Bolin Hu

Engineered magnetic shape anisotropy in BiFeO3-CoFe2O4 self-assembled thin films.

We report growth of various phase architectures of self-assembled BiFeO3-CoFe2O4 (BFO-CFO) thin films on differently oriented SrTiO3 (STO) substrates. CFO forms segregated square, stripe, and triangular nanopillars embedded in a coherent BFO matrix on (001)-, (110)-, and (111)-oriented STO substrates, respectively. Nanostructures with an aspect ratio of up to 5:1 with a prominent magnetic anisotropy were obtained on both (001) and (110) STO along out-of-plane and in-plane directions. Magnetic easy axis rotation from in-plane to out-of-plane directions was realized through aspect ratio control. An intractable in-plane anisotropy was fixed in CFO on (111) STO due to the triangular shape of the ferromagnetic phase nanopillars. These studies established a detailed relationship of magnetic anisotropy with specific shape and dimensions of ordered magnetic arrays. The results suggest a way to effectively control the magnetic anisotropy in patterned ferromagnetic oxide arrays with tunable shape, aspect ratio, and elastic strain conditions of the nanostructures.

Structure, morphology and magnetic properties of Mg(x) Zn(1 − x)Fe2O4 ferrites prepared by polyol and aqueous co-precipitation methods: a low-toxicity alternative to Ni(x)Zn(1 − x)Fe2O4 ferrites

The synthesis and properties of Mg((x))Zn((1 - x))Fe(2)O(4) spinel ferrites as a low-toxicity alternative to the technologically significant Ni((x))Zn((1 - x))Fe(2)O(4) ferrites are reported. Ferrite nanoparticles have been formed through both the polyol and aqueous co-precipitation methods that can be readily adapted to industrial scale synthesis to satisfy the demand of a variety of commercial applications. The structure, morphology and magnetic properties of Mg((x))Zn((1 - x))Fe(2)O(4) were studied as a function of composition and particle size. Scanning electron microscopy images show particles synthesised by the aqueous co-precipitation method possess a broad size distribution (i.e. ∼ 80-120 nm) with an average diameter of the order of 100 nm ± 20 nm and could be produced in high process yields of up to 25 g l(-1). In contrast, particles synthesised by the polyol-based co-precipitation method possess a narrower size distribution with an average diameter in the 30 nm ± 5 nm range but are limited to smaller yields of ∼ 6 g l(-1). Furthermore, the polyol synthesis method was shown to control average particle size by varying the length of the glycol surfactant chain. Particles prepared by both methods are compared with respect to their phase purity, crystal structure, morphology, magnetic properties and microwave properties.

Low loss factor Co2Z ferrite composites with equivalent permittivity and permeability for ultra-high frequency applications

Ferrite composites of nominal composition Ba3Co2+xIrxFe24−2xO41 were studied in order to achieve low magnetic and dielectric losses and equivalent permittivity and permeability over a frequency range of 0.3–1 GHz. Crystallographic structure was characterized by X-ray diffraction, which revealed a Z-type phase accompanied by increasing amounts of Y-type phase as the iridium amount was increased. The measured microwave dielectric and magnetic properties showed that the loss tan δe and loss tan δμ decreased by 80% and 90% at 0.8 GHz with the addition of iridium of x = 0.12 and 0.15, respectively. An effective medium approximation was adopted to analyze the composite ferrites having mixed phase structures. Moreover, adding Bi2O3 enabled equivalent values of real permittivity and real permeability over the studied frequency range. The resultant data give rise to low loss factors, i.e., tan δe/e′ = 0.008 and tan δμ/μ′ = 0.037 at 0.8 GHz, while characteristic impedance was the same as that of free space.

Domain rotation induced strain effect on the magnetic and magneto-electric response in CoFe2O4/Pb(Mg,Nb)O3-PbTiO3 heterostructures

The present work shows good control of both magnetic and electric properties with electric and magnetic fields, respectively, for epitaxial CoFe2O4 (CFO) films on Pb(Mg,Nb)O3-PbTiO3 (PMN-PT). X-ray reciprocal space mapping revealed a transformation between a- and c-domains in the PMN-PT under electric field (E). Magnetic hysteresis loop and magnetic force microscopy (MFM) measurements showed a considerable change in the magnetic properties in specific areas of CFO layers poled by MFM probe tips. Furthermore, a pulsed electric field applied to the substrate was found to switch the magnetization of CFO between high and low values, depending on the polarity of E.

Large-scale synthesis of high moment FeCo nanoparticles using modified polyol synthesis

Binary alloys of Fe and Co have among the highest magnetizations of any transition metal alloy systems, but their affinity to form oxides act to reduce the magnetization of nanoparticles as their size is reduced below ∼30 nm. Here, we demonstrate the synthesis of single phase, size-controlled FeCo nanoparticles having magnetization greater than 200 emu/g via a non-aqueous method in which ethylene glycol served as solvent and reducing agent as well as surfactant. Experiments indicated pure-phase FeCo nanoparticles, having saturation magnetization up to 221 emu/g for sizes of 20–30 nm, in single batch processes resulting in > 2 g/batch. Post-synthesis oxidation of nanoparticles was investigated until very stable nanoparticles were realized with constant magnetization over time.

