Transcript Magnetism in (Sr,Ca)RuO4 and FeAs

The Magnetic Phase Diagram of
(Sr,Ca)2(Ru,Ti)O4 Revealed by mSR
Jeremy P. Carlo
[email protected]
Columbia University
Canadian Neutron Beam Centre,
National Research Council
June 2, 2010
Outline
• Overview
– Correlated electron materials
• Magnetic order
• Superconductors
• The mSR method
– Local probe of magnetism
• (Sr,Ca)2RuO4 & Sr2(Ru,Ti)O4
– Superconductivity
– Magnetic Phase Diagram
Overview
• Relation between magnetic order & superconductivity
– BCS: Cooper pairs: electron-phonon interaction
– High-Tc: magnetic fluctuations more important
– “Canonical” cuprate
phase diagram:
– Parent compound: AF
– Magnetic order close to
SC dome
Overview
• Ongoing questions:
• Behavior of different families of unconventional SCs?
Cuprates
Sr2RuO4
Heavy fermion SCs
Fe pnictides
Organic SCs
etc.
• How do magnetism / magnetic fluctuations relate?
• “Normal” state behavior, M-I / structural links?
• Holy Grail:
• What is the comprehensive theory
of unconventional superconductivity?
• Present Study
The mSR method
•
Production of muons
– Protons extracted from cyclotron/synchrotron
– p + low Z production target → + + stuff
– + → m+ + m
– parity violation: beam is spin polarized
– separate out positrons, etc.
– collimate / steer beam to sample
Polarized muon sources:
TRIUMF, Vancouver BC
PSI, Switzerland
ISIS, UK (pulsed)
KEK, Japan (pulsed)
Continuous-beam mSR
• Muon beam
– Positive muons m+
– Can rotate
polarization
– Insert muons
one at a time
– Come to rest
• Interstitial sites
• Near anions
• Along bonds
Decay Asymmetry
Muon spin
at decay
Detection:
m+ → e+ +m + e
e = E / Emax normalized e+ energy
e+ detector U
incoming
muon counter
sample
e+
m+
detector
e+ detector D
D
time
2.5
e+ detector U
incoming
muon counter
sample
e+
m+
detector
e+ detector D
time
D
2.5
U
1.7
e+ detector U
incoming
muon counter
sample
e+
m+
detector
e+ detector D
time
D
2.5
U
1.7
D
1.2
e+ detector U
incoming
muon counter
sample
e+
m+
detector
e+ detector D
time
D
2.5
U
1.7
D
1.2
D
9.0
+ 106-107 more…
Histograms for
opposing counters
asy(t) = A0 Gz(t)
Total asymmetry
~0.2-0.3
Muon spin
polarization
function
(+ baseline)
a
135.5 MHz/T
Represents
muons in a
uniform field
Field configurations
• ZF-mSR:
m+
–
sees: field due to nearby moments
vs. out-of-plane doping
– Spontaneous ordering?
Example
La
NbO6
[CuCl]+
• Precession
• Rapid relaxation
(CuCl)LaNb2O7
vs. in-plane doping
T-dependence
(in-plane doping)
T-dependence
(out-of-plane doping)
Field configurations
Example
• LF-mSR:
H  initial muon spin
– m+ sees: skewed
local field distribution
– Static order
– Decoupling if Happl ~ Bint
– Dynamic order
– No decoupling
– Drift of “1/3 tail”
Field configurations
• wTF-mSR:
H  initial muon spin
– Calibration of baseline (a), total asymmetry (A0)
– m+ sees:
– (mostly) applied field (paramagnetic state),
– appl. + internal fields (ordered state)
Example
Determine ordered,
PM fractions
Field configurations
• (strong) TF-mSR:
H  initial muon spin
– Order induced by applied field
– Metamagnetism, etc.
