Showing papers on "Antiferromagnetism published in 1992"

Magnetic impurities in half-integer-spin Heisenberg antiferromagnetic chains

Abstract: We study the effect of an isolated impurity on the low-energy properties of a half-odd-integer-spin Heisenberg antiferromagnetic chain using both numerical and conformal field theory techniques. The impurity corresponds to the substitution of a magnetic ion by a different ion with the same or different spin, or else to the coupling of a magnetic impurity to a single spin on the chain. Depending on which kind of impurity is present, the low-energy behavior corresponds either to a ``healing'' of the defect or else to a severing of the chain at the impurity location. In both cases there is a coupling-dependent length scale over which magnetic screening takes place. Analogies with the Kondo effect are elucidated.

Semi-Metal-Insulator Transition of the Hubbard Model in the Honeycomb Lattice

Abstract: Using quantum Monte Carlo and finite-size scaling for the Hubbard model, we find evidence of a zero-temperature transition between the nonmagnetic semi-metal and an antiferromagnetic insulator in the 2D honeycomb lattice for a nontrivial value of U/t?=?4.5???0.5. The corresponding transition in Hartree-Fock mean field is at U/t?=?2.23, which indicates the importance of quantum fluctuations. This represents the first example of Mott-Hubbard transition in a 2D bipartite lattice. Similar transitions are predicted for special lattices in higher dimensions, in particular for the 3D diamond lattice.

Low-temperature magnetic properties of the ferromagnetic organic radical, p-nitrophenyl nitronyl nitroxide.

Abstract: Low-temperature magnetic properties of the \ensuremath{\beta} and \ensuremath{\gamma} phases of p-nitrophenyl nitronyl nitroxide (p-NPNN) were determined by measuring the specific heat, the magnetic susceptibility, and the hysteresis curve of magnetization above $^{3}\mathrm{He}$ temperature in external magnetic fields. The \ensuremath{\beta}-phase crystal undergoes a bulk ferromagnetic transition at 0.60 K, which was confirmed by the magnetic entropy of ln2 due to one unpaired electron on the radical molecule and the hysteresis curve. The \ensuremath{\gamma} phase, on the other hand, revealed an antiferromagnetic transition at 0.65 K and one-dimensional ferromagnetic fluctuations above it. The specific-heat data of the \ensuremath{\gamma} phase in external fields were analyzed by a mean-field theory incorporated in the one-dimensional Heisenberg model. The details of sample characterization of each phase based on thermal analysis are also given.

Macroscopic quantum tunneling in magnetic proteins.

Abstract: We report low-temperature measurements of the frequency-dependent magnetic noise and magnetic susceptibility of nanometer-scale antiferromagnetic horse-spleen ferritin particles, using an integrated dc SQUID microsusceptometer. A sharply defined resonance near 1 MHz develops below T\ensuremath{\sim}0.2 K. The behavior of this resonance as a function of temperature, applied magnetic field, and particle concentration indicates that it results from macroscopic quantum tunneling of the N\'eel vector of the antiferromagnets.

Possible Néel orderings of the Kagomé antiferromagnet.

Abstract: Possible Neel orderings of antiferromagnetically coupled spins on a kagome lattice are studied using linearspin-wave theory and high-temperature expansions. Spin-wave analysis, applied to q=0 (three spins per magnetic unit cell) and to √3 × √3 (nine spins per cell) Neel orderings yield identical excitation spectra with twofold-degenerate linear modes and a dispersionless zero-energy mode. This dispersionless mode is equivalent to an excitation localized to an arbitrary hexagon of nearest-neighbor spins. Second( J2) and third( J3) neighbor interactions are shown to stabilize the q=0 state for J2>J3 and the √3 × √3 state for J23. A hightemperature expansion of the spin-spin susceptibility χαβ(q) is performed to order 1/T8, for n-component, classical spins with nearest-neighbor interactions only. To order 1/T7 the largest eigenvalue of the susceptibility matrix is found to be independent of wave vector with an eigenvector that corresponds to the dispersionless mode of the ordered phase. This degeneracy is removed at order 1/T8. For n=0, the q=0 mode is favored; for n=1, the band is flat; and, for n>1, the maximum susceptibility is found for a √3 × √3 excitation. Similar results are found for the three-dimensional pyrochlore lattice. The high-temperature expansion is used to interpret experimental data for the uniform susceptibility and powder-neutron-diffraction spectrum for the kagome-lattice system SrCr8−xGa4+xO19.

