Showing papers on "Antiferromagnetism published in 1989"

Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange

Abstract: The electrical resistivity of Fe-Cr-Fe layers with antiferromagnetic interlayer exchange increases when the magnetizations of the Fe layers are aligned antiparallel. The effect is much stronger than the usual anisotropic magnetoresistance and further increases in structures with more than two Fe layers. It can be explained in terms of spin-flip scattering of conduction electrons caused by the antiparallel alignment of the magnetization.

Conductance and exchange coupling of two ferromagnets separated by a tunneling barrier.

Abstract: A theory is given for three closely related effects involving a nonmagnetic electron-tunneling barrier separating two ferromagnetic conductors. The first is Julliere's magnetic valve effect, in which the tunnel conductance depends on the angle \ensuremath{\theta} between the moments of the two ferromagnets. One finds that discontinuous change of the potential at the electrode-barrier interface diminishes the spin-polarization factor governing this effect and is capable of changing its sign. The second is an effective interfacial exchange coupling -J cos\ensuremath{\theta} between the ferromagnets. One finds that the magnitude and sign of J depend on the height of the barrier and the Stoner splitting in the ferromagnets. The third is a new, irreversible exchange term in the coupled dynamics of the ferromagnets. For one sign of external voltage V, this term describes relaxation of the Landau-Lifshitz type. For the opposite sign of V, it describes a pumping action which can cause spontaneous growth of magnetic oscillations. All of these effects were investigated consistently by analyzing the transmission of charge and spin currents flowing through a rectangular barrier separating free-electron metals. In application to Fe-C-Fe junctions, the theory predicts that the valve effect is weak and that the coupling is antiferromagnetic (Jl0). Relations connecting the three effects suggest experiments involving small spatial dimensions.

Electronic structure of the high-temperature oxide superconductors

Abstract: Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

Theory of giant magnetoresistance effects in magnetic layered structures with antiferromagnetic coupling.

Abstract: We present a simple theoretical description of recently measured giant magnetoresistance effects in Fe/Cr layered structures. The resistivity is calculated by solving the Boltzmann transport equation with spin-dependent scattering at the interfaces. The magnitude of the effect depends on the ratio of the layer thickness to the mean free path and on the asymmetry in scattering for spin-up and spin-down electrons. Good agreement with experiment is found for both sandwich structures and superlattices.

Preroughening transitions in crystal surfaces and valence-bond phases in quantum spin chains.

Abstract: We show that disordered flat phases in crystal surfaces are equivalent to valence-bond-type phases in integer and half-integer spin quantum chains. In the quantum spin representation the disordered flat phase represents a fluid-type phase with long-range antiferromagnetic spin order. This order is stabilized dynamically by the hopping of the particles and short-range spin-exchange interactions. The mass of N\'eel solitons is finite. Numerical finite-size-scaling results confirm this. We identify the order parameter of the valence-bond phase. The Haldane conjecture suggests a fundamental difference between half-integer and integer antiferromagnetic Heisenberg spin chains. We find that disordered flat phases are realized in both cases, have exactly the same type of long-range antiferromagnetic spin order, and are stabilized by exactly the same mechanism. They differ only in the mathematical formulation of broken symmetry in the spin representation. We suggest experimental methods of observing disordered flat phases in crystal surfaces.

Magnetovolume instabilities and ferromagnetism versus antiferromagnetism in bulk fcc iron and manganese

Abstract: Total-energy band calculations, including an antiferromagnetic extension of the fixed-spin-moment procedure, are used to study magnetovolume effects in bulk fcc iron and maganese. By constraining these systems to have a fixed total magnetic moment in a single-atom fcc unit cell, we find magnetovolume instabilities in the form of first-order transitions from nonmagnetic to ferromagnetic behavior. Constraining the moments to have fixed values in a CuAu unit cell of two atoms to allow for antiferromagnetic (and field-induced ferrimagnetic) order alters these instabilities and yields second-order transitions from nonmagnetic to antiferromagnetic behavior at volumes coincident with the equilibrium volumes for both metals.

Magnetic susceptibility scaling in La2-xSrxCuO4-y.

