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Showing papers on "Antiferromagnetism published in 1997"


Book
01 Jan 1997
TL;DR: Magnetic properties of magnetism have been studied in a wide range of applications, including magnetism of amorphous materials, magnetism and magnetostriction as mentioned in this paper, spin distribution and domain walls.
Abstract: 1. Magnetostatic phenomena 2. Magnetic measurements 3. Atomic magnetic moments 4. Macroscopic experimental techniques 5. Magnetic disorder 6. Ferromagnetism 7. Antiferromagnetism and ferrimagnetism 8. Magnetism of metals and alloys 9. Magnetism of ferromagnetic oxides 10. Magnetism of compounds 11. Magnetism of amorphous materials 12. Magnetocrystalline anisotrophy 13. Induced magnetic anisotropy 14. Magnetostriction 15. Observation of domain structures 16. Spin distribution and domain walls 17. Magnetic domain structure 18. Technical magnetization 19. Spin phase transition 20. Dynamic magnetization 21. Various phenomena association with magnetization 22. Engineering applications of magnetic materials

1,486 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the presence of local Ising anisotropy leads to a geometrically frustrated ground state, preventing long-range magnetic order down to at least 0.05 K. But unlike in the case of a frustrated antiferromagnet, this disorder is principally static.
Abstract: We report a detailed study of the pyrochlore ${\mathrm{Ho}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, in which the magnetic ions $({\mathrm{Ho}}^{3+})$ are ferromagnetically coupled with $J\ensuremath{\sim}1\mathrm{K}$. We show that the presence of local Ising anisotropy leads to a geometrically frustrated ground state, preventing long-range magnetic order down to at least 0.05 K. However, unlike in the case of a frustrated antiferromagnet, this disorder is principally static. In a magnetic field, the ground-state degeneracy is broken and ordered magnetic phases are formed which display an unusual history dependence due to the slow dynamics of the system. These results represent the first experimental evidence for geometrical frustration in a ferromagnetic system.

910 citations


Journal ArticleDOI
TL;DR: In this article, a model of spin configurations in NiO nanoparticles yields 8-, 6-, or 4-sublattice configurations, indicating a new finite size effect, in which the reduced coordination of surface spins causes a fundamental change in the magnetic order throughout the particle.
Abstract: Antiferromagnetic NiO nanoparticles exhibit anomalous magnetic properties, such as large moments, and coercivities and loop shifts of up to 10 kOe. This behavior is difficult to understand in terms of 2-sublattice antiferromagnetic ordering which is accepted for bulk NiO. Numerical modeling of spin configurations in these nanoparticles yields 8-, 6-, or 4-sublattice configurations, indicating a new finite size effect, in which the reduced coordination of surface spins causes a fundamental change in the magnetic order throughout the particle. The relatively weak coupling between the sublattices allows a variety of reversal paths for the spins upon cycling the applied field, resulting in large coercivities and loop shifts.

764 citations


Journal ArticleDOI
TL;DR: In this paper, a microscopic explanation of exchange bias in thin films with compensated ferro/antiferromagnetic interfaces is presented, and full micromagnetic calculations show the interfacial exchange coupling to be relatively strong with a perpendicular orientation between the ferro and antiferrome axis directions, similar to the classic spin-flop state in bulk antiferromagnets.
Abstract: A microscopic explanation of exchange bias in thin films with compensated ferro/antiferromagnetic interfaces is presented. Full micromagnetic calculations show the interfacial exchange coupling to be relatively strong with a perpendicular orientation between the ferro/antiferromagnetic axis directions, similar to the classic ``spin-flop'' state in bulk antiferromagnets. With reasonable parameters the calculations predict bias fields comparable to those observed and provide a possible explanation for both anomalous high field rotational hysteresis and recently discovered ``positive'' exchange bias.

