Showing papers on "Antiferromagnetism published in 1994"
TL;DR: The properties of spin-1/2 antiferromagnetic chains with various types of random exchange coupling are studied via an asymptotically exact decimation renormalization-group transformation, which is a generalization of that introduced by Dasgupta and Ma.
Abstract: The properties of spin-1/2 antiferromagnetic chains with various types of random exchange coupling are studied via an asymptotically exact decimation renormalization-group transformation, which is a generalization of that introduced by Dasgupta and Ma. Random-singlet phases occur in which each spin is paired with one other spin that may be very far away; more exotic phases also occur. The behavior of typical and mean correlation functions is analyzed and found to be very different, with very small sets of spins dominating the latter at long distances as well as the low-temperature thermodynamics. Some of the phase transitions that occur between antiferromagnetically ordered phases and random singlet or other antiferromagnetic phases are also analyzed. For example, if a small uniaxial anisotropy perturbation is added to a random Heisenberg antiferromagnetic chain, a transition occurs from a random-singlet phase to an Ising antiferromagnetic phase, as the anisotropy changes sign from easy plane to easy axis. The staggered magnetization vanishes at the transition with critical exponent \ensuremath{\beta}=8/(1+ \ensuremath{\surd}7 ). Possible implications for the properties of random quantum magnetic systems in higher dimensions are briefly discussed.
453 citations
TL;DR: In this paper, the spin-glass behavior is attributed to the frustration of random competing exchange interactions, namely the ferromagnetic double-exchange interaction between Co 3+ and Co 4+ and the antiferromagnetic one between like spins.
Abstract: Magnetic properties of La 1- x Sr x CoO 3 (0 ≤ x ≤0.5) have been studied by magnetization measurements in low magnetic fields. It has been found that there exist spin-glass (0< x ≤0.18) and cluster-glass (0.18 ≤ x ≤0.5) regions. Ferromagnetic long-range order previously reported does not take place. Spin-glass behavior is ascribed to the frustration of random competing exchange interactions, namely the ferromagnetic double-exchange interaction between Co 3+ and Co 4+ and the antiferromagnetic one between like spins.
352 citations
338 citations
TL;DR: Magnetic wedges of Fe on Cu(100) are explored magneto-optically and the phase boundaries are delineated by cusps in the coercivity along the wedge.
Abstract: Magnetic wedges of Fe on Cu(100) are explored magneto-optically. In the region of 6--11 monolayers (ML) of fcc Fe grown at room temperature, in addition to a surface ferromagnetic layer with a Curie temperature \ensuremath{\sim}250 K, the Fe bulk is antiferromagnetic with a N\'eel temperature of \ensuremath{\sim}200 K. This fcc phase is stable only above a growth temperature of order \ensuremath{\sim}200 K, below which the ferromagnetic spin-reorientation transition occurs at \ensuremath{\sim}6 ML. The phase boundaries are delineated by cusps in the coercivity along the wedge.
228 citations
TL;DR: A classical spin-one lattice gas model is used to study the competition between short-range ferromagnetic coupling and long-range antiferromagnetic Coulomb interactions in high-temperature superconductors.
Abstract: A classical spin-one lattice gas model is used to study the competition between short-range ferromagnetic coupling and long-range antiferromagnetic Coulomb interactions. The model is a coarse-grained representation of frustrated phase separation in high-temperature superconductors. The ground states are determined for the complete range of parameters by using a combination of numerical and analytical techniques. The crossover between ferromagnetic and antiferromagnetic states proceeds via a rich structure of highly symmetric striped and checkerboard phases. There is no devil's staircase behavior because mixtures of stripes with different period phase separate.
221 citations
TL;DR: In this article, the authors demonstrate that NiMn and NiNn-Cr films exhibit strong and thermally stable antiferromagnetism, as well as high corrosion resistance.
Abstract: Dc magnetron sputtered Ni‐Mn and Ni‐Mn‐Cr films are demonstrated to exhibit strong and thermally stable antiferromagnetism, as well as high corrosion resistance. For a 25.2 nm thick 53.3 Ni‐46.7 Mn (in atomic percent) film deposited on top of a 28.5 nm thick 81 Ni‐19 Fe film, a unidirectional anisotropy field (HUA) of 120.6 Oe is obtained at room temperature after annealing in vacuum. The equivalent interfacial exchange coupling energy (JK) is 0.27 erg/cm2, three times higher than that of bilayer Ni‐Fe/50Fe‐50Mn films. This strong exchange coupling appears correlated with the presence of an antiferromagnetic θ (NiMn) phase with a CuAu‐I‐type ordered face‐centered‐tetragonal structure. The blocking temperature, at which the exchange coupling disappears, is higher than 400 °C. The Cr addition to the Ni‐Mn film dilutes the exchange coupling, but the JK for the Cr content ≤10.7 at. %, is still higher than that of the Ni‐Fe/Fe‐Mn films. Both Ni‐Mn and Ni‐Mn‐Cr films exhibit corrosion behaviors much better than...
