Fabrication and magnetic properties of Fe3O4 nanowire arrays in different diameters

Fe₃O₄ nanowire arrays with different diameters of D=50, 100, 150 and 200 nm were prepared in anodic aluminum oxide (AAO) templates by an electrodeposition method followed by heat-treating processes. A vibrating sample magnetometer (VSM) and a Quantum Design SQUID MPMS magnetometer were used to investigate the magnetic properties. At room temperature the nanowire arrays change from superparamagnetism to ferromagnetism as the diameter increases from 50 to 200 nm. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements show that the blocking temperature T_B increases with the diameter of nanowire. The ZFC curves of D=50 nm nanowire arrays under different applied fields (H) were measured and a power relationship between T_B and H were found. The temperature dependence of coercivity below T_B was also investigated. Mössbauer spectra and micromagnetic simulation were used to study the micro-magnetic structure of nanowire arrays and the static distribution of magnetic moments of D=200 nm nanowire arrays was investigated. The unique magnetic behaviors were interpreted by the competition of the demagnetization energy of quasi-one-dimensional nanostructures and the magnetocrystalline anisotropy energy of particles in nanowires.     

高矫顽力的Co70Cu30合金纳米线阵列的制备及磁性研究

利用直流电化学沉积法, 在多孔阳极氧化铝模板中首次制备出了具有[220]取向的单晶 面心立方结构的CoCu固溶体合金纳米线阵列, 其Co含量高达70%.透射电子显微镜显示纳米线均匀连续, 具有较高的长径比, 约为300. 磁性测量表明所制备的Co₇₀Cu₃₀ 合金纳米线具有超高的矫顽力H_c//=2438 Oe(1 Oe=79.5775 A/m)和较高的矩形比S//=0.76, 远高于以往报道的CoCu合金纳米线的磁性, 分析表明磁性好的主要原因是由于较高Co含量和高形状各向异性. 通过磁性测量和模型计算, 得到Co₇₀Cu₃₀ 合金纳米线阵列在反磁化过程中遵从对称扇型转动的球链模型, 并从结构的角度分析了Co₇₀Cu₃₀合金纳米线阵列的反磁化行为.     

Effects of annealing temperature on structure and magnetic properties of amorphous Fe61Co27P12 nanowire arrays

Arrays of Fe₆₁Co₂₇P₁₂ nanowire with an aspect ratio about 70 were prepared in anodic aluminum oxide templates by electrodeposition. The influences of annealing temperature on structure and magnetic properties of Fe₆₁Co₂₇P₁₂ nanowires were studied. When the specimens were annealed below 400 °C, there are no obvious changes in structure except relaxation. With the annealing temperature increasing from 400 to 600 °C, the Fe-Co phase is detected by X-ray diffraction and Mössbauer spectra. The crystalline fraction and hyperfine field can be derived from Mössbauer spectra. The room temperature magnetic hysteresis loops show that the coercivity and squareness of the nanowire arrays in parallel to the wire axis increase with the increasing of annealing temperature, which mainly attributes to the strengthening of anisotropy.     

Synthesis and magnetic properties of ordered barium ferrite nanowire arrays in AAO template

BaFe₁₂O₁₉ nanowire arrays having single magnetic domain size (≤460 nm) in anodic aluminum oxide (AAO) templates were prepared by sol-gel and self-propagating high-temperature synthesis techniques. The diameter of the nanowire arrays is approximately 70 nm and the length is about 2-4 μm. The specimens were characterized using X-ray diffraction, vibrating sample magnetometer, field emission scan electron microscope, atomic force microscopy and microwave vector network analyzer. The magnetic properties of BaFe₁₂O₁₉ nanowire arrays embedded in AAO templates were measured by VSM with a field up to 1274 KA/m at room temperature. The results indicate that the nanowire arrays exhibit large saturation magnetization and high coercivity in the range of 6000 Oe and an obvious magnetic anisotropy with the easy magnetizing axis along the length of the nanowire arrays, probably due to the shape anisotropy and magneto-crystalline anisotropy. Finally the microwave absorption properties of the nanowires were discussed.     

Out-of-plane coercive field of Ni80Fe20 antidot arrays

The out-of-plane magnetic anisotropy and out-of-plane magnetization reversal process of nanoscale Ni₈₀Fe₂₀ antidot arrays deposited by magnetron sputtering technique on an anodic aluminum oxide (AAO) membrane are investigated. The angular dependence of out-of-plane remanent magnetization of Ni₈₀Fe₂₀ antidot arrays shows that the maximum remanence is in-plane and the squareness of the out-of-plane hysteresis loop follow a |cos θ| dependence. The angular dependence of out-of-plane coercivity of Ni₈₀Fe₂₀ antidot arrays shows that the maximum coercivity lies on the surface of a cone with its symmetric axis normal to the sample plane, which indicates a transition of magnetic reversal from curling to coherent rotation when changing the angle between the applied magnetic field and the sample plane.     