627 citations


Journal ArticleDOI
21 Feb 1997-Science
TL;DR: The complex phase diagram of high-critical temperature (Tc) superconductors can be deduced from an SO(5) symmetry principle that unifies antiferromagnetism and d-wave superconductivity, resulting in a quantum nonlinear σ model that describes the phase diagram and the effective low-energy dynamics of the system.
Abstract: The complex phase diagram of high-critical temperature (Tc) superconductors can be deduced from an SO(5) symmetry principle that unifies antiferromagnetism and d-wave superconductivity. The approximate SO(5) symmetry has been derived from the microscopic Hamiltonian, and it becomes exact under renormalization group flow toward a bicritical point. This symmetry enables the construction of a SO(5) quantum nonlinear σ model that describes the phase diagram and the effective low-energy dynamics of the system. This model naturally explains the basic phenomenology of the high-Tc superconductors from the insulating to the underdoped and the optimally doped region.

582 citations


Journal ArticleDOI
TL;DR: In this article, a model based on a calculation of the density of these interfacial uncompensated spins predicts the correct magnitude of the exchange field, as well as the observed inverse dependence on interfacial grain size.
Abstract: The uncompensated spins on the surfaces of antiferromagnetic CoO films exhibit a thermoremanent magnetization after field cooling from ${T}_{N}$ that has the same temperature dependence as the exchange field of $\mathrm{Ni}{}_{81}\mathrm{Fe}{}_{19}/\mathrm{Co}\mathrm{O}$ bilayers after field cooling. This suggests that these interfacial uncompensated spins are responsible for unidirectional anisotropy. A model based on a calculation of the density of these interfacial uncompensated spins predicts the correct magnitude of the exchange field, as well as the observed inverse dependence on interfacial grain size.

400 citations


Journal ArticleDOI
24 Apr 1997-Nature
TL;DR: In this article, it was shown that in one such compound, Pr0.7Ca0.3MnO3, a transition from the insulating antiferromagnetic state to the metallic ferromagnetic states can be driven by illumination with X-rays at low temperatures (<40 K).
Abstract: Manganese oxides of the general formula A1–xBxMnO3 (where A and B are trivalent and divalent cations, respectively) have recently attracted considerable attention by virtue of their unusual magnetic and electronic properties1–9. For example, in some of these materials magnetic fields can drive insulator-to-metal transitions where both the conductivity and magnetization change dramatically—an effect termed 'colossal magneto-resistance'1–3—raising hopes for application of these materials in the magnetic recording industry1–9. Here we show that in one such compound, Pr0.7Ca0.3MnO3, a transition from the insulating antiferromagnetic state to the metallic ferromagnetic state can be driven by illumination with X-rays at low temperatures (<40 K). This transition is accompanied by significant changes in the lattice structure, and can be reversed by thermal cycling. This effect, undoubtedly a manifestation of the strong electron–lattice interactions believed to be responsible for the magnetoresistive properties of these materials6–9, provides insights into the physical mechanisms of persistent photoconductivity, and may also find applications in X-ray detection and X-ray lithographic patterning of ferromagnetic nanostructures.

376 citations


Journal ArticleDOI
TL;DR: In this paper, several samples of antiferromagnetic NiO nanoparticles with average sizes ranging from 50 to >800 A were investigated and the reversible magnetization could not fit with a Langevin function that was consistent with the physically reasonable moment representing the uncompensated spins.
Abstract: As first noted by Neel, antiferromagnetic nanoparticles could exhibit superparamagnetic relaxation of their spin lattices as well as permanent moments arising from uncompensated surface spins. Several samples of antiferromagnetic NiO nanoparticles with average sizes ranging from 50 to >800 A were investigated in the present study. In addition to the inverse dependence on average particle size of the susceptibility predicted by Neel, and previously reported, some unusual behavior was observed. Above the blocking temperatures (TB) of the particles, the reversible magnetization could not be fit with a Langevin function that was consistent with the physically reasonable moment representing the uncompensated spins. For the 53 A diameter particles, both zero-field-cooled (ZFC) and field-cooled (FC) loops below TB exhibit large coercive forces (several kOe) and the loops showed irreversibility up to 50 kOe. In addition, in the FC state below TB the hysteresis loops were strongly shifted. The latter behavior may ...

316 citations


Journal ArticleDOI
TL;DR: In this article, a two-phase region separating an O'-orthorhombic phase stable over 0 ≤ δ ≤ 0.06 and a rhombohedral phase, stable in the range 0.10 − 0.13 − δ ≥ 0.18, is revealed.

290 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the doping of perovskite manganites with either chromium or cobalt induces an insulator to metal transition, which is exceptional since such a transition has not been observed to date in charge ordered manganite Ln 0.5 Ca 0.3 with an A-site cation of such a small size.