186 citations
TL;DR: In this paper, the authors discuss two methods that quantify the contribution of different groups to the AMS: (1) comparative measurements of the magnetic susceptibility in low fields and high fields and (2) heating curves from 77 K to room temperature (low temperature magnetic susceptibility, LTMS).
148 citations
TL;DR: In this paper, a spin valve configuration is presented in which an unpinned ferromagnetic film is separated from exchange-pinned magnetagnetic films on either side by two nonmagnetic spacers, thereby creating a symmetric spin valve structure.
Abstract: A spin valve configuration is presented in which an unpinned ferromagnetic film is separated from exchange-pinned ferromagnetic films on either side by two nonmagnetic spacers, thereby creating a symmetric spin valve structure. The symmetric spin valve is shown to increase the magnetoresistance by 50% over the values of individual spin valves. The increase is attributed to a reduction of spin-independent outer boundary scattering and doubling the number of spin-dependent scattering interfaces. The magnetoresistance and coupling fields of the spin valves that comprise the symmetric structure have been measured as a function of Cu spacer thickness. Oscillatory antiferromagnetic to ferromagnetic coupling was observed in standard spin valves, which had /spl Delta/R/R as high as 13%. >
145 citations
TL;DR: The Kondo eA'ect in a repulsively interacting electron system (Tomonaga-Luttinger liquid) is studied and it is shown that the Kondo coupling is connected to the strong-coupling regime not only for the antiferromagnetic but also for the ferromagnetic case.
Abstract: The Kondo effect in a repulsively interacting electron system (Tomonaga-Luttinger liquid) is studied. By using the poor man's scaling method it is shown that the Kondo coupling in this model flows to the strong-coupling regime not only for the antiferromagnetic but also for the ferromagnetic case. The ground state is governed by stable strong-coupling fixed points where the impurity spin is completely screened; the fixed-point Hamiltonian consists of two semi-infinite Tomonaga-Luttinger liquids and a spin singlet. Specific heat, susceptibility, and conductance are calculated for low temperatures.
132 citations
Patent•
30 Sep 1994TL;DR: A random access memory element utilizes giant magnetoresistance as mentioned in this paper, which consists of at least one pair of ferromagnetic layers sandwiching a nonmagnetic conductive layer, each of which has a magnetic moment oriented within its own plane.
Abstract: A random access memory element utilizes giant magnetoresistance. The element includes at least one pair of ferromagnetic layers sandwiching a nonmagnetic conductive layer. At least one of the two ferromagnetic layers has a magnetic moment oriented within its own plane. The magnetic moment of at least the first ferromagnetic layer of the pair has its magnetic moment oriented within its own plane and is typically fixed in direction during use. The second ferromagnetic layer of the pair has a magnetic moment which has at least two preferred directions of orientation. These preferred directions of orientation may or may not reside within the plane of the second ferromagnetic layer. The bit of the memory element may be set by applying to the element a magnetic field which orients the magnetic moment of the second ferromagnetic layer in one or the other of these preferred orientations. Once the bit is set, the value of the determined by the relative alignment of the magnetic moments of the first and second ferromagnetic layers. This value may be read by applying an interrogating current across the memory element, perpendicular to the plane within which the magnetic moment of the first ferromagnetic layer is oriented, and observing the variation in resistance. These ferromagnetic elements may be fabricated using conventional photolithography. Groups of these ferromagnetic element may be organized into word trees and other arrays.
129 citations
TL;DR: In this paper, the magnetic properties of the spin-1/2 Heisenberg chain were studied theoretically and the high temperature susceptibilty and ground state saturation magnetic field were calculated and the exchange energies of the trimer compound 3CuCl 2 ·2dx were determined.
