Dynamic self-organization of magnetic domains in amorphous films with perpendicular anisotropy

Dynamic self-organization of magnetic domains is observed in amorphous gadolinium-cobalt films in narrow temperature intervals on both sides of the magnetic compensation point. Spiral dynamic domains form in a limited range of ac magnetic field amplitudes and frequencies. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 688–692 (10 December 1997)     

Paramagnetic and spin-glass properties of pyrochlore-like oxides Ln2Mn2/3Mo4/3O7 (Ln=Sm, Gd, Tb, or Y)

The magnetic properties of complex oxides Ln ₂Mn_2/3Mo_4/3O₇ (Ln=Sm, Gd, Tb, or Y) with a pyrochlore-type structure are studied in the temperature range 2–300 K. For all compounds in the paramagnetic state, the temperature dependence of the magnetic susceptibility is described by a generalized Curie-Weiss law with a temperature-independent component of ∼10⁻⁶ cm³/g and with a Weiss constant Θ<0 and |Θ|<16 K. At low temperatures (T<10–12 K), the compounds have spin-glass properties; they exhibit magnetic and temperature hysteresis and the typical dependences of the imaginary and real parts of the dynamic magnetic susceptibility on temperature and the frequency of an ac magnetic field in a wide range of magnetization relaxation times. The data obtained suggest that d electrons are responsible for the formation of frustrated exchange interactions in the compounds and that 4f electrons in the compounds with Sm or Tb provide strong magnetic-anisotropy effects. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 2, 2004, pp. 287–295. Original Russian Text Copyright ? 2004 by Korolev, Bazuev.     

Temperature dependence of the magnetic properties and magnetoimpedance of nanocrystalline Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> ribbons

The temperature dependences of the magnetic properties and the magnetoimpedance effect of soft magnetic nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy ribbons are studied in the temperature range 24–160°C. A high temperature sensitivity of the impedance and the magnetoimpedance effect of the ribbons are detected in the ac frequency range 0.1–50 MHz. At an ac frequency of 50 MHz, the change in the impedance reaches 0.2 Ω/°C (0.5%/°C) in the temperature range 85–160°C. When the temperature increases, a monotonically decreasing character of the dependence of the magnetoimpedance effect on the applied magnetic field changes into a dependence having an increasing initial segment. The effect of temperature on the magnetoimpedance properties of the soft magnetic nanocrystalline ribbons is shown to result from temperature-induced changes in their electrical conductivity, magnetization, and effective magnetic anisotropy.     

Radio-frequency absorption study of the parameters of high-T c materials irradiated with fast reactor neutrons

The magnetic properties of YBa₂Cu₃O_7−x ceramics of various densities and Bi₂Sr₂CaCu₂O₈ quasicrystals irradiated with neutrons at fluences of 10¹⁶–10¹⁹ cm⁻² are investigated by a radio-frequency method (in the frequency range f=100–150 MHz). The electromagnetic absorption is used to estimate the parameters of a Josephson medium: the lower critical field, the critical current density, the grain size and intergranular distances, and the penetration depth of the magnetic field in the sample. The hysteresis of electromagnetic absorption in the indicated superconductors is investigated; the hysteresis effect can be used to obtain data on flux pinning processes and flux creep dynamics. It is shown that an increase in the critical current density is observed as a result of the injection of radiation-induced pinning centers. Fiz. Tverd. Tela (St. Petersburg) 39, 28–34 (January 1997)     

Characterization of ablation plumes and carbon nitride films produced by reactive pulsed laser deposition in the presence of a magnetic field

x ) films in a nitrogen atmosphere within the range 5×10^-4–4×10^-1 Torr. In the presence of a magnetic field, the emission intensities of N₂ (second positive system) and CN species in the graphite ablation plumes were altered significantly, depending on the pressure of the N₂ environment. Corresponding to an intense CN emission, a magnetic field-induced enhancement of N incorporation – for example, up to 37% at an N₂ pressure of 300 mTorr – and the formation of sp³ tetrahedral CN bonding were both observed in the films. This suggests that the arrival of CN species at the substrate surface with kinetic energies is important for film deposition. Received: 27 August 1997/Accepted: 8 September 1997     

