Abstract:
Lateral semiconductor-based spin valve structures reveal all physical and technological challenges of all-electrical semiconductor-based spintronic devices. In the first part of my talk I am focussing on the findings within my PhD thesis concerning the magnetic and magnetoelectric properties of lateral microscale ferromagnet/semiconductor spin-valve structures based on Si, InAs and GaAs [1-3]. In ballistic Py/Oxide/InAs structures the observation of spin-valve operation is obstructed by local Hall effects. However, for diffusive transport in Py/Mg/SiO2/Si devices a positive magnetoresistance (MR) is observed in the field interval for the occurrence of spin blockade. The consideration of all MR contributions strongly implies the dominant effect of spin-dependent tunnelling. The functionalities of semiconductor-based spintronic devices rely on the manipulation of spin polarization and coherence. The understanding of spin dephasing mechanisms that may strongly influence the spin coherence is essential. Spin-orbit (SO) interaction plays a crucial role in spin dephasing and transport phenomena in low-dimensional semiconductor systems. Therefore within the second part of this talk I am reporting on our numerical study based on the D’yakonov-Perel’ mechanism for spin dephasing [4, 5]. We simulate the decay of spin coherence for an ensemble of electrons in the regime where electron motion within the ensemble is diffusive, with classical trajectories and confinement down to micron scales (no quantum confinement). The spin coherence has been investigated under the influence of the Rashba and Dresselhaus SO interaction. In realistic devices these SO contributions can be comparable in magnitude [4], which can result in very weak SO coupling for certain direc-tions of electron motion in the crystallographic lattice. For specific quasi-1D channels this can result in a dramatic increase of spin dephasing time [5]. To verify these numerical predictions time-resolved Kerr rotation measurements on quasi-1D wire gratings are under way.
[1] S. Hacia, et al., J. Phys. D: Appl. Phys., 37, 1310 (2004).
[2] T. Last, et al., J. Appl. Phys., 96, 6706 (2004).
[3] T. Last, PhD thesis (2007).
[4] J.B. Miller, et al., Phys. Rev. Lett., 90, 076807 (2003).
[5] J. Liu, et al., JMMM, submitted.

Abstract:
The study of the dynamics of magnetic systems remains a very exciting field despite decades of intense research. With the discovery of the spin-transfer torque effect the influence of spin-polarized currents on magnetic domain walls and magnetic multilayers have become hot topics, both to understand the fundamental physics as well as to prepare the ground for innovative applications in magnetic data storage or magneto-electronics. In this talk a will present our group’s latest results in this field. I will explain how we found by X-ray microscopy that spin-torque driven domain-wall motion is a stochastic process, and that resonant excitation of domain walls by spin-polarized currents can be described by a harmonic oscillator model. I will further show that magnetic vortices can also be excited to gyrate by alternating spin-currents. Vortices are good test objects to study the interaction of currents and magnetization as they have a highly symmetric restoring potential. Current- and field-driven vortex gyration can be described by a harmonic oscillator. Vortices are known to switch their polarization due to short field pulses, and I will show that alternating currents will do the same. Finally, I will show that the vortex oscillator model can be extended to magnetic antivortices, also called cross-Bloch lines. They too gyrate in alternating currents or fields and switch if the excitation amplitude becomes sufficiently large.

Abstract:
There is currently much interest in the development of ‘spintronic’ devices, in which harnessing the spins of electrons (rather than just their charges) is anticipated to provide new functionalities that go beyond those possible with conventional electronic devices. One widely studied example of an effect, that has its roots in the electron’s spin degree of freedom is the torque exerted by a spin-polarized electric current on the spin moment of a nanometer-scale magnet. This torque can be used for making new devices such as microwave oscillators, microwave detectors and even to switch the magnetization in MRAM. In this talk I will present some recent results on the imaging of magnetization reversal by spin current in magnetic nano-pillars. I will also present results on the ferromagnetic resonance excited by oscillatory spin current in these devices.

