Optical and magneto-optical properties of ZnMnO and ZnMnFeO single crystals and thin films

Article abstract of DOI:10.1002/pssa.200673013

Single crystals of oxide diluted magnetic semiconductor Zn _1-xMn_xO have been grown by chemical vapour transport and thin films of Zn_1-xMn_xO, Zn_1-x-yMn _xFe_yO have been deposited b

Full text of DOI:10.1002/pssa.200673013

phys. stat. sol. (a) 204, No. 1, 106–111 (2007) / DOI 10.1002/pssa.200673013
Optical and magneto-optical properties of ZnMnO
and ZnMnFeO single crystals and thin films
*
A. I. Savchuk , V. I. Fediv, G. I. Kleto, S. V. Krychun, and S. A. Savchuk
Department of Physics of Semiconductors and Nanostructures, Chernivtsi National University,
Kotsubinsky str. 2, 58012 Chernivtsi, Ukraine
Received 19 August 2006, revised 22 September 2006, accepted 25 September 2006
Published online 20 December 2006
PACS 71.35.Ji, 75.30.Hx, 75.50.Pp, 78.20.Ls, 78.40.Fy
Single crystals of oxide diluted magnetic semiconductor Zn Mn O have been grown by chemical vapour
x
1–x
transport and thin films of Zn Mn O, Zn Mn Fe O have been deposited by pulsed laser ablation and rf
1–x
x
1–x–y
x
y
magnetron sputtering methods. Optical absorption spectra of the obtained crystals and films have features
associated with band-to-band direct type transitions and d–d crystal-field transitions due to presence of
2+
Mn ions. The observed peculiarities of the Faraday rotation for Zn Mn O crystals and films suggest of
x
1–x
paramagnetic behaviour. In contrast, Zn Mn Fe O film samples exhibit ferromagnetic-like behaviour
1–x–y
x
y
which is confirmed on typical magnetic field dependence of the Faraday rotation at room temperature.
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
At present there is a growing interest to diluted magnetic semiconductors (DMS) containing transition
metals or rare earth elements. Special attention is devoted to DMS which can exhibit room temperature
ferromagnetism due to their possible applications in spintronic devices. Among many candidates of fer-
romagnetic semiconductors zinc oxide ZnO doped with transition elements became especially popular
for theoretical and experimental studies. First, Dietl et al. [1] theoretically predicted room temperature
ferromagnetism in ZnO doped with 5% Mn ions. Wang et al. [2] by using first principle calculations
based on the density functional theory and generalized gradient approximation show that the ground state
of Mn doped ZnO thin films can change from antiferromagnetic to ferromagnetic when codoped with N.
However, experimental results on magnetism in ZnO based DMS are rather controversial. In fact, these
results can be divided on two parts. While one part of publications confirms ferromagnetism, in the
second part it is not confirmed experimentally. For example, Jung et al. [3] found ferromagnetism in
Zn Mn O epitaxial films with Curie temperature of 45 K. Sharma et al. [4] revealed ferromagnetism in
1–x
x
Zn Mn O films with Mn content x = 0.02 at room temperature. Confirmation for ferromagnetic behav-
1–x x
iour of Mn-doped ZnO was also reported by Kim et al. [5], Diaconu et al. [6], Zhang et al. [7], Mofor et
al. [8], Joseph et al. [9], and Liu et al. [10]. On the other hand, Kolesnik et al. [11], Kane et al. [12], and
Alaria et al. [13] showed a typical paramagnetic Curie–Weiss behaviour without indication of ferromag-
netism. It should be mentioned that high temperature ferromagnetism in ZnO was observed after codop-
ing of Mn with Sn [14, 15]. An important point in understanding of ferromagnetism is to elucidate
whether the magnetism is originated from the substitutional dopant on cation sites or from formation of a
secondary phase. Obviously for spintronic purposes one needs confirmation for carrier mediated ferro-
magnetism. In our previous works [16, 17] paramagnetic behaviour of Zn Mn O single crystals and thin
1–x
x
*
Corresponding author: e-mail: savchuk@chnu.cv.ua, Phone: +380 372 584755, Fax: +380 372 515360
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original
Paper
phys. stat. sol. (a) 204, No. 1 (2007)
107
films has been observed. In the present work we include to study new DMS material Zn Mn Fe O. We
1–x–y x y
report here on ferromagnetism for such DMS films.
2 Experimental details
Bulk Zn Mn O crystals with content of Mn 0 < x < 0.1 were grown by the chemical vapour transport
1–x x
method. One part of the quartz ampoule coated by carbon was filled with mixture of ZnO and Mn O
3
4
powder and kept at temperature of 1050 °C. The opposite side of the ampoule has been kept at 1010 °C.
High purity (5N) carbon rods were served as a transport agent. Growth time was about 120 hours. The
3
dimensions of the grown crystals were about 10 mm and their colour varied from yellow to orange.
