Transcript Наноматериалы для спинтроники

Magnetic and transport
properties of SiMn films with
the high Mn content
Aronzon B.A.
Rylkov V.V.
Tugushev V.V.
Nikolaev S.N.
.
Perov N.S.
Semisalova A.
S. Caprara
Podolskii V.V.
Lesnikov V.P.
PRB, 84, 075209 (2011)
RRC “Kurchatov Institute”, Moscow,
Russia
Moscow State University, Russia
Dipartamento di Fisica, Universita di Roma
NIFTI, N. Novgorod , Russia
1
A. Lashkul
Lappeenranta University of Technology, Finland
Outline
1 Introduction.
What is known about SiMn structures?
2 Transport, AHE
3 Magnetic properties
4 Model
5 Conclusion
2
MnxSi1-x
The equilibrium solubility of Mn in Si is very low (~1016 cm-3).
It is needed to reach higher manganese concentration
(1021cm-3). Mn impurities in Si favor interstitial position and
act as donors, that results in very weak exchange interaction.
Si
Si
Mn
Mn
While strong hybridization of Mn 3d states with s,p states in Si
occurs if Mn enter substitutional (MnSi) positions as
acceptors
Binary compounds of 3d metals with Si are weak itinerant
magnets of helicoidal type with Curie temperature <50K
(no hysteresis loop).
What is known about magnetic properties of MnxSi1-x
1. Ion beam implantation Mn (х ≈ 0.8 %) : Tc > 400 K (M. Bolduc et al., Phys. Rev. B
71, 033302 (2005)).
Magnetism is due to paramagnetic defects T. Dubroca et al., Appl. Phys. Lett. 88, 182504
(2006); A.F. Orlov, A.B. Granovsky et al. JETP 109, 602 (2009)
2. Magnetron sputtering. (х ~ 5%): ТС ≈ 250К (X.C. Liu, Z.H. Lu et al., J. Appl. Phys.
100, 073903 (2006); 102, 033902 (2007)), p ≈1016cm-3
3. MBE [Si(20Å)/Mn(1 - 2Å)] (х ~ 5-10%):ТС ≈ 300К (S.H. Chiu et al., J. Appl. Phys.
103, 07D110 (2008)). While (х<17,5%):ТС ≈ 3К (L. Zeng, PRB, 77, 073306
(2008)) Magnetiztion; no AHE.
4. Magnetron co-sputtering. Mn-doped amorphous Si:H (х ~ 10 %):Т ≈ 150К (J.H.
Yao, S.C. Li et al., Appl. Phys. Lett. 94, 072507 (2009)).
5. Mn –Si complexes (2-3) B/Mn (Q. Liu et al. Phys. Rev. B 77, 245211 (2008)) and
self- organized in Si1-xMnx molecular clusters (S. Zhou et al. PRB 75, 085203
(2007); 80, 174423 (2009)) (> 0.2 B/Mn)
Mn4Si7 ТС ≈ 50К (A. Sulpice et al., JMMM. 272-276, 519 (2004)).
Method: Anomalous Hall Effect
The Hall resistance RHd= yx = R0B + RsM
R0 and Rs are the ordinary and anomalous Hall coefficients.
Anomalous Hall Effect is proportional to magnetization.
Two types of mechanisms:
skew –scattering - Rs Rxx and side-jump - Rs R2xx
For both mechanisms AHE depends on the strength of the spin-orbit
interaction and spin polarization of carriers. The sign of either of the
two contributions can be positive or negative depending on an
interplay between the orientations of orbital and spin momenta as well
as on the character (repulsive vs. attractive) of scattering potentials.
[T. Dietl (2007)]
AHE current arises because the impurity v(k) = grad [ε(k)]/h + (e/h)E(k)
cross-section seen by beam of electrons z(k) = 2Im[<u/ky|u/kx>]
possesses right-left asymmetry
 - does not depend on scattering
T. Jungwirth et al.(2006), T. Dietl et al.(2003), S.Y. Liu et al. (2005),
V.K. Dugaev et al. (2005)
Why Anomalous Hall Effect ?