Abstract:
The current-induced motion of magnetic domain walls confined to nanostructures is of interest for applications in magnetoelectronic devices in which the domain wall serves as the logic gate or memory element. The phenomena have been primarily studied in magnetic elements with in-plane magnetic anisotropy. Here, I describe our experimental demonstration of domain wall (DW) manipulation driven by spin-currents in wires with perpendicular anisotropy. I show that the control of the random pinning potential due to intrinsic defects of the films and defects introduced by the lithography process is a key factor to further reduce the threshold current and increase the DW velocity. Finally, I propose our strategy to develop ultra high density magnetoelectronic devices based on domain wall motion.

Abstract:
The development of magnetic Heusler compounds specifically designed as materials for spintronic applications has made tremendous progress in the very recent past [1]. Heusler compounds can be made as half-metals, showing a high spin polarization of the conduction electrons of up to 100% in tunnel junctions [2]. High Curie temperatures were found in Co2-Heusler compounds with values up to 1120 K in Co2FeSi [3]. The latest results at the time of writing are a TMR device made from the Co2FeAl0.5Si0.5 Heusler compound and working at room temperature with a TMR effect higher than 200% [4]. Good interfaces and a well ordered compound is the precondition to realize the predicted half-metallic properties. Using XRD, it was shown conclusively that Co2FeAl crystallizes in the B2 structure whereas Co2FeSi crystallizes in the L21 structure [5].The series Co2FeSi1-xAlx is found to exhibit half-metallic ferromagnetism over a broad range, and it is shown that electron doping stabilizes the gap in the minority states for x=0.5 [6]. This might be a reason for the exceptional temperature behaviour of Co2FeSi0.5Al0.5-TMR devices [4]. For compounds Co2FeGa or Co2FeGe, with Curie temperatures expected higher than 1000 K, the XRD technique cannot be used to easily distinguish between the two structures. For this reason, the EXAFS technique and anomalous XRD was used to elucidate the structure of these two compounds. Analysis of the data indicated that both compounds crystallize in the L21 structure [7]. Mn3Ga with Heusler structure was predicted to be a half metallic compensated ferrimagnet [8]. However the synthesized material is tetragonal distorted, but still a suitable material for spin torque transfer applications. The material is hard magnetic and has a saturation magnetization in average about ¼ mB / at. The Curie temperature is above the decomposition temperature of about 730 K. The electronic structure calculation indicates a ground state with ferrimagnetic order and 88% spin polarization at the Fermi Energy [9].

References:
[1] C. Felser, G. H. Fecher, B. Balke, Angew. Chem. (int. ed.) 46, 668 (2007)
[2] Y. Sakuraba M. Hattori, M. Oogane, Y. Ando, H. Kato, A. Sakuma, T. Miyazaki, and H. Kubota, Appl. Phys. Lett. 88, 192508 (2006)
[3] S. Wurmehl, G. H. Fecher, H. C. Kandpal, V. Ksenofontov, C. Felser, H.-J. Lin, and J. Morais, Phys. Rev. B 72, 184434 (2005)
[4] N. Tezuka, N. Ikeda and S. Sugimoto, K. Inomata, Appl. Phys. Lett. 89, 252508 (2006)
[5] B. Balke, G. H. Fecher, C. Felser, Appl. Phys. Lett. 90, 242503 (2007)
[6] G. H. Fecher and C. Felser, J. Phys. D: Appl. Phys. 40, 1582 (2007)
[7] B. Balke, S. Wurmehl, G. H. Fecher, C. Felser, et al. Appl. Phys. Lett. 90, 172501 (2007)
[8] S. Wurmehl, G. H. Fecher, H. C. Kandpal, and C. Felser J.Phys. Cond. Matter. 18 (2006) 6171
[9] B. Balke, G. H. Fecher, J. Winterlik, C. Felser, Appl. Phys. Lett. 90, 152504 (2007)

Abstract:
Chemically synthesized nanomagnets are ideal systems to study the decoherence of quantum spin systems in the presence of a well-characterized phonon and spin bath. However, the observation of coherent spin tunneling in these systems poses extreme challenges in terms of low-temperatures and microwave equipment. After a discussion of the scientific significance of this experiment, I shall give a detailed and hands-on description of the design, construction and performance of a home-built pulse-ESR apparatus specifically conceived for this challenge. The system reaches a base temperature T = 7 mK and produces microwave pulses with 20 dBm power and 10 ns duration. Special care is put in thermalizing the sample and aligning the external magnetic field with respect to the magnetic anisotropy of the molecules.

