The structure dependence of exchange bias in ferromagnetic/antiferromagnetic bilayers

The structure dependence of exchange bias in ferromagnetic/antiferromagnetic （FM/AF） bilayers has been investigated in detail by extending Slonczewski＇s ＇proximity magnetism＇ idea. Here three important parameters are discussed for FM/AF bilayers, i.e. interracial bilinear exchange coupling J1, interracial biquadratic （spin-flop） exchange coupling J2 and antiferromagnetic layer thickness tAF. The results show that both the occurrence and the variety of the exchange bias strongly depend on the above parameters. More importantly, the small spin-flop exchange coupling may result in an exchange bias without the interracial bilinear exchange coupling. However, in general, the spin-flop exchange coupling cannot result in the exchange bias. The corresponding critical parameters in which the exchange bias will occur or approach saturation are also presented.     

On one pion exchange potential with quark exchange in the resonating group method

The effect of quark exchange between different nucleons on the one pion exchange potential is studied in the framework of the resonating group method. The calculated phase shifts including the one pion exchange potential with quark exchange in addition to the one gluon plusσ meson exchange are shown to be consistent with experiments. Especially thep-wave phase shifts are improved by taking into account the quark exchange on the one pion exchange potential.     

铁磁/反铁磁/铁磁三层膜系统中反铁磁自旋结构及其交换各向异性

研究铁磁/反铁磁/铁磁三层膜中界面存在二次以及双二次交换耦合下反铁磁磁矩转动及其交换各向异性.结果表明,其反铁磁膜中的磁矩转动存在可逆"恢复行为"、不可逆"连续倒转行为"以及不可逆"中断倒转行为"三种情形,三种情形的出现强烈地依赖于两界面处的线性耦合和双二次耦合.钉扎界面的交换耦合与旋转界面的交换耦合相互竞争,当钉扎界面耦合占主导时,反铁磁磁矩发生可逆"恢复行为",系统出现交换偏置.在旋转界面耦合占主导情形下,其线性耦合与双二次耦合也相互竞争,分别导致反铁磁磁矩发生不可逆"连续倒转行为"和不可逆"中断倒转行为",系统都不出现交换偏置,但矫顽场都得以增强.相关结论为实验上观测的磁滞能耗以及界面垂直耦合提供了可能的解释.     

Slow-down of 13C spin diffusion in organic solids by fast MAS: a CODEX NMR Study.

One- and two-dimensional 13C exchange nuclear magnetic resonance experiments under magic-angle spinning (MAS) can provide detailed information on slow segmental reorientations and chemical exchange in organic solids, including polymers and proteins. However, observations of dynamics on the time scale of seconds or longer are hampered by the competing process of dipolar 13C spin exchange (spin diffusion). In this Communication, we show that fast MAS can significantly slow down the dipolar spin exchange effect for unprotonated carbon sites. The exchange is measured quantitatively using the centerband-only detection of exchange technique, which enables the detection of exchange at any spinning speed, even in the absence of changes of isotropic chemical shifts. For chemically equivalent unprotonated 13C sites, the dipolar spin exchange rate is found to decrease slightly less than proportionally with the sample-rotation frequency, between 8 and 28 kHz. In the same range, the dipolar spin exchange rate for a glassy polymer with an inhomogeneously broadened MAS line decreases by a factor of 10. For methylene groups, no or only a minor slow-down of the exchange rate is found.     

磷脂跨膜交换机制的研究

膜间磷脂交换是一项重要的生理活动, 其对药物运输及膜功能研究有重要意义. 本文用石英晶体微天平及耗散系数测试仪研究囊泡与囊泡、囊泡与支撑膜间磷脂交换行为, 荧光光谱仪用来测量膜表面电性与膜组分对磷脂交换的影响. 实验结果表明: 磷脂跨膜交换速率与交换时间成反比, 膜表面异电性磷脂的增加会加速膜内相互作用和磷脂跨膜交换速率, 以及改变膜表面组分会对囊泡与支撑膜间的磷脂交换产生影响. 本文研究有助于加深理解磷脂跨膜交换机制, 并对药学研究提供参考.     

