Showing papers on "Antiferromagnetism published in 1978"
TL;DR: The existence of spontaneous magnetization at sufficiently low temperature, and hence of a phase transition, in a variety of quantum spin systems in three or more dimensions was proved in this article.
Abstract: We prove the existence of spontaneous magnetization at sufficiently low temperature, and hence of a phase transition, in a variety of quantum spin systems in three or more dimensions. The isotropic spin 1/2 x−y model and the Heisenberg antiferromagnet with spin 1, 3/2,...and with nearest neighbor interactions on a simple cubic lattice are included.
442 citations
TL;DR: In this paper, specific heat measurements of (La1−xCeghazi x )Al2 samples, with 1.5 a/o≦x≦100 a/O, performed in magnetic fields of up to 5 T between 0.3 and 10 K, are reported.
Abstract: We report on specific heat measurements of (La1−x
Ce
x
)Al2 samples, with 1.5 a/o≦x≦100 a/o, performed in magnetic fields of up to 5 T between 0.3 and 10 K. In the Ce rich alloys, and especially in CeAl2, aλ-type peak of an antiferromagnetic phase transition, and at lower temperatures spin waves and very large electronic contributions are clearly visible. In higher magnetic fields, that is when antiferromagnetic order can be suppressed, the specific heat of the alloys exhibits a broadened Schottky peak. All these phenomena add up tok ln 2, i.e. to the correct entropy change per single Ce3+ ion in itsΓ
7 crystal field ground state. We interpret experimental results as an interplay between cooperative magnetism and the single-ion Kondo effect which describes a gradual turning off of one magnetic moment. The broadening of the Schottky peak is directly related to the Kondo temperatureT
K
, which we determine with a simple “resonance level model”.T
K
increases by an order of magnitude whenx increases from 1.5 a/o to 100 a/o. This is interpreted as caused by a lattice contraction. A quadraticx dependence of the Neel temperature suggests that (forT≲T
K
) stable Ce moments can only exist through pair interactions. The very large (and almost field independent) specific heat term linear in temperature with a coefficientγ=135 mJ/K2 mole for CeAl2 is attributed to the Kondo effect—still present in the antiferromagnetically ordered state. Our evaluation of the experimental data is backed by a molecular field theory for a simplified antiferromagnetic structure combined with the simplest possible Kondo theory.
176 citations
TL;DR: In this article, a quasi-one-dimensional antiferromagnet of Ising-like spins of CsCoCl 3 was measured in the long range ordered region at 4.2 K and the critical exponent of the magnetization was found to be 0.34 around the Neel temperature 20.82 K.
Abstract: Neutron diffraction measurements were carried out on single crystals of CsCoCl 3 , a quasi-one-dimensional antiferromagnet of Ising-like spins, in the long range ordered region. The determined magnetic structure at 4.2 K is composed of antiferromagnetic stacking of ferrimagnetic c -plane with three sublattices as determined by Melamud et al. The spins are aligned along c -axis and were evaluated to be 3.11 µ B at 0 K by extrapolation. Unusual temperature dependence of magnetic reflection intensity was observed and interpreted as a partial disordering transition in which one of the three sublattices in a c -plane loses its magnetization above 13.5 K resulting in antiferromagnetic c -planes. This transition is discussed in connection with the antiferro-ferrimagnetic transition in the triangular Ising lattice. The critical exponent of the magnetization was found to be 0.34 around the Neel temperature 20.82 K.
163 citations
TL;DR: In this paper, the recently reported phase transitions of Mo(001) and W(001), in terms of surface charge-density waves, are discussed and the expected two-dimensional critical behaviour is also investigated.
108 citations
TL;DR: In this article, the magnetic moment of the compound Bi2Fe4O9 was found to be (4.95 ± 0.08) μB, compared with the value of 5 μB for the Fe3+ free ion.
