MnF2 Crystals Research Articles

A special TEM Li-ion battery sample preparation and application technique for investigating the nano structural properties of the SEI in lithium ion batteries

AbstractHerein we describe a technique for preparing samples from cycled Li-ion batteries with minimal contact with atmospheric water for examination by TEM and to provide the results of an analysis of the SEI in Li-ion cells. The electrode samples were prepared in a glove box by manipulation with a diamond tipped pen to carefully dislodge particles directly onto the TEM sample holder. Electrodes were extracted from Li-ion cells that contained a cathode of high capacity, manganese rich NCM (HCMR-NCM). Nanometer-sized MnF2 crystal particles embed themselves in the SEI layer of the anodes as observed and confirmed by HRTEM lattice image analysis and EELS. Cross-sections of the SEI layer reveal that the thickness of the SEI and propensity for MnF2 crystal deposition is non-uniform.

Magnetic linear dichroism of an antiferromagnetic MnF2 crystal: Oddness in two different antiferromagnetic states

Dichroism of linearly polarized light propagating along the tetragonal C4 axis, induced by a longitudinal magnetic field, has been revealed in a tetragonal antiferromagnetic manganese fluoride crystal in the region of optical transitions. The magnetic linear dichroism observed is proportional to the magnetic field strength and odd with respect to the field sign. The value of the effect is sufficient for optical observation of antiferromagnetic domains in MnF2 crystals.

ダイヤモンドアンビルで見る高圧物理の世界

Various physical phenomena which can be easily observed in diamond anvil are reviewed. The phenomena include crystal growth and phase transformation in H2O ice, growth and the formation of negative crystal in CCl4 ice, index matching of MnF2 crystal and alcohol under pressure, and the color change in AgI associated with the phase transformations. All these phenomena can be observed easily below 2 GPa and can be used as a good example to teach physics.

Exciton luminescence in orthorhombic-structure MnF2 epitaxial films

The low-temperature photoluminescence (PL) of 100 to 300-nm thick MnF2 epitaxial films of the α-PbO2-type orthorhombic structure was studied. The PL spectrum consists mainly of a broad band peaking around 575 nm and a slowly decaying long-wavelength wing. The short-wavelength part of the main band revealed relatively weak spectral features, which are due to magnon replicas of the Mn2+ excitonic line perturbed by Mg and Ca impurities. These features were found to shift toward shorter wavelengths by 12 nm relative to their position in bulk MnF2 crystals. The shift can be accounted for by a change in the crystal field acting on the Mn2+ ions in the orthorhombic phase.

Magnetic polaritons in XF2 (X=Mn, Fe) canted antiferromagnets

We discuss the magnetic polariton modes which propagate in a two-sublattice, uniaxial Heisenberg antiferromagnet, for the case where an external magnetic field is applied perpendicular to the easy axis of the material. At high magnetic fields, a large magnetization component normal to the easy axis will be induced, and we may expect to find a magnetic field induced surface polariton mode, which propagates parallel to the easy axis, with nonreciprocal propagation characteristics. We confine our discussion to antiferromagnets, which have a body-centered tetragonal structure (e.g., XF2, with X=Mn and Fe). The spectra show that the nonreciprocal propagation of the surface polariton is much more evident in MnF2 crystals.

Spin wave solitons in an antiferromagnetic film (abstract)

The propagation of nonlinear dipole spin waves in a film consisting of a two sublattice, uniaxial, antiferromagnetic material has been investigated. The system when the external magnetic field is assumed to be parallel to the anisotropy axis of the antiferromagnetic film and is directed parallel, or perpendicular, to the film surface is considered. For the first case, surface and volume waves can propagate in the film, and for the second case, volume waves can propagate in the film for a rather weak external magnetic field when the magnetization of the sublattices are counter parallel to each other and are perpendicular to the film surface. The linear dispersion relations for all three types of waves are analyzed and their group velocity dispersion is calculated. The nonlinear shift of the frequency, due to the finite power of the wave, is also obtained for the three types of dipole waves for the case of a thin antiferromagnetic film, when kd≪1 (k is the wave number and d is the thickness of the film). The nonlinear Schrödinger equation governing the propagation of the nonlinear spin wave in the film is investigated. It is shown that the criterion for the existence of spin wave solitons is fulfilled, for parallel magnetization, which permits the existence of surface waves (one branch) and volume waves (both branches), but the criterion is not fulfilled for perpendicular magnetization. The power threshold for soliton creation is also calculated and estimates are given for the data appropriate to a MnF2 crystal.

