Two-dimensional magnetism in the pnictide superconductor parent material SrFeAsF probed by muon-spin relaxation

Article (PDF Available)inPhysical review. B, Condensed matter 79(6) · November 2008with13 Reads
DOI: 10.1103/PhysRevB.79.060402 · Source: arXiv
Abstract
We report muon-spin relaxation measurements on SrFeAsF, which is the parent compound of a newly discovered iron-arsenic-fluoride based series of superconducting materials. We find that this material has very similar magnetic properties to LaFeAsO, such as separated magnetic and structural transitions (TN = 120 K, Ts = 175 K), contrasting with SrFe2As2 where they are coincident. The muon oscillation frequencies fall away very sharply at TN, which suggests that the magnetic exchange between the layers is weaker than in comparable oxypnictide compounds. This is consistent with our specific heat measurements, which find that the entropy change S = 0.05 J/mol/K largely occurs at the structural transition and there is no anomaly at TN. Comment: 4 pages, 3 figures

Figures

arXiv:0811.4598v1 [cond-mat.supr-con] 27 Nov 2008
Two-dimensional magnetism in the pnictide superconductor parent material
SrFeAsF probed by muon-spin relaxation
P. J. Baker,
1
I. Franke,
1
T. Lancaster,
1
S. J. Blundell,
1
L. Kerslake,
2
and S. J. Clarke
2
1
Oxford University Department of Physics, Clarendon Laboratory,
Parks Road, Oxford OX1 3PU, United Kingdom
2
Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory,
South Parks Road, Oxford, OX1 3QR, United Kingdom
(Dated: November 27, 2008)
We rep ort muon-spin relaxation measurements on SrFeAsF, which is the parent compound of a
newly discovered iron-arsenic-fluoride based series of superconducting materials. We find that this
material has very similar magnetic properties to LaFeAsO, such as separated magnetic and structural
transitions (T
N
= 120 K, T
s
= 175 K), contrasting with SrFe
2
As
2
where they are coincident. The
muon oscillation frequencies fall away very sharply at T
N
, which suggests th at the magnetic exchange
between the layers is weaker than in comparable oxypnictide compounds. This is consistent with
our specific heat measurements, which find that the entropy change S = 0.05 Jmol
1
K
1
largely
occurs at the structural transition and there is no anomaly at T
N
.
PACS numbers: 76.75.+i, 74.10.+v, 75.30.Fv, 75.50.Ee
Quasi-two-dimensional magnets on square lattices are
the subject of considerable theoretical and experimental
attention.
1,2,3
This has primarily been due to the success
of models of the spin-1/2 Heisenberg antiferromagnet in
describing the physics of La
2
CuO
4
, which is the proto-
typical parent compound of high-T
c
cuprate supercon-
ductors. La
2
CuO
4
shows a tetragonal to orthorhombic
structural tra nsition at T
o
53 0 K and eel ordering at
T
N
325 K. That T
N
is far smaller than the antiferro-
magnetic exchange constant J 1500 K demo nstrates
that this compound has a remarkably large magnetic
anisotropy, with weak c oupling between the CuO
2
lay-
ers.
2,4
The magnetic parent c ompounds of FeAs-based su-
perconductors such as LaFeAsO
1x
F
x
5
have Fe atoms on
a layered square lattice, and it is interesting to note that,
like La
2
CuO
4
, these have a tetragonal to orthorhombic
structural distortion followed by antiferromagnetic o rder-
ing (e.g. Ref. 6). Here we study the magnetic proper -
ties of a newly discovered parent compound to a s e ries
of fluoropnictide superconductors, SrFeAsF,
7,8
where the
fluoride ions should provide weaker magnetic exchange
pathways between the FeAs layers than for LnFeAsO or
AFe
2
As
2
compounds.
Doped fluoropnictide compounds based on CaFeAsF
and SrFeAsF have recently been found to supercon-
duct,
9,10,11,12,13
with comparable transition temperatures
to the previously discovered oxypnictide compounds
based on LnFeAsO. These have similar FeAs layers to
the oxypnictides, but divalent metal - fluoride layers re-
place the rare-earth - oxide layers. Fluoropnictides c an
be doped on the Fe site, as for CaFe
0.9
Co
0.1
AsF (T
c
=
22 K),
9
or the divalent metal site, as for Sr
0.5
Sm
0.5
FeAsF
(T
c
= 56 K),
10
and several approaches have alr e ady b e e n
explored.
9,10,11,12,13
The magnetic, electronic, and struc-
tural properties o f the parent compounds CaFeAsF and
SrFeAsF, and also EuFeAsF, have alrea dy been investi-
gated. All three show transitions evident in resistivity
and dc magnetization measurements, at T
s
= 120 , 175,
and 155 K respectively.