Abstract: The compound Bi2Fe4O9 belongs to the space group Pbam ( D92h), with two formula units per unit cell. Neutron diffraction measurements showed that it is paramagnetic at room temperature and undergoes a transition to an antiferromagnetic state at TN = (264 ± 3) K in agreement with previous susceptibility and Mossbauer measurements. Analysis of the 80 K neutron diffraction pattern yielded a magnetic structure with the following features: (a) The basic translations ao, bo, co of the chemical lattice change into antitranslations in the magnetic lattice. (b) The spins are perpendicular to co. (c) The magnetic structure belongs to the PC2/m space group and is a basis vector to an irreducible space under the Pbam irreducible representations, in accord with Landau's theory of second-order phase transition. The position parameters of the Fe3+ ions in the unit cell were refined. The magnetic moment of the compound was found to be (4.95 ± 0.08) μB, compared with the value of 5 μB for the Fe3+ free ion. The temperature dependence of the { 131 } magnetic reflection peak intensity was measured and found to be in agreement with the sublattice magnetization predicted by the molecular field approximation.
104 citations
TL;DR: In this article, a detailed analysis of X-ray intensities yields all ion locations and interatomic distances in the body centered tetragonal structure (space group I4/MMM).
98 citations
TL;DR: In this article, the analysis of the magnetic structures of type-II antiferromagnets (a FCC lattice with k = 1 2, 1 2, 1 2 1/2, 1 3, 1 1/ 2 ) is presented.
Abstract: The authors deal with the analysis of the equivalent magnetic structures of type-II antiferromagnets (a FCC lattice with k=(1/2, 1/2, 1/2)). This analysis is essentially based on symmetry considerations taking into account the symmetry of the group G(k) corresponding to a particular set of k-vectors and the symmetry group Gm of the ordered phase, which results from the direction of the Fourier component associated with each k-vector. Only a few magnetic structures are possible if two physical restrictions are imposed: the angle between ki and mki, is the same for each ki and each magnetic moment has the same value. It is concluded that MnO, NiO and FeO have a collinear magnetic structure whereas in CoO the collinear structure and the multiaxis Van Laar model are consistent with previous experiments. From neutron diffraction experiments on single crystals of CoO, under uniaxial stresses applied out of a tetragonal axis one can conclude that CoO is also a collinear antiferromagnet.
91 citations
TL;DR: In this paper, it was shown conclusively that the Dy atoms form a simple cubic sublattice (with a slight rhombohedral distortion) in which the observed magnetic structure consists of (100) planes with moments of 8.77${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$ alternating parallel and antiparallel to the [111] direction.
Abstract: Although Dy${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$ is a superconductor below ${T}_{c}=2.05$ K previous studies of the resistivity, susceptibility, and upper critical field strongly suggest the development of magnetic order at ${T}_{M}=0.4$ K. The present neutron study shows conclusively that this superconducting compound also develops coexistent long-range (greater than 300 \AA{}) antiferromagnetic order at ${T}_{M}$. The Dy atoms form a simple cubic sublattice (with a slight rhombohedral distortion) in which the observed magnetic structure consists of (100) planes with moments of 8.77${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$ alternately parallel and antiparallel to the [111] direction.
87 citations
TL;DR: In this article, the (001) surface electronic structure of chromium was investigated within the tight-binding approximation in the non-magnetic and in the antiferromagnetic states, and the paramagnetic local surface density of states showed a large peak near the middle of the d band just below the Fermi level.
83 citations
TL;DR: In this article, the short-range spin correlations in USb, a metallic compound that orders antiferromagneticically, were measured and the results of this and other expeiments can be understood with simple concepts involving the bonding of 5f electrons.
Abstract: Measurements have been made of the short-range spin correlations in USb, a metallic compound that orders antiferromagnetically. The system has no transverse fluctuations, and the longitudinal spin correlations are anisotropic, showing stronger interactions within the ferromagnetic sheets than between them. The results of this and other expeiments can be understood with simple concepts involving the bonding of 5f electrons.
63 citations
TL;DR: Chynoweth experiments conducted at low temperatures reveal a weak pyroelectric effect which disappears above 28 K, but no anomalies in the electric permittivity or electric conductivity occur at the Neel point as mentioned in this paper.
