanowire (0.58 ≤ x ≤ 0.80) with highly (111)-oriented fcc Co-Pt crystal was 0.082–0.23 %, and the estimated value of DW width was 17–25 nm. These findings provide valuable information for constructing three-dimensional magnetic memory devices.

Ionically driven synthesis and exchange bias in Mn4N/MnNx heterostructures

Ferrimagnets have received renewed attention as a promising platform for spintronic applications. Of particular interest is the Mn4N from the ε-phase of the manganese nitride as an emergent rare-earth-free spintronic material due to its perpendicular magnetic anisotropy, small saturation magnetization, high thermal stability, and large domain wall velocity. We have achieved high-quality (001)-ordered Mn4N thin film by sputtering Mn onto η-phase Mn3N2 seed layers on Si substrates. As the deposited Mn thickness varies, nitrogen ion migration across the Mn3N2/Mn layers leads to a continuous evolution of the layers to Mn3N2/Mn2N/Mn4N, Mn2N/Mn4N, and eventually Mn4N alone. The ferrimagnetic Mn4N, indeed, exhibits perpendicular magnetic anisotropy and forms via a nucleation-and-growth mechanism. The nitrogen ion migration is also manifested in a significant exchange bias, up to 0.3 T at 5 K, due to the interactions between ferrimagnetic Mn4N and antiferromagnetic Mn3N2 and Mn2N. These results demonstrate a promising all-nitride magneto-ionic platform with remarkable tunability for device applications.

Cubic-type Heusler compound Mn2FeGa thin film with strain-induced large perpendicular magnetic anisotropy

This study reports the development of ferrimagnetic cubic Heusler compound ${\mathrm{Mn}}_{2}{\mathrm{Fe}}_{x}\mathrm{Ga}$ (MFG) thin films with large perpendicular magnetic anisotropy (PMA). By growing MFG on a Cr buffer layer, a cubic ${\mathrm{X}}_{\mathrm{a}}$ phase with a tetragonal distortion $c/a\ensuremath{\approx}1.04$ induced by the buffer layer was obtained in the range of $x=1.0--1.3$, leading to large PMA which exceeds $0.75\phantom{\rule{4pt}{0ex}}\mathrm{MJ}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}$ in stoichiometric ${\mathrm{Mn}}_{2}\mathrm{FeGa}$ $(x=1)$. Synchrotron M\"ossbauer spectroscopy revealed that these cubic MFG thin films show good chemical ordering close to the ${\mathrm{X}}_{\mathrm{a}}$-ordered state. First-principles calculations demonstrated that, in ${\mathrm{X}}_{\mathrm{a}}$-ordered cubic MFG, the characteristic electronic structure of Fe around the Fermi level causes a large uniaxial magnetocrystalline anisotropy under the tetragonal strain consistent with experiment. The half-metallic-like band structure of cubic MFG was also shown to be preserved under the strain. Cubic ${\mathrm{Mn}}_{2}\mathrm{FeGa}$ with its small saturation magnetization, large PMA, and possibility of being highly spin polarized make this material an ideal candidate for the development of magnetic random-access memory and other spintronic devices.

Operating characteristics of domain walls in perpendicularly magnetized ferrimagnetic cylindrical nano-wires for three-dimensional magnetic memory

We observed field- and current-induced domain wall motion (DWM) in a ferrimagnetic cylindrical nano-wire using a micromagnetic simulation. Two types of domain walls, namely the Hedgehog Bloch point (HBP) and transverse domain wall (TDW), can be generated by controlling the initial state of these systems. In the case of the field-induced HBP motion, depinning and Walker breakdown were observed, whereas they did not appear in the field-induced TDW motion. According to the observation of current-induced DWM, we found that the domain wall mobility in ferrimagnetic cylindrical nano-wires is higher than that in conventional ferromagnetic cylindrical nano-wires owing to the small saturation magnetization. The current-induced HBP motion exhibits depinning, unlike the TDW motion, which corresponds to field-driven DWM. These differences are attributed to the symmetry of the magnetization in the domain walls. These results provide notable insight into the design of three-dimensional memory based on current-induced DWM.

Synthesis, crystal structure, and magnetic properties of CoMoFeAl and related compounds

We have carried out joint theoretical and experimental investigations of three Heusler compounds CoMoFeAl, CoMo0.5Fe1.5Al, and Co1.5Mo0.5FeAl. Our first-principle calculations show that all three compounds show either ferro- or ferrimagnetic order with CoMoFeAl and CoMo0.5Fe1.5Al exhibiting high spin polarization of almost 80%. The investigated samples were prepared using arc melting and high vacuum annealing. All the samples show cubic crystal structure with disorder. The parent compound CoMoFeAl shows a small saturation magnetization of 12 emu/g, and a Curie temperature of 440 K. The other two compounds, namely, Co1.5Mo0.5FeAl and CoMo0.5Fe1.5Al, show much higher saturation magnetizations of 62 emu/g and 59 emu/g, and substantially higher Curie temperatures of 950 K and 780 K, respectively.

