Showing papers on "Ferromagnetism published in 1999"

Percolative phase separation underlies colossal magnetoresistance in mixed-valent manganites

Abstract: Colossal magnetoresistance1—an unusually large change of resistivity observed in certain materials following application of magnetic field—has been extensively researched in ferromagnetic perovskite manganites. But it remains unclear why the magnetoresistive response increases dramatically when the Curie temperature (T C) is reduced. In these materials, T C varies sensitively with changing chemical pressure; this can be achieved by introducing trivalent rare-earth ions of differing size into the perovskite structure2,3,4, without affecting the valency of the Mn ions. The chemical pressure modifies local structural parameters such as the Mn–O bond distance and Mn–O–Mn bond angle, which directly influence the case of electron hopping between Mn ions (that is, the electronic bandwidth). But these effects cannot satisfactorily explain the dependence of magnetoresistance on T C. Here we demonstrate, using electron microscopy data, that the prototypical (La,Pr,Ca)MnO3 system is electronically phase-separated into a sub-micrometre-scale mixture of insulating regions (with a particular type of charge-ordering) and metallic, ferromagnetic domains. We find that the colossal magnetoresistive effect in low-T C systems can be explained by percolative transport through the ferromagnetic domains; this depends sensitively on the relative spin orientation of adjacent ferromagnetic domains which can be controlled by applied magnetic fields.

Role of Metal-Oxide Interface in Determining the Spin Polarization of Magnetic Tunnel Junctions

Abstract: The role of the metal-oxide interface in determining the spin polarization of electrons tunneling from or into ferromagnetic transition metals in magnetic tunnel junctions is reported. The spin polarization of cobalt in tunnel junctions with an alumina barrier is positive, but it is negative when the barrier is strontium titanate or cerium lanthanite. The results are ascribed to bonding effects at the transition metal-barrier interface. The influence of the electronic structure of metal-oxide interfaces on the spin polarization raises interesting fundamental problems and opens new ways to optimize the magnetoresistance of tunnel junctions.

Lecture notes on electron correlation and magnetism

Abstract: Atoms, ions, and molecules crystal field theory Mott transition and Hubbard model Mott insulators Heisenberg magnets itinerant electron magnetism ferromagnetism in Hubbard models the Gutzwiller variational method the correlated metallic state mixed valence and heavy fermions quantum hall effect hydrogen atom single-spin-flip ansatz Gutzwiller approximation Schrieffer-Wolff transformation.

Origin of the Invar effect in iron–nickel alloys

Abstract: In 1897 Guillaume1 discovered that face-centred cubic alloys of iron and nickel with a nickel concentration of around 35 atomic per cent exhibit anomalously low (almost zero) thermal expansion over a wide temperature range. This effect, known as the Invar effect, has since been found in various ordered and random alloys and even in amorphous materials2. Other physical properties of Invar systems, such as atomic volume, elastic modulus, heat capacity, magnetization and Curie (or Neel) temperature, also show anomalous behaviour. Invar alloys are used in instrumentation, for example as hair springs in watches. It has long been realized that the effect is related to magnetism2,3; but a full understanding is still lacking. Here we present ab initio calculations of the volume dependences of magnetic and thermodynamic properties for the most typical Invar system, a random face-centred cubic iron–nickel alloy, in which we allow for non-collinear spin alignments—that is, spins that may be canted with respect to the average magnetization direction. We find that the magnetic structure is characterized, even at zero temperature, by a continuous transition from the ferromagnetic state at high volumes to a disordered non-collinear configuration at low volumes. There is an additional, comparable contribution to the net magnetization from the changes in the amplitudes of the local magnetic moments. The non-collinearity gives rise to an anomalous volume dependence of the binding energy, and explains other peculiarities of Invar systems.

