The magnetic properties across the martensitic transition in the Co38Ni34Al28 alloy

The magnetic properties of the Co₃₈Ni₃₄Al₂₈ alloy have been studied. The alloy exhibits a first order austenite-martensite phase transition in the temperature region between 155 and 247 K. A strain of 0.07% is produced across this phase transition. The Arrott plots obtained from the isothermal magnetic field dependence of magnetization indicate the presence of spontaneous magnetization both in the austenite and martensite phases, confirming the ferromagnetic character of the alloy up to room temperature. The temperature dependence of the high field magnetization indicates the presence of spin wave excitations, spin wave excitation gap and spin wave-spin wave interactions in the martensite phase. The magnetic anisotropy energy constant for the Co₃₈Ni₃₄Al₂₈ alloy is estimated both with the help of the standard law of approach to saturation of magnetization, and also from the field dependence of magnetization using the field for technical saturation of magnetization. The temperature dependences of these energy terms are compared. The estimated values of the magnetic anisotropy constant seem to be in agreement with the magnitude of the spin wave excitation gap estimated from the temperature dependence of high field magnetization.     

Magnetic exchange driven magnetoelastic properties in GdCu2

The magnetoelastic properties of GdCu₂ have been investigated by thermal expansion and magnetostriction measurements. GdCu₂ orders antiferromagnetically with a noncollinear magnetic structure. The anisotropic magnetostriction is of similar magnitude as in other RCu₂ compounds and can be explained by a contribution of the bilinear exchange interaction to the magnetoelastic energy. For several compounds this contribution is as important as the single ion magnetoelastic exchange. The pressure dependence of the Néel temperature of GdCu₂ is found to be in agreement with the data of thermal expansion.     

Inhomogeneous magnetostriction states in uniaxial ferromagnetic films

Surface magnetoelastic Love waves and nonuniform distributions of the magnetization and elastic strains are investigated in a uniaxial ferromagnetic film on a massive nonmagnetic substrate in a tangential external magnetic field. A new inhomogeneous phase is predicted having spatial modulation of the order parameter, arising from magnetostrictive coupling of the magnetization with lattice strains near the interface of the magnetoelastic and elastic media. It is shown that, at some critical magnetic field H _c, different from the orientational transition field in an isolated sample, a magnetoelastic Love wave propagating parallel to the magnetization vector in the film plane becomes unstable. The frequency and group velocity of the wave vanish at wave number k=k _c≠0 and the wave freezes, forming a domain structure localized in the film and adjoining substrate. Fiz. Tverd. Tela (St. Petersburg) 41, 665–671 (April 1999)     

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.     

Interchain impurity substitution effects on the magnetoelastic coupling in CuGeO3

We report the polarized far-infrared transmittance of Si-doped CuGeO₃ single crystals as a function of impurity concentration and applied magnetic field at low temperature. We use the behavior of the 44 cm⁻¹ spin gap excitation and the 98 cm⁻¹ zone-folding mode structure to investigate the interaction between the magnetic system and the lattice distortion. We find that interchain impurity substitution collapses the spin gap before it suppresses the lattice dimerization, a result that is understood in terms of the relative length scales for lattice distortion and spin singlet formation as well as criteria for magnetoelastic coupling in chains.     

On the long-range magnetic order and the preferred spin orientation of the layered magnetic oxides Sr2MnSi2O7 and Ba2MnGe2O7

The causes for the three-dimensional magnetic ordering and the observed spin orientation of the layered oxides Sr₂MnSi₂O₇ and Ba₂MnGe₂O₇ were investigated by evaluating the spin exchange interactions and the preferred spin orientation on the basis of density functional calculations and by calculating the magnetic dipole–dipole interaction energies.     