The deposition of Zn Mn O films with x = 0.1 was performed by pulsed laser ablation technique. In
1–x x
2
this method a XeCl excimer laser (λ = 308 nm, τ = 30 ns) with fluence of (7–10) J/cm was used. The
films with a thickness of (500–900) nm were deposited on sapphire substrates at (300–360) °C. The
targets in form of ceramic pellets were fabricated by mixing the appropriate amount of ZnO and Mn O
3
4
powders.
For deposition of new quaternary Zn Mn Fe O films on glass substrate the method of rf magnetron
1–x–y
x
y
sputtering has been used. The targets for such kind of deposition were prepared in similar way by mixing
ZnO, Mn O , and Fe O oxide powders.
3
4
2
3
The crystal structure of the grown crystals and films was studied by X-ray diffraction (XRD) meas-
urements using a Cu K radiation. Optical transmission, absorption and magneto-optical Faraday rotation
α
were measured on a set-up with a grating monochromator for wavelengths (200–2200) nm, photomulti-
plier tube and a water-cooled electromagnet which provides magnetic field up to B = 5 T.
3 Results and discussion
Structural analysis was realized by XRD spectra which show (002) and (004) peaks of wurtzite structure
for the obtained single crystalline and film samples [17]. No Mn or Fe separate phases have been found
during structural studies.
Figure 1 shows high photon energy part of the absorption spectra of Zn Mn O thin film at two dif-
0.9 0.1
ferent temperatures. One can see weakly temperature dependent near band gap absorption edge together
with broad absorption band at 3.15 eV, which is absent for undoped ZnO film. In fact, the energy posi-
tion of this peak for different samples (with different content of Mn) varied from 2.85 to 3.20 eV. This
5 2+
absorption peak is associated with transitions between multiplets of 3d configuration of the Mn ions.
2+
According to the calculations in framework of the cluster model [18] the ground state of the Mn ions is
6
6 4
A . The estimated d–d transitions from A to one of the lowest excited terms A corresponds to ener-
1
1
1
5,0
4,8
4,6
4,4
4,2
4,0
3,8
3,6
1
2
Fig. 1 Absorption spectra of Zn Mn O film
0.9
0.1
at different temperatures (the curves 1 and 2
correspond to 300 and 80 K, respectively).
2,8
3,0
3,2
3,4
3,6
3,8
4,0
Photon energy (eV)
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108
A. I. Savchuk et al.: Optical and magneto-optical properties of ZnMnO
0,8
0,6
0,4
0,2
0,0
Fig. 2 Transmission spectra of Zn Mn
0.98
O
0.02
(curve 1) and Zn Mn Fe O (curve 2) films
0.88
0.05
0.07
at room temperature.
1
2
1,2
1,6
2,0
2,4
2,8
Photon energy (eV)
gy of 2.99 eV that can be designed to the observed absorption peak. Weak temperature shift of the ab-
sorption edge in range of (80–300) K suggests of low value of temperature coefficient of band gap
dE /dT = 0.3 meV/K. The estimated value of this coefficient is in good agreement with value of dE /dT
g
g
reported by Hauschild et al. [19] for undoped ZnO.
Low photon energy part of transmission spectra for samples of Zn Mn O and Zn Mn Fe
0.9 0.1 0.88 0.05
O
0.07
films is shown in Fig. 2. The average optical transparency for the studied films is nearly 80%. Fringes
seen in both curves are cause by interference effect which suggests of high optical quality of the film
surfaces.
Magneto-optical Faraday effect is widely used powerful tool for different bulk DMS materials from
point of view to probe magnetisation processes and it is useful for variety of optical applications. To our
knowledge in case of oxide DMS the Faraday rotation spectra were measured only in Zn Co O films
1–x
x
[20]. These experiments demonstrated giant Faraday affect in Zn Co O at low temperature as large as in
1–x x
Cd Mn Te crystals.
1–x x
Figure 3 shows the Verdet constant of Zn Mn O single crystals as a function of photon energy at
1–x x
T = 300 K, B = 1.5 T. As seen in Fig. 3 the difference between the presented curves is the change in the
sign of the rotation angle θ for two different x. In fact, this is typical case for DMS materials type of
F
Cd Mn Te.
1–x x
1,0
0,5
0,0
1
-0,5
-1,0
-1,5
-2,0
-2,5
-3,0
-3,5
2
Fig. 3 Spectral dependence of the Verdet constant
for Zn Mn O (curve 1) and Zn Mn O (curve 2)
0.98
0.02
0.9
0.1
single crystals.
