Optically induced magnetization in diluted magnetic quantum dots

We report the effect of intense laser field on donor impurities in a semimagnetic Cd_1-xinMn_xinTe/Cd_1-xoutMn_xoutTe quantum dot. The spin polaronic energy of different Mn²⁺ is evaluated for different dot radii using a mean field theory in the presence of laser field. Magnetization is calculated for various concentrations of Mn²⁺ ions with different dot sizes. Significant magnetization of Mn spins can be obtained through the formation of polarized exciton magnetic polarons (EMPs). A rapid decrease of the laser dressed donor ionization energy for different values of dot sizes with increasing field intensity is predicted. Also, it is found that the polarization of EMPs increases rapidly at higher excitation energies.     

Properties of laser gain of Li:CdZnO/ZnMgO quantum well

The many-body optical gain in Li:CdZnO/MgZnO quantum well (QW) structures with spontaneous polarization and piezoelectric polarization, and ferroelectric dipole moment is investigated by using the non-Markovian gain model with many-body effects. The CdZnO/MgZnO QW structure with high Cd composition is found to have smaller optical gain because the strain-induced piezoelectric polarization and the spontaneous polarization in the well increase with the inclusion of Cd. The internal field is reduced due to the additional polarization by Li in the CdZnO/MgZnO QW structure. These results show that Li:CdZnO-based QW lasers are promising candidates for optoelectric applications in visible and UV regions.     

High concentration of hematite nanoparticles in a silica matrix: Structural and magnetic properties

The α-Fe₂O₃/SiO₂ nanocomposite containing 45 wt% of hematite was prepared by the sol-gel method followed by heating in air at 200 °C. The so-obtained composite of iron(III) nanoparticles dissolved in glassy silica matrix was investigated by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry. XRPD confirms the formation of a single-phase hematite sample, whereas TEM reveals spherical particles in a silica matrix with an average diameter of 10 nm. DC magnetization shows bifurcation of the zero-field-cooled (ZFC) and field-cooled (FC) branches up to the room temperature with a blocking temperature T_B=65 K. Isothermal M(H) dependence displays significant hysteretic behaviour below T_B, whereas the room temperature data were successfully fitted to a weighted Langevin function. The average particle size obtained from this fit is in agreement with the TEM findings. The small shift of the T_B value with the magnetic field strength, narrowing of the hysteresis loop at low applied field, and the frequency dependence of the AC susceptibility data point to the presence of inter-particle interactions. The analysis of the results suggests that the system consists of single-domain nanoparticles with intermediate strength interactions.     

Electromodulation of the magnetoresistance in diluted magnetic semiconductors based heterostructures

We study the properties of heterostructures formed by two layers of a diluted magnetic semiconductor separated by a nonmagnetic semiconductor layer. We find that there is a RKKY-type exchange coupling between the magnetic layers that oscillates between ferromagnetic and antiferromagnetic as a function of the different parameters in the problem. The different transport properties of these phases make that this heterostructure presents strong magnetoresistive effects. The coupling can be also modified by an electric field. We propose that it is possible to alter dramatically the electrical resistance of the heterostructure by applying an electric field. Our results indicate that in a single gated sample the magnetoresistance could be modulated by an electrical bias voltage.     

The influence of thermal annealing on the structural and magnetic properties of gold nanoparticles

In the past few years ferromagnetic-like behavior has been reported in metal gold nanoparticles coated with diverse organic surfactants. In this work we report on the effect of thermal annealing on the ferromagnetic-like behavior of oleic acid and oleylamine coated gold nanoparticles of about 7 nm size. The magnetic moment of the “as prepared” sample is about 3×10⁻² emu/g and the coercive field is 200 Oe at 10 kOe and 5 K, after the annealing the behavior changes from ferromagnetic-like to paramagnetic and the magnetization at 10 kOe decreases at a factor of 10. These results are compared with those obtained for oleylamine coated gold nanoparticles, which are diamagnetic at room temperature.     

Synthesis and magnetic properties of antiferromagnetic Co3O4 nanoparticles

Antiferromagnetic Co₃O₄ nanoparticles with diameter around 30 nm have been synthesized by a solution-based method. The phase identification by the wide-angle X-ray powder diffraction indicates that the Co₃O₄ nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm. The image of field emission scanning electron microscope shows that the nanoparticles are assembled together to form nanorods. The magnetic properties of Co₃O₄ fine particles have been measured by a superconducting quantum interference device magnetometer. A deviation of the Néel temperature from the bulk is observed, which can be well described by the theory of finite-size scaling. An enhanced coercivity as well as a loop shift are observed in the field-cooled hysteresis loop. The exchange bias field decreases with increasing temperature and diminishes at the Néel temperature. The training effect and the opening of the loop reveal the existence of the spin-glass-like surface spins.     

Low-Temperature Magnetic Properties of Co Antidot Array

Cobalt antidot arrays with different thicknesses are fabricated by rf magnetron sputtering onto porous alumina substrates. Scanning electron microscopy and grazing incidence x-ray diffraction are employed to characterize the morphology and crystal structure of the antidot array, respectively. The temperature dependence of magnetic properties shows that in the temperature range 5K--300K, coercivity and squareness increase firstly, reach their maximum values, then decrease. The anomalous temperature dependences of coercivity and squareness are discussed by considering the pinning effect of the antidot and the magnetocrystalline anisotropy.     

