Solid State Communications,Vol. 15, pp. 1707-17 I0, 1974.
PergamonPress.
Printed in Great Britain
NEUTRON DIFFRACTION STUDY OF THE (l - x)a Fe2O3 -xAl2O3 SYSTEM
E. Krén,B. Molnár, E. Sváb and É.Zsoldos
Central ResearchInstitute for Physics, Budapest,Hungary
(Received 20 May 1974, in revisedform 25 Augast 1974 by A. Zawadowski)
The magneticstructureof the (1 - x) a Fe2O3 -xNzO3 systemwith
x : 0-0.1 has been investigatedon polycrystalline samplesby neutron
diffraction method. The Morin transition temperatureand the Néeltemp.
erature are observedto decreaseon increasingx. The angleby which the
magnetic moments turn out of the basal plane in the Morin transition also
decreaseswith increasingx. The Morin transitiondoes not occur above
x:0.09.
1. INTRODUCTION
THE ANTIFERROMAGNETIC structure of rhombohedral hematite(o Fe203) with spacegroup R34D8d)
undergoesa first order transformation near room
temperature.The transformation is usually referredto
as Morin transition.Below the NéeltemperatureTiy,
the iron spins are ordered in the sequence+ - - +
along the [1I I ] axis.1 Above the Morin temperature
Ty , the spins lie in the (1 I 1) plane and a slight canting
produces a weak ferromagnetism.2Below Ty the spins
are directedalongthe [111] axis.l More accurate
measurementssshowed that the spins are tilted from
the [ 1 I I ] axis by 7" .In order to understandthe
mechanismof the Morin transition,the effect of
magneticdilution, achievedby introducingAl3*, Ga3*,
CÍ.*,Mn'*, Rh3*,Ti4* in o Fe203 has been studied
by severalauthors.3-e The trartsitiontemperature
was found to be highly sensitiveto the presenceof
foreign cations which usually lower it. Recent Mcissbauer
studieson.the(1 -x) aFe2O3-xAl203 systemhave
revelaedan anomalous variation of the angleg between
the spin direction and the [111] axis for both the
concentrationand the temperature.Tg: 30o was
found below Ty for all diluted samplesin the
1-8 mol % NzO3 concentration rangewhereasin a
specimenwithT .62 mol% AlzOt an intermediatephase
with g: 55o was observedbetween 100 and 140oK.
neutron diffraction study was performed on polycrystallinesampleswith x between0 and 0.1 in the
(1 -x) aFe2O3-xAl203 system.The reported
solubility of aluminain heniatiteis about lO mol%.Lo
2. EXPERIMENTAL
The sampleswere preparedchemically as the
usual ceramic method yielded no pure hematite-type
phase.Fe(OH)3 and AI(OH)3 were co-precipitated
from the aqueoussolution of FeCl3. 6H2O and AlCl3
using NH4OH. Repeated washing removedCl-from
the precipitateswhich were then annealedin air at
1200"C for 2 hr. X-ray diffraction Debye-scherrer
photographsshowed singlehematite-typephasesfor
all samples.Magnetite type impurity phasecould not
be detectedby the neutron diffraction measurements,
too, thus its quantity, if any, must be below 5 per cent.
The monotonous variation of the lattice parameters
and magnetic transition temperatureslisted in Table I
shows that solid solutions are formed.
The neutron diffraction measurementswere
.carriedout in the 6-1000"K temperaturerangeat the
WRS-M reactor in Budapest using neutrons with a
wavelengthof 1.144.
3. RESULTS
The magnetic transition temperaturesTya and T1s
In order to further investigatethese anomalies,a
were determinedby measuringthe temperature
1707
1708
NEUTRON DIFFRACTION STUDY OF THE (1-x) o Fe2O3-xAl2O3 SYSTEM Vol. 15, No. 10
Table I. Values of the lattice parameters a, c, the transition temperatures Ty, Ty and angle gs1 in the
( 1 - x)aFe2O3-xAl2O3 system
x
a(A)
0
0.016
0.038
o.u6
0.054
0.061
0.080
0.102
5.034
5.031
5.026
s.026
5.024
5.019
5.011
5.008
á(A)
Tu(K)
13.749
13.741
13.721
13.714
t3.7rt
13.698
13.669
13.663
F
LN
z
L!
F
z_
2
o
F
l
u
z
=
0
100
200
(OK)
-JEMPERATURE
300
FIc. I . Temperature dependenceof the (1 11) neutron
diffraction reflection for samples with different x
concentrationsin the (1 -x) c Fe2O3-xAl203
system.
gttr (o)
at T 1Tya
263
224
180
170
156
148
137
dependenceof the (1 11) neutron diffraction reflection
which is wholly magnetic in origin. The results arelisted
in Table 1. Since the intensity of the (11l) reflection
is proportionalto sin2 9111,where grrr is the angle
of the magneticmoment to the Il I 1] direction,the
Morin transition, associatedwith a flip of the magnetic
momentbetweenthe Il I l] directionand (111) plane,
can be easily followed up by studying the variation
of this reflection with the temperature.The results
obtained in the 6-300'K temperaturerange are shown
in Fig. 1.
