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The search for ferromagnetism above room temperature in dilute magnetic semiconductors has been intense in recent years. We report the first observations of ferromagnetism above room temperature for dilute (4 at.%) Mn-doped ZnO. The Mn is found to carry an average magnetic moment of 0.16 mu(B) per ion. Our ab initio calculations find a valance state of Mn(2+) and that the magnetic moments are ordered ferromagnetically, consistent with the experimental findings. We have obtained room-temperature ferromagnetic ordering in bulk pellets, in transparent films 2-3 microm thick, and in the powder form of the same material. The unique feature of our sample preparation was the low-temperature processing. When standard high-temperature (T700 degrees C) methods were used, samples were found to exhibit clustering and were not ferromagnetic at room temperature. This capability to fabricate ferromagnetic Mn-doped ZnO semiconductors promises new spintronic devices as well as magneto-optic components.

The technique of straining substrates by bending to determine the magneto-elastic constants in thin films is discussed. It is shown that the data may be corrected for the random anisotropy fields, which are present in such deposited films. The analysis will highlight an important point of how accurately one can measure the magnetostriction, even in the regime where the strain induced anisotropy imposed by such bending methods does not completely dominate the random anisotropies. This method may be extended to measure the magneto-elastic coupling in patterned media where a local measurement of magnetostriction is essential.

Brillouin light scattering spin wave studies show that the frequency and linewidth of ultrathin Co/CoO films are strongly dependent on the temperature, so that when the temperature increases, both the frequency and linewidth decrease. In this paper, the frequency and linewidth in Co/CoO films are considered theoretically. We consider the case of coupled ferromagnetic and anti-ferromagnetic films with the exchange coupling at the interface weak compared to the intra-film exchange. The frequency is calculated as two coupled spins with weak exchange coupling. The magnetic fields applied on each spin are the effective magnetic fields on Co in metallic Co and in CoO. Fluctuations in the interface give rise to a random field that causes the additional linewidth. Within this model we can explain the observed increase of the frequency and linewidth with decreasing temperature.

The charge and orbital states of the two-orbital spinless Hubbard model with U → ∞ in two-dimensional small clusters are investigated. This is a simplified model for cubic manganite in the ferromagnetic phase. The modified Lanczos method is used to calculate the ground state energy and wavefunction. Cluster sizes of 8, 10 and 16 are studied with different boundary conditions and electron concentrations. We find a tendency toward the charge uniform state for all cases. The orbital state is highly correlated with the cluster symmetry. There is no strong evidence for orbital ordering at low electron concentrations for high-symmetry clusters. However, there is orbital ordering at high electron concentrations. It has been found that the system develops strong charge and orbital correlations without long-range ordering at all electron concentrations. This demonstrates the possibility of orbital order in the ferromagnetic metal phase of the manganites which is electronically induced.

Recent experiments on magnetic tunnelling devices based on magnetite film show an unexpectedly low tunnel magnetoresistance despite a high magnetic transition temperature and a high spin polarization of the conduction electrons. The explanation is given in two parts. The first reason for the reduction is the quantum mechanical coupling and relaxation of the conduction electron and core spin. This can reduce the tunnel magnetoresistance substantially. The second is the surface effects in which the magnetic spins at the surface are frustrated as a result of fewer nearest neighbours, surface reconstructions, anti-phase boundaries and the superparamagnetic behaviour. This explains the rapid fall of the magnetization and the tunnel magnetoresistance with temperature.

The p-d hybridised single-hole states of the transition-metal-oxygen tetrahedron (ΤΜΟ4) are collectivised due to the direct p-p hopping between oxygens of different clusters. The lowest-lying energy band is always narrow and fully occupied. The first excited band gets occupied as an effect of valence-uncompensated doping, so it can be almost localised. The possible hole excitations to the two higher energy bands, which are wider, may imply the Mott-like hopping form of charge transport in these systems. PACS numbers: 71.10.Fd, 72.20.-i

The application of real space renormalization group methods to quantum lattice models has become a topic of great interest following the development of the density matrix renormalization group by White. This method has been used to find the ground and low

