PowerPoint Presentation

Thomas Seebeck

Jean Charles Peltier

New Directions in Energy Research or a Magnetic Quirk?1

SuperconductorsMagnetocaloric EffectThermoelectric effect

SuperconductorsMagnetocaloric EffectThermoelectric effectResearch Interests

http://www.superconductors.org/INdex.htm Waste heat harvestingLow loss power transferMagnetic refrigeration2

Superconductivity

Zero Resistance

Magnetic Levitation

Applications:MRIsCERNCourtesy Dr Gaifullin

3Superconductors at CERN

2008: Magnets quenched at the LHC (CERN), putting back discovery of the Higgs boson by approximately a year.http://youtu.be/BEnaEMMAO_s http://press.web.cern.ch/press-releases/2008/10/cern-releases-analysis-lhc-incident 4"The LHCs spectacular run got off to a shaky start in 2008. Shortly after operators fired it up, a single bad electrical connection caused coolant to vaporize, triggering an explosion that damaged an entire sector of the machine (see Nature 455, 436437; 2008). Repairs took more than a year, and a subsequent review revealed potentially dangerous flaws in the original design, according to Steve Myers, CERNs director for accelerators. The worst lay in a system of copper bars designed to draw current away from delicate superconducting cables in the event of an emergency shutdown or failure. The way in which the bars had been installed made them vulnerable to failure, Myers says". - Scientific AmericanMagnetocaloric Effect(Solid State Magnetic Refrigeration)

http://www.sseec.eu/Solid_State_Energy_Efficient_Cooling.html Another example of physics research for Energy is magnetic refrigeration. Advantages of magnetic cooling include higher efficiency; less noise; no need for toxic refrigerants (i.e. easier disposal).

The magnetocaloric effect can be used in a similar way as standard Vapour compression cycle, where compressed gas results in temperature increase, which is released to surroundings; decompression leads to temperature decrease which absorbs heat from load (i.e. inside of fridge).

Magnetic refrigeration, or solid state cooling utilizes the cyclic nature of the magnetocaloric effect where there is typically an increase in temperature when magnetic material is magnetised (and decrease when demagnetised). 5

Magnetocaloric Effect(Solid State Magnetic Refrigeration)

Need to find the right materialhttp://www.sseec.eu/Solid_State_Energy_Efficient_Cooling.html

Adiabatic demagnetisation of paramagnetic salts has been used for years to cool below 1K and this works quite well with a standard Carnot cycle where Cp is low. At higher temperatures, however, a larger lattice Cp limits the usefulness of this. This is where alternative refrigeration cycles become useful.For this to work we need large changes in temperature (equivalent to large entropy changes) which require rather complex alloys which exhibit a first order phase transition (i.e. there is some latent heat) and so the majority of research has been focussed on discovering useful materials. This research is currently in the engineering stage designing prototype systems and upscaling material production.

6The Thermoelectric Effect

Thomas Seebeck

Jean Charles Peltier

JCJQLastly the thermoelectric effect describes the interplay of current and temperature difference in a material.Peltier: A voltage applied across a sample results in a temperature change.Seebeck: A temperature difference across a sample generates a voltage.Both these effects can be characterised by S, and if we want to harness this then we want to increase S,.This is what we aim for in a standard Peltier cell, which consists of n and p doped semiconductor pillars arranged so that electrical current flows through and heat current perpendicular.7Onsager ReciprocitySpin dependant Seebeck effectSpin dependant Peltier effectLars Onsager received the Nobel prize for Chemistry in 1968 "for the discovery of the reciprocal relations bearing his name, which are fundamental for the thermodynamics of irreversible processes

8Harvesting HeatPowered by plutonium-238http://mars.jpl.nasa.gov/files/mep/MMRTG_Jan2008.pdf

NASA Mars RoverThis type of technology is reliable as there are no moving parts BUT costly and inefficient.Perhaps this is why it has found a niche with NASA where reliability is vital.9Peltier Cells to Recover Waste Heat

Skudderites are a popular TEG material:

10Cubic

Limited Thermoelectric Efficiency?Cost

Efficiency

Figure of merit

Wiedeman Franz Lawhttp://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2004 /session4/2004_deer_fairbanks2.pdf Ideally we want ZT>3 for the benefits to offset the cost.11

NW=NanowireNDSL=nanodot superlatticeSL=superlattice12Current State of the Art

K. Biswas et al., Nature, 489, 414-418 (2009)

Nanostructuring or bulk engineering to improve ZT

Vineis et al., Adv. Mater., 22, 3970-3980 (2010)13

The Spin Seebeck Effect

Co2MnSiNiFeGaMnAs

YIGLaY2Fe5O12

K. Uchida et al., Nature Letters, 455, 778-781 (2008)14Onsager ReciprocitySpin dependant Seebeck effectSpin dependant Peltier effect152007 Nobel Prize: Fert and Grnberg (GMR)

Giant Magneto Resistance (GMR) achieved by thin films magnetised anti-parallel (with respect to each other). 16Analogy with SSE description.Moores Lawhttp://www.mooreslaw.org/

Intel Corp.Number of transistors doubles every 2 years

Data storage density doubles every 2 years

Processing power doubles every 2 yearsGMR17What Exactly is Spintronics?Using charge and spin to contain information: Four possible states (qubits).