Abstract:
Historically, magneto-electronic devices, i.e. magnetic memory devices, have played a critical role in the development of non-volatile computer data storage. Over the last 50 years, five major magnetic memory technologies have been demonstrated: ferrite core, magnetic recording, thin film memory, bubble memory, and MRAM. Ferrite core was a commercial success and magnetic recording continues as an enormous commercial enterprise. Thin film magnetic memories and magnetic bubble memories had limited market success due to the emergence of competing semiconductor technologies and MRAM technology has yet to establish a market presence. This talk focuses on three magnetic memory topics 1) a discussion of the basic elements of thin film magnetic memory, 2) a more comprehensive retelling of the development and then the collapse of bubble technology and 3) a discussion of the technology lessons learned from magnetic bubble memories and how these lessons should be critically applied to any new magnetic devices. The key theme in this review is that the past history of magnetic memory device technology, and in particular the history of magnetic bubble technology, provides key learning that can guide or provide valuable signposts for the current and future successes of magneto-electronic devices.

Abstract:
I will present the results of low temperature (120-1200 mK) measurements performed on thin film superconducting niobium resonators fabricated on Silicon substrate. The devices studied use coplanar waveguide (CPW) transmission line and have resonance frequencies of around 4 GHz and quality factors in the range of Q ~ 10^4 to 10^6 . These resonators are similar to those used to make novel photon detectors and read out charge qubits. The experiments indicate that the resonators show excess frequency noise which varies as approximately f^-1/2 . This excess noise limits the sensitivity of our photon detectors and likely effects the qubit performance as well. Two level systems (TLS) in amorphous thin-film dielectrics and oxide tunnel barriers have recently been shown to cause dissipation and decoherence in phase qubits. We suggest that noise in our resonators is also caused by TLS most likely near the surfaces of the substrate and metal films. To test this idea, we have measured the frequency shift, the quality factor and the frequency noise as a function of the device temperature and the microwave readout power.The frequency shift data is shown to agree remarkably well with existing weak field TLS theory. We also find that the frequency noise decreases with increasing readout power and temperature with power law index of approximately -1/2 and -2 respectively; This corresponds to decrease in noise by over an order of magnitude as temperature is changed by same amount. I will discuss some mechanisms that might be causing this noise.

Abstract:
The nature of the anomalous Hall effect (AHE) has been a subject of experimental and theoretical debate. Recent theoretical development based on the Berry phase indicated that AHE can arise from the intrinsic spin-orbit interaction in ferromagnets in the regime of large relaxation rates. The "dissipationless Hall current" has been experimentally observed in some ferromagnets (e.g. spinel material). In this talk, I will present out recent work on both AHE and its thermoelectric counterpart, anomalous Nernst effect (ANE) in Mn-doped GaAs ferromagnetic semiconductors or Ga1-xMnxAs. Due to the perpendicular anisotropy in GaMnAs films engineered by strain in the buffer layer, both AHE and ANE can be measured at zero external magnetic field as a function of temperature. Although the relaxation rate changes by a factor of 50, the anomalous Hall conductivity remains constant, which supports the intrinsic mechanism. In addition, this finding is independently confirmed by the ANE results measured in the same samples. Our AHE and ANE results in all samples have quantitatively validated Mott's relation for the intrinsic mechanism.

