Frequency modulation (FM) spectroscopy

Frequency modulation (FM) spectroscopy is a new method of optical heterodyne spectroscopy capable of sensitive and rapid measurement of the absorption or dispersion associated with narrow spectral features. The absorption or dispersion is measured by detecting the heterodyne beat signal that occurs when the FM optical spectrum of the probe wave is distorted by the spectral feature of interest. A short historical perspective and survey of the FM spectroscopy work performed to date is presented. Expressions describing the nature of the beat signal are derived. Theoretical lineshapes for a variety of experimental conditions are given. A signal-to-noise analysis is carried out to determine the ultimate sensitivity limits.     

Nanomagnetic droplets and implications to orbital ordering in LA(1-x)Sr(x)CoO3

Inelastic cold-neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate S = 1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La(1-x)Sr(x)CoO3 , the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.     

Auditory processing of real and illusory changes in frequency modulation (FM) phase

Auditory processing of frequency modulation (FM) was explored. In experiment 1, detection of a tau-radians modulator phase shift deteriorated as modulation rate increased from 2.5 to 20 Hz, for 1- and 6-kHz carriers. In experiment 2, listeners discriminated between two 1-kHz carriers, where, mid-way through, the 10-Hz frequency modulator had either a phase shift or increased in depth by deltaD% for half a modulator period. Discrimination was poorer for deltaD = 4% than for smaller or larger increases. These results are consistent with instantaneous frequency being smoothed by a time window with a total duration of about 110 ms. In experiment 3, the central 200-ms of a 1-s 1-kHz carrier modulated at 5 Hz was replaced by noise, or by a faster FM applied to a more intense 1-kHz carrier. Listeners heard the 5-Hz FM continue at the same depth throughout the stimulus. Experiments 4 and 5 showed that, after an FM tone had been interrupted by a 200-ms noise, listeners were insensitive to the phase at which the FM resumed. It is argued that the auditory system explicitly encodes the presence, and possibly the rate and depth, of FM in a way that does not preserve information on FM phase.     

Detection of frequency modulation (FM) in the presence of a second FM tone

A series of three experiments was undertaken to investigate detection of sinusoidal frequency modulation (FM) in the presence of FM at a separate frequency. The first experiment measured detection of modulation for an FM tone with a modulation frequency (fm) of 6 Hz as a function of carrier frequency (fc) under three conditions: (1) in quiet, (2) in the presence of a 2500-Hz pure tone, and (3) in the presence of a 2500-Hz FM tone with fm = 6 Hz, modulating in phase with the signal. Detection of FM in the presence of the second FM tone was worse than for either the signal presented in quiet or in the presence of the unmodulated tone. Threshold varied as an inverse function of frequency separation between the signal and the masker. In the second experiment, FM detection for a signal with fc = 1900 Hz and fm = 6 Hz was measured as a function of the modulation frequency (fm = 2-18 Hz) of the 2500-Hz masker tone. FM detection improved significantly with increasing difference between the modulation frequencies of the signal and the masker. The final experiment measured detection of FM for a signal (fc = 1900 Hz, fm = 6 Hz) in the presence of a second FM tone (fc = 2500 Hz, fm = 6 Hz) as a function of the relative phase of the 6-Hz modulators. Detection of FM improved monotonically as a function of increasing phase difference between the two modulators. The results are discussed in terms of modulation detection interference and perceptual grouping.     

Magnetic structure of (Fe0.97Cr0.03)2P

Magnetic behaviour of di-metal iron phosphide with a small substitution of iron by chromium, (Fe_0.97Cr_0.03)₂P, has been studied using SQUID magnetometry and powder neutron diffraction. It is paramagnetic at temperatures above ∼180 K with persisting short range ferromagnetic (FM) order. At lower temperatures three different regions of magnetic behaviour are identified. FM order evolves in the region 180 K-120 K but much more slowly and with much less magnetic moments than in Fe₂P. In the region 120 K-50 K negative exchange interactions gain some importance leading to a loss of FM order. Below 50 K FM interactions again dominate. Pinning centres influence the behaviour at low temperature up to ∼100 K.     

