Paramagnetism in Mn Fe implanted ZnO

Abstract

Prompted by the generally poor understanding of the nature of magnetic phenomena in 3d-metal doped ZnO, we have undertaken on-line (57)Fe Mossbauer spectroscopy on ZnO single crystals in an external magnetic field of 0.6 T, following the implantation of radioactive (57)Mn ions at room temperature. The Mossbauer spectra of the dilute Fe impurities are dominated by sextets whose angular dependence rules out an ordered magnetic state (which had been previously proposed) but are well accounted for on the basis of Fe(3+) paramagnetic centers on substitutional Zn sites with unusually long relaxation times (> 20 ns). (C) 2010 American Institute of Physics. [doi:10.1063/1.3490708]

Full text

Paramagnetism in Mn/Fe implanted ZnO
H. P. Gunnlaugsson,1,a兲T. E. Mølholt,2R. Mantovan,3H. Masenda,4D. Naidoo,4
W. B. Dlamini,5R. Sielemann,6K. Bharuth-Ram,5,7 G. Weyer,1K. Johnston,8
G. Langouche,9S. Ólafsson,2H. P. Gíslason,2Y. Kobayashi,10 Y. Yoshida,11
M. Fanciulli,3,12 and ISOLDE Collaboration8
1Department of Physics and Astronomy, Aarhus University, DK-8000 Århus C, Denmark
2Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavík, Iceland
3CNR-IMM MDM Laboratory, Via C. Olivetti 2, 20041 Agrate Brianza (MB), Italy
4School of Physics, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa
5School of Physics, University of KwaZulu-Natal, Durban 4001, South Africa
6Helmholtz-Zentrum Berlin für Materialien und Energie, D-14109 Berlin, Germany
7iThemba Laboratories, P.O. Box 722, Somerset West 7129, South Africa
8PH Department, ISOLDE/CERN, 1211 Geneva 23, Switzerland
9Instituut voor Kern-en Stralings fysika, University of Leuven, B-3001 Leuven, Belgium
10The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan
11Shizuoka Institute of Science and Technology, Shizuoka 437-8555, Japan
12Department of Material Science, Università degli Studi di Milano Bicocca, Via R. Cozzi 53, 20125
Milano, Italy
共Received 14 July 2010; accepted 28 August 2010; published online 4 October 2010兲
Prompted by the generally poor understanding of the nature of magnetic phenomena in 3d-metal
doped ZnO, we have undertaken on-line 57Fe Mössbauer spectroscopy on ZnO single crystals in an
external magnetic field of 0.6 T, following the implantation of radioactive 57Mn ions at room
temperature. The Mössbauer spectra of the dilute Fe impurities are dominated by sextets whose
angular dependence rules out an ordered magnetic state 共which had been previously proposed兲but
are well accounted for on the basis of Fe3+ paramagnetic centers on substitutional Zn sites with
unusually long relaxation times 共⬎20 ns兲.©2010 American Institute of Physics.
关doi:10.1063/1.3490708兴
Dilute magnetic semiconductors, obtained in compound
semiconductors by partial replacement of cations by mag-
netic transition-metal ions, are of current interest as potential
semiconductor-compatible magnetic components for spin-
tronic applications. For ZnO, room temperature dilute
magnetism has been predicted by theory1and observed
experimentally.2However, the origin of magnetism in
transition-metal doped ZnO is poorly understood3,4and there
are inconsistent reports in the literature. Recently, a possible
role of defects has been discussed by several authors5–8while
others have suggested unintentional precipitation.9,10
In continuation of our previous study,11 we have applied
57Fe Mössbauer spectroscopy in an external magnetic field to
study the nature of the magnetism in ion implanted ZnO
single crystals. Radioactive ion beams of 57Mn+共T1/2
=1.5 min兲ions are produced at the ISOLDE facility, CERN,
following proton induced fission in a UC2target and
element-selective laser ionization.12 After acceleration to 60
keV, high purity beams of up to 共1–3兲⫻108ions/s are ob-
tained. These have been implanted to fluences ⬍1012/cm2
into ZnO single crystals held in an external magnetic field.
Commercial 关0001兴single crystals 共CrysTec兲, hydrother-
mally grown, with typical contaminations ⬍20 ppm for 3d
elements 共⬃4⫻1016 3d/cm3兲and n-type conductivity
共102–103⍀cm兲were used.
