Large Saturation Magnetization Research Articles

Stoichiometric study of metastable TbCu7 type Sm-Fe phase synthesized by HDDR treatment

• Compositional variety of HDDR-synthesized TbCu 7 type Sm-Fe alloys is examined. • HDDR treatment is carried out for raw Sm-Fe alloys with various Fe-rich compositions. • TbCu 7 type Sm-Fe alloys synthesized by HDDR behaves like stoichiometric SmFe 8.5 alloy. • Transition from Fe-rich compositions to SmFe 8.5 occurs in a stage of HD reaction. Hydrogenation-disproportionation-desorption-recombination (HDDR) treatment of Fe-rich SmFe x alloys with x > 8.5 was conducted in order to investigate the possibility that the HDDR treatment can synthesize metastable TbCu 7 Sm-Fe phases with a Fe-rich composition, which are expected to provide large saturation magnetization . Fe-rich alloys prepared by melt spinning were HDDR-treated at 600 °C. As a result, the constituent phases of all the alloys transformed into non-Fe-rich TbCu 7 type SmFe 8.5 and iron phases in spite of the Fe-rich compositions of the original Sm-Fe phases. Thus, the TbCu 7 type Sm-Fe phase synthesized by HDDR treatment behaved as the stoichiometric compound of SmFe 8.5 . Electron microscope analysis revealed that the exclusion of excess iron from the original Sm-Fe phase occurred in the stage of disproportionation. In addition, the Sm-Fe phase prepared via a combination process of melt spinning and HDDR treatment had a very refined microstructure with grain sizes smaller than 100 nm, resulting in a large coercivity of 14.1 kOe after nitrogenization.

MnGa (001)-Textured Film Fabricated on Thermally Oxidized Si Substrate for Application to Ion Beam Bit Patterned Media

(001)-oriented MnGa films were grown on thermally oxidized Si substrates, and ion-beam patterned films using the MnGa grown on Si substrates were fabricated. The buffer layer of Cr (20 nm)/MgO (5 nm)/CrB (5 nm)/NiTa (25 nm) was used, and highly (001)-oriented and flat Cr buffer layer was obtained by high-temperature annealing of 800 °C. The large saturation magnetization $M_{\mathbf {s}}$ of 0.58 T, which is 74% of that of the epitaxial MnGa grown on MgO (001) substrate, was confirmed by growing the MnGa on the Cr buffer layer. As well as the large $M_{\mathbf {s}}$ , the MnGa on Si substrate exhibited a large perpendicular anisotropy. By using the (001)-oriented MnGa on Si substrate, ion-beam patterned structures whose pitch sizes down to 100 nm, corresponding to the areal density of 65 Gb/in2 were obtained. These results indicate that low-cost and high-density bit patterned media (BPM) were fabricated by the ion irradiation on MnGa grown on Si substrate. Furthermore, improvement of the crystal orientation of the buffer layer will be necessary to realize the MnGa BPM with the areal density of 1 Tb/in2 and more.

Structural and magneto-dielectric anomalies in Mn-doped Bi2Fe4O9 nanograins

We attempt to modify the magnetoelectric properties of antiferromagnetic Bi2Fe4O9 nanograins by substituting multivalent Mn ions at Fe sites. The doped Bi2Fe4O9 nanograins are synthesized by a hydrothermal method with a different Mn content x (x is the mole ratio of Mn to Mn plus Fe; x = 0.00, 0.02, 0.04, 0.06, and 0.10). Among them, the x = 0.04 sample shows the anomalous decrease of lattice parameters a and b, the lowest antiferromagnetic transition temperature, and the largest saturated magnetization. Besides, the magneto-dielectric effect (Δɛ/ɛ ∼ −4.9%) and the magneto-striction constant (λ ∼ −1.1 × 10−4) of the x = 0.04 sample are also the largest compared with others (220 K and 0.6 T). Combined with Raman scattering and x-ray photoelectron measurement, it is considered that the substitution site of Mn ions varies with an increase of doping amount x, which also leads to the change of valence state of Mn ions and the lattice structure, and further affects the strength of magnetic interaction and spin-lattice coupling.

