Zn-Cu-codoped SnO_2 nanoparticles: Structural,optical, and ferromagnetic behaviors

Zn–Cu-codoped SnO_2 nanoparticles have been synthesized by chemical precipitation method. All nanoparticles are crystalline, with the average size increases from 2.55 nm to 4.13 nm as the calcination temperature increases from 400℃ to 600℃. The high calcination temperature can enhance the crystalline quality and grain growth. The oxygen content decreases with decreasing calcination temperature; at a low temperature of 400℃, Zn–Cu-codoped SnO_2 nanoparticles are in a rather oxygen-poor state having many oxygen vacancies. The optical band gap energies of Zn–Cu-codoped SnO_2 nanoparticles calcined at 400℃ and 600℃ are decreased from 3.93 eV to 3.62 eV due to quantum confinement effects.Both samples exhibit room-temperature ferromagnetism, with a larger saturation magnetization at 400℃ due to the presence of large density of defects such as oxygen vacancies. Zn–Cu-codoped SnO_2 nanoparticles exhibit large optical band gap energies and room temperature ferromagnetism, which make them potential candidates for applications in optoelectronics and spintronics.     

Spin-dependent tunneling and Coulomb blockade in ferromagnetic nanoparticles

In this paper we review studies on spin-dependent transport in systems containing ferromagnetic nanoparticles. In a tunnel junction with a nanometer-scale-island, the charging effect leads to an electric current blockade phenomenon in which a single electron charge plays a significant role in electron transport, resulting in single-electron tunneling (SET) properties such as Coulomb blockade and Coulomb staircase. In a tunnel junction with a ferromagnetic nano-island and electrode, it was expected that the interplay of spin-dependent tunneling (SDT) and SET, i.e., spin-dependent single-electron tunneling (SD-SET), would give rise to remarkable tunnel magnetoresistance (TMR) phenomena. We investigated magnetotransport properties in both sequential tunneling and cotunneling regimes of SET and found the enhancement and oscillation of TMR. The self-assembled ferromagnetic nanoparticles we have employed in this study consisted of a Co–Al–O granular film with cobalt nanoparticles embedded in an Al–O insulating matrix. A _Co36Al22O42${\mathrm{Co}}_{36}{\mathrm{Al}}_{22}{\mathrm{O}}_{42}$ film prepared by a reactive sputtering method produced a TMR ratio reaching 10% and superparamagnetic behavior at room temperature. The TMR ratio exhibited an anomalous increase at low temperatures but no indication of change with bias voltage. In Section 4, we show that the anomalous increase of the MR provided evidence for higher-order tunneling (cotunneling) between large granules through intervening small granules. We emphasize that the existence of higher-order tunneling is a natural consequence of the granular structure, since broad distribution of granule size is an intrinsic property of granular systems. In Section 5, we concentrate on SD-SET properties in sequential tunneling regimes. We fabricated two types of device structures with Co–Al–O film using focused ion-beam milling or electron-beam lithography techniques. One had a granular nanobridge structure: point-shaped electrodes separated by a very narrow lateral gap filled with the Co–Al–O granular film. The other had a current-perpendicular-to-plane (CPP) geometry structure: a thin Co–Al–O granular film sandwiched by ferromagnetic electrodes with the current flowing in the direction perpendicular to the film plane through a few Co particles. We found the enhancement and oscillation of TMR due to spin-dependent SET in sequential tunneling regimes. In Section 6, we report experimental evidence of a spin accumulation effect in Co nanoparticles leading to the oscillation of TMR with alternate sign changes. Furthermore, we discovered that the spin relaxation time in the nanoparticles is unprecedentedly enhanced up to the order of more than hundreds of nanoseconds, compared to that evaluated from the spin-diffusion length of ferromagnetic layers in previous CPP-GMR studies, i.e., the order of tens of picoseconds.     

Defects in room-temperature ferromagnetic Cu-doped ZnO films probed by x-ray absorption spectroscopy

We report a comprehensive study of the defects in room-temperature ferromagnetic (RTFM) Cu-doped ZnO thin films using x-ray absorption spectroscopy. The films are doped with 2 at.% Cu, and are prepared by reactive magnetron sputtering (RMS) and pulsed laser deposition (PLD), respectively. The results reveal unambiguously that atomic point defects exist in these RTFM thin films. The valence states of the Cu ions in both films are 2(+). In the film prepared by PLD, the oxygen vacancies (V(O)) form around both Zn ions and Cu ions in the hexagonal wurtzite structure. Upon annealing of the film in O(2), the V(O) population reduces and so does the RTFM. In the film prepared by RMS, the V(O)s around Cu ions are not detected, and the V(O) population around Zn ions is also smaller than in the PLD-prepared film. However, zinc vacancies (V(Zn)) are evidenced. Given the low doping level of spin-carrying Cu ions, these results provide strong support for defect-mediated ferromagnetism in Cu-doped ZnO thin films.     

