Half-metallic ferromagnets have become one of the major research interests
in the spintronics community as key materials to develop spin polarized
current sources for spintronics devices. Among various classes of half-metallic
materials, full Heusler alloys have attracted significant interest due
to their theoretically predicted half-metallicity, structural matching
with substrate materials and high Curie temperatures. Galanakis et al.
predicted some of the X2YZ type Heusler alloys to be half-metallic [1], where X is a high valent
transition metal atom, Y is a low valent transition metal atom, and Z is
a sp atom. The half-metallicity of a Heusler alloy was most convincingly
demonstrated by the high tunneling magnetoresistance (TMR) value of 570%
in Co2MnSi/Al-O/Co2MnSi magnetic tunneling junctions (MTJs) by Sakuraba et al. [2] However,
the TMR ratio decreases to only 67% at room temperature, indicating that
the half-metallicity of the Co2MnSi alloy is degraded at room temperature. To overcome this problem, several
theoretical and experimental works have shown the shifting of the Fermi
level by quaternary element addition is effective [3,4,5]. So far, only
a limited number of Heusler alloys have been explored as ferromagnetic
electrodes for MTJs, and none of the tested alloys have given satisfactory
results as expected from the theoretically predicted perfect half-metallicity
of Heusler alloys. Hence, a more promising half-metallic material for the
application of spin polarized electron sources must be sought. In this
respect, we have been working systematically on the spin polarization of
various ferromagnetic Heusler alloys using the point contact Andreev reflection
(PCAR) method. However, the spin polarization of Heusler alloys deduced
by the PCAR all fall within the range of 0.5 - 0.65. The PCAR is a quick
and easy method to estimate spin polarizations of various samples with
different compositions, thus it is suitable for finding new alloys with
high spin polarizations.
Figure 1 shows how we sought for highly spin polarized alloys. Based on
ab-initio density of state (DOS) calculations, we parepare bulk quaternary
alloys. After examining structure by x-ray diffraction and Curie temperatures
and order-disordering temperatures by thermal analysis, the spin polarizaiton
of the alloys are measured by PCAR. Promising alloys are selected for thin
film and device fabrications.
Figure 1 Method of searching for highly spin polarized quaternary Heusler
alloys.
One example of spin polarization enhancement by quaternary alloying of
Co2MnSn and Co2MnGa ternary full-Heusler alloys is shown in Fig. 2. According to the ab
initio calculations by Galanakis [1], Co2MnSn is a half-metal with the Fermi level lying in the center of the band
gap, whereas Co2MnGa has a large spin up density of states at the Fermi level even though
the net spin polarization is low. Hence, an increase in the spin up states
can be expected at the Fermi level by substituting Ga for Sn in Co2MnSn.
Figure 2 The conductance curves of Co2MnGa, Co2MnGa0.5Sn0.5 and Co2MnSn alloys respectively, measured by PCAR at T=4.2 K. Solid lines are
the fits using the modified BTK model and open circles are the experimental
data.
Figure 2 shows the conductance curves that were measured at 4.2K using
the PCAR technique. Only selected curves with the lowest Z values among
the many measured conductance curves are shown here.From the PCAR conductance
curves shown in Fig. 2(a) and (b), the P values for Co2MnGa and Co2MnSn
were deduced to be 0.6 ¡Þ 0.02 whereas that for Co2MnGa0.5Sn0.5 was 0.72
¡Þ 0.02. This is the highest value ever reported for Heusler alloys using
the PCAR. This alloy was found to be a quaternary intermetallic compound,
in which the L21 phase is stable up to the melting temperature. Since it
has a strong driving force for L21 ordering, a high degree of order and
the resulting high spin polarization can be readily obtained. Thus, it
can be a very promising material as spin polarized electron sources for
CPP-GMR, TMR and other spintronics devices.
Table 1 shows the spin polarizations deduced by PCAR through our systematic
search. Compared to conventional ferromagnetic alloys, ternary Heusler
alloys tend to show higher spin polarizations because of half-metallic
tendency of the band structure. The reason why the spin polarizaiton is
only around 0.6 is due to the site disorder, as the perfect L21 structure
cannot be achieve in these ternary alloys. Several quaternary alloys tend
to show much higher spin polarizations comapred to ternary alloys. The
highest so far measured is 0.72. Some of these alloys have been tested
for device applications, and we found Co2Mn(Ge,Ga) and Co2Fe(Ge,Ga) are
particulary promising.
Figure 3 Spin polarizations of ferromagnetic alloy, ternary and quaternary
Heusler alloys estimated by PCAR measurments.
References
1) I. Galanakis, P.H. Dederichs and N. Papanikolaou, Phys. Rev. B 66, 174429
(2002).