Ferromagnetism in ZnO with（Mn,Li） codoping

First-principles calculations were performed to investigate the magnetic properties of Zn(Mn,Li)O based on the Perdew-Burke-Ernzerhof form of generalized gradient approximation. Antiferromagnetic (AFM) ordering is the ground state in Mn-doped ZnO system without the codopant of Li, while seven different geometrical configurations of Zn(Mn,Li)O prefer stable ferromagnetic (FM) ordering. We found that dopant Li can effectively change the magnetic coupling in the ZnMnO system. The Curie temperature (T_C) of FM ordering depends on the geometric configuration, and the highest T_C is about 1388 K. The FM stabilization is greatly affected by Mn-Mn distance rather than by the position of dopant Li. We propose that dopant Li mediates FM coupling through a double exchange interaction or an RKKY interaction when Li is located, respectively, near or far from Mn ions.     

Thermal and electronic transport properties and two-phase mixtures in La(5/8-x)Pr(x)Ca(3/8)MnO3

We measured thermal conductivity kappa, thermoelectric power S, and electric conductivity sigma of La(5/8-x)Pr(x)Ca(3/8)MnO3, showing an intricate interplay between metallic ferromagnetism (FM) and charge ordering (CO) instability. The change of kappa, S, and sigma with temperature (T) and x agrees well with the effective medium theories for binary metal-insulator mixtures. This agreement clearly demonstrates that with the variation of T as well as x, the relative volumes of FM and CO phases drastically change and percolative metal-insulator transition occurs in the mixture of FM and CO domains.     

Room-temperature ferromagnetism induced by Cu vacancies in Cu_x(Cu_2O)_(1-x) granular films

Cux(Cu2O)1-x(0.09 x 1.00) granular films with thickness about 280 nm have been fabricated by direct current reactive magnetron sputtering. The atomic ratio x can be controlled by the oxygen flow rate during Cux(Cu2O)1-x deposition. Room-temperature ferromagnetism(FM) is found in all of the samples. The saturated magnetization increases at first and then decreases with the decrease of x. The photoluminescence spectra show that the magnetization is closely correlated with the Cu vacancies in the Cux(Cu2O)1-x granular films. Fundamentally, the FM could be understood by the Stoner model based on the charge transfer mechanism. These results may provide solid evidence and physical insights on the origin of FM in the Cu2O-based oxides diluted magnetic semiconductors, especially for systems without intentional magnetic atom doping.     

Resolution enhancement in a light-sheet-based microscope (SPIM)

Light-sheet-based microscopy [single-plane illumination microscope (SPIM)] performs very well at low numerical apertures. It complements conventional (FM), confocal (CFM), and two-photon fluorescence microscopy (2hnu-FM) currently used in modern life sciences. Lateral and axial SPIM point spread function (PSF) extents are measured by using fluorescent beads to determine the 3D resolution. The results are compared with values derived from an analytical theory and numerical simulations. The discrepancies are found to be less than 5%. The axial extent of a SPIM-PSF (10x/0.3 W) is approximately 5.7 microm. This value is almost a factor of 2 smaller than in CFM, more than 2.5 times smaller than in FM, and more than three times smaller than in 2hnu-FM. SPIM outperforms 2hnu-FM and FM, while CFM has a better axial resolution at NAs above 0.8.     