The phenomenon of `moment canting`, where the atomic moments in a soft magnetic material do not completely align with an applied field of substantial magnitude, has been measured as a function of field in field annealed Fe78Si9B13 metallic glass ribbons. Probability distributions for the Fe moment directions were deduced from unpolarized 57Fe Mossbauer experiments. A method of analysis was devised to account for spectral saturation effects, and validated with reference to polycrystalline alpha -Fe. Fields of up to 0.5 T were applied at room temperature in the ribbon plane. Even in 0.5 T the in-plane moment canting did not fall to zero, with the standard deviation of the Gaussian spread in moment directions remaining at 12 degrees +or-1 degrees . Fields of up to 9 T were applied at 4.2 K, normal to the ribbon plane. Even in 9 T appreciable canting was observed, with a Gaussian standard deviation of 11 degrees +or-1 degrees with respect to the applied field direction. An a priori `combed hair` model was developed to describe the moment canting that would result from the presence of a few frustrated exchange interactions between some magnetic atoms. Long-distance moment canting was predicted, with a truncated inverse sine probability distribution. The data were found to be adequately described using this model.

We investigate optical properties starting from the Anderson model using the slave-boson method in a mean field and random phase approximation (RPA) method. It is shown that the structure of the low-frequency optical spectrum persists up to a temperature which is higher than the Kondo temperature.

This is a tutorial paper. We first consider why one should be interested in the origin of the magnetoelectric effect in ordered magnetic compounds. The non-zero elements of the tensor may be deduced from group theory arguments alone. A microscopic understanding is important if one wishes to learn which systems are likely to have large effects and possibly to design new combinations of materials which should show strong or unusual effects. It also contributes to the overall understanding of the materials involved. Another important feature is that different microscopic origins do give rise to very different behaviour with temperature and hence a microscopic understanding of a particular material is essential to understand the temperature dependence of the magnetoelectric tensor. This is particularly important if one actually has the situation in which the magnetoelectric effect may be used to measure a critical exponent just below the transition temperature. A final reason for being interested in ...

The solution of the mean field equations of the periodic Anderson model are studied in detail in order to compute the temperature dependence of the thermodynamic quantities for heavy-fermion compounds. The mean field solution is valid only for temperatures T >TK because of the shift of the chemical potential which occurs as nf is changed. We show that it is useful to introduce a further characteristic temperature, tau , such that kB tau approximately=(V2)2/W3 where (V2)12 / is the average hybridization over the Fermi surface and W is the band width. We show that a linear approximation may be used for the full Fermi function which allows mean field theory to be evaluated easily. The susceptibility is evaluated in full mean field theory and using the linear approximation. The agreement is good. At high temperatures T>TK the mean field predicts quasi-classical behaviour chi =C/T+ theta . This arises from a non-interacting Fermi gas because the band energies are functions of temperature. This result can be readily understood using the analytic approach.

For part I see ibid., vol.5, p.5277 (1993). The elastic anomaly observed in the coherent Kondo state of Ce heavy-fermion compounds is analysed using the Anderson lattice model simulating the energy level scheme of the CeTe cubic crystal. The Gamma 7 doublets and Gamma 8 quartets of the 4f states are considered in the model. We solve the mean field equations to derive the temperature dependencies of elastic constants, using the random phase approximation for the interaction between the elastic strain and the crystalline field splitting. We compare the calculation with the (c11c12)/2 and c44 constants of CeTe. The presence of the downward dip and the observed overall temperature variations of the two constants are well described by the present theory. The origin of the dip is the coupling between the elastic strain and the splitting of the Gamma 8 quartets.

We report the experimental observation of strong exciton–photon coupling in a planar microcavity composed of an organic semiconductor positioned between two metallic (silver) mirrors. Via transmission and reflectivity measurements, we observe a very large, room temperature Rabi splitting in excess of 300 meV. We show that the Rabi-splitting is enhanced in all-metal microcavities by a factor of more than 2 compared to an organic film positioned between a silver mirror and a dielectric mirror. This enhancement results from the significantly larger optical fields that are confined within all-metal microcavities.

A simple model of the dipolar origin of magnetostriction for thin polycrystalline BCC films has been proposed, which is based on the angular averaging of the magnetoelastic energy tensor. The cases of polycrystalline layers with [1 0 0] and [1 1 0] texture, as well as entirely polycrystalline films, have been considered. The surface effects in dipolar magnetostriction have been estimated for FeCo systems.