Relaxation and shift of the precession frequency of the spin of a negative muon in n-type silicon

The residual polarization of negative muons in n-type silicon with impurity density (1.6±0.2) · 10¹³ cm⁻³ is investigated as a function of temperature in the range 10–300 K. The measurements are performed in an external magnetic field of 0.08 T oriented transversely to the spin of the muons. Relaxation of the muon spin and a shift of the precession frequency are observed at temperatures below 30 K. The relaxation rate at 30 K equals 0.25±0.08 μs⁻¹. The shift of the precession frequency at 20 K equals 7 · 10⁻³. Both the relaxation rate and the shift of the precession frequency increase as the temperature decreases. At temperatures below 30 K the relaxation rate is described well by the relation Λ=bT ^−q , where q=2.8±0.2. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 539–543 (10 April 1996)     

Thermal stability of the magnetic parameters of epitaxial iron garnet films subjected to planar radial stresses

The effect of external planar radial pressure on the thermal stability of the magnetic parameters of epitaxial iron garnet films is investigated in the temperature range 200–500 K for external mechanical stresses in the range 0–40 kgf/mm². It is shown that external planar radial pressure can be used to improve the thermal stability of these magnetic parameters by a factor of 1.5–2, and also to alter significantly the temperature interval of single-domain behavior for orientational phase transitions near a compensation point. Zh. Tekh. Fiz. 67, 114–116 (September 1997)     

Dislocations used to probe the defect state of an ionic crystal lattice excited by a pulsed magnetic field

This paper presents the results of an investigation into the effect of a pulsed magnetic field on the state of linear and point defects in ionic crystals. For different amplitudes (1–7 T) and pulse lengths (3×10⁻⁵ to 10² s) of the pulsed field the kinetics of the transformation of defects into a new state and their relaxation after the field is turned off are studied in the temperature range 77–400 K. It is found that the relaxation of the states of point defects is mainly through recombination, and the change of state of the dislocations and of the point defects contribute nonadditively to the change in the dislocation mobility. The exposure of the crystal to a magnetic field leads to an increase in the dislocation mobility when the sample is mechanically stressed and to a decrease in the dislocation displacement with a second field pulse. Fiz. Tverd. Tela (St. Petersburg) 39, 634–639 (April 1997)     

Continuous frequency tuning of a picosecond neodymium-glass laser in the UV and VUV

The results of an investigation of continuous frequency tuning of a neodymium laser in the UV and VUV ranges are reported. Generation of the sum frequency of second harmonic radiation and the radiation from a parametric light generator in the UV region (338–366 nm) is achieved. The optimal conditions for tuning UV radiation in the range 113.5–117.0 nm in third-harmonic generation processes in xenon and its mixtures with other gases are investigated. A third-harmonic generation efficiency of ∼5×10⁻⁴ and a tuning range >2600 cm⁻¹ are obtained in the VUV range investigated. Zh. Tekh. Fiz. 68, 82–89 (May 1998)     

Natural ferromagnetic resonance in the disordered alloy Pd2AuFe

A contribution to electromagnetic power losses, additional to the losses due to eddy currents and exhibiting a resonance frequency dependence with the main maximum near 1 GHz, has been observed for the ferromagnetic alloy Pd₂AuFe in the frequency range 0.9 MHz–10 GHz in the absence of an external constant magnetic field. Investigations performed in a dc magnetic field show that this effect is a natural ferromagnetic resonance due to intradomain magnetization precession in the effective magnetic-anisotropy field. Fiz. Tverd. Tela (St. Petersburg) 40, 1900–1904 (October 1998)     