The magnetic properties of Be-codoped GaMnN grown via molecular beam epitaxy

GaMnN and Be-codoped GaMnN were grown via molecular beam epitaxy using a single GaN precursor and their structural and magnetic properties were examined. X-ray diffraction and superconducting quantum interference device (SQUID) measurements revealed that the grown layers are homogeneous without precipitates. The saturation magnetization of GaMnN has increased from ∼4 to ∼16 emu/cm³ via codoping of Be. The d–d exchange interaction between Mn atoms was discussed for the ferromagnetism of GaMnN.     

Formation of room-temperature ferromagnetic Zn1−xCoxO nanocrystals

Optical and magnetic properties of Co²⁺-doped ZnO nanocrystals were studied. Optical measurements confirm the incorporation of Co²⁺ in ZnO lattice with tetrahedral geometry. Optical absorption spectra also reveal the partial bleaching of the excitonic feature attributable to an increase in electron concentration. Magnetization measurements indicate the ferromagnetic ordering in Co²⁺-doped ZnO nanocrystals with saturation magnetization . No structural changes were observed in lightly doped ZnO nanocrystals. The present investigations are important in obtaining the ferromagnetic Zn_1−xCo_xO nanocrystals.     

The effect of structure on magnetic properties of Co nanowire arrays

Co nanowire arrays with three typical diameters of 20, 50 and 120 nm have been fabricated into anodic alumina oxide templates using an ac electrodeposition method. It is found that the crystal texture of the Co nanowires depends on the pH value of the deposition electrolyte. X-ray diffraction results show that the (1 0 0) texture appears at pH 6.2, while the diffraction peaks of (1 0 0) and (1 0 1) appear at pH 6.4 with the diameter of 20 nm. In addition, the (0 0 2), (1 0 0) and (1 0 1) peaks appear with an increase of pH value for the nanowire arrays with diameters of 50 and 120 nm, respectively. Magnetic measurements indicate the effect of structure on the magnetic properties of the nanowire arrays, which depend strongly on the different diffraction peaks, as adjusted by the pH value.     

On the understanding of the microscopic origin of the properties of diluted magnetic semiconductors by atom probe tomography

Spintronics, in which both the spin and charge of electrons are used for logic and memory operations, promises to revolutionize the current information technology. Just as silicon supports microelectronics, diluted magnetic semiconductors (DMSs) will be the platform of spintronics. Ideal DMSs should maintain ferromagnetic and semiconducting properties at operating temperatures to realize the spintronic functions. Although many high-temperature Curie temperature DMSs have been reported, the origin of ferromagnetism remains controversial. Currently, this is a major obstacle to the development of spintronic devices. The solution to this problem depends on a more complete understanding of DMS microstructure, especially the distribution of doped magnetic ions at atomic resolution and any defects introduced. Therefore, an analysis technique is required, possessing both high spatial and elemental resolutions, which is beyond the capability of conventional techniques, such as electron microscopy. However, atom probe tomography (APT), which recently has been successfully applied to nanoscale characterization of structural materials, has the potential to provide the unique combination of near atomic spatial and elemental resolutions needed for such an investigation.     

Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles

CoFe₂O₄ ferrite nanoparticles were prepared by a modified chemical coprecipitation route. Structural and magnetic properties were systematically investigated. X-ray diffraction results showed that the sample was in single phase with the space group . The results of field-emission scanning electronic microscopy showed that the grains appeared spherical with diameters ranging from 20 to 30 nm. The composition determined by energy-dispersive spectroscopy was stoichiometry of CoFe₂O₄. The Curie temperature in the process of increasing temperature was slightly higher than that in the process of decreasing temperature. This can be understood by the fact that heating changed Co²⁺ ion redistribution in tetrahedral and in octahedral sites. The coercivity of the synthesized CoFe₂O₄ samples was lower than the theoretical values, which could be explained by the mono-domain structure and a transformation from ferrimagnetic to superparamagnetic state.     

Magnetic force microscopy applied in magnetic data storage technology

Microstructured thin-film elements with critical dimensions of 1 μm or less play an increasingly important role in magnetic components for information technology applications. Devices that are directly based on such microstructures are key components in magnetoelectronics for storage and sensor applications as well as modern concepts which are likely to substitute today’s hard disk drives. Basic research on magnetic materials as well as industrial applications create an increasing demand for high-resolution magnetic imaging methods. One such method is magnetic force microscopy (MFM). In spite of considerable achievements, MFM also has some serious shortcomings, which have not been overcome to date. Under normal circumstances, the method yields only qualitative information about the magnetic object and it is difficult to improve the resolution to values below 100 nm. In this paper, we will report on advanced MFM probe preparation, based on electron beam methods, and discuss the possibilities for batch fabrication of such advanced MFM tips. We show that the advanced probes allow high-resolution imaging of fine magnetic structures within thin-film permalloy elements without perturbing them. Additionally, we present high-frequency MFM measurements on a hard disk write head. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-681/302-3790, E-mail: m.koblischka@mx.uni-saarland.de     

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.     

