The attraction of + to O2-:using muons to study

oxides

Steve Blundell

Clarendon Laboratory, Dept. Physics, University Of Oxford, UK

Why muons?Susceptibility is a bulk measurement

measures “volume-averaged” magneticproperties

Muon-spin rotation is a local measurementmeasures magnetic properties at alocal level

…so what is a muon?

Particle properties

e=eBe

p=pBp

=B

STEP 1:

STEP 2:

implantation 4

STEP 3: decay

2.2 s

Muon decay Muon decays into a positron:

Positron decay is asymmetricwith respect to the initialmuon-spin polarizationbecause of parity violation (weak interaction)

(see S.J. Blundell, Contemp. Phys. 40, 175 (1999))

MUON POSITRON NEUTRINOS

Muon experimentSPIN PRECESSION

MUON IMPLANTATION

SPIN PRECESSION AND DECAY

Experiments

at ISIS pulsed muon facility

Experiments here

Experiments at PSImuon facility

Paul Scherrer Institute, Villigen, Switzerland

GPS spectrometer

In the presence of magnetic order, muonssense the internal magnetic field in a material,

measured at the muon stopping site.

The muon spin precession frequency,

ωμ=2πνμ,

is given by

ωμ=γμBμ.

This allows us to follow the temperature dependenceof the magnetic order.

EuB6

A ferromagnet

M.L. Brooks, T. Lancaster,S.J. Blundell and F.L. Prattin preparation.

SR and ordered organic ferromagnets and antiferromagnets

Ferromagnet

Antiferromagnet

SR and ordered organic ferromagnets and antiferromagnets

or

Uniformly weakly magnetic Non-magnetic, with strongly magnetic impurities

Susceptibility gives average information and thereforecan give the same response for the situations sketchedabove (hence many false claims of room temperature organic ferromagnetism…)

SR gives local information and therefore can distinguishbetween these two situations.

AFM order in LiVGe2O6

SJB et al. Phys Rev. B 67, 224411 (2003)

LiVGe2O6

SJB et al. Phys Rev. B 67, 224411 (2003)

2 clear frequenciespersist below theso-calledorderingtemperature...

LiVGe2O6

SJB et al. Phys Rev. B 67, 224411 (2003)

Dipole-dipole field

Dipole-dipole field

Dipole-dipole field

Problem:

0

Dipolar fields

Dipolar fieldcalculations:

For cuprates, kill AFM with afew % of dopant and achievemaximum superconductivityat x~0.15. The normal stateis a (weird) metal.

For these nickelates, onlymetallic at x~1. No superconductivity. Evidence for 2D orderedarray of holes below ~230 K.

SR used to find groundstate for 0<x<1.

PRB 59 3775 (1999)

PRB 59 3775 (1999)

Sr2CuO3

Chains of-Cu-O-Cu-O-Cu-O-Cu-along x-axis

superexchange throughoxygen anions

chains well separatedand J’/J small

J ~ 1300 K, TN=5 K

Muon data

Sr2CuO3

Ca2CuO3Kojima et alPRL 78 1787 1997

(ingenious chemistry by Rosseinsky, Hayward et al - Liverpool)

Muon data

LaSrCoO3H0.7

Our data:Science 295 1882 2002

Oscillations imply static, large, local field corresponding to the whole of the sample

Muon data

The internal magneticfield is very high (~0.5 T)which is much greaterthan in Sr2CuO3 (~0.01 T)

TN is well above room Tin our compound,much greater than ~5 Kin Sr2CuO3 and ~10 Kin Ca2CuO3

Conclusion:

LaSrCoO3H0.7 contains the hydride ion

H- (1s2)

Hydride ions can transmit exchange interactionsvery effectively! This leads to the separated chainsbeing bridged, raising the transition temperatureof our compound to well above room temperature!

Sr2CuO3

Chains of-Cu-O-Cu-O-Cu-O-Cu-along x-axis

superexchange throughoxygen anions

chains well separatedand J’/J small

C.D. Ling et al PRB 62, 15096 (2000)

La2-2xSr1+2xMn2O7

C.D. Ling et al PRB 62, 15096 (2000)

La2-2xSr1+2xMn2O7

La2-2xSr1+2xMn2O7

C.D. Ling et al PRB 62, 15096 (2000)

Bilayer manganates

A. Coldea et al. PRL 89 277601 (2002)

Relaxation functionsOne caninterpolatebetweenstaticsand dynamicsusing adynamicalKubo-Toyabefunction

Muons and spin glassesMuons that stop closerto magnetic ions“see” a wider local fielddistribution (whichextends to higherfields) than muonswhich stop at a greaterdistance

Y.J. Uemura et al,PRB 31, 546 (1985)

La1.5Sr0.5MnRhO6

ferromagnetic insulatorwith large MR, evidencefor magnetic polaronsabove Tc.

A. Coldea, I.M. Marshall, S.J. Blundell, J. Singleton, L.D. Noailles, P.D. Battleand M.J. Rosseinsky,

PRB 62, R6077 (2000)

ZnCr2O4

Gd3Ga5O12

JPCM 14 L157 (2002)

ZnCr2O4

JPCM 14 L157 (2002)

Gd3Ga5O12

JPCM 14 L157 (2002)

Kajimoto et al, PRB 67, 014511 (2003)

P.G. Freeman et al, PRB 66, 212405 (2002)

La1.5Sr0.5NiO4

La1.5Sr0.5NiO4

ChrisSteer et al.

The attraction of + to O2-:using muons to study

oxides

Thanks to members of the Oxford muon group, ICL Oxford, Chemistry in Liverpool, ISIS + many

othersand to you for your attention!