L.J. de Jongh

Magnetic properties of layered transition metal compounds

to Low-Dimensional Magnetic Systems.- 1. Experimental realizations of 2-d magnetic systems.- 2. Magnetic model Hamiltonians.- 3. Survey of the predicted magnetic behaviour.- 4. Lattice- and spin-dimensionality crossovers in quasi 2-d magnetic systems.- 5. Magnetic and nonmagnetic impurity doping in quasi 2-d magnets.- References.- Theory of Two-Dimensional Magnets.- 1. Introduction.- 2. Ising magnets.- 2.1. Ising model. Excitations and phase transitions.- 2.2. Onsager solution.- 2.3. Critical exponents and scaling.- 2.4. Dual transformation. Order and disorder.- 3. Planar magnets.- 3.1. XY model.- 3.2. Excitations.- 3.3. Scaling and correlations.- 3.4. Phase transition.- 3.5. Magnetic vortices as a Coulomb gas.- 3.6. Relationships with other models.- 3.7. Planar antiferromagnets.- 4. Heisenberg magnets.- 4.1. Heisenberg model and real magnets.- 4.2. Renormailzation of the temperature.- 4.3. Heisenberg ferromagnets in an external magnetic field.- 4.4. Excitations of the 2-d Heisenberg model.- 4.5. Dipolar interactions.- 5. Experimental layered magnets.- 5.1. Ising layered magnets. ANNNI model: application to CeSb and CeBi.- 5.2. Layered planar magnets.- 5.3. Layered Heisenberg magnets.- 6. Dynamics of 2-d magnets.- 6.1. Equations of motion.- 6.2. Spin-wave dynamics.- 6.3. Spin-diffusion dynamics.- 6.4. Dynamics of localized excitations.- 6.5. Resonant paramagnetic cxcitation of vortex pairs.- 6.6. Summary.- Acknowledgement.- References.- Application of High- and Low-Temperature Series Expansions to Two-Dimensional Magnetic Systems.- 1. Introduction.- 1.1. Series expansions.- 1.2. Methods applied in series analysis.- 1.2.1. Ratio methods.- 1.2.2. Pade approximant methods.- 1.2.3. Other methods of series analysis.- 2. Series expansions and predictions for the 2-d Ising model.- 2.1. Spin 1/2 model with nearest neighbours only (simple 2-d lattices).- 2.1.1. High-temperature series.- 2.1.2. Low-temperature series.- 2.1.3. Properties in nonzero parallel field.- 2.1.4. Properties in nonzero perpendicular field.- 2.2. Ising model with general S.- 2.3. Other series for I (1/2).- 2.3.1. Restricted dimensionality systems.- 2.3.2. Further-neighbour interactions.- 2.3.3. Crossover from 2-d to 3-d behaviour.- 3. Series expansions and predictions for the Heisenberg model.- 3.1. Series for S = 1/2, arbitrary S and S = ?.- 3.1.1. Properties at nonzero field.- 3.2. Other series for the Heisenberg model.- 3.2.1. Restricted dimensionality.- 3.2.2. Further-neighbour interactions.- 3.2.3. Crossover from 2-d to 3-d behaviour.- 4. Series expansion in the X Y and Ising-to Low-Dimensional Magnetic Systems.- 1. Experimental realizations of 2-d magnetic systems.- 2. Magnetic model Hamiltonians.- 3. Survey of the predicted magnetic behaviour.- 4. Lattice- and spin-dimensionality crossovers in quasi 2-d magnetic systems.- 5. Magnetic and nonmagnetic impurity doping in quasi 2-d magnets.- References.