Miguel Kiwi

Exchange bias theory

Research on the exchange bias (EB) phenomenon has witnessed a flurry of activity during recent years, which stems from its use in magnetic sensors and as stabilizers in magnetic reading heads. EB was discovered in 1956 but it attracted only limited attention until these applications, closely related to giant magnetoresistance, were developed during the last decade. In this review, I initially give a short introduction, listing the most salient experimental results and what is required from an EB theory. Next, I indicate some of the obstacles in the road towards a satisfactory understanding of the phenomenon. The main body of the text reviews and critically discusses the activity that has flourished, mainly during the last 5 years, in the theoretical front. Finally, an evaluation of the progress made, and a critical assessment as to where we stand nowadays along the road to a satisfactory theory, is presented.

Exchange-bias systems with compensated interfaces

When a ferromagnetic metal (F) is in contact with an antiferromagnet (AF), often a shift of the hysteresis loop away from its normal, symmetric position around H=0, to HE≠0 does occur. This phenomenon is known as exchange bias (EB). We put forward an analytic model, for compensated AF interfaces, based on the AF interface freezing into a metastable canted spin configuration. The EB energy is reversibly stored in a spring-like magnet, or incomplete domain wall, in the F slab. Our theory yields the right values of HE and its F thickness dependence HE∝tF−1. It also predicts the F layer by layer magnetization profile.

Positive exchange bias model: Fe/FeF2 and Fe/MnF2 bilayers

Positive exchange bias (PEB) is a remarkable phenomenon, which was recently observed experimentally. Normal (negative) exchange bias (NEB) was discovered more than 40 years ago. Its signature is the shift of the hysteresis loop along the applied field axis by HE<0, in systems where a ferromagnet (FM) is in close contact with an antiferromagnet (AFM). This occurs after the system is cooled below the Neel temperature in an external field Hcf of a few kOe. As Hcf is substantially increased HE adopts positive values. Here we explain this rather unexpected behavior on the basis of an incomplete domain wall model that develops in the FM, for Fe/FeF2 and Fe/MnF2 systems. A consistent and unified picture of both NEB and PEB, and satisfactory quantitative agreement with experimental results are obtained on the basis of our theory.

Alternative search strategy for minimal energy nanocluster structures: The case of rhodium, palladium, and silver

An alternative strategy to find the minimal energy structure of nanoclusters is presented and implemented. We use it to determine the structure of metallic clusters. It consists in an unbiased search, with a global minimum algorithm: conformational space annealing. First, we find the minima of a many-body phenomenological potential to create a data bank of putative minima. This procedure assures us the generation of a set of cluster configurations of large diversity. Next, the clusters in this data bank are relaxed by ab initio techniques to obtain their energies and geometrical structures. The scheme is successfully applied to magic number 13 atom clusters of rhodium, palladium, and silver. We obtained minimal energy cluster structures not previously reported, which are different from the phenomenological minima. Moreover, they are not always highly symmetric, thus casting some doubt on the customary biased search scheme, which consists in relaxing with density functional theory global minima chosen among high symmetry structures obtained by means of phenomenological potentials.

Is NMR the tool to characterize the structure of C20 isomers

Abstract We investigate the feasibility of using nuclear magnetic resonance (NMR) chemical shift calculations as a tool to provide structural information for C 20 fullerene type molecules. NMR chemical shifts are extremely sensitive to the local chemical environment of an atom, reflecting unambiguously its bond lengths and angles as well as its hybridization. Thus, they can distinguish between the different isomers that are candidates for the ground state of this molecule. We calculate the NMR shifts for several C 20 isomers and show that NMR constitutes a potential tool to discriminate and identify experimentally a particular C 20 molecular conformation, and also the level of theory which best describes the experimental structure.

Electronic versus phononic friction of xenon on silver

Molecular dynamics simulations of a Xe monolayer sliding on Ag(001) and Ag(111) are carried out in order to ascertain the microscopic origin of friction. For several values of the electronic contribution to the friction of individual Xe atoms, the intra-overlayer phonon dissipation is calculated as a function of the corrugation amplitude of the substrate potential, which is a pertinent parameter to consider. Within the accuracy of the numerical results and the uncertainty with which the values of the relevant parameters are known at present, we conclude that electronic and phononic dissipation channels are of similar importance. While phonon friction gives rise to the rapid variation with coverage, the electronic friction provides a roughly coverage-independent contribution to the overall sliding friction.

Dipolar interaction and its interplay with interface roughness

We present a comparison of the strength of the classical dipolar interaction, relative to quantum-mechanical coupling mechanisms like RKKY and comp lete confinement, between two ferromagnetic films separated by a paramagnetic spacer. The classical dipolar coupling, which vanishes if the two interfaces are perfectly continuous and fiat, builds up strength as the interface roughness grows for several models of interface topography. These numerical estimates, carried out for a Co/Cu/Co trilayer show that, in the presence of substantial surface roughness, the dipole-dipole interaction strength is comparable, and at times even larg,;r, than those obtained using other well established mechanisms. These results are also in qualitative agreement with experintental measurements in a variety of multilayer systems. Thus, for rough interfaces, the dipolar interaction cannot be ignor,~d.

Internal rotation of disilane and related molecules: a density functional study

DFT calculations performed on Si2H6 ,S i 2F6 ,S i 2Cl6 and Si2Br6 are reported. The evolution of the energy, the chemical potential and the molecular hardness, as a function of torsion angle, is studied. Results at the DFT-B3LYP/6311++G** level show that the molecules always favor the stable staggered conformations, with low but significant energy barriers that hinder internal rotation. Internal rotation is always accompanied by weakening and lengthening of the central Si–Si bond. In most cases this lengthening seems to be due to an interplay of electrostatic and hyperconjugative interactions. The chemical potential and hardness of Si2H6 remains quite constant as the sylil groups rotate around the Si–Si axis, whereas the other systems exhibit different degrees of rearrangement of the electronic density as a function of the torsion angle. A qualitative analysis of the frontier orbitals shows that the effect of torsional motion on electrophilic attack is negligible, whereas this internal rotation may generate different specific mechanisms for nucleophilic attack. 2003 Elsevier Science B.V. All rights reserved.

Analytic treatment of the incomplete ferromagnetic domain-wall model for exchange bias

Abstract The incomplete ferromagnetic domain-wall model we proposed recently is solved analytically. We derive the dependence of the exchange bias field ( H EB ) on the different parameters that characterize the magnetic bilayer system. Excellent agreement with the numerical solutions is achieved. Moreover, the model yields a crossover from a t F −1 dependence of H EB for thin ferromagnetic films, to a t F −1.9 dependence for thick films, where t F is the ferromagnetic film thickness. Our results are in agreement with experiment.

The structure and properties of small Pd clusters

The zero-temperature minimal energy structure of small free-standing Pd clusters (14≤N≤21, where N is the number of atoms in the cluster), their characteristics and their magnetic configurations are investigated. Results obtained using five different phenomenological many-body potentials (implemented in combination with a genetic algorithm search) are refined by means of various density functional theory (DFT) techniques. The agreement and differences between the results obtained with our procedure, using these five potentials, are displayed in detail. While phenomenological potentials yield values that approach the minimal energies of larger clusters, as compared with DFT results, they fail to predict the right symmetry group for some of the clusters with N>14. We find that the minimal energy configurations are not necessarily associated with high symmetry of the atomic arrangement. Actually, several cases of previously overlooked low symmetry structures turn out to have lower energies than more symmetric ones.