Structure and magnetic properties of Gd 4 Mn 0.05 Sb 0.95 3a Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea b Center for Materials Science, National
Trang 1Structure and magnetic properties of Gd 4 (Mn 0.05 Sb 0.95 ) 3
a Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea
b Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam
Abstract
This work studies the structure and magnetic properties of Gd4(Mn0.05Sb0.95)3with the aim to clarify the role played
by the magnetic Mn atom.Upon substitution of Mn for Sb in the parent Gd4Sb3 compound, it is found that the inverted Th3P4-like structure has been somewhat expanded.The Curie temperature is increased while magnetic moment, measured in the field of 50 kOe at 200 K, is slightly reduced.The ferromagnetic semiconductor,
Gd4(Mn0.05Sb0.95)3, undergoes a ferromagnetic to paramagnetic transition at 270 K.Observed anomalies occur in the magnetization vs.temperature curves measured in a very low magnetic field.They are attributed to magnetic inhomogeneities resulting from a structural modification in Gd4(Mn0.05Sb0.95)3
r2002 Elsevier Science B.V All rights reserved
PACS: 75.50.Pp; 75.30.Cr
Keywords: Gd 4 (Mn 0.05 Sb 0.95 ) 3 ; Structure; Inhomogeneity; Magnetic phase transition
1 Introduction
Following the recent discovery of III–V-based
diluted magnetic semiconductors, which can be
prepared by molecular-beam epitaxy by
substitu-tion of Mn for Ga in GaAs, many interesting
phenomena that are combinations of electrical,
optical and magnetic properties are observed.Of
special interest is the possibility to incorporate
magnetic effects in semiconductors to develop
effects needed for devices applications [1–2].As
reported in Ref.[3], the Curie temperature of the
diluted magnetic semiconductor (Ga,Mn)N can be
tuned in the wide temperature range of 228–370 K
by varying the Mn content.It is noteworthy that,
in the ferromagnetic semiconductors (Ga,Mn)N
[3] and Ga0.98Mn0.02As [4], it cannot be shown which are the Mn atoms participating in the ferromagnetism and which are the paramagnetic
Mn atoms.This is thought to be attributed to magnetic inhomogeneity in the material, which is caused by the partial participation of Mn atoms in the ferromagnetic ordering[4].Thus, it is desirable
to further clarify the role of Mn in magnetic semiconductors
In the present work, the effects of Mn substitu-tion for Sb on the magnetic and structural properties of Gd4(Mn0.05Sb0.95)3were investigated
2 Experimental
Gd4(Mn0.05Sb0.95)3 was prepared by arc-melt-ing.The alloy was annealed at 11001C.The details
*Corresponding author.Tel : 43-261-2270; fax:
+82-43-274-7811.
E-mail address: skoh@chungbuk.ac.kr (S.K Oh).
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 7 5 - 1
Trang 2of the sample preparation will be reported
else-where[5].Structural analyses were made based on
the extended X-ray absorption fine structure
(EXAFS) and X-ray diffraction.EXAFS
experi-ments were carried out at the 7C EC EXAFS beam
line of the Pohang Light Source (PLS) in Korea
The magnetization was measured in a Quantum
Design MPMS-5 SQUID magnetometer from 5 to
300 K in magnetic fields up to 50
kOe.Measure-ments of the real and imaginary parts of the AC
susceptibility were carried out from 70 to 300 K in
a Lakeshore 7000 Susceptometer
3 Results and discussions
Compared with the parent Gd4Sb3 compound
[7–9], we found that the inverted Th3P4-like
structure has been somewhat expanded and the
lattice constant of Gd4(Mn0.05Sb0.95)3 was
deter-mined to be 9.402 (A.For comparison, we have
summarized in Table 1 the lattice constants of