Abstract:
Materials are promising for future oxide electronics, e.g.,ingate insulators, ferroelectric, magnetic, superconducting, or electrode films. The interfaces to substrates or within multilayers are of great importance. Strontium titanate, SrTiO3, is a widely used substrate material for oxide thin film devices. It has recently received great interest when combined with a similar oxide material, LaAlO3,. It has been suggested that a novel two-dimensional electron gas is formed at the non-polar-to-polar interface between SrTiO3 and LaAlO3, resulting in high electrical conductivity and mobility. It is even possible to control the charge density and the conductance in a field effect transistor. However, other works indicate that the large conductivity is three dimensional and due to the doping by oxygen vacancies. In this report we demonstrate that the transport properties in the oxide heterostructures are very sensitive to the deposition parameters during thin film growth. Using cathode- and photo-luminescence studies in conjunction with measurements of electrical transport properties and microstructure, we show that the electronic properties observed at a LaAlO3/SrTiO3 interface may be explained by oxygen reduced SrTiO3. Oxygen can be pushed in and out of the sample but the re-oxygenation of an initially oxygen depleted LaAlO3/SrTiO3 heterostructure is partly prevented by the presence of the cap film.

Strains and dislocations at the interface may be seen by transmission electron microscopy. It has been claimed that charge depletion extends hundreds of micrometers into the SrTiO3; on the other hand, large conductivities have been seen in thin multilayers.

A.S. Kalabukhov¹, R. Gunnarsson¹, J. Börjesson², E. Olsson², D. Winkler¹, and T.Claeson^1
1 Quantum Device Physics Laboratory, Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Göteborg, Sweden
² Microscopy and Microanalysis, Department of Physics, Chalmers University of Technology, Göteborg, Sweden Perovskite

Abstract:
Multiferroics are singular materials that can exhibit simultaneously electric and magnetic orders, with in some cases a coupling between them [1]. As such, these compounds bring new functionalities to spintronics and new device possibilities, such as multinary memory elements or magnetic random access memories with electrical write operation. For practical purposes, the main problem of multiferroics is their scarcity. Notable examples in the simple perovskite family include BiMnO3, a weakly ferroelectric low-temperature ferromagnet, and BiFeO3, a room-temperature ferroelectric antiferromagnet.

We will present experimental results on PLD-grown oxide heterostructures combining multiferroic materials like BiFeO3 (BFO) and LaxBi1-xMnO3 (x 0.1, LBMO), and ferromagnetic metals like SrRuO3 (SRO) and the La2/3Sr1/3MnO3 (LSMO) half-metal. For both BFO and LBMO, the growth pressure-temperature window for obtaining single phase films is very narrow [2,3,4].

We have used 2-4 nm thick ferromagnetic BMO and LBMO layers as tunnel barriers in junctions with a LSMO bottom electrode and a Au top electrode. In these junctions, the tunnel current is modulated by the ferromagnetic order parameter of the barrier material via the spin-filter effect, giving rise to a TMR of up to ~100% [5,6]. Remarkably, 2 nm-thick LBMO films are also ferroelectric [7]. Accordingly, the concomitance of ferromagnetism and ferroelectricity in a unique tunnel barrier allows obtaining a four level resistance state from the combination of the spin-filter effect and the influence of ferroelectricity on tunneling [7].

The interest of BFO is for room-temperature control of magnetization by the application of an electric field. We will show that BFO films can be used to establish a robust exchange-bias effect on CoFeB layers [8] and present the characterization of the magnetic and ferroelectric domain structure of BFO films by neutron diffraction, piezoresponse force microscopy and X-PEEM. Perspectives for electrically-driven magnetization reversal will be given.

H.Béa¹, M. Gajek¹, X.H. Zhu¹, S. Fusil¹, S. Chérifi³, B. Warot-Fonrose⁴, S. Petit⁵, F. Ott⁵, K. Bouzehouane¹, G. Herranz¹, C. Deranlot¹, E.Jacquet¹, J. Fontcuberta⁶, A. Barthélémy¹ and A. Fert²

