Dominant Magnetic Interactions Research Articles

Magnetic properties and magnetic phase diagram of frustrated Co1−xFexPt3 compounds

The investigation of the magnetic properties of the pseudobinary Co1−xFexPt3 L12 ordered compounds resulting from alloying ferromagnet CoPt3 and frustrated antiferromagnet FePt3 is reported. The magnetic phase diagram of this system shows the presence of a pure spin glass phase that separates the ferromagnetic region from the antiferromagnetic one. On the Co-rich side (x<0.6), two re-entrant spin glass phases are found to emerge at low temperature in the ferromagnetic region. On the iron-rich side, an antiferromagnetic region with two antiferromagnetic [12 120] and [1200] structures is observed for 1⩾x⩾0.8. This magnetic phase diagram is discussed comparatively with the previously determined (Fe–Mn)Pt3 and (Co–Mn)Pt3 phase diagrams. The randomness of the average exchange interaction is suggested to arise from a competition between the three dominant magnetic interactions JCoCo, JFeFe, and JCoFe of 3d atoms in sites of second nearest neighbors in the L12 structure.

The effect of the microstructure on the magnetic interactions in CoFe–AgCu granular films: From demagnetizing to magnetizing interactions

The sign and strength of the dominant magnetic interactions in Co34Fe8Ag54Cu4 granular films were modified by changing the microstructure through annealing. Magnetic force micrographs showed that in the as-cast sample the magnetic moments of neighboring grains tended to be arranged parallel along a direction out of the film plane, forming elongated magnetic clusters that were themselves aligned antiparallel, with dominant demagnetizing interactions. This is a direct evidence that an uncompensated antiferromagneticlike microstructure is stabilized below the volume percolation threshold. However, in the sample annealed at 750 °C the particle growth led to large in-plane ferromagneticlike clusters with dominant magnetizing interactions. Thus, in this letter we present direct correlation of interactions effects with magnetic measurements and show that ΔM plots correlate with changes in the magnetic microstructure in these systems.

Comparison between the magnetic specific heat of NdBa2Cu3O x and DyBa2Cu3O x

Low temperature heat-capacity measurements have been performed to study the magnetic ordering in NdBa2Cu3O x and DyBa2Cu3O x . In order to investigate the type of ordering, magnetic fields up to 6 T and hydrostatic pressures up to 4.5 kbar were applied. While for NdBa2Cu3O x and DyBa2Cu3O x both the fully oxidized and the fully oxygen reduced samples display long rang antiferromagnetic ordering, only magnetic short range interaction could be found for samples in an intermediate range of oxygen concentration. For NdBa2Cu3O x , where superexchange is believed to be the dominant magnetic interaction,T N is three times higher in the sample withx=6.26 compared to the one withx=6.98. In contrast,T N of DyBa2Cu3O x is almost oxygen independent as expected for this mainly dipolar interacting system.

Anisotropic spin-glass behavior and random-field effects in EuxSr1-xAs3.

A detailed study of the dc magnetization of the anisotropic diluted antiferromagnet ${\mathrm{Eu}}_{\mathit{x}}$${\mathrm{Sr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{As}}_{3}$ is reported. For x=0.1 and 0.24 anisotropic spin-glass freezing is observed. The different behavior found for these two concentrations is attributed to the different dominant magnetic interactions, viz., long-range Ruderman-Kittel-Kasuya-Yosida and short-range superexchange interactions. For fields parallel and perpendicular to the easy direction, the observed irreversibility lines ${\mathit{T}}_{\mathit{i}}^{\mathrm{\ensuremath{\parallel}}}$(B) and ${\mathit{T}}_{\mathit{i}}^{\mathrm{\ensuremath{\perp}}}$(B) are compatible with a de Almeida-Thouless line. For x\ensuremath{\approxeq}0.5 irreversibilities in the longitudinal magnetization (field parallel to the easy axis) also occur, in spite of the fact that here the samples exhibit long-range order, i.e., incommensurate for x=0.47, and incommensurate (at high temperature) and AF1 antiferromagnetic (at lower temperature) for x=0.49. For x=0.49 evidence for random-field Ising behavior is presented, including an irreversibility line suggestive of random-field behavior for low fields and a transition to spin-glass-like irreversibility at higher fields. Also, a decay of the field-cooled magnetization due to domain-wall relaxation is observed for this concentration. Finally, the transverse spin glass is shown to exist well into the regime of long-range magnetic order, i.e, at least up to x=0.64.

