Magnetic anisotropy and metal-insulator transition in SrRuO3 thin ...

113925-4 Wang et al. J. Appl. Phys. 107, 113925 2010
(a)
(c)
(a)
(b)
FIG. 4. Color online a and b Temperature dependence of resistivity of
SRO films grown at different temperatures 600, 650, 700, and 750 °C.
Inset in b: R-T curve of film grown at 650 °C, replotted in the temperature
range from 150 to 300 K.
(b)
FIG. 3. Color online a–d Hysteresis loops of SRO films grown at
different temperatures 600, 650, 700, and 750 °C in the OOP and in the IP
directions at 10 K. Inset in d: the growth temperature T g dependence of
magnetic moment recorded at 30 kOe in the OOP direction at 10 K.
theory that the magnetic anisotropy is modified by a magnetoelastic
coupling depending on its magnitude and sign. In
addition, Barkhausen jumps were observed in the hysteresis
loop between two regions of opposite magnetic field.
Barkhausen jumps, commonly due to the irreversible motion
of the domain walls between the two regions of opposite
magnetizing forces 15 are not very spectacular in themselves
and have been reported in other references. It can be seen
from the inset of Fig. 3d that the magnetic moment of films
grown at 600 and 650 °C is almost the same with a value of
about 0.57 B /Ru. at 30 kOe. The magnetic moment increases
with increasing growth temperature and to about
1.39 B /Ru. for films grown at 750 °C, agreeing closely
with the calculated value of 1.45 B /Ru. 16 The temperature
dependence of the magnetization not shown here was
found to show that the Curie temperature of film grown at
600 °C is around 158 K, and increases to about 165 K for
films grown at 750 °C. The decrease in the magnetic moment
and the Curie temperature with decreasing growth temperature
indicates that these factors are affected inversely by
the greater strain produced at lower temperatures. Namely,
larger induced strain in films at lower growth temperatures
changes the spin-spin coupling through a change in the Ru–
O–Ru interatomic distance or bonding angles, consequently
resulting in changes in the exchange energy among
spins. 3,4,16
Figures 4a and 4b represent the temperature dependence
of the electrical resistivity of SRO films grown at different
temperatures. When grown at 750 °C, the film exhibits
a typical metallic behavior in the whole temperature range
from 5 to 310 K. Its resistivity is about 210 cm at 300
K, which is comparable with that of bulk single crystal SRO
about 195 cm. 16 A minimum of resistivity appears at
15 K in the film grown at 700 °C. It is commonly thought
(d)
that this phenomenon is due to disorder produced during
initial growth. However, the temperature corresponding to
the minimum of its resistivity shifts to 250 K with films
grown at 650 °C. Moreover, the resistivity behavior is insulator
in the whole temperature range from 5 to 310 K when
the film was grown at the lowest temperature 600 °C. Resistivity
behavior in our case is similar to that reported in
Refs. 1 and 17, where SRO films grown on MgO and SRO
films grown on STO experience a transition from metal to
insulator when the growth temperature decreases. However,
as mentioned in the introduction, when SRO films grown on
MgO, 3 no significant change in either Curie temperature or
lattice constant is observed with variation in growth temperature.
But in our case and Ref. 17, OOP lattice constant increases
gradually with decreasing growth temperature, indicating
that the film has more OOP tensile strain when grown
at lower temperature possibly indicating a better epitaxial
quality. Furthermore, the Curie temperature in Ref. 17 125
K and our case 158 K for the film grown at lower temperatures
690 and 600 °C both decrease, compared with
ones grown at a higher temperature. In addition, in our case,
the magnetic moment of the film grown at 600 °C decreases
slightly more than 60% as compared with the value of the
film grown at 750 °C. So we argue that metal-insulator transition
and insulator behavior in our films grown at lower
temperature are mainly due to larger strain produced at lower
growth temperature. In addition, 3D islands observed in our
films grown at lower temperatures inevitably introduce microstructure
disorder, which also contributes to metalinsulator
transition or insulator behavior.
IV. CONCLUSIONS
In summary, magnetic and transport properties of SRO
films grown with a variation in growth temperature have
been investigated. With decreasing growth temperature, OOP
lattice constants increase from 0.395 nm at 750 °C to 0.403
nm at 600 °C, correspondingly, OOP tensile strain increases.
As a result, metallic behavior appears in the whole temperature
range from 5 to 300 K for the film grown at 750 °C
and a metal-insulator transition occurs at 15 K and 250 K for
films grown at 700 °C and 650 °C, respectively, and then a
complete insulator behavior appears in whole measured temperature
range for film grown at 600 °C. At the same time,
OOP magnetic anisotropy gradually transforms into mag-
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