Abstract: The magnetic properties of the ferromagnetic-ferromagnetic-antiferromagnetic trimerized spin-1/2 Heisenberg chain are studied theoretically. The high temperature susceptibilty and the ground state saturation magnetic field are calculated and the exchange energies of the trimer compound 3CuCl 2 ·2dx are determined. The magnetization curve is obtained by numerical diagonalization of finite size systems. The result explains the low temperature magnetization data for 3CuCl 2 ·2dx with the exchange energies obtained as above. It is predicted that the magnetization curve has a plateau at 1/3 of the saturation magnetization if the ferromagnetic exchange energy is comparable to or smaller than the antiferromagnetic exchange energy.
TL;DR: Experimental and theoretical studies of the magnetization curves of Fe/Cr(211) superlattices with antiferromagnetic interlayer coupling and uniaxial in-plane anisotropy show substantial differences between structures with an even and an even number of Fe layers, when the magnetic field is applied along the easy axis.
Abstract: We report experimental and theoretical studies of the magnetization curves of Fe/Cr(211) superlattices with antiferromagnetic interlayer coupling and uniaxial in-plane anisotropy. There are substantial differences between structures with an even and an odd number of Fe layers, when the magnetic field is applied along the easy axis. For even layered superlattices, the inequivalence of the terminal Fe layers gives rise to a surface spin-flop transition that evolves into a bulk spin-flop arrangement with increasing magnetic field, as originally envisioned by Keffer and Chow [Phys. Rev. Lett. 31, 1061 (1973)].
TL;DR: For the spin-1/2 Heisenberg antiferromagnet on the kagom\'e lattice the high-temperature series for the specific heat and the structure factor is calculated and the uniform susceptibility has a maximum at T\ensuremath{\approxeq}1/6 and vanishes exponentially for lower temperatures.
Abstract: For the spin-1/2 Heisenberg antiferromagnet on the kagom\'e lattice we calculate the high-temperature series for the specific heat and the structure factor. A comparison of the series with exact-diagonalization studies shows that the specific heat has further structure at lower temperature in addition to a high-temperature peak at T\ensuremath{\approxeq}2/3. At T=0.25 the structure factor agrees quite well with results for the ground state of a finite cluster with 36 sites. At this temperature the structure factor is less than two times its T=\ensuremath{\infty} value and depends only weakly on the wave vector q, indicating the absence of magnetic order and a correlation length of less than one lattice spacing. The uniform susceptibility has a maximum at T\ensuremath{\approxeq}1/6 and vanishes exponentially for lower temperatures.
TL;DR: In this article, a special correlation function in the isotropic spin-1 2 Heisenberg antiferromagnet is considered, i.e., the probability of finding a ferromagnetic string of (adjacent) spins in the ground state.
TL;DR: In this paper, the authors measured the specific heat and magnetic susceptibility of GGG to obtain the first accurate map of the low temperature phase diagram and found a critical exponent at the ordering transition, consistent with a transition in a random magnetic environment.
Abstract: The geometrically frustrated magnet gadolinium gallium garnet (GGG) has a unique low temperature (0.38 K.) antiferromagnetic phase which exists only in finite fields ($H\ensuremath{\sim}1$ T). We have measured the specific heat and magnetic susceptibility of GGG to obtain the first accurate map of the low temperature phase diagram. A ground state magnetic structure in this field regime has been determined by simulations including both the exchange and dipole interactions. We find a critical exponent $\ensuremath{\alpha}0$ at the ordering transition, consistent with a transition in a random magnetic environment.
TL;DR: The magnetization of single-crystal HoNi 2 B 2 C has been measured as a function of applied field (H ) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system as discussed by the authors.
Abstract: The magnetization of single-crystal HoNi 2 B 2 C has been measured as a function of applied field ( H ) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system. The normal-state magnetic susceptibility of HoNi 2 B 2 C is highly anisotropic with a Curie-Weiss-like temperature dependence for H applied perpendicular to the c -axis and with a much weaker temperature dependence for H applied parallel to the c -axis, indicating that the Ho +3 magnetic moments lie predominately in the tetragonal a − b plane below 20 K. High-field magnetization (2000 Oe), low-field magnetization (20 Oe) and zero-field specific heat all give an antiferromagnetic ordering temperature of T N =5.0 K. Remarkably, in 20 Oe applied field both superconductivity ( T c =8.0 K) and antiferromagnetism ( T N =5.0 K) clearly make themselves manifest in the magnetization data. From these magnetization data a phase diagram in the H − T plane was constructed for both directions of applied field. This phase diagram shows a non-monotonic temperature dependence of H c2 with a deep minimum at T N =5 K. The high-field magnetization data for H applied perpendicular to the c -axis also reveal a cascade of three phase transitions for T H H versus T phase diagram for HoNi 2 B 2 C at low temperatures.
