Mn Single Crystals Research Articles

Dynamic strain ageing and the boundaries of stable plastic deformation inCu-Mn single crystals

Abstract The boundaries of stable plastic deformation, characterized by critical stresses τi(T), are investigated for single crystals of Cu-2 at.% Mn and compared with those of polycrystals. The single crystals were oriented along [100], i.e. for symmetric multiple glide, or for single glide. Strain rate sensitivity was measured between about 300 K and 500 K and for different basic strain rates to deduce activation parameters. The main result is that the form of τi(T) for the [100]-crystals is very similar to that for polycrystals in all respects, whereas it differs appreciably from that for the single-glide crystals. In all cases two different T intervals, labelled α and β, can be distinguished. The activation parameters obtained are the exponent n of the strain rate and the activation energy Ea of the diffusion process inducing jerky flow at τi. Whereas n always equals 1/3, we find for both the single-glide and the [100]-multiple-glide crystals that E α a=0.9eV in the interval α and E β a=1.6eV in the i...

On the superposition of the effects of dynamic strain ageing and a varying mobile dislocation density -strain rate change experiments on [formula omitted]Mn single crystals-

Investigations on dynamic strain ageing (DSA) in single glide oriented Cu 2 at% Mn crystals have demonstrated that the strain rate sensitivity (SRS) is influenced by DSA as well as by variations in mobile dislocation density, ϱ m , after strain rate changes, in general. There exists a transition temperature T tr : DSA-effects are dominating the SRS above T tr , ϱ m -effects below. For stage II of deformation a joint analysis of both effects has been developed allowing on the one hand a quantitative separation of both, on the other hand an analysis of DSA-processes at temperatures far below T tr .

Transition between planar and wavy slip in cyclically deformed short-range ordered alloys

Fatigue experiments in Ni20 at.%Cr and Cu17 at.%Mn single crystals are reported. Both alloys show short-range order but have different solid-solution hardening parameters. A transition from planar to wavy glide is observed in the late stages of fatigue life. In Cu17 at.%Mn extrusions occur. Transmission electron microscopy shows that this is due to the development of three-dimensional dislocation arrays from the planar arrays observed in earlier stages. A similar tendency is found for Ni20 at.%Cr, but the development of extrusions is only seen in the vicinity of cracks. The planarity of the glide and the transition from planar to wavy glide are discussed on the basis of short-range order and the parameters characterizing solid-solution hardening.

In situ HVEM-investigations of the early stages of recrystallization in Cu-0.2 at.% Mn-single crystals

The influence of foreign atoms on the early stages of recrystallization in Cu-0.2 at.% Mn-single crystals were investigated in situ in a high voltage electron microscope (HVEM). The stacking fault energy (SFE) of copper does not change with Mn addition. Along the grain boundaries the orientations of the recrystallized grains and of the deformation microstructure were determined to establish the local orientation relationships. The new orientations can be described by multiple twinning of the deformation microstructure with additional small angle grain boundaries. Frequently Σ13b- (27.8°〈111〉) and a Σ29b- (46.39° 〈221〉) orientation relationships were found. As is known from growth selection experiments these orientation relationships produce the final recrystallization texture. Because of the relatively high SFE and the large misfit between Cu and Mn these orientation relationships were formed rapidly after nucleation. Thus, the orientation formation process is finished after several μm of growth. The experimental results are discussed in terms of nucleation, twinning and growth of the recrystallized grains. An explanation is given why particular grain boundaries are mobile in Cu Mn.

Preparation and Structural Study of Metastable Mn Phase Grown on GaAs(OOl) Substrate

Single crystal Mn thin films have been successfully grown on GaAs(001) substrates for the first time by molecular beam epitaxy. The bulk atomic structure of the manganese films is determined to be the face-centered-cubic metastable phase with the lattice parameter of 0.362 nm, by in situ reflection high energy electron diffraction (RHEED) and ex situ x-ray diffraction (XRD).

Mn 0.9Co 0.1P in field parallel to hard direction: phase diagram and irreversibility of CONE phase

A single crystal of Mn 0.9Co 0.1P, a homologue of MnP with disordered metal sublattice, has been studied by the ac susceptibility method in a steady field H . This report concerns H parallel to the orthorhombic a axis ( a > b > c ). The magnetic phase diagram is qualitatively similar to that of MnP, including the presence of a Lifshitz multicritical point ( T L = 98 K, H L = 42 kOe) at the confluence of the paramagnetic, ferromagnetic and modulated FAN phases. Contrary to pure MnP, irreversible behaviour was observed in the susceptibility of the modulated CONE phase. This phenomenon develops only for fields above 30 kOe, in contrast to the irreversibility of the FAN phase (reported previously for H ‖ b in the whole field range down to H = 0). New features of the presumably continuous CONE-FAN transition were also found.

