Pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds
Pressure effects on electronic structure and magnetic properties of anisotropic metallic materials (CrB₂, FeGe₂, LaFeAsO) are investigated. Magnetic susceptibilities of CrB₂ and FeGe₂ compounds were studied under hydrostatic pressure at fixed temperatures, 77 and 300 K. In order to analyze the exper...
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Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України
2008
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| Cite this: | Pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds / G.E. Grechnev, A.S. Panfilov, A.V. Fedorchenko, I.V. Svechkarev, V.B. Filippov, A.B. Lyashchenko, A.N. Vasiliev // Физика и техника высоких давлений. — 2008. — Т. 18, № 4. — С. 112-118. — Бібліогр.: 19 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859608504567857152 |
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| author | Grechnev, G.E. Panfilov, A.S. Fedorchenko, A.V. Svechkarev, I.V. FIlippov, V.B. Lyashchenko, A.B. Vasiliev, A.N. |
| author_facet | Grechnev, G.E. Panfilov, A.S. Fedorchenko, A.V. Svechkarev, I.V. FIlippov, V.B. Lyashchenko, A.B. Vasiliev, A.N. |
| citation_txt | Pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds / G.E. Grechnev, A.S. Panfilov, A.V. Fedorchenko, I.V. Svechkarev, V.B. Filippov, A.B. Lyashchenko, A.N. Vasiliev // Физика и техника высоких давлений. — 2008. — Т. 18, № 4. — С. 112-118. — Бібліогр.: 19 назв. — англ. |
| collection | DSpace DC |
| container_title | Физика и техника высоких давлений |
| description | Pressure effects on electronic structure and magnetic properties of anisotropic metallic materials (CrB₂, FeGe₂, LaFeAsO) are investigated. Magnetic susceptibilities of CrB₂ and FeGe₂ compounds were studied under hydrostatic pressure at fixed temperatures, 77 and 300 K. In order to analyze the experimental magnetovolume effects, the electronic structures were calculated ab initio in external magnetic field as a function of atomic volume by employing a full-potential LMTO method. The calculated field-induced magnetic moments and their volume derivatives compare favorably with the experimental pressure effects in magnetic properties of CrB₂ and FeGe₂. The main role of pressure in high-Tc superconductivity of LaFeAsO is found to reduce density of states at the Fermi level and to suppress ferromagnetic spin fluctuations.
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Физика и техника высоких давлений 2008, том 18, № 4
112
PACS: 71.20.–b, 75.10.Lp, 75.20.En, 75.80.+q
G.E. Grechnev1, A.S. Panfilov1, A.V. Fedorchenko1, I.V. Svechkarev1,
V.B. Filippov2, A.B. Lyashchenko2, and A.N. Vasiliev3
PRESSURE EFFECTS ON ELECTRONIC STRUCTURE AND MAGNETIC
PROPERTIES OF ANISOTROPIC METALLIC COMPOUNDS
1B. Verkin Institute for Low Temperature Physics and Engineering, National Academy of
Sciences
47 Lenin Ave., Kharkov, 61103, Ukraine
2I. Frantsevich Institute for Problems of Materials Science, National Academy of Sciences
3 Krzhyzhanovsky Str., Kiev, 03680, Ukraine
3Department of Low Temperature Physics and Superconductivity, Physics Faculty
M.V. Lomonosov Moscow State University, Moscow, 119899, Russia
Pressure effects on electronic structure and magnetic properties of anisotropic metallic
materials (CrB2, FeGe2, LaFeAsO) are investigated. Magnetic susceptibilities of CrB2
and FeGe2 compounds were studied under hydrostatic pressure at fixed temperatures, 77
and 300 K. In order to analyze the experimental magnetovolume effects, the electronic
structures were calculated ab initio in external magnetic field as a function of atomic
volume by employing a full-potential LMTO method. The calculated field-induced mag-
netic moments and their volume derivatives compare favorably with the experimental
pressure effects in magnetic properties of CrB2 and FeGe2. The main role of pressure in
high-Tc superconductivity of LaFeAsO is found to reduce density of states at the Fermi
level and to suppress ferromagnetic spin fluctuations.