Multiple-pulse NMR of oriented radioactive nuclei under electric-quadrupolar interactions

The formation of the nuclear spin echo in the Angular Distribution of Nuclear Radiations (ADNR) in the presence of combined magnetic-dipole and electricquadrupole interactions (EQI) is theoretically considered. The case of a perturbative EQI in addition to the dominant magnetic hyperfine interaction is analysed. When both interactions are extremely large, with the EQI much greater than the magnetic inhomogeniety, the excitation of spin-echo in ADNR by pulsed rf field, including transitions between a definite pair of quadrupole shifted Zeeman levels is considered.

Low-dimensional magnetic ordering and Ising- and XY-like anisotropy of Er3+ in the system ErBa2Cu3Ox with 6

Magnetic ordering of ${\mathrm{Er}}^{3+}$ ions in ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{8}$${\mathrm{O}}_{\mathrm{x}}$ with 6.12\ensuremath{\le}x\ensuremath{\le}6.91 has been studied for both orthorhombic and tetragonal phases by means of magnetic-susceptibility and heat-capacity measurements. Superconductivity in the orthorhombic phase samples was confirmed by the onset of a diamagnetic susceptibility. The superconducting transition temperature ${T}_{c}$ was observed to decrease from 92 to 55 K as x was reduced from 6.91 to 6.31. The tetragonal samples with x=6.23 and 6.12 remained paramagnetic down to 1.2 K and showed no overt trace of diamagnetism. The states of ${\mathrm{Er}}^{3+}$ ions in ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ and ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ were calculated by means of crystalline electric field (CEF) theory using both a point-charge model and scaled inelastic neutron scattering results for ${\mathrm{HoBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$.The calculated susceptibility is in excellent agreement with data on the tetragonal specimens for 1.2 K \ensuremath{\le}T\ensuremath{\le}160 K. For the orthorhombic samples, sharp heat-capacity peaks which resemble those for a two-dimensional Ising system and evidently correspond to the magnetic ordering of ${\mathrm{Er}}^{3+}$ moments are found. The transition temperature ${T}_{m}$ decreases smoothly from 0.604 to 0.545 K as x is varied from 6.91 to 6.31. In contrast, tetragonal samples show only a broad rounded heat-capacity anomaly having its maximum at 0.5 K and closely resembling that for a one-dimensional XY model. The change of ${\mathrm{Er}}^{3+}$ ion anisotropy from Ising-like to XY-like is explained by the CEF calculation which shows the ground state to be planar for the tetragonal phase but with the b axis slightly favored in the orthorhombic structure. Randomness in the coupling of ${\mathrm{Er}}^{3+}$ moments caused by structural disorder may effectively reduce the dimensionality of magnetic ordering as has been found in the case of random dilution. These results imply that the dominant magnetic interactions of ${\mathrm{Er}}^{3+}$ ions in these materials are of short range.

Two-dimensional af ordering or spin-glass freezing at 2.23K in oxygen-doped Ba 2GdCu 3O 7−y

Specific heat was measured with adiabatic method for samples of Ba 2GdCu 3O 7−y with different oxygen compositions. Oxygen doping was no effects on magnetic transition temperature ( Tm) and specific heat above Tm but a little effects on specific heat below Tm. At the temperatures well below Tm, specific heat follows a law, Cm = AT s , with s ∼ 2.0, which is not compatible with 3-D antiferromagnetic or ferromagnetic ordering but with 2-D antiferromagnetic ordering or spin-glass freezing. The unaffected magnetic ordering temperature by oxygen doping indicates that the dominant magnetic interaction in these materials is the dipole-dipole interaction.