7,8,9,12,13
In SrFeAsF the struc-
tural transition has been probed us ing X-ray diffraction,
and changes in the magnetism using ossbauer spec-
troscopy.
7
The structural change varies smoothly below
175 K w hereas the ossbauer spectra b e c ame increas-
ingly complicated as the temperature is reduced. It is
also interesting that the sign of the Hall coefficient R
H
in SrFeAsF is reported to be positive below T
s
, whereas
it is negative in the undop ed LaFeAsO and BaFe
2
As
2
parent compounds.
8
This could result from a different
electronic structure near the Fermi surface, which might
have implica tions for both the magnetism of the undoped
compound and the supe rconductivity that emerges when
it is dope d.
The magnetism of LnFeAsO compounds has already
been intensively investigated by a wide range of tech-
niques. Neutron diffraction measurements have been
carried out on some of the undoped oxypnictides:
LaFeAsO,
14
NdFeAsO,
15
PrFeAsO,
16
and CeFeAsO.
6
The results in each case indicate similar structural
transitions at around T
s
= 150 K, followed by long
range, three dimensional antiferro magnetic ordering of
the iron spins with s ignificantly reduced moments <
1 µ
B
/Fe at T
N
, around 20 K below T
s
, confirmed
by other techniques.
17,18
These features move to lower
temper ature with increasing doping and are absent in
the superconducting phase for LaFeAsO
1x
F
x
19
and
CeFeAsO
1x
F
x
,
6
although magnetism and superconduc-
tivity seem to coexist over a small doping range in
SmFe AsO
1x
F
x
.
20
In constrast, SrFe
2
As
2
has coincident
magnetic and structural ordering occurring in a first-
order phase transition at T
o
= 205 K.
21
It seems that
in general AFe
2
As
2
materials have more c losely related
structural and ma gnetic phase transitions, and more
three-dimensional magnetism than the single layer FeAs
materials. With the discovery of new fluoroarsenide par-
ent materials it is important to compare the magnetic
structures and the separation between T
s
and T
N
in the
2
oxide-arsenide and fluoro-arsenide materials. Here we ad-
dress these comparisons in SrFeAsF using the techniques
of muon-spin relaxation, which is a local probe of the
magnetic fields inside the sample and their dynamics,
and also specific heat measurements which examine the
changes in entropy at the transitions.
The SrFeAsF sample was synthesized in a two step
process similar to that described in Ref. 7. Stoichiomet-
ric quantities of sublimed strontium metal (Alfa 99.9 %),
strontium fluoride powder (Alfa 99.9 %), iron powder
(Alfa, 99.998 %), a nd arsenic pieces (Alfa, 99.9999 %;
ground into powder) were ground to gether and sealed in a
9 mm diameter niobium tube. This was heated at 1
/min
to 500
C and this temperature was maintained for 12
hours to ensure complete reaction of the volatile compo-
nents before heating at 1
/min to 900
C. After 40 hours
at 900
C the product was removed from the Nb tube,
ground to a fine powder, pressed into a pellet, and placed
into an alumina crucible which was then sealed in a pre-
dried e vacuated silica tube. This was heated at 1
C/min
to 1000
C for 48 hours and then coo led at the natural
rate of the furnace to room temperature. All manipula-
tion was carried out in an argon-filled glove box. Analy-
sis of the product by lab oratory X-ray p owder diffraction
(PANAlytical X-pert PRO) [Fig 1(a)] revealed that the
sample consisted of about 97 % by mass SrFeAsF; SrF
2
was identified as a crystalline impurity phase, but no
other crystalline binary or ternary impurity phases were
identified. The refined room temperature lattice parame-
ters of SrFeAsF were a = 4.00059(3)
˚
A, c = 8.9647(1)
˚
A,
V = 143.478(4)
˚
A
3
consistent with other repor ts.
7
Mea-
surement of the dc susce ptibility was carried out in a
Quantum Design MPMS5 instrument [Fig. 1(b)]. The
magnetization of the sample as a function of field at 300
K showed no significant level of ferromagnetic impurity.
Measurements as a function of temperature in an applied
field of 1000 Oe revealed very similar behaviour to that
reported pr e viously.
7
A broad feature at around 175 K is
consistent with the closely associated a ntiferromagnetic
ordering and structural phase transitions which occ ur in
related compounds.
14,22,23
Heat capac ity measure ments
were carried out using a Quantum Design Physical Prop-
erties Measurement System (PPMS) using a sta ndard re -
laxation time approach. A small part of the sample used
for µSR measurements was attached to the sample plat-
form using Apiezon N-grease. Measurements were cor-
rected for the heat capacity of the sample platform and
grease. Muon-spin rotation (µSR) experiments
24
were
performed using the General Purpose Surface-Muon In-
strument (GPS) at the Swiss Muon Source (Paul Scherrer
Institute, Switzerland). The measured parameter is the
time-dependent muon decay asymmetry, A(t), recorded
in positron detectors on opposite sides of the sample.
