Magnetic properties and electronic structure of LaFeAsO₀.₈₅F₀.₁
The magnetic properties of LaFeAsO₀.₈₅F₀.₁ compound were investigated by measurements of dc magnetization at different values of the magnetic field, H = 0.02, 1.0 and 2.0 T, in the temperature range 4.2–300 K. The superconducting behavior was found below 26 K, whereas a distinct peculiarity in the l...
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irk-123456789-1169662017-05-19T03:03:12Z Magnetic properties and electronic structure of LaFeAsO₀.₈₅F₀.₁ Fedorchenko, A.V. Grechnev, G.E. Desnenko, V.A. Panfilov, A.S. Volkova, O.S. Vasiliev, A.N. Низкотемпеpатуpный магнетизм The magnetic properties of LaFeAsO₀.₈₅F₀.₁ compound were investigated by measurements of dc magnetization at different values of the magnetic field, H = 0.02, 1.0 and 2.0 T, in the temperature range 4.2–300 K. The superconducting behavior was found below 26 K, whereas a distinct peculiarity in the low-field dependence of the magnetic susceptibility χ(T) was clearly observed at TM ≈ 135 K, which resembles a weak ferromagnetic (FM) response with saturation magnetic moment of about 10⁻⁴ μB per formula unit at 50 K. The transition at TM is presumably not governed by magnetic impurities but rather correlated with the antiferromagnetic (AFM) transition in the undoped LaFeAsO compound at about the same temperature. We suggest, that observed magnetic properties of the LaFeAsO₀.₈₅F₀.₁ sample are due to an interplay of FM and AFM transitions, and presumably related to an intrinsic feature of a small portion of the undoped LaFeAsO phase inherent in our sample. In order to shed light on the problem of magnetic instabilty of the LaFeAsO, the ab initio DFT calculations of electronic structure and paramagnetic susceptibility were performed within the local spin density approximation. It is shown, that a V-shaped peculiarity in the density of electronic states in a close vicinity of the Fermi level can govern magnetic properties of LaFeAsO under fluorine doping and/or oxygen deficiency. 2010 Article Magnetic properties and electronic structure of LaFeAsO₀.₈₅F₀.₁ / A.V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, A.S. Panfilov, O.S. Volkova, A.N. Vasiliev // Физика низких температур. — 2010. — Т. 36, № 3. — С. 292-298. — Бібліогр.: 39 назв. — англ. 0132-6414 PACS: 71.20.–b, 74.25.Ha, 74.25.Jb, 75.30.Cr http://dspace.nbuv.gov.ua/handle/123456789/116966 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм |
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Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм Fedorchenko, A.V. Grechnev, G.E. Desnenko, V.A. Panfilov, A.S. Volkova, O.S. Vasiliev, A.N. Magnetic properties and electronic structure of LaFeAsO₀.₈₅F₀.₁ Физика низких температур |
| description |
The magnetic properties of LaFeAsO₀.₈₅F₀.₁ compound were investigated by measurements of dc magnetization at different values of the magnetic field, H = 0.02, 1.0 and 2.0 T, in the temperature range 4.2–300 K. The superconducting behavior was found below 26 K, whereas a distinct peculiarity in the low-field dependence of the magnetic susceptibility χ(T) was clearly observed at TM ≈ 135 K, which resembles a weak ferromagnetic (FM) response with saturation magnetic moment of about 10⁻⁴ μB per formula unit at 50 K. The transition at TM is presumably not governed by magnetic impurities but rather correlated with the antiferromagnetic (AFM) transition in the undoped LaFeAsO compound at about the same temperature. We suggest, that observed magnetic properties of the LaFeAsO₀.₈₅F₀.₁ sample are due to an interplay of FM and AFM transitions, and presumably related to an intrinsic feature of a small portion of the undoped LaFeAsO phase inherent in our sample. In order to shed light on the problem of magnetic instabilty of the LaFeAsO, the ab initio DFT calculations of electronic structure and paramagnetic susceptibility were performed within the local spin density approximation. It is shown, that a V-shaped peculiarity in the density of electronic states in a close vicinity of the Fermi level can govern magnetic properties of LaFeAsO under fluorine doping and/or oxygen deficiency. |