The correlation between mechanical stress and magnetic anisotropy in ultrathin films

Abstract: The impact of stress-driven structural transitions and of film strain on the magnetic properties of nm ferromagnetic films is discussed. The stress-induced bending of film-substrate composites is analysed to derive information on film stress due to lattice mismatch or due to surface-stress effects. The magneto-elastic coupling in epitaxial films is determined directly from the magnetostrictive bending of the substrate. The combination of stress measurements with magnetic investigations by the magneto-optical Kerr effect (MOKE) reveals the modification of the magnetic anisotropy by film stress. Stress-strain relations are derived for various epitaxial orientations to facilitate the analysis of the substrate curvature. Biaxial film stress and magneto-elastic coupling coefficients are measured in epitaxial Fe films in situ on W single-crystal substrates. Tremendous film stress of more than 10 GPa is measured in pseudomorphic Fe layers, and the important role of film stress as a driving force for the formation of misfit distortions and for inducing changes of the growth mode in monolayer thin films is presented. The direct measurement of the magneto-elastic coupling in epitaxial films proves that the magnitude and sign of the magneto-elastic coupling deviate from the respective bulk value. Even a small film strain of order 0.1% is found to induce a significant change of the effective magneto-elastic coupling coefficient. This peculiar behaviour is ascribed to a second-order strain dependence of the magneto-elastic energy density, in contrast to the linear strain dependence that is valid for bulk samples.

Model for exchange bias in polycrystalline ferromagnet-antiferromagnet bilayers

Abstract: This paper describes a model for polycrystalline ferromagnet-antiferromagnet bilayers. Independent antiferromagnetic grains are coupled to a ferromagnetic film both by direct coupling to the net moments at the interfaces of the grains and by spin-flop coupling. Rotation of the ferromagnetic magnetization applies a torque to the antiferromagnetic spins at the interface of each grain which winds up partial domain walls in the antiferromagnet. The model explains both the unidirectional anisotropy that gives rise to the well-known shifted hysteresis loops, and the hysteretic effects observed in rotational torque and ferromagnetic resonance experiments. The unidirectional anisotropy comes from grains in which the antiferromagnetic order is stable as the magnetization is rotated. The hysteretic effects come from grains in which the antiferromagnetic order irreversibly switches as the domain wall is wound up past a postulated critical angle. For all of the models considered here, spin-flop coupling does not contribute to the unidirectional anisotropy.

High-temperature weak ferromagnetism in a low-density free-electron gas

Abstract: The magnetic properties of the ground state of a low-density free-electron gas in three dimensions have been the subject of theoretical speculation and controversy for seven decades. Not only is this a difficult theoretical problem to solve, it is also a problem which has not hitherto been directly addressed experimentally. Here we report measurements on electron-doped calcium hexaboride (CaB6) which, we argue, show that-at a density of 7× 1019 electrons cm-3-the ground state is ferromagnetically polarized with a saturation moment of 0.07 µB per electron. Surprisingly, the magnetic ordering temperature of this itinerant ferromagnet is 600 K, of the order of the Fermi temperature of the electron gas.

Magnetic properties of organic materials

Abstract: Genesis of the concept: intermolecular ferromagnetic spin exchange structural determinants of the chemical and magnetic properties of non-kekule molecules antiaromatic triplet ground state molecules - building blocks for organic magnets. Theory of exchange in organic systems: resonating valence bond theory and magnetic properties qualitative and quantitative predictions and measurements of singlet-triplet splittings in non-Kekule hydrocarbon diradicals and heteroatom derivatives. Exchange in small organic open-shell systems: conformational exchange modulation in trimethylenemethane-type biradicals use of dinitrenes as models for intramolecular exchange radical ion building blocks for organic magnetic materials - computational and experimental approaches. Exchange in polymeric organic systems: design and experimental investigation of high-spin organic systems synthesis and properties of organic conjugated polyradicals pendant and conjugated organic polyradicals. Theory of organic magnetism in condensed phases: band theory of exchange effects in organic open shell systems orbital interactions determining the exchange effects in organic molecular crystals molecular orbital theoretical elucidation of spin alignments in organic magnetic crystals. Magnetism in organic condensed phases: exchange effects in three dimensions - real materials bulk magnetic properties of organic verdazyl radical crystals neutron diffraction studies of spin densities in magnetic molecular materials heat capacity studies of organic radical crystals. Summation: an integrated approach to organic-based molecular magnetic materials.