– Vortex lattice in Type-II SC
• Rlx  √<B2>  1/2  ns /m*
•  = penetration depth
• ns /m* = superfluid density
• Polyxtal samples:
distribution broadened
~ Gaussian
=> Gaussian rlx
=> 1/2
=> sf. density
J. E. Sonier, 1998 & 2007
Srn+1RunO3n+1
• Ruddlesden-Popper series
– n=: SrRuO3
(113)
• perovskite structure
• Ferromagnetic, Tc  165K
– n=3: Sr4Ru3O10 (4-3-10)
• multi-layered structure
• FM, Tc  105K
– n=2: Sr3Ru2O7 (327)
• quantum metamagnetism
• FM, AF fluctuations
• mag. ordering w/ Mn
– n=1: Sr2RuO4
(214)
• Unconventional SC Tc  1.5K
• Spin-triplet pairing, p-wave
• isostructural to LBCO, LSCO
RuO66
Sr
(Sr,Ca)RuO3 = ‘113’
• n=
• 3-D structure
• Ca/Sr substitution
– SrxCa1-xRuO3
– isoelectronic doping
– FM suppressed x  0.25
– Phase separation, QPT
Maeno et al. 1994
Sr2RuO4 = ‘214’
MacKenzie & Maeno, 2003
Fermi surface:
• n=1
• SC state (Maeno et al. 1994)
• Tc up to 1.5 K
• NMR: Spin-triplet pairing
• TRSB – (Luke et al. 1996)
distinguish between p-wave states
• Incommensurate spin fluctuations
q ≈ (0.6/a, 0.6/a, 0)
• Normal state: 2-D Fermi liquid
• Doping:
– “Out-of-plane:” Ca on Sr site: SrxCa2-xRuO4
– “In-plane:” Ti on Ru site: Sr2Ru1-yTiyO4
• Small doping on either site suppresses SC
Luke et al. 1996
Ca2RuO4
– AF insulator, moment 1.3mB
•
•
•
•
•
•
•
Competition between A- and B- type ordering
TN  110-150K
Ca doping induces Mott transition
Decreased bandwidth
Increased on-site Coulomb repulsion
→ Increased U/W
Ru-Ru in-plane
dist > Sr2RuO4
• RuO6 flattening, tilting
Ca2-xSrxRuO4
Susceptibility @ 2K:
M-I transition near x=0.2 (I-II)
Near x=0.5: (II-III)
Sharp increase in susceptibility
Correlations more FM-ish
Low susc @ higher x
Old Picture: Ordering at low x only
Antiferro. near x=0
Susc. peak near x=0.5
Paramagnetic at higher x
SC at x=2
mSR:
Rapid relaxation observed 0.2 ≤ x ≤ 1.6
Peaks near x  0.5, 1.5
Ordered ground state throughout!
Nakatsuji & Maeno, 2000.
Nakatsuji & Maeno, 2003.
Sr2Ru1-yTiyO4
• y=0: SC Sr2RuO4
from MacKenzie et al. 2003
• <0.2% Ti doping suppresses Tc
• >2.5% doping induces magnetic ground state
• neutrons: Braden et al. (2002)
– Incommensurate AF in y=0.09
• q  (0.3, 0.3, qz)
• mSR: rapid relaxation
with increasing y.
Experiments
Samples
(Ca2-xSrx)2RuO4 x = 0.0, 0.2, 0.3, 0.5, 0.57, 0.65,
0.9, 1.0, 1.4, 1.5, 1.6, 1.8, 1.95
Sr2(Ru1-yTiy)O4 y = 0.01, 0.03, 0.05, 0.09
single xtals from Kyoto U. (Maeno et al. or
Tsukuba (Yoshida et al.
He gas-flow cryo
1.7K < T < 300K
Dilution fridge
15mK < T < 10K
ZF- & LF-mSR: M20 (LAMPF) and/or M15 (DR)
DC Susceptibility: ZFC, FC, H ~ 50-100 G
Ca2RuO4
• mSR spectra:
• Sum of 2 frequencies
Ca2RuO4 ZF-mSR
ZF-mSR
Temperature Scans
(Ca,Sr) system
ZF-mSR Temperature Scans
(Ru,Ti) system
Edwards-Anderson
order parameter
Uemura “spin glass” function (Uemura, 1985):
Field
width
dynamic
+
d
“root-exponential”
static
as
“Lorentzian Kubo-Toyabe”
as = a √ Q
d = 4a2(1-Q)/
Fluctuation
rate
• ZF Relaxation vs. Temp: Magnetic ordering!
Define: Rlx = sqrt ( d2 + as2 )
Fit to:
zoom
all
Ti
only
Ca
only
Rlx(T) = R [ 1 – (T/To)g ]
• LF @ base temp: decoupling → static order
Fit to tanh(H/Ho)
Static ordering at base temp!
• LF temp scans: map out dynamics
• Comparison of ZF & LF field estimates
R [ 1 – (T/To)g ]
Adapted from Braden et al. (2002)
Neutrons: Braden
Muons: present study
DC Susceptibility
Curie-Weiss:
more
AF
Old view:
New View:
Summary: (Sr,Ca)2(Ru,Ti)O4
Past:
Sr2RuO4 p-wave SC
New:
Sr2-xCaxRuO4
Tc  1.5K, TRSB
muons : magnetic order over almost entire range
magnetic fluctuations
x = 0: commensurate AF, gone by x = 0.2
Sr2Ru1-yTiyO4
peaks x  0.5 (FM-ish?), 1.5 (more AF)
y  0.002 suppresses SC
incommensurate AF / SDW ?
neutrons: incommensurate AF y = 0.09
need long-range magnetic probe!
Ca2RuO4 AF insulator
Sr2Ru1-yTiyO4
TN  100-150K
muons: rapid relaxation y ≥ 0.03
Sr2-xCaxRuO4
susc: large negative w → AF
M-I transition x  0.2
susceptibility peak x  0.5