Magneto-transport properties of MnGeP2 and MnGeAs2 films

Yunki Kim. Int. Journal of Engineering Research and Application www.ijera.com
ISSN : 2248-9622, Vol. 6, Issue 12, ( Part -3) December 2016, pp.79-83
www.ijera.com 79 | P a g e
Magneto-transport properties of MnGeP2 and MnGeAs2 films
Yunki Kim *, J. B. Ketterson **
*(Department of Electrical and Biological Physics, Kwangwoon University, Seoul 01897, Republic of Korea
** (Department of Physics & Astronomy, Northwestern University, Evanston, IL 60208, USA
ABSTRACT
MnGeAs2 and MnGeP2 thin films were deposited on GaAs and Si substrates. For these film samples, room-
temperature ferromagnetism was observed from magnetization and resistance measurements and verified from
hysteresis in magnetization measurements. Hysteresis as well as anomalous behavior in Hall effect
measurements was found in the deposited MnGeAs2 and MnGeP2 films, implying spin polarization of the mobile
carriers in the films. The Hall resistance measurements above the ferromagnetic transition temperature showed
that the carriers are n-type in MnGeAs2 and p-type in MnGeP2.
Keywords: MnGeAs2, MnGeP2, room-temperature ferromagnetism, anomalous Hall effect, MR hysteresis
I. INTRODUCTION
For the development of devices utilizing the
properties of spins as well as charges of carriers in
various materials, spin polarized carrier transport is
essential [1]. Currently commercially available spin
dependent devices such as magneto-resistive (MR)
head and magnetic tunnel junction (MTJ) use the
spin transport of carriers. In the last decades,
technological advances in the field of magneto-
electronics has grown rapidly, motivated by the
findings of various spin-dependent phenomena such
as giant magneto-resistance (GMR) and by the
invention of spin-valve structure which is
industrially successful as a magnetic sensor in hard
disk drive systems [2]. Currently available materials
used in the spin-dependent devices are mostly
ferromagnetic metals, while to reduce the size and
enhance the performance of spin dependent devices,
fully established processing technology in
semiconductor industry as well as some useful
properties of semiconductors may be necessary.
However, it is not efficient to flow spin polarized
current through the semiconductors from
ferromagnetic metals due to large and rapid spin
polarization loss during spin injection between
nonmagnetic semiconductors and ferromagnetic
metals [3,4]. Incidence of diluted magnetic
semiconductors (DMS), such as GaAs doped with
Mn [5,6] attracted much attention since they can
give one of the alternate ways to overcome the rapid
spin polarization loss Spin injection from a
ferromagnetic semiconductor to a lattice and Fermi-
level matched nonmagnetic semiconductor should
significantly reduce the spin-flip scattering rate.
DMS is a useful strategy to achieve control
over the spin degree of freedom [7], an extra
variable we can utilize, substituting magnetically
active transition metal ions such as Mn, Ni, Fe, Cr,
and Co into non-magnetic semiconductor hosts.
Various DMS including, IV [8,9], III-V [10,11], and
II-VI [12] semiconductors have been reported. In
these classes of semiconductors, the low solubility of
magnetic impurity ions in host semiconductors can
limit the possibility to acquire high magnetic
moments and high transition temperatures.
Chalcopyrites (II-IV-V2), acknowledged as
promising for solar cells, detectors, and nonlinear
optical devices [13], with a similar structure and
close lattice constants with zinc-blende (III-V)
materials, drew much attention as a promising
candidate replacing DMS since the magnetic 2+
ions
can occupy all the group II sites (25% of the lattice
sites) in a chalcopyrite structure. Mn-doped
chalcopyrites such as CdGeP2 [14], ZnSnAs2 [15]
and ZnGeP2 [16] were reported to have
ferromagnetic (FM) orderings near or above room
temperature. These ferromagnetic semiconducting
chalcopyrites, with high transition temperatures and
similar crystal and electronic structures to the
popularly used zinc-blende semiconductors, have
potential to improve the performance of spintronic
devices. Here we report the magnetic and magneto-
transport properties of chalcopyrite films of
MnGeAs2 and MnGeP2 on GaAs and Si substrates,
with room-temperature ferromagnetism. And the
spin-dependent I-V characteristics of a p-i-n diode
structure with the MnGeAs2 and MnGeP2 multi-
layers.