Tailoring the magnetic properties of non-magnetic Cd doped BaTiO3 nanostructures

In order to explore the potential role of non-magnetic cadmium (Cd) in establishing room temperature ferromagnetism (RTFM) in diamagnetic BaTiO3, the analyses of the structural, optical and magnetic properties of non-magnetic ions Cd-doped BaTiO3 nanoparticles of the series Ba1-xCdxTiO3 (x = 0.1 %, 0.3 % and 0.5 %) are successfully accomplished making use of the samples synthesized via facile sol–gel route. The X-ray diffraction and Raman scattering analyses demonstrate that doping with Cd leads to a change in the phase composition of the tetragonal BaTiO3 with the dominance of the BaTi2O5 phase. The core-level XPS study carried out gives direct evidence of cation vacancies due to the richness of lattice oxygen content in the investigated sample at x > 0.1 %. The Cd2+ substitution significantly reduces the optical bandgap of the BaTiO3 nanoparticles from 3.14 to 2.78 eV such that it enhances the PL visible emission. Ferromagnetism originates from the contributions of lattice contractions due to the structural deformation, defect formation and cation vacancies. Among all these samples, 0.3 % Cd doped BaTiO3 leads to the gaining of small saturation magnetization (Ms = 5.40 ± 0.02 × 10-3 emu/g).

Ultra-wide-band millimeter-wave generator using spin torque oscillator with strong interlayer exchange couplings

Recent increased development interest in millimeter-wave oscillator devices has necessitated realization of small oscillators with high frequency, wide frequency tunability, and room-temperature operation. Spin-torque oscillators (STOs) are fascinating candidates for such applications because of their nanometer size and suitability for room-temperature operation. However, their oscillation frequency and tunable range are limited to the order of 100 MHz–10 GHz. Here, we propose use of bilinear (J1) and biquadratic (J2) interlayer exchange couplings between ferromagnets in STOs to overcome these problems. The bilinear coupling contributes to oscillation frequency enhancement, whereas the biquadratic coupling facilitates frequency tunability via a current. Using micromagnetic simulation with parameters estimated from a material with small saturation magnetization, for J1 = 0 and J2 = − 1.0 mJ/m2, respectively, we find that the STO exhibits high frequency from 23 to 576 GHz and that its tunability reaches 61 GHz/(1011 A/m2) for current densities of − 0.5 to − 9.5 × 1011 A/m2. An analytical theory based on the macrospin model is also developed, which exhibits good quantitative agreement with the micromagnetic simulations. These results introduce new possibilities for spintronics applications in high-frequency devices such as next-generation mobile communications.

Magnetization Precession at Sub-Terahertz Frequencies in Polycrystalline Cu2 Sb-Type (Mn-Cr)AlGe Ultrathin Films.

A ferromagnetic metal nanolayer with a large perpendicular magnetic anisotropy, small saturation magnetization, and small magnetic damping constant is a crucial requirement for high-speed spintronic devices. Fabrication of these devices on Si/SiO2 amorphous substrates with polycrystalline structure is also strongly desired for the mass production industry. This study involves the investigation of sub-terahertz (THz) magnetization precessional motion in a newly developed material system consisting of Cu2 Sb-type MnAlGe and (Mn-Cr)AlGe films by means of an all-optical pump-probe method. These materials exhibit large perpendicular magnetic anisotropy in regions of a few nanometers in size. The pseudo-2D crystal structures are clearly observed in the high-resolution transmission electron microscopy (TEM) images for the film samples grown on thermally oxidized silicon substrates. The TEM images also show a partial substitution of Cr atoms for the Mn sites in MnAlGe. A magnetization precession frequency of 0.164 THz with a relatively small effective magnetic damping constant of 0.012 is obtained for (Mn-Cr)AlGe. Theoretical calculation infers that the modification of the total density of states by Cr substitution decreases the intrinsic magnetic damping constant of (Mn-Cr)AlGe.

