Inhomogeneous magnetic-field response of YBa2Cu3Oy and La2-xSrxCuO4 persisting above the bulk superconducting transition temperature

We report that in YBa2Cu3Oy and La2-xSrxCuO4 there is a spatially inhomogeneous response to the magnetic field for temperatures T extending well above the bulk-superconducting transition temperature Tc. An inhomogeneous magnetic response is observed above Tc even in ortho-II YBa2Cu3O6.50, which has highly ordered doping. The degree of the field inhomogeneity above Tc tracks the hole-doping dependences of both Tc and the density of the superconducting carriers below Tc, and therefore is apparently coupled to superconductivity.     

Refraction of spin waves by bifocal surface ferromagnetic lens in external magnetic field

Behavior of spin-wave propagation in ferromagnetic medium with non-uniform distribution of magnetic parameters is studied. In particular, the influence of external magnetic field, spin-wave frequency and exchange parameter on behavior of surface spin-wave propagating through inhomogeneity made in form of lens (lens is biaxial ferromagnet placed into uniaxial ferromagnetic medium) is studied.     

Active ferromagnetic shimming of the permanent magnet for magnetic resonance imaging scanner

This paper presents an approach of active ferromagnetic shimming for C-type permanent magnetic resonance imaging (MRI) magnet.It is designed to reduce inhomogeneity of magnetostatic field of C-type permanent magnet to meet the stringent requirement for MRI applications.An optimal configuration (locations and thicknesses) of active ferromagnetic pieces is generated through calculation according to the initial field map and the demanded final homogeneity specifications.This approach uses a minimisation technique which makes the sum of squared magnetic moment minimum to restrict the amount of the active ferromagnetic material used and the maximal thickness of pieces stacked at each hole location in the shimming boards.Simulation and experimental results verify that the method is valid and efficient.     

Origin of ferromagnetism and nano-scale phase separations in diluted magnetic semiconductors

This paper reviews various origins of ferromagnetic response that has been detected in diluted magnetic semiconductors (DMS). Particular attention is paid to those ferromagnetic DMS in which no precipitation of other crystallographic phases has been observed. It is argued that these materials can be divided into three categories. The first consists of (Ga,Mn)As and related compounds. In these solid solutions the theory built on p–d Zener's model of hole-mediated ferromagnetism and the Kohn–Luttinger kp theory of semiconductors describes quantitatively thermodynamic, micromagnetic, optical, and transport properties. Moreover, the understanding of these materials has provided a basis for the development of novel methods enabling magnetisation manipulation and switching. To the second group belong compounds, in which a competition between long-range ferromagnetic and short-range antiferromagnetic interactions and/or the proximity of the localisation boundary lead to an electronic nano-scale phase separation that results in characteristics similar to colossal magnetoresistance oxides. Finally, in a number of compounds a chemical nano-scale phase separation into the regions with small and large concentrations of the magnetic constituent is present. It has recently been suggested that this spinodal decomposition can be controlled by the charge state of relevant magnetic impurities. This constitutes a new perspective method for 3D self-organised growth of coherent magnetic nanocrystals embedded by the semiconductor matrix.     

Magnetic domains perpendicular to the AC demagnetizing field in ordered Ni3Mn

Stripe domains with walls perpendicular to the AC demagnetizing field (PACD) are observed in atomically ordered, ferromagnetic NiMn with slightly more than 25 at.% Mn. Based on Lorentz microscopy and magnetic measurements. PACD is explained in terms of an inhomogeneity in the magnetic phase and coherent rotation of the magnetization in the domain.     

非匀强磁场中双量子位Heisenberg XY链的基态纠缠和热纠缠(英文)

研究了非匀强磁场中各向异性Heisenberg XY链的基态纠缠和热纠缠.结果表明对双量子位情形,纠缠与格点间耦合常数J、外部磁场B、各向异性参数γ和b的正负无关.对绝对零度情形,我们给出了纠缠C的解析表达式,并指出临界磁场Bc随磁场各向异性参数b的增大而增大.对有限温度情形,我们给出了γ=0时C的解析表达式和γ≠0时的数值模拟结果,结果发现引进非匀强磁场可以使纠缠在某些区域明显增大;同时我们还指出当γ=0时,纠缠存在的临界温度Tc仅是b的增函数,而当γ≠0时,它却由B和b共同决定.     

Disordered RKKY lattice mean field theory for ferromagnetism in diluted magnetic semiconductors

We develop a lattice mean field theory for ferromagnetic ordering in diluted magnetic semiconductors by taking into account the spatial fluctuations associated with random disorder in the magnetic impurity locations and the finite mean free path associated with low carrier mobilities. Assuming a carrier-mediated indirect RKKY exchange interaction among the magnetic impurities, we find substantial deviation from the extensively used continuum Zener model Weiss mean field predictions. Our theory allows accurate analytic predictions for Tc and provides simple explanations for a number of observed anomalies, including the non-Brillouin function magnetization curves, the suppressed low-temperature magnetization saturation, and the dependence of Tc on conductivity.     

