Showing papers on "Antiferromagnetism published in 2001"

Magnetism and Magnetic Materials

Abstract: 1. Introduction 2. Magnetostatics 3. Magnetism of electrons 4. Magnetism of localized electrons on the atom 5. Ferromagnetism and exchange 6. Antiferromagnetism and other magnetic order 7. Micromagnetism, domains and hysteresis 8. Nanoscale magnetism 9. Magnetic resonance 10. Experimental methods 11. Magnetic materials 12. Applications of soft magnets 13. Applications of hard magnets 14. Spin electronics and magnetic recording 15. Special topics Appendixes Index.

Magnetic properties of Mn-doped ZnO

Abstract: We report on the magnetic properties of an oxide-diluted magnetic semiconductor (DMS), Zn0.64Mn0.36O. The temperature dependence of the magnetization shows a spin-glass behavior with the large magnitude of the Curie–Weiss temperature, corresponding to a stronger antiferromagnetic exchange coupling than other II–VI DMSs. The small effective Mn moment (x∼0.02) under high field also represents a strong antiferromagnetic exchange coupling in this compound.

Coupling between magnetism and dielectric properties in quantum paraelectric EuTiO 3

Abstract: The dielectric constant of quantum paraelectric ${\mathrm{EuTiO}}_{3},$ which contains ${\mathrm{Eu}}^{2+}$ with $S=7/2$ spin and ${\mathrm{Ti}}^{4+},$ has been measured under a magnetic field. The dielectric constant shows a critical decrease at the antiferromagnetic ordering of the Eu spins at 5.5 K, as well as a substantial change under a magnetic field (by $\ensuremath{\sim}7%$ with 1.5 T), indicating a strong coupling between the Eu spins and dielectric properties. We show that the variation of the dielectric constant is dominated by the pair correlation of the nearest-neighbor Eu spins, likely via the variation of the soft-phonon-mode frequency.

Ising models of quantum frustration

Abstract: We report on a systematic study of two-dimensional, periodic, frustrated Ising models with quantum dynamics introduced via a transverse magnetic field. The systems studied are the triangular and kagome{prime} lattice antiferromagnets, fully frustrated models on the square and hexagonal (honeycomb) lattices, a planar analog of the pyrochlore antiferromagnet, a pentagonal lattice antiferromagnet, as well as two quasi-one-dimensional lattices that have considerable pedagogical value. All of these exhibit a macroscopic degeneracy at T=0 in the absence of the transverse field, which enters as a singular perturbation. We analyze these systems with a combination of a variational method at weak fields, a perturbative Landau-Ginzburg-Wilson approach from large fields, as well as quantum Monte Carlo simulations utilizing a cluster algorithm. Our results include instances of quantum order arising from classical criticality (triangular lattice) or classical disorder (pentagonal and probably hexagonal) as well as notable instances of quantum disorder arising from classical disorder (kagome{prime}). We also discuss the effect of finite temperature, as well as the interplay between longitudinal and transverse fields{emdash}in the kagome{prime} problem the latter gives rise to a nontrivial phase diagram with bond-ordered and bond-critical phases in addition to the disordered phase. We also note connections to quantum-dimer models and therebymore » to the physics of Heisenberg antiferromagnets in short-ranged resonating valence-bond phases that have been invoked in discussions of high-temperature superconductivity.« less

Spins in the vortices of a high-temperature superconductor.

Abstract: Neutron scattering is used to characterize the magnetism of the vortices for the optimally doped high-temperature superconductor La2-xSrxCuO4 ( x = 0.163) in an applied magnetic field. As temperature is reduced, low-frequency spin fluctuations first disappear with the loss of vortex mobility, but then reappear. We find that the vortex state can be regarded as an inhomogeneous mixture of a superconducting spin fluid and a material containing a nearly ordered antiferromagnet. These experiments show that as for many other properties of cuprate superconductors, the important underlying microscopic forces are magnetic.