269 citations


Journal ArticleDOI
TL;DR: Single-crystal field dependent magnetization measurements as well as elastic neutron scattering experiments on the manganese(II)-chromium(III) samples show the existence of long-range ferromagnetic ordering behavior below T(c) = 6 K.
Abstract: Bimetallic, oxalate-bridged compounds with bi- and trivalent transition metals comprise a class of layered materials which express a large variety in their molecular-based magnetic behavior. Because of this, the availability of the corresponding single-crystal structural data is essential to the successful interpretation of the experimental magnetic results. We report in this paper the crystal structure and magnetic properties of the ferromagnetic compound {[N(n-C3H7)4][MnIICrIII(C2O4)3]}n (1), the crystal structure of the antiferromagnetic compound {[N(n-C4H9)4][MnIIFeIII(C2O4)3]}n (2), and the results of a neutron diffraction study of a polycrystalline sample of the ferromagnetic compound {[P(C6D5)4][MnIICrIII(C2O4)3]}n (3). Crystal data: 1, rhombohedral, R3c, a = 9.363(3) A, c = 49.207(27) A, Z = 6; 2, hexagonal, P63, a = 9.482(2) A, c = 17.827(8) A, Z = 2. The structures consist of anionic, two-dimensional, honeycomb networks formed by the oxalate-bridged metal ions, interleaved by the templating cations. Single-crystal field dependent magnetization measurements as well as elastic neutron scattering experiments on the manganese(II)−chromium(III) samples show the existence of long-range ferromagnetic ordering behavior below Tc = 6 K. The magnetic structure corresponds to an alignment of the spins perpendicular to the network layers. In contrast, the manganese(II)−iron(III) compound expresses a two-dimensional antiferromagnetic ordering.

Journal ArticleDOI
TL;DR: In this paper, a quasi-two-dimensional organic system, (ET)2X, has been studied by NMR, susceptibility and specific heat measurements in the light of role of electron correlation.
Abstract: A family of quasi-two-dimensional organic systems, (ET)2X, have been studied by NMR, susceptibility and specific heat measurements in the light of role of electron correlation A systematic variation of antiferromagnetic spin fluctuations toward metal(superconductor)-to-insulator transition and the ordered spin structure in the insulating phase are uncovered Based on the results, we propose a conceptual phase diagram for (ET)2X, where the λ-type of family span a critical region of Mott transition The superconductivity, which is enhanced near the Mott transition, is suggested to have line nodes in gap parameter by NMR relaxation and specific heat profiles

Journal ArticleDOI
TL;DR: In this paper, it was shown that the q-state Potts antiferromagnet on a lattice of maximum coordination number r exhibits exponential decay of correlations uniformly at all temperatures (including zero temperature) whenever q>2r.
Abstract: We prove that theq-state Potts antiferromagnet on a lattice of maximum coordination numberr exhibits exponential decay of correlations uniformly at all temperatures (including zero temperature) wheneverq>2r. We also prove slightly better bounds for several two-dimensional lattices: square lattice (exponential decay forq≥7), triangular lattice (q≥11), hexagonal lattice (q≥4), and Kagome lattice (q≥6). The proofs are based on the Dobrushin uniqueness theorem.

Journal ArticleDOI
TL;DR: In this article, the magnetic structure of antiferromagnetic ordered phases of quasi-one-dimensional organic conductors is studied theoretically at absolute zero based on the mean field approximation to the quarter-filled band with on-site and nearest-neighbor Coulomb interaction.
Abstract: The magnetic structure of antiferromagnetically ordered phases of quasi-one-dimensional organic conductors is studied theoretically at absolute zero based on the mean field approximation to the quarter-filled band with on-site and nearest-neighbor Coulomb interaction. The differences in magnetic properties between the antiferromagnetic phase of (TMTTF) 2 X and the spin density wave phase in (TMTSF) 2 X are seen to be due to a varying degrees of roles played by the on-site Coulomb interaction. The nearest-neighbor Coulomb interaction introduces charge disproportionation, which has the same spatial periodicity as the Wigner crystal, accompanied by a modified antiferromagnetic phase. This is in accordance with the results of experiments by Nakamura et al on (TMTTF) 2 Br and (TMTTF) 2 SCN. Moreover, the antiferromagnetic phase of (DI-DCNQI) 2 Ag is predicted to have a similar antiferromagnetic spin structure.