Itinerant electron metamagnetic transition in La(FexSi1−x)13 intermetallic compounds

Abstract: A first-order transition above the Curie temperature for ferromagnetic La(FexSi1−x)13 (x=0.86 and 0.88) compounds has been confirmed by applying a magnetic field. The magnetic state changes from the paramagnetic to the ferromagnetic state and the transition field increases with temperature, indicating an itinerant electron metamagnetic (IEM) transition. The IEM transition is broad in x=0.86 and becomes clearer in x=0.88, which takes a negative slope of the Arrott plot. The volume change just above the Curie temperature for x=0.88 is huge, about 1.5%, which is caused by a large magnetic moment induced by the IEM transition.

First-principles investigation of ferromagnetism and ferroelectricity in bismuth manganite

Abstract: We present results of local spin density approximation (LSDA) pseudopotential calculations for the perovskite structure oxide, bismuth manganite (BiMnO3). The origin of the differences between bismuth manganite and other perovskite manganites is determined by first calculating total energies and band structures of the high symmetry cubic phase, then sequentially lowering the magnetic and structural symmetry. Our results indicate that covalent bonding between bismuth cations and oxygen anions stabilizes different magnetic and structural phases compared with the rare earth manganites. This is consistent with recent experimental results showing enhancement of charge ordering in doped bismuth manganite.

Ab initio full-potential study of the structural and magnetic phase stability of iron

Abstract: We have investigated the magnetic and structural phase diagram of iron employing the full potential linear augmented plane-wave method within the generalized gradient approximation. Therefore, total-energy calculations have been performed together with investigations with varying $c/a$ ratio to check the phase stability. This study focuses on the structural and magnetic properties relevant to Invar and anti-Invar and structural phase transitions occurring in these materials. We show that the properties of antiferromagnetic fcc iron can be understood by collinear full potential calculations. In order to do this, the antiferromagnetic structure has been distorted by short-range ferromagnetic nearest-neighbor coupling. From this we can conclude that the classical low-spin behavior can be replaced by antiferromagnetic ordering. Additionally, the thermal properties of iron, especially the free-energy and thermal-expansion coefficient $\ensuremath{\alpha}(T)$ have been analyzed, which is important for the understanding of the anti-Invar effect. The free energy and $\ensuremath{\alpha}(T)$ were estimated from a Debye scheme for which ab initio results were given as input. Besides the more common cubic phases we have investigated hcp Fe at large volumes in view of its magnetic structure.

Spin-interference device

Abstract: We propose a spin-interference device which works even without any ferromagnetic electrodes and any external magnetic field. The interference can be expected in the Aharonov–Bohm (AB) ring with a uniform spin-orbit interaction, which causes the phase difference between the spin wave functions traveling in the clockwise and anticlockwise direction. The gate electrode, which covers the whole area of the AB ring, can control the spin-orbit interaction, and therefore, the interference. A large conductance modulation effect can be expected due to the spin interference.

Handbook of Modern Ferromagnetic Materials

Abstract: Foreword Takeshi Takei. Preface. Acknowledgements. 1. Applications and Functions of Ferromagnetic Material. 2. Basics of Magnetism - Source of Magnetic Effect. 3. The Magnetization in Domains and Bulk Materials. 4. AC Properties of Magnetic Materials. 5. Materials for Permanent Magnet Applications. 6. DC and Low Frequency Applications. 7. Soft Cobalt-Iron Alloys. 8. Metallic Materials for Magnetic Shielding Applications. 9. High Permeability-High Frequency Metal Strip. 10. Metal Powder Cores for Telecommunications. 11. Crystal Structure of Ferrites. 12. Chemical Aspects of Ferrites. 13. Microstructural Aspects of Ferrites. 14. Ferrite Processing. 15. Ferrite Inductors and Transformers for Low Power. 16. Soft Magnetic Materials for EMI Suppression. 17. Ferrites for Entertainment Applications. 18. Ferrite Transformers and Inductors at High Power. 19. Materials for Magnetic Recording. 20. Ferrites for Microwave Applications. 21. Miscellaneous Magnetic Material Applications. 22. Physical-Thermal Aspects of Magnetic Materials. 23. Magnetic Measurements-Materials and Components. Bibliography. Appendix 1: Abbreviations and Symbols. Appendix 2: List of World's Major Ferrite Suppliers. Appendix 3: Units Conversion from CGS to MKS(SI) System. Index.

