Negative Exchange Interaction Research Articles

Site occupancies of Si atoms and Curie temperatures for Sm2Fe17-xSix.

The system ${\mathrm{Sm}}_{2}$${\mathrm{Fe}}_{17\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Si}}_{\mathit{x}}$ (x=0, 0.5, 1, 2, and 3) has been studied by $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy. The distributions of Fe and Si atoms over the four sites have been deduced by convolution of the M\ossbauer spectra. The results show that Si atoms are excluded from the 9d site and prefer the 18f sites at all concentrations; Fe atoms preferentially occupy the 6c and 18h sites at low Si concentrations, but at x=3 the occupancy of the Si atoms increases rapidly. The Curie temperatures increase at first, then decrease with Si substitution; a maximum occurs at a Si concentration of x=2. The Curie temperature results are interpreted by a model that considers the competition between the positive and negative exchange interactions.

Lineshape Analysis of Intersubband Transitions in Multiple Quantum Wells

AbstractConduction intersubband transitions between the ground and first excited states in Al0.3Ga0.7As/GaAs multiple quantum wells (MQWs) are studied as a function of the twodimensional electron gas density (0.75 × 1012 ≤ σ ≤ 3.75 × 1012 cm−2) and temperature (5 ≤ T ≤ 300 K). There is no electron tunneling between the wells and well regions are uniformly doped with silicon donors. Theoretically, we have solved the Schrödinger equation containing the self-consistent Hartree potential, in which the z-dependencies of both electron effective mass and dielectric constant, as well as the non-parabolicity in the conduction energy subband dispersion have been taken into consideration. By applying many-body theory which includes the depolarization-shift from a collective dipole moment and the excitonicshift from the negative exchange interaction, we calculate the absorption spectrum as a function of the incident photon energy hw for different values of T and σ. From this, we can quantitatively analyze both T- and σ-dependencies of the peak position and the full width at half-maximum (FWHM) of peak values. The blue-shift or red-shift in the absorption peak position are quantitatively reproduced as either T or σ is reduced. The exchange interaction which depends on σ, will modify the energy subband dispersion. Therefore, the absorption peak will be broadened by the exchange interaction. The T-dependence of broadening from the optical-phonon scattering is also taken into account by a phenomenological model. From the calculated absorption spectrum as a function of o, we have successfully reproduced and explained the σ-dependence of FWHM measured in recent experiments.

57Fe Mössbauer study of Y2Fe16M compounds and their nitrides with M = Ga, V

Mossbauer effect measurements on the substitutional solid solutions Y2Fe16M (M = Ga, V) and their nitrides are carried out. The preferential substitution of Fe by Ga and V in the 12k site leads to a diminution of the negative exchange interactions and an increase of the site hyperfine fields. After nitrogenation a greater hyperfine field increase is attributed to the lengthening of FeFe bonds.

Magnetic properties of 1,5-dimethylverdazyl radicals. I. Evidence for the ferromagnetic exchange interaction in 3-(4-chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl radical solid

Magnetic susceptibilities of 3-(4-chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl ( p-CDTV) and 3-(4-bromophenyl)-6-thioxoverdazyl ( p-BDTV) radicals have been measured in the temperature range between 4.0 and 300 K. The susceptibility of p-CDTV follows the Curie—Weiss law with a Curie constant of 0.381 K emu/mol and a positive Weiss constant of θ = + 3.2 K, suggesting intermolecular ferromagnetic exchange interaction between neighboring radicals. On the other hand, the magnetic susceptibility of p-BDTV exhibits a broad maximum at 26.5 K. This susceptibility can be explained by the one-dimensional Heisenberg model with negative exchange interaction of 2 J/ k= −41.3 K.

Mössbauer investigation of solid solutions Y 2Fe 16M, with M = Mo, Ti and their Y 2Fe 16MN y, interstitials

Mössbauer parameters of Y 2Fe 17, its solid solutions Y 2Fe 16M (M = Mo, Ti) and their nitrides Y 2Fe 16MN y are reported. The increase of site hyperfine fields in the substitutional compounds Y 2Fe 16Mo, Y 2Fe 16Ti is related to the preferential substitution of Fe into the 12k sites, hence to the diminution of negative exchange interactions. A site-dependent enhancement of the ferromagnetic coupling, in the interstitial compounds Y 2Fe 16MoN y , Y 2Fe 16TiN y is evidenced, the largest increase of the hyperfine field corresponding to 4f sites, which remain fully occupied by iron.