1. Unité Mixte de Physique CNRS/Thales, route départementale 128, 91767 Palaiseau, France
   
2. Institut d'Electronique Fondamentale, Université Paris-Sud, 91405 Orsay, France
   
3. Institut Néel, CNRS & Université Joseph Fourier, BP166, 38042 Grenoble Cedex 9, France
   
4. CEMES-CNRS, 29, rue Jeanne Marvig, B.P. 94347, 31055 Toulouse, France
   
5. Laboratoire Léon Brillouin, CEA Saclay, 91191 Gif-sur-Yvette, France
   
6. Institut de Ciència de Materials de Barcelona, CSIC, Campus de la UAB, 08193 Bellaterra, Catalunya, Spain
   

Abstract:
In 2004 it was discovered how to fabricate a novel material - a single layer sheet of graphite, called "graphene". The material exhibits high electron mobility and at the same time allows to change carrier concentration by applying a gate voltage. The combination of the above properties makes graphene an intersting material for future electronic devices. In this colloquium we will review the current state of graphene research. The peculiar band structure of graphene and its implications for the electronic properties, e.g. Klein tunneling paradox, minimal conductivity and unusual quantum Hall effect will be discussed. We than describe the unusual properties of graphene in contact with a superconducting electrode. Finally, a connection between the charge carrier dynamics in graphene and optical properties of the negative refraction index materials will be explained.

Abstract:
Recently, we presented evidence supporting the existence of a novel quantum phase in the vicinity of a metamagnetic quantum critical point in Sr₃Ru₂O₇ [1].In this talk, I describe an extensive follow-up project which shows that within this phase, the resistivity becomes highly anisotropic when a small in-plane field component is added to the large out-of-plane field that promotes the metamagnetic transition. The characteristics of this anisotropy will be discussed, and compared with previous intriguing work on semiconductor 2DEGs [2].

[1] S.A. Grigera, P. Gegenwart, R. A. Borzi, F. Weickert, A. J. Schofield, R.S. Perry, T. Tayama, T. Sakakibara, Y. Maeno, A. G. Green & A. P. Mackenzie, Science 306, 1155 (2004) and references therein.

[2] M. P. Lilly, K. B. Cooper, J. P. Eisenstein, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 82, 395 (1999).

Abstract:
Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. As a result they have a spontaneous magnetization which can be switched by an applied magnetic field, a spontaneous polarization which can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature, or have been synthesized in the laboratory, but there is some incentive to produce new multiferroics for specific technological applications. In this talk we use the study of multiferroics to illustrate the utility of theoretical and computational materials methods in the design of new technologically relevant materials. First we determine the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence, and show that in general the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Then we identify the chemistry behind the additional electronic or structural driving forces that must be present for ferromagnetism and ferroelectricity to occur simultaneously. Finally we describe the prediction and synthesis of some new multiferroic material classes, and outline the many exciting possibilities for future work in the field.

Abstract:
Semiconductor spintronics has now reached a stage where the basic physical mechanisms controlling spin injection and detection are understood. Moreover, some critical technological issues involved in the growth and lithography of the magnetic semiconductors have been solved. This has allowed us to explore the physics of meanwhile quite complex spintronic devices.

As examples of such devices, I will discuss resonant tunneling diodes (RTDs) fabricated from paramagnetic II-VI semiconductors that can be operated as a voltage controlled spin-switch. A quantum dot version of these RTDs exhibits, unexpectedly, remanent magnetism at zero external fieldm which we interpret as resulting from tunneling through a single magnetic polaron.

Abstract:
"Spin-flop transistor is a planar spintronic structure with magnetization M of the free layer kept in the (x,y) plane by a strong easy-plane anisotropy and fixed along ±x̂-direction by a weak easy-axis anistropy. The tranistor is operated by pumping current with spin-polarization perpendicular to the easy axis through the free layer. The spin-transfer torque generated in this device attracts the magnetization of the free layer to the +ŷ direction. At a critical value of the current the configuration with M along the (+ŷ) is stabilized. We show that in this device the confituration with M along the (-ŷ) can also be stabilized by current of the same direction, even though the spin-transfer torque repels the magnetization away from this direction. Such stabilization happens when the Gilbert damping α satisfies the inequalities √ (K_a/K_p) << α << 1 where K_a and K_p are the energies of the easy-axis and easy-plane anisotropies.