| format |
Article |
| author |
Fedorchenko, A.V. Grechnev, G.E. Desnenko, V.A. Panfilov, A.S. Volkova, O.S. Vasiliev, A.N. |
| author_facet |
Fedorchenko, A.V. Grechnev, G.E. Desnenko, V.A. Panfilov, A.S. Volkova, O.S. Vasiliev, A.N. |
| author_sort |
Fedorchenko, A.V. |
| title |
Magnetic properties and electronic structure of LaFeAsO₀.₈₅F₀.₁ |
| title_short |
Magnetic properties and electronic structure of LaFeAsO₀.₈₅F₀.₁ |
| title_full |
Magnetic properties and electronic structure of LaFeAsO₀.₈₅F₀.₁ |
| title_fullStr |
Magnetic properties and electronic structure of LaFeAsO₀.₈₅F₀.₁ |
| title_full_unstemmed |
Magnetic properties and electronic structure of LaFeAsO₀.₈₅F₀.₁ |
| title_sort |
magnetic properties and electronic structure of lafeaso₀.₈₅f₀.₁ |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2010 |
| topic_facet |
Низкотемпеpатуpный магнетизм |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/116966 |
| citation_txt |
Magnetic properties and electronic structure of LaFeAsO₀.₈₅F₀.₁ / A.V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, A.S. Panfilov, O.S. Volkova, A.N. Vasiliev // Физика низких температур. — 2010. — Т. 36, № 3. — С. 292-298. — Бібліогр.: 39 назв. — англ. |
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Физика низких температур |
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2025-07-08T11:23:33Z |
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| fulltext |
© A.V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, A.S. Panfilov, O.S. Volkova, and A.N. Vasiliev, 2010
Fizika Nizkikh Temperatur, 2010, v. 36, No. 3, p. 292–298
Magnetic properties and electronic structure
of LaFeAsO0.85F0.1
A.V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, and A.S. Panfilov
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine
47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: grechnev@ilt.kharkov.ua
O.S. Volkova and A.N. Vasiliev
Department of Low Temperature Physics and Superconductivity, Physics Faculty,
M.V. Lomonosov Moscow State University, Moscow 119899, Russia
Received October 30, 2009
The magnetic properties of LaFeAsO0.85F0.1 compound were investigated by measurements of dc magne-
tization at different values of the magnetic field, H = 0.02, 1.0 and 2.0 T, in the temperature range 4.2–300 K.
The superconducting behavior was found below 26 K, whereas a distinct peculiarity in the low-field dependence
of the magnetic susceptibility χ(T) was clearly observed at TM 135 K, which resembles a weak ferromagnetic
(FM) response with saturation magnetic moment of about 10–4 μB per formula unit at 50 K. The transition at TM
is presumably not governed by magnetic impurities but rather correlated with the antiferromagnetic (AFM)
transition in the undoped LaFeAsO compound at about the same temperature. We suggest, that observed
magnetic properties of the LaFeAsO0.85F0.1 sample are due to an interplay of FM and AFM transitions, and
presumably related to an intrinsic feature of a small portion of the undoped LaFeAsO phase inherent in our
sample. In order to shed light on the problem of magnetic instability of the LaFeAsO, the ab initio DFT
calculations of electronic structure and paramagnetic susceptibility were performed within the local spin density
approximation. It is shown, that a V-shaped peculiarity in the density of electronic states in a close vicinity of the
Fermi level can govern magnetic properties of LaFeAsO under fluorine doping and/or oxygen deficiency.
PACS: 71.20.–b Electron density of states and band structure of crystalline solids;
74.25.Ha Magnetic properties including vortex structures and related phenomena;
74.25.Jb Electronic structure (photoemission, etc.);
75.30.Cr Saturation moments and magnetic susceptibilities.
Keywords: FeAs superconductors, magnetic susceptibility, electronic structure.
