Magnetic properties of SmFeAsOF

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15 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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Magnetic properties of SmFeAsO
1
-
x
F
x

superconductors for 0.15 ≤
x

≤ 0.2

G. Prando
1,2
, P. Carretta
1
, A. Lascialfari
1
, A. Rigamonti
1
, S. Sanna
1
, L.
Romanò
3
,

A. Palenzona
4
, M. Putti
4
, M. Tropeano
4

1
Department of Physics “A.Volta”
, University of Pavia, I
-
27100, Pavia (Italy)


CNISM, u.d.r.
Pavia

2
Department of Physics “E.Amaldi”
, University of Roma Tre, I
-
00146, Roma (Italy)

3
Department of Physics
, University of Parma, I
-
43100
, Parma (Italy)


CNISM, u.d.r. Parma

4
Department of Physics
, University of Genova, I
-
16146, Genova (Italy)


CNR/INFM
-
LAMIA

e
-
mail: prando@fisicavolta.unipv.it

Abstract

The

recent

discovery

of

high
-
T
c

superconductivity

in

Fe
-
based

oxypnictides

has

renewed

the

interest

in

the

study

of

the

interplay

between

superconductivity

and

magnetism

in

layered

materials
.

We

performed

static

magnetization

and

19
F

nuclear

magnetic

resonance

(NMR)

measurements

on

loose

powder

samples

of

superconducting

SmFeAsO
1
-
x
F
x

(
x

=

0
.
15

and

x

=

0
.
2
)
.

Our

results

indicate

that
:



the

low
-
energy

excitations

of

the

magnetic

Sm
3
+

sublattice

are

not

directly

involved

in

the

pairing

mechanisms

leading

to

the

superconducting

state
.



the

4
f

electrons

of

Sm
3
+

ions

are

coupled

to

a

sea

of

weakly

itinerant

and

antiferromagnetically

(AF)

interacting

fermions
.





Fe
-
based pnictides: the 1111
-
phase

SmO
(F)

tri
-
layer

FeAs

tri
-
layer

Sm



SmFeAsO
1
-
x
F
x

is composed of insulating tri
-
layers of Sm
-
O
-
Sm alternating metallic tri
-
layers of As
-
Fe
-
As.



Sm
3+

ions have incomplete electronic shells leading to net magnetic moments.



For one F
-

ion replacing one O
2
-

ion the injection of an electron into the Fe
-
As layers is obtained.



x

< 0.07 : magnetic behaviour characterized by a transition to a spin density wave (SDW) phase.



x

> 0.1 : superconductivity confined onto the As
-
Fe
-
As tri
-
layers. T
c
(
x
) depends very smoothly on
x
. AF
ordering of Sm
3+

ions at T ≈ 5K.



0.07 <
x

< 0.1 : properties still under debate [1].

Magnetization measurements

Analysis of SC fluctuations above Tc



Precursor diamagnetism characterized by an upturn field (H
UP

≈ 200


300 Oe) in the isothermal fluctuative contributions.



The behaviour of H
UP

vs. T (see inset) is reminescent of the
anomalous diamagnetic contributions (superimposed to the
conventional Ginzburg
-
Landau terms) already observed in
underdoped samples of cuprate materials.



Dashed lines (fitting curves): hypothesis that small stable
superconducting droplets lacking of phase coherence appear
already above T
c
.

Estimate of the bulk T
c



x

=

0
.
15

:

broad

superconducting

(SC)

transition
.

Low

contribution

of

magnetic

impurities
.



x

=

0
.
2

:

sharper

SC

transition
.

The

effect

of

magnetic

impurities

causes

the

difference

between

zero
-
field

cooled

(ZFC)

and

field
-
cooled

(FC)

curves

above

T
c
.

19
F
-
NMR linewidth, shift and relaxation rate

NMR linewidth




Increase at T < T
c

due to both dynamical 1/T
2

processes and

spatial

modulation of the magnetic field as
induced by the fluxoids lattice
.



Fluxoids contribution possibly implies motional narrowing effects due to the liquid phase of the vortices
lattice [3].



Direct comparison with the
μ
+
SR linewidth
σ

in the x = 0.2 sample [1]: unexplained huge difference of
values. Further examinations are planned in order to investigate the details of such a discrepancy.

Nuclear spin
-
lattice relaxation rate




1/T
1

is unaffected by the transition to the SC phase, being
possibly entirely driven by the low
-
energy fluctuations of the
magnetic moments associated with the Sm
3+

ions.



1/T
1

vs. T is fairly well described by a power
-
law over the
entire explored range independently of the sample and of the
magnetic field.

The picture Sm
3+

moments decoupled from the Fermi sea is
not

satisfactory.

Interpretation of the results:



4
f

electrons of the rare
-
earth ions are weakly delocalized and
develop AF correlations.



Moriya’s theory of magnetic relaxation in weakly itinerant
metals is valid.



The magnetic correlation length follows the


proportionality


Similar trends have already been observed in some heavy
fermions systems characterized by antiferromagnetic ground
states, like CeCu
6
-
x
Au
x
[4].

Shift in resonance frequency




Negative

paramagnetic

shift

Δ
K

strictly

obeing

a

Curie
-
Weiss
-
like

trend
.



Direct

comparison

with

susceptibility

data
:

value

of

the

scalar

hyperfine

coupling

A
hyp

between

the

magnetic

moments

associated

with

Sm
3
+

ions

and

the

NMR

probes,

namely

the

19
F

nuclei
.

References


[1] S. Sanna
et al
., Phys. Rev. B
80

052503
(2009)


[2] A. Lascialfari
et al
., Phys. Rev. B
65

144523
(2002)


[3] P. Carretta
et al
., Phys. Rev. Lett.
68

1236
(1992)


[4] P. Carretta
et al
., Phys. Rev. B
68

220404
(2003)