Effect of a JahnTeller ion on the ground states of Fe2+ and Ni2+ ions in
spinel ferrites
C. M. Srivastava, M. J. Patni, and T. T. Srinivasan
Citation: J. Appl. Phys. 53, 2107 (1982); doi: 10.1063/1.330973
View online: http://dx.doi.org/10.1063/1.330973
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Published by the American Institute of Physics.
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Effect of a Jahn-Teller ion on the ground states of Fe2 + and
Ni2 + ions in spinel ferrites
C. M. Srivastava, M. J. Patni, and T. T. Srinivasan
Indian Institute o/Technology, Bombay-400076, India
The influence of crystal distortion due to a Jahn-Teller mechanism on lattice constant, Curie temperature,
magnetization, and geff has been investigated by substituting Cu 2+ ions in FeZn and NiZn ferrites. The
observed results have been analyzed on the basis of exchange, spin orbit, and Jahn-Teller mechanisms. The
changes exhibited on copper substitution in the two spinels have sharp differences. These have been attributed
to the difference in the ground state of the divalent ions.
PACS numbers: 76.50.
+ g, 75.50.Gg
INTRODUCTION
geff was obtained at 77 and 300K using a x-band micro-
The magnetic ordering and ferromagnetic resonance
.
f N'Z
l,Z
3,4 spinel ferrites have
~ n a n d FeZn
propert~es 0
been studied by a number of workers. These have been
satisfactorily explained 5 on the basis of Yafet-Kittel
type of spin ordering on the B-sublattice. It is well678
known "
that crystal distortion arising from JahnZ
Teller effect occurs whenever an ion like Cu + is
present at the octahedral site. In an attempt to
investigate the effect of such a crystal distortion on
the properties of exchange coupled magnetic ions in
magnetically ordered systems, we have introduced small
Z
quantities of Cu + ions in Zn Nil Fe 0 and
x
-x Z 4
Zn Fe _ 0 ferrites. The two systems were so chosen
x 3 x 4
that in one all the magnetic ions are in an orbital
singlet state (NiZn) while in the other these are
magnetic ions with orbital degeneracy (FeZn). This
was done to see if the effects due to spin-orbit,
exchange and Jahn-Teller mechanisms which are of
comparable strength can be separated out.
Samples with composition ZnxCuO.lFeZ.9_x04'
ZnxNiO.9_xCuO.lFe204 and ZnxNiO.8_xCuO.2FeZ04
(0 < x < 0.8) have been prepared and their lattice
constant (a), magnetization (M )' Curie temperature
s
(Tc) and geff compared with those of the corresponding
ferrites without Cu substitution.
The change in Ms' a,
Tc' and geff on Cu substitution in NiZn and FeZn
wave bench and applying the usual corrections for size
Z
effect . The data on FeZn and NiZn ferrites were taken
from references 4,9 and 2,5 respectively.
RESULTS AND DISCUSSION
The change in lattice constant on copper substitution has been shown for FeZn and NiZn ferrites in
Fig.(l). On copper substitution the lattice constant
has increased slightly in FeZn while it has decreased
significantly in NiZn. In both the cases the change,
6a, is very nearly independent of the Zinc content
(Fig.2) and it is -0.028 AO for NiZn and +0.005 AO for
FeZn.
The variation of magnetization, Ms with Zn concentration for FeZnCu, NiZnCu, FeZn and NiZn ferrites at
room temperature and at OOK is shown in Fig.(3). With
0.1 Cu in NiZn, H is higher at room temperature coms
2+
pared to NiZn for x < 0.3. Since Cu
has lower moment
than Fe 2+ and Ni 2+ this is not readily understood. In
presence of Y-K angles at 0 K for FeZn and NiZn the
magnetization is given by:
MS=NJ-IB[(l-x)geff S coset. + (l+x)5 coset. - (1-x)5]
(1)
where g ff and S are the values for Lande g factor and
2
2
spin fo: the divalent magnetic ion Ni + and Fe + in
NiZn and FeZn respectively, N is the number of formula
systems have sharp differences. At room temperature,
Ms on 0.1 copper substitution in NiZn increases for
x < 0.5 while in FeZn it decreases for almost all
values of x. On Cu substitution the lattice constant
decreases in NiZn but it increases in FeZn. The Curie
temperature is almost unaffected by a substitution in
NiZn while it decreases for all values of x in FeZn.
