Showing papers on "Antiferromagnetism published in 1987"

Random-field model of exchange anisotropy at rough ferromagnetic-antiferromagnetic interfaces.

Abstract: A field-asymmetric offset of the hysteresis loop in ferromagnetic-antiferromagnetic sandwiches, one of the manifestations of so-called exchange anisotropy, can be predicted from the presence of random interface roughness giving rise to a random field acting on the interface spins. The antiferromagnet breaks up into domains of size determined by the competition of exchange and an additional uniaxial in-plane anisotropy, and this size sets the scale for averaging of the random field.

Simple model for thin ferromagnetic films exchange coupled to an antiferromagnetic substrate

Abstract: The exchange field Hex transferred from a thick antiferromagnetic substrate to a thin exchange coupled ferromagnetic film is shown to reach a limiting value no matter how large the exchange coupling is. The limit is due to domain‐wall formation in the antiferromagnet. Numerical results based on a simple model for the interface are presented and compared to experimental results.

Antiferromagnetism in La 2 CuO 4-y

Abstract: Powder neutron diffraction studies of undoped ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{y}}$ have revealed new superlattice peaks below \ensuremath{\sim}220 K. The absence of corresponding x-ray superlattice lines and an observed susceptibility anomaly near 220 K suggest the occurrence of antiferromagnetism. From the magnetic peak intensities we deduce a structure consisting of ferromagnetic sheets of Cu spins alternating along the [100] orthorhombic axis, with the spins aligned along the [001] orthorhombic axis, The low-temperature magnetic moment is approximately 0.5${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$/(Cu atom). The tetragonal-orthorhombic transition at 505 K has also been studied.

Two-dimensional antiferromagnetic quantum spin-fluid state in La2CuO

Abstract: Elastic, quasielastic, and inelastic neutron scattering measurements in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ reveal novel two-dimensional antiferromagnetic behavior. At high temperatures the spins are ordered instantaneously over long distances two dimensionally but there is no measurable time-averaged staggered moment. The energy scale of the spin fluctuations is large, corresponding to an effective dispersion of \ensuremath{\gtrsim}0.4 eV A\r{}. The relevance to the high-temperature superconductivity in this class of materials is noted.

Magnetism of the heteropolymetallic systems

Abstract: The field of heteropolymetallic systems with magnetic metal centers occupies a crossing point between biology and physics. For instance the Cu(II)-Fe(III) interaction in cytochrome oxidase is of the same nature as the Cu(II)-Mn(II) interaction in a novel system which could be the first molecular ferromagnet. The mechanism of the interaction is discussed, both from a phenomenological view point using a spin Hamiltonian, and from an orbital view point. An orbital model for the isotropic interaction is presented. It is based on the concept of natural magnetic orbitals. The mechanism of the anisotropic and antisymmetric interactions is more briefly treated. The role of the Zeeman perturbation is then considered in relation with the magnetic and EPR properties of the heterobimetallic complexes. Several examples are presented to emphasize that the nature, ferro- or antiferromagnetic of the isotropic interaction is controlled by the symmetry of the magnetic orbitals. The concept of overlap density is introduced. It permits an estimation of the magnitude of the ferromagnetic stabilization in the case of orthogonality of the magnetic orbitals. The Cu(II)-Fe(III) interaction, in relation to the situation encountered in cytochrome oxidase, the Cu(II)-Ni(II) interaction and a few additional selected examples are discussed. A section deals with the case where one of the interacting ions has an orbital degeneracy. Afterwards, the heterotrinuclear complexes are studied. The important concept of regular and irregular spin state structure is developped and the Mn(II)Cu(II)Mn(II) triad is presented as a spectacular example of irregular spin state structure. A section is devoted to the ordered bimetallic chains. The theory is presented, both at a qualitative and quantitative levels and the already reported compounds of this kind are discussed. One of them may be considered as one of the first molecular ferromagnets. The last but one section concerns the systems with even more subtle spin orders. In conclusion, the vast perspectives of this area are outlined.

Antiferromagnetism, localization, and pairing in a two-dimensional model for CuO2.

Abstract: The properties of a model to describe the ${\mathrm{CuO}}_{2}$ planes in the high-${\mathrm{T}}_{\mathrm{c}}$ superconductors are discussed in the strong-coupling limit. On the assumption that doping creates holes on the O rather than the Cu sites, following the recent suggestion of Emery, it is shown that antiferromagnetic ordering of the Cu moments will lead to either localization or pairing of the O holes. The effect of magnetic anisotropy on the pairing interaction is discussed, and it is suggested that it will strongly enhance superconductivity in these systems.

Antiferromagnetism of La 2 CuO 4 − y studied by muon-spin rotation

Abstract: Zero-field spin precession of positive muons has been observed in the antiferromagnetic state of La2CuO(4-y). Sharp onsets of the sublattice magnetization are found at temperatures close to those of the susceptibility maxima of different specimens. The long-lived precession signal indicates a microscopically homogeneous distribution of spin density at each Cu atom below the Neel temperature. A combination of the present results and neutron-scattering studies indicates the ordered moment per Cu atom to be significantly less than 1 mu(B).

