Low-dimensional Magnetic Systems Research Articles

Characterization of Langmuir—Blodgett films using electron spin resonance spectroscopy

The role of electron spin resonance (ESR) spectroscopy in the characterization of Langmuir—Blodgett (LB) films is discussed. Early examples of the application of ESR spectroscopy in LB films are the studies of the structural and magnetic properties of fatty acid LB films containing metallic ions such as Cu 2+ and Mn 2+ and those of photoinduced electron-transfer processes in films containing donor and acceptor monolayers. In recent years, many varieties of molecules have been incorporated into LB films to develop films having new functions related to optical, electrical and other properties. LB films have also been employed as model systems for the study of the basic properties of molecular aggregates. As a result, there are increasing numbers of cases where the ESR technique has played a major role in elucidating the microscopic properties of these new LB films. Three cases are discussed here as illustrative examples. The first topic is the analysis of the in-plane anisotropy of the ESR spectra of the stable radical in merocyanine dye LB films, from which a flow orientation of the dye aggregates has been suggested as the origin of the in-plane anisotropy often observed in LB films. The hyperfine coupling due to a nitrogen in the dye molecule has been identified. The second case introduces the studies of conducting LB films composed of charge-transfer complexes. ESR can provide direct evidence for the formation of the conducting columnar structures of the constituent molecules through the temperature-dependent spin susceptibilities, which show the characteristics of low-dimensional metals or magnetic systems associated with structural disorder. In some systems, the molecular orientation and the magnetic interactions of quasi-one-dimensional spin chains have been deduced from the analysis of the spectral line shapes. The third case refers to LB films containing paramagnetic metallic ions, which still represent an important class of materials. The metallic ions act as well-localized probes in revealing the local orientation of molecules, the aggregation structure of molecules (such as dimer formation) and so on. Most of the research so far has been based on the standard continuous-wave ESR technique. A few works have been reported using the more advanced techniques of ESR, such as ENDOR (electron nuclear double resonance) and ESE (electron spin echo). These techniques, as well as others, are expected to extend the area of ESR application in the studies of LB films.

Excitation spectrum of Heisenberg spin ladders.

Heisenberg antiferromagnetic spin ``ladders'' (two coupled spin chains) are low-dimensional magnetic systems which for S=1/2 interpolate between half-integer-spin chains, when the chains are decoupled, and effective integer-spin one-dimensional chains in the strong-coupling limit. The spin-1/2 ladder may be realized in nature by vanadyl pyrophosphate, (VO${)}_{2}$${\mathrm{P}}_{2}$${\mathrm{O}}_{7}$. In this paper we apply strong-coupling perturbation theory, spin-wave theory, Lanczos techniques, and a Monte Carlo method to determine the ground-state energy and the low-lying excitation spectrum of the ladder. We find evidence of a nonzero spin gap for all interchain couplings ${\mathit{J}}_{\mathrm{\ensuremath{\perp}}}$g0. A band of spin-triplet excitations above the gap is also analyzed. These excitations are unusual for an antiferromagnet, since their long-wavelength dispersion relation behaves as (k-${\mathit{k}}_{0}$${)}^{2}$ (in the strong-coupling limit ${\mathit{J}}_{\mathrm{\ensuremath{\perp}}}$\ensuremath{\gg}J, where J is the in-chain antiferromagnetic coupling). Their band is folded, with a minimum energy at ${\mathit{k}}_{0}$=\ensuremath{\pi}, and a maximum between ${\mathit{k}}_{1}$=\ensuremath{\pi}/2 (for ${\mathit{J}}_{\mathrm{\ensuremath{\perp}}}$=0) and 0 (for ${\mathit{J}}_{\mathrm{\ensuremath{\perp}}}$=\ensuremath{\infty}). We also give numerical results for the dynamical structure factor S(q,\ensuremath{\omega}), which can be determined in neutron scattering experiments. Finally, possible experimental techniques for studying the excitation spectrum are discussed.

Magnetism and Magnetic Interactions at Transition Metal Surfaces and Interfaces

ABSTRACTIn the exciting field of low dimensional magnetic systems including surfaces, interfaces and thin-films, local spin density (LSD) functional ab initio electronic structure calculations have played a key role by not only providing a clearer understanding of the experimental observations but also predicting new systems with desired properties. Our extensive calculated results demonstrate that: (i) Magnetic clean surfaces or interfaces with inert substrates undergo strong magnetic moment enhancements; (ii) the strong interaction with nonmagnetic transition metals diminishes (completely in some cases) the ferromagnetism and usually stabilizes the antiferromag-netic configuration. By including spin-orbit coupling as a perturbation, (i) reliable results for the Magneto-crystalline anisotropy of ultra-thin films can be obtained using the state-tracking procedure, although the anisotropy energy is a few tenths of a MeV; (ii) spectra of the Magneto-optical Kerr effects and magnetic circular dichroism in the soft x-ray region can be determined.