1. Introduction
The recent report on superconductivity with 26cT K
in LaFeAsO1–xFx [1] has opened a new class of the quar-
ternary «1111» iron-based layered superconductors and
stimulated intensive studies of their physical properties. A
large increase of the superconducting transition tempera-
ture in LaFeAsO1–xFx was later observed under high
pressure (up to 43 K at 4P∼ GPa [2]) and with substitution
of rare earths for La (the highest 55cT K was reported
for SmFeAsO1–xFx [3]). The comprehensive reviews of ex-
perimental and theoretical reports on properties of the iron-
pnictide superconductors were recently published (see,
e.g., Refs. 4–7).
The parent undoped compound LaFeAsO is not
superconducting, but shows a number of peculiarities of its
physical properties (e.g., electrical resistivity, magnetic
susceptibility, specific heat, etc. [1,8–10]), which are caus-
ed by a structural tetragonal–orthorhombic transition at
160ST ≈ K and antiferromagnetic (AFM) spin ordering at
140NT ≈ K [8,11–13]. Upon electron or hole doping with
F substitution at the O site [1,9,10,12–14], or with oxygen
vacancies [15–17] both transformations are suppressed and
superconductivity emerges that suggests a relationship bet-
ween magnetic (and structural) properties and supercon-
ductivity. The electron–phonon interaction is estimated to
be small in the pure and F-doped LaFeAsO to provide the
conventional superconductivity, and there is growing evi-
dence that proximity to magnetic instability plays an essen-
tial role in the nature of superconductivity in these com-
pounds [4–7]. It has been suggested that superconductivity
Magnetic properties and electronic structure of LaFeAsO0.85F0.1
Fizika Nizkikh Temperatur, 2010, v. 36, No. 3 293
is driven by spin-fluctuations due to strong AFM [18,19]
or FM [20] instability in LaFeAsO and related compounds.
However, a nature of the complex interplay between
magnetism and superconductivity in RFeAsO is still not
well characterized and understood. Also, the recent expe-
rimental data on behavior of magnetic susceptibility of
«1111» systems in the normal state are still incomplete and
contradicting [9,12,20,21]. Therefore, further studies of the
magnetic and superconducting properties and their evo-
lution with doping, pressure, temperature, etc. can help to
elucidate a mechanism of the high- cT superconductivity in
the new class of the Fe-based superconductors.
In this contribution we report the experimental results
on the magnetic susceptibility in the normal state for the
superconductor LaFeAsO0.85F0.1, which is fluorine-doped
but contains about 0.05 of vacancies in the oxygen sub-
lattice. The present experimental study is supplemented by
the ab initio DFT calculations of the electronic structure
and magnetic susceptibility for a number of LaFeAsO
based systems in the normal state within the local spin
density approximation (LSDA). The aim of this inves-
tigation is to shed more light on the relation between the
chemical composition, magnetic properties, and the inter-
play between superconductivity and magnetic instability in
the «1111» systems.
2. Experimental details and results
Polycrystalline sample of LaFeAsO0.85F0.1 was prepar-
ed using a two-step solid state reaction technique, similar
to that described in Ref. 22. The crystal structure and the
composition were investigated by powder x-ray diffraction
and wavelength-dispersive x-ray spectroscopy. The basic
phase content was determined to be about 97%.
Magnetic properties of LaFeAsO0.85F0.1 were studied
by an in-house SQUID magnetometer in the temperature
range 4.2–300 K. The sample of mass 0.3∼ g had the
cylindrical form with diameter of 5 mm and height of
2.5 mm. The temperature dependence of the magnetic
susceptibility ( )Tχ was measured under cooling to helium
temperature in zero magnetic field (ZFC) followed by a
slow heating with the rate of about 1.5 K/min at applied
magnetic fields of =H 0.02, 1.0 and 2.0 T. The ( )Tχ
dependence was also measured by cooling in the magnetic
field = 0.02H T (FC). As can be seen in Fig. 1, the
studied sample shows the superconducting transition at
temperature 26cT K, which is typical for the fluorine
doped LaFeAsO systems.
For the normal state, the ( )Tχ dependence, measured
in 1H ≥ T, appears to be relatively flat and roughly
described by the mean value of 45 10avχ −⋅∼ emu/mol.
