High-frequency and -field electron paramagnetic resonance of high-spin manganese(III) in axially symmetric coordination complexes

High-frequency and -field electron paramagnetic resonance (HFEPR) has been used to study several complexes of high-spin manganese(III) (3d⁴,S = 2): [Mn(Me₂dbm)X] and [Mn(OEP)X] (X = Cl^?, Br^?), where Me₂dbm^? is the anion of 4,4′-dimethyldibenzoylmethane and OEP^2? is the dianion of 2,3,7,8,12,13,17,18-octaethylporphine. These non-Kramers (integer spin) systems are not EPR-active with conventional magnetic fields and microwave frequencies. However, use of fields up to 15 T in combination with multiple frequencies in the range of 95–550 GHz allows observation of richly detailed EPR spectra. Analysis of the field- and frequency-dependent HFEPR data allows accurate determination of the following spin Hamiltonian parameters for these complexes: [Mn(Me₂dbm)Cl],D = ?2.45(3) cm^?1; [Mn(Me₂dbm)Br],D = ?1.40(2) cm^?1; [Mn(OEP)Cl],D = ?2.40(1) cm^?1; [Mn(OEP)Br],D = ?1.07(1) cm^?1 (E ≈ 0, andg ≈ 2.0 in all cases). Comparison of structural data with the electronic parameters for these and related complexes shows quantitatively the effects of axial and equatorial ligation on the electronic structure of Mn(III). These high-spin complexes can be employed as building blocks in the construction of single-molecule magnets. Thus the accurate determination and understanding of the electronic properties, best obtainable by HFEPR, of these monomeric units is important in understanding and improving the properties of the polynuclear single-molecule magnets which can be formed from them.     

Hall constants and magnetoresistance in aluminium single crystals,studied by means of helicon resonances

The Hall coefficient and the transverse magnetoresistance in aluminium single crystals were determined from measurements on standing helicon waves, in the three main crystallographic directions [100], [110] and [111]. The magnetic fields ranged from 1 to 7T and the temperature from 4.2 to 20 K. A Legendy-type theory with parameters adapted to the separation of the satellites as measured at high fields gave good fittings over the whole field range. The Hall coefficients, determined in this work, decrease with magnetic field as mentioned in earlier publications, but are generally lower than those published previously for comparable field ranges.The transverse magnetoresistance as a function of temperature was determined from a continuous recording of the helicon response at resonance. The temperature dependent part of the resistivity varies according to a T³ law for all fields. The field dependence is close to linear, but indications of quadratic and saturation behaviour are also present.     

The possibility of the “giant” isotope effect for ultrasound absorption in crystals

The Zeeman effect, magnetization M(H), and differential magnetic susceptibility dM/dH of ErVO₄ crystals in a pulsed magnetic field have been experimentally and theoretically studied. In magnetic fields H ∥ [001] and H ∥ [100], the energy levels of Er³⁺ ions exhibit mutual approach and crossing (the crossover effect), which results in the peaks in dM/dH and the jumps in M(H) curves at low temperatures. The anomalies in the magnetic properties related to the crossover in ErVO₄ for H ∥ [001] are highly sensitive to the electronic structure of Er³⁺ ion, which allows this effect to be used for refining the crystal field parameters. The influence of the temperature, field misorientation from the symmetry axis, parameters of pair interactions, and other factors on the magnitude and character of magnetic anomalies in ErVO₄ crystals is considered.     

Magnetic characteristics of PrCd single crystal

Magnetic measurements have been made along the [100], [110] and [111]-axes of PrCd single crystal in magnetic fields up to 70 kOe in the temperature range from 4.2 to 300 K. PrCd shows metamagnetism below T_t = 25 K in which two critical fields are observed in the magnetization processes along the [110] and [111]-axes and a critical field along the [100]-axis. The metamagnetic behaviour is interpreted by a noncollinear antiferromagnetic model with the Pr moments of ～ 2μ_B directed to the 〈111〉-axes.     

Effects of electron correlation and spin-orbit coupling on the electronic and magnetic properties of TbCu3Mn4O12

Electronic and magnetic properties of the three magnetic-sublattice double perovskite TbCu₃Mn₄O₁₂ (TCMO) are investigated by performing first-principles density-functional theory calculations. Our electronic structure calculations show that TCMO is half-metallic and its half-metallicity can only be correctly described when the electron correlation on Tb³⁺ 4f⁸ electrons are considered. The energies of different magnetic configurations among the three magnetic sublattices are also calculated, revealing that the magnetic configuration with Mn and Cu spins in the antiparallel arrangement and with the Tb magnetic moments ferromagnetically/antiferromagnetically (FM/AFM) coupled to Cu/Mn spins (that is Tb^↓Cu₃^↓Mn₄^↑O₁₂) is the lowest energetic magnetic state, which is consistent with recent experimental results. The magnetic anisotropy is further calculated for the [1 1 1], [1 1 0], and [0 0 1] spin quantization directions. It is found that the [1 1 1]-direction is more stable than the [1 1 0]- and [0 0 1]-directions by 123 and 135 meV per formula unit, respectively, indicating a significant magnetic anisotropy. Our detailed projected partial density of states analysis finally shows that Cu and Mn are antiferromagnetically coupled by superexchange interaction and Tb is expected to interact FM with A-site Cu and AFM with B-site Mn sublattices by way of 4f-2p-3d.     

