Antiferromagnetic bipolar semiconductor LaMnPO with ZrCuSiAs-type structure

Abstract

Electronic and magnetic properties of a layered compound LaMnPO are examined in relation to a newly discovered iso-structural superconductor LaFeAs(P)O. Neutron diffraction measurements, together with temperature dependent magnetic susceptibility, clarify that LaMnPO is an antiferromagnet at least up to 375 K. The spin moment of a Mn ion is determined to be 2.26 μ_B at room temperature, and the spin configuration is antiparallel in the Mn–P plane and parallel between the Mn–P planes, which is rather different from that of LaFeAsO. Optical absorption spectra, photoemission spectra, and temperature dependent electrical conductivity indicate that LaMnPO is a semiconductor. Furthermore, nominally undoped LaMnPO exhibits n -type conduction while the conduction type is changed by doping of Cu or Ca to the La sites, indicating that LaMnPO is a bipolar conductor. Density functional calculation using the GGA + U approximation supports the above conclusions; the electronic band structure has an open band gap and the antiferromagnetic spin configuration is more stable than the ferromagnetic one.

Full text

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Kyushu Institute of Technology Academic Repository
九州工業大学学術機関リポジトリ九州工業大学学術機関リポジトリ
Title
Antiferromagnetic bipolar semiconductor LaMnPO with
ZrCuSiAs-type structure
Author(s)
Yanagi, Hiroshi; Watanabe, Takumi; Kodama, Katsuaki;
Iikubo, Satoshi; Shamoto, Shin-ichi; Kamiya, Toshio; Hirano,
Masahiro; Hosono, Hideo
Issue Date 2009-05-07T00:00:00Z
URL http://hdl.handle.net/10228/2881
Rights
Copyright © 2009 American Institute of Physics. This article
may be downloaded for personal use only. Any other use
requires prior permission of the author and the American
Institute of Physics.

Page 2

Antiferromagnetic bipolar semiconductor LaMnPO with ZrCuSiAs-type
structure
Hiroshi Yanagi,1,a?Takumi Watanabe,1Katsuaki Kodama,2Satoshi Iikubo,2,b?
Shin-ichi Shamoto,2Toshio Kamiya,1,3Masahiro Hirano,3,4and Hideo Hosono1,3,4
1Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama
226-8530, Japan
2Japan Atomic Energy Agency (JAEA), 2-4 Shirane Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195,
Japan
3ERATO-SORST, JST, in Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama 226-8530, Japan
4Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8530,
Japan
?Received 30 January 2009; accepted 26 March 2009; published online 7 May 2009?
Electronic and magnetic properties of a layered compound LaMnPO are examined in relation to a
newly discovered iso-structural superconductor LaFeAs?P?O. Neutron diffraction measurements,
together with temperature dependent magnetic susceptibility, clarify that LaMnPO is an
antiferromagnet at least up to 375 K. The spin moment of a Mn ion is determined to be 2.26 ?Bat
room temperature, and the spin configuration is antiparallel in the Mn–P plane and parallel between
the Mn–P planes, which is rather different from that of LaFeAsO. Optical absorption spectra,
photoemission spectra, and temperature dependent electrical conductivity indicate that LaMnPO is
a semiconductor. Furthermore, nominally undoped LaMnPO exhibits n-type conduction while the
conduction type is changed by doping of Cu or Ca to the La sites, indicating that LaMnPO is a
bipolar conductor. Density functional calculation using the GGA+U approximation supports the