This value is comparable with the recently published data
for magnetic susceptibility of the F-doped LaFeAsO at
room temperature [9,20]. It should be noted, however, that
the temperature dependencies of χ for the F-doped
LaFeAsO are essentially different between the above two
reports. The data obtained in Ref. 20 provide an increasing
χ with decreasing temperature, whereas that reported in
Ref. 9 show a gradual decrease of χ with decreasing tem-
perature, as in the undoped LaFeAsO compound. The latter
( )Tχ behavior is consistent with the temperature depen-
dence of the 75As-Knight shift in LaFeAsO:F [23], whereas
the former dependence (increasing χ with decreasing tem-
perature [20]) is considered to be due to impurity phases
which amount to a few wt.% [12]. One can note an
intermediate character of the relatively flat ( )Tχ depen-
dence for our sample measured in high magnetic fields (see
Fig. 1).
As is obvious from Fig. 1, the peculiar features of the
low-field magnetic susceptibility of LaFeAsO0.85F0.1 is a
clear cusp at 135T K with a sharp rise of χ with cool-
ing from 135MT K to 26cT K. Based on the mag-
Fig. 1. Temperature dependencies of dc magnetic susceptibility
of LaFeAsO0.85F0.1 at different magnetic fields: = 2H T (Δ );
1 T ( ); 0.02 T (ZFC — , FC — ).
H = 1.0 T
H = 2.0 T
H = 0.002 T [ZFC]
H = 0.002 T [FC]
T 135 KM
�
16
12
8
4
0
–4
�
,
1
0
em
u
/m
o
l
–
4
0 50 100 150 200 250 300
T, K
Fig. 2. Magnetization versus field for LaFeAsO0.85F0.1 at = 50T K.
M
,
em
u
/m
o
l
12
10
8
6
4
2
0 0.5 1.0 1.5 2.0
H, T
A.V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, A.S. Panfilov, O.S. Volkova, and A.N. Vasiliev
294 Fizika Nizkikh Temperatur, 2010, v. 36, No. 3
netization data in Fig. 2, we can assume the emergence
of weak FM response at .MT T≤ The corresponding
saturation moment at = 50T K amounts to approximately
1 emu/mol or 41.5 10 Bμ
−⋅∼ /f.u., being about four orders
of magnitude smaller than the magnetic moment at Fe site
for the undoped LaFeAsO in the AFM state ( 0.36 Bμ∼
[11]). A similar nonlinear behavior of ( )M H in the low-
field region, and the same order of the ferromagnetic
moments were reported in Refs. 12, 21, that were con-
sidered to be caused by impurity phases. Typically, in the
F-doped LaFeAsO samples one can expect the formation
of such AFM iron-bearing secondary phases as FeAs, FeF2
(both with 78NT ≤ K) and Fe2As ( 350NT ∼ K) [12,21,
24,25]. Consequently, the observed FM transition at
135MT K is presumably not affected by the above
secondary magnetic phases. On the other hand, the FM
transition evidently correlates with a point of the AFM
transition in the undoped LaFeAsO compound [7,11].
3. Computational details and results
The undoped parent LaFeAsO compound, as well as the
LaFeAsO1–xFx and LaFeAsO1–y systems, possess the
tetragonal ZrCuSiAs-type crystal structure (space group
4 /P nmm ) [1,2]. With the /c a ratio close to 2.16, the
tetragonal structure of LaFeAsO exhibits strong two-
dimensional features. The crystal lattice is composed by
alternating La–O and Fe–As triple-layer slabs, which are
stacked along the c axis. Each iron (oxygen) layer is
sandwiched between two nearest-neighbor As (La) atomic
layers, which form edge-shared tetrahedrons around the
iron (oxygen) sites. The positions of As or La sheets are
fixed by internal parameters Asz and Laz , which define
the inter-layer distances of Fe–As and La–O, respectively.
It is suggested that there is partial ionic bonding between
Fe–As and La–O layers, which is increased upon fluorine
doping [4,6].