AHE depends on the strength of the spin-orbit
interaction and spin polarization of carriers
AHE is not affected by the magnetism of
substrate
AHE mainly is not affected by the inclusion of
second phase
AHE is not need an expensive equipment and
could be measured easily
Parameters of MnxSi1-x samples, x ≈ 0.35
Samples
number/
substrate
Rxx(77K)/
Rxx(290K)
Growth
temperature, Tg
°C
d, nm
Hc- coercitivity at 80
K (Oe)
Nо1
Al2O3
0.94
300
40
2900
Nо2
Al2O3
0.93
300
57
2000
Nо3
Al2O3
0.85
350
55
4200
№4
GaAs
0.85
300
80
0
№5
GaAs
0.84
300
50
0
№6
GaAs
0.97
200
75
330
№7
GaAs
0.89
300
300
650
Hole concentration p ≈ (2 – 3)1022 cm-3
AHE
sign
+
+
+
-
2000
Microanalysis Report
GaAs substrate
Counts
1500
1000
Mn-L
Si-K
500
0
Mn-K
2
4
6
Energy, keV
EDAX ZAF Quantification Standardless SEC Table : Default
8
Anomalous Hall effect up to room temperature
0.04
77K
100K
5K
230K
0.00
RH , 
a
RH, 
0.03
77K
300K
0.00
AHE of
strongly doped SiMn.
-0.04
-2
Maximum Tc of
-0.03
MnSi silicides
not exceed 50 K.
-0.7
0
B,T
2
Hall resistance
0.0
0.7
B, T
The Hall resistance is determined mainly by the anomalous component even at
room temperature and has negative sign while normal Hall effect is positive.
Hysteresis is observed up to  230 К.
Hole concentration obtained from the normal Hall effect p  21022 cm-3.
Rs  2.410-8 Ohmcm/Gs (10-7 Ohmcm/Gs for GaMnAs with p  1021 cm-3,
S.H. Chun, et al., Phys. Rev. Lett. 98, 026601 (2007) ).
Comparison with Mn4Si7
U. Gottlieb at el., JMMM (2004) and
Our results
Rxx(T)/Rxx(290K)
1.0
2'
0.9
7 11
1
0.8
0.7
12
2
The growth temperature:
o
o
o
1, 2, 2', 7, 11 - 300 C; 3- 350 C, 12-530 C
0.6
0.5
3
0.4
0
50
100
150
T, K
200
250
300
Comparison with (Si:H)Mn
Our results
0.004
300 K
RH, 
0.03
77K
0.000
RH, 
-0.004
0.00
230K
-0.7
0.0
B,T
0.7
-0.03
-0.7
J.H. Yao et al., Appl. Phys.Lett. 94, 072507 (2009)
0.0
0.7
B, T
JETP Letters 89, 707, (2009)
Comparison with Mn4Si7
Mn4Si7 Tc<50K
U. Gottlieb at el., JMMM (2004)
MnxSi1-x TC> 300K
Hall effect
0.02
RH , 
0.01
Sample 1:
Tg=300C
56K
0.01
Sample 3:
Tg = 350 C
5K
RH, 
0.02
0.00
0.00
-0.01
59K
-0.01
5K
-0.02
-3
-2
-1
0
B, T
1
2
3
-3
-2
-1
0
1
2
3
B, T
For sample grown at Tg =300 C coercive field Bc strongly rises (2.8 times)
when temperature lowering from 56 K down to 5 K.
It is so also for Ga1-x MnxAs (at Т  ТС).
Contrary to that for sample grown at Tg =350 C coercive field Bc diminishes
with temperature lowering from 59 K down to 5 K.
Magnetization
B, T
Magnetic moment per Mn atom  0.1 B/Mn.
In Mn4Si7  0.012 B/Mn.
Correlation between AHE and magnetization
-7
20
M, G
0
0.0
a
xy , Ohm*cm
2.0x10
-7
-20
-2.0x10
-1
0
B, T
Si1-хMnх/Al2O3
(№2) d=57 nm
1
Coercitivity and saturation magnetization
vs. temperature measured by AHE and SQUID
Curie temperature.
Magnetization. Temperature dependence
Coupling between local magnetic
moments of MnD defects in the
MnnSim host mediated by spin
fluctuations (SF).
For DMS M(T) could be fitted by
F(y) = 1 − yn,
with y = T/TC ( n ≈ 2 for GaMnAs)
In the SF mode
y = T (T − ThC)/Tc(TC − ThC)
ThC = 50 K – Curie temperature of
matrix (host).
n = 1.3–1.5
Model
Si1-xMnX
MnSiy
Mn4Si7
MnSi1.75
35%Mn
MnSi1.86
Magnetic defects, molecular cluster with
HOST
magnetic moment (2-3) B/Mn
Weak itinerant magnet of
Q. Liu et al. [Phys. Rev. B 77, 245211 (2008)]
helicoidal type
Spin density is delocolized
due to hybridization of Mn 3d
– states and Si (s,p) - states
Magnetic moment ~ 0.1 B/Mn
Mn atoms in molecular clusters ~ (3-5) %. Distance between them a0 ~ 10-12 Å.
