When simple alloys turn into complicated J.I. Espeso, J.C. Gmez - - PowerPoint PPT Presentation

When simple alloys turn into complicated

J.I. Espeso, J.C. Gómez Sal, N. Marcano Universidad de Cantabria, INA Zaragoza G.M. Kalvius, D.R. Noakes, A. Amato TU Munich, Virginia State Univ., PSI

• V. Zlatić, I. Aviani, M. Očko

Institute of Physics, Zagreb

• S. Haines, R. Smith, S.S. Saxena

Cavendish Laboratory, Cambridge

Acknowledgment to the COST P16 - ECOM action

• Three STSM financed by ECOM

■ Veljko Zlatić

Universidad de Cantabria, Santander, Spain

■ Jose I. Espeso

Institute of Physics, Zagreb, Croatia

■ Ivica Aviani

Cavendish Laboratory, Cambridge, UK

Origin of the study

Looking for a simple antiferromagnet with FeB-type of structure to study the muon stopping site

Candidates:

• CeCu (TN=3.5K)

burns spontaneously

when powdering

• CeGe (TN=10K)

reported as a simple antiferro

Buschow et al., Phys. Stat. Sol. 16 (1966) 467

Previous literature on CeGe

Buschow et al., Phys. Stat. Sol. 16 (1966) 467

Tipical of a simple AF

Magnetic study of RGe alloys Study of the CeGe1-xSix system

Schobinger-Papamantellos et al., Physica B 349 (2004) 100

• q = (0, 1/2, 0)

Obtaining the sample

• Polycrystalline sample prepared at an arc furnace
• Perfect agreement with the FeB-type of structure (Pnma space group)
• a = 8.350 Å

b = 4.078 Å c = 6.022 Å

500 1000 1500 20 30 40 50 60 70

Intensity 2θ (degrees)

CeGe

Cu Kα λ = 1.5418 Å

Heat capacity results

Cp = γT + 9nR T θD 3

θD T

x4ex (ex − 1)2 dx + R∆2

i − ∆i2

T 2

10 20 30 40 50 60 50 100 150 200 250 300

C

p (J/K mol)

Temperature (K)

CeGe

1 2 3 4 5 6 50 100 150 200 250 300

C

mag (J/K mol)

Temperature (K)

Δ1 Δ2

θD = 240 ± 5 K γ = 10.3 ± 2 mJ/K2mol ∆1 = 53.0 ± 0.4 K ∆2 = 137 ± 1 K

Low T Heat capacity

• The ordering temperature is in agreement with that previously reported (TN = 10.8 K)
• ΔCmag = 6.7 J/K mol ⇒ µCe ≈ 1.1 µB, in agreement with neutron data
• The Sommerfeld coefficient at low T is not much enhanced
• Smag at TN is not much reduced with regard to R·ln2

Classical antiferromagnet (till now ...)

2 4 6 8 10 1 2 3 4 5 6 7 2 4 6 8 10 12 14 C

mag (J/K mol)

S

mag (J/K 2mol)

Temperature (K) R ln2

CeGe

0.2 0.4 0.6 0.8 1 1.2 50 100 150 200 C

p/T (J/K 2mol)

T

2 (K 2)

CeGe

First surprising results

• Three different precession frequencies in the µSR response

Ratio of frequencies and intensities is T-independent All three converge at TN

Muon spectroscopy

5 10 15 20 25 30 35 4 6 8 10 12 Frequency (Hz) T(K)

TN

CeGe

ac and dc magnetic susceptibility

0.02 0.03 0.04 0.05 0.06 0.07 0.08

1000 Hz 100 Hz χ' (emu/mol) h

ac = 1 Oe

0.0005 0.001 0.0015 10 20 30 40 50

1000 Hz 100 Hz χ'' (emu/mol) Temperature (K)

CeGe

0.04 0.06 0.08 0.1 0.12 0.14 FC ZFC

M/H (emu/mol Oe) 200 Oe

0.04 0.05 0.06 FC ZFC

M/H (emu/mol Oe) 1000 Oe

0.04 0.05 0.06 5 10 15 20 FC ZFC

M/H (emu/mol Oe) Temperature (K) 3000 Oe

CeGe

• χac must be taken with caution
• Ferromagnetic contribution in M/H
• Strong irreversibility associated to

this FM contribution

0.2 0.4 0.6 0.8 1 20 40 60 80 M(µB/Ce) Magnetic Field (kOe)