Gd4Sb3 and Gd4(Mn0.05Sb0.95)3 from several
different studies.One can see that partial
replace-ment of Sb by Mn in the parent Gd4Sb3compound
increases the volume of the unit cell.Similar
behavior was observed in Ga1xMnxAs [6] and
Gd4(SbxBi1x)3 [7] compounds.As reported in
Ref [7], in Gd4(SbxBi1x)3 compounds, as the
doping concentration x is increased, the lattice
constant a0 decreases from 9.38 (A for Gd4Bi3 to
the value 9.22 (A for Gd4Sb3.Additionally, a
decrease of the Curie temperature TC with
increasing Sb content has been reported [7].The
decrease of TC and a0 with increasing Sb content
in Gd4(SbxBi1x)3 may be due to the smaller
atomic radius of Sb compared to Bi.The larger
atomic radius of Mn in comparison with Sb is probably the origin of the increased lattice constant of Gd4(Mn0.05Sb0.95)3 with respect to the pure Gd4Sb3compound.In order to study the influence of Mn substitution in Gd4Sb3, we have studied the magnetic properties of both Gd4Sb3
and Gd4(Mn0.05Sb0.95)3.InFig.1, the temperature dependence of the magnetization of both com-pounds is shown, measured in an applied field of
100 Oe from 5 to 300 K.For both Gd4Sb3 and
Gd4(Mn0.05Sb0.95)3 samples, there is a prominent difference between the FC (field-cooled) and ZFC (zero-field-cooled) magnetization below TC; indi-cative of magnetic frustration arising from the competition between ferromagnetic and antiferro-magnetic interactions [10].Obviously, both
Gd4Sb3and Gd4(Mn0.05Sb0.95)3are ferromagnetic
in the whole temperature range below the Curie temperature.The TCvalues were determined to be
259 and 270 K for Gd4Sb3and Gd4(Mn0.05Sb0.95)3, respectively.Substitution of Mn for Sb in the parent Gd4Sb3 compound not only increases the volume of the unit cell but also increases the TC: This is consistent with the result reported earlier
by Holtzberg et al [7].The M vs T curve
Gd4(Mn0.05Sb0.95)3 exhibits slightly anomalous behavior in the temperature region from 150 to
255 K suggesting some kind of phase transition
To further clarify this situation, measurements of the real and imaginary parts of the AC suscept-ibility were performed.As can be seen in Fig.2, the real (w0) and imaginary (w00) parts of the AC susceptibility of the sample reveal similar unusual behavior.Since no anomalous behavior is found
in the temperature dependence of the parent compound Gd4Sb3, the small anomalies in the
M vs T curves of Gd4(Mn0.05Sb0.95)3 can safely
Table 1
Lattice constant a0; the magnetization M50 kOe; measured in 50 kOe at 200 K and the Curie temperature TC for Gd 4 Sb 3 and
Gd 4 (Mn 0.05 Sb 0.95 ) 3
Trang 3be attributed to the presence of Mn in this sample The presence of very small amount of about 0.14% Mn (confirmed by EXAFS analysis) causes
a structural modification in the sample, which in turn causes magnetic inhomogeneities.They may
be the origin of the anomalous behavior of the magnetization in the temperature region from 150
to 255 K, before the onset of ferromagnetic-paramagnetic phase transition.The value of magnetic moment of Gd4(Mn0.05Sb0.95)3, mea-sured in the field of 50 kOe at 200 K, is only slightly a bit smaller than that of Gd4Sb3(Fig.3), but the presence of Mn in Gd4Sb3 causes the material to exhibit much softer ferromagnetic behavior (Fig.1).A similar effect has been reported for Gd4(SbxBi1x)3 compounds by dop-ing with Bi[7]
0
1
2
3
4
5
6
T
C = 259 K
0
1
2
3
4
Gd4Sb3
T (K)
ZFC
FC
Gd
3
TC= 270 K
H = 100 Oe
ZFC
FC
T (K)
Fig.1 Temperature dependence of the magnetization at 100 Oe
for both zero-field-cooled (ZFC) and field-cooled (FC) Gd 4 Sb 3
(top panel) and Gd 4 (Mn 0.05 Sb 0.95 ) 3 (bottom panel).