- Theory of Two-Dimensional Magnets.- 1. Introduction.- 2. Ising magnets.- 2.1. Ising model. Excitations and phase transitions.- 2.2. Onsager solution.- 2.3. Critical exponents and scaling.- 2.4. Dual transformation. Order and disorder.- 3. Planar magnets.- 3.1. XY model.- 3.2. Excitations.- 3.3. Scaling and correlations.- 3.4. Phase transition.- 3.5. Magnetic vortices as a Coulomb gas.- 3.6. Relationships with other models.- 3.7. Planar antiferromagnets.- 4. Heisenberg magnets.- 4.1. Heisenberg model and real magnets.- 4.2. Renormailzation of the temperature.- 4.3. Heisenberg ferromagnets in an external magnetic field.- 4.4. Excitations of the 2-d Heisenberg model.- 4.5. Dipolar interactions.- 5. Experimental layered magnets.- 5.1. Ising layered magnets. ANNNI model: application to CeSb and CeBi.- 5.2. Layered planar magnets.- 5.3. Layered Heisenberg magnets.- 6. Dynamics of 2-d magnets.- 6.1. Equations of motion.- 6.2. Spin-wave dynamics.- 6.3. Spin-diffusion dynamics.- 6.4. Dynamics of localized excitations.- 6.5. Resonant paramagnetic cxcitation of vortex pairs.- 6.6. Summary.- Acknowledgement.- References.- Application of High- and Low-Temperature Series Expansions to Two-Dimensional Magnetic Systems.- 1. Introduction.- 1.1. Series expansions.- 1.2. Methods applied in series analysis.- 1.2.1. Ratio methods.- 1.2.2. Pade approximant methods.- 1.2.3. Other methods of series analysis.- 2. Series expansions and predictions for the 2-d Ising model.- 2.1. Spin 1/2 model with nearest neighbours only (simple 2-d lattices).- 2.1.1. High-temperature series.- 2.1.2. Low-temperature series.- 2.1.3. Properties in nonzero parallel field.- 2.1.4. Properties in nonzero perpendicular field.- 2.2. Ising model with general S.- 2.3. Other series for I (1/2).- 2.3.1. Restricted dimensionality systems.- 2.3.2. Further-neighbour interactions.- 2.3.3. Crossover from 2-d to 3-d behaviour.- 3. Series expansions and predictions for the Heisenberg model.- 3.1. Series for S = 1/2, arbitrary S and S = ?.- 3.1.1. Properties at nonzero field.- 3.2. Other series for the Heisenberg model.- 3.2.1. Restricted dimensionality.- 3.2.2. Further-neighbour interactions.- 3.2.3. Crossover from 2-d to 3-d behaviour.- 4. Series expansion in the X Y and Ising-Heisenberg models.- 4.1. Series for the 2-d XY model.- 4.2. Series for the 2-d Ising-Heisenberg model.- 5. Applications to magnetic systems.- 5.1. Ising model.- 5.2. Heisenberg model.- 5.2.1. Spin 1/2.- 5.2.2. Spin 1.- 5.2.3. Spin 3/2 and spin 2.- 5.2.4. Spin 5/2.- 5.2.5. Restricted dimensionality.- 5.3. XY and Ising-Heisenberg models.- Acknowledgements.- References.- Spin Waves in Two-Dimensional Magnetic Systems: Theory and Applications.- 1. Introduction.- 2. Magnetic structures and spin Hamiltonians.- 3. Spin wave theory of model systems.- 4. Dispersion relation.- 5. Thermodynamic properties.- 6. Impurities in antiferromagnets.- References.- Neutron Scattering Experiments on Two-Dimensional Heisenberg and Ising Magnets.- 1. Introduction.- 2. 