Spin-spin interaction and spin squeezing in an optical lattice

Abstract: We show that, by displacing two optical lattices with respect to each other, we may produce interactions similar to the ones describing ferromagnetism and antiferromagnetism in condensed matter physics. We also show that particularly simple choices of the interaction lead to spin squeezing, which may be used to improve the sensitivity of atomic clocks. Spin squeezing is generated even with partially, and randomly, filled lattices, and our proposal may be implemented with current technology.

Magnetic circular dichroism studies of carrier-induced ferromagnetism in (Ga1-xMnx)as

Abstract: Magnetic circular dichroism is used to investigate the evolution of ferromagnetism in the $p$-type magnetic semiconductor $({\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x})\mathrm{As}$. Local Mn moments and holes produce two spectroscopically distinct contributions, whose properties reveal an antiferromagnetic Mn-hole alignment in the ferromagnetic state. These components are present in both metallic and insulating samples with different temperature and field dependences, suggesting that the holes play a more active role in mediating the ferromagnetic exchange than in traditional RKKY systems.

Diffuse Phase Transition and Phase Separation in Cr-Doped Nd 1/2 Ca 1/2 MnO 3 : A Relaxor Ferromagnet

Abstract: We have investigated the diffuse phase transition accompanying the formation of submicrometric ferromagnetic-metallic (FM) domains embedded in the antiferromagnetic charge-ordered state in ${\mathrm{Nd}}_{1/2}{\mathrm{Ca}}_{1/2}{\mathrm{MnO}}_{3}$ crystals, which is caused by quenched random field originating from Cr impurities substituted on the Mn sites. The fraction of the FM phase volume or the size of the FM cluster can be controlled to a large extent by the magnetic-field annealing as well as the Cr content. The observed phenomena are reminiscent of those of relaxor ferroelectrics composed of ferroelectric clusters embedded in paraelectric matrix.

First-principles calculation of the effect of atomic disorder on the electronic structure of the half-metallic ferromagnet NiMnSb

Abstract: The electronic structure of the half-metallic ferromagnet NiMnSb with three different types of atomic disorder is calculated using the layer Korringa-Kohn-Rostoker method in conjunction with the coherent potential approximation. Results indicate the presence of minority-spin states at the Fermi energy for degrees of disorder as low as a few percent. The resulting spin polarization below 100% is discussed in the light of experimental difficulties confirming the half-metallic property of NiMnSb thin films directly.

Interlayer coupling in ferromagnetic semiconductor superlattices

Abstract: We develop a mean-field theory of carrier-induced ferromagnetism in diluted magnetic semiconductors. Our approach represents an improvement over standard Ruderman-Kittel-Kesuya-Yosida model allowing spatial inhomogeneity of the system, free-carrier spin polarization, finite temperature, and free-carrier exchange and correlation to be accounted for self-consistently. As an example, we calculate the electronic structure of a ${\mathrm{Mn}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}$As/GaAs superlattice with alternating ferromagnetic and paramagnetic layers and demonstrate the possibility of semiconductor magnetoresistance systems with designed properties.

Carrier Density Collapse and Colossal Magnetoresistance in Doped Manganites

Abstract: A novel ferromagnetic transition, accompanied by carrier density collapse, is found in doped charge-transfer insulators with strong electron-phonon coupling. The transition is driven by an exchange interaction of polaronic carriers with localized spins; the strength of the interaction determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of bound pair formation in the paramagnetic phase close to the transition, is extremely sensitive to an external magnetic field. This carrier density collapse describes the resistivity peak and the colossal magnetoresistance of doped manganites.

