This PDF file contains the front matter associated with SPIE
Proceedings Volume 7398, including the Title Page, Copyright
information, Table of Contents, Introduction (if any), and the
Conference Committee listing.
We demonstrate spin noise spectroscopy as an efficient and surprisingly sensitive experimental tool to measure
the spin dynamics of free and localized carriers in semiconductors. The technique suppresses perturbations and
gives access to intrinsic spin relaxation times by omitting optical excitation. We show the power of spin noise
spectroscopy for basic physics by measurements on n-type modulation doped (110) GaAs quantum wells. The
measurements reveal that the spin relaxation times are limited by stochastic spin-orbit fields and that the spin can
be used as marker for the observation of electron diffusion processes at thermal equilibrium. We show the power
of spin noise spectroscopy for applied physics, by three dimensional measurements of the doping distribution in
We show that femtosecond time-resolved optical techniques are ideally suited to excite and probe collective spin
organization and collective spin modes, in diluted magnetic semiconductors based quantum wells. The spin
systems that we consider are low concentration CdMnTe quantum wells, which exhibit various spin phenomena
under optical pulsed excitation, such as spontaneous magnetization patterning, soft mode of magnetization
precession in p-doped quantum wells, and mixed electron-Mn precession in n-doped quantum wells. We examine
also how the carrier-carrier Coulomb interactions affect these collective spin excitations.
We discuss the creation and evolution of the optically induced spin coherence in an ensemble of singly charged
(In,Ga)As/GaAs quantum dots. The sample is periodically excited by the pulsed laser radiation which causes the creation
of the discrete spin precession spectrum of electrons in an external magnetic field, known as mode-locking. The nuclei
additionally help to focus all precessing spins into the discrete modes and even make it possible to realize the singlemode
regime, where the whole inhomogeneous ensemble precesses without dephasing.
In this paper we describe the study of the magnetization dynamics after a short magnetic pulse, down to
zero field and with a temporal resolution of a few ns, in (Cd,Mn)Te QWs with 0.2% to 1.5% Mn, and various
densities of carriers. Short pulses of magnetic field were applied in the Faraday configuration by a small magnetic
coil mounted at the surface of the sample. We analyzed the temporal evolution of the giant Zeeman shift of
spectroscopic lines after the pulse with resolution down to a few nanoseconds. This evolution reproduces the
dynamics of the magnetization of the Mn system. The dynamics in absence of magnetic field was found to be up
to three orders of magnitude faster than that at 1 T. Hyperfine interaction and strain are mainly responsible for
this fast decay. The influence of a hole gas is clearly visible: at zero field anisotropic holes stabilize the system
of Mn ions, while in a magnetic field of 1 T they are known to speed up the decay by opening an additional
The passage of spin-polarized currents through magnetic nanocontacts can lead to the excitation of self-sustained
vortex oscillations in the free layer of a spin-valve stack. These oscillations involve the large amplitude translational
motion of the vortex core about the contact region, with oscillation frequencies typically in the 200-500
MHz range. Here, we present a detailed experimental study of such current-driven vortex oscillations. In particular,
we show that the oscillation mode is possible in zero applied magnetic field and is only stable within a
range of in-plane applied fields.
We present the last developments of the
approach of spin-transfer in terms of mesoscopic non-equilibrium
thermodynamics. The modification of the ferromagnetic
states due to the presence of a spin-accumulation process is investigated. The spin accumulation (also responsible for
giant magnetoresistance) is generated by an electric current at the interfaces with a
ferromagnetic nanostructure. The dynamical coupling that links the
ferromagnetic degree of freedom to the spin of the conduction electrons is
investigated with taking into account longitudinal and transverse spin
accumulation. It seems that the coupling with the transverse spin-accumulation
may be equivalent to the usual spin-transfer-torque mechanisms deduced from
microscopic transmission- reflection coefficients at the interface. In
contrast, the coupling with the longitudinal spin-
accumulation generates fluctuations and seems to play important
role in the activation process at larger time
Magnetoelectronic devices usually incorporate at least two magnetic layers, a "fixed" reference layer and a "free"
active layer. We analyze the stabile configurations of such a bilayer in the presence of spin transfer torque acting
on both layers. In the approximation of uniaxial anisotropy, we derive analytical stability conditions dependent
on the parameters of the bilayer as well as the applied field and current. We illustrate our results with stability
diagrams for several simple cases. We also determine the conditions for stable dynamical current-induced states.
Our analysis provides criteria for the design of magnetic nanodevices with optimized response to electric current.
Spin-transfer torques (STT) provides a new mechanism to alter the magnetic configurations in magnetic heterostructures, a
feat previously only achieved by an external magnetic field. A current flowing perpendicular through a magnetic
noncollinear spin structure can induce torques on the magnetization, depending on the polarity of the current. This is
because an electron carries angular momentum, or spin, part of which can be transferred to the magnetic layer as a torque.