above conclusions; the electronic band structure has an open band gap and the antiferromagnetic
spin configuration is more stable than the ferromagnetic one. © 2009 American Institute of Physics.
?DOI: 10.1063/1.3124582?
I. INTRODUCTION
Layered transition-metal compounds have been studied
intensively as correlated electron systems originating from
the 3d electrons in the transition metal ions. Their distinct
electronic and magnetic properties invoke unconventional
functions such as high transition temperature ?high-Tc?
superconductivity,1tunneling magnetoresistance,2and large
thermoelectric power.3Our group has studied mixed anion
layered compounds containing transition metals, which in-
clude a LnMXO system ?Ln=lanthanoid; M=Fe, Co, Ni; X
=P, As? with the ZrCuSiAs-type crystal structure. Electronic
transport and magnetic properties of the mixed anion layered
compounds vary in a wide range by changing the combina-
tion of the transition metal and the anions even if the same
crystal structure is maintained. Since the crystal structure of
LnMXO is composed of alternate stacks of Ln-O and M-X
layers as shown in Fig. 1, the chemical composition is ex-
pressed as ?LnO??MX? in a structural formula. First-
principles calculations have revealed that the 3d electrons in
LnMXO are concentrated in the M-X layer and form the
Fermi level,4–6which are thought to be related closely to the
appearance of high-Tcsuperconductivity in electron-doped
LaFeAsO and related compounds.
Actually, we have reported that electronic transport and
magnetic properties of LaMXO change largely with the tran-
sition metal element. For example, LaFePO and LaNiPO,
whose transition metal ions ?nominally Fe2+and Ni2+? have
even numbers of 3d electrons ?six for Fe2+and eight for
Ni2+?, undergo superconducting transitions.7,8
LaFeAsO is a poor metal with an antiferromagnetic ?AFM?
spin configuration at the ground state. While, electron doping
induces superconductivity at Tc?26 K,9which is further en-
hanced up to ?55 K by an external pressure10or replacing
La with other lanthanide ions ?chemical pressure?.11–15These
findings triggered the new fever in high-Tcsuperconductor
research. On the other hand, the magnetic moments of
LaCoXO ?seven 3d electrons for Co2+? do not vanish com-
pletely, leading to an itinerant ferromagnetic ?FM? phase at
?43 K for LaCoPO.16Further, LaZnXO exhibit a nonmag-
netic semiconductive behavior due to the closed shell ?3d?10
configuration of the Zn2+ion.17,18
The Mn compound LaMnPO is also of great interest
because Mn2+has an odd number of 3d electrons and is
expected to form a spin configuration different from those of
the other LaMXO compounds because many Mn2+com-
pounds have a half-filled pseudoclosed shell configuration
?3dup?5. In addition, investigation of LaMnPO is considered
to provide complementary information to understand the
electron correlation of 3d electrons in these layered com-
pounds and further the superconducting mechanism in
LaFeXO and LaNiXO.
Undoped
a?Author to whom correspondence should be addressed. Electronic mail:
yanagi@lucid.msl.titech.ac.jp.
b?Present address: WPI Advanced Institute for Materials Research Tohoku
University, 2-1-1 Katahira Aoba-ku Sendai, 980-8577, Japan.
JOURNAL OF APPLIED PHYSICS 105, 093916 ?2009?
0021-8979/2009/105?9?/093916/8/$25.00 © 2009 American Institute of Physics
105, 093916-1
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Page 3