Influence of oxygen defects on the physical properties of La2CuO

Abstract: We present the results of magnetic-susceptibility, heat-capacity, electrical-resistivity, and thermoelectric-power measurements on La/sub 2/CuO/sub 4-//sub y/. The crystallographic, magnetic, and electronic transport properties are found to be extremely sensitive to the oxygen defect concentration y. Coupled with x-ray and neutron diffraction data presented in a companion paper, we find that long-range antiferromagnetic order develops with increasing y out of a nonmagnetic ground state at y--0 into a state with a maximum Neel temperature of 290 K at yapprox. =0.03.

Novel method for determining the anisotropy constant of MnFe in a NiFe/MnFe sandwich

Abstract: We show that an in situ Kerr rotation measurement is a very effective technique for the study of antiferromagnetic (AF) ferromagnetic (F) film couples. Magnetic signals can be obtained even in the case where the (AF) is the top layer up to at least 200 A of AF thickness. We have used this in situ approach combined with ion milling to study the thickness dependence of the magnetic properties of Mn50Fe50/Ni80Fe20 systems. We observe that the exchange bias field has a surprisingly sharp onset at a critical thickness of AF∼50 A. We show that this is consistent with a simple model and that the magnetic anisotropy of MnFe can be estimated from the observed critical thickness to be ∼1.35×105 erg/cm3. The exchange field showed the predicted proportionality to the inverse of the F thickness from ∼50 to 400 A. Auger spectroscopy and spin polarized secondary electron emission have been used to rule out gross artifacts due to ion milling.

Magnetic properties of diluted magnetic semiconductors: A review (invited)

Abstract: Diluted magnetic semiconductors (DMS) are semiconducting alloys containing a random distribution of substitutional magnetic ions (e.g., Mn++ in Cd1−xMnxSe). Magnetic properties of DMS—such as spin glass behavior due to lattice frustration, magnon excitations in a random system of spins, superexchange, and short range antiferromagnetic order—are of considerable interest in their own right. In addition, understanding these properties is important because they strongly influence the electronic phenomena in DMS via the sp‐d exchange interaction between the band electrons and the localized magnetic moments. We review the most recent results concerning magnetic susceptibility, Mn++‐Mn++ nearest‐neighbor exchange, and magnetic short‐range order in these materials. On this basis, we can present a fairly complete and unified picture of magnetic properties of DMS. In addition, we point out some of the issues and challenges that lie ahead.

Charge-transfer complexes as potential organic ferromagnets

Abstract: One proposal for the synthesis of an organic ferromagnetic material has been investigated. Two new derivatives of hexaazatritetralin have been prepared carrying electron-withdrawing substituents on nitrogen. These form crystalline one-to-one charge-transfer complexes with tris(dicyanomethylene)cyclopropane. The physical and chemical data for these systems indicate that one of them exists as the cation radical/anion radical complex in the solid, but the other has additional partial second charge transfer. Although the dications of the electron donors have triplet spin multiplicity and the charge-transfer solid appears to fulfill the requirements of our model for ferromagnetism, the charge-transfer solids show antiferromagnetic, not ferromagnetic, ordering.

Electronic and magnetic properties of the fcc Fe(001) thin films: Fe/Cu(001) and Cu/Fe/Cu(001)

Abstract: All-electron total-energy local-spin-density studies of the electronic and magnetic properties of fcc Fe(001) as overlayers or sandwiches with Cu(001) were undertaken in order to understand the following: (1) the surface (interface) magnetism of fcc Fe(001), (2) the effect of nonmagnetic Cu on the magnetization of Fe and (3) the effect of the reduced coordination number on the magnetic coupling of Fe layers near the surface and interface. From our systematic studies of (i) one and two layers of Fe on Cu(001) and (ii) one and five layers of Fe sandwiched by Cu, it is concluded that the Fe magnetic moment is enhanced on the surface (to 2.85${\mathrm{mu}}_{B}$) and the surface (interface) Fe layer is predicted to couple ferromagnetically to the subsurface (subinterface) Fe layer (in contrast to the antiferromagnetic behavior in the bulk fcc Fe). The effect of the nonmagnetic Cu overlayers decreases slightly (by 0.25${\ensuremath{\mu}}_{B}$) the magnetic moments of Fe at the Fe/Cu interface from those of the free-standing surfaces, indicating the persistence of the two-dimensional magnetization at the interface. Magnetic hyperfine fields are compared among various magnetic states; the interface Fe atoms are found to experience a larger hyperfine field than the inner layers for the magnetic ground state due to the retention of antiferromagnetic coupling between Fe layers away from the interface. Electronic charge-density, work-function, and single-particle spectra are presented and discussed. The calculated energy dispersions agree well with a recent photoemission measurement by Onellian et al.

First Observation of an Antiferromagnetic Phase in the Y1Ba2Cu3Ox System

Abstract: The magnetic properties of four different phases of the YBa2Cu3Ox system, including the famous 90 K superconductor, have been probed by the positive muon spin relaxation method (µ+SR). The oxygen-deficient tetragonal insulating phase (x~6.2) was found to be magnetically ordered, thus most likely antiferromagnetic, near room temperature.

