Magnetic excitations of Fe1 + ySexTe1 − x in magnetic and superconductive phases
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2010 J. Phys.: Condens. Matter 22 142202
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JOURNAL OF PHYSICS: CONDENSED MATTER
J. Phys.: Condens. Matter 22 (2010) 142202 (6pp)doi:10.1088/0953-8984/22/14/142202
FAST TRACK COMMUNICATION
Magnetic excitations of Fe1+ySexTe1−xin
magnetic and superconductive phases
P Babkevich1,2, M Bendele3,4, A T Boothroyd1, K Conder5,
S N Gvasaliya2, R Khasanov3, E Pomjakushina5and B Roessli2
1Department of Physics, Clarendon Laboratory, Oxford University, Oxford OX1 3PU, UK
2Laboratory for Neutron Scattering, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
3Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, CH-5232 Villigen PSI,
4Physik-Institut der Universit¨ at Z¨ urich, Winterthurerstrasse 190, CH-8057 Z¨ urich,
5Laboratory for Developments and Methods, Paul Scherrer Institut, CH-5232 Villigen PSI,
Received 15 February 2010, in final form 1 March 2010
Published 19 March 2010
Online at stacks.iop.org/JPhysCM/22/142202
We have used inelastic neutron scattering and muon-spin rotation to compare the low energy
magnetic excitations in single crystals of superconducting Fe1.01Se0.50Te0.50and
non-superconducting Fe1.10Se0.25Te0.75. We confirm the existence of a spin resonance in the
superconducting phase of Fe1.01Se0.50Te0.50, at an energy of 7 meV and a wavevector of
(1/2,1/2,0). The non-superconducting sample exhibits two incommensurate magnetic
excitations at (1/2,1/2,0) ± (0.18,−0.18,0) which rise steeply in energy, but no resonance is
observed at low energies. A strongly dispersive low energy magnetic excitation is also observed
in Fe1.10Se0.25Te0.75close to the commensurate antiferromagnetic ordering wavevector
(1/2 − δ,0,1/2), where δ ≈ 0.03. The magnetic correlations in both samples are found to be
quasi-two-dimensional in character and persist well above the magnetic (Fe1.10Se0.25Te0.75) and
superconducting (Fe1.01Se0.50Te0.50) transition temperatures.
(Some figures in this article are in colour only in the electronic version)
Considerable effort has been devoted over the past two
years to investigating the basic properties of the Fe-based
family of superconductors [1–3].
whether magnetism plays an important role in the formation
of the superconducting state. A useful strategy for tackling
this problem is combining neutron scattering and muon-
spin rotation (μSR) measurements on one and the same
sample. Neutron scattering provides information on magnetic
correlations and on the nature of the magnetic excitations,
while μSR can determine whether static magnetic order and/or
bulk superconductivity exists.
The Fe1+ySexTe1−x system is a convenient one to study
with this methodology as large high quality crystals can be
A central question is
grown [4, 5] and the tetragonal crystallographic structure is
relatively simple to analyse and model. Single crystals are
easier to grow if there is a small excess of Fe (i.e. y > 0),
especially for small x [6–9].
The pure FeSe compound is a superconductor with a
transition temperature Tc∼ 8 K . The Tccan be increased
by partial substitution of Te for Se such that Tc ∼ 14 K for
0.4 ? x ? 0.8 and y ≈ 0 [10, 11]. The application of pressure
has also been found to raise Tc, with values as high as 37 K
observed for FeSe [12–15]. Compounds with x ? 0.4 do not
exhibit bulk superconductivity but order magnetically below
a temperature which has a maximum of 67 K at x = 0 and
which decreases with x and vanishes at x ∼ 0.4. We recently
found evidence for coexistence of incommensurate magnetic
order and partial superconductivity for x ∼ 0.25 .
© 2010 IOP Publishing LtdPrinted in the UK & the USA
J. Phys.: Condens. Matter 22 (2010) 142202 Fast Track Communication
Figure 1. (a) Temperature dependence of the zero-field-cooled magnetization of Fe1.01Se0.50Te0.50and Fe1.10Se0.25Te0.75normalized to the
ideal 1/4π value. The onset Tonset
of the superconducting transition is determined from the intersection of straight lines fitted to the data
above and below the transition. (b) Representative ZF and TF μSR time spectra (upper plots) and temperature-dependentinitial TF
asymmetry of the slowly relaxing component (ATF
) and the mid-point (Tmid
and below the transition and as the point where the asymmetry decreases by a factor of two from its maximum value, respectively.