A spin-polarized current of a substantial current density (e.g., 108 A/cm2) is required to observe the effect of the spin
transfer torques. Consequently, switching by spin-polarized currents is often realized in small structures with sub-micron
cross sections made by nanolithography. Here we demonstrate spin transfer torque effects using point-contact spin
injection involving no lithography. In a continuous Co/Cu/Co trilayer, we have observed hysteretic reversal of sub-100 nm
magnetic elements by spin injection through a metal tip both at low temperature and at room temperature. A small
magnetic domain underneath the tip in the top Co layer can be manipulated to align parallel or anti-parallel to the bottom
Co layer with a unique bias voltage. In an exchange-biased single ferromagnetic layer, we have observed a new form of
STT effect which is the inverse effect of domain wall magnetoresistance effect rather than giant magnetoresistance effect.
We further show that in granular solids, the STT effect that can be exploited to induce a large spin disorder when combined
with a large magnetic field. As a result, we have obtained a spectacular MR effect in excess of 400%, the largest ever
reported in any metallic systems.
Spin-transfer devices that incorporate a polarizer with its magnetization orthogonal to a switchable (free) layer
offer the potential for ultra-fast switching, low power consumption and reliable operation. The non-collinear
magnetizations lead to large initial spin-transfer torques, eliminating the incubation delay seen in devices with
collinear magnetization. Here we present the basic electrical and magnetic characteristics of spin-valve nanopillars
that incorporate a perpendicularly magnetized polarizer and demonstrate current-induced switching with
short current pulses, down to 100 ps in duration. We have fabricated devices that have a CoNi polarizer with
perpendicular magnetization and an in-plane magnetized 3 nm thick Co free layer and a 12 nm thick Co reference
layer, each separated by thin (~ 10 nm) Cu layers. The magnetization of the reference layer is collinear with that
of free layer to read out the device state. The reference layer also contributes to the spin-accumulation acting on
the free layer and leads to a spin-torque that favors the parallel (P) or antiparallel (AP) state depending on the
current pulse polarity, reducing the requirement of precise pulse timing in precessional reversal. The anisotropy
field of the perpendicular polarizer is 1.3 T, i.e. it is high enough so that in-plane fields (< 0.3 T) applied to
switch the magnetizations of the reference and free layers do not reorient the polarizer. Our typical nanopillar
device lateral dimensions are between 60 nm and 300 nm and nanopillars are positioned on coplanar waveguides
to allow for broadband electrical connections and studies with fast rise time pulses, generated by an arbitrary
waveform generator. The switching probability has been determined for variable pulse amplitude and duration,
from 0.1 to 10 ns at room temperature.
A recent experiment determined the magnetic moment /Mn, M, in the dilute MnxSi1-x with x = 0.1% to be 5.0 µB/Mn.
The existing calculated M values range from 2.37 to 3.1µB/Mn except the case with a fixed charge state, Mn2+, which
gives 5.0µB/Mn. We address the issue: Can a single Mn at its neutral charge state in dilute MnxSi1-x alloys have M = 5.0
µB/Mn? After carrying out extensive calculations, the only model giving this M value involves a supercell having a total
of 513 atoms with a Mn at a substitutional site and a Si at a tetrahedral interstitial site serving as a second neighbor to the
Mn. Physically, the Mn contributes 4.0 µB due to the weakening of the d-p hybridization between the transition metal
element and its nearest neighbor Si caused by the presence of the second neighbor Si. The additional 1.0 µB is the
consequence of the exchange interaction through the remaining weak overlap of the wave functions between the d-state
of the Mn and the sp3 state of the nearest neighbor Si atom. Evidences for the weakening of the d-p hybridization are
Single ion spins in semiconductors with sizable spin-orbit interaction can be optically or electrically manipulated.
The highly extended anisotropic wave function of the hole bound to each magnetic ion is susceptible to external
non-magnetic control fields. The spin-orbit coupling between the orbital and spin character of the hole permits
indirect manipulation and readout of the ionic spin state. The spin-spin interaction between the magnetic ions
is electrically or structurally controllable.
Spin-polarized currents are able to change the magnetic configuration of nanostructures through the spintransfer
effect proposed more than a decade ago. Intensive research is currently directed at understanding the
basic physics of this non-equilibrium interaction and designing magnetic nanodevices controlled by electric
current. In those devices spin transfer torques play a key role creating dynamic regimes that are not
present in conventional magnetic systems. Unfortunately full dynamic study of the phenomenon is not
straightforward even for simple spin transfer devices due to the nonlinearity of the Landau-Lifshitz-Gilbert
equation governing the magnetization motion. Devices with complicated magnetic anisotropy feature many
dynamic regimes: "canted states", multiple precession states, "magnetic fan" regimes, etc. which are often
studied by numeric methods. One special case of magnetic anisotropy, a dominating easy plane, turns out
to be ubiquitous in experimental designs. This case is characterized by a simplification of the dynamic
equations which permits analytical treatment by means of an effective planar equation. The planar equation
is presented and discussed for a number of regimes. It is shown how planar description gives an intuitively
clear picture of magnetic dynamics and allows to predict new phenomena.