Density functional studies on the ferromagnetic properties of (Zn,Cr)Te

We have investigated the electronic and magnetic properties of the room temperature ferromagnetic diluted magnetic semiconductor (DMS) (Zn,Cr)Te with density functional calculations. The electronic and magnetic properties of (Zn,Cr)Te are very similar with those of the typical DMS (Ga,Mn)As. The stronger ionicity of ZnTe plays a key role in the electronic and magnetic properties of (Zn,Cr)Te.     

Investigation of ferromagnetic resonance and damping properties of CoFeB

In this study, the influences of thin film thickness and post-annealing process on the magnetic properties of CoFeB thin films were investigated. The angular dependency and linewidth of the ferromagnetic resonance signal were used to explore the magnetic behavior of sputtered single-layer and trilayer thin film stacks of CoFeB. A micromagnetic simulation model was employed based on the metropolis algorithm comprising the demagnetizing field and in-plane induced uniaxial anisotropy terms with all relevant contributions. Our results reveal that the direction of magnetization changes from in-plane to out-of-plane as a result of the annealing process and induces a perpendicular magnetic anisotropy in the 1-nm thick CoFeB thin film. The ferromagnetic resonance (FMR) linewidth can be defined well by the intrinsic Gilbert damping effect and the magnetic inhomogeneity contribution in both as-grown and annealed samples. The difference between the linewidths of the single and trilayer film is mainly caused by the spin pumping effect on damping which is associated with the interface layers.     

Ferromagnetic resonance linewidth in exchange-biased ferromagnet/antiferromagnet bilayers with ac field-cooling procedure

Out-of-plane ferromagnetic resonance spectra of Co₂₀Ni₈₀/FeMn bilayers with a ground state consisting of stripe ferromagnetic domains as accommodated by ac field-cooling process were measured and analyzed by the Landau–Lifshitz–Gilbert equation. One weak resonance peak that might be the surface spin-wave was observed beside the main resonance peak when the applied field was perpendicular to the film plane. In order to explain the linewidth, the extrinsic magnetic relaxation must be considered, in addition to the intrinsic Gilbert damping effect and the broadening induced by magnetic inhomogeneity. The characteristic of the extrinsic magnetic relaxation is consistent with the two-magnon scattering model. PACS 75.70.-i; 75.70.Cn; 76.50.+g; 75.30.Gw     

添加Dy_2O_3的YBa_2Cu_3O_(7-δ)的临界电流密度特征

掺入Dy2O3的YBa2Cu3O7-δ(YBCO)粉末样品采用固相反应方法制备获得。XRD的Rietveld精修和SEM都显示Dy2O3和YBCO并存。随着添加相成分的增加,YBCO的开始转变温度Ton、超导临界温度Tc和零电阻温度Tc0都单调减小。对于某一固定的磁场,温度为77K,当x=0.05时,样品的临界电流密度Jc有最大值。磁场中,微量的Dy2O3的添加,使得YBCO临界电流密度增大。Jc的这些特征行为来自D2O3的添加导致的两方面的共同作用,一方面是超导性的变化,另一方面纳米尺寸范围内的空间分布的非均匀性。     

Localization properties of bulk-dead and comb magnetostatic modes in Brillouin light scattering spectrum

We show that magnetostatic modes with amplitude localized in different regions of the sample along the direction of the magnetic field occur in ferromagnetic stripes: bulk-dead modes, with amplitude localized in two regions and comb modes localized in its central part. We also demonstrate these localization properties can be studied by Brillouin light scattering techniques and applied in practice. Having established that the localization of these modes varies with their frequency, we use this finding as the basis of a trial reinterpretation of experimental results obtained by Demidov et al. [Appl. Phys. Lett. 92 (2008) 232503].     

Effect of defects on room-temperature ferromagnetism in Co and Na co-doped ZnO

The role of defects in the room temperature ferromagnetism of the Co–ZnO based diluted magnetic semiconductor (DMS) was investigated by co-doping the DMS with Na. The structure characterizations indicate that both Na and Co ions enter into the ZnO lattice without the formation of secondary phase. The oxygen vacancy of ZnCoNaO increased while the carrier concentration decreased compared with that of ZnCoO, leading to the enhancement of the ferromagnetic property in the ZnCoNaO. The observed ferromagnetism introduced by Na ions is attributed to the exchange interaction via the electron trapped oxygen vacancies coupled with the magnetic Co ions.     

First-principles studies of interlayer exchange coupling in (Ga,Co)N-based diluted magnetic semiconductor multilayers

Interlayer exchange coupling (IEC) in a series model diluted magnetic semiconductor (DMS) multilayer consisting of two magnetic (Ga, Co)N layers separated by non-doped or Mg-doped GaN non-magnetic spacers has been studied by first-principles calculations. The effects of the spacer thickness and hole doping to the IEC were studied systematically. It is observed that (1) for the GaN spacers without Mg doping, the IEC between two magnetic (Ga, Co)N layers is always ferromagnetic, which is clarified as an intrinsic character of the Ruderman–Kittle–Kasuya–Yoshida (RKKY) interaction based on a two-band model for a gapped system; and (2) for the Mg-doped GaN spacers, the IEC is tunable from ferromagnetic to antiferromagnetic by varying the spacer’s thickness and the dopant’s site.     