( l - x ) s F e 2 0 3- x A t 2 0 3
rNcK),
968
953
928
920
908
900
873
8s3
15
17
30
28
s0
78
83
90
grr, (o)
at T)
Ty
90
90
90
90
90
90
90
90
It can be seen in Fig. 1, that the Morin transition
is complete at l00oK in all samples.The transition is
gradually broadenedand its telnperatureis shifted to
lower temperatureson increasingconcentration x. No
further changein the neutron diffraction patternqcan
be observedbetweenliquid nitrogen and helium
temperatures.Table I givesthe values of Tya which are
taken as the centre of the transition range.The thermal
hysteresismeasuredin pure o Fe2O3 cannot be
observedin the solid solutions.
If the magnetic moments are directed along the
I
I
[1 ] direction at low temperaturesas given by
Shull et al.L, the (111) reflectionwould have zero
However, as is apparent from
intensity below 2n1y.
Fig. l, the intensity does not fall to zero even in pure
o Fe2O3 and thereby indicating that the magnetic
moments are not aligned along the [1 I 1] direction
but are inclined to it by an anglewhich increaseson
increasingx. The values of gr rr , given in Table I , were
determined from the neutron diffraction patterns
taken above and below the transition range.At x : 0
the residual(111) reflectionis very small,thus the
accuracyof grrr is ratherpoor. The value of 15 t lO"
is consistentwith the more precisevalue of 7" obtained
on the single crystal.3 The magneticmoment of the
Fe3* ions at 77"Kwas found to be 4.9 t 0.1 pa
independently of the composition.
The results are summaized in Fig. 2.\\e results
of the magnetic measuÍementson the weak ferro.
magnetic moment are cohsistent with the neutron
diffraction data.ll
Vol. 15,No. 10 NEUTRON DIFFRACTION STUDY OF THE (1 -x) o Fe2O3-xAl2O3 SYSTEM
plane. ln addition to the strift of 77,1, the angle by which
the moments turn out of the basal plane in the Morin
transition is also decreasedwith decreasing dilution.
At about x : 0.09 the Morin transition no longer
occurs' the magnetic moínents lie in the basal plane at
all temperatures below Tlv. The results in Fig. 2 clearly
show that the Morin transition disappears as the angle
by which the moments turn in the transition strongly
decreaseswith increasing x and not as a result of the
transition temperature shifting to very low temperatures.
(l-x)aFe20.-xAl203
1000r
É soot
g, : t
Í.-"..
Txr
_o-o-o-oH
___o_
|
5 aoor
É :ooT
HÍ^ly
E 200
Fl
100 r
9
0-o_
o
The nature of the Morin transition is also changed
by the dilution. The abruptness of th6 transition together with the thermal hysteresis indicate a first order
transition in samples with x near 0, whereas the transition seernsto be of second order for higher values
of x.
T >Tpr
o
=60
5
UJ
"{
230
(mole % A12Q)
FIc.2. Concentration dependenceof the transition
temperaturesand anglegrrr in the (l -x) o Fe2O3xAl2O3 system.
4. DISCUSSION
As seen in Table I and Fig.2, the dilution of
c Fe2o3 by Al.* res Íts in a decreaseinthé.magneiic
transition tanperaturcs Ty and fN and lattice parameteni a and c. The variation is linear for [y. A peculiar
feature of the rystem is that in the low temperature
magnetic structure, the magpetic moments deviate
from the [111] direction being in an intermediate
position between the [111] direction and the basal
The presence of an intermediate magnetic phase
during the Morin transition was not observed in the
studied samples, in contrast with the observation of
Srivastava aird Sharma.T The present restrltsconcerning the shift of the transition temperature fM wittr
^ dilution are in agreementwith the reported datas-8
and can be interpreted by the anisotropy theory.8
However, the observation of the incomplete turn of
the moments in the Morin transition and the stability
of concentration dependentintermediatemoment
directions between the [111] axis and the basal plane
requires an extension of the existing theory.
Acknowledgments - The authors are indebted to
Prof. L. Pál for his constant interest in this work and
to Dr. M. Pardavi.Horváth for magnetic meazurements.
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t7@
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11. . PARDAVI.HORVÁTH M. (privatecommunciation).
(1 - x)cFe2o3_xAl2o3 IIpI4.x: 0_0,1
MarHntnafl cTpyKrypa c}ÍcTervrn
6uaa uccJIeAoBaHa uefirporrorpafiuqecKl,ÍMMeToAoMHa IIo
noJII{Kp}ícTaJlJII,íqecxux
o6pa3qa)G btno onpegeJleno, qTo TeMnepaTypa
x
Mopnna }Í TeMnepaTypa Heelg npl,ÍBo3pacTanIIIeM3HaI{eHIII{
MoMeHTa
I,í36asl,Ícltofi
ItoH}íx(aeTcfl
. Vro.n'noBopoTa MaI-HI{THo|o
rrJrocKocrfi npfi rrepexoAe qepe3 ToqKy MopuHa roxe nonuxaercfl
npl,íBo3pacTanuu x. llepexog MopuHa sooőqe He IIpoI4cxo.4{TBbIIIle
x:0.09.
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