Suppression of tunneling by interference in half-integer-spin particles.

Abstract: Within a wide class of ferromagnetic and antiferromagnetic systems, quantum tunneling of magnetization direction is spin-parity dependent: it vanishes for magnetic particles with half-integer spin, but is allowed for integer spin. A coherent-state path-integral calculation shows that this topological effect results from interference between tunneling paths.

Oscillations in giant magnetoresistance and antiferromagnetic coupling in [Ni81Fe19/Cu]N multilayers

Abstract: We report giant magnetoresistance in [Ni81Fe19/Cu]N multilayers. Saturation magnetoresistance values exceeding 16% for saturation fields of only 600 Oe are found at 300 K. In addition, we show evidence for well‐defined oscillations in saturation magnetoresistance as a function of Cu spacer layer thickness at 4.2 K, with an oscillation period and phase similar to that in Co/Cu multilayers. However, the temperature dependence of the magnetoresistance, while weak for thin Cu layers, is much stronger for thicker Cu layers. Consequently at 300 K only a single oscillation in magnetoresistance for thin Cu layers is found. We show that the properties of the Ni81Fe19/Cu multilayers are very sensitive to annealing at moderate temperatures, which may limit the possible technological applications of such structures.

Classical antiferromagnets on the Kagomé lattice.

Abstract: We examine the classical antiferromagnet on the Kagom\'e lattice with nearest-neighbor interactions and n-component vector spins. Each case n=1,2,3 and ng3 has its own special behavior. The Ising model (n=1) is disordered at all temperatures. The XY model (n=2) in the zero-temperature (T\ensuremath{\rightarrow}0) limit reduces to the three-state Potts model, which in turn can be mapped onto a solid-on-solid model that is o/Iat its roughening transition. Exact critical exponents are derived for this system. The spins in the Heisenberg model (n=3) become coplanar and more ordered than the XY model as T\ensuremath{\rightarrow}0. Thus we argue that the Heisenberg model has long-range antiferromagnetic order in the limit T\ensuremath{\rightarrow}0. For ng3 the system appears to remain disordered for T\ensuremath{\rightarrow}0.

Dual spin valve magnetoresistive sensor

Abstract: A magnetoresistive read sensor based on the spin valve effect and having a multilayered, dual spin valve structure is described. The sensor read element includes first, second and third layers of ferromagnetic material separated from each other by layers of non-magnetic metallic material. The first and third layers of ferromagnetic material, i.e., the outer layers of the structure, have their magnetization orientation fixed, while the second, intermediate ferromagnetic layer is magnetically soft and has its magnetization oriented perpendicular to that of both the outer ferromagnetic layers in the absence of an applied magnetic field. In one preferred embodiment, the two outer ferromagnetic layers have their magnetizations fixed parallel to each other by exchange coupling with adjacent antiferromagnetic layers.

Neutron diffraction study of the heavy fermion superconductors UM2Al3 (M=Pd, Ni)

Abstract: An elastic neutron scattering study was performed on the new superconducting heavy fermion systems UPd2Al3 and UNi2Al3. The neutron diffraction patterns reveal unambiguously long range antiferromagnetic order in UPd2Al3 with an ordered magnetic momentμ U = (0.85±0.03)μ B , which coexists with the superconducting state. This is by far the largestμ U value observed for any heavy fermion superconductor. For UNi2Al3, no long-range magnetic order could be observed for temperaturesT≧1.5 K, yielding an upper limit of the ordered moment of 0.2μ B .

