Cr on f raco f - Mössbauer Spectroscopy Division

Crraco
ow Collloq
quiu
um
on ff electtron
n sy
yste
ems
s
t
Crracow
w, 24tth- 27th
Jun
ne 20
015
CCF
FES20
015
Pro
ogra
am
a
and
d
abs
stra
acts
P-06
Cracow
w Colloquium
m on f-electroon systems, 24-27
2
June 20
015, Cracow, Poland
CCFES2015
5
P-06
Maggnetic spirral order in iron pn
nictides FeAs
F
and F
FeSb
A
A. K. Jasek1, K. Komęd
dera1, A. Błłachowski1, K. Rueben
nbauer1, J. Ż
Żukrowski2,33,
Z.
Z Bukowskki4, A. Krzto
oń-Maziopaa5
1
Mössbauerr Spectrosco
opy Divisionn, Institute of
o Physics, Pedagogica
P
al Universityy,
ul. Pod
dchorążych 22, PL-30-08
84 Kraków, Poland
2
A
AGH Univeersity of Scieence and Teechnology, Academic
A
Center
C
for M
Materials an
nd
Nanottechnology, Av. A. Mickkiewicza 30
0, PL-30-059
9 Kraków, P
Poland
3
AGH
GH Universitty of Sciencce and Technnology, Facculty of Phyysics and Appplied Comp
puter
Sciennce, Departm
ment of Soliid State Phyysics, Av. A. Mickiewicz
cza 30,
PL-30-0059 Kraków
w, Poland
4
Innstitute of Low Temperature and SStructure Reesearch, Polish Academ
my of Sciencces,
ul. Okólna
O
2, PL
L-50-422 Wrocław,
W
Po
oland
5
Warsaw University
U
off Technologgy, Faculty of
o Chemistrry, ul. Noakkowskiego 3,
PL-00-6664 Warsaw
w, Poland
IIron pnictiddes FeAs an
nd FeSb exhhibit very complex
c
maagnetic ordeer pattern du
ue to the
competiition betweeen super-ex
xchange coovalent coup
pling and ittinerant maagnetism. Generally
G
one obbserves antti-ferromagn
netic orderr in the form
f
of in
ncommnsurrate planarr spirals
propagaating along one of thee crystalloggraphic axess. There are two spiraals differing
g by the
shape of the spiral in the planee perpendicuular to the propagation
p
axis.
F
FeAs crystallizes with
hin Pnma ssymmetry group.
g
It was
w found tthat the spiin spiral
propagaating along the c-axis leads to thee complex variation of the hyperrfine magneetic field
amplituude with thee spin orienttation varyiing in the a-b
a plane. The magnituude of the hyperfine
h
field poointing in the direction of the locall magnetic moment
m
dep
pends on thhe orientatio
on of this
momentt in the a-b plane. Pattterns are vaastly different for iron located in tthe [0 k 0] positions
p
and for iron in the [0 k  12 0] positions within the orthorhom
mbic cell. L
Lattice softeens upon
transitioon to the paaramagnetic state at 69..2 K primarrily in the a--c plane as seen by iron atoms.
This efffect is quiite large considering lack of th
he structuraal transitionn. Two prreviously
mentionned iron siites are disscernible inn the param
magnetic
region ttill 300 K by
b different electron ddensities on the iron
nu
uclei. The anisotropy
y of the
iro
on vibrationns developeed at the
traansition too the param
magnetic
staate
increeases
with
the
tem
mperature in accordan
nce with
the harmonnic approx
ximation,
alb
beit tends tto saturation
n at high
tem
mperatures indicating gradual
on
nset of thhe quasi-h
harmonic
co
onditions [11]. Adjacen
nt figures
sh
how unit celll of FeAs and
a spiral
sh
hapes.
T
The Fex+1Sbb compound
d has been synthesized
d close to
stoichioometry withh x = 0.023(8). basicallyy it conform
ms to the
P63/mm
mc symmetryy group witth some inteerstitial iron
n present.
The antti-ferromagnnetic ordering temperrature was found as
232 K i.e. much higher thaan for the less stoich
hiometric
materiall. Regular iron was found
f
to occcupy two different
P-06
Cracow
w Colloquium
m on f-electroon systems, 24-27
2
June 20
015, Cracow, Poland
CCFES2015
5
P-06
positionns in proporrtion 2:1. Th
hey differ byy the electriic quadrupo
ole couplingg constants and
a both
of them
m exhibit exttremely anissotropic eleectric field gradient
g
ten
nsor (EFG) w
with the asy
ymmetry
parametter   1 . The
T negativ
ve componeent of both EFGs is aligned
a
withh the c-axiis of the
hexagonnal unit celll, while the positive coomponent is aligned with the <1200> direction
n. Hence,
a modell describingg deviation from the P
P63/mmc sym
mmetry gro
oup within Fe-planes has
h been
proposeed. Spectra in the magn
netically orddered state could be ex
xplained byy introductio
on of the
incomm
mensurate sppin spirals propagating
p
g through th
he iron atom
ms in the dirrection of th
he c-axis
with a complex pattern
p
of the hyperffine magneetic fields distributed within a-b
b plane.
Hyperfiine magnetiic field patteern of spiraals due to major
m
regulaar iron is sm
moothed by the spin
polarizeed itinerant electrons, while the minor regu
ular iron ex
xhibits hypperfine field
d pattern
characteeristic of thhe highly covalent boonds to thee adjacent antimony atoms. Thee excess
interstittial iron ordders magnetically at thee same temp
perature as the regularr iron, and magnetic
m
momentts of these atoms
a
are liikely to form
m two-dimeensional spiin glass with
th moments lying in
the a-b plane. The upturn of the
t hyperfinne field for minor regu
ular iron annd interstitiaal iron is
observeed below 80
8 K. Magn
neto-elastic effects arre smaller than for FFeAs, howeever the
recoillesss fraction increases
i
significantly upon transiition to the magneticallly ordered state
s
[2].
Figures below shoow unit celll of FeSb with the proposed
p
deeformation of the iron
n planes,
magnetiic spiral shaapes and corrrespondingg average hy
yperfine field distributiion.
This projject was finanncially supporrted by the N
National Scien
nce Center off Poland undeer the Grant No.
N DEC2011/03/B
/B/ST3/00446.
Referencees
[1] A. Błachowski, K. Ruebenbauer, J. Żukrowskki, and Z. Buk
kowski, J. Allo
oys Comp. 5882, 167 (2014)).
Komędera, A. K. Jasek, A.. Blachowski, K. Ruebenbauer, and A. Krztoń-Mazziopa, arXiv:1
1406.7767
[2] K. K
(20144).
Email oof the presennting authorr: [email protected]
om