In the molecular cluster 4 - 5 Si atoms per Mn. Tetrahedral arrangement of Si
surrounding Mn.
Model for long-range order FM
Two contributions
RKKY (through free carriers 21022 cm-3)
TC
 20  30K
The long-range ferromagnetic order at high temperatures is
mainly due to the Stoner enhancement of the exchange coupling
between magnetic defects through thermal spin fluctuations
(“paramagnons”) in the matrix.
Tugushev et al. Physica B (2006); Nikolaev et al. JETP letters (2009)
(Rij) – local susceptibility.
SF(Rij)≈RKKY(ξSFkF)2 ≈N(EF)(ξSFkF)2 -
ξSF – correlation length is about 1.5 nm, (kF)-1– 0.5 nm.
Results for MnxSi1-x/Al2O3
The Hall resistance in MnxSi1-x is determined
mainly by the anomalous component.
Hysteresis is observed up to  230 К.
Magnetic moment is about 0.1B per Mn, that is ten
times higher than in Mn4Si7 0.01B /Mn.
At temperatures below 50 K resistivity decreases
drastically.
Properties of our structures differ from Mn4Si7 .
Tc is about 300 K.
Comparison between MnxSi1-x on Al2O3 and GaAs
Comparison between MnxSi1-x on Al2O3 and GaAs
Comparison between MnxSi1-x/Al2O3 and MnxSi1-x/GaAs
samples
MnxSi1-x/GaAs
xy,10 cm
0.2
0.2
T=283K
N5
d=50nm
0.0
N2
d=57nm
-0.1
-0.2
MnxSi1-x/Al2O3
-0.8
-0.4
0.0
B, T
0.4
0.8
-6
T=300K
xy, 10 cm
0.1
N4
d=80nm
-6
xy, 10 *cm
-6
0.0
0.1
0.0
-0.1
-0.7
0.0
0.7
B, T
0.0
N5
Rxx(5)/Rxx(290) = 0.63
-0.1
-0.2
-3
T=186K
T=43K
T=5K
-0.0
-2
-1
0
B, T
1
2
3
For MnxSi1-x/GaAs Hall resistance ρxy is remarkably higher then in MnxSi1-x/Al2O3
AHE in MnxSi1-x/GaAs is clearly observed at 300K and its amplitude weakly
depends on temperature between 5 K and 190 K, while slope diminishes.
The Hall angle tangent  = xy/ xx is ~ 10-2 (at 200 К), that corresponds to  20 Т for
normal Hall effect if mobility 5 cm2/Vs .
Comparison between MnxSi1-x on Al2O3 and GaAs
At saturation the magnetic moment
per Mn atom is for
MnSi/Al2O3
≈0.07 μB/Mn (200 K)
≈0.03 μB/Mn (300 K)
MnSi/GaAs
≈0.3 μB/Mn (200 K)
≈0.08 μB/Mn (300 K)
Parameters of MnxSi1-x samples, x ≈ 0.35
Samples
number/
substrate
Rxx(77K)/
Rxx(290K)
Growth
temperature, Tg
°C
d, nm
Hc- coercitivity at 80
K (Oe)
Nо1
Al2O3
0.94
300
40
2900
Nо2
Al2O3
0.93
300
57
2000
Nо3
Al2O3
0.85
350
55
4200
№4
GaAs
0.85
300
80
0
№5
GaAs
0.84
300
50
0
№6
GaAs
0.97
200
75
330
№7
GaAs
0.89
300
300
650
Hole concentration p ≈ (2 – 3)1022 cm-3
AHE
sign
+
+
+
-
Conclusion
AHE is observed at room temperature being the main
contribution to the Hall resistance. Hysteresis is observed up to
 230 К.
Tc reaches more then 300 K.
Curie temperature and saturation magnetization is much higher
than in Mn4Si7 and in previously studied Si based structures.
Properties of these films depend on substrate
We explain experimental results within the model of exchange
through the spin fluctuations
PRB, 84, 075209 (2011)
Thank you for attention
video-2011-03-25.3gp