On copper substitution the geff is almost uniformly
t
changed for all values of x in NiZn while in FeZn the
change at x = 0 and for 0.5 < x < 0.8 is negligible
and has a peak at x = 0.3. An attempt to explain these
observations on the basis of spin-orbit, excr.allge and
Jahn-Teller mechanisms has been made.
... 8.39
18.50
IA3
._..r-
IAl
FeZnCu
~}115
~__
~-- FeZn
Hi Zn
I
1.37
~.
__- - -
J!IO
__. - -
.--~
.-.JJ
8,35
cr'NiZnCu
-~
EXPERIMENTAL
o~____~____~__~~__~uo
Polycrystalline samples of the above mentioned
compositions were prepared by the usual ceramic technique. Presintered oxides were finally sintered between
.6
Fig. (1)
1250 and l350°C in nitrogen atmosphere for FeZnCu and
in air for NiZnCu samples. The samples were found to
be single phase cubic by x-ray analysis. The density
was greater than 95% of the x-ray density for all the
samples. The magnetization measurements were made between 77K and Tc using a vibrating sample magnetometer.
2107
J. Appl. Phys. 53 (3). March 1982
0021-8979/82/032107-03$02.40
.1
The variation of cell constant with
Zinc concentration in
Znx Fe _ x 0 4 (FeZn)Zn x CU O• l
3
Fe 2 • 9 _ x 0 4 (FeZnCu) Nil_xznxFe204
(NiZn) and Ni • _ x zn CU O• l Fe 2 0 4
x
O 9
(NiZnCu) spinel ferrites.
© 1982 American Institute of Physics
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2107
.02
unlCS per unit volume and
1
.4
- -, -...---
is the Bohr magneton.
In
M~~N~B[(0.9-x)g~ffs cos a + 0.1 gc 1 cosa '
UZ
*a(lfillCu)-a(Ni In)
_f~A~--*--~*~J*'--~*~~.~~.~-*
.O~
Fig. (2)
~B
this equation we have assumed that Fe 3+ has a moment of
5~B'
On 0.1 copper substitution, we have
a a(FeIIlCu)-a(Feln)
The change in cell constant, ~a, on
copper substitution for NiZn and
FeZn ferrites.
700
+ (1+x)5 casa' - (1-x)5j
(2 )
where g~ff and a ' indicate the new values of g factor
and Y-K angle on copper substitution.
llM
s
~
Thus,
M - M'
s
s
~ N~B[0.9-x) S (geff cosa - g~ff cosa ' )
+ O.l(geff S cosa - 1 gc
"2
600
u
cosa')
+ 5(1+x) (cosa - cosa')]
(3)
In NiZn as shown in Fig.(4), Tc is not affected significantly on copper substitution. We may then assume
that a - a'. In that case we would expect
0'
N~B cosa[(0.9-x) (geff - g~ff)
llMs
u
'-::J
1
+ O.l(geff - "2 gCu)]
E
300
w
~ N~B cosa[nl
(4)
fie then expect that 0 K, (llMs/Ns) = n/2 " - 0.04.
III
~
200
Fe Zn Cu
Fe Zn
100
o
900
0.8
O.L.
0.6
x-
0·2
Fig. (3) (a) The variation of magnetization with
Zinc concentration, x=(O ~ x ~ 0.8)
in Zn x Fe 3 _ x 0 4 and znxcuO.lFe2.9_X04
at
° and
300K.
~600
700
u
I-
I
I
600
I
I
P
500
I
d
500
I
1.00
I
v
f
L.OO
Ni
Ni
l Ni
I
I
~
v
'--
300 K
III
~
\
200
\
N i ZI1 Cu
Ni Zn
100
\
\
\
x ....
\
b
0
0.2
0·4
x
Fig. (3) (b) The variation of magnetization with
Zinc concentration, x=(O ~ x ~ 0.8)
in Znx Ni _ Fe 0 and
l x
2 4
Znx Ni • _ CU n• I Fe 2 0 4 at 0 and300K
o 9 x
2108
J. Appl. Phys. Vol. 53, No.3, March 1982
Fig. (4)
The variation of Curie constant, Tc'
with Zinc concentration, x =
(0 ~ x ~ 0.8) for znx CU y Fe 3 _ xy 04
(y = 0, .1) and znx Ni I _ x _ y cU y
Fe 0
(y = 0, .1 and .2). The
2 4
.
.
changes in T on subst~tut~ng eu in
c
.
.
Nizn system are small a~d ~re w~th~n
the experimental error ~nd~cated by
the triangles.
