Heat treatment effects on microstructure and magnetic properties of Mn-Zn ferrite powders

Mn-Zn ferrite powders (Mn_0.5Zn_0.5Fe₂O₄) were prepared by the nitrate-citrate auto-combustion method and subsequently annealed in air or argon. The effects of heat treatment temperature on crystalline phases formation, microstructure and magnetic properties of Mn-Zn ferrite were investigated by X-ray diffraction, thermogravimetric and differential thermal analysis, scanning electron microscopy and vibrating sample magnetometer. Ferrites decomposed to Fe₂O₃ and Mn₂O₃ after annealing above 550 °C in air, and had poor magnetic properties. However, Fe₂O₃ and Mn₂O₃ were dissolved after ferrites annealing above 1100 °C. Moreover, the 1200 °C annealed sample showed pure ferrite phase, larger saturation magnetization (M_s=48.15 emu g⁻¹) and lower coercivity (H_c=51 Oe) compared with the auto-combusted ferrite powder (M_s=44.32 emu g⁻¹, H_c=70 Oe). The 600 °C air annealed sample had the largest saturation magnetization (M_s=56.37 emu g⁻¹) and the lowest coercivity (H_c=32 Oe) due to the presence of pure ferrite spinel phase, its microstructure and crystalline size.     

Structure and magnetic properties of CoxPb1−x nanowire arrays

Ordered Co_xPb_1−x nanowire arrays embedded in the porous anodic aluminum oxide (AAO) template have been fabricated by electrodeposition. XRD experiments prove that neither hexagonal-close-packed (hcp) nor face-centered-cubic (fcc) Co peaks are detected when the Co component (x) is below 0.91. The coercivity (H_c) and squareness (M_r/M_s) are found to increase with ferromagnetic Co component and the maximum value is at the position x=1 (pure Co nanowires). Annealing effects cause H_c and M_r/M_s increase, which surpasses the pure Co nanowires in the 0.2<x<0.6 at the annealing temperature of 700 °C. Microstructure change during annealing process is proposed to explain the magnetic properties change of samples.     

Synthesis of α-Fe2O3 nanobelts and nanoflakes by thermal oxidation and study to their magnetic properties

α-Fe₂O₃ nanobelts and nanoflakes have been successfully synthesized by oxidation of iron-coated ITO glass in air. The X-ray diffraction, Raman spectrum and scanning electron microscopy are carried out to characterize the nanobelts and nanoflakes. The formation mechanism has been presented. Significantly, the magnetic investigations show that the magnetic properties are strongly shape-dependent. The magnetization measurements of belt-like and flake-like α-Fe₂O₃ in perpendicular exhibit ferromagnetic feature with the coercivity (H_c) and saturation magnetization (M_s) of 334.5 Oe and 1.35 emu/g, 239.5 Oe and 0.12 emu/g, respectively. For the parallel, belt-like and flake-like α-Fe₂O₃ also exhibit ferromagnetic feature with the H_c and M_s of 205.5 Oe and 1.44 emu/g, 159.6 Oe and 0.15 emu/g, respectively.     

Structural and magnetic properties of self-assembled Sm-Co spherical aggregates

Self-assembled Sm-Co nanoparticles in the form of spherical aggregates (referred as nanospheres) with diameter ranging from 50 to 180 nm were achieved by means of polyol technique. The size distribution of the Sm-Co nanospheres can be regulated close to ∼100 nm by controlling the molar ratio of Sm:Co precursor. The spherical aggregates exhibited Sm₂Co₇ phase as a major constituent; while the aggregates obtained at higher Co concentration showed co-existence of Co-phase with Sm₂Co₇ phase. Upon annealing, the biphasic nature of nanospheres (Sm₂Co₇/Co) transformed into Sm₂Co₁₇ structure. By varying the Sm:Co precursor ratio from 1:5 to 1:9, the coercivity (H_c) and magnetization (M_s) values of the as-synthesized nanospheres can be tuned from 336 to 140 Oe and from 63.7 to 108 emu/g, respectively, and these values significantly improved after annealing. Maximum values of H_c (1050 Oe) at the Sm:Co molar ratio of 1:5 and M_s of 184.6 emu/g at the Sm:Co molar ratio of 1:9 were achieved in the annealed samples.     