Magneto-optical study of slanted Co nanowires embedded in CeO2/SrTiO3(0 0 1)

Sequential pulsed laser deposition of CoO and CeO₂ at 650 °C under vacuum leads to the formation of a slanted Co nanowires assembly embedded in CeO₂/SrTiO₃(0 0 1) epilayers. High temperature magneto-optical Faraday measurements were performed, which revealed a Faraday ellipticity of 1.3° at a wavelength of 450 nm for 300 nm thick samples and which allowed to access the magnetic properties. From the analysis of the coercivity dependence on temperature, it is shown that the magnetic anisotropy of the slanted Co nanowires is dominated by shape anisotropy and that their magnetization reversal is localized.     

Ultrafine Fe-Co nanowires: Fabrication and heat treatment influence on the structure and magnetic properties

Ultrafine nanowires of Fe-Co with a diameter around 15 nm have been fabricated by electrodeposition method using anodic porous alumina as a template. The alloy nanowires were in the form of arrays and consisting of polycrystalline structures. They showed obvious shape anisotropy parallel to the axis of nanowires and the perpendicular coercivity (Hc_⊥) was found to be 2576.8 Oe which is higher than any coercivity value reported in the literature. The effects of critical factors such as heat treatment and temperature of annealing on the structure and magnetic properties of the ultrafine nanowire arrays were studied and discussed.     

Synthesis and magnetic properties of spinel CoFe2O4 nanowire arrays

Spinel CoFe₂O₄ nanowire arrays were synthesized in nanopores of anodic aluminum oxide (AAO) template using aqueous solution of cobalt and iron nitrates as precursor. The precursor was filled into the nanopores by vacuum impregnation. After heat treatment, it transformed to spinel CoFe₂O₄ nanowires. The structure, morphology and magnetic properties of the sample were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The results indicate that the nanowire arrays are compact. And the individual nanowires have a high aspect ratio, which are about 80 nm in diameter and 10 μm in length. The nanowires are polycrystalline spinel phase. Magnetic measurements indicate that the nanowire arrays are nearly magnetic isotropic. The reason is briefly discussed. Moreover, the temperature dependence of the coercive force of the nanowire arrays was studied.     

The size and space arrangement roles on coercivity of electrodeposited Co1−xCux nanowires

Magnetization curves with various magnetic field orientations and nanowire diameters were measured at room temperature. The measured coercivity as a function of angle (θ) between the field and wire axis reveals that the coercivity decreases with increasing value of θ for various nanowires. Theoretically, based on Monte Carlo simulation we investigated the magnetization reversal modes of the Co_1−xCu_x nanowires and obtained also the θ dependence of the coercivity. Comparing the simulated with the experimental results, we find that the magnetocrystalline anisotropy plays an important role on the magnetic properties of Co_1−xCu_x nanowires, and the magnetization reversal process in the Co_1−xCu_x nanowires could not be understood by the classical uniform rotation mode in the chain-of-sphere model.     

Temperature dependence of the coercivity in Nd60Fe30−xCoxAl10, x=0 or 10, and Pr58Fe24Al18 bulk amorphous ferromagnets

Bulk amorphous ferromagnet alloys of composition Nd₆₀Fe₃₀Al₁₀, Nd₆₀Fe₂₀Co₁₀Al₁₀ and Pr₅₈Fe₂₄Al₁₈ have been prepared by argon arc melting and quenching into a copper mould. General insight into the magnetic behaviour of the alloys was gained from measurement of the major hysteresis loop at room temperature, and from zero-field cooled and field-cooled magnetisation measurements in the range 10-400 K. Measurements of the coercivity were made from 10 to 400 K, and for all alloys, the coercivity is seen to increase steeply with decreasing temperature to a peak at a temperature in the range 25-50 K, before decreasing. For all alloys, the temperature dependence of the coercivity between 50 and 400 K is well explained by the strong pinning model of domain walls of Gaunt [Philos. Mag. B 48 (1983) 261]. Quantities deduced from the Gaunt model, along with other relevant magnetic parameters, are used to estimate values for the exchange and anisotropy constants.     

Thermomagnetic transitions and coercivity mechanism in bulk composite Nd60Fe30Al10 alloys

The thermomagnetic behaviour (within the temperature range 553-300 K) for the bulk composite Nd₆₀Fe₃₀Al₁₀ alloy is described in terms of a transition from paramagnetic to superferromagnetic state at T=553 K, followed by a ferromagnetic ordering for T<473 K. For the superferromagnetic regime, the alloy thermomagnetic response was associated to a homogeneous distribution of magnetic clusters with mean magnetic moment and size of 1072 μ_B and 2.5 nm, respectively. For T<473 K, a pinning model of domain walls described properly the alloy coercivity dependence with temperature, from which the domain wall width and the magnetic anisotropy constant were estimated as being of ≈8 nm and ≈10⁵ J/m³, typical values of hard magnetic phases. Results are supported by microstructural and magnetic domain observations.     