Magnetoresistance and upper critical fields in single crystal Y1−xPrxBa2Cu3O7−y

Abstract The magnetoresistance and upper critical fields of superconducting single crystal Y 1− x Pr x Ba 2 Cu 3 O 7− y alloy system with varying Pr-content have been measured. The crystals showed a remarkable narrowing of superconducting width and positive curvature in upper critical fields. The results were interpreted in terms of the negative exchange interaction between the Pr 4ƒ electrons and superconducting charge carriers.

High field magnetization of single-crystal γ-phase hydrides HoCo 3H 4.3 and ErCo 3H 4.2

Magnetization measurements have been performed for single-crystal γ-phase hydrides ErCo 3H 4.2 and HoCo 3H 4.3 in high magnetic fields up to 40 T. Both the hydrides become antiferromagnetic with a Néel temperature of about 200 K due to the appearance of the negative Co-Co intersite exchange interaction after hydrogenation. As a result of the significant weakening of the R-Co intersublattice exchange interaction, the R and Co sublattices behave quite independently: With increasing magnetic field along the c-axis, the antiferromagnetic ordering of the R sublattice breaks at first and then the metamagnetic transition occurs at a higher field. After the transition the spin arrangement of R and Co becomes almost aligned. This transition field decreases with increasing temperature and becomes zero at the Néel temperature.

Radical pair mechanism of CIDEP generation. Contribution of the dipole—dipole interaction

The effect of the electron dipole—dipole interaction (DDI) in multiplet chemically induced dynamic electron polarization (CIDEP) generated in radical pair (RP) recombination is investigated. It is demonstrated within some models that DDI results in multiplet CIDEP like exchange interaction (EI) but of the sign opposite to that of EI induced one, corresponding to negative EI. It is shown that in principle DDI effect can be responsible for change of the sign of multiplet CIDEP at large times recently observed for a number of RPs.

Effects of structure on exchange interactions in crystalline and amorphous alloys Gd xY 50−xAg 50

Magnetic interactions and effects of dilution with nonmagnetic Y on the magnetic properties of crystalline and amorphous alloys Gd x Y 50− x Ag 50 (10 ≤ x ≤ 50) have been investigated by measurements of bulk magnetization and susceptibility and by Mössbauer spectroscopy with 155Gd. The crystalline alloys order antiferromagnetically for all Gd concentrations with a noncollinear arrangement of Gd moments induced by negative biquadratic exchange interactions. In amorphous alloys, ferromagnetic order is found for large Gd concentrations ( x ≥ 40). Below the critical concentration x cr, in the range 30< x cr<40, properties typical for magnetic cluster glasses are observed. Magnetic hyperfine fields B hf at 155Gd nuclei vary with x in opposite directions in amorphous and in crystalline alloys. In crystalline alloys, the variation is due to a positive transferred hyperfine field. In amorphous alloys, a reduction of | B hf| with decreasing Gd concentration is caused by a reduction of the frozen Gd moments in the cluster glass phase.

On narrowing of the resistive superconducting transition in applied magnetic fields and positive curvature in Hc2( T) in the (Gd 1− xPr x)Ba 2Cu 3O 7−δ system

The superconducting transition temperature T c and the temperature dependence of the upper critical field H c2 were measured resistively on polycrystalline samples in the (Gd 1− x Pr x )Ba 2Cu 3O 7−δ system. The experimental results reveal that T c decreases monotonically with increasing x and vanishes at x≈0.49. Whereas H c2 of a sample with x=0.10 has an almost linear temperature dependence and the superconducting transition broadens with increasing magnetic field, a sample with x=0.45 shows strong positive curvature in the H c2 versus temperature curve and narrowing of the superconducting transition width with increasing magnetic field. Both phenomena observed in the sample with x=0.45 can be explained qualitatively in terms of a negative exchange interaction between the superconducting charge carriers and the Pr 4f electrons.

57Fe Mössbauer studies of Nd2Fe14B and Nd2Fe12.5Si1.5B (T=77-300 K).