Magnetic memory array with magnetic tunnel junction memory cells having flux-closed free layers

Abstract: An improved magnetic tunnel junction (MTJ) memory cell for use in a nonvolatile magnetic random access memory (MRAM) array has a free layer formed as two ferromagnetic films that are magnetostatically coupled antiparallel to one another by their respective dipole fields. The magnetostatic or dipolar coupling of the two ferromagnetic films occurs across a nonferromagnetic spacer layer that is selected to prevent exchange coupling between the two ferromagnetic films. The magnetic moments of the two ferromagnetic films are antiparallel to another so that the multilayer free layer structure has a reduced net magnetic moment. In the presence of an applied magnetic field, such as during writing to the cell, the moments of the two ferromagnetic films switch directions substantially simultaneously, so that the net magnetic moment of the multilayer free layer structure can have two possible orientations relative to the orientation of the fixed or pinned layer of the MTJ cell, thus resulting in the two stable magnetic states of the MTJ cell. The reduced net magnetic moment of the multilayer free layer structure reduces the magnetostatic coupling between the multilayer free layer and the pinned ferromagnetic layer in the MTJ cell, as well as the magnetostatic coupling between adjacent MTJ cells in the array. As a result, the cells, and thus the MRAM array, can be made smaller.

Spin current in ferromagnet-insulator-superconductor junctions

Abstract: A theory of spin polarized tunneling spectroscopy based on a scattering theory is given for tunneling junctions between ferromagnets and d-wave superconductors. The spin filtering effect of an exchange field in the insulator is also treated. We clarify that the properties of the Andreev reflection are largely modified due to the presence of an exchange field in the ferromagnets, and consequently the Andreev reflected quasiparticle shows an evanescent-wave behavior depending on the injection angle of the quasiparticle. Conductance formulas for the spin current as well as the charge current are given as a function of the applied voltage and the spin polarization in the ferromagnet for arbitrary barrier heights. It is shown that the surface bound states do not contribute to the spin current and that the zero-bias conductance peak expected for a d-wave superconductor splits into two peaks under the influence of the exchange interaction in the insulator.

Coexistence of ferroelectricity and ferromagnetism in BiFeO3–BaTiO3 thin films at room temperature

Abstract: (Bi0.7Ba0.3) (Fe0.7Ti0.3)O3 films have been constructed on Nb-doped SrTiO3 (100) by the pulsed-laser deposition technique, and their physical properties have been examined. The films exhibit both ferroelectricity and ferromagnetism (weak ferromagnetism) with Pr=2.5 μC/cm2 and Mr=0.2 emu/g at room temperature. The film-thickness dependence of their magnetic and electric properties (size effect) is also discussed simultaneously. The dielectric constants of the films decrease with reducing film thickness below 1000 A. The magnetic Curie temperature of the films, on the other hand, does not change at all down to 250 A.

Charge and Orbital Order in Half-Doped Manganites

Abstract: Manganese oxides with the general composition R12xAxMnO3 (where R and A are rare- and alkaline-earth ions, respectively) have attracted considerable attention because of their unusual magnetic and electronic properties. In some of these materials, metal-insulator transitions can be observed where both conductivity and magnetization change markedly. The x 0 and x 1 end members of the R12xAxMnO3 family are insulating and antiferromagnetic (AF) with the Mn ion in the Mn 31 and Mn 41 state, respectively. For intermediate x, the average Mn valence is noninteger and the material is generally metallic or semiconducting. Most of the perovskite manganites show a ferromagnetic (FM) ground state when the holes are optimally doped (usually 0.2 , x , 0.5) and anisotropic antiferromagnetic (AFM) phases for x . 0.5. The half-doped manganites, with x 1 , are very particular. Magnetically these systems form FM