Exchange interaction and photomagnetization in dilute magnetic semiconductor heterostructures and/or superlattices

The Heisenberg exchange interaction between mobile carriers and the localized magnetic ions opens new aspects for modelling low-dimensional magnetic systems in context with electronic confinement effects. The magnetic ions in dilute magnetic semiconductors (DMS) can be polarized by optical pumping with circularly polarized light via exchange interaction with spin-polarized carriers. The magnetization of optically-oriented Mn-ions were detected by a superconducting quantum interferometer (SQUID) in modulated hetero- or superlattice structures like CdTd/Cd1-xMnxTe, CdTd/Cd1-xMnxTe, Cd1-xMnxTe/Cd1-yMnyTe, ZnTe/Cd1-xMnxSe, Pb1-xMnxTe/PbTe and Pb1-xEuxTe/PbTe. In wide-gap modulated semi-conductor structures time-resolved studies of the photomagnetization (PM) were performed by other investigators, whereas in narrow-gap systems only our CW measurements are available. The fundamentals of optically induced magnetization depend critically on the exchange parameters which by themselves reflect essential properties of the crystal system and band-structure. In narrow gap systems like Pb1-xMnxTe/PbTe or Pb1-xEuxTe/PbTe a quite unique model system is available to investigate the exchange interaction in a rocksalt-structure and in a many-valley electronic bandstructure. Therefore, the dependence of photomagnetization and of spin-relaxation of optically pumped carriers of the dimensionality and on the penetration of wavefunctions into DMS barriers is also distinct in these systems. The different behaviour concerning mean-field approach or dynamic polarization of magnetic ions is critically reviewed in context of earlier and new studies of optically induced magnetization.

Electronic structure theory of surface, interface and thin-film magnetism

Low dimensional magnetic systems including surfaces, interfaces and thin-films, have attracted a great amount of attention in the past decade because, as expected, the lowered symmetry and coordination number offer a variety of opportunities for inducing new and exotic phenomena and so hold out the promise of new device applications. Local spin density functional (LSDF) ab initio electronic structure calculations played a key role in the development of this exciting field by not only providing a clearer understanding of the experimental observations but also predicting new systems with desired properties. Extensive calculated results reviewed here demonstrate that (1) weakened interatomic hybridization at clean surfaces or interfaces with inert substrates give rise to strong magnetic enhancement and (2) the strong interaction with nonmagnetic transition metals diminishes (entirely in some cases) the ferromagnetism and usually stabilizes the antiferromagnetic configuration. Surprisingly, experimentally observed surface (interface) magnetic anisotropy can be reproduced correctly in the theoretical calculations, although the anisotropy energy is only ∼10 −4−10 −5 eV.

High-temperature dipolar local field correlations in two-dimensional exchange-coupled systems

High-temperature local field correlations are needed to calculate the angular variation of the electron spin resonance (ESR) linewidth of exchange-coupled systems in the paramagnetic phase. In low-dimensional magnetic systems, these time correlations are usually calculated by appealing to arbitrary intermediate functions to interpolate between short- and long-time regimes. In this work, the authors calculate the correlation function of the dipolar field in a two-dimensional (2D) square lattice of spins 1/2, for several orientations of an external magnetic field. Arbitrary interpolation functions are avoided by assuming that the normal modes of the spin correlations behave diffusively as soon as quantum coherence is broken. The effects of interlayer exchange in an actual system are considered by introducing a cut-off in the 2D diffusion. The angular variation of the resulting ESR linewidth is analysed as a function of the 'quantum coherence time' tau 0 and the cut-off time tau c. Comparison with experimental data in layered Cu-amino acid crystals allows the authors to estimate tau 0<or=0.5h(cross)/J and tau c approximately=200h(cross)/J, where J is the in-layer exchange.

Low-dimensional magnetic systems; from 1D to 3D ferrimagnets

One-dimensional (1D) ferrimagnetism has recently been reported in ordered bimetallic compounds. Actually, such a behavior is not exclusive of this type of system. Ferrimagnetism can also occur in homometallic chains formed by alternating sites, or by discrete species containing an odd number of interacting metal ions. We report in this paper several examples of bimetallic or homometallic chains featuring 1D ferrimagnetism. We will also discuss the conditions for the occurrence of a long-range ferrimagnetic ordering at low temperatures.