Fabrication and magnetic properties of ordered Fe60Pb40 nanowire arrays electrodeposited in AAO templates

Ordered ferromagnetic-nonmagnetic heterogeneous Fe₆₀Pb₄₀ nanowire arrays were successfully fabricated by alternating current (AC) electrodeposition into nanoporous alumina templates. Transmission electron microscopy (TEM) image and selected-area diffraction (SAED) pattern analysis showed that the Fe₆₀Pb₄₀ nanowires are polycrystalline with an average diameter of 22 nm and lengths up to several micrometers. X-ray diffraction (XRD) observations indicated that α-Fe and fcc Pb phase coexist and do not form metastable alloy phase. The as-deposited samples were annealed at 200, 300, 400 and 500 ^°C, respectively. Magnetic measurements showed that nanowires have high magnetic anisotropy with their easy axis parallel to the nanowire arrays, and the coercivity of the samples increased with the annealing temperature up to 400 ^°C and reached a maximum (2650 Oe). The change of magnetic properties associated with the microstructure was discussed.     

Dependence of magnetic characteristics on sputtering-power and substrate-temperature in amorphous Tb40(FeCoV)60 films

The amorphous Tb₄₀(Fe₄₉Co₄₉V₂)₆₀ films were deposited at different sputtering powers and substrate temperatures. The microstructural and magnetic characteristics were investigated by means of field emission scan electron microscope, magnetic force microscope and vibrating sample magnetometer. Our results show that with increasing sputtering power, out-of-plane coercivity decreases monotonically while saturation magnetization has a maximum value of 231 kA/m for the sample prepared at 50 W. The as-deposited alloy films are amorphous, whereas the coercivity and saturation magnetization are strongly dependent on the substrate temperature. An out-of-plane hysteresis loop with coercivity below 22 mT and saturation magnetization over 290 kA/m is obtained combining dc power and substrate temperature. The dominant mechanism of room temperature coercivity appears to be domain wall pinning, rather than nucleation under all conditions measured. The variation of saturation magnetization is similar to that of perpendicular magnetic anisotropy with either sputtering power or substrate temperature according to the difference of magnetic domain structure.     

Superparamagnetic behavior of La0.67Sr0.33MnO3 nanoparticles prepared via sol-gel method

Magnetic nanoparticles of La_0.67Sr_0.33MnO₃ (LSMO) manganite were prepared by sol-gel method. Phase formation and crystal structure of the synthesized powder were examined by the X-ray diffraction (XRD) using the Rietveld analysis. The mean particle size was determined by the transmission electron microscopy (TEM). Infrared transmission spectroscopy revealed that stretching and bending modes are influenced by calcinations temperature. The temperature dependence of the ac magnetic susceptibility was measured at different frequencies and ac magnetic fields in the selected ranges of 40-1000 Hz and 80-800 A/m, respectively. The temperature dependence of ac susceptibility shows a characteristic maxima corresponding to the blocking temperature near room temperature. The frequency dependence of the blocking temperature is well described by the Vogel-Fulcher law. By fitting the experimental data with this law, the relaxation time τ₀=1.7×10⁻¹² s, characteristic temperature T₀=262±3 K, anisotropy energy E_a/k=684±15 K and effective magnetic anisotropy constant k_eff=2.25×10⁴ erg/cm³ have been obtained. dc Magnetization measurement versus magnetic field shows that some of LSMO nanoparticles are blocked at 293 K. The role of magnetic interparticle interactions on the magnetic behavior is also investigated.     

Enhancement of L10 ordered FePt by Pt buffer layer

L₁₀-ordered FePt thin films prepared by molecular-beam epitaxy on MgO (0 0 1) substrate at 320 °C with different thickness of Pt buffer layer have been investigated. The out-of-plane coercivity increases with increasing thickness of Pt buffer. The maximum values of the long-range order parameter and uniaxial magnetic anisotropy energy are 0.72 and 1.78×10⁷ erg/cm³, respectively, for films with 12 nm thick Pt buffer layer, where the c/a ratio (0.976) shows the minimum value. The reason for the enhancement in ordering is due to the proper lattice strains Pt buffer bestows on FePt layer, these strains are equal to the contraction in lattice parameter c and the expansion in a. Studies of angular-dependent coercivity revealed that the magnetization reversal behaviour shifts from a domain-wall motion dominated case towards a near rotational mode with increasing thickness of Pt buffer layer.     