Physical properties of microwave and solid state synthesized La0.7Na0.3MnO3

We report the structural, magnetic, electrical and broadband microwave absorption in La0.7Na0.3MnO3 sample synthesized by microwave (MW) irradiation (Na0.3LMO_MW) and compare them to the sample synthesized by solid-state (SS) reaction method (Na0.3LMO_SS). Single phase Na0.3LMO_MW was synthesized at 800 °C in 30 min, whereas, Na0.3LMO_SS sample was obtained by sintering at 1200 °C for 48 h. Although both these samples show ferromagnetic transition at TC ∼324.8 K, the MW-synthesized sample shows distinct physical properties: broad ferromagnetic transition, smaller saturation magnetization, a large difference between the magnetic ordering and metal-insulator transition temperatures, a large high-field magnetoresistance, a table top-like magnetocaloric effect, and a large low-field microwave absorption compared to the solid state synthesized sample. These differences are suggested to arise from magnetic heterogeneity induced by smaller grain size and surface spin disorder in the MW synthesized La0.7Na0.3MnO3.

Molecular beam epitaxial growth and magneto-transport properties of Mn4−xInxN films on SrTiO3(001) and MgO(001) substrates

Ferrimagnetic Mn4N films have a small saturation magnetization with perpendicular magnetic anisotropy (PMA) and are thereby considered suitable materials for ultrafast current-induced domain wall motion (CIDWM). We recently achieved ultrafast CIDWM, reaching 3,000 m/s at room temperature, in compensated ferrimagnet Mn4−xNixN (x = 0.2) films on SrTiO3[STO](001) substrates. In this work, we grew nonmagnetic element In-doped Mn4N epitaxial films (Mn4−xInxN) on two different substrates, STO(001) and MgO(001), by molecular beam epitaxy. The epitaxial growth of these Mn4−xInxN films was demonstrated up to x = 1.40 for those on MgO(001), but limited only to x = 0.12 for those on STO(001). The PMA vanished for greater amounts of In for both substrates. We attribute such a large difference in x between the two substrates to the difference in lattice mismatch between Mn4−xInxN/STO(001) and Mn4−xInxN/MgO(001). We also investigated anomalous Hall effect of the Mn4-xInxN epitaxial films, and discussed their anomalous Hall angles and coercive fields dependence on x.

Robustness of Binary Stochastic Neurons Implemented With Low Barrier Nanomagnets Made of Dilute Magnetic Semiconductors

Binary stochastic neurons (BSNs) are excellent hardware accelerators for machine learning. A popular platform for implementing them are low- or zero-energy barrier nanomagnets possessing in-plane magnetic anisotropy (e.g. circular disks or quasi-elliptical disks with very small eccentricity). Unfortunately, small geometric variations in the lateral shapes of such nanomagnets can produce large changes in the BSN response times if the nanomagnets are made of common metallic ferromagnets (Co, Ni, Fe) with large saturation magnetization. Additionally, the response times are also very sensitive to initial conditions. Here, we show that if the nanomagnets are made of dilute magnetic semiconductors with much smaller saturation magnetization, then the variability in their response times (due to shape variations and variation in the initial condition) is drastically suppressed. This significantly reduces the device-to-device variation, which is a serious challenge for large scale neuromorphic systems.

Investigation of the surface tension of water-based Fe3O4 magnetic fluids under magnetic field: The effect of surfactants

In this work, water-based Fe3O4 magnetic fluids were prepared and the effect of surfactants on magnetic surface tension was investigated under a constant magnetic field. Different surfactants include anionic surfactants citric acid and sodium dodecyl sulfate (SDS); nonionic surfactants polyvinyl pyrrolidone (PVP K30) and polyethylene glycol(PEG-600) were employed to modify Fe3O4 nanoparticles. After the modification, the stability of Fe3O4-SDS magnetic fluid improved greatly, with the zeta potential improved to −40.5 mV compared to Fe3O4 magnetic fluid (−22 mV) without surfactants. X-ray diffraction and Fourier transform infrared spectroscopy were employed to characterize the crystalline and the surface changes. The results showed that the surfactant was successfully coated on the surface of Fe3O4 without changing the crystallinity. The surface coverage of nanoparticles by the surfactants caused a smaller saturation magnetization than uncoated Fe3O4 nanoparticles while keeping the superparamagnetic behavior. More importantly, the surface tension of the magnetic fluid was measured by the Du Noüy ring method, and an in-depth study of the magnetization time and direction on the magnetic surface tension has been carried out. Visual characterization of the cluster structure was carried out for an in-depth analysis of the influence of the external magnetic field on magnetic surface tension.