Crystal field and quadrupole interaction effects in YbXO4 Zircons (X = V, P)

The magnetic and magnetoelastic properties of YbPO₄ and YbVO₄ crystals are investigated experimentally and theoretically; the crystal field parameters are determined, as well as the magnetoelastic coefficients B ^μ and total quadrupole coupling constants G ^μ for all symmetry modes. It is found that, for H ∥ [100], γ-symmetric quadrupole interactions predominate and are responsible for a significant contribution to the third-order susceptibility, magnetization, magnetostriction, and elastic constant. It is demonstrated that, in the absence of an external field, these interactions do not lead to quadrupole ordering, because the respective deformation susceptibility χ_γ is several times less than the critical value of 1/G ^γ. The influence of an external magnetic field along different symmetry axes on the quadrupole effects and quadrupole interactions in Yb zircons is investigated. It is demonstrated that, for H ∥ [110], the susceptibility χ_γ increases with the field, so that in a fairly strong field in the investigated crystals one can expect a γ-symmetric stimulated phase transition.     

Giant magnetothermopower oscillations in indium: Evidence for magnetic breakdown

Large amplitude, low frequency oscillations in the transverse magnetothermopower of indium have been observed below 2.5 K for magnetic fields between 10 kG and 20 kG directed along either [010], [$\text{1}$01], or [1$\text{1}$0] crystallographic axes. The oscillations are attributed to magnetic breakdown.     

Magnetism of ErAgSn studied by 119Sn Mössbauer spectroscopy

Antiferromagnetic ErAgSn compound was investigated in detail by ¹¹⁹Sn Mössbauer spectroscopy in a temperature range between 2.2 and 300 K. The ¹¹⁹Sn spectra recorded below 4.2 K can be well fitted with a single main magnetic component in agreement with recent neutron diffraction studies [1]. A broad distribution of magnetic hyperfine fields observed above 4.2 K and enhanced spin correlations among Er³⁺ ions at T > T _N = 5.6 K are the remarkable features of the investigated system.     

Spin-wave resonance and magnetic anisotropy in FePt thin films

Ferromagnetic Resonance (FMR) measurements at room temperature and X-band microwave frequency were performed on polycrystalline FePt thin films with face-centered cubic structure. With the external field perpendicular to the film plane, the absorption fields H_n of the odd and even spin-wave resonance modes n detected for the Fe_0.44Pt_0.56(45 nm)/Si(1 0 0) and Fe_0.51Pt_0.49(105 nm)/Pt(55 nm)/MgO(1 0 0) films, were found to obey the well-known H_n×n² ratio, giving for these films the exchange stiffness constant values of 3.9×10⁻⁸ and 4.4×10^–7 erg/cm, respectively. The study of the out-of-plane angular dependence of the absorption field of the uniform FMR mode allowed the measurement of the effective magnetic anisotropy constants of 5.3×10⁶ , 6.4×10⁶ , and 6.7×10⁶ erg/cm³, related, respectively, to the [1 1 1], [1 0 0], and [1 1 0] textures present in the films.     

Determination of the effective g-value of n-InAs by four-wave mixing spectroscopy

Spin resonant four-wave mixing of the radiation of two single frequency CO₂ lasers is observed in n-InAs. By this method the effective g-value of the material is determined in magnetic fields up to 6.5 T. The temperature of the sample is 1.5 K; the carrier concentration is 1.5 × 10¹⁶/cm³. The observed g-values lie between 14.7 at 0.5 T and 13.7 at 6.3 T and extrapolate to 15.0 at B = 0. We observed symmetric lineshapes at high magnetic fields with linewidths of the order of 1.3 kG corresponding to 0.9 cm^-1. At low magnetic fields the lines show an asymmetric shape with a steep decrease at the high field side of the resonance.     