The heavy-fermion compound [Formula: see text] has been studied using the fully relativistic spin-polarized mean muffin-tin orbital method within the local density approximation. Two separate calculations, one where the f electron is treated as a valence electron and the other where it is treated as part of the core, have been performed and the Fermi surface is obtained. The angular-dependent de Haas - van Alphen (dHvA) frequencies are calculated in both cases and they are compared with the experimental dHvA frequencies. We also calculated the electron momentum densities and compared them with the electron - positron momentum densities measured from the two-dimensional angular correlation of electron - positron annihilation radiation. The spin polarization of the Fermi surface is analysed and we present a new interpretation of the experimental data of Harrison et al.

We study the domains in an ultrathin magnetic layer with perpendicular magnetization as a function of temperature. The domain spacing is given by the balance between the interface energy and the demagnetizing energy. The demagnetizing energy is calculated in a continuum theory which is valid provided that the domains are large compared with the lattice spacing. The temperature dependence of the interface energy arises from the entropy of the rough walls. This is obtained from the exact results for the two-dimensional Ising model. We predict that the domain size shrinks rapidly as the temperature is raised towards the transition.

The application of real space renormalization group methods to quantum lattice models has become a topic of great interest following the development of the density matrix renormalization group by White. This method has been used to find the ground and low

We present the first approach to a numerical simulator that extends the traditional resolution of the discrete Poisson and continuity equations, to solve a p-n ferromagnetic and degenerate junction. As it is degenerate, the carriers within the p-n junction can recombine through a direct tunnelling process from the valence band to the conduction band and viceversa. Ferromagnetism produces a spin split of the conduction band in n-type semiconductors and of the valence band in p-type semiconductors. The simulator evaluates the tunnelling asymmetry for an ideal junction with arbitrary spin polarisation on each side and computes how the current depends on the voltage across the junction. The obtained results predict that the strongest influence of the spin polarisation over the current value occurs in the negative resistance region, main characteristic of the tunnel diode.

The authors examine the role played by dipolar interactions in the equilibrium critical behaviour of the dilute antiferromagnet in a uniform field (DAFF). The DAFF has been known to map onto a random field model, and we perform a similar mapping, only this time including dipolar interactions. They conclude that dipolar forces affect the DAFF in a way no different from the case for the corresponding pure system. Dilution does not generate any additional long-range forces or other effects arising from dipolar forces. In particular, in the final random field model which the DAFF maps onto, they find that the mean-field propagator is unaffected by the inclusion of dipolar interactions save for some constant terms. They restrict the work to the critical region close to the transition temperature and assume a domain-free state.

The longitudinal susceptibility of the Heisenberg ferromagnet, with short-range interactions only, is known to diverge below Tc at H=0 as 1/k and at k=0 as H-12/. This divergence has never been observed in neutron scattering experiments. The authors compute the longitudinal and transverse susceptibilities including dipolar effects at low temperatures and their variation with k and H. They use linear spin-wave theory and find that for k2+H >m, the two susceptibilities are asymptotically identical. Thus they conclude that it is the applied magnetic field that destroys the divergence. Numerical results are presented for the susceptibilities as a function of k and H.

A variational method based on the nearest-neighbour Ising model is used to derive an expression for the low-temperature correlation function in the Ising model with long-range ferromagnetic interactions. Also by expanding the correlation function directly, the authors demonstrate that the characteristic power-law behaviour at large distances is irrelevant in the critical region, when above the marginal range of interaction.

The properties of rare-earth hexaborides have been studied within the framework of density functional theory. The LMTO-ASA method has been used to study the electronic structure of paramagnetic LaB6, CeB6, PrB6 and NdB6 and of ferromagnetic CeB6, PrB6 and NdB6. In the ferromagnetic case, which treats the f electrons as true band states, the authors find good quantitative agreement between the experimental and theoretical Fermi surfaces. In addition, comparison of spin moments and total energies is used to predict the zero-field susceptibility within a Landau free energy argument. This indicates that the experimentally heavy fermion system CeB6 does indeed have a substantially enhanced effective electron mass.