Molecular/Organic Ferromagnets

Quantitative bulk ferromagnetic behavior has been established for the molecular/organic solid [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Above 16 K the dominant magnetic interactions are along a 1-D chain and, near T(c), 3-D bulk effects as evidenced by the value of the critical exponents dominate the susceptibility. The extended McConnell model was developed and provides the synthetic chemist with guidance for making new molecular materials to study cooperative magnetic coupling in systems. Assuming the electron-transfer excitation arises from the POMO, ferromagnetic coupling by the McConnell mechanism requires stable radicals (neutral, cations/anions, or ions with small diamagnetic counterions) with a non-half-filled POMO. The lowest excited state formed via virtual charge transfer (retro or forward) must also have the same spin multiplicity and mix with the ground state. These requirements limit the structure of a radical to D(2d) or C>/=(3) symmetry where symmetry breaking distortions do not occur. Intrinsic doubly and triply degenerate orbitals are not necessary and accidental degeneracies suffice. To achieve bulk ferromagnetism, ferromagnetic coupling must be established throughout the solid and a microscopic model has been discussed. These requirements are met by [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Additionally this model suggests that the Ni(III) and Cr(III) analogs should be antiferromagnetic and ferrimagnetic, respectively, as preliminary data suggest. Additional studies are necessary to test and further develop the consequences of these concepts. Some molecular/organic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) with spin state S = 1/2 (...D.(+)A.(-)D.(+)A.(-)...) exhibit cooperative magnetic phenomena, that is, ferro-, antiferro-, ferri-, and metamagnetism. For [Fe(III)(C(5)Me(5))(2)].(+)[TCNE](-). (Me = methyl; TCNE = tetracyanoethylene), bulk ferromagnetic behavior is observed below the Curie temperature of 4.8 K. A model of configuration mixing of the lowest charge-transfer excited state with the ground state was developed to understand the magnetic coupling as a function of electron configuration and direction of charge transfer. This model predicts that ferromagnetic coupling requires stable radicals with a non-half-filled degenerate valence orbital and a charge-transfer excited state with the same spin multiplicity that mixes with the ground state. Ferromagnetic coupling must dominate in all directions to achieve a bulk ferromagnet. Thus, the primary, secondary, and tertiary structures are crucial considerations for the design of molecular/organic ferromagnets.

Electronic Theory of Mn - Alloyed Diluted Magnetic Semiconductors

AbstractRecent calculations of the electronic structure and magnetic interactions in Mn - alloyed II-VI diluted magnetic semiconductors (DMS) are summarized. Detailed band structure results are obtained using an empirical tight-binding, coherent potential approximation approach with input from experiment and local spin density band calculations. The dominant magnetic interactions in these systems result from hybridization between spin-split Mn d states and sp valence bands. Superexchange between Mn moments is well described by a simple three-level model which yields accurate Mn - Mn exchange constants for a variety of II-VI DMS as well as the rocksalt insulators MnO and α-MnS.

Asymmetry of the quadrupole satellites and determination of the sign of the quadrupole coupling constant of 23Na in NaNO3

The satellite resonances in the quadrupole-split NMR spectrum of 23Na in a single crystal of NaNO3 have been studied at various crystal orientations. They are quite asymmetric near an orientation where the dominant magnetic dipole interaction is between adjacent 23Na nuclei situated in linear chains lying parallel to the external magnetic field. The orientation dependence of the third moment of the satellites is in quite good agreement with the theoretical dipolar third moment, and the transition associated with each satellite has been unambiguously determined. It is found that the quadrupole coupling constant of 23Na in NaNO3 is negative, rather than positive as previously believed. The temperature and pressure dependence of the quadrupole coupling constant measured by others is more easily understood when account is taken of the negative sign. The discrepancy between the measured and calculated third moment values was found to be mainly due to a small admixture of x′ in the spectrometer output. No significant asymmetry of the inhomogeneous quadrupole broadening was found.

Magnetic interactions between nitroxide free radicals and lanthanides or Cu2+ in liquids

Heisenberg exchange has been found in all previous work to be the dominant intermolecular magnetic interaction between paramagnetic species in solutions of low viscosity, and this conclusion is confirmed here in analysis of interactions between Cu2+ and a nitroxide free radical. Exchange between a fast relaxing metal ion and a slow relaxing radical contributes equally to the transverse and longitudinal relaxation probabilities, T−12 and T−11, of the radical. A new result that could be of significance in investigations of molecular dynamics in liquids is that dipole–dipole interactions between ions of the lanthanide series and free radicals are dominant in all conditions over Heisenberg exchange. In the liquid phase, Gd3+ is particularly effective in inducing free radical longitudinal relaxation.

Magnetic exchange interactions in `ilmenites' MeTiO3(Me = Mn, Fe, Co and Ni)

In FeTiO3, CoTiO3 and NiTiO3 the dominant magnetic exchange interactions are held to be of super-superexchange type, using two oxygens or one oxygen and one diamagnetic Ti4+ ion between the pairs of paramagnetic cations. In order to explain the antiferromagnetic ordering of manganese spins in the same layer in MnTiO3 it is suggested that the metal cations are Mn4+ and Ti2+, the interactions being of the same superexchange types as those previously suggested for Cr2O3.