Introduction
The discovery of high-Tc superconductivity in MgB2 and ferropnictides
(RFeAsO, LiFeAs, AFe2As2), charge density waves in NbSe2, intriguing magnetic
properties of borides and germanides have initiated considerable interest in elec-
tronic structure and properties of compounds of transition metals with metalloids
and nonmetals. The properties of metalloids themselves are between metals and
nonmetals, and when involved in chemical bonding, the metalloids also exhibit
middle-of-the-road qualities. They are capable of taking electrons from most met-
als and will readily lose electrons to most nonmetals. Their electronegativity val-
ues are also mid-range. Consequently, it is unlikely for them to be involved in
ionic bonding, and it is believed they will establish covalent bonding when found
in compounds.
Физика и техника высоких давлений 2008, том 18, № 4
113
On the other hand, under high pressure a metallic behavior of such compounds
often becomes more pronounced, and such a pressure effect can provide drastic
changes in electronic and magnetic properties. In this contribution we report re-
sults of theoretical and experimental studies of pressure effects on electronic spec-
tra and magnetic properties of selected anisotropic compounds with outstanding
physical properties: CrB2, FeGe2 and LaFeAsO.
Experimental and theoretical details
For certification of the samples quality, the magnetic susceptibility χ was
measured as a function of temperature using a Faraday microbalance method. The
pressure effect on the susceptibility was measured under helium gas pressure up to
2 kbar at fixed temperatures, 77.3 and 300 K, using a pendulum-type magne-
tometer placed into the nonmagnetic pressure cell [1], and the experimental de-
pendencies χ(P) for CrB2 and FeGe2 clearly demonstrated linear behavior over the
range 0–2 kbar.
The polycrystalline sample of CrB2 was initially prepared by arc-melting of the
stoichiometric amounts of Cr and B elements of better than 99.8% purity in a wa-
ter cooled crucible under protective argon atmosphere. The study of X-ray powder
diffraction revealed that the sample has the AlB2-type hexagonal crystal structure,
and obtained data on the lattice parameters agree closely with that published in
literature. In this work we used the same single-crystalline sample of FeGe2 as in
the magnetostriction studies of Ref. [2]. The susceptibility measurements were
performed for magnetic fields in the basal plane and along [001] direction, and the
averaged χ values were compared with theoretical calculations.
Ab initio calculations of the volume dependent electronic structures were per-
formed by employing a modified full-potential relativistic LMTO method (FP-
LMTO) [3,4] for nonmagnetic, ferromagnetic (FM), and antiferromagnetic (AFM)
phases of studied compounds in the LSDA approximation of the density functional
theory. No shape approximations were imposed on the charge density or potential,
what is especially important for the open and anisotropic crystal structures.
To analyze the observed magnetovolume effect values, the magnetic suscepti-
bility and its volume dependence were evaluated by means of the Zeeman opera-
tor, ˆˆ(2 1)H s + , incorporated in FP-LMTO Hamiltonian for calculations of the
field-induced spin and orbital magnetic moments. The corresponding contributions
to χ were derived from the field-induced moments, which have been calculated in
an external magnetic field of 10 T.
CrB2
CrB2 is an itinerant-electron antiferromagnet with the Néel temperature TN =
= 85–88 K [5–7], possessing a hexagonal structure of AlB2 type. The neutron diffrac-
tion studies revealed a complicated helicoidal magnetic structure of CrB2, with mag-
netic moment of about 0.5μB per Cr atom (at T = 0) turning in a–c plane [6]. The sus-
Физика и техника высоких давлений 2008, том 18, № 4
114
ceptibility of CrB2 in the PM phase is an order of magnitude higher than that of other
diborides, presumably demonstrating a large exchange-enhancement effect.