In this paper, we measured electronic transport, optical
properties, and magnetization properties of LaMnPO with
single-phase polycrystalline samples. Further the magnetic
structure and the electronic structure around the Fermi level
were examined by neutron diffraction and synchrotron x-ray
photoemission spectroscopy, respectively. We also performed
density functional theory ?DFT? calculations using the
GGA+U approximation to know the stable crystal structure
and the spin configuration along with the electronic band
structure. These results show LaMnPO is an indirect
transition-type semiconductor with an AFM spin configura-
tion at room temperature. It is also found that undoped
sample exhibits n-type conduction, which is converted to
p-type by hole doping.
II. EXPERIMENTAL AND CALCULATIONS
The single-phase polycrystalline samples were obtained
through a two-step solid-state reaction process using La
?Shin-etsu Chemical, purity 99.5%?, P ?Rare Metallic,
99.9999%?, and MnO ?Soekawa Chemical, 99.9%? as start-
ing materials. This process is different from that reported by
Nientiedt et al.19In the first step of the synthesis, single-
phase LaP was prepared by heating a mixture of powdered
La and P with an atomic ratio of 1.00:0.97 in an evacuated
silica tube at 400 °C for 12 h and then temperature was
increased to 700 °C and kept for 6 h; this was because the
resulting LaMnPO samples synthesized with stoichiometric
LaP ?La:P=1:1? contained an impurity phase of MnP. Then,
a mixture of the LaP and the MnO was pressed into a pellet
and heated at 1000 °C for 12 h in an evacuated silica tube. It
was confirmed that the residual MnP contents in the samples
were below the detection limits of our high-power x-ray dif-
fraction ?XRD? equipment ?detection limit ?0.1%8,20,21? and
magnetic measurements ??0.1%?. Ca3P2and CuO were
added to the starting materials in the second step to form
doped LaMnPO samples,
LaMn1−xCuxPO, respectively.22
The prepared samples were characterized by a high-
power XRD ?D8 ADVANCE-TXS, Bruker AXS? with
Cu K? radiation at 24 °C. Rietveld analyses were carried
out using the code TOPAS3 ?Ref. 23? to refine crystallo-
graphic parameters of LaMnPO. Neutron diffraction experi-
ments were performed using a high-resolution powder dif-
fractometer installed at JRR-3 of the Japan Atomic Energy
Agency ?JAEA?. The collimations were open ?the effective
value was 35??-open-40?-6? and the neutron wavelength ?
was 0.182 33 nm. The data were collected at room tempera-
ture ?RT?. Neutron Rietveld analyses were carried out using
the code RIETAN-2000.24
Diffuse reflectance spectra were measured on fine-
powdered undoped LaMnPO samples. Optical absorption
spectra were taken on 150 nm thick ?001? oriented epitaxial
LaMnPO films on MgO ?001? single-crystal substrates pre-
pared by pulsed laser deposition in order to evaluate the op-
tical band gap. Details in the film deposition procedure will
be reported elsewhere.25
The electrical conductivities of the sintered pellets were
measured in the temperature range 1.8–305 K with a four-
probe technique ?PPMS, Quantum Design?. Ohmic elec-
trodes were formed using sputtered Au films. The magnetic
susceptibility were measured with a vibrating sample mag-
netometer ?PPMS, Quantum Design? from 2.5 to 375 K un-
der a magnetic field of 5000 Oe in a zero field cooling pro-
cedure.
Resonant photoemission spectroscopy ?RPES? measure-
ments were performed with several excitation photon ener-
gies from 636 to 639 eV, which correspond to the energy at
the Mn L3absorption edge, in the BL23SU beam line at the
Japan Synchrotron Radiation Research Institute ?SPring-8?.
The energy resolution estimated from the Fermi edge broad-
ening of a gold reference was ?130 meV. To prevent a
charging effect, Ca-doped p-type conductive LaMnPO was
employed for the measurements. Before the measurement,
polycrystalline samples were fractured in a vacuum prepara-
tion chamber, which is attached to the measurement cham-
ber, to obtain clean surfaces. All the measurements were per-
formed at 20 K in an ultrahigh vacuum of ?10−8Pa.
DFT periodic calculations were performed with the Vi-
enna ab initio simulation package ?Ref. 26? code using a
projector augmented plane wave method27,28and PBE96
generalized gradient approximation ?GGA? functionals.
Crystal structure parameters obtained by the XRD Rietveld
analyses were employed as initial values, and then the lattice
parameters and internal ionic coordinates were optimized so
as to take the minimum total energy. Two different spin con-
figurations were examined; one was a FM configuration
where both of the two Mn ions in the unit cell were assumed
to be identical, and the other was an AFM configuration
where the two Mn ions were treated independently with the
La1−xCaxMnPO,and
a
b
c
O
Ln
M / Cu
X / Ch
FIG. 1. Crystal structure of LnMXO ?Ln=lanthanoid; M=Mn, Fe, Co, Ni,
Zn; X=P. As? and LaCuChO ?Ch=S, Se, Te?, which have layered crystal
structures composed of Ln-O and M-X/Cu-Ch layers.
093916-2Yanagi et al.J. Appl. Phys. 105, 093916 ?2009?
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Page 4

