Effect of substrate temperature on ordering and soft magnetic properties of Co2FeSi/Si(001)
thin films deposited by UHV magnetron sputtering
T. Prasanna Kumari1,2, Binoy Krishna Hazra2, M. Manivel Raja1, S. Srinath2, S.V. Kamat1
1
Defence Metallurgical Research Laboratory, Hyderabad-500058, India
2
School of Physics, University of Hyderabad, Hyderabad-500048, India
Email: binoyserampore@gmail.com
Abstract: Co2FeSi thin films were grown onto Si(001) by UHV magnetron sputtering at different substrate
temperatures (RT, 300, 400, 450, 500 and 550oC) keeping other sputtering conditions same. X-Ray reflectivity
analysis reveals an increase in surface roughness with increase in substrate temperature while the thickness of
films remains constant (45 nm). The grain size was found to increase with increase in substrate temperature. The
films are disordered at low substrate temperatures and may transform into partially ordered B2 and fully ordered
L21 phases upon increasing substrate temperatures. The soft magnetic properties were also evaluated and their
trend with substrate temperature is discussed.
1. INTRODUCTION
Spin polarization at the surface is the pre-dominant
factor that can improve performance of spintronics
devices. Now-a-days, spintronic materials which exhibit
this phenomenon are extensively studied for improving
TMR ratios for fabrication of spintronics devices. In
view of this Co-based half-metallic full Heusler alloys
were theoretically proven for 100% spin polarization.
However, to exhibit this nature chemical and magnetic
disorder have been found [1] to play a significant role
and intensified research is being carried out to improve
spin polarization values in particular at room
temperature. Experimentally, it has been verified that
Co2FeSi crystallizes in the ordered L21 structure in the
bulk alloy by quenching followed by annealing at a high
temperature [2]. Therefore, in this present work we have
deposited Co2FeSi thin films using UHV sputtering
technique at elevated substrate temperatures followed by
sudden cooling after deposition. The structural and
magnetic aspects of these films are systematically
investigated and presented.
2. RESULTS AND DISCUSSIONS
Co2FeSi/Si(001) films were characterized using
different techniques to study the effect of substrate
heating on structure and soft magnetic properties.
Fig.1(a) shows the typical XRR patterns obtained for
these thin films. From the figure, it is obvious that with
increase in substrate temperature, the roughness values
also increase and they are in the range of (1.5–4 nm). The
thicknesses of the films obtained are also almost similar
with average value (~45nm). The compositional analysis
revealed that film quality is very good without any
oxygen content. From microstructural images (Fig. 1.(b),
it is observed that grains have spherical shape and the
size increases with increase in substrate temperature. The
grain size values range from 5–65 nm. The particle size
calculated from the microstructural images are almost
same for all films irrespective of their grain size, which
is typically around (~ 4 nm). The crystal structure of the
films were determined using GI-XRD, the typical
patterns for the films deposited at RT and 500oC are
shown in Fig.1(c). It is clear that sample deposited at
500oC has L21+B2 structure and the crystallite size
calculated from XRD pattern is matching with particle
size of FESEM images. The magnetic measurements
were performed by employing vibrating sample
magnetometer (VSM) and all samples exhibited good
soft magnetic properties.
Fig. 1(d) indicates the trends of coercivity and
saturation magnetization value of these films with respect
to substrate temperature. It is observed that sample
deposited at 500oC shows low coercivity and good soft
magnetic properties. The magnetization values are
increase with increase in substrate temperature except for
that of for sample deposited at 550oC. This can be
attributed to its high interface roughness and also to Fe3Si
formation at the interface due to high substrate
temperature.
(b) films
Fig.1(a) Typical XRR pattern for Co2FeSi/Si(001) thin
deposited at various substrate temperatures. (b) Microstructure
showing large grains of 550oC with particle growth in cross
sectional inset, another inset is showing small grain size which
is deposited at room temperature. (c) Typical XRD pattern for
RT and 500oC and (d) Trend of coercivity and magnetisation
with substrate temperature.
3. CONCLUSION
From these studies it is concluded that Co2FeSi/Si(001)
film deposited at 500oC is ideal for obtaining good soft
magnetic properties and well ordered Heusler alloy
behavior for tunnel junction applications.
REFERENCES
[1].M. P. Rapheal, B. Ravel et al, Phys.Rev.B 66,
(2002)104429.
[2].S. Wurmehl, G. H. Fecher et al, Appl. Phys. Lett. 88
(2006) 032503.