This on-line method of populating the 57Fe Mössbauer
state via
␤
-decay of 57Mn allows for the study of truly dilute
samples with local concentration of the implanted species
below 2⫻1016 cm−3 共⬃5⫻10−5 at. %兲. The probe nucleus
is sensitive predominantly to its local atomic environment
共up to second next nearest neighbors兲and to electric and
magnetic hyperfine interactions13 and the resultant Möss-
bauer spectrum gives information on the magnetic and va-
lence state of the Fe ions and their electronic configurations.
In previous publications based on similar measurements
in ZnO 共Refs. 11 and 14兲the spectra were analyzed differ-
ently. Although the nature of the magnetism could not be
deduced, ferromagnetism was favored.11 This interpretation
is revised here in the light of the Mössbauer measurements
performed with the ZnO sample held in an external magnetic
field, which prove paramagnetism.
In the case of ordered magnetism, e.g., ferromagnetism,
the relative line intensities of the resulting sextet are ex-
pected to show a well-known dependence on the angle
␪
between the
␥
-detection direction and the magnetic field di-
rection thus enabling one to substantiate or discard such an
interpretation. Spectra of paramagnetic states, on the other
hand, may be more complex, consisting of the superposition
of more than one sextet,15 as is shown to be the case here.
Two samples were employed in this study, one for mea-
surements in an external magnetic field Bext=0.6 T, and an-
other in zero field. For the magnetic field measurements the
sample was placed directly on a permanent magnet mounted
on a rotation stage with the sample c-axis collinear with the
external field. Mössbauer emission spectra were recorded us-
ing a resonance detector equipped with a stainless steel ab-
sorber enriched in 57Fe. The detector was mounted on a con-
ventional velocity drive unit outside the implantation
chamber at 90° relative to the beam direction. First, Möss-
a兲Electronic mail: hpg@phys.au.dk.
APPLIED PHYSICS LETTERS 97, 142501 共2010兲
0003-6951/2010/97共14兲/142501/3/$30.00 © 2010 American Institute of Physics97, 142501-1

bauer spectra were recorded during an implantation period of
4 min with the crystal mounted in such a way that the angle
between the 共0001兲direction and the detected
␥
-rays was
␪
⬃60° 关Fig. 1共b兲兴. The implantation was then halted, the
sample rotated toward the detector and a Mössbauer spec-
trum recorded with
␪
⬃0° 关Fig. 1共a兲兴for four minutes. The
sample measured in zero external magnetic field was
mounted with the 共0001兲direction at
␪
⬃60° toward the de-
tector 共Fig. 2兲. Both samples were preimplanted with ⬎1011
57Mn/cm2to avoid dose dependence effects.14 Velocities and
isomer shifts are given relative to the spectrum of
␣
-Fe at
room temperature.
Mössbauer spectra obtained in the external magnetic
field are shown in Fig. 1. Compared to previously reported
spectra,11,14 the sextet parts of the spectra, as visibly best
represented by the four outermost lines, show much sharper
spectral lines. Hence the consideration of a magnetic
hyperfine field distribution in the analysis is not required.
The relative intensities of the lines change with the angle
␪
, but do not follow the behavior expected for the case
of an ordered magnetic state. A notable difference between
the two spectra is that spectral line intensities at
v⬃−2.5 mm/s and v⬃3mm/s observed in the
␪
⬃60°
spectrum 关Fig. 1共b兲兴seem to have disappeared in the
␪
⬃0°
spectrum 关Fig. 1共a兲兴. This strongly suggests the disappear-
ance of the ⌬mI=0 nuclear transition for a SZ=⫾3/2 elec-
tronic state, related to the splitting of the middle member
of the crystal field Kramers doublets expected for Fe3+
in a paramagnetic state.15 Paramagnetic 6S5/2states have
been reported earlier by electron paramagnetic resonance
spectroscopy16 and photoluminescence17 in nonimplanted
ZnO:Fe3+.