spin configuration obtained by the neutron Rietveld analyses.
For the latter case, self-consistent cycles converged to a total
spin moment of zero, indicating an AFM configuration.
GGA+U calculations were also performed because the
simple GGA calculation did not reproduce the energy posi-
tion of the Mn 3d states observed by RPES. The simplified
rotationally invariant approach of Dudarev et al.29was used,
where two empirical parameters, the Coulomb parameter U
and the exchange parameter J, are required but appear as one
independent parameter ?effective Coulomb parameter? Ueff
=U−J, for each localized orbital. Uefffor Mn 3d electrons
was varied from 0 to 6 eV, and that for La 4f was fixed at 11
eV.30
III. RESULTS AND DISCUSSION
A. Structure analyses
Figure 2 shows a powder XRD pattern for LaMnPO. All
the peaks agree with those of a simulated pattern by the
Rietveld analyses and no extra peak was observed, which
confirms the sample was a single phase. Table I summarizes
the refined structural parameters, which substantiates LaMn-
PO has the same crystal structure as those of LaMXO and
NdMnXO.19It belongs to the tetragonal ZrCuSiAs-type
structure ?P4/nmm?, composed of alternating stacks of
Mn–P and La–O layers. The Mn–P layer is built from the
edge-sharing networks of MnP4tetrahedrons, which is dis-
torted from the regular tetrahedron. The distortion is evalu-
ated from deviation of the P-M-P bond angles from that of
the regular tetrahedron ?109.47°?. Those for LaMnPO are
111.24° and 108.59°, whose distortions are smaller than
those of LaFePO ?120.2° and 104.4° ?Ref. 7??, LaCoPO
?127.4° and 101.3° ?Ref. 31? or 121.7° and 103.7° ?Ref. 16??,
and LaNiPO ?126.4° and 101.7° ?Ref. 8??. On the other hand,
the ZnP4tetrahedron in LaZnPO ?108.1° and 110.2° in Ref.
32 and 108.4° and 110.0° in Ref. 17? is the closest to the
regular tetrahedron among the LaMPO compounds.
Figure 3?a? shows a neutron diffraction pattern ?crosses?,
a simulated one ?red line?, and a difference between the ob-
served and simulated patterns ?gray line at the bottom?. Table
II summarizes refined structural and magnetic parameters.
The obtained structural parameters agree with those obtained
from the XRD measurements with the differences less than
0.5%. Figure 3?b? shows decomposition of the simulated pat-
tern into nuclear ?blue? and magnetic ?red? scattering contri-
butions. It indicates that the spin ordering information is
clearly obtained from the 100 and 101 diffractions at ?25.9°
and ?28.5°, respectively. The 100 diffraction disappears in
the nuclear scattering due to the extinction rule originating
from the n glide plane along the ?110? direction in the space
group P4/nmm, but is clearly observed in the magnetic scat-
tering, indicating that the two Mn ions in the unit cell have
different spin moments. Furthermore, when we assumed the
direction of the magnetic moment on Mn was perpendicular
to the c-axis, a weighted profile reliability factor, Rwp, be-
came 7.18%. The value decreased to 6.18% provided that the
direction was assumed to be parallel to the c-axis. Conse-
quently, we concluded that the direction of the magnetic mo-
ment is parallel to the c-axis. The refined magnetic moment
is 2.26?2? ?B/Mn, which is much smaller than the value
expected from the high-spin configuration in the localized
spin scheme ?5 ?B/Mn?, but much larger than that expected
from the low-spin configuration ?1 ?B/Mn?. The most reli-
able magnetic structure obtained is illustrated in Fig. 3?c?,
where the in-plane magnetic structure is AFM; the magnetic
coupling with the nearest Mn ions, which lie in the ?110?
direction, is antiparallel to each other, whereas the spins
couple in parallel with the second nearest Mn ions, which lie
in the ?100? and ?010? directions. On the other hand, the
interlayer spin coupling is parallel. Note that the spin con-
figuration of LaMnPO is different from that of LaFeAsO,
where the Fe spin moments are perpendicular to the c-axis,
orders in the stripe type configuration in the Fe–As layer, and
are antiparallel between the adjacent layers.33These results
indicate that magnetic interactions among the transition
metal ions in LnMXO are sensitive to kind of 3d transition
metal elements ?i.e., number of d electrons?.
2040 60
2θ θ θ θ (degree)
80 100120140
0
1
2
3
4
5
Intensity (10
4counts)
FIG. 2. ?Color online? Powder XRD patterns of the LaMnPO sample and
the results of the Rietveld analysis. ?Upper row? Observed pattern ?+? and
simulated pattern obtained by the Rietveld analysis ?red line?. ?Middle row?
The difference profile between the observed and simulated patterns. ?Bottom
row? The positions of the Bragg reflections from LaMnPO.
TABLE I. Crystallographic parameters of LaMnPO refined by powder x-ray
Rietveld analysis. The XRD data were collected at 24 °C. Occupancy and B
values of O were fixed at 1.0 and 0.9, respectively. The reliability factor was
Rwp=11.91% and the goodness of fit parameter was S=2.39.
a
?nm?
c
?nm?
V
?nm3?
0.405786?1?
0.884341?3?
0.1456181?9?
SiteWycoff position
xyz
B
?Å2?
La
Mn
O
P
2c
2b
2a
2c
1/4
3/4
3/4
1/4
1/4
1/4
1/4
1/4
0.13879?4?
1/2
0
0.6569?2?
0.146?8?
0.13?2?
0.90
0.25?3?
093916-3Yanagi et al.J. Appl. Phys. 105, 093916 ?2009?
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Page 5