Abstract: Chromium chrysoberyl undergoes a phase transformation from a paramagnetic state to a complex antiferromagnetic state at 28 K. The spiral spin structure of the antiferromagnetic state violates all the crystallographic symmetry elements, making Cr2BeO4 potentially ferroelectric. Chynoweth experiments conducted at low temperatures reveal a weak pyroelectric effect which disappears above 28 K. Cr2BeO4 ceramics can be poled electrically between 24 and 28 K, giving rise to remnant polarizations four to six orders of magnitude smaller than normal ferroelectrics. The pyroelectric coefficient and the remnant polarization reverse in sign with the poling field, but no anomalies in the electric permittivity or electric conductivity occur at the Neel point.
TL;DR: In this article, the authors studied transition and noble metal site NMR spectra of stoichiometric, off-stoichiometric and substituted Heusler alloys and analyzed the hyperfine fields in terms of local and transferred contributions.
Abstract: The authors have studied transition and noble metal site NMR spectra of stoichiometric, off-stoichiometric and substituted Heusler alloys. The hyperfine fields can be analysed consistently in terms of local and transferred contributions. The transferred fields are negative at near neighbour and second neighbour to a Mn site, and positive at third neighbour. In many cases excess Mn atoms substituted onto Z sites form antiferromagnetic good local moments in these ferromagnetic hosts with strongly anomalous hyperfine field temperature dependences. Both the transferred fields and the Mn-Mn coupling follow the predictions of a Friedel-Anderson d resonance model correctly treated in the preasymptotic region. The local hyperfine fields at Mn sites are very similar to fields in dilute Mn alloys, while the local fields at Co sites are positive indicating unquenched local orbital moments on the Co atoms.
TL;DR: In this paper, a preferential location of Fe 2+ and Fe 3+ ions on each antiferromagnetic sublattice is proposed, which is the first example of an annealed random spin system.
Abstract: Magnetic properties and electrical conductivity of the YFe 2 O 4 single crystal were investigated. It was disclosed that YFe 2 O 4 is an antiferromagnet whose Neel point is 205 K, and the spin lies parallel to the hexagonal c axis. Below T N , the magnetization parallel to the c axis was not proportional to the magnetic field. A large rotational hysteresis was observed at temperatures between 170 and 120 K. Below 140 K, a weak but very hard ferromagnetic moment parallel to the c axis resulted from field cooling. On the basis of these facts, a model of parasitic ferromagnetism, i.e., preferential location of Fe 2+ and Fe 3+ ions on each antiferromagnetic sublattice, is proposed. If such a preferential location is caused by the difference of the exchange interaction constant, YFe 2 O 4 is the first example of an annealed random spin system.
TL;DR: In this paper, the Hartree-Fock approximation of the periodic Anderson model for a lattice of magnetic ions is investigated in Hartree Fock approximation, and the effect of hybridization leading to reduced magnetic moments strongly depends on the position of the localized f levels relative to the conduction band.
Abstract: The periodic Anderson model for a lattice of magnetic ions is investigated in Hartree-Fock approximation. Attention is paid to different solutions of the self-consistency equations corresponding to ferromagnetic or antiferromagnetic ordering of the local magnetic moments. The effect of hybridization leading to reduced magnetic moments strongly depends on the position of the localizedf levels relative to the conduction band. For paramagnetic solutions with a non-integer value for thef level occupation number comparison is made with properties of intermediate valence rare earth compounds. The mean-field results for the susceptibility and specific heat agree with essential features found for these substances.
TL;DR: In this article, the crystal and molecular structure of the title cubane-type molecules have been determined from single-crystal X-ray diffraction data and refined to R 0.068 for 1 423 independent reflections.