Copper-doped AlN polycrystalline powders: A class of room-temperature ferromagnetic materials

Cu-doped AlN polycrystalline samples were synthesized by a solid-state reaction. X-ray diffraction analysis revealed the hexagonal structure of the doped samples. Photoluminescence measurements reveal the substitution of Cu for Al in the AlN lattice. Clear hysteresis loops are observed in the M-H curves for the samples at 300 K, revealing room temperature ferromagnetism of the samples. Our experimental results verify that room temperature ferromagnetism is an intrinsic property of Cu-doped AlN.     

Magnetic and magnetooptical properties of ferromagnetic semiconductor GaN : Cr

Magnetic and magnetooptical properties of the Cr-doped GaN layers have been investigated in the temperature range 50–400 K. A high saturation magnetization of 25 G has been observed in the obtained material. Spectra of the magnetooptical transversal Kerr effect have revealed strong magnetic response in the energy range less than 3.0 eV due to the appearance of new spin-polarization states in the band gap of GaN upon Cr doping.     

Co掺杂的ZnO室温铁磁半导体材料制备与磁性和光学特性研究

采用溶胶-凝胶法制备出具有室温铁磁性的Co掺杂的ZnO稀磁半导体材料. 通过对样品的结构、磁性和发光特性的研究发现,样品具有室温铁磁性,并发现其铁磁性源于磁性离子对ZnO中Zn离子的取代. 对不同温度制备的样品的磁性以及其发光特性的变化研究发现,样品的铁磁性与样品中锌间隙位（Zn_i）缺陷的密度有关. 关键词： ZnO 稀磁半导体 铁磁性     

Collective states of interacting ferromagnetic nanoparticles

Discontinuous magnetic multilayers [CoFe/Al₂O₃] are studied by use of magnetometry, susceptometry and numeric simulations. Soft ferromagnetic Co₈₀Fe₂₀ nanoparticles are embedded in a diamagnetic insulating a-Al₂O₃ matrix and can be considered as homogeneously magnetized superspins exhibiting randomness of size (viz. moment), position and anisotropy. Lacking intra-particle core-surface ordering, generic freezing processes into collective states rather than individual particle blocking are encountered. With increasing particle density one observes first superspin glass and then superferromagnetic domain state behavior. The phase diagram resembles that of a dilute disordered ferromagnet. Criteria for the identification of the individual phases are given.     

Nature of room-temperature ferromagnetism from undoped ZnO nanoparticles

ZnO nanoparticles were synthesized by a sol–gel calcination process following being annealed in air at 400, 600, 800 and 1000°C. X-ray diffraction pattern and X-ray photoelectron spectroscopy show that all the samples present a typical wurtzite structure and no other impurity phases are observed. Room-temperature ferromagnetism from all the samples is confirmed by the vibrating sample magnetometer measurements, which shows that the RTFM decreases with the annealing temperature increasing from 400 to 800°C, and then became larger for the sample annealed at 1000°C. According to the Raman scattering spectra and electron paramagnetic resonance, the RTFM of samples annealed at 600, 800 and 1000°C might be related to the oxygen vacancy related defects. However, the RTFM from the sample annealed at 400°C, presenting nearly the same value as that of the sample annealed at 1000°C, could originate with the interstitial zinc defects associated with XPS and photoluminescence analyses.     

Spin excitations in granular structures with ferromagnetic nanoparticles

Spin excitations and relaxation in a granular structure which contains metallic ferromagnetic nanoparticles in an insulating amorphous matrix are studied in the framework of the s-d exchange model. As the d system, we consider the granule spins, and the s system is represented by localized electrons in the amorphous matrix. In the one-loop approximation with respect to the s-d exchange interaction for a diagram expansion of the spin Green’s function, the spin excitation spectrum is found, which consists of spin-wave excitations in the granules and of polarized spin excitations. In polarized spin excitations, a change in the granule spin direction is accompanied by an electron transition with a spin flip between two sublevels of a split localized state in the matrix. We considered polarized spin relaxation (relaxation of the granule spins occurring by means of polarized spin excitations) determined by localized deep energy states in the matrix and the thermally activated electronic cloud of the granule. It is found that polarized spin relaxation is efficient over a wide frequency range. Estimates made for structures with cobalt granules showed that this relaxation could be observed in centimetric, millimetric, and submillimetric wavelength ranges.     