The electronic and magnetic properties of （Mn,C）-codoped ZnO diluted magnetic semiconductor

The electronic and magnetic properties of (Mn,C)-codoped ZnO are studied in the Perdew-Burke-Ernzerhof form of generalized gradient approximation of the density functional theory. By investigating five geometrical configurations, we find that Mn doped ZnO exhibits anti-ferromagnetic or spin-glass behaviour, and there are no carriers to mediate the long range ferromagnetic (FM) interaction without acceptor co-doping. We observe that the FM interaction for (Mn,C)-codoped ZnO is due to the hybridization between C 2p and Mn 3d states, which is strong enough to lead to hole-mediated ferromagnetism at room temperature. Meanwhile, we demonstrate that ZnO co-doped with Mn and C has a stable FM ground state and show that the (Mn,C)-codoped ZnO is FM semiconductor with super-high Curie temperature (T C = 5475 K). These results are conducive to the design of dilute magnetic semiconductors with codopants for spintronics applications.     

氮掺杂(1120) ZnO 薄膜磁性质研究

采用基于密度泛函理论(DFT)和局域密度近似(LDA)的第一性原理分析了氮掺杂(1120) ZnO 薄膜的磁性质.首先,研究了一个N原子掺杂ZnO薄膜的磁性质,结果表明N 2p,O 2p和Zn 3d 发生自发自旋极化.其次,研究了二个N原子掺杂ZnO薄膜的磁性质,9个不同几何结构的计算结果表明N原子之间具有FM耦合稳定性,而且具体分析了N掺杂ZnO铁磁稳定性的产生原因.最后,讨论了氮 关键词： 第一性原理 半导体 铁磁性     

Modulation of ultrafast laser-induced magnetization precession in BiFeO_3-coated La_(0.67)Sr_(0.33)MnO_3 thin films

The ultrafast laser-excited magnetization dynamics of ferromagnetic(FM) La_(0.67)Sr_(0.33)MnO_3(LSMO) thin films with BiFeO_3(BFO) coating layers grown by laser molecular beam epitaxy are investigated using the optical pump-probe technique.Uniform magnetization precessions are observed in the films under an applied external magnetic field by measuring the time-resolved magneto-optical Kerr effect.The magnetization precession frequencies of the LSMO thin films with the BFO coating layers are lower than those of uncoated LSMO films,which is attributed to the suppression of the anisotropy field induced by the exchange interaction at the interface between the antiferromagnetic order of BFO and the FM order of LSMO.     

非磁性掺杂下的铁磁/反铁磁系统中的交换偏置和矫顽场

采用Monte Carlo方法,分别讨论了在铁磁/反铁磁双层膜和铁磁/反铁磁单层混合膜中,掺入非磁性物质后,掺杂浓度对交换偏置以及矫顽场的影响.计算结果表明:随着掺杂浓度的增大,双层膜和单层膜交换偏置都有先增大后减小的现象,而其矫顽场则先减小后增大.在相同掺杂浓度下,对随机掺杂和规则掺杂两种不同掺杂方式的结果比较发现:铁磁/反铁磁双层膜中,规则掺杂下产生的交换偏置和矫顽场都得到了增强;对于单层混合膜,随机掺杂下的交换偏置更强,规则掺杂下的矫顽场更大.研究发现对于双层膜规则掺杂可明显地导致其磁滞回线的不对称性,说明铁磁/反铁磁系统中磁滞回线的不对称性与界面自旋微结构密切相关.     

Exchange-coupling-induced fourfold magnetic anisotropy in CoFeB/FeRh bilayer grown on SrTiO3(001)

Exchange coupling across the interface between a ferromagnetic (FM) layer and an antiferromagnetic (AFM) or another FM layer may induce a unidirectional magnetic anisotropy and/or a uniaxial magnetic anisotropy, which has been extensively studied due to the important application in magnetic materials and devices. In this work, we observed a fourfold magnetic anisotropy in amorphous CoFeB layer when exchange coupling to an adjacent FeRh layer which is epitaxially grown on an SrTiO₃(001) substrate. As the temperature rises from 300 K to 400 K, FeRh film undergoes a phase transition from AFM to FM phase, the induced fourfold magnetic anisotropy in the CoFeB layer switches the orientation from the FeRh$\langle 110\rangle $ to FeRh$\langle 100\rangle $ directions and the strength is obviously reduced. In addition, the effective magnetic damping as well as the two-magnon scattering of the CoFeB/FeRh bilayer also remarkably increase with the occurrence of magnetic phase transition of FeRh. No exchange bias is observed in the bilayer even when FeRh is in the nominal AFM state, which is probably because the residual FM FeRh moments located at the interface can well separate the exchange coupling between the below pinned FeRh moments and the CoFeB moments.     