Perpendicular exchange bias of Co/Pt multilayers.

Abstract: Exchange bias measurements of ferromagnetic/antiferromagnetic (F/AF) bilayers are typically performed with the magnetization of the F layer parallel to the AF interface. We describe measurements of Co/Pt multilayers with out-of-plane magnetic easy axis that are exchange biased with CoO. Field-cooling experiments with the applied field perpendicular and parallel to the sample plane exhibit loop shifts and enhanced coercivities. Modeling and comparison to biasing of samples with planar easy axis suggests such measurements provide a way to probe the spin projections at F/AF interfaces.

Orbital ferromagnetism and anomalous Hall effect in antiferromagnets on the distorted fcc lattice.

Abstract: The Berry phase due to the spin wave function gives rise to the orbital ferromagnetism and anomalous Hall effect in the noncoplanar antiferromagnetic ordered state on face-centered-cubic (fcc) lattice once the crystal is distorted perpendicular to the $(1,1,1)$ or $(1,1,0)$ plane. The relevance to the real systems $\ensuremath{\gamma}$-FeMn and ${\mathrm{NiS}}_{2}$ is also discussed.

Spin Reorientation at the Antiferromagnetic NiO(001) Surface in Response to an Adjacent Ferromagnet

Abstract: Polarization dependent x-ray photoemission electron microscopy was used to investigate the spin structure near the surface of an antiferromagnetic NiO(001) single crystal in response to the deposition of a thin ferromagnetic Co film. For the cleaved NiO surface we observe only a subset of bulklike antiferromagnetic domains which is attributed to minimization of dipolar energies. Upon Co deposition a spin reorientation near the NiO interface occurs, with the antiferromagnetic spins rotating in plane, parallel to the spins of the Co layer. Our results demonstrate that the spin configuration in an antiferromagnet near its interface with a ferromagnet may significantly deviate from that in the bulk antiferromagnet.

Carrier-induced ferromagnetism in p − Zn 1 − x Mn x Te

Abstract: We present a systematic study of the ferromagnetic transition induced by the holes in nitrogen doped ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ epitaxial layers, with particular emphasis on the values of the Curie-Weiss temperature as a function of the carrier and spin concentrations. The data are obtained from thorough analyses of the results of magnetization, magnetoresistance, and spin-dependent Hall effect measurements. The experimental findings compare favorably, without adjustable parameters, with the prediction of the Rudermann-Kittel-Kasuya-Yosida (RKKY) model or its continuous-medium limit, that is, the Zener model, provided that the presence of the competing antiferromagnetic spin-spin superexchange interaction is taken into account, and the complex structure of the valence band is properly incorporated into the calculation of the spin susceptibility of the hole liquid. In general terms, the findings demonstrate how the interplay between the ferromagnetic RKKY interaction, carrier localization, and intrinsic antiferromagnetic superexchange affects the ordering temperature and the saturation value of magnetization in magnetically and electrostatically disordered systems.

Crystallographic symmetry and magnetic structure of CoO

Abstract: The crystallographic symmetry of antiferromagnetic CoO was studied using high-resolution synchrotron powder diffraction in the temperature range 10--300 K. The high-quality powder patterns unambiguously revealed a monoclinic symmetry (space group $C2/m)$ and allowed the extraction of accurate values for the lattice constants. The temperature dependence of the monoclinic deformation scales with the much stronger tetragonal distortion as determined from laboratory x-ray diffraction. Magnetic ordering is associated with a cubic-to-monoclinic transition that is, thus, of first order. Neutron powder-diffraction data are compatible with a collinear magnetic structure with the moments ordered in the monoclinic ac plane.