Current device technology requires high magnetostriction in low-coercivity, low-anisotropy samples. It has long been suspected that surface interfaces provide a method for tuning the magnetostriction. In this paper we explore the effects of a surface configuration in granular iron-cobalt silver systems. Significant reduction is seen in the granular material, which can not be explained simply by theory or in terms of the surface to volume ratio. A detailed discussion is made of the possible effects.

The proper understanding of spin electronics in metallic systems relies on an understanding of some basic metal physics. The material presented here is very brief and is covered in more detail in several excellent textbooks [1],[2]. The discussion starts with the definition of a metal and the justification for the frequently used independent particle model even when the interactions between the electrons are included. The role of impurities is also discussed and localized and extended states are defined. Because we are concerned with magnetic properties the most relevant perturbation will be that of a periodic magnetic field. This is characterized by the generalized susceptibility, which is a very useful concept as it allows us to consider the instability of a metal to ferromagnetism or to spindensity waves as well as the response of a paramagnetic metal to a magnetic impurity. Finally we consider strong coupling theory. This includes a discussion of the formation of local moments in a metal. Such a moment will be coupled to the conduction electrons and may be screened out by the Kondo effect. A dense array of Kondo type impurities will form a heavy Fermion compound, which is also described briefly. Alternatively, a dense array of such moments can interact with each other to form a spin glass or a ferromagnet.

In this work we have implemented a numerical simulator and analytical model to study the dependence of the tunnelling current on the polarization ratio of the carrier spin for a degenerate and ferromagnetic heterojunction. We have applied these models to study the behaviour of a magnetically doped GaAs/ZnO PN junction and the current transport in a PN heterojunction where the polarization of the spin of the charge carriers is also a control variable.

Calculations of the tunneling current as a function of voltage for a Zener diode where both sides are ferromagnetic have been performed. The current is evaluated as a function of the voltage and of the magnetization on each side of the diode. The tunneling magnetoresistance is analyzed. Two cases are considered, one that corresponds to Mn doped GaAs in which the ferromagnetism is stronger on the p side of the diode and the other that corresponds to ZnO where there are likely to be many more carriers on the n side of the diode.

A theoretical investigation of the structure, magnetism and thermal conductivity of LaMn1−xGaxO3 has been made. A local, cluster model is introduced. It is proposed that gallium substitution may cause neighbouring Mn3+ (3x2 − r2)/(3y2 − r2) orbitals to transform into the 3z2 − r2 state. This unique idea is the basis of a successful description of the structure and magnetism of LaMn1−xGaxO3. The evolution of the crystal lattice parameters has been simulated; agreement with experimental data is excellent; changes in orthorhombic strain and cell volume are also well predicted. The behaviour of the magnetization has been successfully described by a spin-flipping model: over the entire range of doping, good qualitative consistency between simulation predictions and experimental data is observed. Ultimately, the aforementioned results indicate that lattice distortions alone may be responsible for the observed magnetic order in these manganites. Additionally, the thermal conductivity of LaMn1−xGaxO3 has been analysed. The results are fitted with a kinetic model. It appears that thermal currents in LaMn1−xGaxO3 may be limited by scattering of phonons from the structural changes near the Ga ions.

The authors discuss the domain structure in a ferromagnetic plate with strong uniaxial anisotropy in the perpendicular direction. The inclusion of dipolar interactions produces anisotropic magnetisation correlations above the ordering temperature T0. They calculate the dependence of the width D of incipient domains on plate thickness L in the temperature region from slightly above to just below the transition temperature. They find that D approximately L1/3 in this region, in contrast to the Kittel relation D approximately L1/2 for stripe domains at temperatures well below T0. At low temperatures the width of the walls is very small compared with the distance between them and so they do not interact with each other. They determine the range of validity of the Kittel theory and they use this to suggest substances upon which it might be possible to make appropriate observations.