1, lower plot) for single crystals of Fe1.01Se0.50Te0.50and Fe1.10Se0.25Te0.75. The
N) of the magnetic transition are determined from the intersection of straight lines fitted to the data above
In this study, we present neutron scattering and μSR mea-
surements on two single-crystal samples: (i) Fe1.01Se0.50Te0.50,
a bulk superconductor, and (ii) Fe1.10Se0.25Te0.75, a non-
superconducting sample which exhibits magnetic order. For
superconducting Fe1.01Se0.50Te0.50 we observe a resonant
magnetic excitation consistent with that reported previously
for compounds with similar compositions [16–19].
Fe1.10Se0.25Te0.75we observe strongly dispersive, low energy
magnetic excitations associated with the magnetic ordering
wavevector (1/2 − δ,0,1/2), δ ≈ 0.03, and also with the
wavevectors (1/2,1/2,0) ± (0.18,−0.18,0).
evidence for a resonance peak in the excitation spectrum of
We find no
2. Experimental details
Single crystals of Fe1+ySexTe1−x were grown by a modified
Bridgman method as reported by .
measurements were carried out on the triple-axis spectrometer
TASP  at the SINQ  spallation source (Paul Scherrer
Institut, Switzerland). Bragg reflections from a pyrolytic
graphite PG(002) monochromator and analyser were used
at a fixed final wavevector of 2.66 ˚ A−1.
placed after the sample to reduce contamination from higher
order harmonics in the beam and the instrument was set up
in the open–open–open–open collimation with the analyser
focusing in the horizontal plane.
A PG filter was
The crystals were single
rods with masses of approximately 4 g. The Fe1.10Se0.25Te0.75
sample was orientated in two settings to give access to (h,0,l)
and (h,k,0) planes in reciprocal space. Measurements on
Fe1.01Se0.50Te0.50 were made in the (h,k,0) plane only. In
this report we index the reciprocal lattice with respect to the
primitive tetragonal unit cell described by the P4/nmm space
groupwith unitcell parameters a ≈ 3.8˚ A and c ≈ 6.1˚ A along
lines joining the nearest neighbour Fe atoms.
Zero-field-cooled magnetization measurements were per-
formed on a Quantum Design MPMS magnetometer with a
measuring field μ0H = 0.3 mT using the direct current
method. To reduce the effects of demagnetization, thin plate-
like pieces of Fe1+ySexTe1−x, cleaved from the main single
crystals, were oriented with the flat surface (ab plane) parallel
to the applied field.
Zero-field (ZF) and transverse-field (TF) muon-spin
rotation (μSR) experiments were performed on the πM3 beam
line at SμS (Paul Scherrer Institut, Switzerland).
experimentsa magneticfieldof 11.8mTwasappliedparallelto
the crystallographic ab plane of the crystal and perpendicular
to the muon-spin polarization.
3.1. Magnetization and μSR measurements
Zero-field-cooled magnetization data normalized to the ideal
1/4π value (Mnorm) are shown in figure 1(a). The
J. Phys.: Condens. Matter 22 (2010) 142202Fast Track Communication
Figure 2. Elastic neutron measurements of Fe1.10Se0.25Te0.75at the magnetic order propagation vector q = (0.47,0,0.5). (a) Map showing the
incommensurate peak at q in the (h,0,l) plane at 2 K. (b) Temperature dependence of the intensity at q. The inset shows scans along
(h,0,0.5) measured at 2, 20, 30 and 50 K. A sloping background function has been subtracted from the data and the dashed lines show a
Gaussian fit through the peaks. For clarity, the scans have been displaced vertically.
Fe1.01Se0.50Te0.50sample is seen to be a bulk superconductor
with the onset of the transition Tonset
−0.8 at T
exhibits superconductivity (Tonset
superconducting fraction of order 10% at low temperature.
