Origin of Interface Magnetism in BiMnO_{3}/SrTiO_{3} and LaAlO_{3}/SrTiO_{3} Heterostructures

Possible ferromagnetism induced in otherwise nonmagnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between SrTiO3 and two insulating polar oxides, BiMnO3 and LaAlO3. By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetism can be stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated with magnetic Ti3+ ions at the interface itself for LaAlO3/SrTiO3 and to Mn3+ ions in the overlayer for BiMnO3/SrTiO3. In LaAlO3/SrTiO3 the induced magnetism is quenched by annealing in oxygen, suggesting a decisive role of oxygen vacancies in this phenomenon.

Magnetic properties of surface-adsorbed single rare earth atoms, molecules, and atomic scale clusters

Aparajita Singha

This thesis presents combined experimental and theoretical investigations of nanoscale, surface-supported magnets based on rare earths (RE) to understand and control the magnetic properties down to the scale of single atoms. We present the effects of adatom-substrate interaction on the magnetic properties of isolated single RE atoms using x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and multiplet analysis. Our systematic investigations of Dy, Ho, Er, and Tm adatoms adsorbed on Pt(111), Cu(111), Ag(100), and Ag(111), reveal that the REs can possess two types of 4f occupations, namely divalent 4f^n and trivalent 4f^(n-1). The trivalent state is realized in presence of low promotion energy of the RE and strong hybridization of their external spd shell with the surrounding environment. Notably, none of the REs exhibit magnetic hysteresis, suggesting that magnetic relaxation is faster than about 10 seconds. We also report on the effect of adatom-adatom interaction by studying the size-dependent magnetic properties of Er clusters adsorbed on Cu(111). Combining XMCD, scanning tunneling microscopy (STM), and mean-field nucleation theory we reveal that the adatom-adatom interaction dominates over the adatom-substrate interaction in Er clusters starting from the size of three atoms. Consequently the easy axis of Er changes from in-plane for the single atoms and dimers to out-of-plane for trimers and bigger clusters. In addition, the out-of-plane magnetic anisotropy of 2.9 meV/atom results in magnetic hysteresis at 2.5 K in all clusters starting from trimers. With a magnetic lifetime of approximately 2 minutes at 0.1 T, the Er trimers are one of the smallest metal-supported ferromagnetic clusters observed so far. The investigation of adatom-adatom interaction is further extended by studying 4f-3d heterodimers namely, Ho-Co adsorbed on thin insulating layers of MgO. Their magnetic easy axis is oriented out-of-plane. Using spin-polarized STM we have detected spin-excitations in these heterodimers at ±20 and ±8 meV. We have identified the origin behind these spin-excitations using an effective spin Hamiltonian model, which indicates that, given a ferromagnetic exchange interaction between Ho and Co, the most intense feature at ±20 meV corresponds to a transition in which the spin moment of Co is strongly diminished. This is further accompanied by an overall change of the total magnetic moment of the heterodimer, i.e., ¿J=-1. In contrast, the weaker transition at ±8 meV occurs following a change in the out-of-plane moment of Ho while the total moment of the heterodimer remains intact i.e., ¿J=0. Notably, we observe an effective g factor of 3.1 for the ±20 meV transition, which significantly exceeds the free electron value of 2. In addition, the ferromagnetic coupling between Ho and Co is very unusual compared to the ferrimagnetic exchange interaction known for the bulk 4f-3d compounds, especially those derived from the late lanthanides. Nonetheless, this marks the first evidence of spin-excitations in the smallest 4f containing cluster. The knowledge gained on the fundamental aspects of magnetism in surface-supported REs combined with the continued parallel search for magnetic stability down to single atoms, enabled us to achieve magnetic remanence in single adatoms with lifetimes of the order of 1000 s at 2.5 K. In addition we have achieved significantly enhanced hysteresis and magnetic lifetime in TbPc2.

EPFL2017

Optimization of Embedded Atom Method Interatomic Potentials to Simulate Defect Structures and Magnetism in [alpha]-Fe