Spectroscopic and specific heat measurements are reported on two singlet magnets, TbAsO4 and TbVO4. The ratios between the antiferromagnetic ordering temperature TN and the splitting Δ between the two singlets are TN/Δ = 1.58 and TN/Δ = 0.45, respectively. The experimentsshow considerable deviationsfrom a molecular field theory but remarkably better agree with a self-consistent RPA calculation in which the experimentally determined exchange and dipole fields are used. Es wird uber spektroskopische Messungen und uber Messungen der spezifischen Warme an den beiden Singulett-Magnetika TbAsO4 und TbVO4 berichtet. Das Verhaltnis der antiferromagnetischen Ordnungstemperatur TN zum Abstand Δ zwischen den beiden Singuletts betragt TN/Δ = 1,58 fur TbAsO4 und TN/Δ = 0,45 fur TbVO4. Die Experimente zeigen betrachtliche Abweichungen von der Molekularfeldtheorie; die ubereinstimmung mit einer selbstkonsistenten RPA-Rechnung, in welcher die experimentell bestimmten Austauschund Dipolfelder eingesetzt werden, ist wesentlich besser.

The study of changes in the refractive indices (with consequent changes of birefringence) of a transparent magnetic crystal which accompany changes in the magnetic order is becoming more popular. The authors review why this is. The first reason is that birefringence can be measured very accurately: the different experimental arrangements are reviewed. The second reason is because a birefringence measurement is an integrational spectroscopic technique and therefore it is studied both experimentally and theoretically as a branch of magneto-optics and hence gives information on the detailed energy level structure of the solid. The third reason is that in a number of interesting systems the birefringence is proportional to the magnetic energy over a wide temperature range and it is often a more convenient method of obtaining the magnetic specific heat than direct specific heat measurements; this is particularly true in magnetic crystals which show low dimensional ordering. The last reason is that in all magnetic crystals the birefringence change should vary like one of the thermodynamic critical exponents near to the phase transition. They review in detail the reasons why birefringence studies have become so successful for measuring critical exponents in pure and particularly mixed crystals.

The temperature dependence of the domain structure of an Ising ferromagnetic slab is examined near to the critical temperature. It is found that the values of the domain size and surface magnetisation evaluated as the transition temperature is approached from below agree nicely with the values obtained from a theory of the dominant fluctuations in the paramagnetic phase. The theory is compared with available experimental data.

The detailed mechanisms which can give rise to changed hyperfine anomalies and electric field gradients in ferromagnetic metals and alloys are discussed. It is shown that both of these effects are caused by spin orbit coupling. Calculations of these effects are discussed for alloys and pure metals. A relation between the electric field gradient and the Anomalous Hall Effect is discussed.

Optical side bands are observed in the spectrum of TmAsO4. Measurements of their polarisation and intensity are made as a function of magnetic field. These results are interpreted using a tight binding theory developed earlier. It is found that the most intense side band is caused by off-diagonal exchange (Γ3 − Γ1 and Γ4 − Γ1) between the ground doublet and the excited singlet arising from virtual phonon exchange. The less intense side bands are caused by a large number of exchange interactions and the analysis is less clear. Es wurden die Polarisation und die Magnetfeldabhangigkeit der Intensitat von Nebenlinien im Absorptionsspektrum von TmAsO4 untersucht. Die gewonnenen Ergebnisse werden mit Hilfe eines „tight-binding”-Modells interpretiert. Die intensivste Nebenlinie wird durch einen virtuellen Phononaustausch zwischen dem Grundzustandsdublett und einem angeregten Singulett ver-ursacht. Bei anderen Nebenlinien, die durch eine Vielzahl von Austauschwechselwirkungen induziert werden, fuhrte die Analyse der Wechselwirkungen nicht zu einer eindeutigen Zuordnung.

The co-operative Jahn-Teller effect is a phase transition which is driven by the interaction between localized orbital electronic states and the crystal lattice. The possible origins, symmetries and properties of the electron-lattice interactions and how they lead to possible hamiltonians for the coupled system are discussed. The relation of these interactions with quadrupolar interactions and magnetostriction is included. The methods of solution of the hamiltonians lead to an understanding of the electronic states, the phonon spectrum and the mixed normal modes. The wide variety of experimental techniques used on this problem are reviewed in detail and the results are compared with theoretical expressions whenever possible. The application of external stress and magnetic field is of particular significance in the case of the rare earth compounds, because they can product effects which are larger than the low-temperature spontaneous effects.

The authors consider, using a combination of variational and 1/N techniques, an actinide impurity whose lowest ionic configurations are f2 and f3 embedded in a free electron matrix. They show that the ground state is a singlet, in agreement with recent experiments on dilute uranium systems.