The band structure calculations revealed that p–d hybridization and filling of the
conduction band are predominantly responsible for properties of transition metal
diborides. As is seen in Fig. 1,a, in CrB2 the Fermi level EF lies at the steep slope
of the N(E) peak where DOS rapidly grows with energy, and the main contribu-
tion to N(EF) comes mostly from d-states of Cr. The Stoner criterion is nearly ful-
filled for CrB2, and the calculated susceptibility enhancement factor S appeared to
be about 8, which is comparable with earlier estimations (S ~– 10, [8]).
a b
c
The magnetic susceptibility of CrB2 decreases linearly with pressure at both
temperatures, and the corresponding pressure derivatives, d ln / dPχ , are listed in
Table together with the values of χ measured at ambient pressure. In order to con-
vert the pressure derivative into the volume derivative, we used the calculated
bulk modulus value, B = 2.3 Mbar. A moderate value of the pressure effect is
Fig. 1. Density of electronic states for
paramagnetic CrB2 (a), FeGe2 (b) and
LaFeAsO (c) at ambient pressure
Физика и техника высоких давлений 2008, том 18, № 4
115
found to be negative in sign and weakly dependent on the magnetic state of the
compound. The main contributions to χ and their volume dependences are calcu-
lated ab initio, and appeared to be in close agreement with experimental data (see
Table). In addition, the measured pressure dependence of the Néel temperature,
dTN/dP = 0.1 ± 0.1 K/kbar, differs essentially from that for pure Cr where the
strong suppression of the AFM state under pressure with dTN/dP ~– –5.1 K/kbar
has been reported [9].
Table
Magnetovolume effects in CrB2, FeGe2 and LaFeAsO
χ, 10–3 emu/mole dlnχ/dP, Mbar–1 dlnχ/dlnV dlnN(EF)/dlnVCom-
pound T, K
exp theor (PM) exp exp theor (PM) theor (PM)
CrB2
77.3
300
0.642
0.511
–
0.7
–1.82 ± 0.3
–1.65 ± 0.2
4.2 ± 0.7
3.8 ± 0.5
–
4
–
1.5
FeGe2
77.3
300
2.7
1.7
–
2
–2.73 ± 0.3
–2.23 ± 0.3
4.6 ± 0.5
3.8 ± 0.5
–
4.5
–
1.6
LaFeAsO 30 0.5 [16] 0.6 – – 6 1.9
The measured pressure derivative of the susceptibility, dlnχ/dP, can be used to
evaluate the spontaneous volume change in CrB2 due to the AFM ordering:
2( )m
V C M T
V B
Δ
≡ ω = , (1)
where C is the magnetoelastic coupling constant, B – the bulk modulus, 2 ( )M T – the
squared local magnetic moment. According to the phenomenological relation [10]:
1 d ln
2 dm
C
B V P
χ
= −
χ
, (2)
where χ and Vm are the molar susceptibility and volume, respectively. Using in Eq. (2)
the experimental values from the Table and the calculated Vm = 13.8 cm3, we esti-
mated the magnetoelastic constant as: C/B = (1.1 ± 0.1)·10–10 (emu/mole)–2. The
substitution in Eq. (1) of the above C/B value and experimental value of the magnetic
moment, M(0) ~– 0.5μB/Cr [6], gives the volume change at T = 0 to be ωm(0) ~– 0.086%.
This estimate agrees closely with the experimental value of ωm(0) = 0.085% [11].
FeGe2
FeGe2 is an itinerant electron antiferromagnet of tetragonal C16-type crystal
structure that has a second-order transition from a PM phase to an incommensu-
rate (IC) phase at 289 K with a long-wavelength modulation in the basal plane,
and the other first-order transition from an IC to a commensurate AFM phase at
TM = 263 K [2,12]. Nearest-neighbour (NN) Fe atoms are along the c-axis and
their moments have FM alignment, whereas the next-NN Fe atoms are along the
[110] direction with AFM alignment of the moments below TM = 263 K [13].
Физика и техника высоких давлений 2008, том 18, № 4
116
According to our calculations, the electronic structure at EF is governed by a
strong hybridization of the 3d (Fe) and 4p (Ge) states, which leads to a peculiar
structure of DOS with the Fermi level located at the V-shaped minimum of N(E)
(see Fig. 1,b). FeGe2 has a Fermi surface driven instability which forces it into an
incommensurate spin density wave state [13,14]. The calculated bulk modulus,
Btheor = 1.7 Mbar, is in reasonable agreement with the reported experimental value,
Bexp = 1.5 Mbar [12].