II. EXPERIMENT
Thin layers of MnGeAs2 and MnGeP2 were
grown on GaAs(100) and Si(100) substrates with a
molecular beam epitaxy (MBE) system. GaAs and Si
substrates were heated before the film deposition
upto 650 C substrate to remove surface oxide at the
surface and to obtain smooth surface. For GaAs
substrates, an arsenic flux was maintained during the
RESEARCH ARTICLE OPEN ACCESS

pre-heating process to compensate As out-gassing
from the GaAs substrates. Then thin GaAs buffer
layer deposition (50-150 Å) was followed, if
necessary. The deposition rate was maintained to be
around 0.5 Å/s. The substrate temperature during the
growth was 350 C when GaAs was used as a
substrate. MnGeAs films on Si substrate were
deposited at a higher substrate temperature of 500-
700 C. To assure the correct final composition the
flux of As was maintained at about 20 times that of
the manganese and germanium. To monitor crystal
orientation and mode of growth of the deposited
film, reflection high-energy electron diffraction
(RHEED) was used. The thickness of the thin layers
was measured in-situ using a quartz thickness
monitor system.
III. RESULTS AND DISCUSSION
As shown in Fig.'s 1(a), (b) and (c), streaky
RHEED patterns before and during the film
deposition at 20 and 500 seconds were observed for
a GaAs substrate and for a MnGeP2 film on GaAs.
The patterns were becoming weaker during the
deposition as shown in the figures, implying that as
the deposition proceeds, the surface of the deposited
film becomes coarse. MnGeAs2 films were grown on
GaAs(100) and Si(100) substrates. The substrate
temperatures were 350 C for GaAs and 500-700 C
for Si. RHEED patterns for the films deposited on
GaAs were streaky as shown in Fig.'s 1(d)
throughout the deposition for over 1000 seconds.
RHEED patterns taken from a Si substrate after pre-
heating and a MnGeAs2 film Si(100) were streaky,
as shown in Fig.'s 1(e) and (f) .
Fig. 1 RHEED images of (a) GaAs(100)
substrate before the deposition and those of a
MnGeP2 film on GaAs(100) during the deposition in
(b) 20 and (c) 500 seconds. (d) A RHEED image of
a MnGeAs2 film on GaAs(100) during the
deposition and those of (e) Si(111) substrate before
the deposition and of (f) a MnGeAs2 film on Si(100)
during the deposition.
MnGeAs2 has lattice constants of a = 5.782 and
c = 11.323 Å in bulk, while those for MnGeAs2 are a
= 5.655 and c = 11.269 Å [17]. The lattice mismatch
between MnGeAs2 and GaAs (a = 5.65315 Å) is
2.28% for a (0.15% for c/2) (with Si (a = 5.4307 Å),
6.47% (4.25%)) and that between MnGeP2 and
GaAs is 0.0327% for a (0.330% for c/2) (with Si,
4.13% (3.75%)). Hence for our very thin MnGeAs2
and MnGeAs2 layers, we could not resolve the film
peaks from the GaAs substrate peaks in x-ray -2
diffraction (XRD) measurements, as shown in Fig. 2.