Samuele Chiesa

Magnetism is largely responsible for the body centered cubic to face centered cubic structural phase transition occurring in iron at 1185 K and to many anomalies in the vicinity of the ferromagnetic to paramagnetic phase transition at 1043 K, as for instance an anomalous softening of the tetrahedral shear modulus. Current atomistic models including magnetism are either limited to the treatment of perfect lattice models or to zero temperatures, while research and development of candidate materials for future fission and fusion power plants requires the modeling of irradiation induced defects in ferritic/martensitic steels at high temperatures. An attempt to fill this gap is the Dudarev-Derlet potential, which includes zero temperature magnetism in an embedded atom method formalism, together with a more recent extension of the method to the inclusion of spin rotations at non zero temperature with nearly half the computational speed of an embedded atom method potential. In this work, we report on the optimization of the Dudarev-Derlet potential to the zero temperature bulk properties of the non-magnetic and ferromagnetic bcc and fcc phases, including the third order elastic constants of the ferromagnetic bcc phase, the point defects formation and migration energies and the core structure of the screw dislocation with Burgers vector 1/2[111], either from experiments or from density functional theory calculations, where we develop a method to fit the core structure of the screw dislocation based on the Suzuki-Takeuchi model. Three representative fits from the optimization of the Dudarev-Derlet potential are compared with recent semi empirical potentials for iron, with density functional theory and experiments. The migration energies of the self-interstitial range from 0.31 eV to 0.42 eV, compared to a density functional theory value close to 0.35 eV and an experimental value close to 0.3 eV, and the vacancy migration energies range from 0.85 eV to 0.94 eV, compared to a density functional theory value close to 0.65 eV. Clusters composed of parallel self-interstitials are oriented along ‹110› if the number of interstitials composing the cluster is smaller or equal than 3, while for bigger clusters the ‹111› orientation is more stable, in qualitative agreement with density functional theory. Depending on which one of the three representative fits is chosen, the formation entropy of one ‹110› dumbbell calculated by the thermodynamical integration method in the range from 300 K to 600 K varies from 0.28 kB to 4.02 kB. The diffusion coefficient of the ‹110› dumbbell at 600 K ranges from 1×10-6 cm2/s to 10×10-6 cm2/s, while at room temperatures the scatters extends over three orders of magnitude. The main difficulties, common to all the semi empirical potentials considered in the work, are related to the description of the fcc phase and the migration mechanism of the screw dislocation. The semi empirical potentials are unable to distinguish the anti-ferromagnetic fcc from the low spin ferromagnetic fcc or the high spin ferromagnetic fcc. Considering the equilibrium volume and the bulk magnetic moment, the high spin phase is the one which most resembles the ferromagnetic fcc phase of the Dudarev-Derlet potentials. Finally, for those fits with a non-degenerate core structure, we investigate some fundamental aspects of the migration mechanism of the screw dislocation with Burgers vector 1/2[111] at zero temperature and at zero applied stress, by calculating the Peierls potential in the [211] direction between two structurally equivalent soft cores. This confirms the existence of a stable core structure in the middle of the migration path not observed in density functional theory, which is actually found to be energetically degenerate with the soft core. The consequences of this are discussed in terms of formation energies of double kinks in the [211] direction.

EPFL2010

Magnetization Reversal Dynamics in Ferromagnetic Semiconductors

Liza Herrera Diez

This thesis is dedicated to the study of the magnetization reversal dynamics in compressively strained (Ga,Mn)As and differently functionalized (Ga,Mn)As materials. In the first part the domain wall dynamics of pure (Ga,Mn)As/GaAs materials is in the focus. The changes in the magnetic anisotropy energy landscape occurring as a function of temperature are monitored in detail by different experimental techniques. These measurements provide the necessary information for the identification of the temperature ranges corresponding to different magnetic anisotropy regimes. Knowing the biaxial anisotropy and uniaxial anisotropy dominated regimes the dependence of the domain wall dynamics on the magnetic anisotropy landscape is studied. Critical changes in domain wall alignment observed by Kerr microscopy are found upon changing from biaxial to uniaxial anisotropy. These changes could be partially attributed to the tendency of the system to minimize magnetic free poles at the domain boundaries. To complement the space resolved studies of the magnetization reversal, magnetic time relaxation effects are addressed. In particular the magnetic aftereffect in (Ga,Mn)As/GaAs is studied in detail. The irreversible aftereffect is evidenced as a critical reduction of the domain wall velocity with time under a constant applied magnetic field below coercivity. This time relaxation is tracked as a function of both time and magnetic field. The measurements show that the overall relaxation is composed of two relaxation processes acting in parallel at short and long time scales. By fitting these experimental results the values of the relaxation times of both relaxation processes were obtained. By this modeling also the values of the activation volumes for two independent (Ga,Mn)As materials are estimated. In the second part of this thesis the possibilities of tuning the electrical and magnetic properties of (Ga,Mn)As by volume and surface treatments are explored. As for the volume modification, oxygen species were incorporated into the (Ga,Mn)As films by means of exposure to an oxygen plasma. The incorporation of oxygen was evidenced by depth profile X-ray photoelectron spectroscopy. This treatment is found to weaken the ferromagnetism, visible as a reduction in the Curie temperature, and to hinder the electrical transport evidenced as an increase in the electrical resistance. In agreement with theories accounting for the hole mediated ferromagnetism in (Ga,Mn)As this behaviour was related to a hole compensation mechanism arising from the presence of oxygen impurities sitting in interstitial positions of the (Ga,Mn)As Zinc blende structure. In the last part, the modulation of the electrical and magnetic properties via surface functionalization is presented. The effects produced by the adsorption of molecular species on (Ga,Mn)As are similar to those found after oxygenation, namely a strong weakening of the magnetic properties and an increase in the values of the electrical resistance. However, in this case the distinctive feature is provided by the possibility of regulating the hole quenching effect by exploiting the additional degree of freedom provided by the chemical properties of the adsorbates. The adsorbate chosen for these experiments are dye molecules that absorb light in the visible range, in this way allowing for a light modulated interaction with the substrate. Using this concept is was possible to observe a modulation of the ferromagnetic transition temperature, the coercive field and the electrical resistance upon illumination. These observations provide a proof of principle for the realization of photo-sensitized (Ga,Mn)As devices where the magnetic properties can be regulated by light.

EPFL2011