Abstract: The crystal and molecular structure of the title complex has been determined from single-crystal X-ray diffraction data and refined to R 0.068 for 1 423 independent reflections. Four oxygen-bridged cubane-type molecules with a Cu4O4 core, having crystallographically imposed S4 point symmetry, occupy a tetragonal unit cell, space group /2d, of dimensions a= 17.85(1) and c= 11.92(1)A. The four copper atoms in the tetranuclear cluster are situated in an almost tetrahedral arrangement, the two independent copper–copper separations being 3.194(2)(× 2) and 3.193(2)A(× 4). Copper–oxygen distances within the Cu4O4 framework [1.936(6)(× 4), 2.243(6)(× 4), and 2.113(6)A(× 4)] are different and show unsymmetric bridging of the copper atoms. The co-ordination around copper (amine nitrogen, isocyanate nitrogen, and three alkoxide triply bridging oxygens) may be described as between distorted square pyramidal and distorted trigonal bipyramidal. The magnetic susceptibility measured over the range 2.6—300 K shows a maximum at ca. 35 K and thus indicates overall antiferromagnetic spin coupling. The magnetism of the complex can be explained on the basis of the isotropic Heisenberg–Dirac–van Vleck model. The resulting values for the magnetic exchange integrals: [J12-21.4 ± 0.5 (× 2), J13 12.3 ± 1.0 cm–1(× 4)] indicate the simultaneous presence of antiferromagnetic and ferromagnetic interactions between copper atoms within the same molecule.
TL;DR: In this article, the antiferromagnetic phase boundary in the Cr-Fe system has been determined by neutron diffraction, and it is shown that no long-range order exists in alloys between 16 and 19 at.% Fe.
Abstract: The antiferromagnetic phase boundary in the Cr-Fe system has been determined by neutron diffraction. Antiferromagnetism disappears at a concentration of 16.0+or-0.5 at.% Fe. Resistivity measurements indicate that TN is marked by the maximum in the temperature coefficient of resistivity rather than the minimum in the resistivity. On the basis of these measurements it is apparent that no long-range order exists in Cr-Fe alloys between 16 and 19 at.% Fe.
TL;DR: In this article, it has been shown that nearly stoichiometric BaVS 3 behave like a quasi ID antiferromagnet and the sulphur deficient BaVS3 is a quasi-ID ferromagnetic order takes place below 16 K.
TL;DR: The magnetic properties of the NiAs-type iron selenides have been investigated by susceptibility measurements between 100 and 450 K as discussed by the authors, and the transition to the paramagnetic state is assumed to occur in the unstable range between room temperatur and about 573K. Hexagonal γ-Fe1-xSe exhibits both antiferromagnetic and ferrimagnetism depending on composition.
Abstract: The magnetic properties of the NiAs-type iron selenides have been investigated by susceptibility measurements between 100 and 450 K. Hexagonal γ-Fe1-xSe exhibits both antiferromagnetism and ferrimagnetism depending on composition. For antiferromagnetic alloys with 0.02≤x≤0.10 (50.5 to 52.5 at % Se) the transition to the paramagnetic state is assumed to occur in the unstable range between room temperatur and about 573K. Ferrimagnetism is observed at compositions near Fe7Se8 (0.10≤x≤0.16; 52.5 to 54.3 at % Se) withCurie temperatures varying only slightly with composition. In Fe7Se8 the ferriparamagnetic transition is observable at 453 K. The characteristic discontinuities in the magnetization curves of Fe7Se8 in both superstructures (3c, 4c) are related to the rotation of the magnetic moments from a direction close to [001] into (001).
TL;DR: In this paper, low temperature measurements of the specific heat, the thermal expansion, the electrical resistivity and the magnetization of single crystal specimens are found to be very sensitive to crystal perfection and chemical composition in both the paramagnetic and magnetically ordered state.
Abstract: CeP and CeAs are characterized by low temperature measurements of the specific heat, the thermal expansion, the electrical resistivity and the magnetization of single crystal specimens. These physical properties are found to be very sensitive to crystal perfection and chemical composition in both the paramagnetic and the magnetically ordered state. In the intrinsically antiferromagnetic CeP, imperfections can give rise to a remanent magnetization belowT
N
.