Abstract:
In contemplating the headlong rush toward miniaturization represented by Moore's Law, it is tempting to think only of the progression toward molecular sized components. There is a second aspect of Moore's Law that is sometimes overlooked. Because of miniaturization, the energy efficiency of information processing steadily improves. We anticipate that the energy required to process a single bit of information will eventually become as tiny as 1 electron-Volt per function, truly indeed a molecular sized energy. Inevitably most logic functions including storage, readout, and other logical manipulations will eventually be that efficient. However there is one information-processing-function that bucks this trend. That is communication, especially over short distances. Our best projections of improvements in the short distance communication function show that it will still require hundreds of thousand of electron-Volts, just to move a bit of information the tiny distance of only 10 micro-meters. Why this energy per bit discrepancy for communications? It is caused by the difference in length scale between the wires and the molecular size devices. Wires are long and that leads to a low impedance, while tiny transistors have a high impedance. The challenge then is to find a new switch that operates at lower voltages and lower impedance. The suggestion has been made that a current gated magneto-resistor can be regarded a three terminal device called the "trans-spinnor". It might bring the energy efficiency of communications in line with the excellent efficiency of all the other bit-functions.

Abstract:
Fast fabrication of devices based on nano-scale features in complex arbitrary patterns, coherent over extended areas (such as a semiconductor chip or a magnetic storage disk), has been demonstrated using ion projection systems for parallel processing, with versatility and reliability. The current European project CHARPAN for maskless ion projection will extend the original fixed-pattern prototype to enable arbitrarily programmable patterns with few-nanometer features to be used readily in manufacturing.

I will describe the ion projection system in Berlin, which uses an open stencil mask and 10x demagnification to give improved speed and resolution compared to serial writing with a focused ion beam. Four applications are presented:

- 3D-structures in resist,
- Patterned ion mixing of magnetic Co/Pt multi-layers, for high density disk storage,
- Selective electro plating after ion surface damage,
- Ion exposure of stretched polymers.

Abstract:
We have recently completed a series of experiments in which we have pushed the electrical detection and manipulation of ferromagnetic resonance (FMR) into the nanoscale regime, by embedding a single, nanoscale ferromagnetic element in an on-chip, lateral microwave transmission line device. Strong, resonant driving of large-angle magnetization precession in the element is achieved by locating the element in close proximity to the shorted end of a coplanar strip waveguide (CSW), which generates an intense, local microwave magnetic field. The FMR uniform precession mode is detected by measuring the microwave transmission across an inductively coupled detection CSW. Moreover, we demonstrate a method by which the cone angle of the precessing magnetization is precisely measured via the anisotropic magnetoresistance effect of the ferromagnet. I will conclude with some new results from a novel spin electronics device (the "spin battery"), whose realization was made possible by our advances in the control and understanding of FMR at the nanoscale.

Abstract:
Understanding giant magnetoresistance (GMR), discovered in the late 1980s, led directly to the development of IBM's 1997 record-breaking 16.8 gigabyte hard disk drive for desktop computers, and paved the way for a more than 30-fold increase in hard disk drive capacity from then to now. Similarly, a detailed understanding of magnetotunnel junctions (MTJs), exhibiting high tunneling magnetoresistance TMR), is currently proving vital for the development of magnetic random access memory (MRAM).

Abstract:
Due to the enormous increase in computing power and storage space in thelast decades, computers are now able to numerically solve problems thatwere hitherto too complex to study. Computer simulation has been established as "a third pillar of research next to analytical and experimental science"[1]. The micromagnetic model[2] is founded on fundamental quantum mechanic and thermodynamic considerations and is well accepted in the magnetism community as a valid description of the static and dynamic behavior of ferromagnetic objects between a few nanometers and hundreds of micrometers large. We extended the popular micromagnetic code OOMMF by several packages, resulting in aperformance gain of more than an order of magnitude, including the local anisotropic magnetoresistance and both spin-torque terms as proposed by Zhang and Li [3]. We can now use the code to combine micromagnetic simulations with magnetoresistance measurements, ferromagnetic resonance measurements, magnetic force microscopy, and magnetic X-ray microscopy. In my talk I will discuss some results on the stray-field interaction of submicrometer-sized structures, the local anisotropic magnetoresistance as studied by MR-measurements, simulation, and X-ray microscopy, spatially resolved spin-wave eigenmodes of confined structures, and the current-induced domain wall dynamics of permalloy nano- and microstructures.