The previous ab initio calculations of the electronic
structure of the «1111»-type iron-based oxypnictides were
predominantly related to studies of the ground state AFM
ordering in the undoped «1111» compounds (see
[18,19,26–28] and references therein). In this paper the
detailed calculations of the electronic structure are carried
out for the paramagnetic phase of LaFeAsO based systems
with the aim to study a paramagnetic response in an
external magnetic field, and to shed more light on the na-
ture of paramagnetism and interplay between super-
conductivity and magnetic instability in «1111» systems.
In the present ab initio calculations of the electronic
structure we employed a full-potential all-electron relati-
vistic linear muffin-tin orbital method (FP-LMTO, code
RSPt [29,30]). No shape approximations were imposed on
the charge density or potential, what is especially
important for the anisotropic layered crystal structures. The
exchange-correlation potential was treated in the LSDA
[31] and generalized gradient (GGA, [32]) approximations
of the density functional theory (DFT). The effect of the
spin–orbit coupling, included in the Hamiltonian, appeared
to be important for the precise calculations of atomic
volume dependent density of states (DOS) at the Fermi
level FE . The FP-LMTO calculations were carried out for
the LaFeAsO compound and also for LaFeAsO with
oxygen replaced by virtual atoms with Z = 7.9 and Z =
= 8.1 in order to simulate doping.
The calculations were performed for a number of lattice
parameters a close to the experimental ones [1,5], with
parameters /c a , Asz and Laz fixed to their experimental
ambient pressure values. The total energies E were cal-
culated as a function of volume V, and were fitted to the
Murnaghan analytical parametrization for the equation of
state [29]. Then the theoretical lattice spacings and the
bulk moduli B were determined from calculated volume
dependencies of the total energy ( )E V with both LSDA
[31] and GGA [32] exchange-correlation potentials. The
theoretical lattice parameter a appeared to be close to the
experimental one [1,2,5] (about 1% smaller for the LSDA
potential). The estimated bulk moduli of LaFeAsO,
LSDAB 1.3 Mbar and GGAB 1 Mbar, are in agreement
with the earlier reported theoretical value of theorB =
= 0.98 Mbar [33].
According to our calculations, in the vicinity of the Fer-
mi level FE the electronic structure of LaFeAsO is gover-
ned by a moderate hybridization of d (Fe) and p (As)
states within the Fe–As layers. The Fe d states provide
the dominant contribution to the density of states (DOS)
( )N E extending between –2 eV and 2 eV around = 0FE ,
where the partial contributions of other states are
substantially smaller. The p states of As are predominant-
ly extended in the interstitial region, and there is no clear
separation of the 3d states of Fe into the ge and 2gt ma-
nifolds in the crystal field of As tetrahedron. In fact,
a pseudogap at 0.3 eV above FE corresponds to the
occupation 6d of Fe2+. These calculated basic features of
electronic structure and DOS( E ) of LaFeAsO are in a
qualitative agreement with results of earlier calculations
[18,26,28].
As is seen in Fig. 3, in LaFeAsO the Fermi level FE
lies at the steep slope of N(E), rapidly decreasing with
energy. We found that FE is located very close to the V-
shaped local minimum of N(E), which is also seen in
Fig. 3. This V-shaped feature of DOS is related to a Fermi
surface driven instability, reported in Refs. 18, 27, 28,
which forces the undoped LaFeAsO into the stripe AFM
state.
The calculated ( )N E for LaFeAsO with oxygen re-
placed by virtual atoms with Z = 7.9 and Z = 8.1 are also
presented in Fig. 3. Within the virtual crystal approx-
imation (VCA) the true atom in the phase is replaced by an
«average» atom which is interpolated linearly in charge
between the corresponding pure atoms. Hence, the VCA
Magnetic properties and electronic structure of LaFeAsO0.85F0.1
Fizika Nizkikh Temperatur, 2010, v. 36, No. 3 295
calculations have taken into account the hybridization and
band-filling effects, but neglected randomness and have
not properly taken into account the charge transfer effect.
The chosen approximation has an advantage due to its
simplicity and hence we can be able to study small
concentrations of F in the oxygen sublattice. The scattering
effect of disorder is expected to be weak, since oxygen
ions do not participate in the states at FE . For all studied
systems one can see a peculiar ( )N E behavior, with the V-
shaped feature at the Fermi level. Also, the evaluated
volume derivatives of ( )FN E were found to be positive
(d ln ( ) / d ln 0.9FN E V for the undoped LaFeAsO), and
this suggests the reduction of ( )FN E with pressure for
the LaFeAsO1–xFx systems.