TL;DR: The existence of ferromagnetic γ-Fe-films can be explained by the epitaxial expansion of the lattice parameter and will be discussed in terms of different concepts of the magnetic order as mentioned in this paper.
Abstract: Epitaxial f.c.c. γ-Fe-films on Cu (111) with a well-defined structure are measured magnetometrically. Layer-grown as well as island-grown films prepared with a film thickness between 3 and about 85 A, are found to be ferromagnetic with a magnetic moment of (0.58 ± 0.13) μB/atom. The temperature dependence of the anisotropy field differs fundamentally for layer- and islandgrown films. The existence of ferromagnetic γ-Fe-films, which seems to be controversal to the antiferromagnetism of γ-Fe-precipitates, can be explained by the epitaxial expansion of the lattice parameter and will be discussed in terms of different concepts of the magnetic order in γ-Fe.
Epitaktische k.f.z.-γ-Fe-Filme auf Cu (111) mit wohldefinierter Struktur werden magnetometrisch vermessen. Im untersuchten Schichtdickenbereich von 3 bis ca. 85 A sind diese Filme ferromagnetisch mit einem magnetischen Moment von (0,58 ± 0,13) μB/Atom, sowohl fur flachenhafte als auch fur Inselschichten. Die Anisotropiefelder dagegen zeigen fur flachenhafte und Inselscher γ-Fe-Filme im Gegensatz zu den antiferromagnetischen γ-Fe-Ausscheidungen wird auf die epitaktisch gedehnte Gitterkonstante zuruckgefuhrt und im Rahmen verschiedener Modelle der magnetischen Ordnung des γ-Fe diskutiert.
TL;DR: Using n-butyl lithium techniques, it was found that CrPS4 can readily accomodate lithium as discussed by the authors, but this insertion is not allowed by any parameter change in the host structure nor reduction of the Cr3+ magnetic moment.
TL;DR: In this article, the Mossbauer effect was applied to EuMn2Ge2 and GdM2Gd and the results showed that the easy axis of magnetization is close to the c-axis, while in the Gd compounds it is in the basal plane.
TL;DR: In this article, the metamagnetic behavior of the low temperature properties of single crystal Co(pyridine)2Cl2 is discussed, and the basic features are consistent with a six-sublattice model for the ordered antiferromagnetic system.
Abstract: The metamagnetic behavior of the low temperature properties of single crystal Co(pyridine)2Cl2 is discussed. At 1.25 K oriented single crystals exhibit a two‐step metamagnetic transition at applied fields ∼0.8 and 1.6 kG along the b‐axis, a single transition at ∼0.7 kG for applied fields along the a* axis, and a single transition at ∼4.2 kG for an applied field along the c axis. Just above the transition fields a moment of 2μB/Co atom is measured for B0 parallel to the a* axis or b axis, and 0.4μB/Co atom is measured for the B0 parallel to the c axis. A large field dependent moment is observed at high fields. Many features of this compound closely mirror the behavior of CoCl2⋅2H2O. However, the Co(pyridine)2Cl2 has a much smaller interchain exchange, so that many features can be examined at lower fields. The basic features are consistent with a six‐sublattice model for the ordered antiferromagnetic system. Measurements of magnetic moment versus temperature show that Co(pyridine)2Cl2 does not obey a Curie–...
TL;DR: In this article, powder neutron diffraction data on superconducting (Ts∼6 K) samples of ErMo6Se8 taken in the temperature range 0.05-2.0 K show that magnetic Bragg peaks develop at TM=1.1 K, in agreement with specific heat studies.
Abstract: Development of new ternary superconductors has led to materials which also show a strong tendency toward magnetic order when one of the constituent elements is a rare earth. Powder neutron diffraction data on superconducting (Ts∼6 K) samples of ErMo6Se8 taken in the temperature range 0.05–2.0 K show that magnetic Bragg peaks develop at TM=1.1 K, in agreement with specific heat studies. However, it is not possible to index these new reflections using a simple antiferromagnetic unit cell based on the ErMo6Se8 lattice. It is also not possible to index the magnetic reflections based on a single modulation vector, or with a modulation vector along a high symmetry direction including higher order harmonics. Thus either the long range magnetic order corresponds to a more complicated magnetic structure in the ErMo6Se8 lattice, or at least some of the peaks develop in impurity phases. These results are compared with the recent neutron data on the reentrant superconductors HoMo6Se8 and ErRh4B4, in which the develop...
