KKR-CPA-LDA计算方法对CuMnAl和CoMnAl以及CuCoMnAl合金的磁性研究

采用KKR-CPA-LDA方法研究了CuMnAl,CoMnAl和CuCoMnAl四元合金中磁性原子磁矩和Co-Mn间的交换作用.通过与实验结果对比,揭示了Mn的磁矩和Co的磁矩以及它们的相互作用随成分变化的规律.研究发现,在Cu₅₀Mn_25+xAl25-x合金中超过化学配比并占据Al位的Mn原子是反铁磁的,而且由于近邻环境的不同,其磁矩大于原有Mn原子的磁矩.在Co₅₀Mn_{25+< 关键词： KKR-CPA-LDA计算 Co-Mn间交换作用}     

<Superscript>27</Superscript>Al NMR Study of the Structure and Dynamics of Natrolite

The temperature dependences of nuclear magnetic resonance and magic angle spinning nuclear magnetic resonance spectra of ²⁷Al nuclei in natrolite (Na₂Al₂Si₃O₁₀· 2H₂O) have been studied. The influence of water molecules and sodium ions mobility on the shape of the ²⁷Al NMR spectrum and framework dynamics have been discussed The temperature dependences of the spin–lattice relaxation times T₁ of ²⁷Al nuclei in natrolite have also been studied. It has been shown that the spin–lattice relaxation of the ²⁷Al is governed by the electric quadrupole interaction with the crystal electric field gradients modulated by translational motion of H₂O molecules in the natrolite pores. The dipolar interactions with paramagnetic impurities become significant as a relaxation mechanism of the ²⁷Al nuclei only at low temperatures (<270 K).     

Non-spherical magnetic moment in MnAlGe

The magnetic structure factors of MnAlGe (space groupP4/nmm) measured with polarised neutrons have been expressed in terms of the magnetic moment of the Mn atom (site symmetry tetrahedral with tetragonal distortion), the Bessel transforms 〈j _n〉 of the Mn radial functions and the fractional occupancies of the moment density in the various crystal field orbitals. The measured structure factors were least-squares fitted with the theoretical expression involving 〈j _n〉 appropriate to the Mn⁰, Mn⁺ and Mn²⁺ atoms. The best fit was got using Mn⁰ transforms, yielding 1·45µ _B as the Mn magnetic moment. The fractional occupancies of the moment density in the crystal field orbitalsA _1g,B _1g E _g andB _2g were obtained. This analysis shows the magnetic moment to be highly non-spherical with a large fractional occupancy (38%) in theA _1g orbital directed along the tetragonal axis while the fractional occupancies ofB _1g andB _2g are found to be 31% and 30% respectively. The fractional occupancy of the moment in theE _g orbital directed towards the Ge and Al atoms is very low (1%). The spatially averaged moment density of Mn in MnAlGe is more diffuse than that of Mn I and Mn II in isostructural Mn₂Sb.     

Symmetry-dependent Mn-magnetism in Al<Subscript>69.8</Subscript>Pd<Subscript>12.1</Subscript>Mn<Subscript>18.1</Subscript>

We investigated the stability of magnetic moments in Al_69.8Pd_12.1Mn_18.1. This alloy exists in both, the icosahedral (i) and the decagonal (d) quasicrystalline form. The transition from the i- to the d-phase is achieved by a simple heat treatment. We present the results of measurements of the ²⁷Al NMR-response, the dc magnetic susceptibility, and the low-temperature specific heat of both phases. In the icosahedral compound, the majority of the Mn ions carries a magnetic moment. Their number is reduced by approximately a factor of two by transforming the alloy to its decagonal variety. For both compounds, we have indications for two different local environments of the Al nuclei. The first reflects a low density of states of conduction electrons and a weak coupling of the Al nuclei to the Mn-moments. The second type of environment implies a large d-electron density of states at the Fermi level and a strong coupling to the magnetic Mn moments. Spin-glass freezing transitions are observed at T^deca_f=12 K for the decagonal, and T^ico_f=19 K for the icosahedral phase.     

