Structural and magnetic properties of nanostructured Mn–Al–C magnetic materials

Pre-alloyed Mn_50+x−yAl_50−xC_y   (x=0$x=0$, 2, 4, 6, 8; y=0$y=0$, 1.7, 3) powders were mechanically milled (MM), and the as-milled powders subsequently annealed at temperatures from 350 to 600 °C to produce the ferromagnetic metastable L1₀-structured τ-phase. Bulk Mn₅₄Al₄₆ specimens were also annealed under the same conditions for comparison. The effects of the Mn concentration and C additions on phase formation, microstructure, magnetic properties, as well as on the magnetization mechanism of the Mn–Al–C alloys were systematically investigated. It was found that the magnetic properties are strongly dependent both on the fraction of the τ-phase and its microstructure. There exists a strong influence of the microstructural refinement, due to the ball milling, on the rate of ε-phase to τ-phase transformation and on the stability of the τ-phase. The kinetics of formation and subsequent decomposition of the magnetic τ-phase were markedly different in the MM and bulk alloys. Both remanence curves and δM plots showed no exchange coupling among the τ-phase nanograins. The mechanism for the magnetization process was determined to be domain wall pinning.     

Evolution of phase,texture, microstructure and magnetic properties of Fe–Cr–Co–Mo–Ti permanent magnets

Magnetic phase evolution, crystallographic texture, microstructure and magnetic properties of Fe–28Cr–15Co–3.5Mo–1.8Ti alloy have been investigated by X-ray diffractometry, scanning transmission electron microscopy and magnetometry techniques as a function of processing conditions. Heat treatment conditions for obtaining optimum textural, microstructural and magnetic properties have been established by the experimentations. The Goss {110}〈001〉 and cube type {001}〈010〉 textures have been developed in an optimal treated Fe–28Cr–15Co–3.5Mo–1.8Ti magnets. The coercive force in Fe–28Cr–15Co–3.5Mo–1.8Ti magnets depends critically on the shape anisotropy of rod-like Fe Co Ti-rich α₁ particles and remanence on the alignment and elongation of α₁ particles parallel to applied magnetic field 〈100〉 directions. The optimum magnetic properties obtained in Fe–28Cr–15Co–3.5Mo–1.8Ti alloy are intrinsic coercive force, _iH_c, of 78.8 kA/m (990 Oe), remanence, B_r of 1.12 T (11.2 kG) and energy product, (BH)_max of 52.5 kJ/m³ (6.5 MGOe). The development of Fe–28Cr–15Co–3.5Mo–1.8Ti magnets as well as characterization of texture, microstructural and magnetic properties in the current study would be helpful in designing the new Fe–Cr–Co–Mo based magnets suitable for scientific and technological applications.     

Synthesis and magnetic properties of Co–Zn magnetic fluid

Co_1−xZn_xFe₂O₄ (with x varying from 0 to 0.7) nanoparticles to be used for ferrofluid preparation were prepared by chemical co-precipitation method. The fine particles were suitably dispersed in transformer oil using oleic acid as the surfactant. The magnetization (M_s) and the size of the particles were measured at room temperature. The magnetization (M_s) was found to decrease with the increase in zinc substitution. The magnetic particle size (D_m) of the fluid was found to vary from 11.19 to 4.25 nm decreasing with the increase in zinc substitution.     

Electromagnetic waves reflection,transmission and absorption by graphene–magnetic semiconductor–graphene sandwich-structure in magnetic field: Faraday geometry

Electrodynamic properties of the graphene–magnetic semiconductor–graphene sandwich-structure have been investigated theoretically with taking into account the dissipation processes. Influence of graphene layers on electromagnetic waves propagation in graphene–semi-infinte magnetic semiconductor and graphene–magnetic semiconductor–graphene sandwich-structure has been analyzed. Frequency and field dependences of the reflectance, transmittance and absorbtance of electromagnetic waves by such structure have been calculated. The size effects associated with the thickness of the structure have been analyzed. The possibility of efficient control of electrodynamic properties of graphene–magnetic semiconductor–graphene sandwich-structure by an external magnetic field has been shown.     

In situ formation of magnetic–luminescent, bi-functional, polymer-stabilized cerium sulfide nanoparticles

Polymer-stabilized paramagnetic and fluorescent rare-earth metal sulfide (cerium sulfide, Ce₂S₃) nanoparticles have been synthesized by using an ??in situ polymerization and composite formation?? (IPCF) technique (Mallick et al. in J. Appl. Phys. 106:074303, 2009) at room temperature. Encapsulated cerium sulfide nanoparticles showed photoluminescence when excited with laser irradiation. The composite material exhibited a paramagnetic behavior due to the in situ formation of magnetic Ce³⁺ ionic species at the reaction condition.     