自旋为1/2的XY模型亚铁磁棱型链的物性和有序-无序竞争

利用不变本征算符法, 计算低温下自旋为1/2的XY模型一维亚铁磁棱型链系统的元激发谱, 讨论在此系统中不同的特殊情形下的元激发能量, 从而给出体系的三个临界磁场强度的解析解H_C1, H_C2, H_peak. 分析不同外磁场下 体系的磁化强度随温度的变化规律, 发现三个临界磁场强度的解析解H_C1, H_C2, H_peak是正确的, 并从三个元激发对磁化强度的贡献进行了说明. 低温下磁化强度随外磁场的变化呈现1/3磁化平台. 体系的磁化率随温度或者外磁场的变化都出现了双峰现象. 这说明双峰源于二聚体分子内电子自旋平行排列的铁磁交换作 用能和二聚体与单基体分子间电子自旋反平行排列的反铁磁交换作用能, 热无序能, 外磁场强度相关的自旋磁矩势能之间的竞争.     

阻挫诱导的亚铁磁性Heisenberg系统中的量子相变

利用密度矩阵重整化群(DMRG)方法研究磁性阻挫对一种S=1/2准一维反铁磁自旋链但却具有亚铁磁性的Heisenberg系统基态的影响.计算了单个晶胞的基态能、自旋关联函数以及自旋能隙.研究表明这种Heisenberg自旋系统的基态随着阻挫α的增强将从磁有序相变化到自旋无序相,并且伴随着自旋能隙的出现,量子相变点为α≈0.412.同时线形链上格点间自旋长程关联值的计算结果表明在磁有序区间体系的磁有序性质随着α的增强而减弱,阻挫在0≤α< 关键词： 准一维反铁磁自旋链 亚铁磁性 密度矩阵重整化群 自旋能隙     

圆柱状铁磁体中的偶极-交换自旋波

对于轴向磁化的圆柱状铁磁体,同时考虑偶极作用和交换作用,得到了偶极-交换自旋波的本征方程,给出了频谱的数值结果。当自旋波波矢β较小,即偶极能起主要作用时,相当于静磁模的结果,β较大,即交换能起主要作用时,过渡到宏观自旋波交换模理论的结果。     

Investigation of magnetoelastic waves in dependence on the parameters of ferromagnetic and antiferromagnetic interactions

The Bogoliubov method of canonical transformations is used to study the magnetoelastic waves in long-period modulated structures. The dispersion equation for a magnetoelastic wave and the parameters of magnetoelastic interaction for variable directions of propagation of a coupled wave are found. The effect of I ₁/I ratio on the value of a dimensionless constant of magnetoelastic interaction is examined.     

Incommensurate phases in ferromagnetic spin-chains with weak antiferromagnetic interchain interaction

We study planar ferromagnetic spin-chain systems with weak antiferromagnetic inter-chain interaction and dipole-dipole interaction. The ground state depends sensitively on the relative strengths of antiferromagnetic exchange and dipole energies κ = J′a ² c/(g _L μ _B )². For increasing values of κ, the ground state changes from a ferromagnetic via a collinear antiferromagnetic and an incommensurate phase to a 120° structure for very large antiferromagnetic energy. Investigation of the magnetic phase diagram of the collinear phase, as realized in CsNiF₃, shows that the structure of the spin order depends sensitivly on the direction of the magnetic field in the hexagonal plane. For certain angular domains of the field incommensurate phases appear which are seperated by commensurate phases. When rotating the field, the wave vector characterizing the structure changes continously in the incommensurate phase, whereas in the commensurate phase the wave vector is locked to a fixed value describing a two-sublattice structure. This is a result of the competition between the exchange and the dipole-dipole interaction.     