A magnetic and Mossbauer study of quasi-one-dimensional antiferromagnetic chain silicates: the cummingtonites (FexMg1-x)7Si8O22(OH)2

Magnetic properties and hyperfine parameters of the cummingtonites (FexMg1-x)7Si8O22(OH)2 have been examined using susceptibility, magnetisation measurements and Mossbauer spectroscopy. The magnetic ordering temperatures of three cummingtonite samples with x=0.95 (grunerite sample), x=0.49 and x=0.36 have been measured by Mossbauer spectroscopy to be 47+or-1 K, 6.5+or-1 K and 3.5+or-0.5 K, respectively. The magnetic measurements performed on the grunerite sample indicate a magnetic ordering temperature of 44+or-0.5 K, a positive value of the paramagnetic Curie temperature theta p=+47 K and a metamagnetic transition at a critical magnetic field Hc of about 1.4 T accompanied by a small hysteresis. From the analysis of these data, one can deduce the values of the effective magnetic coupling, between the Fe2+ moments on adjacent ribbons z'J' and of the Fe2+ moments inside one ribbon zJ; the ratio z'J'/zJ is about 0.04 which reflects the strong ferromagnetic coupling of moments within each ribbon, and a weaker antiferromagnetic coupling between adjacent ribbons is characteristic of low-dimensional (here 1D) magnetic systems. The Mossbauer spectrum of the grunerite sample recorded at 14 K can be fitted with four different hyperfine magnetic fields Hhf equal to 63, 114, 153 and 245 kOe (with positive values of Vzz) which can be assigned to Fe2+ in the four non-equivalent M1, M2, M3 and M4 sites, respectively. The substantial difference between the Hhf values can be related to the symmetry of the local crystal field connected with the more or less pronounced distortion of the octahedral sites and also to a lower extent to covalency effects.

Renaissance in molecular magnetism; application to the design of low dimensional magnetic molecular materials

In this paper, we would like to bear witness of the Renaissance in molecular magnetism by emphasizing what concerns the low dimensional magnetic systems. More precisely, we will approach two aspects, namely the design of molecular based ferromagnets and the design of spin transition molecules incorporated in Langmuir-Blodgett films. Along these two lines, the latest results will be presented and the perspectives they open will be discussed.

Hall effect and magnetoresistance in low dimensional magnetic systems

We have measured the Hall effect and magnetoresistance of stages 5 and 9 FeCl3-graphite intercalated compounds in a magnetic field up to 20 T and at several temperatures. Our preliminary results indicate that the Hall coefficient (RH) of stage 5 is positive at all field values and has insignificant variation between room and liquid helium temperature. In stage 9, however, RH has a complicated field and temperature dependence. The magnetoresistance of the two stages at all temperature and field values is positive and stage 5 exhibits strong Shubinkov–de Haas oscillations at helium temperature. Fourier transforms of these oscillations are performed and the results indicate that stage 5 has a dominant fundamental frequency which is attributed to the hole carriers, whereas stage 9 exhibits a modulation of several frequencies which are related to different carrier pockets.

Experimental examination of exact solutions for low-dimensional magnetic systems by optical birefringence

By measuring optical birefringence Δn, rigorous solutions of the nearest-neighbour correlation function <SjSi > have been examined for linear-chain Heisenberg and two-dimensional Ising prototype antiferromagnets. The temperature behaviour of Δn above critical points gives important information about the growth of spin correlations associated with magnetic phase transitions.

Shift of Half-Field EPR Lines in Low-Dimensional Magnetic Systems

In the one-dimensional system TMMC, the position of the half-field EPR line changes remarkably with angle between the static field and the c-axis, whereas in the two-dimensional system K 2 MnF 4 , there is almost no change in its position. The large shift in TMMC is thought to come solely from the dynamic effect due to the long-time tail of the spin correlation functions.

Comment on the NMR lineshape in low-dimensional paramagnets

Abstract It will be demonstrated that the model used by some workers to describe the NMR lineshape in low-dimensional magnetic systems is of academic interest only. Furthermore, we will show that this model was not treated properly. The NMR lineshape in low-dimensional systems has a normal single-peaked structure.

Neutron scattering studies of low dimensional magnetic systems

Abstract Within the last few years, neutron scattering studies have been carried out on several new prototypes of low dimensional (2D or 1D) magnetic systems. We will discuss in some detail a 2D Ising antiferromagnet K 2 CoF 4 (Ikeda and Hirakawa), as well as our current results on K 2 NiF 4 , Rb 2 Mn 0.5 Ni 0.5 F 4 and Rb 2 Mn 0.5 Mg 0.5 F 4 . We will also report our new results of high resolution studies of TMMC. Recent neutron experiments at Brookhaven have been carried out in collaboration with J. Als-Nielsen(Ris0), R.A Cowley (Edinburgh), and H.J. Guggenheim (Bell Laboratories).

On the Dynamics in Low-Dimensional Magnetic Systems

On the Dynamics in Low-Dimensional Magnetic Systems