Dielectric relaxation and computer simulation studies of rutile-structured MnF2 crystals

The fluorides of the rutile structure are relatively simple ionic materials with tetragonal symmetry for which the dominant intrinsic defect has not been established. The present experiments involve low-temperature dielectric relaxation measurements on Er3+-and Y3+-doped MnF2 single crystals. Unexpectedly, dielectric loss peaks were observed at cryogenic temperatures, involving very low activation energies,E. For both dopants a prominent peak is observed for samples oriented parallel to thec-axis withE ∼ 6 meV and in perpendicular orientations withE=37 meV for Er3+ and 46 meV for Y3+ doping. Such lowE-values are probably too small to be controlled by lattice migration of a defect. Rather, we expect that they are due to a very low symmetry configuration created when the ions near the defect move “off symmetry” to a more stable configuration. Computer simulation calculations have been carried out which are much improved over early studies of this system in terms of the code used and the F-F interatomic potentials. The results show that the energy per defect for the anion Frenkel (1.53 eV) is lower than that of the Schottky (1.99 eV). It was also shown that the fluorine interstitial, Fi, adopts a split-interstitial form. This defect associates strongly with trivalent dopants Er and Y to produce a low symmetry dipolar structure with the necessary off-symmetry configuration to explain the experimental findings. Since there is no alternative way to explain these low temperature relaxations in terms of impurities associated with Mn vacancies, as would be required by the Schottky model, we conclude that these experiments serve to establish the nature of the intrinsic defect in MnF2 as anion Frenkel.

PIEZOMAGNETISM AND DOMAINS IN MnF2

Neutron topographic experiments show that the simultaneous application of an uniaxial stress and a magnetic field when cooling a MnF2 crystal through TN favours one type of 180° antiferromagnetic domain. This appears to be related to the piezomagnetism of MnF2, in spite of the weakness of this effect, which was measured in our case by a SQUID.

Theoretical model of the Raman intensities of localized and magnon modes in CoxMn1−xF2

We analyze two excited modes due to the presence of Co++ ions embedded as impurities in MnF2. The Raman scattering is not observed in the pure crystal of MnF2 for there is no spin-orbit coupling in the ground state of the Mn++ ion. But the magnon modes become activated by the presence of Co++ ions and the intensity of these magnetic excitations is of the same order of magnitude as that of the localized ones. We calculate separately the scattered intensities of those two types of modes. They depend on the amplitude of the two excited wave functions on the impurity sites. The wave functions are calculated with the help of the Green function formalism and using the Lippman–Schwinger equation. (Here it is not necessary to know the density of magnon states.) Our main assumptions are that the energy level of the localized mode is near the edge of the magnon band and that the concentration of impurities is low enough to treat them as separated scattering centers. So, after explicit calculations of the Green function of the pure crystal we find that the variation of the spin-wave intensity is quadratic in the concentration whereas it is linear for the localized modes. Thus our model yields their ratio in terms of the concentration, the two exchange integrals, the two effective spins, and the distance in energy between the localized and magnon modes, relative to the magnon bandwidth.

Temperature dependence of the linewidth of the first-order Raman spectra for MnF2 crystal

The first-order Raman spectra ofE gandA 1g modes in MnF2 crystal were measured at temperatures from 4.2 to 563 K, and the values of the linewidths obtained. The temperature dependence of the linewidths was analyzed by the phonon dispersion curves based on the rigid ion model, and the result showed that it was caused approximately by the cubic anharmonic term in crystal potential energy.

Critical behavior in random anisotropic antiferromagnets

The role of random uniaxial anisotropy on critical behavior has been studied in the antiferromagnetic system FexMn1−xF2 using linear magnetic birefringence. In the pure FeF2 and MnF2 crystals the deduced magnetic specific heat near TN shows no rounding for reduced temperature t as small as 10−4. Except for the effects of concentration gradients, no rounding is observed in the mixed crystals. In fact, for the Fe.75 Mn.25F2 sample, where such gradient effects are minimal, the transition is as sharp as in pure FeF2. Since MnF2 and FeF2 differ primarily in the size of the single ion anisotropy, it is clear that random anisotropy, by itself, does not broaden the transition. If anisotropy were not considered, MnF2 and FeF2 would be expected to have nearly identical TN. Although MnF2 is itself asymptotically Ising-like as T→TN because of its small but finite anisotropy, the larger TN in the more anisotropic FeF2 and the drastic increase in TN with x near x = 0 is attributable to the effects of crossover from Heisenberg to Ising critical behavior.

Abstract: Observation of the spin-flop transition of MnF2 by neutron diffraction

In a polarized neutron experiment, the diffraction lines of the basal plane of a single crystal of MnF2 were studied as a function of an external field rather well aligned (0.2°) with the c-axis of the crystal; above 92.7 Koe (at 4.2K) the magnetic response of the normal lattice lines increases suddenly as for an induced magnetization of ∼0.4 μB/Mn. In the spin flop state the polarization dependence of the intensities of the superlattice lines indicates that the magnetic moments lie largely in the basal plane. (AIP)

Critical scattering from a disordered antiferromagnet: Mn/ZnF2

Neutron scattering measurements have been undertaken of the temperature dependence of the critical scattering from a disordered antiferromagnet. The sample-Mn/ZnF2-was a single crystal of MnF2 in which 32% of the (magnetic) manganese ions were randomly replaced by (nonmagnetic) zinc ions. The transition temperature of the mixed crystal was TN=38.7K or about 58% of that of pure MnF2. The experimental results for the inverse correlation length and susceptibility in Mn/ZnF2 cannot be equated with simple power-law fits in the reduced temperature. However, if it is assumed that there is a 1% concentration variation throughout the sample, the resulting exponents are consistent with those found in pure MnF2 and with theoretical predictions. Unfortunately, without independent corroboration of a concentration gradient, it is at present impossible to distinguish the system from one in which it is the actual disorder which smears the transition.