2-d systems with Ising and Heisenberg interactions.- 2.1. K2CoF4: a 2-d Ising system.- 2.2. K2FeF4: a 2-d planar antiferromagnet.- 2.3. K2MnF4 and K2NiF4: weakly anisotropic Heisenberg magnets.- 2.4. Rb2CrCl4: a planar Heisenberg ferromagnet with small anisotropy.- 2.5. K2CuF4: a planar Heisenberg ferromagnet.- 3. 2-d random magnetic systems.- 3.1. Phase transitions and critical phenomena.- 3.2. Excitations.- 3.3. Random field effects.- 3.4. Relaxation front 2-d to 3-d order.- 3.5. Competing anisotropics and interactions.- 4. Triangular lattice antiferromagnet (TALAF).- 4.1. Fluctuations.- 4.2. An additional degree of freedom.- 4.3. Perturbation.- 4.4. Quantum effect RbFeCl3 and CsFeCl3 VX2 (X = Cl, Br, I) AMX2 (A = Li, Na, K M = 3d metal ion X = O, S, Se).- References.- Phase Transitions in Quasi Two-Dimensional Planar Magnets.- 1. Introduction.- 2. Phase transition and excitations in the 2-d XY model.- 3. Crystallographic properties of BaM2(X)4)2 compounds.- 4. Magnetic properties of BaNi2(PO4)2.- 4.1. Static properties.- 4.2. Dynamic properties.- 4.3. Critical properties.- 5. Magnetic properties of BaCo2(AsO4)2.- 5.1. Static properties.- 5.2. Magnetic phase diagrams.- 5.3. Dynamic properties.- 6. Magnetic properties of BaNi2(AsO4)2.- 6.1. Static properties.- 6.2. Dynamic properties.- 7. Magnetic properties of BaCo2(PO4)2.- 8. Other experimental realizations of the 2-d planar model.- 8.1. K2CuF4.- 8.2. NiCl2 and CoCL2 graphite intercalated compounds NiCl2-GIC CoCl2-GIC.- 9. Concluding remarks.- Acknowledgement.- References.- Spin Dynamics in the Paramagnetic Regime: NMR and EPR in Two-Dimensional Magnets.- 1. Introduction.- 1.1. Dynamics of the 2-spin correlation functions.- 1.2. Nuclear magnetic resonance (NMR).- 1.3. Electron paramagnetic resonance (EPR).- 2. General formalism.- 2.1. Diffusion and dimensionality.- 2.2. Cut-off and EPR linewidth.- 3. EPR spectrum.- 3.1. Diffusion of 4-spin correlation functions.- 3.2. Secular contribution D0.- 3.3. Nonsecular contributions.- 3.4. Satellite line.- 4. Experiments on quasi 2-d Heisenberg magnets.- 4.1. NMR experiments.- 4.2. EPR experiments.- 4.2.1. Angular dependence of linewidth.- 4.2.2. Frequency dependence of magic angle linewidth.- 4.2.3. Dynamic shift.- 4.2.4. Lineshape of the main line.- 4.2.5. Satellite lines at half resonance field.- 5. Critical dynamcis.- 5.1. Critical behaviour of the NMR line.- 5.1.1. Isotropic regime.- 5.1.2. Anisotropic regime.- 5.1.3. Experiments.- 5.2. Critical behaviour of the EPR linewidth.- 5.2.1. Ferromagnets.- 5.2.2. Antiferromagnets.- 5.3. AC susceptibility.- 6. Conclusions.- References.- Field-Induced Phenomena in Two-Dimensional Weakly Anisotropic Heisenberg Antiferromagnets.- 1. Introduction.- 2. Effective, field-dependent anisotropies.- 3. The phase diagram.- 4. Random fields and domain walls (solitons).- 5. The spin flop transition.- 6. The bicritical point.- 7. Concluding remarks.- Acknowledgements.- References.- Index of Names.- Index of Chemical Compounds.- Index of Subjects.