Magnetism in Cr thin films on Fe(100)

Abstract: The spin polarization of secondary electrons from a Cr film on Fe(100), measured with scanning electron microscopy with polarization analysis, oscillates as a function of Cr thickness with a period near two atomic layers, consistent with incommensurate spin-density-wave antiferromagnetism in the Cr. The position of a phase slip due to incommensurability varies reversibly by 14 layers over the temperature range of 310 to 550 K. The Cr surface magnetic moment persists well above the Neel temperature of bulk Cr

Spin Gap and Antiferromagnetic Correlations in the Kondo Insulator CeNiSn

Abstract: Neutron scattering measurements show that the crossover (at T\ensuremath{\le}10 K) from metallic heavy-fermion to semiconducting behavior coincides with the formation of a gap in the magnetic excitation spectrum of CeNiSn. In contrast to the simple band picture of an insulator, the gap is well defined only at particular values of the momentum transfer Q. While substantial antiferromagnetic correlations in the a-c plane characterize the low-T state, the corresponding zero-frequency response function is Q independent.

Antiferromagnetic order in UPt 3 under pressure: Evidence for a direct coupling to superconductivity

Abstract: We use neutron diffraction to study the effect of hydrostatic pressure on the ordered antiferromagnetic moment in the heavy-fermion superconductor ${\mathrm{UPt}}_{3}$. It is found that hydrostatic pressure suppresses and eventually destroys the antiferromagnetic order. A comparison of our results with specific-heat measurements made under hydrostatic pressure shows that the splitting of the two anomalies in the low-temperature heat capacity correlates well with the collapse in the magnitude of the ordered moment. This suggests a coupling between the superconducting and antiferromagnetic order parameters.

Photomechanical properties of rhodium(I)-semiquinone complexes. The structure, spectroscopy, and magnetism of (3,6-di-tert-butyl-1,2-semiquinonato)dicarbonylrhodium(I)

Abstract: Crystals of Rh(CO) 2 (3,6-DBSQ) [3,6-DBSQ=3,6-di-tert-butyl-1,2-semiquinone] have been reported to bend reversibly when exposed to light in the near-IR. Structural characterization on the complex has confirmed the Rh 1 -DBSQ charge distribution for the complex and shown that molecules are stacked in columns in the solid state with a slightly nonlinear oligomeric rhodium core. Magnetic features show marked temperature dependence with both ferromagnetic and antiferromagnetic interactions apparent over the temperature range between 330 and 5 K

Magnetic properties of LaVO3.

Abstract: Herein, we report on the magnetic properties of ${\mathrm{LaVO}}_{3}$. We have observed diamagnetism in this compound, which is an antiferromagnetic insulator, below the N\'eel temperature under field-cooled conditions. We observe hysteresis in magnetization versus applied field M(H) isotherms below the N\'eel temperature. ${\mathrm{LaVO}}_{3}$ has a ferromagnetic component to the spin ordering, due to canting of spins and/or due to ferrimagnetism. Four-probe resistivity measurements rule out the possibility of superconductivity and the dimagnetism appears to be associated with uncompensated antiferromagnetism (ferrimagnetism). From our $^{51}\mathrm{V}$ NMR measurements on ${\mathrm{LaVO}}_{3}$ we associate the kink in susceptibility with magnetic ordering.

Spin-dependent x-ray absorption of MnO and MnF 2

Abstract: A method to measure spin-dependent x-ray-absorption spectra that does not require long-range magnetic order is presented. It is based on the spin dependence of the emission spectrum. Using a high-resolution spectrometer to separate and select the different spin-polarized final states in the [ital K][beta] fluorescence spectrum of MnO and MnF[sub 2], spin-dependent absorption spectra in the vicinity of the Mn [ital K] edge were measured. Since this method uses the direction of the local magnetic moment as a reference it can be applied to paramagnetic and antiferromagnetic systems as well as to ferromagnetic.