Interaction of superconductivity and magnetism in borocarbide superconductors

Abstract: The interaction of rare-earth magnetism and superconductivity has been a topic of interest for many years. In classical magnetic superconductors (Chevrel phases, ternary rhodium borides, etc) as well as in the high-Tc cuprates the superconducting state usually coexists with antiferromagnetic order on the rare-earth sublattice. In these compounds the magnetic ordering temperature TN is much below the superconducting transition temperature Tc. The discovery of superconducting borocarbides RT2B2C with R = Sc, Y, La, Th, Dy, Ho, Er, Tm or Lu and T = Ni, Ru, Pd or Pt (where not all of these combinations of R and T result in superconductivity) has reanimated the research on the coexistence of superconductivity and magnetic order. Most of these borocarbides crystallize in the tetragonal LuNi2B2C type structure which is an interstitial modification of the ThCr2Si2 type. Contrary to the behaviour of Cu in the cuprates Ni does not carry a magnetic moment in the borocarbides. Various types of antiferromagnetic structures on the rare-earth sublattice have been found to coexist with superconductivity in RNi2B2C for R = Tm, Er, Ho and Dy. Particularly of interest is the case of HoNi2B2C for which three different types of antiferromagnetic structures have been observed: (i) a commensurate one with Ho moments aligned ferromagnetically within layers perpendicular to the tetragonal c axis where consecutive layers are aligned in opposite directions, (ii) an incommensurate spiral along the c axis and (iii) an incommensurate a-axis-modulated structure with a modulation vector τ≈(0.55,0,0). This wave vector emerges in various RNi2B2C compounds with magnetic as well as nonmagnetic R elements and is connected with Fermi surface nesting. Both incommensurate magnetization structures have been shown to be related to the near-reentrant behaviour observed in HoNi2B2C whereas the commensurate structure coexists well with the superconducting state in this compound. The variation of TN and Tc with the de Gennes factor can roughly be drawn on straight lines from Lu to Gd and from Lu to Tb, respectively, with the exception of Yb. Consequently, Tc>TN holds for Tm, Er, Ho and Tc

Spectroscopic Identification and Direct Imaging of Interfacial Magnetic Spins

Abstract: Using x-ray absorption spectromicroscopy we have imaged the uncompensated spins induced at the surface of antiferromagnetic (AFM) NiO(100) by deposition of ferromagnetic (FM) Co. These spins align parallel to the AFM spins in NiO(100) and align the FM spins in Co. The uncompensated interfacial spins arise from an ultrathin CoNiOx layer that is formed upon Co deposition through reduction of the NiO surface. The interfacial Ni spins are discussed in terms of the "uncompensated spins" at AFM/FM interfaces long held responsible for coercivity increases and exchange bias. We find a direct correlation between their number and the size of the coercivity.

Coercivity in exchange-bias bilayers

Abstract: A model is presented for coercivity in polycrystalline exchange-bias bilayers. It includes two contributions for their enhanced coercivity, inhomogeneous reversal, and irreversible transitions in the antiferromagnetic grains. The model can be characterized in terms of a small number of dimensionless parameters, and its behavior has been determined through simulations of magnetic reversal for a range of values of these parameters. In these simulations, the first contribution to the coercivity arises from energy losses in the ferromagnet due to irreversible transitions over small, local energy barriers in the ferromagnetic film due to the inhomogeneous coupling to the antiferromagnet. This inhomogeneous reversal contributes to the coercivity at all temperatures. The second contribution to the coercivity arises from energy losses in the antiferromagnet due to irreversible transitions of the antiferromagnetic order in the grains. In the present model, the antiferromagnetic order only becomes unstable at nonzero temperature, so that this contribution to the coercivity only occurs at nonzero temperatures. In addition to the coercivity, the computed hysteresis loops are found to be asymmetric, and the loop shift is shown to differ from the grain-averaged unidirectional anisotropy.