Microstructure, dielectric and enhanced multiferroic properties of Fe3O4/PbZr0.52Ti0.48O3 composite ceramics

Magnetoelectric composite ceramics Fe3O4/PbZr0.52Ti0.48O3 with different molar ratios (Fe3O4/PbZr0.52Ti0.48O3 = 1:1, 1:2 and 1:8) were prepared by combining hydrothermal method and sol–gel method, effects of molar ratio on the structure, dielectric and multiferroic properties were investigated. The results indicate that the synthesized composites show bi-phase structure, ruling out the presence of any obvious impurity phases. The grains can be divided into two types, the larger grain is attributed to PbZr0.52Ti0.48O3 while the smaller one can be considered to be Fe3O4. With the molar ratio increasing, the grain shape of PbZr0.52Ti0.48O3 changes from stripe to bulk-like while the shape and size of Fe3O4 is near the same. Both the dielectric constant and loss decrease with frequency, the specimen with the molar ratio of 1:8 shows the largest dielectric constant, while the sample has the lowest dielectric loss when the molar ratio is 1:2. With the increase of molar ratio, the height of the relaxation peak decreases and the peak position shifts to higher temperature. When the molar ratio is 1:8, the relaxation peak disappears due to less interface polarization. The remnant polarization increases with increasing the molar ratio, the maximum value is 1.12 μC/cm2, obtained at 1 kHz when the molar ratio is 1:8. Anomalous magnetic properties are observed, in which the magnetization increases first and then decreases with molar ratio, the largest saturation magnetization is ~ 30 emu/g when the ratio is 1:2 due to the strong interface interaction. The sample with the ratio of 1:1 shows the largest magnetic loss because of the highest content of magnetic phase. The magnetization shows a monotonic variation behavior with the molar ratio, indicating the strong interface interaction between the two phases. The maximal ME coupling coefficient is about 5.12 mV/(cm.Oe) for the sample 1:2 due to the stronger magnetic and ferroelectric properties.

Ultrafine FeCo Nanoparticles Isolated by Ultrathin Dielectric Shells for Microwave Application

Low-dimensional soft magnetic materials are technologically important for miniaturization of magnetic devices. It is, however, challenging to maintain ferromagnetic response in nanosized magnetic material. In this work, ultrafine FeCo nanoparticles have been prepared by reactive pulsed laser deposition, which can be exchange coupled through ultrathin dielectric shells, even though their diameters are far below the superparamagnetic blocking limit. This unique core/shell structure guarantees ferromagnetic coupling and insulation of neighboring magnetic nanoparticles simultaneously. The nanoparticle film exhibits large saturation magnetization of 1.13 T, high electrical resistivity of 4200 μΩ·cm, maximum permeability of 120, a cutoff frequency of 1.7 GHz, and it is a promising material for microwave application.

Combinatorial evaluation of phase formation and magnetic properties of FeMnCoCrAl high entropy alloy thin film library

We report on the influence of the Al content (from 3.5 to 54 at.%) on phase formation and magnetic properties in FeMnCoCrAl high entropy alloy thin film libraries. Al additions to FeMnCoCr crystallizing in the alpha-Mn structure cause the formation of the body centered cubic (BCC) structure. This is consistent with density functional theory predictions as Al additions give rise to a larger stability for the BCC phase compared to the face centered cubic phase (FCC) which can be rationalized by the formation of a pseudogap at the Fermi level indicating the stabilization of the BCC phase over the FCC phase. Al additions to paramagnetic FeMnCoCr induce ferromagnetism. The largest saturation magnetization was measured for the film containing 8 at.% of Al. As the concentration of non-ferromagnetic Al is increased beyond 8 at.%, the number density of the ferromagnetic species is decreased causing a concomitant decrease in magnetization. This trend is consistent with ab initio predictions of the Al concentration induced changes in the magnetic moment. Based on the experimental and theoretical results presented here the effect of the Al concentration on the phase formation and the magnetic properties of FeMnCoCrAl thin film library can be rationalized.