1,8
2,0
2,2
2,4
2,6
2,8
3,0
Photon energy (eV)
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
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phys. stat. sol. (a) 204, No. 1 (2007)
109
25
Fig. 4 Spectral dependence of the Faraday rotation
for Zn Mn O film on sapphire substrate (curve 1)
0.9
0.1
and Zn Mn Fe O
film on glass substrate
(curve 2) in magnetic field B = 1.5 T at room tem-
perature.
0.88
0.05
0.07
20
15
2
10
1
5
0
1,8
2,0
2,2
2,4
2,6
2,8
3,0
Photon energy (eV)
The observed reversal of the direction of the Faraday rotation in its spectral dependence is associated
with a positive and a negative part due to pure Zeeman and sp–d spin exchange interaction contribution,
respectively. According to the microscopic model [21, 22], the Verdet constant V = θ /B d can be ex-
F
pressed as
V(E) = Z f(X) + C Yg(X) ,
(1)
where X = E/E , E is photon energy and E is the band-gap energy, and Z, C, Y are fitting parameters.
g g
The f(X) function has a positive sign and g(X) function is negative. Fitting parameters C and Y contain
the oscillator strength, exchange integrals and concentration of magnetic ions. Therefore, in available
competition between two contributions the second negative term of Eq. (1) is larger than the first one if
content of Mn is large enough. However, absolute values of the Verdet constant for Zn Mn O crystal
0.9
0.1
are compared with those for nonmagnetic ZnO because of small value of sp–d exchange integrals
No(α–β) ≈ 0.1 eV in oxide DMS [23].
Main problem during magneto-optical studies of semiconductor thin films is that one should take into
account the substrate contribution to the observed Faraday rotation angle. In Fig. 4 the Faraday rotation
angle is plotted as a function of photon energy for Zn Mn O film on sapphire substrate (thickness
0.9
0.1
d = 0.5 mm) and Zn Mn Fe O film on glass substrate (d = 0.2 mm). Since thicknesses of the used
0.88 0.05 0.07
substrates are much larger than typical thickness of the deposited films (d = 700 nm) we can see more
larger substrate contributions in the resulting Faraday rotation spectra at room temperature. Common
picture which illustrates this situation is presented in Fig. 5.
10
8
6
3
4
Fig. 5 Magnetic field dependence of the Fara-
2
2
day rotation for separate Zn Mn Fe O film
0.88 0.05 0.07
(curve 1), the film together with glass substrate
(curve 2) and separate glass substrate (stright line
3) at wavelength 600 nm.
0
1
-2
0,0
0,5
1,0
1,5
2,0
Magnetic field (T)
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110
A. I. Savchuk et al.: Optical and magneto-optical properties of ZnMnO
Fig. 6 Room temperature hysteresis loop for
1,5
1,0
Zn Mn Fe O film after subtracting contribu-
0.88 0.05 0.07
ZnMnFeO film
T=300 K
tion in Faraday rotation from glass substrate at
wavelength of 600 nm.
0,5
0,0
-0,5
-1,0
-1,5
-1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0
Magnetic field (T)
On the base of such kind of consideration we have revealed interesting peculiarities in magnetic field
dependence of the corrected Faraday rotation from Zn Mn Fe O film. It was observed clear satura-
0.88
0.05
0.07
tion effect in the θ (B) dependence which starts at 0.75 T. In addition, detailed analysis of many curves
F
of θ (B) allowed us to find the room temperature magnetic hysteretic loop namely for composition of
F
Zn Mn Fe O (Fig. 6). In contrast, the observed linear dependence θ (B) and absence of hysteresis
0.88 0.05 0.07 F
for Zn Mn O film suggests of paramagnetism in this DMS.
0.9 0.1
The revealed ferromagnetic ordering in ZnMnFeO films is still far from being well understood. How-
ever, we suggest that such magnetic behaviour can be explained in terms of Dietl’s model [1]. According
to this model intrinsic ferromagnetism is mediated by delocalized or weakly localized holes in materials
of p-type. In this case magnetic and magneto-optical properties of the oxide DMS are very sensitive to
the content of the magnetic ions as well as to presence of other dopants. It should be mentioned the other
examples from literature when codoping of ZnO (Mn with N [2] or Mn with Sn [15]) has been stimu-
lated appearance of ferromagnetism.
4 Conclusions
In conclusion, single crystals of Zn Mn O were grown by chemical vapour transport and thin films of
1–x x
Zn Mn O and Zn Mn Fe O were deposited by pulsed laser deposition and rf magnetron sputtering.
1–x x 1–x–y x y
2+
The observed absorption band near band gap edge is associated with d–d transitions of Mn ions. Mag-
neto-optical Faraday rotation measurements confirmed paramagnetism in Zn Mn O and ferromagnetic
1–x x
behaviour in Zn Mn Fe O film samples at room temperature.
1–x–y x y
Acknowledgements The work has been supported in part by the Ministry of Education and Science of Ukraine
(grants No. M/128-2004 and No. D/152-2005).
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