Abstract: The powder magnetic susceptibility chi(T) of La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ is found to scale with doped hole concentration p = x-2y according to a law of corresponding states for 0less than or equal topless than or equal to0.20, thereby allowing chi/sup Pauli/(p) of the holes and chi/sup 2D/(p,T) of the Cu/sup +2/ spin sublattice to be separated and precisely evaluated. chi/sup Pauli/ increases with p. The shape of chi/sup 2D/(T) is that of the spin-(1/2 square-lattice Heisenberg antiferromagnet; however, by p = 0.20, the in-plane Cu-Cu superexchange coupling constant and effective magnetic moment per Cu ion are both largely suppressed.

Magnetic structures of cubic FeGe studied by small-angle neutron scattering

Abstract: The magnetic ordering of a single crystal of the cubic polymorph of FeGe has been studied by small-angle neutron scattering. The compound orders magnetically at TN=278.7 K into a long-range spiral (period approximately 683-700 AA) propagating along equivalent directions at high temperatures and along equivalent directions at low temperatures. The length of the spiral wavevector is nearly independent of temperature. The transition at TN is first order with very little hysteresis. The transition at which the direction of the spiral turns is rather sluggish. It takes place in a temperature interval of approximately 40 K and shows pronounced temperature hysteresis (T2 down arrow =211 K, T2 up arrow =245 K). Applied magnetic fields of 20-40 mT, depending on the temperature and the field direction, cause the spiral axis to turn into the direction of the applied field. As the field is further increased, the amplitude of the antiferromagnetic spiral decreases and the ferromagnetic component increases until at fields above approximately 200-300 mT cubic FeGe becomes magnetically saturated. The magnetic ordering in cubic FeGe is a Dzyaloshinskii spiral similar to the structure observed in the isostructural compound MnSi. However, in MnSi the spiral propagates along equivalent directions at all temperatures below TN=29.5 K.

Magnetic resonance determination of the antiferromagnetic coupling of Fe layers through Cr.

Abstract: Variable frequency ferromagnetic resonance (FMR) has been used to directly observe the coupled resonance modes in single-crystal Fe/Cr/Fe(001) sandwiches grown by molecular-beam epitaxy. Magnetization M and magnetoresistance measurements also were carried out on these samples, which exhibited antiferromagnetic (AF) layer alignment for 12 A\r{}lt(Cr)25 A\r{}. The FMR data reveal two resonance modes with complex frequency dependences for the AF-aligned samples. The detailed FMR and M vs H behavior can be quantitatively explained by an AF coupling parameter J which has a thickness dependence peaked about t(Cr)=16 A\r{}.

Spiral phase of a doped quantum antiferromagnet.

Abstract: A low density of vacancies in a 2D, spin-1/2, Heisenberg antiferromagnet leads (for a range of effective couplings) to a metallic phase with incommensurate antiferromagnetic order, i.e., with the staggered magnetization rotating in a plane with the wave number proportional to the density. This structure originates from the polarization of the antiferromagnetic dipole moments of the vacancies. The excitation spectrum of this spiral state includes an interesting low-lying mode. Implications for neutron scattering and normal-state resistivity are discussed.

Magnetic properties of 3d transition metal monolayers on metal substrates

Abstract: We report results of systematic calculations for magnetic properties of 3d transition metal monolayers on Pd(001) and Ag(001). We find large similarities to interactions of magnetic 3d impurities in the bulk. Therefore the overlayer results are supplemented with results for 3d dimers in Cu, Ag, and Pd. Differences between the two classes of systems are utilized to reveal the interaction within the overlayers and between overlayers and substrates. In virtually all cases we find both ferromagnetic and antiferromagnetic solutions, showing large magnetic moments and similar densities of states. From the trend of the calculations we conclude that V, Cr, and Mn overlayers favor the antiferromagnetic c(2×2) structure, while Ti, Fe, Co, and Ni prefer the ferromagnetic one.