2) Y. Sakuraba, M. Hattori, M. Oogane, Y. Ando, H. Kato, A. Sakuma, T.Miyazaki,
and H. Kubota, Appl. Phys. Lett. 88, 192508 (2006).
3) N. Tezuka, N. Ikeda, A. Miyazaki, S. Sugimoto, M. Kikuchi, and K.Inomata,
Appl. Phys. Lett. 89, 112514 (2006).
4) G. H. Fecher and C. Felser, J. Phys. D: Appl. Phys. 40 1582 (2007).
5) T. M. Nakatani, A. Rajanikanth, Z. Gercsi, Y. K. Takahashi, K. Inomata,
and K. Hono, J. Appl. Phys. 102, 033916 (2007).
Related Publications
Spin polarization measurements of L21-ordered Co2Mn(Ga0.5Sn0.5)
B. S. D. Ch. S. Varaprasad, A. Rajanikanth, Y. K. Takahashi, and K. Hono,
J. Mag. Mag. Mater. 323, 3092 - 3097 (2011).
Spin polarization of Co/Ni and Co/Pt multilayer thin films
A. Rajanikanth, S. Kasai, N. Ohshima and K. Hono, Appl. Phys. Lett. 97, 022505 (2010).
Enhanced spin polarization of Co2MnGe Heusler alloy by substitution of Ga for Ge
B. S. D. Ch. S. Varaprasad, A. Rajanikanth, Y. K. Takahashi and K. Hono,
Appl. Phys. Express.3, 023002 (2010).
Spin polarization of Fe4N thin films determined by point-contact Andreev reflection
A. Narahara, K. Ito, T. Suemasu, Y. K. Takahashi, A. Rajanikanth, and K.
Hono, Appl. Phys. Lett. 94, 202502 (2009).
Suppression of magnon excitations in Co2MnSi Heusler alloy by Nd doping
A. Rajanikanth, Y. K. Takanashi and K. Hono, J. Appl. Phys. 105, 063916 (2009).
Highly spin polarized Co2MnGa0.5Sn0.5 Heusler compound
B. Varaprasad, A. Rajanikanth, Y. K. Takahashi and K. Hono, Acta Mater. 57, 2702-2709 (2009)..
Antiferromagnetism and Spin Polarization in Double Perovskite SrLaVMoO6
H. Gotoh, Y. Takeda, H. Asano, J.P. Zhong, A. Rajanikanth, and K. Hono, Appl. Phys. Express 2 (2009) 013001 .
Highly spin polarized Co2MnGa0.5Sn0.5 Heusler compound
B. Varaprasad, A. Rajanikanth, Y. K. Takahashi and K. Hono, Acta Mater. (2009), accepted
Spin polarization of Co-Fe alloys estimated by point contact Andreev reflection
and tunneling magnetoresistance
S. V. Karthik, T. M. Nakatani, A. Rajanikanth, Y. K. Takahashi, and K.
Hono, J. Appl. Phys. 105, 07C916 (2009).
The effect of iron addition on the spin polarization and magnetic properties
in Co2CrGa Heusler alloy
T. M. Nakatani, Z. Gercsi, A. Rajanikanth, Y. K. Takahashi and K. Hono,
J. Phys. D: Applied Physics, 41 225002 (2008).
The enhancement of spin polarization of Co2MnSn by Fe doping
A. Rajanikanth, Y.K. Takahashi and K. Hono, J. Appl. Phys. 103, 103904 (2008).
Structure, magnetic property and spin polarization of Co2FeAlxSi1-x Heusler alloy
T. M. Nakatani, A. Rajanikanth, Z. Gercsi, Y. K. Takahashi, K. Inomata,
and K. Hono, J. Appl. Phys. 102, 033916 (2007).
Ab initio predictions for the effect of disorder and quarternary alloying
on the half-metallic properties of selected Co2Fe-based Heusler alloys
Z. Gercsi and K. Hono, J. Physics: Condensed Matter, 19, 326216 (2007).
Effect of Cr substitution for Fe on the spin polarization of Co2CrxFe1-xSi Heusler alloys
S. V. Karthik, A. Rajanikanth, T. M. Nakatani, Z. Gercsi, Y. K. Takahashi,
T. Furubayashi, K. Inomata and K. Hono, J. Appl. Phys, 102, 043903 (2007).
Spin polarization of Co2FeSi full-Heusler alloy and tunneling magnetoresistance of its metallic
tunneling junctions
Z. Gercsi , A. Rajanikanth , Y. K. Takahashi, and K. Hono, M. Kikuchi,
N. Tezuka, and K. Inomata, Appl. Phys. Lett. 89, 082512 (2006).
Spin polarization of quaternary Co2Cr1-xFexAl Heusler alloys
S. V. Karthink, A. Rajanikanth, Y. K. Takahashi, T. Ohkubo, and K. Hono,
Appl. Phys. Lett. 89, 052505 (2006).
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