Strong ferromagnetism and weak antiferromagnetism in double perovskites: Sr2FeMO6 (M = Mo, W, and Re)

Abstract: Double perovskites Sr2FeMO6 (M = Mo and Re) exhibit significant colossal magnetoresistance even at room temperature due to the high Curie temperatures (419 and 401 K). However, such a high Curie temperature is puzzling, given the large separation between

Spin-1/2 Kagome Like Lattice in Volborthite Cu3V2O7(OH)2-H2O

Abstract: A novel cuprate Volborthite, Cu3V2O7(OH)2-2H2O, containing an S-1/2 (Cu 2+ spin) kagome-like lattice is studied by magnetic susceptibility, specific heat, and 51 V NMR measurements. Signs for neither long-range order nor spin-gapped singlet ground states are detected down to 1.8 K, in spite of large antiferromagnetic couplings of ~100 K between Cu spins forming a two-dimensional kagome-like network. It is suggested that Volborthite represents a system close to a quantum critical point between classical long-range ordered and quantum disordered phases.

Large magneto-optical effect in an oxide diluted magnetic semiconductor Zn1−xCoxO

Abstract: Strong magneto-optical effect of Zn1−xCoxO near the optical band gap is reported. Magnetic circular dichroism (MCD) and Faraday rotation of Zn1−xCoxO (x=0.012 and 0.016) at 5 K are as high as 2°/cm Oe at 3.4 eV, which are about two orders larger than those of ZnO. Absence of strong Co2+ d–d* transition near the optical band gap makes Zn1−xCoxO an useful material for short-wavelength magneto-optical applications. Large magneto-optical effects and paramagnetic temperature dependence of MCD show a strong mixing of the sp bands of the host ZnO with Co2+ d orbitals. The polarity of the MCD peak is negative indicating the antiferromagnetic p–d exchange interaction.

Interface ferromagnetism in oxide superlattices of CaMnO3/CaRuO3

Abstract: Oxide superlattices composed of antiferromagnetic insulator layers of CaMnO3 (10 unit cells) and paramagnetic metal layers of CaRuO3 (N unit cells) were fabricated on LaAlO3 substrates by pulsed-laser deposition. All the superlattices show ferromagnetic transitions at an almost identical temperature (TC∼95 K) and negative magnetoresistance below TC. Each magnetization and magnetoconductance of the whole superlattice at 5 K is constant and independent of CaRuO3 layer thickness when normalized by the number of the interfaces between CaMnO3 and CaRuO3. These results indicate that the ferromagnetism shows up only at the interface and is responsible for the magnetoresistance.

Magnetically mediated superconductivity in quasi-two and three dimensions

Abstract: We compare predictions of the mean-field theory of superconductivity for nearly antiferromagnetic and nearly ferromagnetic metals in two and three dimensions. The calculations are based on a parametrization of the effective interaction arising from the exchange of magnetic fluctuations. The results show that for comparable parameters, magnetic pairing is more robust in quasi-two-dimensions than in three dimensions, for either p-wave (spin triplet) pairing in nearly ferromagnetic metals or d-wave (spin singlet) pairing in nearly antiferromagnetic metals. Moreover we find higher mean-field transition temperatures for d-wave pairing than for p-wave pairing (for comparable parameters), regardless of dimensionality. We present intuitive arguments for why quasi-two-dimensional d-wave pairing is a particularly favorable case.

Spin-dependent tunneling sensor suitable for a magnetic memory

Abstract: A method and system for providing a top pinned spin-dependent tunneling sensor is disclosed. The method and system include providing a free layer, a tunneling barrier, a synthetic pinned layer and an antiferromagnetic layer. The free layer is ferromagnetic. The tunneling barrier is an insulator. The tunneling barrier is disposed between the free layer and the synthetic pinned layer. The synthetic pinned layer is ferromagnetic and includes a ferromagnetic top layer. The synthetic pinned layer is between the tunneling barrier and the antiferromagnetic layer. The ferromagnetic top layer acts as a seed layer for the antiferromagnetic layer.