Journal ArticleDOI
TL;DR: In this article, a triangular Ising spin system is considered to be in a partially disordered antiferromagnetic state for 10 K < T < 25 K and in a ferrimagnetic state below 10 K.
Abstract: Magnetization measurements on the one-dimensional oxide Ca 3 Co 2 O 6 having a triangular net of Co 2 O 6 chains have been carried out both in static and pulsed high magnetic fields. The M / H vs. T curve obeys the Curie-Weiss law at high temperatures. Below 25 K, however, M / H increases abruptly, and a plateau is observed at 1/3 of the full moment in the M - H curve, suggesting a ferrimagnetic state of the ferromagnetic chains due to the antiferromagnetic interchain interaction. At low H , this system is considered to be in a partially disordered antiferromagnetic state for 10 K< T <25 K and in a ferrimagnetic state below 10 K. The observed plateau in the M - H curve for T ≤5 K is broader than for 10 K≤ T ≤20 K, indicating spin freezing at lower temperatures. The results can be discussed in terms of the triangular Ising spin systems.

Journal ArticleDOI
TL;DR: In this article, the double exchange mechanism was used to obtain ferromagnetic insulator BiMnO3, which was synthesized under high pressure, and showed that the highly polarizable Bi3+ion with 6s2lone pair would cause local distortion of the perovskite lattice, which presumably reduces the mobility of carriers.

Journal ArticleDOI
TL;DR: In this article, the magnetic structure of antiferromagnetic ordered phases of quasi-one-dimensional organic conductors is studied theoretically at absolute zero based on the mean field approximation to the quarter-filled band with on-site and nearest-neighbor Coulomb interaction.
Abstract: The magnetic structure of antiferromagnetically ordered phases of quasi-one-dimensional organic conductors is studied theoretically at absolute zero based on the mean field approximation to the quarter-filled band with on-site and nearest-neighbor Coulomb interaction. The differences in magnetic properties between the antiferromagnetic phase of (TMTTF)$_2$X and the spin density wave phase in (TMTSF)$_2$X are seen to be due to a varying degrees of roles played by the on-site Coulomb interaction. The nearest-neighbor Coulomb interaction introduces charge disproportionation, which has the same spatial periodicity as the Wigner crystal, accompanied by a modified antiferromagnetic phase. This is in accordance with the results of experiments on (TMTTF)$_2$Br and (TMTTF)$_2$SCN. Moreover, the antiferromagnetic phase of (DI-DCNQI)$_2$Ag is predicted to have a similar antiferromagnetic spin structure.

Journal ArticleDOI
TL;DR: In this paper, the authors show that negative magnetoresistance (CMR) occurs in manganites at a first-order ferromagnetic transition and that the CMR is greatest near t≈0.98 where the transition at TC is first order.
Abstract: A “colossal” negative magnetoresistance (CMR) occurs in manganites at a first-order ferromagnetic transition. The Mn4+ and high-spin Mn3+ ions each contain localized t3 configurations; the t3–pπ–t3 superexchange interactions are antiferromagnetic. The orbital degeneracy of localized Mn3+:t3e1, 5Eg configurations is lifted by cooperative static or dynamic Jahn–Teller deformations. Strong e-electron coupling to oxygen displacements, static or dynamic, introduces ferromagnetic e1–pσ–e0 interactions either via superexchange or, for fast Mn3+ to Mn4+ electron transfer relative to the spin-relaxation time (τh ωR−1 to τh<ωR−1 occurs within mobile molecular units, where Wσ is the bandwidth for states of e-orbital parentage and ωR−1 is the period of the optical-mode lattice vibration that traps a mobile hole as a small-polaron Mn4+ ion. TC increases with the fraction of double-exchange couplings, and this fraction increases with Wσ and ωR at the transition from polaronic to itinerant-electron behavior below TC. The bandwidth Wσ∼eσλσ2 cos φ〈cos(θij/2)〉 depends on the covalent mixing parameter λσ, which increases with pressure, as well as on the Mn–O–Mn bond angle (180°−φ), which increases with the tolerance factor t that measures the equilibrium bond-length mismatch, and on the angle θij between neighboring spins so that Wσ increases with the spontaneous magnetization on cooling below TC. In the compositions Ln0.7A0.3MnO3 with A=Ca or Sr, TC increases with t over the range 0.96⩽t⩽0.98 where the transition at TC is first order. The CMR is greatest near t≈0.96; it reflects a trapping out of mobile holes with decreasing temperature in the paramagnetic phase and their progressive release with decreasing temperature in the ferromagnetic phase where spin entropy is exchanged for configurational entropy.