Magnetoresistance and noncollinear structures of multilayer ferromagnetic nanoparticles

The distribution of the magnetization over multilayer particles, including three ferromagnetic layers separated by insulating spacers, is studied experimentally and theoretically. Experimental data on the magnetic state of these particles are obtained by measuring their magnetoresistance. For the case of zero applied field, it is shown that a multilayer particle with easy-plane magnetic anisotropy is a noncollinear helical state.     

Theoretical study of phonon spectra in ferromagnetic nanoparticles

Based on the spin-phonon model we analyze the influence of surface and size effects on the phonon properties of ferromagnetic nanoparticles. A Green's function technique in real space enables us to calculate the renormalized phonon energy and its damping depending on the temperature and the anharmonic spin-phonon interaction constants. With decreasing particle size the phonon energy can decrease or increase for different surface spin-phonon interaction constants, whereas the damping increases always. The influence of an external magnetic field is discussed, too. The theoretical results are in reasonable accordance to experimental data.     

Possibility of observing chiral-symmetry effects in ferromagnetic nanoparticles

The statistics of left-and right-handed vortical magnetization states in a system of submicron-sized ferromagnetic disks is studied experimentally. Lattices of elliptic cobalt particles with a planar size of 750 × 450 nm and a thickness of 25 nm were formed using high-resolution electronic lithography. To visualize the magnetization distributions in the disks, magnetic force microscopy was used. The experimental results indicate the conservation of magnetic chiral symmetry in cobalt ferromagnetic particles of the above size and shape.     

The important role of Mn in the room-temperature ferromagnetism of Mn-doped GaN films

Mn-doped GaN films (Ga_1−xMn_xN) were grown on sapphire (0 0 0 1) using Laser assisted Molecular Beam Epitaxy (LMBE). High-quality nanocrystalline Ga_1−xMn_xN films with different Mn concentration were then obtained by thermal annealing treatment for 30 min in the ammonia atmosphere. Mn ions were incorporated into the wurtzite structure of the host lattice by substituting the Ga sites with Mn³⁺ due to the thermal treatment. Mn³⁺, which is confirmed by XPS analysis, is believed to be the decisive factor in the origin of room-temperature ferromagnetism. The better room-temperature ferromagnetism is given with the higher Mn³⁺ concentration. The bound magnetic polarons (BMP) theory can be used to prove our room-temperature ferromagnetic properties. The film with the maximum concentration of Mn³⁺ presents strongest ferromagnetic signal at annealing temperature 950 °C. Higher annealing temperature (such as 1150 °C) is not proper because of the second phase Mn_xGa_y formation.     

Possible origin of room-temperature ferromagnetism in undoped GaN epilayers implanted with Mn ions

Manganese ions were implanted into unintentionally doped GaN epilayers grown by metal organic chemical vapor deposition (MOCVD). The (Ga,Mn)N and Ga_xMn_y phases were formed after Mn-implanted undoped GaN epilayers annealed at 700 and 800 °C. The samples showed ferromagnetic behavior at room temperature with the highest magnetization obtained in the sample annealed at 800 °C. Ferromagnetic signal reduces as annealing temperature increased above 900 °C. It is believed that the room-temperature ferromagnetic property of Mn-implanted undoped GaN epilayers are mainly from (Ga_,Mn)N. The Ga_xMn_y phases play a critical role in providing holes and also contribute to increasing the ferromagnetic property.     

Coupling of ferromagnetic nanoparticles through dipolar interactions

We consider two ferromagnetic nanoparticles coupled via long-range dipolar interactions. We model each particle by a three-dimensional array of classical spin vectors, with a central spin surrounded by a variable number of shells. Within each particle only ferromagnetic coupling between nearest neighbor spins is considered. The interaction between particles is of the dipolar type and the magnetic properties of the system is studied as a function of temperature and distance between the centers of the particles. We perform Monte Carlo simulations for particles with different number of shells, and the magnetic properties are calculated via two routes concerning the dipolar contribution: one assuming a mean-field like coupling between effective magnetic moments at the center of the particles, and other one, where we take into account interactions among all the pairs of spins, one in each particle. We show that the dipolar coupling between the particles enhances the critical temperature of the system relative to the case in which the particles are very far apart. The dipolar energy between the particles is smaller when the assumption of effective magnetic moment of the particles is used in the calculations.     