The measured pressure effect on the TM temperature, dTM/dP = 0.03 ± 0.03
K/kbar was found to be weak (see Fig. 2). As is seen in the Table, the dlnχ/dP de-
rivatives appeared to be close both in PM and AFM phases of FeGe2. In order to
convert the pressure derivatives into the volume derivatives, we used the calcu-
lated bulk modulus, B = 1.7 Mbar. The spin and Van Vleck contributions to χ and
their volume dependences for the PM phase were calculated ab initio and were
found in agreement with the experimental data in the Table. It should be noted
that dlnχ/dV derivatives in the itinerant AFM compounds CrB2 and FeGe2 are
somewhat lower than the corresponding derivatives in the PM compounds with
highly enhanced spin susceptibility (CeCo2, YCo2, TiCo, Ni3Al [15]).
LaFeAsO
Reported superconductivity (SC) up to 54 K in RFeAsO (R = La, Pr, Nd, Sm,
Gd) [16,17] has raised a number of questions about the nature of SC state and the
pairing mechanism, and encouraged the studies of high-Tc superconductors out-
side the cuprate family. The tetragonal structure of RFeAsO exhibits strong two-
dimensional features, and SC is presumably confined to the Fe–As layers. Sim-
ple chemical considerations suggest that their formula can be represented as
R+3O–2Fe+2As–3, which is consistent with the observed semi-metallic properties
and spin-density waves in LaFeAsO below 150 K [17]. Actually, in RFeAsO the
high-Tc SC has been achieved with F-doping, and a competition is anticipated
between the spin ordering and SC [16,17].
Fig. 2. Temperature dependencies of the
magnetic susceptibility of FeGe2 around
TM = 263 K in magnetic field 17 kOe ap-
plied along [100] axis for two different
pressures: ● – P = 0, □ – P = 1.6 kbar.
The higher pressure data are shifted up-
wards to about 0.045·10–6 emu/g for
convenience of comparison
Физика и техника высоких давлений 2008, том 18, № 4
117
The band structure of LaFeAsO is determined by hybridization of d (Fe) and p
(As) states within the Fe–As layers. As is seen in Fig. 1,c, the Fermi level is lo-
cated at the steep slope of N(E). The DOS at EF comes essentially from the d-
electrons of Fe, and the partial contributions of other states are substantially
smaller. Also, the LaFeAsO compound is found to be on the verge of magnetic in-
stability, very close to the FM quantum critical point. Though the calculated Stoner
enhancement factor S ~– 5 provides a high spin magnetic susceptibility, it never-
theless can not describe fully the experimental value ( χ ~– 5·10–4 emu/mole, [17]).
On the other hand, the susceptibility calculated in external magnetic field ap-
peared to be very close to the experimental value at the theoretical lattice parame-
ter. The Van Vleck contribution to χ comes mainly from d-electrons of Fe and
amounts up to 10% of total susceptibility.
The experimental susceptibility in LaFeAsO compound is large with relatively
flat temperature dependence, whereas upon F-doping it grows further and be-
comes Curie–Weiss like above Tc [17]. Our FP-LMTO calculations for the F-doped
LaFeAsO, which were performed using the virtual crystal approximation, indicate
that the main function of doping is to shift the system away from FM instability.
This was proved by calculating the Stoner product IN(EF), where I is the exchange
parameter. In the same way, the total DOS N(EF) is also found to be decreasing
with pressure, as well as the enhanced Pauli susceptibility (see the Table).
Based on the experimental observations, a suggestion was put forward [17,18]
that SC state in LaFeAsO is favored by the AFM spin fluctuations (SF). There is a
growing evidence that electron-phonon interaction is too small in the pure and F-
doped LaFeAsO to provide the conventional SC [18]. It should be noted that in the
undoped LaFeAsO the calculated susceptibility is larger, and the experimental one
is smaller than in the doped systems. This suggests that besides FM spin fluctua-
tions there are also other important spin excitations in RFeAsO.