TL;DR: In this paper, the spin fluctuation mechanism for the high temperature superconductivity in cuprates is investigated with a parametrization of the spin fluctuations spectra in terms of the self-consistent renormalization theory of spin fluctuations.
Abstract: The spin fluctuation mechanism for the high temperature superconductivity in cuprates is investigated with a parametrization of the spin fluctuation spectra in terms of the self-consistent renormalization theory of spin fluctuations. The doping concentration dependence of the transition temperature in the best studied cuprates is explained in terms of the parameter values estimated from the normal state experiments. We find that the larger the amplitude and the energy spread of spin fluctuations, the higher T c . In this context the cuprates have much favorable properties compared with some other antiferromagnetic spin fluctuation systems whose parameter values are estimated for comparison.
TL;DR: In this article, the magnetic properties of ThCr 2 Si 2 -type structure LaMn 2 Ge 2 (XSi, Ge) compounds have been reinvestigated by neutron diffraction experiments.
TL;DR: In this paper, the specific heat of single crystals of superconducting YNi 2 B 2 C and TmNi 2B 2 C was measured in zero and 5 T magnetic fields and it was shown that superconductivity coexists with antiferromagnetic order below 1.5 K.
Abstract: We have measured the specific heat of single crystals of superconducting YNi 2 B 2 C and TmNi 2 B 2 C. Analysis of YNi 2 B 2 C data in zero and 5 T magnetic fields indicates modestly strong electron-phonon coupling and a superconducting energy gap Δ ( O ) = 29 K . Superconductivity coexists with antiferromagnetic order below 1.5 K in TmNi 2 B 2 C. An interpretation of very low temperature specific heat indicates substantial magnetic anisotropy, with ferromagnetic planes weakly coupled antiferromagnetically to each other. Two-dimensional ferromagnetic spin waves produce a large linear-in-temperature contribution to the specific heat. We find no compelling evidence for an unconventional pairing mechanism in these systems.
TL;DR: In this paper, a range of hexagonal structures in the MnFeP 1− y As y system have been studied and local transformations accompanying both the hexagonal crystal structure and the magnetic ordering have been measured.
TL;DR: In this paper, the authors determined unusual magnetic ordering of the hexagonal intermetallic uranium compound U${\mathrm{N}}_{\mathm{i}4}$B via neutron diffraction.
Abstract: We have determined unusual magnetic ordering of the hexagonal intermetallic uranium compound U${\mathrm{N}}_{\mathrm{i}4}$B via neutron diffraction. In the easy basal plane the U moments have triangular symmetry with antiferromagnetic interactions. Along the hard $c$ axis ferromagnetic coupling occurs. Below ${T}_{N}=20$ K only two out of every three U moments of $1.2{\ensuremath{\mu}}_{B}$ order in vortexlike arrangements around the third paramagnetic spin. This novel magnetic structure is related to the occurrence of a crystallographic superstructure. Previously observed anomalies in bulk properties below ${T}_{N}$ are attributed to unconventional spin-wave excitations associated with this type of ordering.
TL;DR: In this paper, a spin gap with a value of 680 K was shown to be in quantitative agreement with the theoretical value of Δ∼0.5 J =650 K, if the same value of 1300 K as in the square lattice system is used as the antiferromagnetic exchange coupling constant.
Abstract: Low-energy spin excitation in a spin S =1/2 two-leg ladder system SrCu 2 O 3 has been investigated by Cu NMR. From an activated decrease of the nuclear-spin lattice relaxation rate, 1/ T 1 , in a T -range of 100–300 K, a spin gap, Δ, is demonstrated to open with a value of 680 K, which is in quantitative agreement with the theoretical value of Δ∼0.5 J =650 K , if the same value of 1300 K as in the square lattice system is used as the antiferromagnetic exchange coupling constant, J . It is furthermore deduced from the measurement of the spin-echo decay rate, 1/ T 2G , that a magnetic coherence length, ξ/ a ( a : lattice spacing between Cu atoms along the chain), remains finite with (ξ/ a )∼13, comparable to a value in La 2 CuO 4 near T N . This class of cuprate is thus a frustrated quntum antiferromagnet (AF) with a spin gap.