Deformation of single-crystal Mn 3Sn

Single-crystal Mn 3Sn was chosen to investigate the behavior of a DO 19 intermetallic compound. The alloy of composition 23.75 at.% Sn was made from the pure elemental materials and single crystals were grown using the Bridgman process. Compression samples were prepared using an electric discharge machine and tested at moderate strain rates in an air or argon environment, depending upon the test temperature. Testing temperatures ranged from 77 to 1200 K and various orientations within the standard unit triangle were chosen so as to produce slip on several slip systems. Preliminary results show that the dominant mode of deformation is basal slip accompanied by an anomalous flow stress vs. temperatures behavior. There was, however, also some prismatic and pyramidal slip observed. For most orientations slip traces were observed on the sample surfaces; however, for some orientations with low Schmid factors, local crushing or even brittle fracture were observed.

Growth of Ba1−x Sr x TiO3 and doped BaTiO3 single crystals by the method of floating zone in a thermal image furnace

Ba1−x Sr x TiO3 and BaTiO3∶Mn single crystals were grown in halogen lamp heated thermal image furnace. The method is very advantageous when quick evaluation of properties of a new crystal is needed, since the growth rates are rather high compared to most other methods. The other benefit of this procedure is the possibility to grow refractory crystals in oxygen with higher melting point than 1700 °C.

Structural study of epitaxial MN/AG multilayers

We have studied the epitaxial growth and structural properties of single crystal Mn/ Ag multilayers grown on (001) GaAs substrates using transmission electron microscopy. A buffer layer of Ag (001) was grown on a 5 monolayer Fe seed layer on the (001) GaAs substrate before the growth of the multilayers to obtain good quality films. Mn/Ag multilayers show very sharp and coherent interfaces for Mn layer thicknesses up to ≈ 14 monolayers. The structure of Mn in Mn/Ag multilayers depends on the thickness of the Mn layer. An fee precursor structure of Mn is proposed with a lattice parameter of 3.94Â and is expected to be antiferromagnetic.

Optical absorption and photochromic effect in Cr and Mn doped Bi12SiO20single crystals

Abstract The optical absorption spectra and photochromic effect of Bi12SiO20 pure and doped with Cr and Mn single crystals have been investigated in the wave-length range between 8000–26000 cm−1 at 80 and 300 K. It is shown that Cr and Mn ions are in several charge states in the doped crystals: Cr3+. Cr2+, Mn3+ and Mn4+ substituting of Bi3+ ions in the crystalline lattice. The photochromic effect investigations allow to suggest the mechanism of photogeneration of colour centers in doped crystals consisting in the charge transfer under the action of the light quanta according to the following scheme: Cr-2+ - e → Cr3+; Mn4+ + e → Mn3+.

The site and pseudodonor character of manganese in single-crystal lead telluride

From the analysis of electron paramagnetic resonance (EPR) spectra and the electrical characteristics of as-grown, long-period 'annealed' at room temperature and laser annealed PbTe:Mn single crystals with Mn content NMN approximately=2-7*1018 cm-3 the authors infer that Mn2+ ions in as-grown crystals are incorporated partly interstitially and partly in the metal sublattice sites. In crystals with NMn>or approximately=2*1019 cm-3 Mn ions tend to form clusters. In such cases manganese behaves as an amphoteric impurity, as a small amount of Mn ions in the metal sublattice points cannot compensate for the large number of Pb vacancies (NPb approximately 5*1019 cm-3) that act as acceptors. When creating the conditions that promote migration of Mn ions into PbTe metal sublattice points (long-period 'annealing' at T=300 K, IR laser annealing) which is tested by hyperfine structure and superhyperfine structure of EPR spectra, the Mn2+ ions act as pseudodonors, compensating for two holes from each lead vacancy. The electron concentration in such a case is attributed to Te vacancies, which are donors. The spin-Hamiltonian constants were determined at T=20 K for as-grown, long-period 'annealed' and laser annealed PbTe:Mn crystals.

Dislocation densities in [001]-orientated -Mn single crystals deformed in tension. Determination by X-ray diffraction line broadening

Dislocation densities in [001]-orientated -Mn single crystals deformed in tension. Determination by X-ray diffraction line broadening

Growth and magnetic characterization of Mn films and superlattices on Ag(001).

We have grown single-crystal Mn films and coherent (Mn/Ag) superlattices on Ag(001) and characterized the magnetic behavior with temperature-dependent ferromagnetic resonance and magnetic susceptibility measurements. We find that the Mn films exhibit a 2.89-\AA{} square surface net identical to that of Ag(001), and a tetragonal structure similar to that of quenched $\ensuremath{\gamma}$-Mn. The (Mn/Ag) superlattices exhibit a well-defined high-angle x-ray-diffraction satellite structure. We detect no significant ferromagnetic contribution at temperatures down to 5 K, and conclude that the Mn is antiferromagnetically ordered.

Modification of the top-seeded growth from high temperature solutions

A new modification of the top-seeded growth from high temperature solutions (HTS), allowing simultaneous application of the accelerated crucible rotation technique (ACRT), was developed. The platinum crucible containing the flux was equipped with the platinum reflectors bearing the holder of the seed. A particular interest was devoted to the optimization of the temperature distribution in the flux and to the ACRT program. The apparatus was tested by the growing of YIG: Mn single crystals on the seeds orientated in [111], [110] and [211] directions.