Magnetoelectric effect in composites of magnet,metal-cap,and piezoceramic

We report on the magnetoelectric (ME) effect in three-phase magnet-metal-cap-piezoceramic composites of a thickness-polarized Pb(Zr,Ti)O₃ disk bonded between two truncated conical brass caps and two thickness-magnetized NdFeB magnets along the thickness direction. The effect originated from the product of the magnetic attractive–repellent effect in the magnets and the amplified piezoelectric effect in the cap-disk assembly. The composites exhibited a ME voltage coefficient α_V of ∼2.6 mV/Oe in the non-resonance frequency range of 0.1–10 kHz with good voltage-field linearity in the field range of 10^-3–10 Oe, besides an enhanced α_V of 167.4 mV/Oe at a resonance frequency of 14.4 kHz. Compared to conventional two-phase and three-phase magnetostrictive material-contained ME composites, these composites possessed two distinct benefits of higher property-tailorable flexibility and requiring no external dc bias magnetic field to enable an obvious α_V. PACS 77.84.-S; 75.80.+q; 77.84.Dy; 77.65.-j; 85.80.Jm     

Ac electrical parameters of Al-ZnPc-Al organic semiconducting films

The ac electrical parameters of thermally evaporated zinc phthalocyanine, ZnPc, semiconducting thin films was measured in the temperature range of 180–390 K and frequency between 0.1 and 20 kHz. Aluminum electrode contacts were utilized to sandwich the organic ZnPc semiconducting films. Capacitance and loss tangent decreased rapidly with frequency at high temperatures, but at lower temperatures a weak variation is observed. An equivalent circuit model assuming ohmic contacts could qualitatively and successfully explains capacitance and loss tangent behavior. The ac conductivity showed strong dependence on both temperature and frequency depending on the relevant temperature and frequency range under consideration. Ac conductivity σ (ω) is found to vary with ω, as ω ^s with the index s ≤ 1.35 suggesting a dominant hopping conduction process at low temperatures (< 250 K) and high frequency. The conductivity of some samples did not increase monotonically with temperature. This behavior was attributed to oxygen exhaustion of the sample as its temperature is increased. The ac conductivity behavior at low temperatures of ZnPc films could be described well by Elliott model assuming hopping of charge carriers between localized sites.     

Mn55 NMR investigation of the suppression of quantum fluctuations in quasi-one-dimensional antiferromagnets by a magnetic field

The NMR of Mn⁵⁵ nuclei is measured in the quasi-one-dimensional antiferromagnets CsMnBr₃, RbMnBr₃, and CsMnI₃ in magnetic fields upto 8 T attemperatures in the range 1.3–4.2 K. The average moments of the magnetic sublattices and their field dependences, which turned out to be comparatively strong and different for magnetically non-equivalent Mn²⁺ ions, are determined from the hyperfine-field data obtained. As a result, the magnetizations of separate sublattices in an external magnetic field ∼8 T differ by more than 5%. The results obtained agree qualitatively with the theory of the suppression of quantum fluctuations by a magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 724–729 (10 December 1997) Deceased.     

Magnetooptical microwave spectroscopy of the coherent magnetic state in the mixed valence compound SmB6 in the frequency range 40–120 GHz

In undoped pure single crystals of the mixed valence compound SmB₆ anomalous ESR absorption is observed in the frequency range v=40–120 GHz at temperatures of 1.8–4.2 K. The ESR for the case of the coherent ground state consists of two components corresponding to g-factors g ₁=1.907±0.003 and g ₂=1.890±0.003. The amplitude of both ESR lines strongly depends on temperature in the temperature range studied: the amplitude of the first line with g=g ₁ increases and the amplitude of the second line decreases with temperature. A model based on consideration of intrinsic defects in the SmB₆ crystalline lattice, with a densit ∼10¹⁵−10¹⁶ cm⁻³, is suggested as an explanation for the anomalous ESR-behavior. In the frequency range v>70 GHz at T=4.2 K, in addition to the main ESR lines, a new magnetic resonance with a hysteretic field dependence is discovered. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 707–712 (25 November 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