Study of NiCuZn ferrite powders and films prepared by sol gel method

This paper reports that a series of NiCuZn ferrite powders and films are prepared by using sol-gel method.The effects of raw material composition and the calcinate temperature on magnetic properties of them are investigated.The NiCuZn ferrite powders are prepared by the self-propagating high-temperature synthesis method and subsequently heated at 700 C～1000 C.The results show that NiCuZn ferrite powders with single spinel phase can be formed after heat-treating at 750 C.Powders obtained from Ni 0.4 Cu 0.2 Zn 0.4 Fe 1.9 O 4 gel have better magnetic properties than those from gels with other composition.After heat-treating at 900 C for 3 h,coercivity H c and saturation magnetization M s are 9.7 Oe (1 Oe=80 A/m) and 72.4 emu/g,respectively.Different from the powders,NiCuZn films produced on Si (100) from the Ni 0.4 Cu 0.2 Zn 0.4 Fe 2 O 4 gel formed at room temperature possess high properties.When heat-treating condition is around 600 C for 6 min,samples with low H c and high M s will be obtained.The minimal H c is 16.7 Oe and M s is about 300 emu/cm 3.In comparison with the films prepared through long-time heat treating,the films prepared through short heat-treating time exhibits better soft magnetic properties.     

A study on the spectral,microstructural, and magnetic properties of Eu–Nd double-substituted Ba0.5Sr0.5Fe12O19 hexaferrites synthesized by an ultrasonic-assisted approach

In this study, an examination on the spectral, microstructural, and magnetic characteristics of Eu–Nd double-substituted Ba_0.5Sr_0.5Fe₁₂O₁₉ hexaferrites (Ba_0.5Sr_0.5Nd_xEu_xFe_12−2xO₁₉ (x = 0.00–0.05) HFs) fabricated by an ultrasonic-assisted approach has been presented. An UZ SONOPULS HD 2070 ultrasonic homogenizer with frequency of 20 kHz and power of 70 W was used. The chemical bonding, structure and the morphology of the products were evaluated by Fourier-Transform Infrared (FT-IR) Spectroscopy, XRD (X-ray diffraction), scanning and transmission electron microscopy and techniques. The textural properties of the prepared nanomaterials were examined by using the Brunauer-Emmett-Teller (BET) method. The magnetic properties were studied using a vibrating sample magnetometer (VSM) at room temperature (RT) and low temperature 10 K. The magnitudes of various magnetic parameters including M_s (saturation magnetization), M_r (remanence) and H_c (coercivity) were estimated and evaluated. The M-H loops revealed the hard ferrimagnetic nature for all products at both temperatures. The M_s and M_r values showed a decreasing tendency with increasing degree of Eu³⁺ and Nd³⁺ substitutions whereas H_c values displayed an increasing trend. At RT, M_s, M_r and H_c values lie in the ranges of 63.0–68.8 emu·g⁻¹, 24.6–39.2 emu·g⁻¹ and 2252.4–2782.1 Oe, respectively. At 10 K, the values of M_s, M_r and H_c lie between 87.5–97.1 emu·g⁻¹, 33.5–40.1 emu·g⁻¹ and 2060.6–2417.2 Oe, respectively. The observed magnetic properties make the prepared products promising candidates to be applied in the recording media.     

Synthesis and surface modified hard magnetic properties in Co0.5Pt0.5 nanocrystallites from a rheological liquid precursor

Small crystallites of a metastable phase Co_0.5Pt_0.5 are precipitated by heating a rheological liquid precursor of cobalt–hydrazine complex and platinum chloride H₂PtCl₆·xH₂O in polymer molecules of poly(vinylpyrrolidone) (PVP) in ethylene glycol. The hydrazine co-reduces nascent atoms from the Co²⁺ and Pt⁴⁺ that recombine and grow as Co_0.5Pt_0.5. The PVP molecules cap a growing Co_0.5Pt_0.5 as it achieves a critical size so that it stops growing further in given conditions. X-ray diffraction pattern of a recovered powder reveals a crystalline Co_0.5Pt_0.5 phase (average crystallite size D∼8 nm) of a well-known Fm3m-fcc crystal structure with the lattice parameter a=0.3916 nm (density ρ=14.09 g/cm³). A more ordered L1₀ phase (ρ=15.91 g/cm³) transforms (D≥25 nm) upon annealing the powder at temperature lesser than 700 °C (in vacuum). At room temperature, the virgin crystallites bear only a small saturation magnetization M_s=5.54 emu/g (D=8 nm) of a soft magnet and it hardly grows on bigger sizes (D≤31 nm) in a canted ferromagnetic structure. A rectangular hysteresis loop is markedly expanded on an optimally annealed L1₀ phase at 800 °C for 60 min, showing a surface modified coercivity H_c=7.781 kOe with remnant ratio M_r/M_s=0.5564, and M_s=39.75 emu/g. Crystallites self-assembled in an acicular shape tailor large H_c from ideal single domains and high magnetocrystalline anisotropy of a hard magnet L1₀ phase.     