XRD measurements on a MnGeAs2 film on Si(100)
substrates with a considerable lattice mismatch with
the film showed a couple of XRD films peaks as
shown in the figure. The peaks could be identified as
(200) and (400) from a chalcopyrite ordering and
other peaks from the film layers were not found on a
log scale plot. The (004) and (008) peaks, which are
expected in a crystal with a chalcopyrite ordering
were not found, suggesting that the grains on Si(100)
do not have a chalcopyrite structure but a mixed
(zinc-blend) structure. Scanning electron microscope
(SEM) was used to characterize the surface
morphology of the grown films. MnGeAs2 grains
look to be distributed randomly over the whole area
and not relatively smooth. The composition of Mn,
Ge, and As (or P) for the film samples were
determined by energy dispersive x-ray spectroscopy
(EDX) measurements, as summarized in Table 1.
From the EDX measurements, the composition of
Mn, Ge, and As deviates from stoichiometric value
of (1: 1: 2).
Fig. 2 -2 XRD patterns of a MnGeAs2
film on Si(100) substrate deposited at substrate
temperature of 600 o
C and of a MnGeP2 film on
GaAs(100) at temperature of 350 o
C, on a
logarithmic scale.
(a) (b) (c)
(d) (e) (f)

Table 1 Compositions of Mn, Ge, and As or P of
several MnGeAs2 and MnGeP2 films from EDX
measurements. (Note that the gallium or arsenic
composition of a film on GaAs substrate cannot be
determined.)
Samples (substrate and
growth temperature (o
C))
Composition
Mn Ge As or P
MnGeAs2
(Si(100), 600)
0.72 1 2.02
MnGeAs2
(GaAs(100), 350)
1.23. 1
MnGeP2
(GaAs(100), 350)
1.64 1 1.74
The magnetizations of the film samples
were measured using a SQUID (Quantum Design)
magnetometer. Magnetization (M) measurements on
MnGeAs2 and MnGeP2 film samples were
performed in a small (1000 Oe) external magnetic
field (H) from 5 to 400 K, as shown in Fig. 3. All the
samples display magnetic transitions above room
temperature, around 320 and 350 K. Note that the
negative magnetization values above the transition
temperature for the film samples on GaAs come
from the GaAs substrate which is diamagnetic.
Temperature dependent electrical resistance
measurements from 5 to 400 K in zero magnetic
field show that the MnGeAs2 and MnGeP2 films on
GaAs have a metallic behavior in resistance which
increases with temperature up to the transition
temperature observed in the magnetization
measurement but above the temperature saturates.
This distinct change in the resistance curve seems to
be caused by spin-flip scattering rate between
ferromagnetic (FM) and paramagnetic (PM)
temperature regions, which leads the slope change in
resistance at the transition temperature. Field
dependent magnetization measurements were also
performed for the film samples at 5 and 300 K (and
250 K for the films on GaAs). The hysteric
ferromagnetic M-H curves were observed at room-
temperature (300 K) and below (5 K or 250 K),
suggesting that the transitions around 320 and 350 K
are FM-PM transition. Among them, M-H curves for
the films at 5 K and 300 K are shown in Fig's 4(a)
and (b). The coercive fields of the MnGeAs2 and
MnGeP2 films are summarized in Table 2. The
magnetic moment per Mn atom at 5 K for the
MnGeAs2 and the MnGeP2 films on GaAs were
obtained to be 3.4 B and 2.4 B, respectively, from
the saturation magnetization value at 5 K, which is
in good agreement with the bulk value [17].
Fig. 3 Magnetization of MnGeAs2 films on
GaAs and Si and of a MnGeP2 film on GaAs,
normalized with their value at 5 K, scaled on left
axis. And, electrical resistance of a MnGeAs2 and a
MnGeP2 film on GaAs, normalized with their value
at 300 K, scaled on right axis.
Fig. 4 M-H curves for the MnGeP2 and the
MnGeAs2 films on GaAs and the MnGeAs2 film on
Si at (a) 5 and (b) 300 K. The insets are de-
magnified views for the M-H curve of the film on Si.

Table 2 Coercive fields of the MnGeAs2 and
MnGeP2 films at some temperatures (5 and 300 K
for all the samples, 250 K for the films on GaAs)
with respect to various growth conditions.