For pt.I see ibid., vol.10, p.2937 (1977). The authors have measured changes in linear birefringence ( Delta n) associated with the cooperative Jahn-Teller phase transition of DyVO4 near 14K as a function of temperature and magnetic fields, B, between 0.024 and 0.095 T. Theoretical arguments show that Delta n is directly proportional to the order parameter of the transition and that B2 is equivalent to the conjugate ordering field. By extrapolation to zero field they obtain the temperature dependence of the order parameter and the susceptibility. The data are compared with calculations based on a mean-field 'compressible' Ising model. For a reasonable choice of adjustable parameters this classical description gives a good fit to the data close to TD consistent with general theoretical arguments and more detailed calculations, but it deviates progressively away from TD presumably because of the known importance of short-range interactions in the system.

The form of the Landau-Ginsburg effective Hamiltonian is derived for a spin-phonon coupled system. For TD not=0 it is found that both the quadratic and quartic terms reduce to those for an Ising model with a non-analytic form for J(q). A crossover calculation (to order epsilon ) is performed to evaluate the way in which the effective susceptibility exponent gamma will vary with t as the non-analytic terms drive the system away from the Ising to the gaussian fixed point.

Far-infrared absorption spectroscopy has revealed the presence of a strong absorption line at 38 cm-1 in the Jahn-Teller distorted phase of TbVO4. The fact that this is at a lower energy than any previous observations would indicate, is attributed to a Davydov splitting caused by coupling to odd-parity phonons. The temperature dependence and the splitting of this line in a magnetic field are also reported. The construction of a Hamiltonian incorporating this odd-parity coupling is discussed and solutions are found using Green function techniques.

The authors use a slave-boson technique in the mean-field approximation to obtain the temperature dependence of the valence, nf, the susceptibility, chi S, and the specific heat, CV, for a lattice of mixed-valence ions. They find that the mean-field solution is sufficient to extrapolate between the low- and high-temperature regimes giving for example an enhanced Pauli-like susceptibility for low T and a Curie-Weiss-like susceptibility at high T. They consider an extension to the Anderson model in which direct f-f- hopping is included and show that such a model exhibits a variety of interesting low-temperature behaviour.

The theory of optical side-bands is developed for a crystal with a cooperative Jahn-Teller phase transition containing rare-earth ions. A tight binding scheme is used to describe the rare-earth energy levels over the whole Brillouin zone. The possible two-ion couplings which can give rise to side-bands are discussed; these include the diagonal and off-diagonal contributions of the virtual phonon interaction. Fur Kristalle mit einem kooperativen Jahn-Teller-Phasenubergang, an dem Seltene-Erd-Ionen beteiligt sind, wird eine Theorie der optischen Nebenlinien entwickelt. Das „tight-binding”-Schema wird benutzt, um die Energieniveaus der Seltenen Erden in der gesamten Brillouinzone zu beschreiben. Die moglichen Zwei-Ionen-Kopplungen, die solche Nebenlinien verursachen konnen, werden diskutiert. Dabei werden die diagonalen und nichtdiagonalen Beitrage der virtuellen Phononenwechselwirkung berucksichtigt.

Small electric field gradients in cubic ferromagnets may be observed using nuclear resonance techniques. The hyperfine coupling generates an interaction of the same symmetry known as the pseudoquadrupole effect. The authors show that this effect is related to the difference in transverse and longitudinal magnetic susceptibilities. Model calculations are performed for a Heisenberg ferromagnet using molecular-field theory near Tc, spin-wave theory at low temperatures, and the itinerant model at absolute zero and Tc. They find that the pseudoquadrupole effect in iron, nickel and GdAl2 appears to be too small to be observed using current techniques. They therefore infer that the observed quadrupole effects in cubic magnets are due to real electric field gradients.

The energy of a one-dimensional domain wall (Bloch or Neel) at low and high temperatures is calculated approximately for the application of an external magnetic field along the hard direction. Following Aharoni (1966), Bulaevskii and Ginzburg (1964), the wall is found to vanish at a critical field value h=hc, the wall energy varying as (hc-h)32/. At high temperatures, in an applied magnetic field h which tends to zero, the magnetisation profile of the wall is calculated as a function of T-T* (where T*