Crystallographically textured self-biased W-type hexaferrites for X-band microwave applications

We report the magnetic and structural properties of a series of W-type barium hexaferrites of composition BaZn2−xCoxFe16O27, where x = 0.15, 0.20, and 0.25. The anisotropy field of these barium ferrites (BaW) decreased with the substitution of divalent Co ions, while they maintained crystallographic c-axis texture. The measured anisotropy field was ∼10 kOe, and a hysteresis loop squareness Mr/Ms = 79% was obtained due to well-controlled grain size within the range of single domain scale. These two properties make the BaW suitable for applications in microwave devices at lower frequencies, such as self-biased circulators operated at X-band frequencies.

Tunable fringe magnetic fields induced by converse magnetoelectric coupling in a FeGa/PMN-PT multiferroic heterostructure

The fringe magnetic field, induced by magnetoelectric coupling in a bilayer Fe-Ga/Pb(Mg1/3Nb2/3)O3_PbTiO3 (PMN-PT) multifunctional composite, was investigated. The induced external field is characterized as having a butterfly hysteresis loop when tuned by an applied electric field. A tuning coefficient of the electrically induced fringe magnetic field is derived from the piezoelectric and magnetostrictive properties of the composite. A measured maximum tuning coefficient, 4.5 Oe/(kV cm−1), is found to agree well with theoretical prediction. This work establishes a foundation in the design of transducers based on the magnetoelectric effect.The fringe magnetic field, induced by magnetoelectric coupling in a bilayer Fe-Ga/Pb(Mg1/3Nb2/3)O3_PbTiO3 (PMN-PT) multifunctional composite, was investigated. The induced external field is characterized as having a butterfly hysteresis loop when tuned by an applied electric field. A tuning coefficient of the electrically induced fringe magnetic field is derived from the piezoelectric and magnetostrictive properties of the composite. A measured maximum tuning coefficient, 4.5 Oe/(kV cm−1), is found to agree well with theoretical prediction. This work establishes a foundation in the design of transducers based on the magnetoelectric effect.

Giant magnetoresistance due to magnetoelectric currents in Sr3Co2Fe24O41 hexaferrites

The giant magnetoresistance and magnetoelectric (ME) effects of Z-type hexaferrite Sr3Co2Fe24O41 were investigated. The present experiments indicated that an induced magnetoelectric current in a transverse conical spin structure not only presented a nonlinear behavior with magnetic field and electric field but also depended upon a sweep rate of the applied magnetic field. More interestingly, the ME current induced magnetoresistance was measured, yielding a giant room temperature magnetoresistance of 32.2% measured at low magnetic fields (∼125 Oe). These results reveal great potential for emerging applications of multifunctional magnetoelectric ferrite materials.

Magnetocrystalline Anisotropy and FMR Linewidth of Zr and Zn-Doped Ba-Hexaferrite Films Grown on MgO (111)

Ba M-type ferrite films of composition BaFe₁₁Zn_0.5Zr_0.5O₁₉ were grown on MgO (111) substrates by pulsed laser deposition. Both Zn and Zr divalent ions were introduced to reduce the magnetic anisotropy field to ~7.5 kOe, or 46% lower than the parent Ba M-type compound. Temperature dependence of magnetization over 25 to 350°C was investigated, indicating a Curie temperature of 320°C. Ferromagnetic resonance linewidth, as the peak-to-peak power absorption derivative at 9.55 GHz, was 120 Oe. The substituted sample had a K₁ of ~0.920 × 10⁶ erg/cm³. Such sample properties provide an attractive option for self-biased microwave devices applications operating at X (8-12 GHz) and Ku bands (12-18 GHz) as circulators and isolators.

Magnetic and microwave properties of U-type hexaferrite films with high remanence and low ferromagnetic resonance linewidth

U-type barium hexaferrite films (Ba4Ni1.4Co0.6Fe36O60) were deposited on (0001) sapphire substrates by pulsed laser deposition. Microstructure and magnetic properties of the films were characterized by X-ray diffraction, scanning electron microscopy and vibrating sample magnetometry. Ferromagnetic resonance (FMR) measurements were performed at X-band. The results indicate an anisotropy field of ∼8 kOe, and the saturation magnetization (4πMs) of ∼3.6 kG. An optimal post-deposition annealing of films results in a strong (0 0 n) crystallographic texture and a high hysteresis loop squareness (Mr/Ms = 92%) leading to self biased properties. Furthermore, the highly self-biased ferrite films exhibited an FMR linewidth of ∼200 Oe. The U-type hexaferrite films having low microwave loss, low magnetic anisotropy field, and high squareness are a suitable alternative to Sc or In doped BaM ferrites that have been the choice material for self-biased microwave devices at X-band frequencies.