Journal ArticleDOI
TL;DR: In this article, the double-exchange mechanism is replaced by antiferromagnetic spin fluctuations at a concomitant charge ordering and structural transition in a single crystalline single-crystalline material.
Abstract: Single crystalline ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ ( $0.74\ensuremath{\le}x\ensuremath{\le}0.82$) were studied with neutron scattering, electron diffraction, and bulk magnetic measurement. We discovered dynamic ferromagnetic spin correlations at high temperatures, which are replaced by antiferromagnetic spin fluctuations at a concomitant charge ordering and structural transition. Our results indicate that thermal-activated hopping of the Jahn-Teller active ${e}_{g}$ electrons in these insulating materials, nevertheless, induces ferromagnetic interaction through the double-exchange mechanism. It is the ordering of these charges which competes with the double-exchange ferromagnetic metallic state.

Journal ArticleDOI
TL;DR: The specific heat of perovskite manganite was measured in the doping regime x = 0.0 to 0.3 where at low temperatures the material changes from a layered antiferromagnetic insulator to a ferromagnetic metal as discussed by the authors.
Abstract: The specific heat of the perovskite manganite La{sub 1-x}Sr{sub x}MnO{sub 3+{delta}} was measured in the doping regime x=0.0 to 0.3 where at low temperatures the material changes from a layered antiferromagnetic insulator to a ferromagnetic metal. A term in the specific heat, C{proportional_to}T{sup 2}, is found in LaMnO{sub 3} that is attributable to spin excitations in a layered antiferromagnet. Ferromagnetic samples have the expected specific heat term C{proportional_to}T{sup 3/2} due to ferromagnetic spin waves. The exchange coupling (J), electronic linear term ({gamma}), Debye temperature ({theta}{sub D}), and the local field at the Mn site (H{sub hyp}) are extracted from the specific heat as functions of Sr doping. {copyright} {ital 1997} {ital The American Physical Society}

Journal ArticleDOI
TL;DR: In this article, a systematic study of the magnetic properties of the manganites Ln0.5A 0.5MnO3 (A,A′)0.
Abstract: A systematic study of the magnetic properties of the manganites Ln0.5(A,A′)0.5MnO3 (A,A′=Ba, Ca, Sr) has been carried out. The variations of TC and TN vs two parameters, the average size of the interpolated cations 〈rA〉 and the mismatch effect represented by the variance σ2, have been studied for the series of manganites La0.5(A,A′)0.5MnO3, Pr0.5(A,A′)0.5MnO3, Nd0.5(A,A′)0.5MnO3 and Sm0.5(A,A′)0.5MnO3 (A,A′=Ba, Ca, Sr), involving at most three different cations on the perovskite A site. The results obtained for other (Ln, Ln′)0.5Sr0.5MnO3(Ln, Ln′=Gd, Sm Y, Pr) have been taken into consideration. A σ2−〈rA〉 diagram has been established, which displays the different types of magnetic behaviors that can be exhibited by Ln0.5A0.5MnO3 manganites, i.e., when the temperature is decreasing, either a paramagnetic (PM) to ferromagnetic (FM) transition, or PM to FM followed by FM to antiferromagnetic transitions, or a PM to weak ferromagnetic transition.

Journal ArticleDOI
TL;DR: The magnetic and magnetotransport properties of perovskite La0.67Ca0.33Mn0.9Fe0.1O3 have been investigated, and the spin-glass behavior with a spin freezing temperature of 42 K has been well confirmed for this compound.
Abstract: The magnetic and magnetotransport properties of the perovskite La0.67Ca0.33Mn0.9Fe0.1O3 have been investigated, and the spin-glass behavior with a spin freezing temperature of 42 K has been well confirmed for this compound. A metal-to-insulator transition and colossal magnetoresistance have been observed near its spin freezing temperature; besides, the insulator behavior has been found to reappear at lower temperature. The formation of ferromagnetic and antiferromagnetic clusters and the competition between them with the introduction of Fe3+ ions, which do not participate in the double-exchange process, have been suggested to explain the experimental results.