Optical Excitation of Single- and Multimode Magnetization Precession in Fe - Ga Nanolayers

We demonstrate a variety of precessional responses of the magnetization to ultrafast optical excitation in nanolayers of Galfenol (Fe,Ga), which is a ferromagnetic material with large saturation magnetization and enhanced magnetostriction. The particular properties of Galfenol, including cubic magnetic anisotropy and weak damping, allow us to detect up to 6 magnon modes in a 120-nm layer, and a single mode with effective damping ${\alpha}_{eff}$ = 0.005 and frequency up to 100 GHz in a 4-nm layer. This is the highest frequency observed to date in time-resolved experiments with metallic ferromagnets. We predict that detection of magnetization precession approaching THz frequencies should be possible with Galfenol nanolayers.

Enhanced multiferroic and magnetoelectric properties of Ni0.92(Cu0.05Co0.03)Fe2O4/Ba1-xCaxZr0.10Ti0.90O3 lead-free composite films

The search for lead-free high performance magnetoelectric multiferroic materials at room temperature continues to be stimulated from the perspective of designing environmentally friendly multifunctional devices. In this context, lead-free composite thin films consisting of Ba1-xCaxZr0.10Ti0.90O3 (BCZT, x = 0, 0.10 and 0.15) as the ferroelectric/piezoelectric phase and Ni0.92(Cu0.05Co0.03)Fe2O4 (NCCFO) as the ferromagnetic/magnetostrictive component have been grown on Pt/TiO2/SiO2/Si substrates via chemical solution spin-coating method. The phase and structural evolutions of two distinct pure phases of as grown NCCFO/BCZT composite films were confirmed using X-ray diffractrometry (XRD) and field emission scanning electron microscopy (FE-SEM). The multiferroic, magneto-dielectric and magnetoelectric properties of the NCCFO/BCZT composite films have been investigated. Moreover, significant enhancement of multiferroic properties and magnetoelectric effect was observed in these composite films when compared to those reported recently for other lead-free BCZT based multiferroic composites. The magnetic studies demonstrate that the NCCFO/Ba0.90Ca0.10Zr0.10Ti0.90O3 (NCCFO/BCZT10) composite film exhibits a large saturation and remnant magnetization of Ms ∼ 346.2 emu/cm3 and Mr ∼ 73.5 emu/cm3 respectively having a coercive field of Hc ∼ 73.5 Oe. The NCCFO/BCZT10 composite film also exhibits good ferroelectric properties with saturation polarization Ps ∼26.7 μC/cm2, remnant polarization Pr ∼8.9 μC/cm2 and coercive field Ec ∼82.7 kV/cm. The observed magneto-dielectric effect is of two orders higher than those reported for other lead-free multiferroic composites. A maximum magnetoelectric voltage co-efficient of αME ∼22.4 mVcm−1Oe−1 is obtained for the NCCFO/BCZT10 composite film. The observed results suggest that the NCCFO/BCZT composite films are potential lead-free multiferroic systems at room temperature.

Influence of Li<sub>2</sub>CO<sub>3</sub> and ZnO Nanoparticle on Microstructure and Magnetic Properties of Low-Temperature Sintering LiZnTiBi Ferrites for High-Frequency Applications

LiZnTi ferrite ceramics with high saturation flux density (Bs), large remanence ratio (Br/Bs) and high saturation magnetization (4πMs) is a vital material for high frequency devices. In the present work, we prepared uniform and compact LiZnTiBi ferrite with large average grain size (>30μm) at 900°C. Firstly, the hybrid materials, including Li2CO3, ZnO, TiO2, Bi2O3 and Fe2O3, were pre-sintered at 850°C at O2 atmosphere. Next, composite additives composited of Li2CO3 and ZnO nanoparticles were added to control grain growth. The influences of the Li2CO3 and nano-ZnO (LZ) on the microstructure and magnetic properties of LiZnTiBi ferrite, especially for grain size, have been analyzed. SEM images demonstrated that moderate LZ additives (x=0.75 wt%) can prevent abnormal grains. Also, the ferrite samples possess compact microstructures. The phenomenon indicated that the LZ additive is a good sintering aid for low-temperature sintering LiZnTiBi ferrites. XRD patterns showed that all samples have a pure spinel phase. The magnetic properties, including Bs, Br/Bs and 4πMs, have weak change when LZ additives were added. However, due to smaller average grain size, the coercivity (Hc) gradually increased. Thus, a low-temperature sintering LiZnTiBi ferrite with high saturation flux density (Bs=311.10 mT), large remanence ratio (Br/Bs=0.86), low coercivity (Hc=244.6 A/m) and high saturation magnetization (Ms=75.40) was obtained when 1.00 wt% LZ additive was added. More important, the LiZnTiBi ferrite possessed uniform average grain. Such a sintering method (i.e., adding composite additive to control abnormal grain) should also promote synthesis of other advanced ceramics for practical applications.