Band-structure calculations, and magnetic and transport properties of ferromagnetic chromium tellurides (CrTe, Cr3Te4, Cr2Te3)

Abstract: Electronic band-structure calculations are presented for the ferromagnetic compounds CrTe, Cr3Te4 and Cr2Te4 and Cr2Te3. In these compounds the Cr3d-Te5p covalency and the Cr3d(z2)-Cr3d(z2)-overlap along the c axis are the most important interactions. The magnetic polarisation of Te is parallel to the Cr local moment in CrTe, antiparallel to it in Cr2Te3 and about zero in Cr2Te4. Measurements of electronic transport properties (resistivity, Hall effect and thermo-electric power) and magnetic properties of Cr1- delta Te( delta =0.1) and Cr3+xTe4(x=0.2) indicate that these chromium tellurides are p-type metals, with strong interaction between the holes in the Te5p band and the Cr magnetic moments. In the literature the variation of the magnetic properties of Cr3.2Te4 near Ts approximately=100 K has been attributed to a change from a canted antiferromagnetic to a collinear ferromagnetic structure. However, our Hall-effect measurements indicate that the spin structure is not collinear ferromagnetic above Ts.

Quantitative determination of quantum fluctuations in the spin-1/2 planar antiferromagnet.

Abstract: Frequency moments of the light-scattering spectrum from spin-pair excitations are calculated for the spin-\textonehalf{} planar Heisenberg antiferromagnet. The quantitative agreement with the experimental ${B}_{1g}$ spectrum in ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ demonstrates that the observed linewidth is dominated by quantum fluctuations and yields a value for the exchange parameter $J$ of 1030\ifmmode\pm\else\textpm\fi{}50 ${\mathrm{cm}}^{\ensuremath{-}1}$. Quantum fluctuations also permit light scattering due to diagonal-next-neighbor spin-pair excitations. Observed spectral features in the ${A}_{1g}$ and ${B}_{2g}$ symmetries are consistent with those calculated for this process.

Kondo-stabilised spin liquids and heavy fermion superconductivity

Abstract: The authors consider an SU(2) path integral formulation of a Kondo lattice model for heavy fermions that treats the RKKY interaction explicitly At low temperatures they find the heavy Fermi liquid becomes unstable to the formation of a spin liquid amongst the f spins Kondo coupling to the spin liquid stabilises it against antiferromagnetism, causing the resonating valence bonds of the spin liquid to occasionally escape into the conduction sea This process induces off-diagonal resonant scattering in the conduction sea, thereby generating anisotropic superconductivity in the heavy fermion system

Antiferromagnetism in the two-dimensional Hubbard model.

Abstract: Magnetic properties of the two-dimensional Hubbard-model are inferred from results of Monte Carlo simulations. Lattice sizes up to 8\ifmmode\times\else\texttimes\fi{}8 and temperatures down to $T=\frac{t}{20}$ ($t=\mathrm{hopping}$) were studied. The half-filled system is found to exhibit antiferromagnetic long-range order for all values of the Coulomb repulsion $U$. The low-temperature magnetic properties are found to be well described by spin-wave theory with renormalized local moment and spin-wave velocity. Numerical evidence presented suggests that when doped the system loses the long-range order immediately away from half filling.

Magnetic order in the different superconducting states of UPt3

Abstract: We use neutron diffraction to show that superconductivity affects the magnetic order in ${\mathrm{UPt}}_{3}$. The different superconducting states identified in previous bulk measurements can be associated with different behaviors of the magnetic order parameter. The data suggest that the coupling between multicomponent superconducting and magnetic order parameters leads to the variety of superconducting phases of ${\mathrm{UPt}}_{3}$.