References: [1] K. Binder, Physik Journal 3, 25 (2004).
[2] W. F. Brown jr., Micromagnetics, Interscience Publishers, New York, NY, 1963.
[3] S. Zhang and Z. Li, Phys. Rev. Lett. 93, 127204 (2004).

Abstract:
The development of magnetic devices like Magnetic Random Access Memories (MRAM) is based on the ability to grow optimized magnetic materials. The actual race for the development of ferromagnets above room temperature, which often require out-of-equilibrium growth conditions, justifies plainly the development of highly competitive Molecular Beam Epitaxy methods. To address this concern, a new MBE deposition system with 26 evaporation cells and 5 atoms sources devoted to the study of complex oxides is under construction in the SpinAps Lab. The possibility of laser positioning of the samples for a such system is studied here.

More specifically, an extremely promising type of ferromagnetism has been testified by numerous research teams in the so-called Dilute Magnetic Semiconductors. In such semiconductors, out-of-equilibrium doping by transition metals leads to global ordering of the magnetic moments that is associated to high Curie temperature ferromagnetism. The mechanism involved in this global ordering raises a new question: can the same phenomenon occur in oxides without the intervention of d and f electrons?

The MBE growth of Nitrogen doped Magnesium Oxide in the 111 configuration intends to answer this question. The following work presents theoretical and numerical arguments indicating how high TC in MgO:N may be achieved, as well as preliminary experimental results. The theoretical analysis uses the model of hydrogen-like defect orbitals in semiconductors and the Heisenberg spin-hamiltonian in a localized picture to describe low doping situations. It leads to a maximum TC below 200K, whereas the numerical approach using mean-field theory leads to TC above room temperature for doping concentration above 1%. This promising result should be considered carefully due to reliability issues of ab-initio calculations of magnetic properties.

Abstract:
1. I will review a proposal from Bart van Wees' group of "spin-maser" based on the spin-injection into a paramagnetic metal.

2. The second part of the talk will be about the calculation we just completed for a spin-transfer system in which the spin-polarizer and the spin-analyzer have comparable sized and can be both excited by spin-transfer torques.

Abstract:
Spintronics in ferromagnetic metals is built on a complementary set of phenomena in which magnetic configurations influence transport coefficients and transport currents alter magnetic configurations. For instance, spin transfer phenomenon refers to a novel method to manipulate spins using an electric current [1]. This method offers unprecedented spatial and temporal control of spin distributions and attracts a great deal of attention because it combines poorly understood fundamental science questions with the promise of applications in a broad range of technologies. In our experiments we use point contacts, which were instrumental both for the original observation of spin transfer [2] and in probing the high-frequency manifestation of this phenomenon [3]. The extremely small size, less than a trillionth of a square cm, qualifies point contact as the smallest probe of spin transfer phenomenon today. Moreover, our ability to control the size of the contact opened a new route to the size-dependent studies of spin transfer [4]. An integration of these capabilities leads to a powerful technique with in situ sensitivity to both frequency and wavenumber of current-induced magnetic excitations. Finally, I will discuss our search for novel spin transfer effects in antiferromagnetic metals that have been recently proposed on theoretical grounds [5]. We now demonstrate transfer of spin angular momentum across an interface between ferromagnetic and antiferromagnetic metals [6]. The spin transfer is revealed by variation in the exchange bias at the ferromagnet/antiferromagnet interface which can either increase or decrease depending on the polarity of electric current flowing across the interface. Such a current-mediated variation of exchange bias can be used to control the magnetic state of spin-valve devices, e.g., in magnetic memory applications.

[1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353 (1996)
[2] M. Tsoi, et al., Phys. Rev. Lett. 80, 4281 (1998)
[3] M. Tsoi, et al., Nature 406, 46 (2000)
[4] Z. Wei and M. Tsoi, to be published
[5] A. S. Nunez, R. A. Duine, A. H. MacDonald, cond-mat/0510797
[6] Z. Wei, et al., cond-mat/0606462

Abstract: Within the past decade, theoretical high energy physics has witnessed what is arguably the most tantalizing development in its entire history. It is now widely believed that gravity, and by implication, spacetime is an emergent concept. This belief stems largely from the premise that every non-Abelian gauge theory contains within it a theory of quantum gravity. At present, such an assertion can only be labelled as conjecture. After briefly reviewing the ideas involved, the talk will focus on recent results which lend credence to the validity of the assertion.