Magnetic Properties of Hf0.8Ta0.2（Fe0.97A0.03）2 （A=Al, Co, Mn） Systems

Moessbauer studies on the effect of substitution with 3% Al, Co, Mn atoms in the intermetallic compound of Hf0.8Ta0.2Fe2 are reported. The Al substitution leads to increase of the FM-AFM transition temperature and to decrease of the AFM-PM transition temperature. The Co substitution leads to disappearance of the FM state, only showing some FM impurity component, while Mn substituted compound indicates coexistence of FM and AFM states at low temperature. The phenomena imply complex itinerant electron properties in these magnetic systems.     

First principles study on the electronic structure of the two chain compounds of [ M(N3)2(HCOO)][(CH3)2NH2] (M=Fe and Co)

First principles calculations have been performed to study the electronic structure and the ferromagnetic properties on the two chain compounds of [M(N₃)₂(HCOO)][(CH₃)₂NH₂] (M=Fe and Co). The relative stability of the ground state, the density of states and the electronic band structure are examined. The results reveal that antiferromagnetism (AFM) state is the ground state and ferromagnetism (FM) state is the metastable one for both of them. The two compounds exhibit semiconductor character with small gap in the FM state, while metallic in the AFM state. In the FM state, the magnetic moments mainly arise from the Fe and Co ions with little contribution from the nearest-neighboring N and O atoms due to the hybridization between the Fe or Co 3d states and the nearest-neighboring N and O 2p states.     

分布反馈半导体激光器频率调制特性的速率方程分析

本文引入描述分布反馈半导体激光器中光场纵向分布不均匀对其阈值条件影响的α_p和δ_p因子.建立了包含空间烧孔效应的速率方程模型,并给出小信号强度、频率调制响应表达式.数值求解理想分布反馈半导体激光器的α_(th)L和δL随平均光子密度变化曲线及对应的α_p和δ_p因子.数值模拟表明,空间烧孔对调制特性影响甚大,频率调制相应与偏置电流有关,为得到较宽的调制频带必须适当选择工作点.     

La0.67Ca0.33MnO3/NdGaO3(001)薄膜中外延应变诱导的各向异性磁阻效应

本文使用脉冲激光沉积方法在正交的NdGaO3(001)衬底上外延生长了La0.67Cao.33MnO3薄膜.退火后,由于外延应变引起薄膜正交畸变增强,薄膜在低于250K的整个温度区间都出现了铁磁金属态和反铁磁绝缘态的相分离和相竞争.这两相的竞争强烈依赖于磁场相对晶轴的方向,由此导致存在于很宽温度范围内的低场各向异性磁阻...     

Electronic structure and magnetic properties of(Cu,N)-codoped3C-SiC studied by first-principles calculations

The electronic structures and magnetic properties of the Cu and N codoped 3C-Si C system have been investigated by the first-principles calculation.The results show that the Cu doped Si C system prefers the anti-ferromagnetic(AFM) state.Compared to the Cu doped system,the ionicities of C–Cu and C–Si in Cu and N codoped Si C are respectively enhanced and weakened.Especially,the Cu and N codoped Si C systems favor the ferromagnetic(FM) coupling.The FM interactions can be explained by virtual hopping.However,higher N concentration will weaken the ferromagnetism.In order to keep the FM interaction,the N concentration should be restricted within 9.3% according to our analysis.