Our sample was a press e d powder pellet o f 1 cm diam-
eter mounted inside a silver packet on a silver backing
plate. This arrangement gives a time and temperature
independent ba ckground to the signal which is straight-
forward to subtract.
C T
T
C
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FIG. 1: (Color online.) (a) R ietveld refinement against pow-
der X-ray diffraction data, χ
2
= 1.98, wRp = 0.044. The inset
shows the structure. (b) DC magnetization measurements vs.
temperature in a field of 0.1 T. The small feature at about 50K
is probably due to a small amount of adsorbed O
2
apparent
because of the small sample moment. ( Inset) Magnetization
vs. field at 300 K. (c) Heat capacity C(T ) showing the peak
at T
s
which is highlighted in the inset. The line shows the
lattice heat capacity fit discussed in the text.
The heat capacity measurements shown in Figure 1(c)
show a clear feature at the structura l transition and no
anomalies or e ffects due to la tent heat were evident at
any other temperatures. Our data are in good agreement
with those reported on this compound by Tegel et al.
7
.
To separate the lattice and magnetic contributions to the
heat capacity, we estimated the lattice ba ckground using
the function:
C(T ) = γT + A
D
C
D
(T, θ
D
) + A
E
C
E
(T, θ
E
), (1)
where γ is the Sommerfeld coefficient, and C
D
and C
E
are
Debye and Einstein terms respectively. This was found
3
t
A t
FIG. 2: (Color online.) Muon asymmetry data for
SrFeAsF showing the spin precession signal evident at low-
temperature, t he greater damping of the oscillations close to
T
N
, and the paramagnetic signal at 140 K. The data are fitted
to Eq. 2 with the parameters shown in Figure 3.
to be an effective model fo r oxypnictides in Ref. 25. The
parameters extracted from this fit (excluding data be-
tween 100 and 18 5 K) were γ = 3.44(7) mJmol
1
K
2
,
A
D
= 56.6(5) Jmol
1
K
1
, θ
D
= 237(1) K, A
E
=
52.2(4) Jmol
1
K
1
, and θ
E
= 407(3) K. These are com-
parable with the values determined for oxypnictide ma-
terials without rare-earth magnetic moments.
17,25
The
magnetic contribution is plotted in the inse t to Fig-
ure 1(c) showing that zero-field and 10 T measurements
were effectively identical, and the integrated mag netic
entropy is 0.5 Jmol
1
K
1
. While this is a small en-
tropy change, it is twice the value observed in LaFeAsO,
where features at b oth the structural and magnetic tran-
sitions are evident.
17
SrFe
2
As
2
has a far larger entropy
change at the combined first-or der structural and mag-
netic transition, 1 Jmol
1
K
1
. The majority of the
entropy change in Sr FeAsF occurs c lose to T
s
= 175 K,
but it appears that another much broader feature at lower
temper ature also contributes. Since we find long-range
magnetic ordering at T
N
= 120 K using µSR (described
below), it seems that the bro ad feature is likely to have
a magnetic origin. The lack of a distinct anomaly in the
sp e c ific heat (o r in magnetiza tion or resistivity data),
7
suggests that the re sidue below the structural transition
comes from the build up of 2D correlations within the
FeAs planes. Gaining a rough estimate of the in-plane
exchange constant J 250 300 K from the position of
the hump, and knowing T
N
= 120 K, we can estimate
the out-of-plane exchange constant J
0.05J, consis-
tent with the lack of any observed anomaly at T
N
.
3,4
This is a similar situation to that in La
2
CuO
4
,
26
though
with a lower anisotropy in the ex change constants and a
smaller separatio n between the structural and magnetic
transitions.
In Figure 2 we present muon decay asymmetry data
at temperatures of 10, 116, and 140 K. At low temper-
atures, up to around 75 K, two oscillations are c le arly
T
T
FIG. 3: (Color online.) Parameters extracted from tting
raw asymmetry data using Eq. 2 described in the text. (a)
Oscillation frequencies ν
1
and ν
2
with lines drawn showing the
power law function described in the t ext. It is n oticeable that
the sharp drop-off in the frequencies near to the transition is
poorly described by this function. (b) Linewidths of the two
oscillating components, λ
1
and λ
2
.
resolved but as we approach T
N
the broadening of each
of the oscillations grows until they are both overdamped.
This overdamped behavior is seen in the 116 K data
set. Immediately above the magnetic ordering transition
the muon decay asymmetry takes the exponential form
exp ected for a paramagnet with electronic fluctuations
faster than the characteristic time of the measurement.