The influence of magnetoelastic coupling of exchange spin waves on the spectrum of magnetoacoustic vibrations in planar structures

The influence of coupling between exchange spin waves and acoustic waves on the spectrum of magnetoelastic vibrations in planar structures (such as a ferrite film-dielectric substrate structure) is investigated theoretically. A strong magnetoelastic coupling is observed in a narrow spectrum of magnetoacoustic modes that corresponds to the phase matching of the exchange magnetostatic and acoustic modes. An explanation is offered for the experimental results obtained earlier by the authors, according to which the linear excitation of exchange acoustic and dipole exchange acoustic modes occurs in a spectral range corresponding to the resonance magnetoelastic coupling of exchange modes irrespective of the degree of pinning of surface spins in the ferrite film. It is demonstrated that the exchange acoustic and dipole exchange acoustic modes can be excited in films with free surface spins due to a substantial transformation of the structure of normal modes of the magnetization vector and elastic displacements in the range of the phase matching of the exchange spin and acoustic modes.     

Breakdown of the Siegert theorem and the many-body charge density operators

The exchange charge density operator is studied in the two-boson exchange model with consistent treatment of the exchange current and nuclear wave functions. We find a non-vanishing exchange charge density operator even in the static limit, which leads to the breakdown of the Siegert theorem.     

Symmetry analysis in neutron diffraction studies of magnetic structures: 4. Theoretical group analysis of exchange Hamiltonian

By analyzing the exchange Hamiltonian, we develop the technique of determining the magnetic structures liable to occur in a crystal. The technique rests on Bertaut's idea that the exchange matrix eigenfunction corresponds to some magnetic structure. A technically simple and efficient method of diagonalizing the exchange matrix is worked out using the devices of space group representation theory. A method is presented to find the magnetic structures with equal exchange energy (exchange multiplets). The occurrence of exchange multiplets results from the additional invariance of the exchange Hamiltonian under rotation of all the spins. The degeneracy within the exchange multiplet may be the reason why some magnetic structures arise not according to one irreducible representation of the space group. The theory is illustrated with reference to an example of the magnetic structure of spinels.     

Exchange field induced magnetoresistance in colossal magnetoresistance manganites.

The effect of an exchange field on the electrical transport in thin films of metallic ferromagnetic manganites has been investigated. The exchange field was induced both by direct exchange coupling in a ferromagnet/antiferromagnet multilayer and by indirect exchange interaction in a ferromagnet/paramagnet metallic superlattice. The electrical resistance of the metallic manganite layers was found to be determined by the magnitude of the vector sum of the effective exchange field and the external magnetic field.     

EPR spectra of the double triangular polyoxovanadates with exchange and electron transfer effects: Symmetry breaking and effective motional averaging

The interplay of isotropic exchange, antisymmetric (AS) exchange, and electron delocalization has been theoretically investigated for a system comprising two triangles, one with three localized d¹ spins, which gives rise to spin frustration effects, and the other with one d¹ center delocalized on the three sites. In the localized limit the AS exchange is partially reduced by the low symmetry components of an isotropic exchange (inter-triangle interaction) and splits the spin triplets only in the second order, contrary to what is found for a single triangular system where splitting of the spin doublet levels occurs in the first order. Zero-field splitting parameters depend on both AS exchange parameters and low symmetry components of the isotropic exchange. The EPR spectrum consists of intratriplet (Heisenberg type) and intertriplet (non-Heisenberg type) lines. AS exchange also induces singlet-triplet transitions. The EPR spectra of powder samples are generated for a domain of key parameters. Two different situations occur depending on the symmetry of the electronic states when the electron delocalization is taken into account. The orbital doublets of the localized system indicate a complete averaging of the exchange interactions, which results in effective enhancement of AS exchange. Unlike triplets, the orbital singlets are unaffected by a transfer process, developing only second-order AS exchange. Coexistence of localized and delocalized states is expected to be an intrinsic feature of exchange systems with partial delocalization. The relevance of these effects to the magnetic properties of exchange clusters is briefly discussed.     