For the x = 0.5 sample, the ZF time spectra measured
at T = 1.7 and 20 K are almost identical, thus suggesting
that the magnetic state of this sample is the same above
and below the superconducting transition temperature. The
solid lines correspond to a fit with the function AZF(t) =
is the initial asymmetry and ?ZFis
the exponential relaxation rate. Such behaviour is consistent
with dilute Fe moments as observed recently for FeSe1−x.
The TF data for x
ATF(t) = ATF
135.5 MHz T−1is the muon gyromagnetic ratio, φ is the
initial phase of the muon-spin ensemble, and σ is the Gaussian
relaxation rate.Figure 1(b) shows that the TF asymmetry
at 20 K is due to the formation of the vortex lattice at T < Tc.
Static (within the μSR time window) magnetism develops
in Fe1.10Se0.25Te0.75as signalled by a fast drop of both AZFand
ATFwithin the first 100 ns (see the upper panel of figure 1(b)).
The solid lines correspond to fits with AZF(t) = AZF
initial ZF (TF) asymmetry and the exponential depolarization
rate of the slow (fast) relaxing component, respectively. The
temperature evolutionof ATF
below 20 K magnetism occupies more than 95% of the whole
sample volume. The corresponding values of the onset and the
mid-point of the magnetic transitions, determined as shown in
the figure, are Tonset
? 33.7 K and Tmid
that althoughthe magnetic order is shown to extend throughout
virtually the entire volume of the sample, μSR cannot be used
to determine whether the magnetic order is long range. The
? 14.0 K and Mnorm?
2 K. The Fe1.10Se0.25Te0.75 sample also
? 8.6 K) but has a small
0e−?ZFt, where AZF
0.5 fit well to the function
0e−(?TFt+σ2t2)cos(γμBt + φ). Here, γμ/2π =
0is almost temperature independent. The slightly stronger
2tand ATF(t) = e−σ2t2/2[ATF
2tcos(γμB2t +φ)]. Here, AZF(TF)
1, showninfigure 1(b), reveals that
? 27.6 K. We note
neutron diffraction data presented in the next section show that
the magnetic order is in fact relatively short range.
3.2. Neutron scattering results
Elastic neutron scattering measurements on Fe1.10Se0.25Te0.75
in the (h,0,l) scattering plane at 2 K, as shown in figure 2(a),
reveal a diffuse magnetic peak centred on (1/2 − δ,0,1/2)
with δ ≈ 0.03. The incommensurate peak is much broader
than the resolution of the instrument. From Q scans through
the peak we obtain correlation lengths along the a and
c axes of 11.4(6) ˚ A and 7.5(4) ˚ A respectively at 2 K.
Figure 2(b) shows that the magnetic peak develops below
TN∼ 50 K. The correlation lengths did not change measurably
upon warming through the TN (figure 2(b): inset).
magnetic propagation vector q = (1/2 − δ,0,±1/2) is
similar to that found previously for the similar composition
confirmed that the peak described by q is magnetic in character
using neutron polarization analysis .
consistent with measurements on Fe1.07Se0.25Te0.75for which
the incommensurability is found to be δ ≈ 0.04 .
The magnetic scattering cross-section is directly pro-
portional to the magnetic response function S(Q, E)—the
Fourier transform of the space- and time-dependent spin–spin
correlation function. According to the fluctuation-dissipation
theorem, S(Q, E) is in turn related to the imaginary part of the
dynamical susceptibility χ??(Q, E) by 
For the latter compound we
Our results are
S(Q, E) =1
π[n(E,T) + 1]χ??(Q, E).
TheBose–Einsteinpopulationfactorn(E,T) = [exp(E/kBT)
−1]−1(where kBis the Boltzmannconstant) takes into account
the increase in scattering from bosonic excitations due to the
thermal population at temperatures T > 0. Correction for this
factor allows the temperature dependence of χ??(Q, E) to be
J. Phys.: Condens. Matter 22 (2010) 142202Fast Track Communication
Figure 3. Inelastic neutron scattering from Fe1.10Se0.25Te0.75in the vicinity of the magnetic ordering wavevector q = (0.47,0,0.5).