Optical properties of GaSe: Mn single crystals

To identify the manganese related defect levels in GaSe, GaSe:Mn single crystals were grown with various Mn dopant levels using the Bridgman technique and the photoconductivity and photoluminescence properties were investigated. Peaks introduced by the manganese related defects were observed at 1.916 and 1.724 eV in the photoconductivity spectra and at 1.804 eV in the photoluminescence spectra at 80 K. These results allow the calculation of the energies of the A 1 and A 2 centers at 0.348 and 0.156 eV, respectively, above the valence band and a donor level at 0.112 eV below the conduction band. Also, we find that the A 1 and A 2 centers are pinned within the conduction band from measurements of the temperature dependence of the photoconductivity spectra.

Precision measurements of critical resolved shear stress in CuMn alloys

The critical resolved shear stress τ 0( T) and the activation volume V( T) of Cu Mn single crystals were measured in the temperature range from 4 to 450 K for 2.0 ⩽ c ⩽ 7.6 at.% Mn. For T ⪢ 20 K the temperature dependence of τ 0 and V can be described consistently by the concept of thermally activated dislocation motion. Below about 20 K the curvature of τ 0( T) changes sign and approximates a finite value for T → 0 K. These deviations from ordinary thermally activated glide behaviour are discussed in terms of inertial effects, sample heating and quantum effects like tunneling and zero-point vibrations of dislocation segments. A satisfactory explanation of the new anomaly is possible only by the theory of zero-point vibrations of dislocations.

Anelasticity and relaxation effects in single crystal Mn ferrites

Young's modulus and acoustic loss are investigated in three single crystal Mn-ferrites in the 300 to 650 K temperature range. Anelastic behaviour near 500 K is found to be composed of two parts. The first part, strongly dependent on the external magnetic field, is present when acoustic longitudinal vibrations are applied parallel to any of the main crygtallographic axes 〈100〉, 〈110〉 and 〈111〉, respectively. The other part, independent of external magnetic field, is present only with the vibrations along 〈110〉 and 〈111〉 directions. A close analogy of the latter behaviour with the process III, observed in magnetic aftereffect spectra of Mn-ferrites, is found. Der Elastizitatsmodul und der mechanische Verlust wurden in drei Manganferrit-Einkristallen im Temperaturbereich von 300 bis 650 K gemessen. Um 500 K wird unelastisches Verhalten gefunden, welches zwei Effekten zugeschrieben wird. Bin Effekt, der direkt vom angelegten Magnetfeld abhangig ist, erscheint bei longitudinalen Schwingungen parallel zu den drei Kristallrichtungen 〈100〉, 〈110〉 und 〈111〉. Der andere Effekt der Anelastizitat, der unabhangig vom auseren Magnetfeld ist, ist nur bei Schwingungen in der 〈110〉- und 〈111〉-Richtung vorhanden. Er hat eine grose Ahnlichkeit mit dem magnetischen Desakkommodationsprozes III, der in polykristallinen Mn-Ferriten gefunden worden wird.

Analysis of powder and single-crystal electron paramagnetic resonance spectra for manganese(II) protoporphyrin IX myoglobin at various microwave frequencies

Powder and single-crystal electron paramagnetic resonance spectra for manganese(II protoporphyrin IX myoglobin (Mn 2+Mb) were measured at S-, X-, K-, and Q-band microwave frequencies to determine principal g values, hyperfine coupling constants, and zero-field splitting parameters. The effective EPR parameters for the lowest Kramers doublet were found to be g ‖ = 1.99, g ‖ = 5.90, A ‖ = 8.64 mT, and A ⊥ = 8.24 mT. Zerofield splitting between the lowest Kramers doublet ( M s = ± 1 2 ) and the second Kramers doublet ( M s = ± 3 2 ) was estimated to be 1.39 cm −1. The effective g tensor is approximately axially symmetric. Hyperfine coupling with the 55Mn nucleus was nearly isotropic, but also reflects axial symmetry. The orientation of manganese(II) protoporphyrin plane in single crystals of Mn 2+Mb is nearly identical to that for high-spin iron(III) myoglobin single crystals. The spin-Hamiltonian parameters for Mn 2+Mb are fully consistent with single-crystal or powder spectra obtained at all four microwave frequencies.

Recombination and trapping levels in GaS:Mn single crystals

Recombination and trapping levels in GaS:Mn single crystals

Investigation of thermally stimulated luminescence and its description by a tunneling model

Using a tunneling pair recombination model with the assumption of spatially distributed ‘‘ionized’’ pairs of luminescent centers and traps, a general and analytical expression of thermally stimulated luminescence (TSL) intensity is derived. The Hoogenstraaten and Bube methods of determining the activation energy are still valid in the tunneling model, except they determine the activation energy of the trap tunneling state. Based on explicit expressions for TSL, it is shown that the study of TSL shape yields information on the dependence of the pair distribution on phosphor excitation conditions. Experimental results on Zn2SiO4:Mn single crystals with or without arsenic codoping are presented.