Magnetic reversal mechanism in strong perpendicular magnetic anisotropic CoPt/AlN multilayer film

Magnetic reversal mechanism of the Sub/AlN5 nm/[CoPt2 nm/AlN5 nm]₅ nano multilayer film, which shows strong perpendicular magnetic anisotropy (K_u=6.7×10⁶ erg/cm³), has been studied. The angle-dependent magnetic hysteresis loops of this highly perpendicular anisotropic CoPt/AlN multilayer film were measured in the present work, applying a magnetic field along different angles φ with respect to the film normal. It demonstrates that the magnetic reversal of the CoPt ultrathin layers in the CoPt/AlN multilayer film is occurred by the reversible magnetization rotation and the irreversible displacement of domain walls. The φ-dependent part of coercive field is resulted from the internal stress according to the Kondorsky and Kersten model. The φ-independent part of coercive field implies some random and isotropy pinning centers (e.g., vacancies, dislocations, grain boundaries) in the ultrathin CoPt layers. Our work is useful for coercivity control of metal/ceramics layered structures, in particular the perpendicular magnetic tunneling junctions.     

Microstructure and magnetic properties of FePt:Ag nanocomposite films on SiO2/Si(1 0 0)

FePt:Ag nanocomposite films were prepared by pulsed filtered vacuum arc deposition system and subsequent rapid thermal annealing on SiO₂/Si(1 0 0) substrates. The microstructure and magnetic properties were investigated. A strong dependence of coercivity and ordering of the face-central tetragonal structure on both Ag concentration and annealing temperature was observed. With Ag concentration of 22% in atomic ratio, the coercivity got to 6.0 kOe with a grain size of 6.7 nm when annealing temperature was 400 °C.     

Magnetization reversal process and intrinsic coercivity of sputtered Sm2Co17-based films

The Sm₂Co₁₇-based intermetallic films with additives of Fe, Cu, and Zr have been deposited on Si(1 0 0) substrates by dc magnetron sputtering process. Subsequent thermal treatment and the film thickness are found to have significant contribution to the crystal structure and grain structure, which determines the magnetization reversal process and intrinsic coercivity (H_C) of these films. The conventional thermal annealing (CTA) treatment almost failed to crystallize the as-deposited films, leading to a very low H_C. Continuous and homogeneous domain walls cannot form in this deteriorated microstructure, so that the pinning mechanism can be excluded. Contrarily, the films with thickness exceeding 0.8 μm treated by rapid recurrent thermal annealing (RRTA) show an improved H_C, which is attributed to the observed completed crystallization and compact microstructure. It is suggested that this film structure is responsible for providing continuous and homogeneous domain walls, leading to a magnetization reversal process controlled by domain wall pinning model. In special, the H_C of the RRTA-treated film with thickness of 1.8 μm shows a good temperature dependence from 25 to 300 °C, with intrinsic coercivity temperature coefficient β of −0.23%/°C.     

Structure,magnetic properties and magnetostriction of Fe81Ga19 thin films

The structure, magnetic properties and magnetostriction of Fe₈₁Ga₁₉ thin films have been investigated by using X-ray diffraction analysis, scanning electron microscope (SEM), vibrating sample magnetometer and capacitive cantilever method. It was found that the grain size of as-deposited Fe₈₁Ga₁₉ thin films is 50–60 nm and the grain size increases with increase in the annealing temperature. The remanence ratio (M_r/M_s) of the thin films slowly decreases with increase in the annealing temperature. However, the coercivity of the thin films goes the opposite way with increase in the annealing temperature. A preferential orientation of the Fe₈₁Ga₁₉ thin film fabricated under an applied magnetic field exists along 〈1 0 0〉 direction due to the function of magnetic field during sputtering. An in-plane-induced anisotropy of the thin film is well formed by the applied magnetic field during the sputtering and the formation of in-plane-induced anisotropy results in 90° rotations of the magnetic domains during magnetization and in the increase of magnetostriction for the thin film.     

Microstructure and magnetic properties of nanocomposite FePt/MgO and FePt/Ag films

An in-plane magnetic anisotropy of FePt film is obtained in the MgO 5 nm/FePt t nm/MgO 5 nm films (where t=5, 10 and 20 nm). Both the in-plane coercivity (H_c∥) and the perpendicular magnetic anisotropy of FePt films are increased when introducing an Ag-capped layer instead of MgO-capped layer. An in-plane coercivity is 3154 Oe for the MgO 5 nm/FePt 10 nm/MgO 5 nm film, and it can be increased to 4846 Oe as a 5 nm Ag-capped layer instead of MgO-capped layer. The transmission electron microscopy (TEM)-energy disperse spectrum (EDS) analysis shows that the Ag mainly distributed at the grain boundary of FePt, that leads the increase of the grain boundary energy, which will enhance coercivity and perpendicular magnetic anisotropy of FePt film.