Effects of rapid thermal annealing on structural, magnetic and optical properties of Ni-doped ZnO thin films

XPS depth profiles were used to investigate the effects of rapid thermal annealing under varying conditions on the structural, magnetic and optical properties of Ni-doped ZnO thin films. Oxidization of metallic Ni from its metallic state to two-valence oxidation state occurred in the film annealed in air at 600 °C, while reduction of Ni²⁺ from its two-valence oxidation state to metallic state occurred in the film annealed in Ar at 600 and 800 °C. In addition, there appeared to be significant diffusion of Ni from the bottom to the top surface of the film during annealing in Ar at 800 °C. Both as-deposited and annealed thin films displayed obvious room temperature ferromagnetism (RTFM) which was from metallic Ni, Ni²⁺ or both with two distinct mechanisms. Furthermore, a significant improvement in saturation magnetization (M_s) in the films was observed after annealing in air (M_s = 0.036 μ_B/Ni) or Ar (M_s = 0.033 μ_B/Ni) at 600 °C compared to that in as-deposited film (M_s = 0.017 μ_B/Ni). An even higher M_s value was observed in the film annealed in Ar at 800 °C (M_s = 0.055 μ_B/Ni) compared to that at 600 °C mainly due to the diffusion of Ni. The ultraviolet emission of the Ni-doped ZnO thin film was restored during annealing in Ar at 800 °C, which was also attributed to the diffusion of Ni.     

Nanostructures and magnetic properties of cobalt ferrite (CoFe2O4) fabricated by electrospinning

This paper describes the fabrication of cobalt ferrite (CoFe₂O₄) nanostructures (in the form of nanofibers and nanoparticles) by the electrospinning method using a solution that contained poly(vinyl pyrrolidone) (PVP) and cheap Co and Fe nitrates as metal sources. The as-spun and calcined CoFe₂O₄/PVP composite samples were characterized by TG-DTA, X-ray diffraction, FT-IR, SEM and TEM, respectively. After calcination of the as-spun CoFe₂O₄/PVP composite nanofibers (fiber size of 320±48 nm in diameter) at 500, 600, and 800°C in air for 3 h with different heating rates of 5 or 20°C/min, either NiFe₂O₄ nanofibers of ∼10−200 nm in diameter or nanoparticles with particle sizes of ∼50−400 nm having a well-developed spinel structure were successfully obtained. The crystal structure and morphology of the nanofibers were influenced by the calcination temperature and heating rate. A faster heating rate allowed for a rapid removal of the PVP matrix and resulted in a complete change from fibrous structure to particle in the calcined CoFe₂O₄/PVP composite nanofibers. Room temperature magnetization results showed a ferromagnetic behavior of the calcined CoFe₂O₄/PVP composite nanofibers, having their hysteresis loops in the field range of ± 4500 and 3000 Oe for the samples calcined respectively with heating rates of 5 and 20°C/min. The values of the specific magnetization (M _s) at 10 kOe, remnant magnetization (M _r), M _r/M _s ratio, and coercive forces (H _c) are obtained from hysteresis loops. It was found that the values of M _s, M _r, M _r/M _s, and H _c depended strongly on morphology of the CoFe₂O₄ nanostructures.     

Size analysis and magnetic structure of nickel nanoparticles

The size distribution of an assembly of fcc nickel nanoparticles is studied by measuring the temperature dependent magnetization curves fitted by a uniform model and a core-shell model, both based on the Langevin function for superparamagnetism with a log-normal particle volume distribution. The uniform model fits lead to a spontaneous magnetization M_s much smaller than the M_s for bulk nickel and to particle sizes larger than the ones evaluated by transmission electron microscopy; the core-shell model fits can result in a correct size distribution but the M_s in the core becomes significantly greater than the M_s for bulk nickel. It is concluded that there is a core-shell magnetic structure in nickel particles. Although the enhanced M_s in the core may be related to the narrowing of the energy bands of 3d electrons in small fcc nickel particles, the modeling values of M_s are over large compared with previous calculations on nickel clusters of different structures, which implies possible existence of an exchange interaction between the core and the shell, which is not considered in the simple core-shell model.     

