MAGNETIC SEMICONDUCTORS

BY…..

……B.KIRAN KUMARB.KIRAN KUMAR

• Magnetic semiconductors are the materials that exhibit both ferromagnetic

and semi conducting properties …. Traditional electronic devices are based on

control of electric charge, but magnetic semiconductors allow control of quantum spin-state

…. This would theoretically provide spin polarization, which is important property of SPINTRONIC DEVICES

Preparetion….

• The dilute magnetic semiconductors are studying based on semiconductor properties but are doped with “transition metals” using MBE technic

• These materials offer the advantage of semiconductors combined with “non-volatile” properties of magnetic materials

• The manufacturability of these materials depend on thermal equilibrium solubility of these dopant in the base material

• II-VI, III-V group elements are the most studying materials…

• Atoms of Manganese can be inserted at desired locations by using STM.

• MBE is used for doping

• That material is used to fabricate the storage devices..

• Mn doped ZnSe,CdTe,CdS (2nd-6th) behaves like antiferromagnets (all are like that)

due to super exchange…• Mn doped GaAs and InSb ( 3rd-5th) behaves like

ferromagnets…due to inter exchange mediated by free carriers.

• When divalent dopant like Mn substitute for 3rd group elements like Ga, a hole is introduced, this is the reason GaAs behaves like ferromagnet and ZnSe not.

• we used density-functional theory to compute the effective coupling strength between Mn spins as a function of their separation

• Interestingly, this coupling is antiferromagnetic for nearest-neighbor Mn atoms, but ferromagnetic for most separations greater than this.

• Using modern epitaxy methods, it is now possible to control the growth of semiconductor crystals with atomic-layer precision.

• In particular it is possible to incorporate clean "2-dimensional" planes and arrays of magnetic material into otherwise nonmagnetic semiconductor lattices.

• In the presence of an applied magnetic field, the planes of Mn become magnetized. The magnetic barriers are thus lowered for spin-down electrons and holes (excitons), and are increased for spin-up electrons and holes.

• Alternatively, spin-up excitons "see" a different barrier height than their spin-down counterparts. The net effect is to energy-split the exciton spin-states, shown schematically below. In these magnetic heterostructures, this Zeeman splitting can be made hundreds of times larger than in nonmagnetic quantum structures.

• In the absence of an applied magnetic field, spin-up and spin-down states are degenerate, and their spin relaxation may be seen as mirror images of each other

• Upon the application of a magnetic field, the energy degeneracy is lifted

and the coherently-excited spin polarizations exhibit quantum beating between the Zeeman-split levels at terahertz frequencies, with a frequency proportional to the electron g-factor and a rapid dephasing time.

• examination of the data shows that the system has not returned to equilibrium. A second signal is seen with a different period, corresponding to a g-factor of 2, and is a signature of the manganese ions.

• Angular momentum is coherently transferred from the electronic system to the magnetic spins: Although it has returned to electronic equilibrium, the quantum well is still undergoing dynamical spin processes.

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• This magnetic spin precession persists over far longer timescales - hundreds of picoseconds - eventually relaxing due to phonons in the system.

• These types of studies allow us to disentangle electronic and magnetic spin phenomena in quantum structures, enabling us to perform spin resonance experiments in systems with near atomic thicknesses.

“WHAT THESE SMSC PROVIDES”

• The wide variety of these host crystal and dopants provides materials which ranges from wide gap to zero gap semiconductotrs and variety of interactions

• The coupling between the localised moments results in the existence of different magnetic phases

• The zeeman splitting is due to interaction between the s,p band electrons and d electrons of Mn atoms

• SMSC provides the possibility of tuning parameters such as the band gap and the lattice constant by varying the composition of the material.

change in the concentration of the magnetic ions alters the magnitude of several magnetooptical effects and influences the magnetic properties of the material

• the s-d exchange interaction is almost pure potential (Coulomb) exchange results in the ferromagnetic coupling of the spins of transition metal ions and conduction band electrons.

• The p-d exchange, which couples the spins of the valence band electrons to that of the magnetic ions, results mostly from hybridization and is antiferromagnetic for all Mn, Fe or Co-based II-VI SMSC.