Field and temperature dependences of the specific heat of the La1.85Sr0.15CuO4 superconductor

This paper reports on the temperature and field dependences of the specific heat of high-quality La_1.85Sr_0.15CuO₄ single crystals carried out at low temperatures in magnetic fields of up to 8 T for two magnetic field orientations, namely, along the [100] and [110] crystallographic axes. The field dependence of the electronic density of states (DOS) was found to be anisotropic for different magnetic field orientations in the a–b plane, with the electronic density being the lowest along the a axis (for H ∥ [100]) and maximum for the field inclined at 45° to the a axis (for H ∥ [110]). Electronic specific heat in a magnetic field was observed to depend linearly on temperature T and nonlinearly on the magnetic field H: C _DOS=bTH ^1/2. In a zero field, the electronic specific heat grows quadratically with temperature as C _DOS=αT ². Estimation of the maximum superconducting gap width from the experimentally determined values of the α coefficient of T ² and of the electronic DOS in the normal state yields Δ ₀=300 K. The observed features indicate that La_1.85Sr_0.15CuO₄ is a superconductor with d symmetry of the order parameter.     

Spectroscopic analysis and magnetic susceptibility of CuO-TeO2-V2O5 glasses

This article reports on the structure of the glassy system xCuO-65TeO₂-(35−x)V₂O₅, 5≤x≤10 mol% which was studied using infrared (IR) and Raman spectroscopy methods as well as magnetic susceptibility measurements. IR and Raman spectroscopy analysis reveals the presence of four main absorption bands attributed to [TeO₃], [TeO₄], [VO₄], and [VO₅] structure units. It suggests that Cu²⁺ ions occupy the available open spaces of the Te-O network without straining the bonds too much. Increasing the concentration of Cu²⁺ ions beyond 5 mol% results in the modification of the glass by straining and locally distorting the surrounding of the Te-O network. The magnetic susceptibility of these materials was investigated in the temperature range of 5-200 K revealing the paramagnetic behavior described by the Curie-Weiss law and indicating the presence of weak antiferromagnetic exchange interactions between Cu ions. The magnetic entropy change of the glasses was determined based on the temperature and magnetic field dependence of magnetization.     

Drift theory of charged-particle motion for a strong electric field in a weakly relativistic approximation

Drift equations of motion are derived for a charged particle in the case of a strong electric field with allowance for relativistic effects of order v²/c². The role of these effects is discussed along with the effects of a high-frequency field. The cases of weak and strong electric fields are distinguished [2] in the drift theory of the motion of charged particles in weakly inhomogeneous magnetic and electric fields. In the case of a weak electric field, the electric-drift velocity is v_E v, where v is the characteristic velocity of the particle. For a strong electric field,v _Ev.The drift theory has now been reasonably well developed for the case of weak electric fields in the classical and relativistic cases, for the absence of high-frequency fields and for the presence of these [1–3], Extension of the theory to strong electric fields involves considerable mathematical difficulties, and this has been done only in the classical approximation with and without hf fields [2–4], Here we consider the drift theory of charged-particle motion for the case of a strong electric field in the weakly relativistic approximation, incorporating terms of order v²/c², where c is the velocity of light. Also hf fields may be present.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 7–9, September, 1981.     

Magnetic lenses performance with fields of the form B(Z)∝Z

Performance of magnetic lenses with fields of the form B(z)∝Zⁿ was studied. To compare the performance of different objective lenses the results are presented in W. Tretner's [Existenzbereiche rotations symmetrischer Elektronenlinsen, Optik 16 (1959) 155-184] form.     

Magnetic monopoles in neutron stars

The effects of monopole-antimonopole annihilation on a previously reported bound [1] on the product of the galactic flux of grand unified magnetic monopoles and the cross section for monopole catalyzed nucleon decay: (F_M/cm^?2s^?1sr^?1)(σ_ΔB/10^?2 cm²) ? 10^?22 are examined for several models of neutron star interiors. For neutron stars with superconducting interiors or large internal magnetic fields this bound is unaltered. In the unlikely event that old neutron stars are not superconducting and have internal magnetic fields B_int ? 10⁸ Gauss the effects of monopole-antimonopole annihilation relax the bound to (F_M/cm^?2 s^?1 sr^?1)(σ_ΔB/10^?27 cm^?2)² ? 10^?18. Magnetic monopoles may also have a significant effect on the structure of the interior magnetic field in neutron stars.     

High field susceptibility of Zn1−xCuxCr2Se4

We present the results concerning field dependences of magnetization for Zn_1-xCu_xCr₂Se₄ mixed spinels studied at low temperatures with the use of both high magnetic stationary fields up to 14 T and high pulsed fields up to 25 T. From the isotherms of static magnetization we have obtained the incremental high field susceptibility and from pulsed measurents the differential one. The temperature and concentration (x) dependences of the calculated high field susceptibility χ_hf are interpreted based on the magnetic properties of the spinels under study presented in refs. [4–7]. The greatest changes of χ_hf were observed in the intermediate concentration range exhibiting a conical spin structure, in which the appearance of the double-exchange interaction between Cr³⁺ and Cr⁴⁺ ions plays a main role. It drives the transition from a noncollinear magnetic structure to a ferromagnetic one with increasing copper concentration.     