Synthesis, structure and magnetic properties of multipod-shaped cobalt ferrite nanocrystals

A non-aqueous sol–gel route followed by oriented attachment to make multi-pod CoFe2O4 nanocrystals showing large room temperature saturation magnetization.

Optimizing Composition in MnBi Permanent Magnet Alloys

MnBi is an attractive rare-earth-free permanent magnetic material due to its low materials cost, high magnetocrystalline anisotropy (1.6 × 106 J m-3), and good magnetization (81 emu g-1) at room temperature. Although the theoretical maximum energy product (BH)max of 20 MGOe is lower than that of NdFeB-based magnets, the low temperature phase (LTP) of MnBi has a positive temperature coefficient of coercivity, up to 200 °C, which makes it a potential candidate for high temperature applications such as permanent magnet motors. However, the oxygen sensitivity of the MnBi compound and the peritectic reaction between Mn and Bi make it difficult to synthesize into a material with high purity. This challenge is partly offset by adding excess Mn to the alloy, with composition close to Mn55Bi45 resulting in the highest saturation magnetization after common processing techniques such as arc melting, casting, melt spinning, and ball milling. Here we report a systematic process which reduces the amount of excessive Mn, while simultaneously providing a large saturation magnetization (MS) of 79 emu g-1 at 300 K in the annealed Mn52Bi48 ribbons. We also report excellent magnetic properties in the ball powders, resulting in 0.5-5 µm particles with MS of 75.5 emu g-1, coercivity Hci of 10.8 kOe, and (BH)max of 13 MGOe using 9 T applied field at 300 K. A secondary annealing treatment on various ball milled powders increased Hci by up to 21%, and also resulted in an increase in MS up to 78.8 emu g-1.

Observation of lamellar like fringes and Barkhausen effects in iron-carbon filled vertically aligned carbon nanotubes

Observations of lamellar like fringes in iron-carbon-filled vertically aligned carbon nanotubes (CNTs) produced by pyrolysis of ferrocene and rapid cooling methodologies (controlled quenching) have been investigated. The quantitative phase analysis of encapsulated Fe3C, γ-Fe, and α-Fe by X-ray diffraction reveals that controlling the phase-percentage leads to control of the crystal habit and of magnetic properties. Large coercivities and variable saturation magnetizations were observed. In addition, we reported a thorough study of the hysteresis behavior, the Barkhausen effect which has to be possibly associated with the formation of interfacial-defects in the crystal areas in contact with CNT layers.

Room‐Temperature Ferromagnetic Insulating State in Cation‐Ordered Double‐Perovskite Sr2Fe1+xRe1−xO6Films

Ferromagnetic insulators (FMIs) are one of the most important components in developing dissipationless electronic and spintronic devices. However, FMIs are innately rare to find in nature as ferromagnetism generally accompanies metallicity. Here, novel room-temperature FMI films that are epitaxially synthesized by deliberate control of the ratio between two B-site cations in the double perovskite Sr2 Fe1+ x Re1- x O6 (-0.2 ≤ x ≤ 0.2) are reported. In contrast to the known FM metallic phase in stoichiometric Sr2 FeReO6 , an FMI state with a high Curie temperature (Tc ≈ 400 K) and a large saturation magnetization (MS ≈ 1.8 µB f.u.-1 ) is found in highly cation-ordered Fe-rich phases. The stabilization of the FMI state is attributed to the formation of extra Fe3+ Fe3+ and Fe3+ Re6+ bonding states, which originate from the relatively excess Fe ions owing to the deficiency in Re ions. The emerging FMI state created by controlling cations in the oxide double perovskites opens the door to developing novel oxide quantum materials and spintronic devices.

