The spinning computer era Spintronics Hsiu-Hau Lin National Tsing-Hua Univ.

The spinning computer era Spintronics

Hsiu-Hau LinNational Tsing-Hua Univ

Quantum Information Science WokshopTainan, Oct 18-19, 2002

Hsiu-Hau Lin National Tsing-Hua Univ

Collaborators and References

Jurgen Konig Allan H. MacDonald John Schliemann Steve M. Girvin Thomas Jungwirth

Phys. Rev. Lett. 84, 5628(2000) Appl. Phys. Lett. 78, 1550 (2001) Phys. Rev. Lett. 86, 5637 (2001) Lecture Notes in Physics 579, Springer (2002)



Outline

Introductory spintronics Making ferromagnetic

semiconductors Pushing up the critical temperature Theoretical Understanding Conclusions



Spin and Charge

Spin: data storage inMagnetic materials.

Charge: data processing in Semiconductors.



Spin Valve

Use spin configurations to control the current -- spintronics !

Conventional transistors make use of current or voltageto control the transmitted current -- electronics.



Spin-Valve Transport

metal insulator

Transport properties depend onthe carriers’ spins.



Magnetic Recording

Actual setup

A real spin valve!



Magnetic Read Head

Available commercial product of spintronics -- magnetic read-out head by IBM in 1997.

Now it becomes $1,000,000,000USD market !

Even more surprises on the way!



Nonvolatile Memory (MRAM)

Weak currents flip red spin -- Data processing

Strong currents flip blue spins -- Memory storage



Earlier Tries

(A) Magnetic Semiconductors: Eu chalogenides,semiconducting spinels, …

Hard to grow !

(B) Diluted II-VI Magnetic Semiconductors: CdTe, ZnSe, …

- Easy to replaced by Mn- Hard to dope, no mobile carriers.



Success of III-V Semiconductors

Ferromagnetism in (Ga1-x, Mnx) As:

Ga3+

Mn2+ + hole

Key to ferromagnetism !

Mn2+ (S = 5/2)

Expect nI = ne -- But ! Mobile hole (s=1/2)



Phase Diagram

• Low solubility, easily leads to phase separation.• Molecular Beam Epitaxy (MBE) at low temperatures.



Spin Doctor Ohno

Critical temperature Tc = 110K



Spin Doctor Awschalom

Efficiency over 90% polarized injected current.

Coherent spin transport over 100 m.

Hope for Quantum Computer!



Mean-Field Prediction

The critical temperaturegrows as the doping x increases.



Seems Optimistic?

Weiss mean field theorypredictions !

Wide-band gap semiconductors and oxides are our best bets!



Dream Come True?

Co-doped TiO2:

• Formation of domain wallssuggesting the presence of a longrange magnetic moment

• Tc is estimated as high as 400 Kexceeding the room temperature.



Magnetization Curve

• Hysteresis is very small• Small magnetoR ratio (60% under 8T at 2K)

But, there is always hope!



A Simple Model

Kinetic energy

Exchange energy

• J > 0, Antiferromagnetic coupling• Ignore realistic band structure, disorder, electronic correlation, and so on.• Similar to CMR, Kondo,… BUT ! Not quite the same.



Ferromagnetic Ground State

• Impurity spins aligned by mobile electrons.

• Electronic bands acquire a Zeeman splitting gap.

Carrier-Mediated Ferromagnetism !



Spin-Wave Spectrum

• Gapless spin excitations (Goldstone mode).

• Exchange – minimized

• Kinetic – twisted phase

In the dilute limit,



Optical Spin Waves

E = + Kinetic

• Two-particle continuum.

• Collective mode below the continuum.



Trend of Critical Temperature

• Spin-wave fluctuations included self-

consistently.• Magnetization curve is

computed.• Tc is not monotonically

increasing with respect to doping and carrier density.



Different Regimes

• Weiss mean-field theory is not always correct !

• Friedel oscillations become important and reduce Tc .

• Interesting crossovers !



Conclusions

Diluted magnetic semiconductors are fascinating materials.

Tc already exceeds the room temperature. However, lots of room for improvements.

Deeper theoretical understanding of spin transport at finite temperature is in order.

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The spinning computer era Spintronics Hsiu-Hau Lin National Tsing-Hua Univ.

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