To test this hypothesis, the spectra in Fig. 1have been
analyzed in terms of a superposition of three sextets origi-
nating from the three Kramers doublets belonging to the SZ
=⫾5/2, ⫾3/2, and ⫾1/2 crystal field states. Such an ansatz
was shown to be applicable for the case of dilute Fe3+ in
␣
-Al2O3.18 In sufficiently high external magnetic field, where
level crossings are avoided 共Bext⬎0.3 T according to the
data from Ref. 17兲the magnetic hyperfine splitting of the
three Kramer doublets is proportional to 兩SZ兩共Ref. 15兲and
the relative intensities of each six-line spectrum behave as in
an effective magnetic field. At room temperature all three
Kramers doublets are equally populated and the final spec-
trum is the sum of those. We make the reasonable assump-
tions that each sextet has the same isomer and quadrupole
shifts. In addition, doublets assigned to substitutional Fe2+
共D2兲and interstitial Fe 共D3兲have been included in the si-
multaneous analysis of the two spectra. As is evident, from
the fit presented in Fig. 1, this simple model describes ad-
equately the observed spectra and their angular dependence,
thus demonstrating that the magnetically split components in
the Mössbauer spectra are attributable to Fe3+ paramagnetic
states in ZnO.
The hyperfine parameters obtained from the analysis of
the spectra in Fig. 1are collected in Table I. The parameters
obtained for D2 and D3 are in good agreement with those
obtained in Ref. 11. In addition, in the present work the sign
of the quadrupole interaction of the doublets can be deduced
from their angular dependence. The line-widths of the para-
magnetic sextets are considerably larger than that of D2. The
value of the principal component of the electrical field gra-
dient 共EFG兲deduced from the quadrupole shift in the para-
magnetic sextet, VZZ=+5.5共5兲⫻1020 V/m2, is very similar
to the value VZZ=+6.6⫻1020 V/m2at 4.2 K for isolated
Zn2+ on substitutional sites determined from 67Zn-Mössbauer
spectroscopy,19 共in both cases only a lattice contribution to
the EFG兲suggesting a similar type of configuration and al-
lowing us to conclude that the Fe3+ ions observed after im-
plantation are on substitutional sites in a paramagnetic state.
The assignment of lattice site is in accordance with finding
from emission channeling experiments.20
-12-9-6-303691
2
Velocit
y(
mm/s
)
Re
l
at
i
ve em
i
ss
i
on
(
ar
b
.un
i
t
)
B
ext║c
θ
~0o
D2
D3
±1/2
±3/2
±5/2
B
ext║c
θ
~60o
(a)
(b)
±1/2
±3/2
±5/2
FIG. 1. 共Color online兲Room temperature Mössbauer spectra obtained in
Bext= 0.6 T applied parallel to the c-axis and at two different emission
angles 共
␪
兲relative to the c-axis as indicated.
-12 -9 -6 -3 0 3 6 9 1
2
Velocit
y(
mm/s
)
Em
i
ss
i
on
(
ar
b
.un
i
t
)
B
ext =0T
θ
=60
o
FIG. 2. 共Color online兲Room temperature Mössbauer spectrum obtained
after implantation into ZnO single crystal in zero external magnetic field.
TABLE I. Hyperfine parameters obtained from simultaneous analysis of the
spectra in Fig. 1. The table lists the magnetic hyperfine field 共Bhf兲for the
SZ=⫾5/2 subsextet, isomer-shift 共
␦
兲, quadrupole splitting/shift 共⌬EQ=2␧
for the sextet兲, full width at half maximum line-width 共⌫兲with the detector
line-width subtracted, and the area fractions.
Component D2 D3 Parameter sextet
Assignment FeZn2+ FeIFeZn3+
Bhf共T兲49.3共1兲
␦
共mm/s兲0.91共1兲0.50共3兲0.19共1兲
⌬EQ共mm/s兲⫺0.39共2兲+0.85共4兲+0.12共1兲
⌫共mm/s兲0.17共3兲0.5共1兲0.63共5兲
Area 共%兲17共1兲12共1兲71共1兲
142501-2 Gunnlaugsson et al. Appl. Phys. Lett. 97, 142501 共2010兲

Figure 2shows the Mössbauer spectrum without exter-
nal magnetic field. The spectral shape is considerably differ-
ent as compared with the spectra in Fig. 1. The spectrum has
been analyzed in the same way as in Ref. 11 with a sextet
and two magnetic hyperfine field distributions in addition to
components D2 and D3. These distributions are not observed
in the spectra presented in Fig. 1.