Exchange bias for ferromagnetic/antiferromagnetic bilayers with the uniaxial anisotropy being misaligned with the exchange anisotropy

Using the principle of minimal energy and S-W model, the exchange bias for ferromagnetic/antiferromagnetic bilayers has been investigated when the uniaxial anisotropy is misaligned with the exchange anisotropy. According to the relation between the energy of the bilayer and the orientation of ferromagnetic magnetization, it is found that the bilayer will be in the monostable state or bistable state when the external field is absent in the initial magnetization state. The monostable state or bistable state of the bilayer, which determines the angular dependence of exchange bias directly, is controlled by the competition between the exchange anisotropy and uniaxial anisotropy. When the applied field is parallel to the intrinsic easy axes and intrinsic hard axes, one of the switching fields of the hysteresis loop shows an abrupt change, while the other keep continuous by analyzing the magnetization reversal processes. Consequently, the exchange bias field and the coercivity will show a jump phenomenon. The numerical calculations indicate that both the magnitude and direction of the exchange anisotropy will significantly affect the angular dependence of exchange bias. The jump phenomenon of exchange bias is an intrinsic property of the bilayer, which is dependent on the interfacial exchange-coupling constant, the orientation of the exchange anisotropy, the thickness and uniaxial anisotropy constant of the ferromagnetic layer.     

Exchange currents and nucleon magnetic moments

We investigate the effect of gluon and pion exchange currents on the magnetic moments of the nucleon using the constituent quark model. We also study the effect of scalar exchange currents connected with the confinement and sigma exchange potentials. We conclude that although individual exchange currents are quite large, the total exchange current contribution is relatively small, due to a cancellation of gluon, pion, and scalar exchange currents.Lecture presented at the Indian-Summer School on Interaction in Hadronic Systems, Praha (The Czech Republic), 25–31 August 1993.I would like to thank my colleagues Dr. E. Hernandez and Prof. K. Yazaki, for their collaboration in this field.     

人体胰岛素质子交换反应的核磁共振研究

用线型拟合的方法通过核磁共振谱图测得了人体胰岛素R6六聚体中酰胺质子与重氢的交换速度.因为用这一方法对实验谱图进行计算模拟时考虑了核磁共振谱线所提供的所有信息,所以我们获得了化学位移非常接近的蛋白质酰胺质子与重氢的交换速度,这是用其它方法难以得到的数据.结果表明,用本文所述方法获得的酰胺质子的交换速度与由2DNMR计算所得的蛋白质结构有非常好的吻合.在获得酰胺质子交换速度的基础上,本文详细讨论了交换反应速度与蛋白质结构的关系,结果表明,在该蛋白质中影响酰胺质子交换速度大小的因素以强弱顺序排列为:质子在肽链中的位置 > 是否参与形成氢键 > 蛋白质的二级及三级结构.     

The direct exchange interaction for weakly delocalized multielectron systems in crystals

The multielectron theory of exchange interactions taking into account both electrostatic interactions between electrons (“potential” exchange) and electron transfer between magnetic centers (“kinetic” exchange) has been developed on the basis of Bogolubov's procedure using double irreducible operators. The final hamiltonian contains a number of isotropic and anisotropic terms. The exchange interaction parameter variations with the distance have been calculated. The terms determining the dependence on the occupancy of magnetic shells are shown to be of the same order of magnitude as the “potential” exchange and two orders of magnitude less than the “kinetic” exchange. It is shown that in the model adopted the mechanisms of direct exchange interactions always favour the antiferromagnetic spin ordering     

Practical failures from the inclusion of exact exchange: how much exact exchange is appropriate?

The influence of exact exchange incorporated into exchange—correlation functionals on the predictions of relative energies, structures, electronic states, and vibrational spectra is examined numerically. Failures of widely used hybrid exchange—correlation functionals due to either the physical unacceptability of including exact exchange or an unbalanced mixing of exact exchange are considered. One set of examples involves tetraatomic chalcogen clusters and charge transfer complexes between diatomic chalcogens and diatomic oxygen. Poor energetic predictions from Hartree-Fock rule against the inclusion of exact exchange into the exchange—correlation functionals for these systems with significant left—right electron correlation effects. The energies of the conformers of [10]annulene are considered from an unusual viewpoint, namely, the empirical adjustment of the admixture of exact exchange to match the predictions of very high level theoretical methods. For this annulene with insignificant left-right electron correlation effects, a greater (50%) percentage of exact exchange should be included. The relationship of symmetry breaking to the inclusion of exact exchange is examined for seven linear radicals, OXO (X = B, Al, Ga, In, TI). AIOS, and OAIS. exchange—correlation functionals generate symmetry adapted solutions at the expense of an unusual ordering of the Kohn-Sham orbitals, which can cause uncharacteristic electronic states, incorrect vibrational spectra, and poorer predictions of energetics. These effects are greater when exact exchange is included. In all the examples considered, the appropriate focus for a detailed discussion of molecular properties involves consideration of the effects of exact exchange.     