98K 49K 24K 14K 147K 9.8K 8.3K 4.8K 1.8K

CeGe Isothermal magnetization

• Saturation far to be reached

⇓ strong anisotropy

• Broad metamagnetic

transitions ⇓ not so simple magnetic structure

2 4 6 8 10 12 5 10 15 20

0 T 1 T 3.5 T 5 T 7 T 9 T

C

P (J/K mol)

Temperature (K)

CeGe

Heat capacity under field

• The applied magnetic field

unveils two transitions

• Both of them have

antiferromagnetic character

Electrical resistivity

• Increase of the resistivity at TN
• Two possible explanations:

gap opening at the magnetic superzone (Tb, Dy, Ho, Er, Tm) electron scattering by substitutional spin disorder (HoMn12-xFex)

200 300 400 500 600 700 800 50 100 150 200 250 300

ρ (µΩ cm) Temperature (K)

CeGe

240 250 260 270 280 290 300 2 4 6 8 10 12 14 16 18 ρ (µΩcm) Temperature (K)

Gap opening at the magnetic superzone Strong effect of magnetic field Substitutional spin disorder No effect of magnetic field

Stankiewicz et al., Phys. Rev. Lett. 89 (2002) 106602 Ellerby et al., Phys. Rev. B 57 (1998) 8416

How to distinguish

Our results

200 220 240 260 280 4 8 12 16 20 0 T 3 T 5 T 7 T 9 T

ρ(µΩ cm) Temperature (K)

CeGe

100 110 120 130 140 (b) HoMn

12-xFe x

x=2 x=3 x=4 T

N

T

N

80 100 120 140 100 200 300 400 T(K) (d) x=9 x=8 HoMn

12-xFe x

T

N

Thulium

T (K)

Model for the resistivity

R.J. Elliott and F.A. Wedgwood, Proc. Phys. Soc. Lond. 81 (1963) 846

ρ = α + βT + γ(1 − 1

2M 2)

1 − ΓM

ρphon ∝ T θD 5 θD/T z5 sinh2(z/2)dz

α = 120.6 ± 0.7 β = 110 ± 1 γ = 90.8 ± 0.8 Γ = 0.321 ± 0.001

• Smaller coupling than the

heavy rare-earths:

• Dy ---> Γ = 0.68
• Ho ---> Γ = 0.65
• Er ---> Γ = 0.66
• Tm ---> Γ = 0.73

Elliot63 and Ellerby98

240 250 260 270 280 290 300 310 5 10 15 20

ρ fit fit residual

ρ (µΩ cm) Temperature (K)

CeGe

Thermopower results

• 8
• 7
• 6
• 5
• 4
• 3
• 2
• 1

10 100

CeGe Ce0.9La0.1Ge

S (µV/K) Temperature (K)

Edwards et al., Phys. Rev. 176 (1968) 753

Thermopower of Thulium

• Confirm the gap opening at the magnetic superzone
• Effect of dilution on the Ce site

Measurements under pressure

10 20 30 40 50 60 70 2 4 6 8 10 12 14 ρ(P,T)-ρ(P,0)(µΩ cm) T(K)

CeGe

P = 15 kbar P = 11 kbar P = 4.5 kbar P = 1 kbar 0.06 0.07 0.08 0.09 0.1 5 10 15 20 P = 0 kbar P = 9 kbar P = 12 kbar M (µB/Ce) T(K)

H = 10 kOe

CeGe

• TN does not change with pressure
• Small influence of the Kondo effect

Back to neutron diffraction

Schobinger-Papamantellos et al., Physica B 349 (2004) 100

Back to neutron diffraction

1000 2000 3000 4000 5000 10 20 30 40 50 60 70

Magnetic Intensity 2θ (degrees)

CeGe

D1B λ = 2.52 Å Commensurate structure 15 K - 2.4 K 1000 2000 3000 4000 5000 10 20 30 40 50 60 70

Magnetic Intensity 2θ (degrees)

CeGe

D1B λ = 2.52 Å Incommensurate structure 15 K - 2.4 K

Pattern matching

• q = (0, 1/2, 0)
• q = (0, 0.51, 0)

Schobinger-Papamantellos et al., Physica B 349 (2004) 100

Conclusions

• Partial studies may lead to unsound conclusions

1/χ or Cp ⇒ CeGe is a simple AFM

• The magnetic structure of CeGe is incommensurate

■ Three frequencies in μSR ■ Different contributions in M/H and Cp(T,H) ■ Gap opening at the magnetic superzone (ρ, S) ■ Neutron diffraction ⇒ q = (0, 0.51, 0)

• Small influence of the Kondo hybridization