TL;DR: In this paper, the magnetic properties of uranium pnictides and chalcogenides depend on the orientation of the applied field, and an intermediate ferrimagnetic spin structure of UAs is put into evidence.
Abstract: The magnetic properties of uranium pnictides and chalcogenides depend on the orientation of the applied field. As an example the anisotropic magnetic moments of USe (a ferromagnet) and the anisotropic susceptibility of UAs (an antiferromagnet) are discussed. Furthermore the existence of an intermediate ferrimagnetic spin structure of UAs is put into evidence.
TL;DR: In this article, the spin wave energy dispersion relation has been investigated at three temperatures in a two-dimensional Ising-like antiferromagnet, Rb2CoF4.
Abstract: Using inelastic-neutron scattering techniques, the spin wave energy dispersion relation has been investigated at three temperatures in a two-dimensional Ising-like antiferromagnet, Rb2CoF4. The results show there is relatively little dispersion across the Brillouin zone with a very large energy gap at zero wavevector. As the temperature is increased to just below TN there is relatively little renormalisation of the spin waves over the region of wavevectors examined.
TL;DR: In this article, the magnetoresistance of itinerant electron antiferromagnets is studied on the basis of the two band model where the electrons in one conduction or s-band are the current carriers and are scattered by the spin fluctuations due to the d electrons.
Abstract: The magnetoresistance of itinerant electron antiferromagnets is studied on the basis of the two band model where the electrons in one conduction or s-band are the current carriers and are scattered by the spin fluctuations due to the d electrons. The self-consistent renormalization theory of spin fluctuations is applied to the d electrons. The resistivity shows a cusp at the Neel temperature and becomes linear in T at high temperatures. The resistivity under a perpendicular field shows a negative magnetoresistance for small fields and a narrow peak at the antiferromagnetic to paramagnetic transition point H c . In the case of a parallel applied field with the uniaxial anisotropy, we expect a large increase in the magnetoresistance on both sides of the spin flopping transition point. Numerical calculations are made with the use of the free electron gas model with Umklapp processes.
TL;DR: In this article, the five and six-coordinate dimeric manganese(III) and cobalt (III) complexes derived from benzhydrazide and salicylaldehyde (BSH), 0-hydroxyacetophenone(BAH), o-hydroxpropiophenone (BPH), ohydroxybutyrophenone (BBH) and 2-hydoxy-1-naphthaldehyde(BNH) having the general formulae [Mn(L)X]2 and [Co(L)(OH)H2
Abstract: The five- and six-coordinate dimeric manganese(III) and cobalt(III) complexes derived from benzhydrazide and salicylaldehyde(BSH), 0-hydroxyacetophenone(BAH), o-hydroxypropiophenone(BPH), o-hydroxybutyrophenone (BBH) and 2-hydroxy-1-naphthaldehyde(BNH) having the general formulae [Mn(L)X]2 and [Co(L)(OH)H2O]2 (X = Cl, Br or CH3 COO) are described. These complexes have been characterised by elemental analyses and by conductance, molecular weight, magnetic, electronic and infrared spectral measurements. The complexes of Mn(III) involve phenolic oxygen, while in Co(III) complexes hydroxyl groups are present as bridges between metal atoms as revealed by i.r. spectra. Each unit of dimeric Mn(III) complexes possesses distorted square-pyramidal shape and anions are present on the axial position. The ν(Mn-X) values are consistent with penta-coordinate stereochemistry. These complexes have subnormal magnetic moments (3.40–3.52 BM) explained in terms of antiferromagnetic exchange coupling between two adjac...