铁磁/反铁磁双层膜中交换偏置及矫顽场的温度特性研究

讨论了铁磁-反铁磁双层膜中交换偏置和矫顽场随温度变化的关系。在本模型中,温度的依赖性来源于系统态的热激发以及相关磁学参量的温度依赖性。数值结果显示:低温下,交换偏置和矫顽场随温度的升高而减少,但是随着界面的交换耦合的增强或铁磁层各向异性的减少,其交换偏置变得平坦。随着温度的升高,交换偏置减少直至零;而矫顽场却达到峰值后再减为零。这些结果与实验结果定性一致。根据数值计算结果,可以预见软的铁磁层耦合上硬的反铁磁层,在恰当的交换耦合强度下,可构建具有大的交换偏置、小矫顽场;并在某温度区几乎不随温度变化的磁存贮器件.     

Wealth distributions in asset exchange models

A model for the evolution of the wealth distribution in an economically interacting population is introduced, in which a specified amount of assets are exchanged between two individuals when they interact. The resulting wealth distributions are determined for a variety of exchange rules. For “random” exchange, either individual is equally likely to gain in a trade, while “greedy” exchange, the richer individual gains. When the amount of asset traded is fixed, random exchange leads to a Gaussian wealth distribution, while greedy exchange gives a Fermi-like scaled wealth distribution in the long-time limit. Multiplicative processes are also investigated, where the amount of asset exchanged is a finite fraction of the wealth of one of the traders. For random multiplicative exchange, a steady state occurs, while in greedy multiplicative exchange a continuously evolving power law wealth distribution arises. Received: 13 August 1997 / Revised: 31 December 1997 / Accepted: 26 January 1998     

Influence of an interface layer on the effective coupling at a ferromagnet/antiferromagnet interface

We examine the exchange anisotropy induced at a ferromagnetic/antiferromagnetic interface when an antiferromagnetic interface layer exists. We show that competition between exchange couplings in the interface layer can result in a ferrimagnetic-like compensation point. This leads to a reversal of the effective field acting on the ferromagnet, and a consequent sign change of the exchange bias for temperatures near the Néel temperature of the antiferromagnet. A surprising result is the sensitive dependence of the compensation point on exchange interactions. Even minute modifications of the exchange interactions near the interface can result in a reversal of the effective field, provided certain conditions are met.     

On the applicability of the average exchange model for electron spin polarization phenomena

The applicability of the average exchange model as a tool for describing electron spin polarization in micelles is investigated by comparison with analytical and numerical results. The analytical results are used for the interpretation of the various types of dependence, while the numerical results are used for quantitative tests. It is demonstrated that the dephasing effect of the exchange interaction has a significant influence both on the time dependence and on the stationary value of the polarization. Dephasing arising from the exchange interaction is not included in the average exchange model, and therefore the spin dynamics are not described correctly. The average exchange model can only be used to estimate the frequency and the stationary value of the polarization, and only in the case of very slow singlet-triplet mixing, weak exchange interaction, and low reactivity of the radicals.     

THE CONTRIBUTION OF THE NUCLEON EXCHANGE TO THE TRANSFER OF IN DEEP ANGULAR MOMENTUM AND ENERGY INELASTIC COLLISION

The nucleon exchange can be divided into two different modes, the equal one and unequal one. Assuming that the motion of nucleon is random and according to Einstein relation the number of equal and unequal exchange is discussed. The total energy loss and the angular momentum for the reaction Kr+Kr have been calculated. The results. show that the total energy loss is only partly induced by nucleon exchange, while the. angular momentum dissipation is mainly induced by nucleon exchange and the equal exchange is more important than unequal one in both cases.