Magnetic properties of lanthanide complexes with nitronyl nitroxides

Abstract: 1H-imidazolyl-1-oxy1 3-oxide, NITR; R = phenyl, ethyl) The magnetic properties of Gd(hfac),(NITPh), were investigated in the range 11-300 K, while all the other compounds were studied in the range 4-300 K The gadolinium(II1) ions were found to be weakly coupled in a ferromagnetic fashion to the radical, while the radicals are antiferromagnetically coupled to each other The possible mechanisms responsible for this behavior are discussed Many efforts have currently been devoted to the synthesis of materials with expected magnetic properties In order to arrive at such a result, chemists must have available different building blocks with which it will be possible to arrange complex architectures Therefore, it is of fundamental interest to know the conditions under which, for instance, ferro- or antiferromagnetic coupling will be developed between different spins, and simple molecular clusters need to be synthesized and investigated in order to test theories This procedure is now fairly well established for transition-metal ions:*3 but much less is known for lanthanide ions Indeed, although lanthanides have long been known to develop interesting magnetic properties when used to dope transition-metal ionic the study of discrete magnetically coupled molecular systems has been largely overlooked up to the present Some of us have recently reported the magnetic properties of trinuclear complexes containing one gadolinium(II1) and two copper(I1) ions magnetically coupled in such a fashion that a smooth increase of XT on lowering T was observede12 These data were explained with an isotropic ferromagnetic exchange between the gadolinium and the copper ions The coupling must be determined by superexchange through the oxygen bridges, but no attempt was made to use an orbital model to justify the observed coupling This purpose in principle might be achieved more easily by studying the interaction of lanthanide ions with stable organic radicals such as nitroxides, because in this case the magnetic interaction would be of the direct exchange type, and the observed coupling might be directly connected with the nature of the bond interaction between the x* orbital on the ligand and the f orbitals of the lanthanides In other words, the nitroxides, which are widely used as spin labels and spin probes, in this case might be used as bond probes,13 in the sense that an analysis of their interaction with the metal ions should provide first-hand information on the metal-ligand bond and on the nature of the magnetic orbitals With these considerations in mind, we synthesized a series of stable adducts of lanthanides with nitronyl nitroxides of general formula 0

Effective transfer for singlets formed by hole doping in the high-Tc superconductors

Abstract: By comparing the exact results of an extended multiband Hubbard Hamiltonian for Cu 2 O 7 and Cu 2 O 8 clusters to those obtained from an effective single band Hamiltonian, we show that the low energy scale physics is very well described by a t - t ′- J model which describes the motion of singlets in an antiferromagnetic background of spins. We obtain values for t , t ′ and J for both hole and electron doping and show that these are different. We also study the dispersion relations and density of states within the quasi particle approximation both of which show rather interesting characteristics which could be relevant for high T c superconductors.

Ferromagnetic and antiferromagnetic intermolecular interactions of organic radicals, α‐nitronyl nitroxides. II

Abstract: The temperature dependence of the magnetic susceptibilities and the magnetizations for the three kinds of α‐nitronyl nitroxides, 2‐R‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazolyl‐1‐oxy 3‐oxide [with R=phenyl (I), 3‐nitrophenyl (II), 4‐nitrophenyl (III)] have been measured. It is found that the intermolecular spin interaction is ferromagnetic in the crystal of III, while it is antiferromagnetic in I or II. UV–visible absorption spectra suggests the enhancements of the π‐electron delocalization and the spin polarization in III in comparison with the others. The difference in the magnetic coupling between I or II and III is interpreted based on these two effects.

Properties of magnetic superlattices with antiferromagnetic interfacial coupling: Magnetization, susceptibility, and compensation points.

Abstract: Magnetic superlattices composed of ferromagnetic films (Fe and Gd) which couple antiferromagnetically at the interfaces are studied. In a magnetic field a variety of different spin configurations are possible. We show how these phases are reflected in magnetization and susceptibility measurements. In particular, a twisted spin configuration is characterized by a much larger susceptibility than an aligned spin configuration. We show that small changes in the layering pattern can lead to large changes in /ital M/(/ital T/) and static susceptibility. In contrast, changes in the value of the interface exchange parameter cause large changes only in the weak-coupling limit.

Mean-field theory for the t-J model.

Abstract: We present a new mean-field theory for the t-J model using a representation in which singly occupied sites or spins are represented by bosons and empty sites or holes by fermions. We obtain a phase diagram which includes ferromagnetic, antiferromagnetic, spiral (with the pitch inversely proportional to the density of holes), and disordered spin-liquid phases, with a clear separation of charge and spin excitations. We comment on the conditions for obtaining superconductivity in the model.

Ferromagnetism versus antiferromagnetism of the Cr(001) surface

Abstract: Three different spin arrangements for the (001) surface of Cr are discussed on the basis of self-consistent total-energy calculations: ferromagnetism, inside-the-surface $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetism, and "topological" antiferromagnetism between ferromagnetic terraces separated by single steps. The $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic configuration is found to be energetically unfavorable. Instead, topological antiferromagnetism is compatible with both the absence of magnetization observed by spin-resolved photoemission and the existence of spin-split surface states detected by energy- and angular-resolved photoemission.