(a) Constant energy scans collected at 2, 4 and 6 meV and 2 K along (h,0,0.6). The data have been shifted in χ??(Q, E) by arbitrary amounts
for clarity. (b) Constant energy scans collected at 2 meV and the temperature of 2 K showing χ??(Q, E) along (h,0,0.5), (h,0,0.7) and
(h,0,0.9). The plots have been displaced and the dashed lines show Gaussian peaks through the spectra. (c) Constant energy scans at 2 meV
at temperatures of 2, 40, 150 and 300 K showing χ??(Q, E) along (0.5,0,l). Note that a linear background has been subtracted in all scans.
(d) Diagram of the (h,0,l) plane to show scan directions denoted by roman numerals.
Figure 3(a) shows background corrected scans along
the (h,0,0.6) direction at energy transfers of 2, 4 and
6 meV for the Fe1.10Se0.25Te0.75 crystal.
q is present in each scan, indicating a strongly dispersing
excitation. The broadening of the dispersion in Q may be due
to unresolvable splitting of the mode into two excitations at
higher energies. The measured magnetic response at 2 meV
parallel to (1,0,0) for l = 0.5, 0.7 and 0.9, as shown in
figure 3(b), reveals considerable broadening of χ??(Q, E) in
the out-of-plane direction. Such broadening is characteristic of
a quasi-two-dimensional system with weak interactions along
c. Figure 3(c) shows that spin fluctuations persist up to at least
150 K, well into the paramagnetic state. At 40 K, i.e. close
to the magnetic ordering temperature, χ??(Q, E) is almost the
same as at 2 K.
We now turn to the low energy excitation spectrum in
the vicinity of the wavevector (1/2,1/2,0).
and (b) show maps of χ??(Q, E) measured along (h,1 −
h,0) for Fe1.10Se0.25Te0.75 at 2 and 40 K. The fluctuations
measured at 2 K are consistent with the magnetic excitation
spectrum at higher energiesreported for Fe1.03Se0.27Te0.73.
The excitation spectrum at 2 K is characterized by steep
incommensurate branches arising from (1/2 ± ?,1/2 ∓ ?,0)
where ? ≈ 0.18. The incommensurate excitations are still
present at 40 K. The scans shown in figure 4(c) reveal that
at E = 7 meV, the system response is nearly the same at
2 K as at 40 K. The background corrected χ??(Q, E) for
the Fe1.10Se0.25Te0.75 sample does not appear to change for
energies in the 2–7 meV range measured at these temperatures.
This is also the case for measurements along (1/2,0,l) in
A peak at Q =
figure 3(c) that show χ??(Q, E) data at 2 meV to be similar
at 2 and 40 K.
The results obtained for Fe1.01Se0.50Te0.50 are in stark
contrasttothose forthe non-superconductingFe1.10Se0.25Te0.75
sample just described. Figures 4(d) and (e) show maps of
the magnetic spectrum as a function of wavevector along
(h,1 − h,0) for energies between 2 and 7 meV at 2 and
40 K. At 2 K we find a strong signal in χ??(Q, E) centred on
Q = (1/2,1/2,0) and E ∼ 7 meV. This feature corresponds
to the spin resonance reported previously in superconducting
FeSe0.4Te0.6, FeSe0.46Te0.54 and FeSe0.5Te0.5. At
higher energies, the excitations have been found to disperse
away from (1/2,1/2,0) along (1,−1,0) . However, it is
the low energy response of the system which shows the most
dramatic change on transition into the superconducting state,
as may be seen in figure 4(f). As the sample is cooled from 40
to 2 K, the integrated dynamical susceptibility of the peak at
7 meV increases by more than a factor of two and decreases in
width along (1,−1,0) by ∼30%. Fluctuations continue to be
observed well above Tc.