Experiments on simple magnetic model systems

In this paper we shall review the theoretical and experimental results obtained on simple magnetic model systems. We shall consider the Heisenberg, XY and Ising type of interaction (ferro and antiferromagnetic), on magnetic lattices of dimensionality 1, 2 and 3. Particular attention will be paid to the approximation of these model systems in real crystals, viz . how they can be realized or be expected to exist in nature. A large number of magnetic compounds which, according to the available experimental information, meet the requirements set by one or the other of the various models are considered and their properties discussed. Many examples will be given that demonstrate to what extent experiments on simple magnetic systems support theoretical descriptions of magnetic ordering phenomena and contribute to their understanding. It will also be indicated in which direction there is a need and/or a possibility for future work.

Experimental observation of the transition from weak link to tunnel junction

Abstract An extension to Morelands break junction technique is developed in order to obtain a clean and stable, mechanically adjustable junction. As a function of an externally applied force the coupling of two electrodes can be varied in vacuum. Experiments are described of a junction with niobium electrodes at 4.2 K which undergo a continuous change in normal resistance R N , from 1 to 10 9 Ω upon applying an increasing force. In this resistance range we discern a transition from a weak link regime to a tunnel regime. The current voltage ( I–V ) curves are reproducible upon adjustment changes in the whole resistance range. In the weak link regime the two electrodes of the junction are in physical contact with each other. The product of the critical current and normal resistance is compared with predictions of Ambegaokar-Baratoff and Kulik-Omelyanchuk. The product of the excess current and normal resistance shows a logarithmic increase for low R N values and decreases for the highest R N values in the weak link regime. Subharmonic gap structure, originating from multiple Andreev reflections is observed over a wide range of R N . In the transition regime the two electrodes are not in contact but there is still a large overlap of the superconducting and quasiparticle wave functions. In this regime a finite slope of the “critical current part” in the current voltage curve is observed. The I–V curves show features characteristics for both a weak link and a tunnel junction. In the tunnel regime there exists a vacuum gap between the electrodes and the Josephson coupling is suppressed. A considerable subgap current is observed, where the product of the subgap current and normal resistance is constant over almost four orders of magnitude of R N . A decreasing conductance near zero bias shows up in this regime. The normal resistane exhibits an exponential behaviour upon variations in the vacuum gap. The absolute stability of the distance between the two junction electrodes is estimated to be better than 0.5 pm over a 100 mV voltage range.

Magnetothermal properties of molecule-based materials

We critically review recent results obtained by studying the low-temperature specific heat of some of the most popular molecule-based materials. After introducing the experimental techniques and basic theoretical framework needed for heat capacity determination and understanding, we report on the magnetothermal properties of molecular antiferromagnetic wheels. For selected molecular high-spin clusters, particular emphasis is devoted to magnetic quantum tunnelling and coherence as well as collective phenomena as probed by heat capacity experiments. We discuss also the possibilities of application of molecule-based materials for magneto-cooling at low temperatures and the limitations in other temperature ranges. Perspectives for future developments are mentioned as well.

Low-temperature study of the magnetization reversal and magnetic anisotropy of Fe, Ni, and Co nanowires

We present an extensive study of the magnetic reversal mechanism of Fe and Ni nanowires with diameters down to 6 nm, i.e. smaller than the domain wall width. The coercive field at 5 K is a factor of 3 lower than the prediction for rotation in unison. We also observe that the activation energy associated with the reversal process is proportional to the cross-section of the wires and nearly independent of the wire length. From the temperature dependence of the coercive field and the magnetic viscosity we can conclude that magnetization reversal takes place via a nucleation of a small magnetic domain, probably at the end of the wire, followed by the movement of the domain wall. For Co wires, we observe a different behavior that is dominated by the competition between the shape anisotropy and the temperature-dependent magnetocrystalline anisotropy.

Metal-cluster compounds and universal features of the hopping conductivity of solids

Abstract In this paper we present an overview of data on the DC conductivity and on the frequency dependent (up to 1 MHz) dielectric properties of polynuclear metal cluster compounds. These materials can be viewed as model systems for identical clusters embedded in a dielectric matrix. Comparison is made with results on related narrow band materials, such as ceramic metals (cermets), various other metal-nonmetal composites, and doped or amorphous semiconductors. Remarkable similarities are observed in the behavior of these at first sight quite different systems. In order to understand this apparent universality an overview is made of the available predictions from theoretical models for the hopping conductivity in random composites and other disordered media. Although these models are often based on quite different assumptions regarding the microscopic transport mechanisms, the nature of the carriers, etc.., one finds the same similarities in the predicted behavior as found in the experiments. Thus, for a system of carriers hopping between sites separated by random barriers in a diffusive motion, the nature of carriers, sites and barriers does not appear to be very important, except that external variables such as temperature, strength and frequency of the electric field have to be properly scaled. From the observed dependences of the dielectric properties on these parameters, it is concluded that the behavior is well explained in terms of stochastic models for the hopping conductivity. The possible relevance of an extended Hubbard model for the description of these materials is discussed.