$^{57}\mathrm{Fe}$ M\ossbauer effects of ${\mathrm{Nd}}_{2}$${\mathrm{Fe}}_{14}$B and ${\mathrm{Nd}}_{2}$${\mathrm{Fe}}_{12.5}$${\mathrm{Si}}_{1.5}$B have been studied in a temperature range of 77--300 K. The distribution of silicon atoms over the six sites and the dependences of ${\mathit{H}}_{\mathrm{hf}}$, \ensuremath{\Delta}, and \ensuremath{\delta} on the temperature for the $^{57}\mathrm{Fe}$ isotope have been deduced by fitting the M\ossbauer spectra. The results show that about 80% of the silicon atoms occupy the ${\mathit{k}}_{2}$ site in ${\mathrm{Nd}}_{2}$${\mathrm{Fe}}_{12.5}$${\mathrm{Si}}_{1.5}$B. Meanwhile, silicon substitution for iron raises the Curie temperature and reduces the temperature coefficients of the hyperfine fields at the six iron sites, especially the ${\mathit{j}}_{1}$ site. The results are interpreted in terms of a decrease of the magnitude of the negative exchange interaction between the ${\mathit{j}}_{1}$-${\mathit{k}}_{2}$ atoms.

Magnetic oscillations and many-body effects in cerium compounds

A review is presented of the results of the de Haas-van Alphen measurements in the cerium based intermetallic compounds CeSb and CeB 6. A comparison with similar experiments in the isostructural lanthanum analogues and with band structure calculations indicates, that in both compounds the 4f states are localized. CeSb is an example of a semi-metal which exhibits a delicate balance between positive and negative exchange interactions between Ce ions. CeB 6 is a typical dense Kondo lattice with strongly enhanced quasiparticle masses. The heavy-fermion mass is found to be strongly magnetic field dependent. The field dependence of the cyclotron mass is consistent with specific heat results.

On the Effects of Magnetic Bonding in Rare Earth Transition Metal Interxmetallics

AbstractNeutron diffraction experiments on rare-earth transition metal magnetic alloys Er2Fe14B and Er2Fe17 have been carried out at temperatures above and below the ordering temperature (Tc). An anomalously large magnetic moment is observed at the crystallographic j2 site in Er2Fe14 B which is the intersection point of the major ligand lines in the crystal structure. The interatomic Fe-Fe distances are in the range of strong ferromagnetic bonds (≥2.66 Å). The analogous f site in Er2Fe17 does not develop as large a magnetic moment. In addition, the same sites show strong preference for Fe atoms in the respective substituted compounds. Due to poor phase stability of Er2 (CoxFe1-x)14 B compounds, iron substitution has been studied in detail in Er2(CoxFe1-x)17 alloys for site specific order and lattice distortion effects. However, a nonlinear change in the c lattice parameter observed in the neutron diffraction results cannot be explained on the basis of site preference alone.The neutron refinement results indicate iron rich compositions in Er2(CoxFe1-x)17 materials, which is related to random substitution of Fe dumbbell pairs on the rare earth sites in the lattice. However, extensive electron microscopy (selected area electron diffraction and high resolution imaging) of Er2Fe17 and Er2 (Co .40Fe.60)17 failed to reveal any microscopic inhomogeneity. At this stage, the concept of direct negative exchange interaction between dumbbell Fe atom pairs at very short distances of −2.38 A is invoked and its effect is correlated with phase stability. We further suggest that the addition of boron in Er2Fe14B suppresses the substitution of dumbbell Fe pairs as a result of which Tc is raised significantly.

Magnetic properties of amorphous Gd 4Fe 3 and Y 4Fe 3 alloys

Amorphous alloys with non-S state rare earth ions (Gd 4Fe 3 and Y 4Fe 3) are prepared as ribbons. The amorphism is proved by X-ray diffraction and differential scanning calorimetry. The crystallization of the alloys was studied using a variable heating rate and the results were analysed by Kissinger's method. The phases formed during the crystallization process were identified by means of X-ray diffraction and thermomagnetic measurements. The magnetic properties were investigated from 4.2 to 400 K in steady fields of up to 140 kOe. A spin-glass-like behaviour was observed for the amorphous Y 4Fe 3 alloy and this was due to the presence of both positive and negative exchange interactions. The observed reduction in the iron magnetic moment was discussed in terms of the environmental model of Jaccarino and Walker. The magnetic properties of the alloys investigated were compared with those of similar rare earth alloys containing cobalt instead of iron.