We discuss spin transfer and its reverse process, the generation of electric current by dynamic magnetization
textures. Both these processes acquire corrections due to spin relaxation. In the case of layered structures the
latter contribute to the effective-field-like spin torque, which in the continuum case results in the dissipative
spin transfer torque, or "β-term". The corresponding corrections to spin pumping and spin motive forces are
discussed with emphasis on, and applications to, field-driven domain walls.
Zero-bias spin separation generated by homogeneous optical excitation with terahertz radiation in quantum wells
is reviewed. In gyrotropic semiconductor structures spin-dependent asymmetry of electron scattering induces
a pure spin current which results in a spin separation. We consider the relaxation mechanism yielding the
spin current due to the energy relaxation of a heated electron gas and the excitation mechanism caused by the
scattering assisted free carrier absorption. An experimental access to these phenomena provides the application
of an external magnetic field converting the spin current into a measurable net electric current. We discuss
microscopic and phenomenological theory of these effects, give an overview of experimental data, and address
In non-centrosymmetric semiconductors with zinc-blende structure grown along the  crystallographic direction,
electrons with up and down spins undergo different quantum phase shifts upon tunneling, which can be
wieved as resulting from spin precession around a complex magnetic field. There is no spin filtering but a pure
spin dephasing. The phase shift of the transmitted wave is proportional to the overall barrier-material thickness.
We show that a device incorporating a number of resonant tunnel barriers constitutes an efficient quantum-phase
The spin states of single Mn atoms embedded in InAs quantum dots (QD) were investigated by optical means. In
(In,Ga)As, the Mn impurity acts both as acceptor and a magnetic moment. In the low density limit, which is relevant
here, the acceptor is in the neutral state A0 with an effective spin J = 1. Using magneto-photoluminescence
experiments, we probed the exchange interaction between the dot confined carriers and the Mn spin. Peculiar
properties are found such as a large influence of the QD strain field on the acceptor spin states, a ferromagnetic
hole-Mn spin coupling and a very small interaction with electrons. The Mn atoms were deposited during the
QD growth by molecular beam epitaxy. Although the substrate temperature favored large segregation of the
Mn atoms, we could measure a few dots containing single magnetic impurities. The zero-magnetic field exciton
of a Mn-doped single dot shows a complex spectrum consisting of two doublets and a singlet. They correspond
to the excitonic recombination of electron-hole pairs, affected by the exchange coupling with the Mn magnetic
moment in the Jz = ±1, 0 states. The fine structure of each doublet is due to the QD in-plane anisotropy which
partially mixes the Jz = ±1 states. The magnetic field dependence exhibits even more striking modifications,
with several crossing and anti-crossing again linked to the QD strain field. We developed a model taking into
account the spin interactions between the Mn impurity and the carriers in the dot. A very good agreement is
found with the experimental data.
Compact proximity focused vacuum tubes with GaAs(Cs,O) photocathodes are used for experimental studying spindependent
phenomena. Firstly, spin-dependent emission of optically oriented electrons from p-GaAs(Cs,O) into vacuum
in a magnetic field normal to the surface was observed in a nonmagnetic vacuum diode. This phenomenon is explained
by the jump in the electron g-factor at the semiconductor-vacuum interface. Due to this jump, the effective electron
affinity on the semiconductor surface depends on the mutual direction of optically oriented electron spins and the
magnetic field, resulting in the spin-dependent photoemission. It is demonstrated that the observed effect can be used for
the determination of spin diffusion length in semiconductors. Secondly, we developed a prototype of a new spin filter,
which consists of a vacuum tube with GaAs(Cs,O) photocathode and a nickel-covered venetian blind dynode.
Preliminary results on spin-dependent reflection of electrons from the oxidized polycrystal nickel layer are presented.
Spin transport in graphene devices has been investigated in both local and non-local spin valve geometries. In the nonlocal
measurement, spin transport and spin precession in single layer and bilayer graphene have both been achieved with
transparent Co contacts. Gate controllable non-local spin signal was also demonstrated in this system. For the local
graphite spin valve device, we report MR up to 12% for devices with tunneling contacts. We observe a correlation
between the nonlinearity of the I-V curve and the presence of local MR and conclude that tunnel barriers can be
employed to surmount the conductance mismatch problem in this system. These studies indicate that the improvement of
tunnel barriers on graphene, especially to inhibit the formation of pinholes, is an important step to achieve more efficient
spin injection into graphene.