Spin fluctuation and local magnetism of isolated Fe impurities in Pd<Subscript>1−x</Subscript>V<Subscript>x</Subscript> alloys studied by time differential perturbed angular distribution spectroscopy

The magnetic moment and spin fluctuation temperature of isolated Fe impurity atoms in Pd_1?xV_x (0 ≤ x ≤ 0.15) alloys have been studied by time differential perturbed angular distribution (TDPAD) technique. With increasing V content in Pd matrix, a large non-linear reduction of the local magnetic moment accompanied with an exponential increase of the spin fluctuation temperature T_SF has been observed. At and beyond x = 0.12, the Fe atoms are found to be nonmagnetic. As an important new feature, T_SF is observed to vary quadratically with composition dependent changes in host spin polarization.     

Spin-polarized first-principles study of ferromagnetism in zinc-blende In<Subscript>1−x</Subscript>Mn<Subscript>x</Subscript>Sb

First-principles calculations based on the density functional theory are performed to study the structural properties, spin-polarized electronic band structures, density of states and magnetic properties of the zinc blende In_{1− x} Mn _x Sb (x = 0.125, 0.25, 0.50, 0.75, 1.0). The calculated lattice constants of In_{1− x} Mn _x Sb obey the Vegard’s law with a marginal upward bowing. With the increase of Mn concentration in In_{1− x} Mn _x Sb, a transition from the semi-metallic to the half-metallic behavior happens such that the majority-spin valence states crosses the Fermi level and the minority-spin states have a gap at the Fermi level. A large exchange splitting (∼ 4 eV) is observed between Mn 3d states of the majority-spins and the minority-spins. The total magnetic moment of In_{1− x} Mn _x Sb half-metallic ferromagnets per Mn atom basis is 4μ _B. The total magnetic moment per Mn atom indicate that Mn atoms act as acceptors in InSb and contribute to holes in the lattice of InSb. Due to p-d hybridization, the free space charge of Mn reduces that results a loss in its magnetic moment. The loss in the magnetic moment of the Mn atoms is converted into a small local magnetic moments on the In and Sb sites.     

Spin frustration in Y(Sc)Mn2

The relaxation rate of muon polarization of Y_0.97Sc_0.03Mn₂ with a magnetic lattice composed of corner‐shared tetrahedrons shows a broad peak around 2.5 K indicating a random spin freezing at low temperatures. The substitution by nonmagnetic Al atoms for 10% Mn atoms raises the spin freezing temperature to 45 K. The present work shows clearly that due to the strong geometrical frustration, the ground state of the system is singlet tetrahedrons mixed with frozen paramagnetic tetrahedrons. This revised version was published online in July 2006 with corrections to the Cover Date.     

27Al Magic Angle Spinning Nuclear Magnetic Resonance Study of Al1−x Cr x K(SO4)2·12H2O (x = 0, 0.07, and 0.2)

The physical properties of Al_1?x Cr _x K(SO₄)₂·12H₂O (x = 0, 0.07, and 0.2) were studied as a function of temperature using magic angle spinning nuclear magnetic resonance for ²⁷Al. On the basis of the physical properties of pure AlK(SO₄)₂·12H₂O, the effects of partially replacing Al³⁺ with Cr³⁺ ions were examined. Molecular motion changed with the concentration of Cr³⁺ ions. The relaxation process near 320 K was found to undergo molecular motion as described by the Bloembergen–Purcell–Pound theory. The activation energies, phase transition temperatures, and spin–lattice relaxation times in the rotating frame T _1ρ changed with the concentration of paramagnetic ions.     

Mn site substitution of La0.67Ca0.33MnO3 with closed shell ions: Effect on magnetic transition temperature

Mn site is substituted with closed shell ions (Al, Ga, Ti, Zr and a certain combination of Zr and Al) and also with Fe and Ru ions carrying the magnetic moment (S=5/2 and 2 respectively) at a fixed concentration of 5 at %. Substitution did not change either the crystal symmetry or the oxygen stoichiometry. All substituents were found to suppress both the metal-insulator and ferromagnetic transition temperatures (T _p(ρ) and T _C, respectively) to varied extents. Two main contributions identified for the suppression are the lattice disorder arising due to difference in the ionic radii between the substituent (r _M) and the Mn³⁺ ion (r _Mn ³⁺) and in the case of the substituents carrying a magnetic moment, the type of magnetic coupling between the substituent and that of the neighboring Mn ion.     