The influence of a magnetic field on the inelastic properties of LiF crystals

The effect of weak magnetic fields (0.1–0.8 T) on the internal friction and Young’s-modulus defect of LiF crystals is investigated over a range of relative strain amplitudes ɛ ₀ from 10⁻⁶ to 10⁻⁴ at frequencies of 40 and 80 kHz. Experiments with these fields show that the internal friction increases and the effective elastic modulus decreases, indicating an increase in the plasticity of the samples. Plots are obtained of the internal friction versus the magnitude of the magnetic field at various values of the strain amplitude ɛ ₀. Fiz. Tverd. Tela (St. Petersburg) 41, 1035–1040 (June 1999)     

Optical properties of BiFeO<Subscript>3</Subscript> ceramics in the frequency range 0.3–30.0 THz

Reflection and transmission infrared spectra of BiFeO₃ ceramic samples have been measured using submillimeter spectroscopy (on a backward-wave tube spectrometer) and Fourier-transform infrared spectroscopy in the frequency range from 5 to 1000 cm⁻¹ at temperatures in the range from 10 to 500 K. New resonant modes (probably, magnetic in nature) with the eigenfrequencies decreasing with an increase in the temperature have been recorded in the range 10–30 cm⁻¹ by IR spectroscopy for the first time. An additional absorption with a fairly large dielectric contribution has been revealed in the range 30–60 cm⁻¹. It has been demonstrated that the corresponding oscillators couple with both the lowest frequency phonon mode and the magnetic subsystem.     

Mechanism of field emission from carbon materials

Field emission in diamond and graphite-like polycrystalline films is investigated experimentally. It is shown that the emission efficiency increases as the nondiamond carbon phase increases; for graphite-like films the threshold electric field is less than 1.5 V/μm, and at 4 V/μm the emission current reaches 1 mA/cm², while the density of emission centers exceeds 10⁶ cm⁻². A general mechanism explaining the phenomenon of electron field emission from materials containing graphite-like carbon is proposed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 56–60 (10 July 1998)     

Magnetically stabilized electron-hole liquid in indium antimonide

Luminescence spectra of sufficiently pure n-type indium antimonide crystals (N _D−N _A=(1–22)·10¹⁴ cm⁻³) in a magnetic field of up to 56 kOe, at temperatures of 1.8–2 K, and high optical pumping densities (more than 100 W/cm²) have been studied. More evidence of the existence of electron-hole liquid stabilized by magnetic field has been obtained, and its basic thermodynamic parameters as functions of magnetic field have been measured. When the magnetic field increases from 23 to 55.2 kOe, the liquid density increases from 3.2·10¹⁵ to 6.7·10¹⁵ cm⁻³, the binding energy per electron-hole pair rises from 3.0 to 5.2 meV, and the binding energy with respect to the ground exciton level (work function of an exciton in the liquid) rises from 0.43 to 1.2 meV. Zh. éksp. Teor. Fiz. 111, 737–758 (February 1997)     

Transformation of beams of dia-, para-, and ferromagnetic particles by the magnetic field of a linear current

Some possibilities of transformation and trapping of beams of small (10⁻¹–10⁻⁴ cm) dia-, para-, and ferromagnetic particles by the magnetic field of a linear, constant current are investigated. Zh. Tekh. Fiz. 67, 138–141 (August 1997)     

Investigation of acceptor centers in semiconductors with the diamond crystal structure by the μ − SR method

The residual polarization of negative muons in crystal silicon samples with phosphorus (P: 1.6×10¹³ cm⁻³) and antimony (Sb: 2×10¹⁸ cm⁻³) impurities is investigated. The measurements are made in a 1000 G magnetic field oriented in a direction transverse to the muon spin in the temperature range 4–300 K. The relaxation rate and shift of the precession frequency in the silicon sample with the phosphorus impurity are measured more accurately than previously. It is found that in antimony-doped silicon the acceptor center _μ A1 at temperatures below 30 K can be in both ionized and neutral states. The experimental data are interpreted on the basis of spin-lattice relaxation of the magnetic moment of an acceptor center, formation of acceptor-donor pairs, and recombination of charge carriers at the acceptor. Preliminary measurements showed a nonzero residual polarization of negative muons in germanium. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 61–66 (10 July 1998)