Smart water-based ferrofluid with stable state transition property: Preparation and its application in anionic dye removal

This article reports a smart water-based ferrofluid that can transform from stable state to unstable state for separating the contained magnetic nanoparticles after use. The ferrofluid is synthesized by preparing polyethylenimine modified Fe3O4 nanoparticles via a one-pot method, and then improved by acidification treatment and ultracentrifugation-based washing. The resultant ferrofluid and intermediate products have been systematically characterized, verifying that the ferrofluid possesses superparamagnetism, high saturation magnetization as well as strong colloidal stability, and the magnetic nanoparticles contained in the ferrofluid have small aggregation size, strong electropositivity and high saturation magnetization. It has been confirmed that the acidification treatment and ultracentrifugation-based washing greatly activate the positive charge and reduce the aggregation size of polyethylenimine modified Fe3O4 nanoparticles, which is the key to our successful synthesis of this new type of ferrofluid. Furthermore, the adsorption behavior of the ferrofluid on the anionic Ponceau S dye has been systematically investigated, demonstrating that the ferrofluid can adsorb anionic Ponceau S dye in a short time (<5 min) with a maximum adsorption capacity of 140.26 mg/g. The experimental data show that the adsorption kinetics follows the pseudo-second-order mode and the Langmuir isotherm model is applicable to describe the adsorption processes. Importantly, the magnetic nanoparticles in the ferrofluid can be easy to separate from solution after adsorbing anionic Ponceau S dye, preventing secondary pollution and showing great potential in wastewater treatment.

The effect of particle size on structural and magnetic properties of Sm3+ ion substituted Zn-Mn nanoferrites synthesized by glycol-thermal method

Zn0.5Mn0.5SmxFe2-xO4 (0 ≤ x 0.05) fine powders with average crystallite sizes in the range 12–17 nm were synthesized by glycol-thermal reaction. The as-prepared compounds were subjected to thermal annealing process at 1100 °C, after which the crystallite sizes increased to about 60 nm. XRD analysis confirmed a single-phase cubic spinel structure in all the compounds investigated. TEM images showed nearly spherical particles with uniform particle size distributions. SEM and FTIR have been used to study morphology and confirm phase formation, respectively. The Mössbauer spectrum for Zn0.5Mn0.5Fe2O4 (x = 0) at room temperature may be resolved into two quadrupole doublets indicative of paramagnetic spin state. Sm3+ substituted compounds showed weak sextets in addition to broad doublets attributed to some particle magnetic moments in ordered magnetic phase. The Mössbauer spectra of the compounds annealed at 1100 °C exhibit magnetic split sextets indicative of ordered magnetic phase. The compounds have small coercive fields and high saturation magnetization (40 emu/g to 60 emu/g) which reduce with increasing Sm3+ content due to the paramagnetic nature of Sm3+ ions. Magnetic measurements as a function of field and temperature in as-prepared particles showed superparamagnetic behaviour with average moment of about ≈104μB. The average particle size obtained from the magnetic data corresponds well with that estimated from XRD analysis.

Heat flux sensing by anomalous Nernst effect in Fe–Al thin films on a flexible substrate

We performed a numerical analysis of the material parameters required for realizing a heat flux sensor exploiting the anomalous Nernst effect (ANE). The results showed the importance of high thermopower of ANE (SANE) and small saturation magnetization. This motivated us to investigate the effect of Al substitution of Fe on ANE and found SANE = 3.4 μV K−1 in Fe81Al19 because of the dominant intrinsic mechanism. Using this material, we made a prototype ANE-based heat flux sensor on a thin flexible polyimide sheet and demonstrated accurate sensing with it. This study gives important information for enhancing sensor sensitivity.

Epitaxially grown Cu2Sb-type MnGaGe films with large perpendicular magnetic anisotropy

A perpendicularly magnetized film is essentially important for spintronics applications, such as spin-transfer-torque type magnetoresistive random access memories, STT-MRAMs, and large perpendicular anisotropy energy, Ku, with small saturation magnetization, Ms, is required for reducing the critical current for the STT induced magnetization switching phenomenon. In this study, an equiatomic intermetallic compound, MnGaGe, was investigated in epitaxially grown film samples. Perpendicularly magnetized films with an Ms value of about 260 emu/cm3 at 300 K were fabricated onto MgO (100) single crystal substrates. The maximum value of Ku was 8.1 × 106 erg/cm3 at room temperature. Microstructure analysis using a scanning transmission electron microscope atomically revealed the Cu2Sb type crystal structure in the film sample. First principles calculations were also carried out to discuss the electronic structure. Using the analysis of the projected density of states for 3d-orbitals, the hybridization of the 3d-orbitals was suggested as a possible factor for promoting magnetocrystalline anisotropy. The MnGaGe film with the large Ku and relatively small Ms would be potentially used for spintronics applications.