Magnetic properties and peculiarity of magnetic states in dilute antiferromagnets Mn1−x Zn x F2

The dependence of magnetic moment and susceptibility on temperature, magnetic field and frequency of some single crystals Mn_1?x Zn _x F₂ (x≈x _e=0.75—percolation limit) were experimentally investigated. Our experiments show that (Bazhan and Petrov 1984; Cowleyet al 1984; Villain 1984) in these crystals the nonequilibrium magnetic state of spinglass type with finite correlation length appears as temperature decreasesT<T in weak magnetic fields. This state is determined by fluctuation magnetic moments √nμ (wheren is the number of magnetic ions, corresponding to finite correlation length andμ the magnetic moment Mn⁺¹). In the experiments in low magnetic fields and frequencies there are no peculiarities in the magnetic susceptibility temperature dependence atT≠T _f. At temperaturesT>T _f andT<T _f magnetic susceptibility is determined by 1 $$\chi \left( {T > T_f } \right) = \frac{{N\left\langle \mu \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \frac{N}{n}\frac{{\left\langle {\sqrt n \mu } \right\rangle ^2 }}{{3k\left( {T + \theta } \right)}} = \chi \left( {T< T_f } \right)$$ . In strong magnetic fields and large frequencies there are peculiarities in thex(T) dependence atT=T _f. AtT<T _f and strong magnetic fieldsX(T)=x ₀ andT<T _f and at large frequenciesx(T)=x ₀+α/T. The dependences of magnetic susceptibility on the frequency are determined by the magnetic system relaxation. Calculations and comparison with experiments show that the relaxation of the investigated magnetic systems atT<T _f follows the relaxation lawM(t)=M(0) exp[?(t/τ) ^r ], suggested in Palmeret al (1984) for spin-glasses relaxation taking into account the time relaxation distributionτ ₀....τ _max in the system and its ‘hierarchically’ dynamics.     

X-ray photoelectron spectroscopy and magnetism of Mn1−x Al x alloys

X-ray photoelectron spectroscopy (XPS), magnetization and magnetic susceptibility of Mn_1−x Al _x (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.7) alloys are reported. X-ray diffraction measurements showed that all investigated samples have the same crystallographic structure as the parent compound (AuCu₃-structure type). The alloys are disordered for x ≤ 0.5, but become almost crystallographically ordered for higher Al concentration. This change in the crystallographic order is reflected both in the magnetization and Curie temperature values. The exchange interaction is ferromagnetic between the pairs of the near-neighbour Mn-Ni and Ni-Ni magnetic moments and antiferromagnetic for Mn-Mn pairs. The last one is present only in the disordered alloys, which leads to smaller values of the magnetization of these alloys in comparison with the ordered ones. The Mn magnetic moment has the fully ordered value of 3.2 μB in all investigated alloys. The decrease of the Ni magnetic moment as the Al concentration increases may be explained by the hybridization of the Ni 3d and Al 3sp states, which leads to a partial filling of the Ni 3d band. The magnetic susceptibility measurements pointed out the existence of spin fluctuations on Ni sites.      

Relaxation phenomena and nuclear magnetic resonance of116In(T 1/2=14s) produced by capture of polarized neutrons in the indium III–V compounds