The data set at 140 K shown in Figure 2 is very similar
to all those taken above the magnetic ordering tempera-
ture, and we saw no change in the relaxation signal when
passing through T
s
= 175 K.
Observing two precession frequencies in the magneti-
cally ordered phase and finding that the tempe rature de-
pendent relaxation is Gaussian (suggesting that the fluc-
tuations of electr onic moments are motionally narrowed),
we were able to descr ibe the raw asymmetry data using
the fitting function:
A(t) =
1,2
X
i
A
i
e
λ
i
t
cos(2πν
i
t)+A
3
e
σ
2
t
2
+A
bg
e
Λt
. (2 )
The first two terms describe two damped oscillatio ns , the
third term describes the Gaussian relaxation for muon
spins with their direction along that of the local field
at their stopping site, which are depolarized by a ran-
dom distribution of nuclear moments, and the final term
describes the weak temperature-independent depolariza-
tion observed for muons stopping outside the sample.
Above the magnetic ordering transition there is no oscil-
4
latory signal and we set A
1
= A
2
= 0. In many fluorine
containing magnets a characteristic signal due to the for-
mation of a bound state between a pos itive muon a nd
one or more fluoride ions is observed above the magnetic
ordering transition.
27,28
No such signal is observed in Sr-
FeAsF, probably because the mag netic ordering transi-
tion is at too high a temperature for the muons to be
sufficiently well bound.
The parameters der ived from fitting Eq. 2 to the raw
data are shown in Figure 3. The two precession frequen-
cies plotted in Figure 3(a) are well defined and at low-
temper ature appe ar to follow a conventional power law.
Fitting the upper precession fr equency to the function
ν(T ) = ν(0)(1 (T/T
N
)
α
)
β
leads to T
N
= 120.6(3) K,
α = 3.1(3), and β = 0.20(2). This is a much sharper
magnetic transition than in LaFeAsO
29,30
and this would
suggest that the magnetism is more two-dimensional in
this fluoropnictide. Also, β is between the values ex-
pected for 2D Ising and 2D XY order parameters, though
the sharp drop in the frequencies near to T
N
may mean
that this fitting function is less effective in estimating
the tr ue critical parameters. The higher precession fre-
quency tends to ν
1
(0) = 22.22(5) MHz and, assuming the
same power law, the lower precession frequency tends
to ν
2
(0) = 1.9(1) MHz. These frequencies are a little
lower than in LaFeAsO
29,30
but in a similar proportion.
This suggests the magnetic structure is very similar to
LaFeAsO and the ordered Fe moments µ
Fe
0.3 µ
B
.
14,29
Seeing the lower frequency signal persis ting all the way
to the magnetic or dering transition as Carlo et al.
30
did
in LaFeAsO suggests that this mino rity oscillation sig-
nal is intrinsic to the sa mple, and reflects the antifer-
romagnetic structure being sampled at a different site
within the structure. It had previously been suggested
that the some ma gnetic signals in these pnictide mate-
rials o riginated in FeAs impurities (e.g. Ref. 17) but we
can dis c ount this possibility for our SrFeAsF sample on
the basis of µSR measurements on FeAs and FeAs
2
, both
of which give significantly different signals.
31
In the or-
dered phase the higher frequency oscillation accounts for
about 85 % of the oscillating amplitude. This ampli-
tude ratio for the two oscillating components is similar
to the situation in LaFeAsO, as is the lower frequency
signal becoming overdamped close to the magnetic or-
dering transition.
29,30
The linewidths λ
1
and λ
2
[shown
in Figure 3(b)] are both much smaller than the respective
precession frequencies a t low temperatures, giving rise to
the clear osc illations seen in the 10 K data in Figure 2,
and then grow towards the ordering transition giving the
overdamped oscillations seen in the 116 K data.
Our results have shown that lo ng range d, three-
dimensional antiferromagnetic ordering in SrFeAsF oc-
curs, but with a greater separation between the structural
and magnetic ordering transitions (T
s
T
N
50 K) than
in c omparable oxypnictide compounds (e.g. LaFeAsO).
While the µSR measurements show that the magnetic en-
vironment within the FeAs planes is very similar to that
in oxypnictide compounds, we note that the magnetic
ordering transition is not as clear in the magnetization
and heat capacity measurements. The heat capacity and
µSR measurements, in particular the lack of a heat c a-
pacity anomaly at T
N
and the low va lue of β = 0.2, both
suggest far more two-dimensional magnetic interactions
than in oxypnictide compounds, consistent with the in-
creased separation T
s
T
N
. This is also consistent with
the expectation that the interplanar exchange mediated
by a fluoride layer will be weaker than that mediated by
an oxide layer.
Part of this work was performed at the Swiss Muon
Source, Paul Scherrer Institute, Villigen, CH. We are
grateful to Alex Amato for experimental assistance and
to the EPSRC (UK) for financial support.
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