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
50 100 150 200 250 300 0.000
0.004
0.008
0.012
0.016
T (K)
Gd4(Mn0.05Sb0.95)3
, (arb units)
Temperature (K)
HAC = 10 Oe
f = 80 Hz
Fig.2 Temperature dependence of the real (w 0 ) and imaginary
(w00) parts of the AC susceptibility of Gd (Mn Sb )
0 20 40 60 80 100
0 20 40 60 80 100
H (Oe)
Ga4(Mn0.05Sb0.95)3
T = 200 K
H (Oe)
Gd
3
T = 200 K
Fig.3 Magnetic-field dependence of the magnetization at
200 K for both Gd 4 Sb 3 (top panel) and Gd 4 (Mn 0.05 Sb 0.95 ) 3
(bottom panel).
Trang 4In order to further elucidate the nature of the
magnetic transition, magnetization vs.magnetic
field isotherms of Gd4(Mn0.05Sb0.95)3 were
mea-sured in the vicinity of TC: As can be seen inFig.4,
the M2H curves show a normal monotonic
decrease of the magnetization with increasing
temperature.This might be due to a monotonic
phase transition and a weak coupling between spin
and lattice in the magnetic ordering process arising
from inhomogeneity in the sample [11].Similar
behavior has also been observed for the parent
compound Gd4Sb3[7]
4 Conclusions
The structural and magnetic properties of
Gd4(Mn0.05Sb0.95)3 were investigated.The partial
replacement of Sb by Mn in the parent compound
Gd4Sb3causes an expansion of the inverted Th3P4
-like structure and an increase of the Curie
temperature of about 10 K.The presence of Mn
in Gd4Sb3 causes a small anomaly in the
ferro-magnetic behavior.The ferroferro-magnetic semicon-ductor, Gd4(Mn0.05Sb0.95)3 undergoes the ferromagnetic to paramagnetic transition at
270 K.The observed anomalies in the magnetiza-tion vs.temperature curves, measured in a very low magnetic field, are likely to be attributed to magnetic inhomogeneities resulting from a struc-tural modification in Gd4(Mn0.05Sb0.95)3
Acknowledgements This work was supported by the Korea Re-search Foundation Grant (KRF-2001-005-D20010)
References [1] A.Oiwa, S.Katsumoto, A.Endo, M.Hirasawa, Y.Iye, H Ohno, F.Matsukura, A.Shen, Y.Sugawara, Solid State Commun.103 (1997) 209.
[2] F.Matsukara, H.Ohno, A.Shen, Y.Sugawara, Phys.Rev.
B 57 (1998) R2037.
[3] M.L Reed, N.A El-Masry, H.H Stadelmaier, M.K Ritums, M.J Reed, C.A Parker, J.C Roberts, S.M Bedair, Appl.Phys.Lett.79 (2001) 3473.
[4] H Ohldag, V Sonilus, F.U Hillebrecht, J.B Goedkoop, M.Finazzi, F.Matsukura, H.Ohno, Appl.Phys.Lett.76 (2001) 2928.
[5] Nguyen Ngoc Chau, Manh-Huong Phan, to be published [6] S.J Potashnik, K.C Ku, S.H Chun, J.J Berry, N Samarth, P.Schiffer, Appl.Phys.Lett.79 (2001) 1495 [7] F Holtzberg, T.R McGuire, S Methfessel, J.C Suits, J.Appl.Phys.35 (1964) 1033.
[8] R.J Gambino, J Less-Common Metals 12 (1967) 344 [9] G.Borzone, M L.Fornasini, N.Parodi, R.Ferro, Intermetallics 8 (2000) 189.
[10] J.G Park, M.S Kim, H.C Ri, K.H Kim, T.W Noh, S.W Cheong, Phys.Rev.B 60 (1999) 14804.
[11] S.E Lofland, K.V Ramanujachary, W.H McCarroll, J.Magn.Magn.Mater.238 (2002) 22.
0
50
100
100 K
160 K
200 K
220 K
240 K
260 K
265 K
270 K
275 K
280 K
285 K
290 K
300 K
H (Oe)
Fig.4 Magnetization vs field for Gd 4 (Mn 0.05 Sb 0.95 ) 3 at
various temperatures in fields up to 50 kOe.The temperatures
of the isotherms are indicated.