Polarized¹¹⁶In nuclei have been produced by capture of polarized thermal neutrons in several In compounds. At temperaturesT below 77 °K and magnetic field strengthsH ₀ of several kOe, asymmetries of a few percent of the β^? decay of the¹¹⁶In ground state could be observed in polycrystalline InP, InAs and InSb, thus indicating the nuclear polarization. Nuclear magnetic resonance signals have been measured with the result for the magnetic moment μ _i (¹¹⁶In)=2.7723 (10) nm (uncorrected). β^? decay asymmetry and spin lattice relaxation timeT ₁ have been studied as a function ofH ₀ andT. The effect ofH ₀ is to decouple the hyperfine interaction caused by the capture-γ recoil process. However,H ₀ has no influence uponT ₁, which demonstrates the absence of nuclear relaxation due to paramagnetic impurities.T ₁ is determined by quadrupolar relaxation. A quadrupole momentQ(¹¹⁶In)=0.09 (2) b was calculated by comparison of the¹¹⁶In relaxation rates with those of the stable¹¹⁵In isotope in the same compounds. Above 30 °K the temperature dependence of 1/T ₁ agrees with a recent theoretical investigation. Below 30 °K the relaxation rate shows an anomalous behaviour, which can be explained by resonance modes due to recoil lattice defects.     

NMR study of pure and doped β-LiAl

The NMR spin-lattice relaxation time, T_I, has been measured as a function of temperature for both ⁷Li and ²⁷Al in pure and doped β-LiAl alloys. Compositions with ⁷Li concentration in the range 48.3–54.5% and doping in the form Li₅₀Al_50?xM_x, where M = Ag or In, were studied. The relaxation rates T₁^?1 for the ²⁷Li and the ²⁷Al resonances were found to be peaked functions of temperature with the maxima for ⁷Li appearing at composition dependent temperatures. The ²⁷Al maxima always appeared at a lower temperature, independent of composition, and the ²⁷Al maximum relaxation rate was a strong function of composition in contrast with ⁷Li where the maximum rate was only weakly dependent on composition. The principle relaxation mechanisms are identified as dipole-dipole coupling in the ⁷Li and coupling of the ²⁷Al quadrupole moment to electric field gradients. The temperature dependence of these rates is attributed to the thermally activated diffusion of vacancies of a non-thermal origin in the Li sub-lattice. These vacancies are also responsible for the fluctuating electric field gradients. The results have been analyzed to give the Li diffusion coefficients with associated activation energies and estimates of the vacancy concentration as functions of alloy composition.     

MnxGe1－x稀磁半导体薄膜的结构研究

利用X 射线衍射(XRD)和X射线吸收精细结构(XAFS)方法研究了磁控共溅射方法制备的Mn_xGe_1-x薄膜样品的结构随掺杂磁性原子Mn含量的变化规律.XRD结果表明，在Mn的含量较低(7.0%)的Mn_0.07Ge_0.93样品中，只能观察到对应于多晶Ge的XRD衍射峰，而对Mn含量较高(25.0%, 36.0%)的Mn_0.25Ge_0.75和Mn关键词： 磁控溅射 XRD XAFS xGe_1-x稀磁半导体薄膜')" href="#">Mn_xGe_1-x稀磁半导体薄膜     

Study of spin–phonon coupling in LiFe1 − xMnxPO4olivines

LiFe_{1 − x}Mn_xPO₄ olivines are promising material for improved performance of Li‐ion batteries. Spin–phonon coupling of LiFe_{1 − x}Mn_xPO₄ (x = 0, 0.3, 0.5) olivines is studied through temperature‐dependent Raman spectroscopy. Among the observed phonon modes, the external mode at ~263 cm⁻¹ is directly correlated with the motions of magnetic Fe²⁺/Mn²⁺ ions. This mode displays anomalous temperature‐dependent behavior near the Néel temperature, indicating a coupling of this mode with spin ordering. As Mn doping increases, the anomalous behavior becomes clearly weaker, indicating the spin–phonon coupling quickly decreases. Our analyses show that the quick decrease of spin–phonon coupling is due to decrease of the strength of spin–phonon coupling, but not change of spin‐ordering feature with Mn doping. Importantly, we suggest that the low electrochemical activity of LiMnPO₄ is correlated with the weak spin–phonon coupling strength, but not with the weak ferromagnetic ground state. Our work would play an important role as a guide in improving the performances of future Li‐ion batteries. Copyright © 2015 John Wiley & Sons, Ltd.     