Fabrication and magnetic properties of Fe-Pd nanowire arrays

Ordered 20 nm Fe-Pd nanowire arrays with different compositions have been fabricated by alternating current electrodeposition into nanoporous anodic alumina. The structural and magnetic properties of the arrays were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). When Fe content is lower than 46 at.%, Fe-Pd alloy phase with fcc structure forms for the as-deposited. After annealing the alloy structure remains unchanged, but the coercivity (H_C) and squareness (M_r/M_s) increase. When Fe content is up to 60 at.%, α-Fe and Fe-Pd phases with fcc structure coexist for the as-deposited. After annealing the nanowires consist of a uniform Fe-Pd phase with fcc structure and the coercivity and squareness decrease. The change of the structure and magnetic properties with the alloy composition and annealing are explained reasonably.     

Critical grain size in the γ→α martensite transformation. Thermodynamic analysis with regard to spatial scales characteristic of martensite nucleation

In the work, it is shown that taking into account the ratio of spatial scales characteristic of martensite crystal nucleation in the elastic field of an individual dislocation localized in a grain of diameter D makes it possible (i) to estimate the critical grain size D_c (～1 μm) characteristic of the γ→α transformation at a martensite starting temperature M_s ～ 100 K from the requirements that threshold strain arises in the elastic region, (ii) to consistently describe the D dependence of M_s with M_s(D_c) = 0 from analysis of the phase free energy difference, (iii) to express D_c through macroparameters and interpret the limiting case M_s∞→0, D_c→∞ (D_c ～ (M_s∞)^?1 with M_s∞ ≡ M_s (∞)), and (iv) to more exactly specify the qualitative dynamic model of the effect of dislocation grain boundaries on the onset of initial excitation.     

Magnetization reversal in Fe48Pt34B34 magnetic foils

Exchange-spring magnet L1-FePt/(Fe₂B+α-Fe) is fabricated by flash annealing a melt-spun Fe₄₈Pt₃₄B₃₄ foil. A coercivity of 8500 Oe (1 Oe = 79.5775 A/m), squareness (M_r/M_s) of 0.70, saturation magnetization of 10.2 kGs (1 Gs = 10^-4 T) and an effective anisotropy K_eff =2.0 × 10⁷ ergs/cm³ are obtained. A two-step magnetization reversal feature is characterized in this paper. An exchange bias phenomenon is also observed in a low saturation field.     

Influence of Nb substitution for Fe on the magnetic and magneto-impedance properties of amorphous and annealed Fe76.5−xSi13.5B9Cu1Nbx (x=0–7) ribbons

We report a systematic study of the influence of Nb substitution for Fe on the magnetic properties and magneto-impedance (MI) effect in amorphous and annealed Fe_76.5−xSi_13.5B₉Cu₁Nb_x (x=0, 1, 2, 3, 4, 5, 6, and 7) ribbons. The amorphous ribbons were annealed at different temperatures ranging from 530 to 560 °C in vacuum for different annealing times between 5 and 20 min. We have found that for the as-quenched amorphous ribbons, the substitution of Nb for Fe first increases the saturation magnetization (M_s) and decreases the coercivity (H_c) until x=3, for which the largest M_s∼152 emu/g and the smallest H_c∼1.3 Oe are obtained, then an opposite trend is found for x>3. The largest MI ratio (ΔZ/Z∼38% at f=6 MHz) is achieved in the amorphous ribbon with x=3. A similar trend has been observed for the annealed ribbons. The most desirable magnetic properties (M_s∼156 emu/g and H_c∼1.8 Oe) and the largest MI ratio (ΔZ/Z∼221% at f=6 MHz) are achieved for the x=3 sample annealed at 540 °C for 15 min. A correlation between the microstructure, magnetic properties, and MI effect in the annealed ribbons has been established.     

Effect of nitrogen concentration on the magnetic properties of Fe-Ta-N thin films

Soft magnetic nanocrystalline thin films with a high content of Ta (10 wt %) are synthesized by the method of reactive RF magnetron sputtering with subsequent annealing. It is established that the microstructure and magnetic properties of the films depend on the nitrogen partial pressure during sputtering and on the annealing temperature. Annealing of the amorphous films leads to the formation of α-Fe nanocrystallites whose properties and interactions determine the film parameters. A decrease in the α-Fe grain size to a level below the length of ferromagnetic exchange interaction sharply increases the magnetic softness. The role of nitrogen ions in the formation of a α-Fe nanocrystallite structure and uniaxial magnetic anisotropy of Fe-Ta-N films is established. The optimum technological regimes of deposition and annealing of the Fe-Ta-N films are determined, which ensure the synthesis of Fe-Ta-N nanocrystalline thin films with a high magnetic softness (B _s=1.6 T, H _c=0.2 Oe, and μ₁(1 MHz)=3400).     