Journal ArticleDOI
TL;DR: In this article, the ground and low-lying excited states of uniform and dimerized alternating spin chains were analyzed using spin wave and density-matrix renormalization group (DMRG) studies.
Abstract: We report spin wave and density-matrix renormalization-group (DMRG) studies of the ground and low-lying excited states of uniform and dimerized alternating spin chains. The DMRG procedure is also employed to obtain low-temperature thermodynamic properties of the system. The ground state of a 2N spin system with spin-1 and spin- alternating from site to site and interacting via an antiferromagnetic exchange is found to be ferrimagnetic with total spin ${\mathrm{s}}_{\mathrm{G}}$=N/2 from both DMRG and spin wave analysis. Both the studies also show that there is a gapless excitation to a state with spin ${\mathrm{s}}_{\mathrm{G}}$-1 and a gapped excitation to a state with spin ${\mathrm{s}}_{\mathrm{G}}$+1. Surprisingly, the correlation length in the ground state is found to be very small from both the studies for this gapless system. For this very reason, we show that the ground state can be described by a variational ansatz of the product type. DMRG analysis shows that the chain is susceptible to a conditional spin-Peierls' instability. The DMRG studies of magnetization, magnetic susceptibility (\ensuremath{\chi}), and specific heat show strong magnetic-field dependence. The product \ensuremath{\chi}T shows a minimum as a function of temperature (T) at low-magnetic fields and the minimum vanishes at high-magnetic fields. This low-field behavior is in agreement with earlier experimental observations. The specific heat shows a maximum as a function of temperature and the height of the maximum increases sharply at high-magnetic fields. It is hoped that these studies will motivate experimental studies at high-magnetic fields.

Journal ArticleDOI
TL;DR: In this paper, the transport and magnetic properties of stoichiometric and Li-rich LiMn 2 O 4 spinels are investigated, which are associated with a structural phase transition from cubic to tetragonal.

Journal ArticleDOI
TL;DR: Close to the border of antiferromagnetic order at high pressure, the cubic stoichiometric compound CeIn 3 displays an unconventional normal-state resistivity followed by superconductivity at low temperatures as mentioned in this paper.
Abstract: Close to the border of antiferromagnetic order at high pressure the cubic stoichiometric compound CeIn 3 displays an unconventional normal-state resistivity followed by superconductivity at low temperatures. The superconducting transition temperature varies rapidly with pressure near the critical pressure where the Neel temperature collapses towards absolute zero.

Journal ArticleDOI
TL;DR: In this article, single-phase samples of La1-yMnO3-(3y/2)+δ with 0 ≤ y ≤ 0.09 and LaMn1-zO3(3z/2)-with 0 ≤ z ≤ 0.18 were obtained.
Abstract: Single-phase samples of La1-yMnO3-(3y/2)+δ with 0 ≤ y ≤ 0.09 and LaMn1-zO3-(3z/2)+δ with 0 ≤ z ≤ 0.09 have been prepared via the citrate method with subsequent heating at different temperatures and partial pressures of oxygen. Oxygen parameters 0 ≤ δ ≤ 0.18 were obtained. At room temperature, all compositions y and z have the orthorhombic structure of GdFeO3 (Pbnm) for small values of δ and the rhombohedral structure of LaAlO3 (R3c) for larger values of δ. Below a temperature Tc, all samples show a weak ferromagnetism at δ = 0 that increases with δ, samples with y = 0 approaching a full ferromagnetic alignment of all Mn moments in the range 0.10 < δ < 0.13. At smaller δ, the orthorhombic samples contain randomly distributed superparamagnetic clusters that couple via an antiferromagnetic matrix to form a spin glass below Tc. At higher values of δ, a sharp increase of Tc to a value greater than the Weiss temperature θp for the paramagnetic phase indicates a first-order transition on lowering T through Tc f...