Exploring the Magnetoelectric Coupling at the Composite Interfaces of FE/FM/FE Heterostructures

Multiferroic materials have attracted considerable attention as possible candidates for a wide variety of future microelectronic and memory devices, although robust magnetoelectric (ME) coupling between electric and magnetic orders at room temperature still remains difficult to achieve. In order to obtain robust ME coupling at room temperature, we studied the Pb(Fe0.5Nb0.5)O3/Ni0.65Zn0.35Fe2O4/Pb(Fe0.5Nb0.5)O3 (PFN/NZFO/PFN) trilayer structure as a representative FE/FM/FE system. We report the ferroelectric, magnetic and ME properties of PFN/NZFO/PFN trilayer nanoscale heterostructure having dimensions 70/20/70 nm, at room temperature. The presence of only (00l) reflection of PFN and NZFO in the X-ray diffraction (XRD) patterns and electron diffraction patterns in Transmission Electron Microscopy (TEM) confirm the epitaxial growth of multilayer heterostructure. The distribution of the ferroelectric loop area in a wide area has been studied, suggesting that spatial variability of ferroelectric switching behavior is low, and film growth is of high quality. The ferroelectric and magnetic phase transitions of these heterostructures have been found at ~575 K and ~650 K, respectively which are well above room temperature. These nanostructures exhibit low loss tangent, large saturation polarization (Ps ~ 38 µC/cm2) and magnetization (Ms ~ 48 emu/cm3) with strong ME coupling at room temperature revealing them as potential candidates for nanoscale multifunctional and spintronics device applications.

Microstructure and magnetic properties of low-temperature sintered M-type hexaferrite BaZn0.6Sn0.6Fe10.8O19 for LTCC process

Abstract M-type barium hexaferrite (BaM) of composition BaZn0.6Sn0.6Fe10.8O19 was successfully synthesized through a ceramic process. Sintering aids Bi2O3·B2O3·SiO2·ZnO (BBSZ) were added to lower the sintering temperature. The formation of pure M phase for samples sintered at 900 °C with sufficient BBSZ was confirmed from XRD patterns analysis. Large saturation magnetization (69.02 emu/g) and relatively low magnetic anisotropy field (7.4 kOe) were obtained simultaneously when sample was synthesized with 2.5 wt% BBSZ dosage. These properties make the material suitable for applications in LTCC process. The ferromagnetic resonance (FMR) linewidth (ΔH) decreased efficiently by 719 Oe owing to the sintering aids. This M-type hexaferrite is also potential candidate for the design of microwave devices such as circulators and isolators operated at low frequencies. Meanwhile, the influences of sintering aids upon the microstructure, complex permeability and magnetic properties of M-type hexaferrite BaZn0.6Sn0.6Fe10.8O19 were investigated.

Temperature Dependent Electric Properties and Magnetoelectric Effects in Ferroelectric rich Ni0.8Mg0.2Fe2O4 + BaZr0.2Ti0.8O3 Magnetoelectric Composites

The magnetoelectric composites with the compositions of ferrites & ferroelectrics having the general chemical formula (x) Ni0.8Mg0.2Fe2O4 + (1-x) BaZr0.2Ti0.8O3 (in which x = 0.1, 0.2, 0.3) were synthesized by using double sintering ceramic technique. From XRD, the phase formation of ferrites with cubic structure and ferroelectrics with tetragonal perovskite structure was confirmed through the measurement of XRD. The two phase ME composites have larger saturation magnetization and dielectric constant; it is because of the effect of interfacial strain on BaZr0.2Ti0.8O3 ferroelectric phase. Furthermore, the ME response 4.262 mVcm−1Oe−1 was observed for (30%) Ni0.8Mg0.2Fe2O4 + (70%) BaZr0.2Ti0.8O3 composites at 5.0 kOe applied DC magnetic field; it shows the success of magnetic control of the dielectric response via the mechanical coupling which can be exploited in the future applications of multiferroic composites.

Size-selected Fe3O4-Au hybrid nanoparticles for improved magnetism-based theranostics.