Calculated magnetic properties
The FP-LMTO-LSDA calculations of the field-induced
spin and orbital (Van Vleck) magnetic moments were
carried out self-consistently within the procedure described
in Ref. 30 by means of the Zeeman operator,
ˆˆ= (2 ),Z BH H s l+μ (1)
which was incorporated in the original FP-LMTO Hamil-
tonian. Here H is the external magnetic field, ŝ and l̂ the
spin and orbital angular momentum operators, respec-
tively. The field induced spin and orbital magnetic mo-
ments were calculated in the external field of 10 T and
provided estimation of the related contributions to the
magnetic susceptibility, spinχ and orbχ .
For the tetragonal crystal structure of LaFeAsO, the
components of the paramagnetic contributions spinχ and
orbχ were derived from the magnetic moments obtained
in an external field, applied parallel and perpendicular
to the c axis. The evaluated magnetic anisotropy, which
is determined by the orbital contribution, orbχΔ =
orb orbχ χ ⊥= − , was found to be negligible, in comparison
with the dominant spinχ
contribution. The orbital Van
Vleck contribution orbχ itself is at least an order of mag-
nitude smaller than the strongly enhanced spin suscept-
ibility, and comes almost exclusively from the d states of
Fe. For the theoretical LSDA equilibrium volume the
contributions spinχ and orbχ are found to be equal
to 5.45·10–4 emu/mol and 0.46 410−⋅ emu/mol, respecti-
vely. The Langevin diamagnetism of filled shells,
4
dia 0.54 10χ −− ⋅ emu/mol, was estimated according to
Ref. 35, and assumed to be close to a free-ionic dia-
magnetic susceptibility.
Due to the close proximity of the induced spin-po-
larized state to the spontaneous magnetic ordering for the
experimental lattice parameters, the calculated total
magnetic moment rises to 0.2 Bμ , which in fact is close
to the experimentally observed magnetic moment of
0.36 Bμ at Fe site in the AFM state [11]. This is however
not an induced magnetization, since it turns out that FM
state with quite small moment is actually more stable than
the paramagnetic solution. Actually, as it is seen in Fig. 4,
Fig. 3. LSDA density of states of the paramagnetic LaFeAsO
around FE for the undoped compound (solid line), and for
LaFeAsO with oxygen replaced by virtual atoms with Z = 7.9
(dashed-doted line) and Z = 8.1 (dashed line). Calculations are
performed with the fine energy mesh of 0.001 eV. The Fermi
level position (at 0 eV) is marked by a vertical line.
2.5
2.0
1.5
N
(E
),
eV
–
1
–0.1 –0.05 0 0.05
E, eV
EF
Fig. 4. Magnetic moment induced in the field = 10H T as a
function of volume with lattice parameters /c a , Asz and Laz
fixed to their experimental ambient pressure values for undoped
LaFeAsO. Dashed vertical lines denote the calculated LSDA
equilibrium volume, the experimental volume and the calculated
GGA equilibrium volume of LaFeAsO (from left to right).
0.10
0.05
60 65 70
V, �
3
M
,
�
B
0
A.V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, A.S. Panfilov, O.S. Volkova, and A.N. Vasiliev
296 Fizika Nizkikh Temperatur, 2010, v. 36, No. 3
the spontaneous spin polarization occurs in the field-
induced calculations at lattice parameters above LSDAa ,
whereas for LSDAa a≤ (approximately 2% lower than the
experimental value), a paramagnetic response was
obtained.