Electronic structure of paramagnetic In<Subscript>1-x</Subscript>Mn<Subscript>x</Subscript> As nanowires

The electronic structure, spin splitting energies, and g factors of paramagnetic In_1-xMn_xAs nanowires under magnetic and electric fields are investigated theoretically including the sp-d exchange interaction between the carriers and the magnetic ion. We find that the effective g factor changes dramatically with the magnetic field. The spin splitting due to the sp-d exchange interaction counteracts the Zeeman spin splitting. The effective g factor can be tuned to zero by the external magnetic field. There is also spin splitting under an electric field due to the Rashba spin-orbit coupling which is a relativistic effect. The spin-degenerated bands split at nonzero k_z (k_z is the wave vector in the wire direction), and the spin-splitting bands cross at k_z = 0, whose k_z-positive part and negative part are symmetrical. A proper magnetic field makes the k_z-positive part and negative part of the bands asymmetrical, and the bands cross at nonzero k_z. In the absence of magnetic field, the electron Rashba coefficient increases almost linearly with the electric field, while the hole Rashba coefficient increases at first and then decreases as the electric field increases. The hole Rashba coefficient can be tuned to zero by the electric field.     

Dihyperons in chiral color dielectric model

The mass of the dibaryon having spin, parityJ ^π = 0⁺, isospinI = 0 and strangeness—2 is computed using chiral color dielectric model. The bare wave function is constructed as a product of two color-singlet three-quark clusters and then it is properly antisymmetrized by considering appropriate exchange operators for spin, flavor and color. Color magnetic energy due to gluon exchange, meson self energy and energy correction due to center of mass motion are computed. The calculation shows that the mass of the particle is 80 to 160 MeV less than twice λ mass.     

Spin waves and exchange interactions in the antiferromagnetic garnets with Fe3+ in the octahedral sites

The results of an inelastic neutron scattering study of the spin wave spectrum for the garnet Fe₂Ca₃Si₃O₁₂(FeSiG) are presented. We compare the exchange parameters for this garnet and for the Ge-species (Fe₂Ca₃Ge₃O₁₂(feGeG)) having the same magnetic structure. We relate the differences found with structural information from powder neutron diffraction. In this way the super exchange paths viap orbitals of intermediate oxygen atoms can be identified. We discuss the effect of a small number (3.2(5)%) of Mn²⁺ impurities in the 24c sites. These lead to an effective ferromagnetic exchange between the Fe³⁺ ions and drastically renormalize the average exchange constants. An estimate for the Fe³⁺–Mn²⁺ indirect exchange between a and c sites of 6(1) K is obtained. The exchange parameters for the pure FeSiG are found to beJ ₁=1.16(4) K,J ₁=0.96(4K andJ ₂=–1.24(4) K for nearest and next nearest neighbours, respectively. These values apply for a moment of 4.02(4) _B per iron atom as obtained from a structure refinement of powder diffraction data. Finally we present results for FeSiG of a high resolution study of the excitations at the zone centre in an attempt to verify our earlier findings of a quantum spin wave gap for FeGeG. In contrast to the earlier measurements, we could follow the acoustical branch to much lower energies using a timeof-flight spectrometer. We found indications for a crossing of the two low lying spin wave branches, the acoustical one extrapolating to the anisotropy gap of 0.005 THz and the antiphase branch extrapolating to the quantum gap of 0.02 THz.     

Dynamical interaction of antiferromagnetic subsystems: a neutron scattering study of the spinwave spectrum of the garnet Fe2Ca3(GeO4)3

The Fe³⁺ ions in the garnet Ca₃Fe₂Ge₃O₁₂ form two identical antiferromagnetic subsystems. The interaction between the two subsystems is vanishing within molecular field approximation forq=0. A coupling appears due to the spin fluctuations. The dynamics of the system is described by the Hamiltonian for a Heisenberg antiferromagnet. Symmetry requirements impose two exchange parameters between the sublattices (nearest neighbours)J ₁ in the direction of the 3-fold axis andJ' ₁ in the other three space diagonals. The interaction within each sublattice (second nearest neighbours) is described by the exchange parameterJ ₂. The measured spin wave dispersion curves for the three principal symmetry directions are very well reproduced by a model calculation withJ ₁=-0.909(9) K,J' ₁=-0.307(8) K andJ ₂=-0.615(2)K. The observed intensities are in agreement with predictions from the model. Forq0 the model predicts two acoustic branches going towards zero frequency. A calculation beyond linear spin wave theory forq=0 predicts a quantum gap for the lower acoustic branch. This gap has been found at 0.033(4) THz. An anisotropy gap of 0.007 THz has been taken from the literature.     