D. Pesin and A.H. MacDonald, Nature Materials, 11, 409-416 (2012)Giant MRAndreev ReflectionCurrent induced spin polarisation in PMsSpin Hall effect in PMsImage caption from Pesins article: In all panels, the large spins represent overall magnetization and the small spins represent the transport electrons. The field of spintronics is divided in the first place between the study of magnetically ordered conductors (left panels) and the study of paramagnetic metals or semiconductors (right panels). Within each class, one can study effects in which electric fields alter spin configurations (bottom panels) and complementary effects (top panels) in which effective magnetic fields due to spinorbit, exchange or magnetostatic interactions influence transport properties. The four panels in this figure (anticlockwise from the top left) schematically illustrate (i) giant magnetoresistance in which variation in magnetization direction increases backscattering and hence resistance; (ii) Andreev reflection of spins in non-collinear magnetic systems that leads to spin-transfer torques and current-induced spin reversal; (iii) current-induced spin polarization in paramagnetic conductors; and (iv) the spin Hall effect and spin currents in paramagnetic conductors. Spin-transport effects in paramagnetic conductors always require spinorbit interactions. This Progress Article concentrates on right-panel phenomena in topological insulators and in monolayer graphene, and left-panel phenomena in bilayer graphene.18The Spin Seebeck EffectBVISHETMaterialSpin CurrentCharge CurrentNormal metalFerromagnetic metal

Ferromagnetic semiconductor

Ferromagnetic insulator

VISHET

Transverse spin SeebeckLongitudinal spin SeebeckA spin current may flow in an electric insulator19Aside: How do You Detect a Spin Current?Spin Hall Effect: Generation of a spin polarised current due to charge current flowing from a paramagnet to a ferromagnet.

Inverse Spin Hall Effect: Generation of a voltage EISHE due to a spin polarised current.

Heavy metals such as Pt are typically very good for detection of Js by generation of EISHE.The first step in harnessing a spin polarised current or the SSE is detecting it in the first place this is where the ISHE comes in.Spin currents injected into a PM metal. EISHE generated by the spin-orbit interaction. Voltage builds up normal to the spin current.The ISHE is caused by the bending of electron orbits of up and down spins into opposite directions normal to their group velocity, owing to the spin-orbit interaction.Large voltage with heavy metals such as Pt and scales linearly with wire length.20Can be thought of as the efficiency with which a spin current, JS is converted to a charger current, JC.Aside: Spin Hall AngleElementSpin Hall Angle, SH (%)Al0.02Au0.25 11Bi>0.8Cu0.22Mo-0.05 -0.8Nb-0.87Pd0.64 1Pt1.3 11Ta-0.37 -12W-33The spin Hall angle can be thought of as a measure of efficiency of spin to charge current conversion.The best effciencies/values lie around 10% and for application we require both positive and negative spin Hall angles.21Maximising VISHE, Minimising CostContactCost of host metal ($/g)Cost of dopant($/g)Total cost of contact ($/g)Measured (M), or predicted (P), spin Hall angle (%)Pt50.95-50.951.2 to 11[1] (M)Cu0.12-0.120.22 [2] (M)Cu+1% Pt0.12510.63 2.7 [3] (P)Cu+1% Bi0.120.020.128.1 [3] (P)Cu+1% Ta0.126.30.18-1 [4] (P)Ag0.71-0.710.47 [2] (M)Ag+1% Pt0.71511.211.0 [3] (P)Ag+1% Bi0.710.020.71.4 [3] (P)

[1] A. Hoffman, IEEE Trans. Magn., 49, 5172 (2013).[2] H.L. Wang et al., arXiv:1307.2648 (2013).[3] M. Gradhand et al., Phys. Rev. B, 81 245109 (2010).[4] A. Fert and P.M. Levy, Phys. Rev. Lett., 106 157208 (2011).22Advantage of negative spin Hall angle with small doping = easier production of thermopile.Measuring the Spin Seebeck Voltage

B23The Next Stage?

24Impact of the Spin Seebeck EffectThermal transportSpin transportCharge transportReduced fabrication costsSpintronicsSpin valvesMagnetic heat switchesTunnel junctionsQuantum ComputingPhysical SciencesIncreased figure of merit, ZTThermoelectricsEnergyEnergy efficiencyEnergy storageMaterials for energy applicationsInformation and Communication TechnologiesThermal spin transfer torqueNon-CMOS technology25Available PhD project.As part of the PhD you will be expected to characterise potential spin Seebeck samples using x-ray diffraction, x-ray reflectivity, transport measurements, thermal transport and magnetometry. It is also likely that you will prepare patterned thin films using pulsed laser deposition and physical vapour deposition techniques.

http://homepages.lboro.ac.uk/~phkm2/Phd.htm

26

quBitterPieter Kok (Sheffield) on Quantum ImagingDan Browne (UCL) on D-Waves quantum computer

When: 7pm Wed 26 MarchWhere: Swan in the Rushes (upstairs)

Target Audience: Part C and above but all welcome(space permitting)27