Ab initio calculation and analysis of the properties of digital magnetic heterostructures and diluted magnetic semiconductors of IV and III-V groups

We present the first-principles calculations of digital magnetic heterostructures Si/M, Ge/M. GaAs/M, GaSb/M, GaN/M and GaN/M (50%) with M=Cr, Mn, Fe, and Co. The interaction between magnetic dopants results in a wide spin-polarized two-dimensional band inside the gap. It is found that beginning occupation of the minority-spin band greatly increases the energy of the ferromagnetic (FM) state and leads, as a rule, to the antiferromagnetic (AFM) spin ordering. This mechanism causes transition to the AFM state, when interaction between magnetic atoms is too strong, and defines the optimum of Curie temperature as a function of transition element concentration in magnetic layers.     

The influence of composition and field annealing on magnetic properties of FeCo-based amorphous and nanocrystalline alloys

We report the influence of composition and very high transverse field annealing on the magnetic properties and structure of four FeCo-based amorphous and nanocrystalline alloys. The compositions (Fe₅₀Co₅₀)₈₉Zr₇B₄ and (Fe₆₅Co₃₅)₈₉Zr₇B₄ were investigated changing the Fe:Co ratio from 50:50 to 65:35. (Fe₅₀Co₅₀)₈₅Zr₂Nb₄B_8.5 was chosen to investigate Nb substitution for Zr in an FeCo-based alloy. This substitution is shown to increase the magnetostrictive constant, _λS${\lambda }_{\mathrm{S}}$, of the nanocrystalline alloy from 36×10⁻⁶ to 54×10⁻⁶. The composition (Fe₆₅Co₃₅)₈₄Cr₅Zr₇B₄ was studied to investigate the influence of Cr on intergranular coupling across the amorphous matrix. Samples of each composition were annealed in the amorphous state at 300 °C and in the nanocrystalline state at 600 °C. Field annealing was performed in 17 T transverse field in an inert atmosphere. Frequency-dependent magnetic properties were measured with an automatic recording hysteresisgraph. Static magnetic properties were measured with a vibrating sample magnetometer. The mass-specific power loss of the alloys decreased with field annealing in both the nanocrystalline and amorphous states for some frequency and induction combinations. Furthermore, the hysteresis loops are sheared after field annealing, indicating a transverse magnetic anisotropy. The nanocrystalline (Fe₅₀Co₅₀)₈₅Zr₂Nb₄B_8.5 composition has a lower relative permeability than the other compositions.     

Electrical Properties of Nanostructured Magnetic Colloid and Influence of Magnetic Field

We investigate the electrical properties of the nanostructured magnetic colloid without and with magnetic field. The competition between the directional motion of the charged magnetic nanoparticles and other minor nonmagnetic impurities （also small amount of ions） under applied voltage and their random orientation due to thermal activation is implemented to elaborate the electrically conduction mechanism under zero magnetic field. Two equivalent electric circuits are employed for explaining the charging and discharging processes. The tunnelling conduction mechanism upon application of externally magnetic field may exist in the nanostructured magnetic colloid. The alternation of the two conduction mechanisms accounts for the current spikes when the magnetic field is switched on or off. This work presents the peculiar electrical phenomena of the magnetically colloidal system.     

Morphology and magnetic properties of Co0.8Fe2.2O4 films prepared by the chemical solution deposition

Co_0.8Fe_2.2O₄ ferrite thin films have been prepared on Si(0 0 1) substrates by the chemical solution deposition. Structural characteristics indicate all films are single phase with spinel structure and the space group and the mean grain size increases from 8 to 30 nm with the increase of annealing temperature. The magnetic properties of Co_0.8Fe_2.2O₄ thin films are highly dependent on annealing temperature. The sample annealed at 800 °C possesses high saturation magnetization, moderate coercivity and squareness ratio, making it a promising application candidate in high-density record and magneto-optical materials.     

Formation mechanism of ferromagnetism in Si1−xMnx diluted magnetic semiconductors

Si_1−xMn_x diluted magnetic semiconductor (DMS) bulks were formed by using an implantation and annealing method. Energy dispersive X-ray fluorescence, transmission electron microscopy (TEM), and double-crystal rocking X-ray diffraction (DCRXD) measurements showed that the grown materials were Si_1−xMn_x crystalline bulks. Hall effect measurements showed that annealed Si_1−xMn_x bulks were p-type semiconductors. The magnetization curve as a function of the magnetic field clearly showed that the ferromagnetism in the annealed Si_1−xMn_x bulks originated from the interaction between interstitial and substitutional Mn⁺ ions, which was confirmed by the DCRXD measurements. The magnetization curve as a function of the temperature showed that the ferromagnetic transition temperature was approximately 75 K. The present results can help to improve understanding of the formation mechanism of ferromagnetism in Si_1−xMn_x DMS bulks.     

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.