Structural,electrical and magnetic properties of Zr–Mg cobalt ferrite

Spinel cobalt ferrite, CoFe_2−xM_xO₄ has been synthesized by substitution of the combination of metallic elements M=Zr–Mg by the microemulsion method using polyethylene glycol as a surfactant. Powder X-ray diffraction analysis reveals that the substitution results in shrinkage of the unit cell of cobalt ferrite due to higher binding energy of the synthesized samples. The energy-dispersive X-ray fluorescence analysis confirms the stoichiometric ratios of the elements present. The thermogravimetric analysis shows that the minimum temperature required for the synthesis of these substituted compounds is 700 °C. A two-point probe method was employed for the measurement of the electrical resistivity in a temperature range of 293±5 to 673±5 K. It appears that there is a decrease in the number of Fe²⁺/Fe³⁺ pairs at the octahedral sites due to the substitution and corresponding migration of some of the Fe³⁺ ions to tetrahedral sites, consequently increasing the resistivity and the activation energy of hopping of electron at the octahedral sites. The susceptibility data also suggest migration of Fe³⁺ to tetrahedral site in the initial stage, which results in an increase in A–B interactions leading to large increase in the blocking temperature (T_B) as observed in samples having dopant content x=0.1.     

Bose–Einstein condensates in magnetic waveguides

In this article, we describe an experimental system for generating Bose–Einstein condensates and controlling the shape and motion of a condensate by using miniaturised magnetic potentials. In particular, we describe the magnetic trap setup, the vacuum system, the use of dispenser sources for loading a high number of atoms into the magneto-optical trap, the magnetic transfer of atoms into the microtrap, and the experimental cycle for generating Bose–Einstein condensates. We present first results on outcoupling of condensates into a magnetic waveguide and discuss influences of the trap surface on the ultra-cold ensembles. Received: 21 August 2002 / Revised version: 10 December 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-7071/295-829, E-mail: fortagh@pit.uni-tuebingen.de     

Chemical,magnetic and charge ordering in the system hematite–ilmenite,Fe2O3–FeTiO3

Spin polarized electronic structure calculations of total energies for ordered supercells in the system Fe₂O₃–FeTiO₃ suggest that some layered ordered phases are more stable than an isocompositional mechanical mixture of hematite, Fe₂O₃, and ilmenite, FeTiO₃. This result contradicts established ideas about hematite–ilmenite phase relations because it suggests that there is at least one stable ordered phase with a bulk composition intermediate between hematite and ilmenite. It is not clear if this result is an artifact of the approximations made in generalized gradient spin density functional calculations, or if an intermediate phase, or phases, is in fact stable. The electronic structure of a 30-atom layered supercell was studied by a variety of techniques. The supercell structure is FTFFFT, where F is an Fe layer and T is a Ti layer perpendicular to the hexagonal c axis. The idea was to investigate possible charge ordering on Fe sites, that is a postulate of the ‘lamellar magnetism hypothesis’, but significant Fe²⁺–Fe³⁺ordering is not predicted.     

Synthesis,structure stability and magnetic properties of nanocrystalline Ag–Ni alloy

Silver–nickel alloy nanoparticles with an average size of 30–40 nm were synthesized by chemically reducing the mixture of silver and nickel salts using sodium borohydride. The structure and the magnetic properties of the alloy samples with different compositions were investigated. The phase stability of the material was analysed after annealing the sample in vacuum at various temperatures. The material exhibits single fcc phase which is stable up to 400 °C and Ni precipitation sets in when the sample is annealed to 500 °C. The thermal analysis using DSC was carried out to confirm the same. The alloy compositions are found to be in close correlation with the metal salt ratios in the precursors. The synthesized samples exhibit weak paramagnetic to ferromagnetic behaviour. The magnetic measurements reveal that by adjusting the precursor ratio, the Ni content in the material can be altered and hence its magnetic properties tailored to suit specific requirements. The formation of Ag–Ni alloy is confirmed by the observed Curie temperature from the magneto thermogram. Annealing the sample helps to produce significant enhancement in the magnetization of the material.     

The abnormal direct-current resistivity of Ni–Mn–Ni triple-layered magnetic thin-films and their magnetic wall observations

We have observed an abnormal electron transport characteristic from the Ni–Mn–Ni triple-layered magnetic thin-films. Due to the intercalated Mn ultra-thin interlayer, the magnetic domain structure and the electron transport characteristic differ a lot from their original magnetic (Fe, Co, and Ni) films. As inspected by a magnetic force microscopy (MFM), we observed the variation of the domain configuration with Mn interlayer thicknesses (for 1, 5, and 10 nm). Moreover, we also examined and found that the direct current (DC) resistivity have no significant change as the current conducts from the current-in-wall (CIW) to the current- perpendicular-to-wall (CPW), which are opposite to the results of single layered films.     

Pressure effects on magnetic properties and martensitic transformation of Ni–Mn–Sn magnetic shape memory alloys

The mechanism for the effects of pressure on the magnetic properties and the martensitic transformation of Ni-Mn- Sn shape memory alloys is revealed by first-principles calculations. It is found that the total energy difference between paramagnetic and ferromagnetic austenite states plays an important role in the magnetic transition of Ni-Mn-Sn under pressure. The pressure increases the relative stability of the martensite with respect to the anstenite, leading to an increase of the martensitic transformation temperature. Moreover, the effects of pressure on the magnetic properties and the martensitic transformation are discussed based on the electronic structure.     