Resonant x-ray diffraction near the iron K edge in hematite (α- Fe 2 O 3 )

Abstract: An unexpected resonant enhancement in x-ray diffraction occurs at a forbidden reflection in the antiferromagnet hematite. This fifty fold increase in the diffracted signal, 10 eV below the iron K edge, exhibits a rotation in the x-ray polarization and a 60 o azimuthal symmetry as the crystal is rotated about the reflection vector. The erect is explained to result from electronic quadrupole transitions in the anomalous scattering from iron and demonstrates that diffraction can be a sensitive probe of the electronic environment at specific atomic sites in the crystal

Finite size and temperature effects in the AF Heisenberg model

Abstract: The low temperature and large volume effects in the d=2+1 antiferromagnetic quantum Heisenberg model are dominated by magnon excitations. The leading and next-to-leading corrections are fully controlled by three physical constants, the spin stiffness, the spin wave velocity and the staggered magnetization. Among others, the free energy, the ground state energy, the low lying excitations, staggered magnetization, staggered and uniform susceptibilities are studied here. The special limits of very low temperature and infinite volume are considered also.

Spiral-spin-density-wave states in fcc iron: Linear-muffin-tin-orbitals band-structure approach.

Abstract: A method for electronic-structure calculations of noncollinear magnetic structures has be n developed based on the linear-muffin-tin-orbitals approach. Calculations of total energies and magnetic moments for various spiral-spin-density-wave states of fcc iron have been carried out. The results obtained show that the ground state of fcc iron is the spiral-spin-density-wave state. With the increase of the number of valence electrons (as in fcc Co) ferromagnetic ordering stabilizes essentially, whereas the decrease of that (as in fcc Mn) results in the antiferromagnetic ordering of spin moments

Spin-liquid ground state of the half-filled Kondo lattice in one dimension.

Abstract: By finite-size analysis of exact diagonalization the existence of a finite spin gap is confirmed for both antiferromagnetic and ferromagnetic coupling in the half-filled Kondo lattice model. For all finite values of antiferromagnetic coupling the system belongs to a class of coherent Kondo spin singlet, whereas for any finite ferromagnetic coupling it scales to a spin S=1 antiferromagnetic chain with a Haldane gap.

Magnetic properties of ultrafine Co particles

Abstract: Fine particles of Co in the size range of 50-350 AA by the vapor deposition technique have been prepared. The dependence of magnetization on size and temperature has been studied. A maximum coercivity of 1500 Oe was obtained for a particle size of 350 AA. Magnetic and structural measurements indicate a shell-type structure with an FCC (face centered cubic) Co core surrounded by a Co-oxide shell. The effect of exchange interactions between the antiferromagnetic Co-oxide shell and the ferromagnetic Co core on the magnetic behavior of the powders has been investigated. The core/shell structure is responsible for the large coercivities present at cryogenic temperatures and the nonsaturation effects, which increase with decreasing particle size and with higher oxidation. The disappearance of the shift at 150 K marks the loss of exchange interaction between the Co-core and its CoO oxide coating. Thus the 150 K temperature could either be the blocking temperature or the Neel temperature of CoO crystallites present in the oxide shell. >

A New Antiferromagnetic Heavy Fermion Compound: CePd2Al3

Abstract: We report the first measurements of the electrical resistivity, the magnetic susceptibility and the specific heat of the new hexagonal compound CePd 2 Al 3 . The results indicate that CePd 2 Al 3 h is a new heavy fermion system with an electronic specific heat coefficient (γ) of 380 mJ/mol·K 2 and antiferromagnetic ordering below T N =2.8 K.

Ferromagnetic interaction in poly(m-aniline): Electron spin resonance and magnetic susceptibility

Abstract: Ferromagnetic interaction among the spins in poly(m‐aniline) has been suggested by electron spin resonance and magnetic susceptibility measured at low temperatures. From the g‐value analysis, these spins are found to be located on nitrogens although they are in some degree delocalized on phenyl rings. Doping of this polymer with iodine leads to an increase in the spin concentration up to 1020 spins⋅(g‐polymer)−1. Saturation magnetization of 0.30 emu⋅G⋅g−1 and spin clusters of S=1 are observed in the iodine‐doped polymer from the temperature and field dependence of the magnetization. Through‐bond interaction, depending on the topology of a polymer skeleton, is considered to operate between rather distant spins through the spin polarization with antiferromagnetic array of the phenyl rings.