The enhanced Pauli spin contribution to the magnetic
susceptibility was also calculated within the Stoner model:
2 1
ston = ( )[1 ( )] ,P B F FS N E IN Eχ χ μ −≡ − (2)
where Pχ = 2 ( ),B FN Eμ S is the Stoner enhancement fac-
tor, and Bμ the Bohr magneton. The multi-band Stoner
integral I, representing the exchange-correlation inter-
actions for conduction electrons and appropriate for com-
pounds, can be expressed in terms of the calculated para-
meters of the electronic structure [34]:
2= 1 / ( ) ( ) ( ).F ql F qll ql F
qll
I N E N E J N E′ ′
′
∑ (3)
Here ( )FN E and ( )ql FN E are the total density of
electronic states and site, q, and angular momentum, l ,
projected DOS at the Fermi level, respectively. The para-
meters of the exchange interaction qllJ ′ are defined in
terms of the intra-atomic exchange integrals:
2 2= ( ( )) ( ) ( ) d ,qll ql qlJ g r r r rρ φ φ′ ′∫ (4)
and therefore depend upon the corresponding partial wave
functions φl(r). Here ( ( ))g rρ is a function of the electron
density [31], l and l ′ are the corresponding angular-mo-
mentum quantum numbers.
The calculated value of the enhanced Pauli suscepti-
bility ( 4
ston 5 10χ −⋅∼ emu/mol for expLSDAa a a≤ ≤ ) is
close to the field-induced evaluated spinχ for the same
range of lattice parameters. The Stoner criterion is nearly
fulfilled for LaFeAsO, and the calculated susceptibility
enhancement factor S appeares to be about 8, which is
comparable with the earlier estimation ( 6,S [26]). This
means nearness to a quantum critical point in the pure
LaFeAsO compound and a possibility of competition
between FM and AFM spin fluctuations.
4. Discussion
To our best knowledge, no clear anomalies were ob-
served at 135T K in ( )Tχ dependencies of the super-
conducting doped LaFeAsO1–xFx systems, similar to that
found in the present study (see Fig. 1). The disappearance
of the static AFM order in the doped «1111» super-
conducting materials is widely reported [4–7], and it is
believed to be favorable for the unconventional super-
conductivity [18,19].
One may speculate that the studied sample contains a
fraction of the undoped (or weakly doped) parent phase,
wherein a strong interplay of FM and different AFM spin
fluctuations can take place. Firstly, the Fe–As–Fe positions
angle is larger than 90° and provides a possibility of AFM
superexchange interaction (see, e.g., [36]) via the As p
orbitals. Furthermore, a substantial direct Fe–Fe overlap in
the Fe planes can yield an additional AFM exchange [26].
Finally, the calculated band structure indicates a possibility
of the nesting-related AFM spin-density-wave type
[18,19,28].
The calculated for LSDAa a≤ volume-dependent field-
induced moments allow to describe the experimental sus-
ceptibility of LaFeAsO in the paramagnetic region at tem-
peratures above 140 K. The LaFeAsO compound is found
to be on the verge of magnetic instability. The proximity to
a quantum critical point is clearly seen in Fig. 4, and this
nearness results in strong FM spin fluctuations [26] and
can provide a weak FM ordering in the doped phase of the
LaFeAsO0.85F0.1 sample.
On the other hand, it seems probable that magnetization
found at low temperatures is due to a weak ferromagnetism
(or canted antiferromagnetism) of Dzyaloshinsky–Moriya
(DM) type [37,38]. In this case the DM antisymmetric
exchange interaction can provide incomplete cancellation
of the magnetic moments at Fe sites for the AFM
LaFeAsO phase. Also, a presence of defects in the iron
sublattice, as well as crystal structure distortions, can not
be ruled out. Then the environments of the Fe sites are dif-
ferent, and this can be assumed as another origin of the
observed weak FM. It has been recently shown [39], that
the combined effects of spin orbit coupling, lattice distor-
tion, and p–d hybridization in tetrahedrally coordinated Fe
in LaFeAsO can provide a highly anisotropic magnetic
moment with an in-plane value of 0.25–0.35 Bμ per Fe
with a small z projection 0.05z BM μ . Curiously, the
total moment of about –310 Bμ was roughly estimated for
the ground AFM state of LaFeAsO [12] from the
calculated not fully equivalent densities of spin-up and
spin-down electrons for 16-atom supercell. However, this
result may be rather due to incomplete convergence of the
DFT calculations of Ref. 12 for the AFM phase.