It is not possible to analyze the zero-field spectrum with
the same ansatz as used for the spectra in Fig. 1. A variety of
effects may contribute to the rather “diffuse” or “smeared”
shape, most likely distributions of local perturbing fields
共static兲and/or relaxation effects, due to fluctuating hyperfine
fields. Additional temperature dependent measurements be-
tween 300 K and about 600 K 共Ref. 11兲showed that the
line-width of the sextet is largely temperature independent up
to 400 K and increases above 400 K. From this, further in-
formation on the nature of the spectrum-perturbing interac-
tion may be inferred. Since the Fe probes in the sample have
been implanted, they all find themselves at the end of an
implantation cascade in a local defect concentration of sev-
eral percentages. Since many of the defects produced are
paramagnetic, one has a situation similar to the case of a
concentrated paramagnetic solution 共alloy兲with a paramag-
netic neighbor in at least every second or third neighbor
shell. This can lead to strong broadening and distortion
共“smearing”兲due to dipolar 共electronic-兲spin–spin interac-
tion well known from experiments in chemical solutions and
well described as function of spin dilution.21 These 共mostly兲
static dipolar interactions may result in magnetic fields
reaching 10 mT or more, large enough to strongly perturb the
paramagnetic Mössbauer spectrum. These static fields can be
decoupled by the external magnetic field of 0.6 T, resulting
in an effective quantization of all interactions along the ex-
ternal field direction 共and z-direction of the crystal兲resulting
in the much sharper lines given in Fig. 1. There is, however,
a residual line broadening as mentioned above even when the
magnetic field is applied. This may indicate either an incom-
pletely decoupled interaction which could be removed by
applying a larger magnetic field or a perturbation caused by
time fluctuating spin–spin interaction 共aT
2type relaxation if
described in terms of magnetic resonance spectroscopy兲.
Since the latter is essentially temperature independent it can-
not easily be distinguished from a static perturbation. If the
line-broadening at room temperature is interpreted as due to
relaxation effects it implies relaxation times
␶
⬎20 ns.15,22
At temperatures above 400 K the increasing line-
broadening shows that temperature dependent relaxation is
active. This is most likely the expected spin–lattice relax-
ation which describes the coupling of the Fe ion to the lattice
and in resonance spectroscopy is denoted T1. Since the sextet
spectrum is observable up to 600 K,11 it is found that the
isolated Fe in ZnO in its 3+ state is a system with one of the
longest relaxation time found so far.
Our results show that substitutional Fe assumes both
2+ and 3+ charge states after implantation into ZnO. The
natural charge state for Fe substituting a metal ion in ZnO is
2+. However, even for Fe contaminations occurring uninten-
tionally in ZnO, isolated substitutional Fe3+ has been
detected.16,17 After implantation Fe3+ might easily occur and
might be stabilized by defects occurring from the implanta-
tion induced nearby defects 共electrically active and charge
compensating兲. Implantation may also lead to local changes
in the Fermi level favoring the 3+ state. The spectra pre-
sented here do not provide direct evidence that the Fe3+ state
is due to charge compensating defects nor for a complex
formation with the Zn vacancy as proposed previously.11
However, it has been observed that the 2+ /3+ ratio depends
on implantation dose and temperature, and this has been at-
tributed to charge compensation due to mobile zinc
vacancies.14
In conclusion, the Mössbauer measurements in an exter-
nal magnetic field provide unequivocal evidence that the
magnetic structure observed in the Mössbauer spectra of
Mn/Fe implanted ZnO is attributed to substitutional para-
magnetic Fe3+ impurities with unusually long spin-lattice re-
laxation times, an effect attributable to the weak coupling of
Fe3+ to the lattice. The results also provide direct insight in
the charge states of the Fe ions as well as their lattice sites.
This work was supported by the European Union Sixth
Framework through RII3-EURONS. K. Bharuth-Ram, W.
Dlamini, H. Masenda, and D. Naidoo acknowledge support
from the South African National Research Foundation. T. E.
Mølholt acknowledges support from the Icelandic Research
Fund. Financial support of the German BMBF 共Contract No.
05KK4TS1/9兲is also gratefully acknowledged.
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