Possibility of negative exchange interaction effect in RE-Ba-Cu oxides

Abstract Superconducting transition of (RE) x Ba 1- x CuO y (RE: rare earth or Y; 0.25≦ x ≦0.40) has been studied in magnetic fields up to 6T. Two-step transitions due to higher- T c and lower- T c phases have been observed, depending on the RE ion radius. Anomalous hysteresis of the resistivity in the field sweep has been found in the transition region of the lower- T c phase. This is explained by assuming the exchange interaction between localized Cu spins and conduction electrons to be negative and taking account of the Jaccarino-Peter effect.

The dynamics of spiral magnetic structures near the phase transition

The excitation spectrum of magnetic spiral structures has been investigated in two cases: when the modulation of the initial ferromagnetic structure is caused by the competition of the negative and positive exchange interactions (“exchange spiral”) and by the anisotropic interactions (“relativistic spiral”). The distinction between these two cases is displayed in the form of the Ginzburg-Landau functional for the magnetic energy of a crystal. This functional contains the linear gradient invariants in the case of a relativistic spiral, and does not contain them in the case of an exchange spiral. The excitation spectrum has been studied near the phase transition, where the order parameter dynamics includes the local magnetic moment precession and the oscillation of its modulus. The damping is introduced phenomenologically with the help of the dissipation function, which contains the terms conserving and nonconserving the total spin of the system. Great attention has been paid to the study of the Goldstong mode (phasons) with the wave vector equal to the wave vector of the magnetic structure. The results are in accordance with the hydrodynamical theory of the spin waves. The system response has been studied with the help of the classical retarded Green's function, describing the correlation of order parameter fluctuation. The possibility of studying the Goldstone modes by the method of the inelastic scattering of neutrons and light has been investigated.

A neutron diffraction study of the GdA10 3 magnetic structure

Neutron diffraction experiments have permitted to determine the magnetic order in the GdA10 3 perovskite compound. GdA10 3 is a simple two-sublattice antiferromagnet well described by the Gx-mode. This ordering results from isotropic and negative next neighbours exchange interactions which are more important than dipolar interactions.

Amorphous Heusler alloys: The effect of annealing on structure and magnetic and transport properties

Alloys with the Heusler compositions Cu 2MnIn, Cu 2MnAl and Cu 2MnSn have been prepared as amorphous films for the first time. The structural disorder results in as-deposited films which are not ferromagnetic and, in fact, exhibit spin-glass properties due to the distribution of MnMn distances and co-existence of positive and negative exchange interactions. Annealing of the In and Al alloys can restore single phase ordered Heusler structures which are ferromagnets with Curie temperatures typical of bulk alloys. The calculated Mn moments remain lower than the bulk values of 4 μ B probably due to disorder at grain boundaries. Films annealed under non-optimum conditions crystallized into several non-magnetic structures as well as the magnetic L2 1 Heusler phase. Lattice constants of all phases were determined. The temperature dependence of resistivity through the amorphous-crystalline transformation was found to confirm the results of thermal annealing on structure and magnetization.

Magnetic properties and structure of some ternary chromium chalcogenides with thallium and silver

The structure and magnetic properties of TlCr X 2 ( X= S, Se, Te), Tl 3CrS 3, Tl 3Cr 3S 5, TlCr 3S 5, TlCl 5S 8 and AgCrO 2 have been investigated. TlCrS 2, TlCrSe 2 and Tl 3CrS 3 order ferromagnetically with Curie temperatures in the range 60–140 K. The other compounds are antiferromagnetics with negative paramagnetic Curie temperatures, whereas TlCrTe 2 is antiferromagnetic with a positive paramagnetic Curie temperature. In the case of the layered hexagonal TlCrS 2 and TlCrSe 2 owing to the great radius of the Tl-ion and the large Cr-Cr-distance the intralayer exchange interaction is positive giving rise to the ferromagnetic order. For AgCrO 2 the smaller intralayer Cr-Cr-distance gives rise to negative exchange interaction and antiferromagnetism.

Invar properties in the rare earth-3d transition metal alloys

The RCo 2 and R 2Fe 17 compounds (R = rare earth) exhibit Invar-like thermal expansion anomalies below their ordering temperatures. These spontaneous volume magnetostrictions are discussed by considering their magnetic properties. In RCo 2 compounds there is no intrisinc Co-moment is induced by the exchange and applied fields. The volume expansion anomaly is associated with the onset of the 3d magnetic moment. IN R 2Fe 17 compounds there is an intrisinc Fe- moment. Magnetic structures give evidence for positive and negative exchange interactions between Fe atoms which are strongly distance dependent. The thermal expansion anomaly is a result of this distance of the magnetic interactions.