Local investigation of the energy gap within the incompressible strip in the quantum Hall regime

We experimentally study the energy gap within the incompressible strip at local filling factor ν _c = 1 at the quantum Hall edge for samples of very different mobilities. The obtained results indicate strong enhancement of the energy gap in comparison to the single-particle Zeeman splitting. We identify the measured gap as a mobility gap, so a pronounced experimental in-plane magnetic field dependence can both be attributed to the spin effects as well as to the change in the energy levels broadening.     

Der Einfluß der Dipolwechselwirkung auf die Magnetisierung dünner ferromagnetischer Schichten

The magnetization of thin films is calculated for low temperatures, taking into account the exchange interaction, an external magnetic field, and the dipole interaction. The calculations are performed within a quantized phenomenological spin wave theory. For thin enough films, within the temperature range considered, only the lowest spin wave band contributes to the decrease of the magnetization. The influence of the dipole interaction is as follows: The magnetization decreases less rapidly with growing temperature than predicted by calculations within the Heisenberg model; the decrease depends considerably on the angle between the magnetization and the film plane; even atT=0K there is a small increase of the magnetization with growing external field.     

Rare earth paramagnetism,as studied by μSR

Some recent result of muon spin relaxation measurements in rare earth metals and intermetallic compounds are reviewed. Special emphasis is put on measurements that relate to the properties of correlated regions of spins existing relatively far above the ordering temperature in the rare earth ions. As far as comparable data from paramagnetic neutron scattering exist, they will be discussed in the same framework. For each temperature the correlated regions (or short-lived magnetic clusters) are characterized by their size, possible anisotropy with respect to the crystalline axes and their lifetime. The actual form of the interaction between the rare earth spins themselves and with the crystal fields determine the temperature dependence of these properties; a strong dipole interaction can, for instance, be expected to change the critical behaviour nearT _c . Much of the time will be devoted to experiments on Gd-metal where there are experimental indications that several interesting phenomena occur: (1) a strong effect of a cross-over from a non-conserved dynamics (dipolar) regime to a conserved (exchange dominated) regime some 10 K aboveT _c , (2) an anisotropy of the magnetic clusters with respect to the hexagonalc-axis, and (3), a persistence of spin correlations far aboveT _c . Some attempts to correlate the rare earth spin relaxation times measured in this region with cluster lifetimes deduced from neutron scattering will be reviewed, as well as a model for understanding these lifetimes in terms of temperature dependent cluster wall motion, which is determined by exchange and magnetic anisotropy parameters. Effects of possible quantum correlations originating from the “spin system+bath” interaction will be mentioned.     

Dynamic stress sensitivity of Fe73.5Cu1(Ta or W)3Si15.5B7 ribbons after heat treatments in vacuum

Summary The effects of heat treatments in vacuum on magnetic permeability, Young's modulus and magnetoelastic wave amplitude were investigated in the amorphous ferromagnetic alloys Fe_73.5Cu₁Ta₃Si_15.5B₇ and Fe_73.5Cu₁W₃Si_15.5B₇ where tantalum or tungsten replaces niobium, which was generally used with the same atomic fractions of the other elements. The stress sensitivity coefficient was also theoretically deduced in its developmentvs. the applied magnetic bias field. Structural relaxation and crystallization consequences are evidenced. Moreover an already predicted relation between the maximum of magnetoelastic wave amplitude and the minimum of stress sensitivity coefficient was confirmed.