Journal ArticleDOI
15 Jun 1997
TL;DR: In this article, the effects of magnetic frustation in the stacked triangular lattice were reviewed and the authors described the confrontation of theory and experiment for a number of systems with differing magnetic Hamiltonians.
Abstract: In this article we review the effects of magnetic frustation in the stacked triangular lattice. Frustration increases the degeneracy of the ground state, giving rise to different physics. In particular it leads to unique phase diagrams with multicritical points and novel critical phenomena. We describe the confrontation of theory and experiment for a number of systems with differing magnetic Hamiltonians; Heisenberg, Heisenberg with easy-axis anisotropy, Heisenberg with easy-plane anisotropy, Ising and singlet ground state. Interestingly each leads to different magnetic properties and phase diagrams. We also describe the effects of ferromagnetic, rather than antiferromagnetic, stacking and of small distortions of the triangular lattice.

Journal ArticleDOI
TL;DR: In this paper, the authors measured thermal conductivity versus temperature and magnetic field for perovskite manganites that exhibit ferromagnetic (FM), charge-ordering (CO), antiferromagnetic, and structural phase transitions.
Abstract: Measurements of thermal conductivity versus temperature and magnetic field are reported for perovskite manganites that exhibit ferromagnetic (FM), charge-ordering (CO), antiferromagnetic, and/or structural phase transitions. The data reveal a dominant lattice contribution to the heat conductivity with $\ensuremath{\kappa}\ensuremath{\sim}1\ensuremath{-}2$ W/mK near room temperature. The rather low values, implying a phonon mean free path on the order of a lattice spacing, are shown to correlate with static local distortions of the ${\mathrm{MnO}}_{6}$ octahedra. Modifications of the local structure are responsible for abrupt anomalies in the zero-field \ensuremath{\kappa} at the FM, CO, and structural transitions, and for colossal magnetothermal resistance near the FM transition.

Journal ArticleDOI
TL;DR: In this article, the variation of the Curie temperature and the antiferromagnetic transition temperature as a function of the average size of the A site cations (Ln, Sr) in the manganite perovskites Ln0.5Sr 0.5MnO3 was studied.
Abstract: We have studied the variation of the paramagnetic to ferromagnetic transition temperature TC and of the ferromagnetic to antiferromagnetic transition temperature TN as a function of the average size of the A site cations (Ln, Sr) in the manganite perovskites Ln0.5Sr0.5MnO3 (Ln=Gd, Sm, Y, Pr, La). For our investigation we used dc resistivity, magnetization, susceptibility and magnetoresistance measurements. Results show that the Curie temperature TC increases from 85 to 310 K when 〈rA〉 varies from 1.221 to 1.263 A, whereas TN remains stable around 150 K. A magnetic phase diagram temperature-〈rA〉 has been established, allowing five regions with different magnetic behaviours to be evidenced: paramagnetic insulating, ferromagnetic metallic, antiferromagnetic insulating, canted antiferromagnetic metallic, and weak ferromagnetic insulating. The extension of these regions is determined by the value of the applied magnetic field.

Journal ArticleDOI
TL;DR: In this article, the authors used high-resolution neutron powder diffraction to detect the presence of important tensile and compressive stresses in the crystal structure of, which are progressively released along the series as the rare earth size decreases.
Abstract: The title compounds (R = La, Pr, Nd, Sm, Eu, Tb, Ho, Er) have been prepared in polycrystalline form by a citrate technique and, excepting the Sm and Eu phases, structurally studied by high-resolution neutron powder diffraction. All the materials are isostructural (space group Pbam, Z = 4) and contain infinite chains of octahedra sharing edges, linked together by and units. The size of the three kinds of coordination polyhedron regularly decreases as R cations become smaller. A bond-valence study allowed us to detect the presence of important tensile and compressive stresses in the crystal structure of , which are progressively released along the series as the rare-earth size decreases. The magnetic properties strongly depend on the nature of R, going from the spin-glass behaviour observed at low temperature in to the field-induced transitions exhibited by . A cusp in the susceptibility curves suggests an antiferromagnetic ordering at low temperatures, which is masked in the compounds containing strongly paramagnetic rare earths (Tb, Ho, Er). At high temperatures the paramagnetic moments are consistent in all cases with the presence of high-spin and cations.