Specific features of magnetic phase diagram of an MnSi helimagnet

Magnetization curves of MnSi single crystals have been measured in a range of temperatures T = 5.5–35 K and magnetic field strengths H ≤ 11 kOe for H ∥ [1 1 1], [0 0 1], and [1 1 0]. Special attention has been paid to the temperature interval near T _N = 28.8 K, where MnSi exhibits a transition to the state with a long-period helical magnetic structure. Some new features in the magnetic behavior of MnSi have been found. In particular, in an intermediate temperature region above the transition (28.8 K = T _N ≤ T < 31.5 K), the dM(H)/dH curves exhibit anomalies that are not characteristic of the typical paramagnetic state. It is established that the line of the characteristic field H*(T) of this anomaly is a natural extrapolation of the temperature dependence of the field of the transition from a conical phase to an induced ferromagnetic phase observed at T < T _N. It is concluded that the properties of MnSi in the indicated intermediate region are related to and governed by those of the conical phase (rather than of the A phase). Based on these data, magnetic phase diagrams of MnSi for H ∥ [1 1 1], [0 0 1], and [1 1 0] are plotted and compared to diagrams obtained earlier by other methods.     

Anisotropic pressure in dense neutron matter under the presence of a strong magnetic field

Dense neutron matter with recently developed BSk19 and BSk21 Skyrme effective forces is considered in magnetic fields up to ¹⁰²⁰ G at zero temperature. The breaking of the rotational symmetry by the magnetic field leads to the differentiation between the pressures along and perpendicular to the field direction which becomes significant in the fields H>Hth∼¹⁰¹⁸ G. The longitudinal pressure vanishes in the critical field ¹⁰¹⁸<Hc?¹⁰¹⁹ G, resulting in the longitudinal instability of neutron matter. For the Skyrme force fitted to the stiffer underlying equation of state (BSk21 vs. BSk19) the threshold Hth and critical Hc magnetic fields become larger. The longitudinal and transverse pressures as well as the anisotropic equation of state of neutron matter are determined under the conditions relevant for the cores of magnetars.     

Synthesis of goethite from Fe(OH)2 precipitates: Influence of Fe(II) concentration and stirring speed

Magnetic methods are efficient tools in soil and environmental science. But in such natural environments, several magnetic minerals are generally present. So, synthetic standard samples are necessary for calibration of laboratory techniques. The aim of this study was to synthesise goethite free of magnetic impurities (concentration <∼1 μg kg⁻¹) with different crystal sizes. Goethite was prepared by oxidation of aqueous suspensions of Fe(OH)₂ precipitates. Final products were characterised by X-ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy and magnetic methods. Goethite could be obtained in the absence of any trace of strong magnetic minerals using FeSO₄·7H₂O and NaOH as reactants with the following experimental conditions: temperature=45 °C, [FeSO₄·7H₂O]=0.50 mol L⁻¹, [NaOH]=0.20 mol L⁻¹, stirring speed=760 rpm. The Fe(II) concentration and the stirring speed were varied. It proved possible to modify the size of the goethite crystals by varying the Fe(II) concentration and the stirring speed, but important changes of these parameters induced the formation of other phases, lepidocrocite when the oxidation reaction was drastically accelerated and Fe₃O₄ when the reaction was slowed down. In any case, for weak magnetic fields, a low-coercivity magnetic mineral saturating at weak magnetic fields was observed. It may correspond to traces of δ-FeOOH or to domains structurally similar to δ-FeOOH inside the multidomainic crystals of δ-FeOOH.     

Hyperfine interactions of tin atoms in samarium

The hyperfine interactions of ¹¹⁹Sn impurity atoms in samarium at temperatures from 5 to 70 K are investigated by Mössbauer spectroscopy. The distributions P of magnetic hyperfine fields B _hf for tin atoms at sites of the hexagonal [P _h(B _hf)] and cubic [P _c(B _hf)] samarium sublattices are determined from the experimental absorption spectra. Ion ordering in pairs of magnetic centers located in layers of the cubic sublattice is observed by Mössbauer spectroscopy for the first time. Each magnetic center involves ordered ions at the nearest neighbor sites of the tin atom replacing the samarium ion at the hexagonal lattice site. The quadrupole coupling constant e ² q _hQ=0.59±0.12 mm/s is determined for tin atoms at the hexagonal sublattice sites of samarium. The quadrupole interaction of tin atoms in heavy rare-earth metals (from Tb to Er) with a hexagonal close-packed structure is discussed.