Perpendicular magnetic anisotropy of (001)-textured poly-crystalline MnAlGe films

Cu2Sb-type intermetallic compound, MnAlGe, is known to exhibit uniaxial magnetocrystalline anisotropy and relatively small saturation magnetization, which is suitable for spintronic application, e.g. spin-transfer phenomena requiring small critical current density. Ge-concentration dependence of the crystal structures, saturation magnetization, Ms, and perpendicular magnetic anisotropy, Ku was investigated for MnAlGe films prepared onto silicon substrates with a thermally oxidized amorphous layer. For the stoichiometric and Ge-rich samples, the films exhibited (001)-texture with perpendicular magnetization. The maximum values of Ms and Ku were 270 emu/cm3 and 4.8 × 106 erg/cm3, respectively, which were comparable values with those reported for bulk and epitaxially grown films in literatures.

Structural, Optical and Magnetic Properties of ZnS Co-doped with Cd and Fe

Cd and Fe co-doped nano ZnS samples (Zn0.75−xFexCd0.25S) are prepared by thermolysis method. The effect of Fe doping content on the structural parameters, infrared absorption bands and the energy gap of Zn0.75Cd0.25S are investigated using Rietveld analysis for synchrotron X-ray diffraction data, Fourier transform infrared and UV–Vis spectroscopy. The energy gap first decreases upon doping then it increases with further Fe-doping. The photoluminescence spectroscopy reveals that the emitted colors for Zn0.75−xFexCd0.25S are dependent on the excitation wavelengths and the amount of Fe-doping. Magnetization measurements indicate that Zn0.75Cd0.25S possess a diamagnetic behavior while Zn0.55Fe0.2Cd0.25S exhibits a weak ferromagnetic nature with small coercivity and saturation magnetization obtain from the law of approach to saturation.

Martensitic transformation and magnetic properties of Heusler alloys Co-V-(Al, Ga, In)

The electronic structure, magnetism and martensitic transformation in Co-V based Heusler alloys Co2V1.5Z0.5 (Z = Al, Ga, In) were investigated theoretically. In these alloys, the tetragonal martensitic phase has a higher stability compared with the cubic austenite. This can be explained based on their electronic structures. Here the energy difference ΔEM between the martensite and austenite is large in Co2V1.5Al0.5 and Co2V1.5Ga0.5, suggesting that they are possible candidates for magnetic shape memory alloys (MSMAs). This also agrees with the existing experimental observation in Co-V-Ga. In Co2V1.5Z0.5 a ferromagnetic to paramagnetic transition occurs together with the martensitic transition. The spin polarization ratio also decreases drastically from close to 100% in austenite to almost zero in martensite. All this makes Co2V1.5Z0.5 alloys a cross-over point of MSMA and spintronics, which can be controlled by magnetic/temperature/stress multi-field. In Co2V1.5Ga0.5 alloy, calculations suggest a degree of Co-V disorder in the experimental sample, which may introduce a weak ferromagnetic state in the martensite and lower the phase transition driving force ΔEM. This can explain the small saturation magnetization of the martensite reported in literature. To improve the properties of Co-V-Ga alloys, adjusting their atomic ordering can be of great importance.

Thermal treated synthesis and characterization of polyethylene glycol (PEG) mediated zinc ferrite nanoparticles

Zinc ferrite (ZnF) nanoparticles have been the latest topic with a keen interest in the medical field and industries by virtue of their unique superparamagnetic behavior at room temperature. Many synthesis methods and different doping materials have been used to enhance the particle size for these applications. This work reports a comparison of ZnF as-prepard and PEG-mediated ZnF nanoparticles. A powder X-ray diffraction (XRD) spectroscopy confirms the formation of single-phase cubic spinel structure with PEG-ZnF showing the broadest peaks. The average particle size increases from 23.8 nm to 29.5 nm for ZnF and PEG-ZnF nanoparticles respectively. Fourier transform infrared spectroscopy (FT-IR) confirmed the formation of pure phase ferrites and the presence of metal oxide bands. The scanning electron microscopy (SEM) shows the formation of spherical shape with a reduced size from 22.5 ± 19.6 nm to 19.5 ± 2.9 nm after the mediation of PEG and sintering. The phase purity of PEG-ZnF was confirmed by energy dispersive X-ray spectroscopy (EDX). The magnetic properties confirmed by a vibrating sample magnetometer (VSM), showed that the two samples exhibit superparamagnetic behavior with small saturation magnetization for PEG-ZnF than ZnF. The moderate crystallite size, uniform morphology with 15% polydispersity index and low saturation magnetization makes PEG-ZnF a nanoparticle that is good for sundry applications.