A Novel Antiferromagnetic Organic Superconductor κ-(BETS)2FeBr4 [Where BETS = Bis(ethylenedithio)tetraselenafulvalene]

Abstract: The electrical and magnetic properties of κ-(BETS)2FeBr4 salt [where BETS = bis(ethylenedithio)tetraselenafulvalene] showed that this system is the first antiferromagnetic organic metal at ambient pressure (TN = 2.5 K). The characteristic field dependence of the magnetization at 2.0 K indicates a clear metamagnetic behavior. The small resistivity drop observed at TN clearly shows the existence of the interaction between π metal electrons and localized magnetic moments of Fe3+ ions. In addition, this system underwent a superconducting transition at 1.1 K. That is, κ-(BETS)2FeBr4 is the first antiferromagnetic organic metal exhibiting a superconducting transition below Neel temperature. The magnetic field dependence of the superconducting critical temperature indicated that the superconductivity in this system is strongly anisotropic also in the conduction plane because of the existence of the metamagnetically induced internal field based on the antiferromagnetic ordering of the Fe3+ 3d spins in contrast to...

Charge density studies utilizing powder diffraction and MEM. Exploring of high Tc superconductors, C60 superconductors and manganites

Abstract: The recent progress of the accurate charge density studies by the Maximum Entropy Method (MEM) utilizing X-ray powder diffraction is reviewed with some examples. Results for PrBCO (PrBa 2 Cu 3 O 7-δ ), YBCO (YBa 2 Cu 3 O 7-δ ), C 60 superconductors (Rb 2 CsC 60 , K 2 RbC 60 , Na 2 RbC 60 ) and the layered manganite, NdSr 2 Mn 2 O 7 , which is well known as colossal magnetoresistive (CMR)-related material, are given. For non-super conductor, PrBCO, it is found in the MEM charge density that there exist directional robes of the charge density from Pr atom toward the O atoms in the CuO 2 conduction planes. On the other hand, for a very well known high Tc super conductor, YBCO, appreciable charge densities in the interatomic region around the Y atom is not recognized in the MEM charge density. The distinct difference of the charge densities between PrBCO and YBCO presents clear experimental evidence of the hybridization between Pr(4f)-O(2pπ) orbitals which supports the idea that the hole trapping by the hybridized states suppresses the superconductivity in PrBCO. The MEM charge densities of the fullerene superconductors, Rb 2 CsC 60 , K 2 RbC 60 and Na 2 RbC 60 , show distinct structural differences from that of non-superconductors, C 60 and Li 2 CsC 60 , reflecting the superconducting properties. And the charge deficiencies of the doped metal atoms, which should be associated with charge transfer from the metal atoms to the C 60 molecule seems to have strong correlation to the superconducting transition temperature, Tc. As the bigger the charge transfer, the higher the Tc. The accurate MEM charge density of antiferromagnetic manganite, NdSr 2 Mn 2 O 7 , presents the direct imaging of spontaneous ordering of the dx 2 - y 2 orbital indicating anisotropic exchange couplings between the local-spins on the Mn sites, which causes an unique layered-type spin order. The theoretical background of the MEM is also mentioned in some detail.

Metal-Nonmetal Changeover in Pyrochlore Iridates

Abstract: We report the low temperature properties of a new series of pyrochlore iridates R 2 Ir 2 O 7 ( R =rare-earth elements). We found that the compounds with R =Pr, Nd, Sm, and Eu exhibit metallic conductivity, whereas those with smaller rare-earth ions are nonmetallic. Such metal-nonmetal changeover is attributable to the importance of electron correlation among the Ir 5d electrons. Reflecting the geometrical frustration in the pyrochlore lattice, the rare-earth moments do not exhibit magnetic ordering to temperatures well below the antiferromagnetic Weiss temperature. We did not find any sign of superconductivity down to 0.3 K in these compounds.