Samples
(substrate, growth
temperature (o
C))
Coercive Field (Oe)
5 K 250 K 300 K
MnGeAs2
(Si(100), 600)
494 286
MnGeAs2
(GaAs(100), 350)
2300 260 70
MnGeP2
(GaAs(100), 350)
3900 1400 160
Magnetoresistance (MR) measurements for
the deposited film samples were performed at 5, 305
and 355 K. The MR changes for a MnGeAs2 film in
fields between -5 and 5 T, at 5 and 300 K, were
found not larger than 9%. Hysteresis in MR curves is
clearly seen in the data at 305 K, as shown in Fig.
5(a). The two peak positions, one from a positive
sweep of magnetic field (around 1000 Oe) and the
other (around -1000 Oe) from a negative sweep,
look to be 2000 Oe apart. At 355 K above the
transition temperature, no apparent MR change was
found. At 5 K, MR changes by very small amount,
less than 1%. MR measurement results for a
MnGeP2 film are shown in Fig. 5(b). Though MR
changes from -5 to 5 T, at 5 and 305 K are small,
less than 2%, but hysteresis is clearly seen for both 5
and 305 K data. And no hysteresis was found from
the 355 K data, which is above the transition
temperature. These results give evidence that some
of carriers in the films are spin polarized [18]. When
a low magnetic field is applied anti-parallel to the
magnetization direction of the sample, two peaks in
MR can be usually observed, due to the scattering.
Fig. 5 MR curves for (a) a MnGeAs2 film
and (b) a MnGeP2 film on GaAs at 5, 305 and 355 K.
All the MR values are normalized with a resistance
value at zero field during the sweep-up.
Field dependent Hall resistances have been
measured at various temperatures. The anomalous
Hall effect in a bar-patterned MnGeAs2 film on
GaAs has been observed at temperatures below the
FM-PM transition temperature, as shown in Fig. 6(a),
indicating the presence of spin polarized carriers in
MnGeAs2. At 355 K, above the transition
temperature, the ordinary Hall effect was observed
and the type of charge carriers have been determined
n-type with a effective carrier density is 21020
cm-3
.
A MnGeAs2 film was deposited on an p-type
GaAs(100) substrate and the current-voltage (I-V)
characteristics measured. A typical p-n diode type I-
V curve is observed for the MnGeAs2/p-GaAs
system, confirming that the MnGeAs2 film layer is
n-type. Hall measurement has also been performed
on a patterned MnGeP2 film on GaAs. The
anomalous Hall effect was observed in the film at
various temperatures from 5 to 305 K below the
transition temperature with apparent hysteresis in
Hall resistance curve with respect to the magnetic
field sweeping direction as shown in Fig. 5(b), and
an ordinary Hall effect was viewed at 355 K above
the transition temperature, with a p-type (also
confirmed from I-V characteristics of a MnGeP2/n-
GaAs diode structure) effective carrier density of
61020
cm-3
.
Fig. 6 Hall resistance curves for (a) a
MnGeAs2 film and (b) a MnGeP2 film on GaAs at 5,
305 and 355 K. All the Hall resistance values are
normalized with a normal zero-field resistance value.

IV. CONCLUSION
MnGeAs2 and MnGeP2 films have been
synthesized on GaAs(100) and Si(100) substrates.
The films display room-temperature ferromagnetism
and a high magnetic moment of 3.4 B and 2.4 B
per Mn, respectively. Anomalies in magnetic field
dependent transport of the carriers, presumably due
to spin polarization were observed from Hall and
magnetoresistance measurements. The results of the
present investigation suggest that MnGeAs2 and
MnGeP2 films are potential candidates for room-
temperature spintronic devices.
ACKNOWLEDGEMENTS
The present research has been conducted by the
Research Grant of Kwangwoon University in 2016.
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Magneto-transport properties of MnGeP2 and MnGeAs2 films

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