The present FP-LMTO calculations revealed a peculiar
non-monotonous behavior of the density of states in the
immediate vicinity of FE . As seen in Fig. 3, for the
undoped LaFeAsO the V-shaped minimum of ( )N E is
followed by a local maximum of DOS, about 0.05 eV
above FE . Our calculations for LaFeAsO with oxygen
replaced by virtual atoms with Z = 7.9 and Z = 8.1 also
provided similar behavior of ( )N E (see Fig. 3). Obvious-
ly, the VCA calculations are unable to reproduce the
relative positions of FE and the fine ( )N E features with
resolution better than 0.05 eV, which is basically the
accuracy of LSDA calculations. However, one can expect
that with the up-shifting of the Fermi level upon F doping
in LaFeAsO1–xFx, the density of states ( )FN E slightly
bounces up due to the local DOS maximum, but then
eventually descends down with increased fluorine con-
Magnetic properties and electronic structure of LaFeAsO0.85F0.1
Fizika Nizkikh Temperatur, 2010, v. 36, No. 3 297
tent x . Note, that the local DOS peak in Fig. 3 can be
«smeared» substantially due to the scattering effect of
disorder.
We presume, that within the Stoner model (2) such
( , )FN E x behavior can affect the spin paramagnetic sus-
ceptibility and can explain qualitatively the reported expe-
rimental ( )Tχ and ( )xχ dependencies in LaFeAsO1–xFx
[9,12,20,21]. In particular, it has been shown in Ref. 12,
that the paramagnetic susceptibility in the normal conduct-
ing state of LaFeAsO1–xFx is enhanced with the F-doping,
showing a maximum around F content of 5%. Also, within
the Stoner model (2), the observed weak ( )Tχ behavior
for LaFeAsO0.85F0.1 in the range 140–300 K (see Fig. 1) is
qualitatively consistent with the calculated non-monoto-
nous ( )N E dependence in Fig. 3, provided the FE is vari-
ed slightly in the range about 0.05 eV, which is well within
the accuracy of ab initio calculations.
A detailed theoretical study of ( )Tχ and ( )xχ in
LaFeAsO1–xFx merits a separate examination beyond the
scope of this study. It would obviously require a more
rigorous calculation of ( )N E in the alloys, presumably
employing CPA (coherent potential approximation) me-
thod. Also, the thermal expansion and spin fluctuation
effects have to be taken into account. Finally, according to
the present calculations, the density of states ( )FN E in
LaFeAsO1–xFx is found to be decreasing with pressure, as
well as the enhanced Pauli susceptibility.
5. Conclusions
Magnetic susceptibility of the LaFeAsO0.85F0.1 compo-
und was investigated in the temperature range 4.2–300 K,
and the superconducting transition is detected at 26 K.
A clear cusp at 135MT K was observed in the low-field
dependence of magnetic susceptibility ( )Tχ with a sharp
rise of χ with further cooling to 26cT K. The obtained
magnetization data ( )M H indicate the emergence of FM
phase at MT T≤ .
Ab initio calculations of the electronic structure and
paramagnetic contributions to susceptibility of the parent
LaFeAsO compound have revealed that this system is in
close proximity to the quantum critical point. It is shown,
that the paramagnetic susceptibility calculated in external
magnetic field appeares to be close to the experimental
value. The Van Vleck contribution to χ comes mainly
from d electrons of Fe and amounts up to 10% of total
susceptibility.
The V-shaped minimum and local maximum are reveal-
ed in the density of electronic states in the immediate vici-
nity of the Fermi level, which can govern magnetic pro-
perties of LaFeAsO0.85F0.1 under fluorine doping and/or
oxygen deficiency. Our calculations for LaFeAsO1–xFx
indicate, that the main role of pressure (and F-doping
0.1x ) is to reduce the density of states at the Fermi
level and to shift the «1111» system away from the FM
instability. In general, the reported results point out that
itinerant magnetism theory is relevant to describe magnetic
properties of the LaFeAsO based systems.
The authors thank Drs. A. Beznosov and A. Grechnev
for fruitful discussions and comments.
This work has been supported by the Russian-Ukrainian
RFBR-NASU project 8-2009.
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