Abstract: The aim of the talk is to paint a broad overview of the notion that gravity, and by implication, spacetime is an emergent concept. The basic premise/assertion is that every non-abelian gauge theory contains within it a theory of quantum gravity

Abstract: Magnetic random access memory (MRAM) is a research-stage, high performance, non-volatile, solid-state memory which uses magnetic bits to store data. Toggle MRAM has been proposed to solve the problems of small switching margins and half select activated errors found in Stoner-Wohlfarth MRAM. However it is widely acknowledged that the switching fields required for toggle MRAM are substantially larger than those needed for Stoner-Wohlfarth MRAM. Previously published reports on toggle switching use large fields around 120 Oe as the toggle operating point. This talk will show experimental evidence that by optimizing the free layer material, spacer thickness, device size, and device shape, the operating point can be reduced to below 50 Oe. The experimental data will be compared to the theoretical predictions of a single domain model. Furthermore, high-speed pulsed measurements used to estimate the activation energy will be presented, showing that toggle junctions have the full predicted single domain activation energy, in contrast to the severely reduced activation energy observed in Stoner Wohlfarth junctions of comparable size. Finally, a new idea for scaling involving the use of parallel coupling to reduce the switching field will be theoretically and experimentally discussed. The data show a reduction of switching field by more than a factor of three in 130 nm diameter devices, making this a suitable candidate for a 45 nm node MRAM.

Abstract:
Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. As a result they have a spontaneous magnetization which can be switched by an applied magnetic field, a spontaneous polarization which can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature, or have been synthesized in the laboratory, but there is some incentive to produce new multiferroics for specific technological applications. In this talk we use the study of multiferroics to illustrate the utility of theoretical and computational materials methods in the design of new technologically relevant materials. First we determine the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence, and show that in general the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Then we identify the chemistry behind the additional electronic or structural driving forces that must be present for ferromagnetism and ferroelectricity to occur simultaneously. Finally we describe the prediction and synthesis of some new multiferroic material classes, and outline the many exciting possibilities for future work in the field.

Abstract:
Semiconductor spintronics has now reached a stage where the basic physical mechanisms controlling spin injection and detection are understood. Moreover, some critical technological issues involved in the growth and lithography of the magnetic semiconductors have been solved. This has allowed us to explore the physics of meanwhile quite complex spintronic devices.

As examples of such devices, I will discuss resonant tunneling diodes (RTDs) fabricated from paramagnetic II-VI semiconductors that can be operated as a voltage controlled spin-switch. A quantum dot version of these RTDs exhibits, unexpectedly, remanent magnetism at zero external fieldm which we interpret as resulting from tunneling through a single magnetic polaron.

Abstract: Spin injection in Normal/Ferromagnetic metal junctions is investigated, taking into account Anisotropic Magnetoresistance (AMR) effects. It is shown that there exists an interface resistance contribution originating from anisotropic interband scattering, beyond accumulation and Giant Magnetoresistance (GMR). The expression of the related thermopower is derived. Experimental results which were obtained with electrodeposited Ni nanowires contacted with Ni, Au, and Cu are discussed. It is concluded that, while GMR effects are related to spin-flip processes, the anisotropic magneto-thermopower originates from s-d interband relaxation.

Abstract:Research into spin-dependent phenomena in semiconductors has experienced a renaissance in the past decade, as new experimental and theoretical tools have illuminated the fundamental properties of well- known semiconductors, exotic new materials have been discovered, and potential applications have been identified. The motivations to explore these phenomena include the potential for manipulating magnetism without magnetic fields, and of manipulating quantum states in systems with very long decoherence times. Proposed devices relying on electron spin, such as "spin transistors", take advantage of the non-volatile nature of magnetism coupled with the gated transport of charge-based logic. The long spin decoherence times further indicate the promise of semiconductor spin-based systems for quantum information processing. A selection of recent advances in this area will be described.