Domain state model for exchange bias

Abstract: Monte Carlo simulations of a system consisting of a ferromagnetic layer exchange coupled to a diluted antiferromagnetic layer described by a classical spin model show a strong dependence of the exchange bias on the degree of dilution in agreement with recent experimental observations on Co/CoO bilayers. These simulations reveal that diluting the antiferromagnet leads to the formation of domains in the volume of the antiferromagnet carrying a remanent surplus magnetization which causes and controls exchange bias. To further support this domain state model for exchange bias we study, in the present article, the dependence of the bias field on the thickness of the antiferromagnetic layer. It is shown that the bias field strongly increases with increasing film thickness and eventually goes over a maximum before it levels out for large thicknesses. These findings are in full agreement with experiments.

Spatially resolved electronic structure inside and outside the vortex cores of a high-temperature superconductor

Abstract: Puzzling aspects of high-transition-temperature (high-Tc) superconductors include the prevalence of magnetism in the normal state and the persistence of superconductivity in high magnetic fields. Superconductivity and magnetism generally are thought to be incompatible, based on what is known about conventional superconductors. Recent results, however, indicate that antiferromagnetism can appear in the superconducting state of a high-Tc superconductor in the presence of an applied magnetic field. Magnetic fields penetrate a superconductor in the form of quantized flux lines, each of which represents a vortex of supercurrents. Superconductivity is suppressed in the core of the vortex and it has been suggested that antiferromagnetism might develop there. Here we report the results of a high-field nuclear-magnetic-resonance (NMR) imaging experiment in which we spatially resolve the electronic structure of near-optimally doped YBa2Cu3O7-delta inside and outside vortex cores. Outside the cores, we find strong antiferromagnetic fluctuations, whereas inside we detect electronic states that are rather different from those found in conventional superconductors.

Paramagnetism and antiferromagnetic d–d coupling in GaMnN magnetic semiconductor

Abstract: The magnetization of Ga1−xMnxN (x<0.1) crystals was measured as a function of the magnetic field and temperature. Paramagnetic behavior typical of spin S=5/2 expected for Mn2+ (d5) magnetic centers was observed in the temperature range of 2 K

Synthesis, Structural Characterization, Magnetic Properties, and Ionic Conductivity of Na4MII3(PO4)2(P2O7) (MII = Mn, Co, Ni)

Abstract: The new phases, Na4M3(PO4)2(P2O7) (M = Mn, Co, Ni), have been synthesized by solid-state reactions. Single crystals of Na4M3(PO4)2(P2O7) (M = Mn, Ni) have been isolated and their structure has been determined by X-ray diffraction techniques using as the starting model the structure of the isostructural compound Na4Co3(PO4)2(P2O7). These compounds crystallize in the orthorhombic noncentrosymmetric space group Pn21a with a = 17.991(3) A, b = 6.6483(1), and c = 10.765(2) A for the manganese compound and a = 17.999(2), b = 6.4986(6) A, and c = 10.4200(9) A for the nickel compound, with Z = 4. Magnetic measurements reveal the existence of antiferromagnetic interactions in the nickel compound. The manganese and cobalt compounds show canting antiferromagnetic behavior at low temperatures. Magnetic correlation is also studied from the analysis of possible superexchange pathways in the structure. The ionic conductivity, due to Na+ ions, is measured for the three compounds. The activation energy is nearly the same ...

Spin valve sensors having synthetic antiferromagnet for longitudinal bias

Abstract: Magnetoresistive (MR) sensors are disclosed having mechanisms for reducing edge effects such as Barkhausen noise. The sensors include a pinned layer and a free layer with an exchange coupling layer adjoining the free layer, and a ferromagnetic layer having a fixed magnetic moment adjoining the exchange coupling layer. The exchange coupling layer and ferromagnetic layer form a synthetic antiferromagnetic structure with part of the free layer, providing bias that reduces magnetic instabilities at edges of the free layer. Such synthetic antiferromagnetic structures can provide a stronger bias than conventional antiferromagnetic layers, as well as a more exactly defined track width than conventional hard magnetic bias layers. The synthetic antiferromagnetic structures can also provide protection for the free layer during processing, in contrast with the trimming of conventional antiferromagnetic layers that exposes if not removes part of the free layer.