Ferromagnetism in diluted magnetic semiconductor quantum dot arrays embedded in semiconductors

We present an Anderson-type model Hamiltonian with exchange coupling between the localized spins and the confined holes in the quantum dots to study the ferromagnetism in diluted magnetic semiconductor (DMS) quantum dot arrays embedded in semiconductors. The hybridization between the quantum-confined holes in the quantum dots and the itinerant holes in the semiconductor valence band makes possible hole transfer between the DMS quantum dots, which can induce the long range ferromagnetic order of the localized spins. In addition, it makes the carrier spins both in the quantum dots and in the semiconductors polarized. The spontaneous magnetization of the localized spins and the spin polarization of the holes are calculated using both the Weiss mean field approximation and the self-consistent spin wave approximation, which are developed for the present model.Received: 17 Mars 2003, Published online: 30 January 2004PACS: 75.75. + a Magnetic properties of nanostructures - 75.30.Ds Spin waves - 75.50.Dd Nonmetallic ferromagnetic materials - 75.50.Pp Magnetic semiconductors     

Localized ferromagnetic resonance in inhomogeneous thin films

The effect of sample inhomogeneity on the ferromagnetic resonance linewidth is determined by diagonalization of a spin wave Hamiltonian for ferromagnetic thin films with inhomogeneities spanning a wide range of characteristic length scales. A model inhomogeneity is used that consists of size D grains and an anisotropy field H(p) that varies randomly from grain to grain in a film with thickness d and magnetization M(s). The resulting linewidth agrees well with the two-magnon model for small inhomogeneity, H(p)DpiM(s)d, the precession becomes localized and the spectrum approaches that of local precession on independent grains.     

溶胶凝胶合成锰掺杂ZnO的室温磁性行为

通过溶胶凝胶自燃法合成锰掺杂氧化锌纳米晶体, 研究了Mn掺杂ZnO稀磁半导体(简称DMS)的性质．X射线衍射光谱表明,锰掺杂氧化锌保留纤锌矿型状氧化锌六角晶体结构．采用能量色散X射线能谱和扫描电子显微镜分别对成分和形态进行研究．温度依赖的电阻率显示了DMS的半导体材料行为．振动样品磁强计测定的室温磁性行为,揭示了锰掺杂氧化锌的铁磁性和反磁性特性.     

用电化学沉积方法制备钴掺杂的氧化锌薄膜及其光学性质

通过选用乌洛托品作为络合剂,采用电化学沉积的方法成功地制备出钴掺杂的氧化锌薄膜。通过对样品的XRD表征,得出生长的样品为ZnO纤锌矿结构,并没有其他杂相峰,即没有出现分相;通过对样品XPS的分析显示Co离子在薄膜中以+2价的形式存在;为进一步验证Co²⁺离子进入ZnO的晶格,对掺杂不同Co²⁺浓度的样品进行PL谱的测量,从发光光谱上可以看出随着掺杂Co²⁺浓度的增加,带隙逐渐变窄,发光峰位红移,证明Co²⁺部分取代了Zn²⁺而进入了ZnO晶格中。     

Domain-wall guided nucleation of superconductivity in hybrid ferromagnet-superconductor-ferromagnet layered structures

Domain-wall superconductivity is studied in a superconducting Nb film placed between two ferromagnetic Co/Pd multilayers with perpendicular magnetization. The parameters of top and bottom ferromagnetic films are chosen to provide different coercive fields, so that the magnetic domain structure of the ferromagnets can be selectively controlled. From the dependence of the critical temperature Tc on the applied magnetic field H, we have found evidence for domain-wall superconductivity in this three-layered F/S/F structure for different magnetic domain patterns. The phase boundary, calculated numerically for this structure from the linearized Ginzburg-Landau equation, is in good agreement with the experimental data.     

Magnetization reversal of ferromagnetic nanoparticles under inhomogeneous magnetic field

We investigated remagnetization processes in ferromagnetic nanoparticles under inhomogeneous magnetic field induced by the tip of magnetic force microscope (MFM) in both theoretical and empirical ways. Systematic MFM observations were carried out on arrays of submicron-sized elliptical ferromagnetic particles of Co and FeCr with different sizes and periods. It clearly reveals the distribution of remanent magnetization and processes of local remagnetization of individual ferromagnetic particles. Modeling of remagnetization processes in ferromagnetic nanoparticles under magnetic field induced by MFM probe was performed on the base of Landau–Lifshitz–Gilbert equation for magnetization. MFM-induced inhomogeneous magnetic field is very effective to control the magnetic state of individual ferromagnetic nanoparticles as well as to create different distribution of magnetic field in array of ferromagnetic nanoparticles.