In combination with earlier measurements,
of Fe1+ySexTe1−xvary strongly with x. The magnetic spectra
of the magnetically ordered compound (x = 0.25) and the
bulk superconductor (x
= 0.5) both contain low energy
magnetic fluctuations in the vicinity of the antiferromagnetic
wavevector (1/2,1/2,0). However, at x
J. Phys.: Condens. Matter 22 (2010) 142202Fast Track Communication
Figure 4. Variation of χ??(Q, E) in the (h,1 − h,0) direction for energies between 2 and 7 meV at temperatures of 2 and 40 K. Data in
(a)–(c) are from Fe1.10Se0.25Te0.75and data in (d)–(f) are from Fe1.01Se0.50Te0.50. Constant energy cuts at 7 meV along (h,1 − h,0), measured
at 2 and 40 K for Fe1.10Se0.25Te0.75and Fe1.01Se0.50Te0.50are shown in (c) and (f), respectively. A flat background has been subtracted in all
scans and dashed lines through the data are fits with a Gaussian lineshape.
fluctuations are incommensurate with wavevector (1/2 ±
?,1/2 ∓ ?,0), ?
strongest magnetic signal is commensurate.
x = 0.5 the magnetic spectrum has a gap of ∼6 meV and
the size of the signal just above the gap increases strongly
at low temperatures. This behaviour is consistent with the
superconductivity-induced spin resonance reported recently in
bulk superconducting samples of Fe1+ySexTe1−x of similar
composition to ours [16–19], and also in related Fe pnictide
A further difference is that the x = 0.25 sample exhibits
short-range, static (within the μSR time window) magnetic
order with a characteristic wavevector q = (1/2−δ,0,±1/2),
δ ≈ 0.03, whereas according to our μSR data there is no
staticmagnetic order inthe bulk superconductor. The magnetic
ordering wavevector q found at x = 0.25 is the same as that
in the parent phase Fe1+yTe. The slight incommensurability is
thought to be caused by the small excess of Fe accommodated
ininterstitialsitesinthecrystal structure[8, 31,32], althoughit
is interesting that the incommensurability is the same to within
experimental error at y = 0.10 (the present sample) and at
y = 0.03 (the sample studied by us previously ).
Our results suggest that there are two distinct magnetic
ordering tendencies at x = 0.25, one with wavevector (1/2 −
δ,0,±1/2)andtheother withwavevector (1/2±?,1/2∓?,0).
The μSR data indicate thatthe volumefraction of magnetically
ordered phase is close to 100%, but we cannot say whether the
two characteristic magnetic correlations coexist on an atomic
scale or whether the sample is magnetically inhomogeneous.
Finally, we comment on the fact that for the x = 0.25
sample diffuse peaks are observed in the elastic (within energy
resolution) channel below T ≈ 50 K by means of neutron
0.18, whereas at x
scattering but static magnetic order is only detected below
T ≈ 35 K by μSR. These observations can be reconciled
by means of the difference in fluctuation rates observable by
using muons (∼GHz) and neutrons (∼THz) below which spin
freezing is measured. We infer from this that the characteristic
fluctuations of the spin system lie between ∼GHz and ∼THz
for 35 K ? T ? 50 K.Such a gradual slowing down
of the fluctuations could be a consequence of the quasi-two-
dimensional nature of the spin system, which is also indicated
by the persistence of spin correlations to temperatures well
above the ordered phase. It is also interesting that the size
of the magnetically ordered domains does not significantly
increase with decreasing temperature, which suggests that the
short-range order is never truly static but fluctuates down to the
lowest temperature investigated. This picture is consistent with
for 0.05 < x < 0.55 .
We have observed a resonance-like peak at the antiferromag-
netic wavevector (1/2,1/2,0) in the low energy magnetic
spectrum of Fe1.01Se0.50Te0.50, and shown that this feature is
absent from the magnetic spectrum of Fe1.10Se0.25Te0.75which
instead shows incommensurate peaks flanking (1/2,1/2,0).
Our results reveal a clear distinction between the magnetic
excitation spectra of Fe1+ySexTe1−xsamples which are mag-
netically ordered and those which are bulk superconductors.
We conclude that the existence of a resonance peak at the
commensurate antiferromagnetic wavevector is a characteristic
of bulk superconductivity in Fe1+ySexTe1−x.
J. Phys.: Condens. Matter 22 (2010) 142202Fast Track Communication
Thiswork wasperformed atthe PaulScherrer Institut,Villigen,
Switzerland. PB is grateful for the provision of a studentship
from the UK Engineering and Physical Sciences Research
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