TL;DR: The magnetoelectric effect of Cr2O3 single crystals has been studied in magnetic fields up to 20 T in the range of liquid helium to room temperature as mentioned in this paper, where the magnetic ground state symmetry of the spin-flop phase is either 1 − or 2 − where 2 − can be excluded.
Abstract: The magnetoelectric effect of Cr2O3 single crystals has been studied in magnetic fields up to 20 T in the range of liquid helium to room temperature In the antiferromagnetic phase for low magnetic fields the well known results for the magnetoelectric effect below the Neel temperature were reproduced. In the spin-flop phase, for magnetic fields above a critical field (H > H crit ≃ 10 T), the magnetic field induced electric polarization has been measured along all three crystallographic directions. A magnetoelectric effect, linear in the magnetic field was found for the three orientalions leading to the conclusion that the magnetic ground state symmetry of the spin-flop phase is either 1′ or 2/m′, where 2′/m can be excluded.
TL;DR: In this paper, the sign of magnetic interaction is discussed in connection with the orthorhombic lattice distortion, and the antiferromagnetic interaction is attributed to this distortion.
Abstract: Electrical resistivity ρ and magnetic susceptibility of sintered Ca 1- x Sr x RuO 3 have been measured. The Weiss temperature changes sign at about x =0.5 in consistent with other studies. Temperature dependence of normalized ρ is almost identical for all the materials in the paramagnetic temperature range irrespective of the sign of magnetic interaction. The Rhodes-Wolfarth ratio is 1.3 for x =1. This ratio and the temperature dependence of ρ suggest that 4 d -electrons in this system are in the intermediately localized state as in Ni. The sign of magnetic interaction is discussed in connection with the orthorhombic lattice distortion, and the antiferromagnetic interaction is attributed to this distortion.
TL;DR: In this paper, the properties of the classical Heisenberg pyrochlore lattice antiferromagnet with local planar single ion anisotropy are discussed.
Abstract: The properties of the classical Heisenberg pyrochlore lattice antiferromagnet with local planar single ion anisotropy are discussed. The zero temperature ground state is found to be macroscopically degenerate and without long range order. Monte Carlo simulations are used to investigate the properties of the system at finite temperature. We find that thermal fluctuations select a subset of the ground state manifold, and induce a first‐order phase transition to a conventional Neel ground state.
TL;DR: In this article, magnetization, susceptibility, and ESR measurements of finely dispersed α-Fe 2 O 3 grains were studied by means of magnetization and susceptibility and the results showed a superparamagnetic behavior above T b = 145 K (blocking temperature from dc measurements, applying H = 25 Oe) and gave evidence of the antiferromagnetic nature of the particles, bearing a net uncompensated moment.
TL;DR: In this article, the properties of BaMn 2 P 2 and SrMn2 P 2 are discussed in light of their structures and previous theoretical investigations by Zheng and Hoffmann, and two distinct sets of magnetic reflections are distinguished by different temperature dependencies and indexing schemes.
IBM1
TL;DR: Evidence from magnetic and giant magnetoresistance studies of well-defined antiferromagnetic interlayer coupling in single crystalline (111) permalloy/Au multilayers is presented.
Abstract: The existence or not of oscillatory interlayer exchange coupling of ferromagnetic layers via (111) oriented copper spacer layers is controversial. We present evidence from magnetic and giant magnetoresistance studies of well-defined antiferromagnetic interlayer coupling in single crystalline (111) permalloy/Au multilayers. Four oscillations in the coupling are observed as the Au spacer layer thickness is increased. The oscillation period is \ensuremath{\simeq}10 \AA{} which is significantly shorter than the period of \ensuremath{\simeq}11.5 \AA{} predicted in Ruderman-Kittel-Kasuya-Yosida based models.
TL;DR: Giant magnetoresistance (GMR) effects are observed in several classes of bulk magnetic materials as discussed by the authors, and the most spectacular resistance variations are connected with metamagnetic transitions in U‐intermetallic antiferromagnets.
Abstract: Giant magnetoresistance (GMR) effects are observed in several classes of bulk magnetic materials. The resistance changes at metamagnetic transitions connected with reorientation of 4f moments are only moderate due to the relatively weak coupling of the 4f and conduction electrons. Much larger GMR effects can be achieved by mechanisms involving the d states (RhFe, RCo2), though the most spectacular resistance variations are connected with metamagnetic transitions in U‐intermetallic antiferromagnets. This phenomenon can be interpreted as due to Fermi surface gapping (due to magnetic superzones) and/or due to spin‐dependent scattering in analogy with magnetic multilayers.