At the moment we can suggest that the spectrum and relative distribution of SF
in RFeAsO can arise from different sources. Firstly, the Fe–As–Fe positions angle
is larger than 90° and provides a possibility of AFM superexchange interaction via
the As p orbitals [19]. Furthermore, a substantial direct Fe–Fe overlap in Fe–As
planes can yield an additional AFM exchange. Also, the calculated band structure
indicates a possibility of the nesting-related AFM spin-density-wave type SF.
Further analysis of AFM interactions in RFeAsO requires a detailed study of the
magnetic response and fine band structure features at the Fermi level.
Conclusions
The pressure effect on the AFM ordering temperatures of CrB2 and FeGe2 is
found to be much weaker than in the itinerant AFM chromium. For both com-
pounds the pressure derivatives of χ have been measured for the first time. Based
on these dlnχ/dP derivatives, the estimate of the magnetoelastic coupling constant
describes properly the spontaneous volume change in CrB2 due to the AFM or-
dering.
Физика и техника высоких давлений 2008, том 18, № 4
118
It is found that the Stoner approach substantially underestimates the spin sus-
ceptibility for PM phases of CrB2, FeGe2 and LaFeAsO, whereas the calculated
field-induced spin and orbital moments allowed to describe the large χ and mag-
netovolume effects in these compounds, which are close to the quantum critical
point. This is presumably related to deficiency of the Stoner approach, when both
parameters involved in susceptibility enhancement, N(EF) and I, are calculated
and averaged over the band states separately. On the other hand, such response
function as χ is microscopically not uniform in space, and induced magnetization
density varies considerably within the unit cell.
Our calculations for LaFeAsO indicate that the main role of pressure (and F-
doping) is to shift the system away from the FM instability, which is expected to
be unfavorable for SC, and to suppress FM spin fluctuations.
This work has been supported by the Russian-Ukrainian RFBR-NASU project
8-2008.
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|
| id | nasplib_isofts_kiev_ua-123456789-70464 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0868-5924 |
| language | English |
| last_indexed | 2025-11-28T08:53:20Z |
| publishDate | 2008 |
| publisher | Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України |
| record_format | dspace |
| spelling | Grechnev, G.E. Panfilov, A.S. Fedorchenko, A.V. Svechkarev, I.V. FIlippov, V.B. Lyashchenko, A.B. Vasiliev, A.N. 2014-11-06T18:26:11Z 2014-11-06T18:26:11Z 2008 Pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds / G.E. Grechnev, A.S. Panfilov, A.V. Fedorchenko, I.V. Svechkarev, V.B. Filippov, A.B. Lyashchenko, A.N. Vasiliev // Физика и техника высоких давлений. — 2008. — Т. 18, № 4. — С. 112-118. — Бібліогр.: 19 назв. — англ. 0868-5924 PACS: 71.20.–b, 75.10.Lp, 75.20.En, 75.80.+q https://nasplib.isofts.kiev.ua/handle/123456789/70464 Pressure effects on electronic structure and magnetic properties of anisotropic metallic materials (CrB₂, FeGe₂, LaFeAsO) are investigated. Magnetic susceptibilities of CrB₂ and FeGe₂ compounds were studied under hydrostatic pressure at fixed temperatures, 77 and 300 K. In order to analyze the experimental magnetovolume effects, the electronic structures were calculated ab initio in external magnetic field as a function of atomic volume by employing a full-potential LMTO method. The calculated field-induced magnetic moments and their volume derivatives compare favorably with the experimental pressure effects in magnetic properties of CrB₂ and FeGe₂. The main role of pressure in high-Tc superconductivity of LaFeAsO is found to reduce density of states at the Fermi level and to suppress ferromagnetic spin fluctuations. This work has been supported by the Russian-Ukrainian RFBR-NASU project 8-2008. en Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України Физика и техника высоких давлений Pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds Article published earlier |
| spellingShingle | Pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds Grechnev, G.E. Panfilov, A.S. Fedorchenko, A.V. Svechkarev, I.V. FIlippov, V.B. Lyashchenko, A.B. Vasiliev, A.N. |
| title | Pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds |
| title_full | Pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds |
| title_fullStr | Pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds |
| title_full_unstemmed | Pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds |
| title_short | Pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds |
| title_sort | pressure effects on electronic structure and magnetic properties of anisotropic metallic compounds |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/70464 |
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