Magnetic excitations in antiferromagnetic Mn1-cZncF2at large Zn concentrations

Neutron inelastic scattering techniques have been used to study Zn-substituted MnF2 crystals with average Zn concentrations c=0.22+or-0.02 and 0.7+or-0.1. For c=0.22 the results gave scattered distributions whose peaks were at frequencies less than those of the excitations of pure MnF2 and whose width was considerably larger than the experimental resolution. The other specimen had a concentration gradient such that one end of the crystal ordered magnetically at 4.2K whereas the other end did not. There were no significant differences between the scattering from the different ends, and it consisted of an overdamped distribution the width of which decreased as the wavevector of the excitation decreased.

Precision Thermometer for the Region 10–40 K

A low temperature thermometer based on the temperature dependence of the vF(T) of the 19F NMR frequency in antiferromagnetic MnF2 has been successfully operated in the 10–40 K temperature region. Novel features include the use of a MnF2 crystal with a sharp 19F NMR line as the temperature sensitive element in a solid state frequency-locking autodyne circuit that operates while completely submerged in liquid H2 or He. In the present design the circuit-sample combination takes the form of a compact probe. With this device temperature readings which are reproducible to ±10−4 K at 20 K have been obtained. Various applications for precision thermometry in the 10–40 K range using the device are proposed.

Magnetic Scattering of Neutrons in MnF2 near the Critical Point

The angular distribution of slow neutrons (λ=4.34 Å) scattered from a single crystal of MnF2 has been studied as a function of temperature in the vicinity of the Néel point, about the (100) and the (001) reciprocal lattice points. The (001) data have been analyzed to obtain the values of χs⊥, the transverse staggered susceptibility, and its variation with temperature above the transition point has been examined in the light of the theories of Moriya and Tomita. Moriya's theory shows that χs⊥ is proportional to (T−T⊥)−1 where (TN−T⊥) is calculated by him to be 1.36°K. To give a good fit to our values, an expression of the type (T−T⊥)−γ has been used for χs⊥ with Moriya's value for (TN−T⊥), and γ is found to be 0.85. A much better fit is obtained by using the expression for χs⊥ suggested by Tomita, namely, C{A + B(T − TN)γ}−1, for A=0.016 and γ=1.22. The longitudinal staggered susceptibility, χs∥, has been deduced from the measurements around the (100) point. Its temperature dependence above TN is found to be of the form (T−TN)−1, and is not consistent with a four-thirds power law.

Observation of a Localized Magnon in Co-Doped MnF2

Measurements have been made with neutron scattering techniques of the spin-wave frequencies at 4.2°K in a doped single crystal of MnF2 containing 5% of substituted Co impurity. The triple-axis crystal spectrometer at the C5 facility of the NRU reactor was used throughout in its constant-Q mode of operation. The low-frequency acoustic spin-wave branches for the [110] and [001] directions agree closely with the measured dispersion for pure MnF2 and are largely unaffected by the presence of Co impurities. Additional scattering has been observed at a frequency of 3.57×1012 sec−1, which appears to be independent of the momentum transfer. This scattering is not observed above TN at 110°K and no mode of this frequency has been observed in pure MnF2. This additional scattering is believed to result from a localized spin-wave mode associated with the Co impurities. The frequency agrees closely with calculations of the energy of the first excited state of Co in MnF2 in the molecular-field approximation.

Temperature dependence of the width of the optical absorption bands in MnF2 crystals

Temperature dependence of the width of the optical absorption bands in MnF2 crystals

A Measurement of Spin-Wave Dispersion in MnF2 at 4.2°K

Antiferromagnetic spin waves have been observed in a single crystal of MnF2 at 4.2°K by the inelastic scattering of neutrons. Measurements were carried out using a twin rotor time-of-flight spectrometer having a mean incident neutron wavelength of 3.00 Å and a resolution (half-width at half-height) of 0.06 Å. Spin waves were excited in the {010} plane of the crystal and the dispersion plotted for propagation in the 〈100〉 and 〈001〉 directions. The observed dispersion curves can be accurately described by spin-wave theory using the following values for the parameters concerned: J1=0.32°K (ferromagnetic), J2=−1.76°K (antiferromagnetic), |J3|<0.05°K, and an anisotropy field at zero wave vector HA=1.06°K, where J1 is the exchange coupling between nearest neighbours (in the 〈001〉 direction), J2 acts between next nearest neighbours (〈111〉 directions), and J3 refers to third-neighbour coupling (along the 〈100〉 and 〈010〉 directions). A small effect arising from the wave vector dependence of HA has been calculated and taken into account in the evaluation of these parameters. The values quoted are in excellent agreement with those predicted from measurements of perpendicular susceptibility, spin-wave resonance, paramagnetic resonance in (Mn, Zn)F2, and from paramagnetic neutron scattering at room temperature.