Order-disorder transition-induced twin domains and magnetic properties in ilmenite-hematite

Abstract: Ansrnacr The transition temperature (4) between disordered, R3c, and ordered, R3, ilmenitehematite solid solutions in the ferrian ilmenite composition range between Ilmuo and Ilm'o has been redetermined by observing the presence or absence of transition-induced cationordered domains and the behavior of pre-existing domains annealed below the transition. The transition was reversed for Ilmro and is bracketed between 1000 and 1050 "C. The domains were shown by dark-field transmission electron microscopy to be twin related by a 180" rotation about an axis parallel to a and to vary in size depending on Z" and the temperature of quench. A model of the twin-domain boundary indicates that such boundaries are disordered and partially Fe-enriched. Magnetic and reiu observations on the same samples show that the room-temperature saturation magnetization is related to the surface area of the twin boundaries. Because the cation-ordered domains are ferrimagnetic with a strong magnetic moment and the disordered boundaries are probably antiferromagnetic with a weak magnetic moment, the quenched samples are essentially mixtures of two magnetic phases. The magrretization is therefore related to the proportion of each phase. The twin boundaries act as the classic "x" phase, which allows ferrian ilmenite to acquire a self-reversed thermoremanent magnetization (rnvr). Our measurements indicate that when the surface area of the twin boundaries in Ilmro exceeds approximately 25 x 106 m2/m3, the quenched samples acquire reverse rnu during cooling in a 0.5-Oe field. When the boundary surface area is less than this critical threshold, the quenched samples acquire a normal rnvr.

Magnetocaloric effects in rare-earth magnetic materials

Abstract: A review is given of the experimental investigations of the magnetocaloric effect (MCE) in rare-earth magnetic materials of different classes: heavy rare-earth metals and their alloys, iron garnets, and intermetallic compounds. The results of measurements of the MCE near phase transitions are given for the vicinity of the Curie and Ne?l temperatures, and also near magnetic compensation points in the case of ferromagnetism-helicoidal antiferromagnetism and helicoidal ferromagnetism?paramagnetism transitions. The contributions to the MCE made by the various magnetic sublattices in rare-earth ferromagnets are identified. Measurements of the MCE in single crystals of alloys of heavy rare-earth metals have made it possible to identify the main energy contributions to the helicoidal antiferromagnetism-ferromagnetism phase transitions and their energy dependences. The concluding section of the review deals with the technical applications of the MCE exhibited by rare-earth magnetic materials. An analysis is made of the potential applications as refrigerants in magnetic refrigerators, in thermodynamic cycles, and in structures of various types.

Specific heat and anisotropic magnetic susceptibility of Pr2CuO4, Nd2CuO4 and Sm2CuO4 crystals

Abstract: Specific heat and magnetic susceptibility measurements on single crystals of RE2CuO4 (RE=Pr, Nd, andSm) indicate antiferromagnetic order at TN≲1.5 K and 5.95 K for RE = NdandSm, respectively. The susceptibility of all RE2CuO4 compounds is highly anisotropic and suggests the presence of crystal field effects.

Magnetic phase transitions and structural distortion in Nd2CuO4

Abstract: Neutron and X-ray diffraction have been used to study the magnetic and structural properties of single crystal Nd2CuO4. Long range magnetic order of the Cu moments develops at TN=245 K, with a noncollinear antiferromagnetic arrangement of spins. Additional abrupt transitions are observed at 75 K and 30 K, in which a spin reorientation takes place. Bragg peaks associated with the crystal structure are found at the same positions as the magnetic Bragg peaks, and indicate that a distortion of the basic tetragonal structure has occurred above 300 K.

Model for quasi-one-dimensional antiferromagnets: application to CsNiCl3

Abstract: The approximate mapping of the antiferromagnetic Heisenberg chain onto a (1+1)-dimensional quantum field theory indicates that the ground state is disordered with a finite gap for integer spin. We study a lattice of coupled (1+1)-dimensional field theories as a model for weakly coupled spin chains. This model offers a possible resolution to the mystery of the upper mode in the low-temperature ordered phase of CsNi${\mathrm{Cl}}_{3}$, whose polarization is inconsistent with spin-wave theory.