Muon spin relaxation study on magnetism in high quality single crystal of a high transition temperature superconductor La2−xSrxCuO4−σ (0.11≤x≤0.14)

The positive muon spin relaxation method is applied to probe magnetic ordering in the superconducting phase of a high quality single crystal of La_2−xSr_xCuO_4−σ (0.11≤x≤0.14). The well characterized crystal ofx=0.11 (T _c=34.5 K) with nearly complete flux exclusion exhibits spin freezing at 8K(T _f) with significant spin fluctuation up to 20 K. The onset of spin fluctuation andT _f decrease against increasingx towardsx=0.15, suggesting an existence of a magnetic phase boundary aroundx whereT _c becomes maximum.     

Susceptibility behaviour of Mn dissolved in liquid Cu—Al-alloys

Measurements of the magnetic susceptibility of liquid Mn_0.05—Cu—Al alloys up to 1700 K indicate Curie—Weiss like behaviour at the Cu-rich end and minima of the reciprocal Mn susceptibility values as a function of the temperature 1/_ξMn(T) at the A1-rich end of the series. The occurrence of 1/_ξMn(T) minima at temperatures, which are sensitively depending on the alloy composition, hints at a transition to localized magnetic moment behaviour even in the A1-based alloys at high temperatures. Both, the aspects of extremely increasing Kondo temperature and that of decreasing spin fluctuation times, allow to discuss the susceptibility behaviour uniformly for the whole alloy series. Thus, the results do not support a fundamental distinction (magnetic—nonmagnetic) between the alloy systems $\text{Cu}$—Mn and $\text{A1}$—Mn.     

On the ferromagnetic perovskite with potential GMR properties: The manganite La1−x K x MnO3

We investigate structural features (cell distortions), magnetic and electric properties versus temperature for La_1-x K _x MnO₃ (0.05 < x < 0.2) perovskites. All the phases crystallize in a rhomboedral symmetry (R3c) and are ferromagnetic. Curie temperatures are strongly dependent of x, with a maximum around 308 K for La_0.8K_0.2MnO₃. A sharp decrease of the resistivity is observed just below the Curie temperature, accompanied by a very steep increase of the spontaneous magnetization, characteristic of a conventional Brillouin type curve. The average magnetic moment of Mn is about 88 e.m.u/g at 5 K for the composition La_0.8K_0.2MnO₃. The transition from ferromagnetic-metallic to paramagnetic-semiconducting states is explained from the suggested distribution of the cations (La³⁺ _1-x K⁺ _x )_A(Mn⁴⁺ _1-2x Mn⁴⁺ _2x )_BO₃ by the double exchange of Mn³⁺-Mn⁴⁺ pairs at the B-sublattice. Preliminary magnetoresistance measurements on bulk ceramic samples display an MR effect of the same amplitude as in the case of the alkaline earth substituted La manganites.     

A μSR and neutron polarisation analysis study of Mn moment delocalisation in Fe substituted YMn2

Neutron polarisation analysis measurements reveal antiferromagnetic spin correlations persisting to temperatures of 120 K in Pauli paramagnetic Y(Mn_1−x Fe _x )₂, 0.03≤x≤0.05. The mean moment at the Mn(Fe) site is found to be 0.2μ _B. Transverse field μSR is characterised by weak exponential damping with a rate of 0.02 μs⁻¹ at 300 K increasing according to the power lawT ^−0.75 to only 0.16μ _S ⁻¹ at 12 K. It is suggested that these results are consistent with a slowing down of longitudinal spin fluctuations at the Mn site as temperature decreases.     

Magnetic properties of Mn-rich Rh2Mn1+xSn1−x Heusler alloys

X-ray powder diffraction and magnetization measurements have been carried out on Rh₂Mn_1+xSn_1−x (0≤x≤0.3) alloys. The alloys, which crystallize in the L2₁ structure, were found to exhibit ferromagnetic behavior. The lattice constant a at room temperature decreases with increasing x, whereas the Curie temperature T_C decreases linearly. At 5 K the magnetic moment per formula unit first increases with increasing x and then saturates for x≥0.2. The experimental results are discussed in terms of the influence of the Mn-Mn exchange interactions between the Mn atoms on the Sn and Mn sites.