Magnetic measurements on (C2H5NH3)2 CuCl4: Ferromagnetic layers coupled by a very weak antiferromagnetic interaction

Abstract The differential magnetic susceptibility χ = (∂M/∂H)T of (C2H5NH3)2CuCl4 has been studied as a function of an extra external field (0–2 kOe) and of temperature (1–30 K; Tc = 10.20 K). The compound is a typical layer-type ferromagnet, with a very weak antiferromagnetic coupling between the Cu2+ layers. The magnetic phase diagram of the antiferromagnetic structure has been established. From this we have obtained quantitative information about the small deviations from the ideal 2-dimensional Heisenberg model present in this compound. The effective field associated with the antiferromagnetic interlayer coupling Haf was found to be 800 Oe, whereas the anisotropies within and perpendicular to the ferromagnetic layer have been obtained as HinA ≈ 75 Oe and HoutA ≈ 1000 Oe, respectively. The ferromagnetic intralayer exchange field is some orders larger: Hf = 5.1 x 105 Oe. The fact that χ is large enough to be measured differentially (a consequence of the fact that Haf « Hf) facilitates a detailed study of a number of properties which are interesting from a theoretical point of view, viz. (1) the temperature dependence of χ// near Tc in zero external field and in a number of constant magnetic fields, (2) the divergence of χ at the spin-flop transition, (3) the temperature dependence of the phase boundaries in the phase diagram.

Magnetic measurements on (CH3NH3)2MnCl4, a quasi two-dimensional Heisenberg antiferromagnet

Abstract We have measured the magnetic susceptibility of the first member of a series of tetragonal compounds of general formula (C n H 2 n +1 NH 3 ) 2 MnCl 4 ( n = 1,2,3,…), which may be considered to consist of nearly isolated antiferromagnetic layers. The inter layer coupling J ′ is estimated to be 10 −9 -10 −8 of the intra layer exchange J , a result 10 2 smaller than may be realized within the well-known K 2 NiF 4 structure. Quantitatively, we obtain J / k = −5.0 ± 0.2 K for the intra layer exchange and H A / H E = 1.1 × 10 −3 for the anisotropy, the latter being derived from the spin flop field.

Superconductivity in (Pb, Bi)2Sr2−xLaxCu2O6+δ

Abstract Pb 2 Sr 2− x La x Cu 2 O 6+δ has a structure, which can be derived from that of Pb 2 Sr 2 Ca 1− x Y x Cu 3 O 8+δ , by removing a CuO 2 and (Ca, Y) layer. The until cell parameters for the composition with x = 1.0 are a = 0.5333 (2), b = 0.5421 (2), c = 1.2609 (6) nm with space group P22 1 2. The onset of the superconducting transition temperature is 32 K with zero resistance at 26 K for x = 1.2, whereas for other compositions a broader transition is observed. Partial replacement of Pb by Bi leads to a decrease in T c . Heating in air at 500°C leads to a transformation to tetragonal symmetry, which can be reversed by heating in nitrogen at 500°C.

High-temperature superconductivity in LaBaCaCu3O6.85

Abstract A new superconducting compound with composition LaBaCaCu3O6.85 has been prepared. It crystallizes in a pseudo-tetragonal structure related to YBa2Cu3O7. The onset superconducting temperature is 78 K. Electron diffraction clearly shows the presence of a superstructure in this material. Conductivity and superconductivity data are presented and possible structures of the unit cell are discussed.