Giant magnetoresistance and enhanced antiferromagnetic coupling in highly oriented Co/Cu (111) superlattices.

Abstract: We demonstrate the presence of large oscillatory antiferromagnetic (AF) interlayer coupling and ``giant'' magnetoresistance in crystalline (111)-oriented Co/Cu superlattices grown on Pt buffer layers on (0001) sapphire substrates. The AF coupling strength is \ensuremath{\approxeq}4--5 times larger than previously reported for sputtered polycrystalline Co/Cu multilayers. However, a significant fraction of the superlattice (\ensuremath{\approxeq}70%) remains ferromagnetically coupled. This may account for the comparatively low giant magnetoresistance values observed of \ensuremath{\approxeq}40% at 3.5 K and \ensuremath{\approxeq}26% at 295 K compared to values of g120% at 4.2 K and g65% at 295 K, respectively in sputtered structures. Similar crystalline (111) Co/Cu structures grown on Cu buffer layers on GaAs(110) show no evidence for antiferromagnetic coupling, although they are of nearly comparable structural perfection. The presence or absence of antiferromagnetic coupling in (111) Co/Cu superlattices appears to be a result of subtle structural imperfections giving rise to direct ferromagnetic coupling of neighboring Co layers.

Perpendicular anisotropy and antiferromagnetic coupling in Co/Ru strained superlattices.

Abstract: Despite the large lattice mismatch (\ensuremath{\simeq}8%), we demonstrate the growth of epitaxially ordered Co/Ru superlattices consisting of hcp (0001) Co and Ru sublayers. For small Ru interlayer thickness, a large antiferromagnetic exchange coupling between the Co sublayers is observed that influences strongly the magnetization process, as the magnetization within Co sublayers can no longer be considered to be fully locked ferromagnetically. For small Co and Ru thicknesses, the magnetization is oriented along the film normal, while the adjacent Co layers are strongly coupled antiferromagnetically. For this peculiar magnetic structure, a first-order spin-flop transition from an antiferromagnetic state to a ferromagnetic canted state is observed and clearly revealed by use of a ferromagnetic-resonance technique.

Local spin-density theory of itinerant magnetism in crystalline and amorphous transition metal alloys

Abstract: The authors present self-consistent spin-polarized electronic structure calculations for realistic models of amorphous transition metal alloys. The atomic structure is prepared by a simulated molecular-dynamic quench, based on interatomic forces calculated using hybridized nearly-free-electron tight-binding-bond theory. The electronic structure is calculated in the local spin-density approximation, using a linear muffin-tin orbital (LMTO) supercell approach. Detailed results for crystalline and amorphous alloys of Ni, Co and Fe with Zr are presented. NixZr1-x alloys are predicted to be paramagnetic for x or=0.75 the competition between ferromagnetic and antiferromagnetic exchange interactions leads to the formation of negative moments on isolated Fe sites. The number of negative Fe moments increases strongly for x>or=0.90, leading to a decrease of the average moment in the Fe-rich limit. The authors show that the formation of negative local Fe moments is related to a large number of contracted Fe-Fe pairs. The predictions of local spin-density theory are found to be in excellent agreement with experiment. For all crystalline and amorphous alloys the local exchange splitting of the 3d states is shown to be correlated linearly with the local magnetic moment, with a slope of approximately=1 eV mu B-1. This relation holds for all types of magnetic order and also for the crystalline and amorphous pure metals.

Structural and magnetic properties of ferrihydrite

Abstract: Fe MSssbauer spectra of two synthetic samples of ferrihydrite, recorded at 4.2 K in applied fields of up to 9 T, have been analysed by a mean-field model. The samples exhibit two and six X-ray diffraction peaks. It is shown that only one ferric ion site is present in the mineral, and that in this site the ions are octahedrally coordinated. The spectra show the presence of different magnetic states: ferri- magnetism in two-line ferrihydrite, and antiferromagnetism in six-line ferrihydrite. The ferrimagnetism in two-line ferrihydrite is analysed in terms of random fluctuations arising from the small numbers of ferric ions per particle, and it is shown that the different magnetic states may arise purely as a result of these fluctuations.