Size-selected Fe3O4–Au hybrid nanoparticles with diameters of 6–44 nm (Fe3O4) and 3–11 nm (Au) were prepared by high temperature, wet chemical synthesis. High-quality Fe3O4 nanocrystals with bulk-like magnetic behavior were obtained as confirmed by the presence of the Verwey transition. The 25 nm diameter Fe3O4–Au hybrid nanomaterial sample (in aqueous and agarose phantom systems) showed the best characteristics for application as contrast agents in magnetic resonance imaging and for local heating using magnetic particle hyperthermia. Due to the octahedral shape and the large saturation magnetization of the magnetite particles, we obtained an extraordinarily high r2-relaxivity of 495 mM−1·s−1 along with a specific loss power of 617 W·gFe−1 and 327 W·gFe−1 for hyperthermia in aqueous and agarose systems, respectively. The functional in vitro hyperthermia test for the 4T1 mouse breast cancer cell line demonstrated 80% and 100% cell death for immediate exposure and after precultivation of the cells for 6 h with 25 nm Fe3O4–Au hybrid nanomaterials, respectively. This confirms that the improved magnetic properties of the bifunctional particles present a next step in magnetic-particle-based theranostics.

Fe2MnGe: A hexagonal Heusler analogue

We synthesized bulk polycrystalline samples of Fe2MnGe using arc-melting and vacuum annealing. Our experimental investigations revealed that Fe2MnGe crystallizes in a hexagonal DO19 phase at room temperature, in analogy with its “parent” compound Fe3Ge. We find a large saturation magnetization, even larger than that predicted for the tetragonal phase, of approximately 5μB/f.u. at T = 5 K, with a Curie temperature of Tc∼505 K. The large moment and magnetocrystalline anisotropy, along with the possibility of half-metallicity presents Fe2MnGe as a strong candidate for spin-transfer-torque RAM and other spintronic applications. We do not find evidence for the cubic or tetragonal phases predicted by first-principles theory during these investigations. However, our neutron diffraction investigations were suggestive of a low temperature structural transformation.

Fe-Cr-Nb-B ferromagnetic particles with shape anisotropy for cancer cell destruction by magneto-mechanical actuation

We introduce a new type of magnetic particles (MPs) prepared by wet milling of superferromagnetic Fe-Cr-Nb-B precursor glassy ribbons for cancer treatment by magneto-mechanical actuation in low magnetic fields (1 ÷ 20 Oe). The rectangular shapes of MPs and the superferromagnetism of the glassy alloys of which are made the MPs induce important magnetic shape anisotropies which, in association with a large saturation magnetization, generate an improved torque in a rotating magnetic field, producing important damages on the cellular viability of MG-63 human osteosarcoma (HOS) cells. The specific parameters such as MPs concentration, frequency and intensity of the applied magnetic field, or the time of exposure have a strong influence on the cancer cells viability. The specific behavior of the Fe-Cr-Nb-B MPs offers them destructive effect even in low magnetic fields such as 10 Oe, and this characteristic allows the use of coils systems which provide large experimental spaces. The novel MPs are used for the magneto-mechanical actuation alone or in association with hyperthermia, but also can be transported to the tumor sites by means of stem cells carriers.

Formation of BaFe12-xNixO19 ceramics with considerably high dielectric and magnetic property coexistence

High dielectric and magnetic properties coexisted in one is important for the miniaturization and multifunction of electronic devices. Ni doped barium ferrites were successfully fabricated by sol-gel technique in this work. The microstructures and properties were investigated by the means of XRD, SEM, XPS, impedance analysis and magnetic property measurement systems. Results showed that main phase of M-type barium ferrite with hexagonal plate-like grains and Ni2+ ions substituting for Fe3+ as BaFe12-xNixO19 was formed. Ni2+ doping promotes the formation and grain growth of barium ferrite ceramics. The optimized sintering temperature was 1250 °C at which relatively perfect microstructure was formed. Giant permittivity above 20 k and low dielectric loss of ∼0.1 generated in the ferrite. Low Ha of 14–13 kOe and Hc of 145–111 Oe emerged and large saturation magnetization of 60–72 emu/g presented in Ni2+-doped barium ferrites ceramics. The coexistence of impressive dielectric and magnetic properties in Ni2+-doped barium ferrites ceramics, makes it potential candidate in the area of miniaturization and multifunction of electronic devices.