Magnetism & Magnetic Materials-1981
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2108
This is of the right sign but smaller by a
fa c tor 0 f 3
compared to the observed value of
-0.12. This result indicates that a small percentage
of copper is going on the A site. A value of 2% of Cu
on A site accounts for the observed result. The magnetization data on FeZnCu can be explained assuming
that all of copper is on B sub lattice. On this basis
the values of a' obtained using the observed values of
g~ff and M~ are 5, 18, 36 and 52° for x = 0, 0.2, 0.4
constant J and hence in Tc' Simultaneously it leads to
2
a paramagnetic state of the Fe + ion and hence to an increase in geff as observed. The max i mum 1n
. geff corres-
and 0.6 respectively. The corresponding values of a
for FeZn are 0, 14, 27, and 44. This increase in the
value of a is consistent with the observed decrease in
Tc on copper substitution.
singlet (r 2 ).
The variation of Tc on copper substitution has
been shown in Fig.(4) for both FeZn and NiZn samples.
It is observed that copper substitution does not affect
Tc significantly in NiZn but leads to a substantial decrease in FeZn.
The decrease in Tc from Fig.(4) can be
expressed in the form
Tc (K) = 250 x + 50 where x is the
(5)
fraction
The
FeZn and
NiZn the
of Zn atoms in FeZnCu.
variation of g-factor for Cu substitution in
NiZn is given in Fig.5(a) and Fig.5(b). In
difference I1g
= g(NiZnCu) - g(NiZn) is aleff
most independent of x but in FeZn it has a maximum at
x = 0.3 and almost vanishes at x = 0 and x > 0.5.
The explanation for the difference in geff in
2
NiZnCu and FeZnCu lies in the ground states of the Fe +
2
and Ni 2+ ions. Fe + ion without exchanges is in an
orbital triplet state. With exchange the wave function
in the ground state is real so that the orbital moment
vanishes. Consequently the g-factor drops from 3.24
2
for a paramagnetic Fe + ion with L-S interaction in the
2
ground level to 2.25 for Fe + ion in Fe 0 without it lO
3 4
2+
If a copper atom is present near a Fe
ion the J
exBB
change due to baa, transfer integral vanishes 11
This
ponds to the maximum in the probability of the micro. h a central Fe 2+ 10n
.
state W1t
having 4Fe 3+ • lCu 2+ and
2
lFe + nearest neighbour which proves our assumption.
In NiZn the ground state of Ni 2+ is an orbital
The g factor is changed from 2 to 2.26
r 4 state on account of
the spin-orbit interaction. The local distortion pro2
duced by the Cu + ion due to Jahn-Teller mechanism reduces the crystal field splitting I1 from 8000 cm- l to
due to the mixing of the higher
4000 cm- l raising the contribution due to spin orbit
interaction from 0.26 to 0.46. This is independent of
zinc concentration and does not affect the ground
orbital state and hence the exchange interaction. Consequently, Tc remains unchanged.
CONCLUSION
The effect of crystal distortion due to Jahn-Tel1er
mechanism in Fe-Zn and NiZn spinel ferrites has been
studied by substituting small amounts of copper in
these ferrites. The changes in lattice constant, magnetization Curie temperature and geff on copper substitution in the two spinels show sharp differences. This
has been explained on the basis of the difference in
2+
2+ .
the ground states of the Fe
and Ni
10ns and the
relative strengths of the spin orbit, exchange and
Jahn-Teller mechanisms.
leads to a decrease in the strength of the exchange
2.6
-
..,.
GO
2
2'~
REFERENCES
____~____~____- r____~
x
8
~
Fig. (5) (a) The variation of geff with Zinc
concentration, x, (0 ~ x ~ 0.8) in
Zn Fe 3 _ x 0 4 and znxCu O. 1 Fe 2 ,9-x04·
x
2.6
t
Fig. (5) (b) The variation of geff with Zinc
concentration, x, (0 ~ x ~ 0.8) in
znx Ni 1 _ x _y CU y Fe 2 0 (y=0,.1,.2).
4
2109
J. Appl. Phys. Vol. 53, No.3, March 1982
1)
L.K.Leung, B.J.Evans and A.H.Morrish, Phys. Rev.
B~, 29 (1973).
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Hoshino, Y., Iida, S., and Sugimoto, M., University
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4) C.M.Srivastava~S.N.Shringi and A.S.Bommanavar,
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Magnetism & Magnetic Materials-1981
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2109