Magnetism of mass-filtered nanoparticles on ferromagnetic supports

The magnetic properties of 3d-metal clusters significantly differ from bulk behavior and, for small clusters, strongly depend on the number of atoms within each cluster. Such phenomena are caused by a narrowing of electronic states and the high ratio of surface to volume atoms giving rise to enhanced magnetic orbital moments. However, even large Fe nanoparticles (6–12 nm) deposited onto ferromagnetic surfaces show enhanced orbital moments. At a low coverage large iron clusters on a cobalt film exhibit a nearly doubled value for the orbital moments when compared to bulk behaviour. With increasing coverage, the orbital moment is clearly reduced. Additionally, the spin and orbital moments of iron and cobalt in Fe₅₀Co₅₀ alloy clusters with a size of 7.5 nm on a nickel substrate have been investigated. FeCo alloys are known to exhibit very high magnetic moments for soft magnetic materials. PACS 73.22.-f; 75.75.+a; 81.07.-b     

Direct observation of ferromagnetic spin polarization in gold nanoparticles

We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using x-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms as estimated by XMCD shows a good agreement with results obtained by conventional magnetometry. This evidences intrinsic spin polarization in nanosized gold.     

Fe impurities weaken the ferromagnetic behavior in Au nanoparticles

In this Letter, we report on a crucial experiment showing that magnetic impurities reduce the ferromagnetic order temperature in thiol-capped Au glyconanoparticles (GNPs). The spontaneous magnetization of AuFe GNPs exhibits a fast decrease with temperature that contrasts with the almost constant value of the magnetization observed in Au NPs. Moreover, hysteresis disappears below 300 K. Both features indicate that Fe impurities reduce the high local anisotropy field responsible for the ferromagnetic behavior in Au GNPs. As a consequence, the amazing ferromagnetism in Au NPs should not be associated with the presence of magnetic impurities.     

Order-disorder transformation in FePt nanoparticles studied by ferromagnetic resonance

We have investigated the ferromagnetic resonance (FMR) response of as-made and temperature annealed FePt magnetic nanoparticles. The as-made nanoparticles, which have been fabricated by a chemical route, crystallize in the low magnetic anisotropy fcc phase and have a diameter in the range of 2-4 nm. The annealing of the particles at high temperatures (T_A=550, 650 and C) in an inert Ar atmosphere produces a partial transformation to the high magnetocrystalline anisotropy L1₀ phase, with a significant increase in particle size and size distribution. FMR measurements at X-band (9.5 GHz) and Q-band (34 GHz) show a single relatively narrow line for the as-synthesized particles and a structure of two superimposed lines for the three annealed samples. The origin of this line shape has been attributed to the presence of the disordered fcc phase. Assuming that the system consists of a collection of identical particles with a random distribution of easy axes, we have been able to estimate a mean value for the magnetic anisotropy constant of the particles in the fcc phase, K∼2×10⁶ erg/cm³. The measured line shape in the annealed samples can be explained if we consider that the magnetic anisotropy of the particles has a gaussian distribution with a relatively broad width.     

Search for ferromagnetic ordering in Pd doped wide band gap semiconductors GaN and ZnO

GaN and ZnO are possible candidates for dilute magnetic semiconductors with Curie temperatures above room temperature. Doping with transition metals like Co, Mn or Fe could be a simple way to create such systems. The perturbed angular correlation (PAC) probe ¹⁰⁰Pd/¹⁰⁰Rh is isoelectronic to cobalt and therefore a perfect tool to investigate the incorporation of transition metals into these compounds as well as the influence of other impurities on internal magnetic fields. The (0001) and (10 $\bar{1}$ 10) surfaces of ZnO single crystals, freestanding GaN films, and GaN thin films (6 μm) on sapphire substrates were recoil-implanted with the ¹⁰⁰Pd/¹⁰⁰Rh probe. The probe was produced using the fusion evaporation reaction ⁹²Zr(¹²C, 4n)¹⁰⁰Pd at a beam energy of 69 MeV. Subsequently, the incorporation of the probe was studied by PAC spectroscopy during an isochronal annealing program. First results without and with an applied external magnetic field are indicative of a strongly disturbed lattice vicinity of Pd impurities in both hosts. No signs of spontaneous ferromagnetic ordering were observed.