Soft magnetic properties of FeCo films with Co underlayer

Bilayered Fe₆₅Co₃₅ (=FeCo)/Co films were prepared by facing targets sputtering with 4πM_s∼24 kg. The soft magnetic properties of FeCo films were induced by a Co underlayer. H_c decreased rapidly when the Co underlayer was 2 nm or more. The films showed well-defined in-plane uniaxial anisotropy with the typical values of H_ce=10 Oe and H_ch=3 Oe, respectively. High frequency characteristics of the films show the films can work at 0.8 GHz with real permeability as high as 250.     

Structure and magnetic state of the films deposited by laser ablation of composite nickel and palladium targets

Transmission electron microscopy, electron diffraction, and vibrating-sample magnetometry are used to show that a metastable hcp structure can form in both nickel and Ni-Pd alloy films during alternating sputtering of the Ni and Pd components of composite targets. The hcp lattice parameters increase monotonically when the palladium content in a sputtered target increases in the range 0–75%. The ratio of the hcp lattice parameters c/a is close to the ideal ratio for the hcp lattice (1.63) within the limits of experimental error. In the as-deposited state, nickel and Ni-Pd alloy films with an hcp structure have no magnetic moment. Upon annealing, the films transform into a ferromagnetic state with an fcc structure. The concentration dependence of the lattice parameter of the fcc solid solution a ₀ is found to exhibit a positive deviation from Vegard’s law, which is characteristic of alloys with a concave liquidus line.     

Observation of the “Inverse” spin valve effect in a Ni/V/Ni trilayer system

An experimental study of magnetic and superconducting properties of a trilayer Ni/V/Ni thin film system grown on single-crystalline MgO(001) substrate is reported. The field dependence of the superconducting transition temperature T _c for samples comprising Ni layers with similar values of the coercive field H _c reveals no anomalies. However, in samples with different thicknesses of the nickel layers the difference in H _c amounts up to ΔH _c ∼ 1.8 kOe, thus enabling to manipulate the relative orientations of the layers’ magnetization by an external magnetic field. Surprisingly, for these samples the T _c for the parallel orientation of the magnetizations of the Ni layers is higher, in a certain magnetic field range, than for the antiparallel one, at odds with theoretical predictions. Possible reasons of this contradiction are discussed.     

Magnetic and structural properties of the electrochemically deposited arrays of Co and CoFe nanowires

Magnetic and structural properties of the arrays of 18 nm diameter nanowires of Co and Co₉₀Fe₁₀ electrodeposited in the pores of anodic alumina are investigated. Arrays of Co and Co₉₀Fe₁₀ nanowires show perpendicular magnetic anisotropy and textured crystallographic behaviour. Coercivity H_c (⊥) and remanence M_r/M_s (⊥) values of 2275 Oe (Co₉₀Fe₁₀); 1188 Oe (Co) and 96% (Co₉₀Fe₁₀), 81% (Co) are observed. The continuous films of Co and Co₉₀Fe₁₀ on Cu substrates show in plane magnetic anisotropy and coercivity values between 109 and 288 Oe.     

Benzene adsorption on nickel (100) and (111) faces studied by leed and high resolution electron energy loss spectroscopy

Studies of benzene (C₆H₆ and C₆D₆) adsorption have been performed by high resolution electron energy loss spectroscopy (HRELS) and LEED experiments on nickel (100) and (111) single crystal faces at room temperature. Chemisorption induces ordered structures, c(4 × 4) on Ni(100) and (2√3 × 2√3)R30° on Ni(111), and typical energy loss spectra with 4 loss peaks accurately identified with the strongest infrared vibration bands of the gazeous molecules. Benzene chemisorption preserves the aromatic character of the molecule and involves respectively 8 nickel surface atoms on the (100) face and 12 on the (111) face by adsorbed molecule. The interaction takes place via the π electrons of the ring. Significant shifts of the CHτ bending and CH stretching vibrations show a weakening of the CH bonds due to the formation of the chemisorption bond and a coupling of H atoms with the nickel substrate.