Development of antiferromagnetic correlations in the heavy-fermion system UPt 3

Abstract: We have carried out an inelastic neutron scattering study of ${\mathrm{UPt}}_{3}$. For low temperatures, antiferromagnetic correlations, with a characteristic energy of 5 meV, between spins in adjacent planes of this hexagonal material are observed. They become apparent below the temperature at which the bulk magnetic susceptibility displays a maximum. We conclude that correlations between the localized (f-like) moments are an important aspect of the low-temperature, coherent state of heavy-fermion systems. The relation of our results to the unconventional superconductivity of ${\mathrm{UPt}}_{3}$ is also described.

Antiferromagnetism and oxygen deficiency in single-crystal La2CuO4- delta.

Abstract: Two single crystals of undoped La/sub 2/CuO/sub 4/ are studied by neutron diffraction between T = 5 and 300 K. In one crystal oxygen vacancies are created by heat treatment and the magnetic properties are found to depend strongly on this treatment. While the untreated sample is found to order antiferromagnetically at T/sub N/approx. =50 K, the heat-treated crystal orders at T/sub N/approx. =150 K with a somewhat larger moment. The magnetic structure is the same for the two crystals and supports previously reported powder results.

Superconductivity and magnetism in the high-Tc superconductor YBaCuO.

Abstract: The new high-${T}_{c}$ superconductor YBaCuO was prepared and characterized. Measurements were performed to determine the superconducting and magnetic properties of the samples. Onset of superconductivity was found above 90 K. Antiferromagnetism was also observed, probably associated with the nonsuperconducting pairs of the samples. Modest critical current density as well as large upper critical fields were inferred from the data.

Spin-Peierls Transition with Competing Interactions

Abstract: The one-dimensional spin-1/2 Heisenberg antiferromagnet which has next-nearest-, as well as nearest-neighbor exchange interaction and couples to the lattice distortion is investigated. The next-nearest-neighbor interaction is shown to result from the non-adiabaticity of the lattice distortion or the itineracy of electrons. The groundstate of this system is studied by the renormalization group method. In the absence of the coupling to the lattice distortion, the phase diagram consists of three states, i.e., the spin-fluid, the dimer and the Neel state, in essential agreement with Haldane, while in its presence, there are two states, the spin-Peierls and the Neel state. The energy gain due to the lattice distortion is estimated by the self-consistent harmonic approximation.

Neutron Scattering Study of Successive Phase Transitions in Triangular Lattice Antiferromagnet CsNiCl3

Abstract: Successive phase transitions of triangular lattice Heisenberg antiferromagnet with a small Ising anisotropy, CsNiCl 3 , are studied by neutron scattering We have reexamined the temperature dependences of the z and x y components of sublattice magnetizations and also measured the both components of staggered susceptibilities The character of the transitions is orderings and divergences of the z and x y components at independent temperatures

Intervalence electron transfer and magnetic exchange in reduced nontronite

Abstract: The effects of chemical reduction of structural Fe 3§ in nontronite SWa-1 (ferruginous smectite) on intervalence electron transfer (IT) and magnetic exchange were investigated. Visible absorption spectra in the region 800-400 nm of a chemical reduction series of the SWa-1 nontronite revealed an IT band near 730 nm (13,700 cm-'). Both the intensity and position of this band were affected by the extent of Fe reduction. The intensity increased until the Fe 2+ content approached 40% of the total Fe, then decreased slightly with more Fe 2+. The position of the band also shifted to lower energy as the extent of reduction increased. Variable-temperature magnetic susceptibility measurements showed that the magnetic exchange in unaltered nontronite is frustrated antiferromagnetic, but ferromagnetic in reduced samples. Magnetic ordering temperatures are in the range 10-50 K, depending on the extent of reduction. The ferromagnetic component in the magnetization curve increased with increasing Fe 2§ in the crystal structure. The positive paramagnetic interaction likely is due to electron charge transfer from Fe 2§ to Fe 3§ through such structural linkages as Fe2+-O-Fe 3§ (perhaps following a double exchange mechanism), which is consistent with the visible absorption spectra.

Experimental Study of New Type Phase Transition in Triangular Lattice Antiferromagnet VCl2

Abstract: Critical properties of a Heisenberg antiferromagnet on a triangular lattice are investigated experimentally using a quasi-two-dimensional antiferromagnet VCl 2 by means of neutron scattering. It is found that VCl 2 shows characteristic critical behaviors of two and three dimensional models. A little above T N , the line shape of χ( Q ) is found to be represented by the Ornstein-Zernike form (κ 2 + q 2 ) -1 . This suggests the existence of magnetic point defects which were predicted in the two dimensional model. The measurecl critical exponents are β=0.20±0.02, γ=1.05±0.03, and ν=0.62±0.05, in agreement with the theoretical work for three dimensional SO(3) systems. Successive phase transitions due to a small Ising anisotropy were found at T N1 =35.88±0.01 K and T N2 =35.80±0.01 K. The spin structure and the dispersion relation of the spin wave well below T N are also determined.