• ferromagnetic p-d interaction for transition metal ions with less than half-filled p-d shells.

• Molecular Beam Epitaxy (MBE) now allow zinc blende structure crystals of CdMnTe, ZnMnTe and ZnMnSe to be grown in the entire composition range: 0 < x < 1

• Finally the material has both magnetic and semiconductor behaviour

Applications…….

• SPINTRONIC devices are used in the field of mass-storage devices.

• Storage density is about 1.5 Gbit/mm². means 1 TB on a single sided 3.5" diameter disc

• spintronic device to date is the spin-valve. This device utilizes a layered structure of thin films of magnetic materials

• SPINTRONIC DEVICES changes their electrical resistance depending on applied magnetic field direction

• Future applications may include a spin-based transistor which requires the development of magnetic semiconductors exhibiting room temperature ferromagnetism . The operation of MRAM or magnetic random access memory is also based on spintronic principles.

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• GMR ratios as high as 200%

• (Antiparallel Resistance - Parallel Resistance) / Parallel Resistance x 100% is called the GMR ratio

• So it can detect very week magnetic fields also..

LIMITATIONS….

• Solid solubility;.. Magnetic behaviour of SMSC is depend on the concentration of magnetic material

• Temperature;… most of the SMSCs are having Tc bellow room temp,… due to super paramagnetism effect in small magnets

TRFR scans at 5K with an applied field of 1000G

• for four different ferromagnetic layers. The top trace is for a control sample that does not have any ferromagneti layer. Single layer refers to 1/2 monolayer (ML) of MnAs, "5 layers" to 5 periods of 1/2ML MnAs with GaAs spacings, "GaMnAs" sample has Mn concentration of 5%. The presence of an ferromagnetic layer strongly modifies the coherent electron spin precession.

• TRFR changes as a function of field for the single layer sample at 5K. As we scan the field, we see a hysteretic behavior, and a sharp change in TRFR appears at the coercive field of ferromagnetic layer.

• nuclear spins cannot polarize near zero field. Means nuclei are mediating the effective field,

Cd1-xMnxSe

• Diluted magnetic semiconductors Cd1-xMnxSe for x=0.01, Cd1-xFexSe for x=0.01 and 0.02, and Cd1-xCoxSe for x=0.006, 0.009, and 0.01 were studied in the temperature range of 180-400 K by 113Cd magic-angle-spinning NMR spectroscopy

• The NMR spectra of Cd1-xFexSe and Cd1-xCoxSe contain a set of resonance lines that are shifted away from the line of the undoped CdSe compound by the transferred hyperfine (THF) interaction between the cadmium nuclei and the paramagnetic ion

• The temperature dependence of the THF shifts follows the Curie-Weiss law

• the spin-lattice relaxation times of the shifted lines are significantly shorter than that of the CdSe line

• The 113Cd lines show anisotropies that are smaller than the values evaluated from the dipolar interaction between the paramagnetic ions (M) and their nearest-neighboring cations (M-Se-Cd).

• The detected anisotropies are therefore composed of dipolar and hyperfine contributions from next-nearest-neighboring (NNN) cadmium nuclei (M-Se-Cd-Se-Cd).

• The spin-lattice relaxation times of the spectral lines are determined by the electron-nuclear dipolar interaction with NNN cations.

• The number of observed lines corresponds to the number of nonequivalent NNN cations around each paramagnetic ion.

• Using relaxation times and the amplitudes of the lines, it is possible to correlate each line to a well-defined NNN conformation.

• The NMR spectra of Cd1-xMnxSe did not show any fine structure similar to that observed in the Co and Fe alloys

DNA goes SPINTRONIC…???

• DNA molecules could soon add ‘spintronic’ effects to their repertoire of surprising electronic properties.

• The current flowing through DNA molecules jumped by 26% when the spins of the electrons were flipped. Previous studies have shown that DNA can act as a superconductor and a semiconductor

ThanQ….

……………………………......…kiran kumar M.Sc,M.Tech. iitm.