B. Optical spectra, electrical transports, and
magnetization
Optical absorption spectra were obtained from diffuse
reflectance spectra by using the Kubelka–Munk relation.34,35
To evaluate the band gap energy, two types of plots, a
???/s??h??2-h? plot for the allowed direct transition model
and a ??/s?1/2-h? plot for the indirect transition model,
where ? denotes the optical absorption coefficient, h? de-
notes the photon energy, and s denotes the scattering factor,
are examined in Fig. 4?a?. Straight lines are found in the
???/s??h??2-h? plot in two different regions, between 1.0
and 1.5 eV and between 2.0 and 2.5 eV, which may provide
direct transition gaps of ?0.9 and ?1.4 eV, respectively. On
the other hand, the ??/s?1/2-h? plot shows a straight line in a
narrow photon energy region of 0.9–1.0 eV, which may give
an indirect band gap of ?0.9 eV. The band gap energies,
obtained in this procedure, are not so confirmative because
similar absorption tail structures have frequently been ob-
served in similar compounds including LaCuChO ?Ch=S,
Se, Te?.36For instance, LaCuSeO has the band gap energy of
?2.7 eV and exhibits an absorption tail extended to
?2.5 eV even in undoped samples and to ?2.0 eV in hole-
doped ones, where the tails are assigned to subgap states
related to defects. Therefore, we also measured optical ab-
sorption spectra using epitaxial films of LaMnPO to further
examine the band gap energy. As shown in Fig. 4?b?, the
???h??2-h? plot exhibits a straight line at ?1.6 eV, provid-
ing a direct band gap of ?1.4 eV. On the other hand, the
?1/2-h? plot exhibits a straight line in a wide energy region
2040 60
2θ θ θ θ (degree)
80 100 120 140 160
0
2
4
6
8
10
(a)
(a)
Intensity (10
3counts)
204060
2θ θ θ θ (degree)
80 100 120 140 160
0
2
4
6
8
10
2426 2830
0
1
2
(b)
(b)
Intensity (10
3counts)
101
100
002
(c) (c)
(c)
aaaa
bbbb
cccc
O
La
P
MnMn Mn
O
La
PP
O
La
FIG. 3. ?Color online? ?a? Neutron powder diffraction patterns of the LaMn-
PO sample and the result of the Rietveld analysis. ?Upper row? Observed
pattern ?+? and simulated pattern obtained by the Rietveld analysis ?red
line?. ?Bottom row? The difference profile between the observed and simu-
lated patterns. ?b? The simulated patterns by nuclear scattering ?blue line?
and magnetic scattering ?red line?. Inset shows a magnified view around the
100 diffraction. ?c? Magnetic structure of LaMnPO.
TABLE II. Crystallographic parameters refined by powder neutron Rietveld
analysis. The neutron diffraction data were collected at RT. The reliability
factor was Rwp=6.18% and the goodness of fit parameter was S=1.32.
a
?nm?
c
?nm?
0.40572?2?
0.88415?3?
Site
xyz
B
?Å2?
MMn
??B?
¯
2.26?2?
?2.26?2?
¯
¯
La
Mn 1
Mn 2
O
P
1/4
3/4
1/4
3/4
1/4
1/4
1/4
3/4
1/4
1/4
0.1381?2?
1/2
1/2
0
0.6586?2?
0.52?3?
0.67?4?
0.67?4?
0.67?4?
0.77?4?
0.5 1.0
Energy (eV)
1.5 2.02.5
0.0
0.5
1.0
(α α α αhv)
α α α α
2
(α α α αhν ν ν ν)
2(10
10cm
-2eV
2)
0
2
4
1/2
α α α α
1/2(10
2cm
-1/2)
1.4 eV
(b)
1.3 eV
0.5 1.0
Energy (eV)
1.52.0 2.5
0
10
20
30
40
[(α α α α/s)hν ν ν ν]
(α α α α/s)
2
1/2
[(α α α α/s)hν ν ν ν]
2
0.0
0.5
1.0
1.5
2.0
(α α α α/s)
1/2
(a)
1.4 eV
0.9 eV
FIG. 4. ?a? Absorption spectra of LaMnPO converted from diffuse reflec-
tance spectra with the Kubelka–Munk relation. Direct band gap and indirect
bad gap are estimated from the ???/s??h??2-h? plot ?black line? and the
??/s?1/2-h? plot ?gray line?, respectively. ?b? Optical absorption spectra of a
LaMnPO epitaxial film.
093916-4Yanagi et al. J. Appl. Phys. 105, 093916 ?2009?
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