Fe–Pt–MgO stacked films for perpendicular magnetic recording

Fe–Pt–MgO stacked storage layer constructed by [Fe–Pt/Fe–Pt–MgO/Fe–Pt] trilayered structure was proposed for a next-generation high-density perpendicular magnetic recording medium. The Fe–Pt–MgO composite middle layer was prepared by sputtering the Fe–Pt–MgO composite-type target including relatively large MgO content of 50 vol%. The Fe–Pt(0 0 1) seed layer deposited on MgO underlayer was effective in forming the ordered fct(0 0 1) phase for the Fe–Pt–MgO composite film. The reduction of transition jitter noise and the suppression of signal overlap were observed in the stacked-type medium with the Fe–Pt–MgO middle layer of 1 nm thickness. The improvement of recording properties is attributed to the pinning effect of magnetic domain wall by the Fe–Pt–MgO composite layer inserted into the middle of pure Fe–Pt storage layer.     

Microstructural investigation of Fe–Zr–B–Ag soft magnetic thin films

A microstructural study of DC-sputtered Fe_93−xZr₃B₄Ag_x films on Si(0 0 1) substrates has been carried out using X-ray diffraction (XRD) and transmission electron microscopy (TEM). All samples were deposited as a function of additive Ag content (x=0–6 at%), and annealed in the range of temperature, 300–600°C, for 1 h in order to obtain enhanced soft magnetic properties. Through XRD and TEM investigation, Ag-free Fe₉₃Zr₃B₄ films on Si(0 0 1) substrates consisted of nano-crystalline Fe-based phases. In the presence of Ag additive element, the microstructure of as-deposited Fe_93−xZr₃B₄Ag_x films consisted of a mixture of majority of Fe-based amorphous and Ag crystalline phases. In this case, additive element, Ag played a role in retarding the formation of Fe-based crystalline phases during deposition, and insoluble nano-crystalline Ag particles were dispersed in the Fe-based amorphous matrix. As the content of Ag increased, the intensity of Ag crystalline XRD peak increased. Crystallization of Fe-based amorphous phase in the matrix of Fe₈₈Zr₃B₄Ag₅ thin films occurred at an annealing temperature of 400°C. In the case of Fe₈₈Zr₃B₄Ag₅ films annealed at 500°C, a much enhanced permeability of the Fe-based alloy thin films associated with nano-crystalline phases was achieved.     

Glass formation,magnetic properties and magnetocaloric effect of ternary Ho–Al–Co bulk metallic glass

A ternary Ho–Al–Co system with high glass-forming ability (GFA) was developed and fully glassy rods with diameters up to 1 cm can be produced for the best glass former of Ho₅₅Al_27.5Co_17.5 alloy. The thermal stability and low-temperature magnetic properties of the Ho₅₅Al_27.5Co_17.5 bulk metallic glass (BMG) were studied. The magnetic transition temperature of this alloy is ∼14 K as determined by the thermomagnetic measurement. Two indicators, i.e. isothermal magnetic entropy change (ΔS_M) and the relative cooling power (RCP), were adopted to evaluate the magnetocaloric effect (MCE) of the alloy under a low magnetic field up to 2 T, which can be generated by permanent magnets. The values of |ΔS_M| and RCP are 7.98 J kg⁻¹ K⁻¹ and 191.5 J kg⁻¹, respectively. The Ho₅₅Al_27.5Co_17.5 BMG with good MCE and high GFA provides an attractive candidate for magnetic refrigeration applications, like hydrogen liquefaction and storage.     

Effect of magnetic pulse treatment on the magnetic characteristics of yttrium–iron garnets

The effect weak (10–100 kA m^–1) low-frequency (10–20 Hz) pulsed magnetic fields have on the surface structure and magnetic characteristics of yttrium–iron garnet Y₃Fe₅O₁₂ is studied by means of electron and Mössbauer spectroscopy. A mechanism is proposed for the variation of saturation magnetization in Y₃Fe₅O₁₂ after magnetic pulse treatment. The mechanism is associated with the change in the spin state of iron ions localized in the tetrahedral sublattice.     

Controllable synthesis, characterization, and magnetic properties of nanoscale zerovalent iron with specific high Brunauer–Emmett–Teller surface area

This article reports a novel approach for the controllable synthesis of nanoscale zerovalent iron (NZVI) particles with specific high Brunauer–Emmett–Teller (BET) surface areas. Borohydride reduction is a primary and effective liquid phase reduction method for the synthesis of zerovalent iron nanoparticles. However, previous methods for synthesizing NZVI did not suggest a standard technique for controlling the size of particles during the synthesis process; in addition, previous literature generally reported that NZVI had a BET surface area of <37 m²/g. In this communication, a novel approach for the controllable synthesis of NZVI particles with specific high BET surface areas is presented. As a result, the BET surface areas of the NZVI particles synthesized increased to 47.49 and 62.48 m²/g, and the particle sizes decreased to 5–40 and 3–30 nm. Additionally, the physical and chemical properties of the synthesized NZVI particles were investigated by a series of characterizations, and magnetic analysis indicated that the synthesized NZVI particles had super-paramagnetic properties.