Ferromagnetic Perpendicularly magnetized epitaxial thin films of tau (Mn,Ni)AI have been successfully grown on AlAs/GaAs heterostructures by molecular beam epitaxy. We have investigated the polar Kerr rotation and magnetization of tau MnAl and (Mn,Ni) Al as a function of Mn and Ni concentration. The largest polar Kerr rotation and remnant magnetization were obtained for Mn0.5Al0.5 thin films with values of 0.16-degrees and 224 emu/cm3, respectively. We observed that the Kerr rotation and magnetization remained constant with Ni additions up to about 12 at. % and subsequently decreased with further Ni additions. We discuss these results and one possible method of enhancing the Kerr rotation

Brasil, M. J. S. P.

Cheeks, T. L.

De Boeck, Jo

Harbison, J. P.

Keramidas, V. G.

Sands, T.

Scherer, A.

Tanaka, M.

English

Caltech Authors - Main

Epitaxial I (Mn,Ni)AI/(AI,Ga)As heterostructures: Magnetic 
and magneto-optic properties 
T. L. Cheeks, M. J. S. P. Brasif,a) Jo De Boeck,b) J. P. Harbison, T. Sands, 
M. Tanaka,cl A. Scherer, and V. G. Keramidas 
.BeIlcore, Red Bunk, New Jersey 07701 
c 
Ferromagnetic perpendicularly magnetized epitaxial thin films of r (Mn,Ni)Al have been 
successfully grown on AlAs/GaAs heterostructures by molecular beam epitaxy. We have 
investigated the polar Kerr rotation and magnetization of r MnAl and (Mn,Ni)Al as a function 
of Mn and Ni concentration. The largest polar Kerr rotation and remnant magnetization were 
obtained for Mne.sAl,, thin films with values of 0.16” and 224 emu/cm3, respectively. We 
observed that the Kerr rotation and magnetization remained constant with Ni additions up to 
about 12 at. % and subsequently decreased with further Ni additions. We discuss these results 
and one possible method of enhancing the Kerr rotation. 
INTRODUCTION MATERIALS GROWTH AND CHARACTERlZATiON 
Integration of magnetic devices with optoelectronics 
requires compatible materials that retain both their mag- 
netic and semiconducting properties. Recently, we have 
demonstrated epitaxial ferromagnetic thin films of r (Mn, 
Ni)Al grown on AlAslGaAs (100) heterostructures by 
molecular beam epitaxy (MBE) .‘-j The films were epitax- 
ial, single phase and have their magnetically easy c axis of 
the tetragonal unit cell oriented normal to the ( 100) GaAs 
surface. The perpendicular magnetization of the ferromag- 
netic thin films was demonstrated by the estraordinary 
Hall effect (EHE) measurements. The measurements 
showed a hysteretic behavior with r-e&angular shaped hys- 
teresis loops and 100% remanence.’ Preliminary measure- 
ments of the magnetic and magnet.o-optic properties re- 
vealed a polar Kerr rotation of -0.11” and remnant 
magnetization (M,j of .- 171 enm/cm.3’4 In addition, the 
integrity of the underlying semiconductor and metal/ 
semiconductor interface was maintained,5 thereby, making 
device integration a possibility, 
The Kerr romtion of the epitaxial films measured 
somewhat lower than MnBi,6 and the previous predicted 
values for Mn-based materkk7 Recently, it was reported 
that Ni additions to sputtered MnAl films increased the 
magnetization.” Since the Kerr rotation is related in part to 
the magnetization, one may expect an increase in the Kerr 
rotation for CMn,Ni)Al as well as hlnAl with different Mn 
compositions. It is this issue that we address in the work 
reported here. 
The details of 7 MnAl and MnNiAl epitaxial growth 
on AlAs/CaAs heterostructures have been described in a 
previous publication.* The AlAs/GaAs heterostructure 
was grown using standard III-V MBE growth conditions. 
The metal layers were grown using a template technique 
with a maximum temperature of -400 “C. The Mn,All X 
films were grown with the Mn content ranging from 0.40 
<x < 0.60 and the addition of Ni up to 8 at. % in 
Mno.60-J$A10,,w. The presence of the -5 phase was verified 
using in situ refection high energy electron diffraction 
(RHEED) and ex situ x-ray diffraction. Harbison et al3 
found that for this entire range of samples, the metal Urns 
were coherently strained to the GaAs lattice with an a0 of 
0.283 nm and a 1.22 c/a ratio that remained unchanged up 
to 4 at. % Ni. They showed that only when Ni additions of 
18 at. % were applied, the c/a ratio reduced to 1.09 (a0 
=0.284 nm) resulting in a near cubic lattice structure. In 
contrast, the reported c/a ratios for sputtered polycrystal- 
line films, unconstrained to the lattice constant of the un- 
derlying substrate, decreased with much smaller Ni addi- 
tions, i.e., 4 at. oJo.8 Also, the a0 increased resulting in a 
more cubic rather than tetragonal lattice. 
The heterostructures investigated in this study con- 
sisted of a GaAs ( 100) semi-insulating substrate with an 
undoped GaAs buffer layer grown above it. A thin 10 nm 
layer of undoped AlAs was then grown in order to leave a 
chemically stable aluminum-containing layer in direct con- 
tact with MnAl. To comp1et.e the structure a 10 nm metal 
layer was grown. 
We discuss the magnetic and magneto-optic properties 
of epitaxial P MnAl and MnNiAl films grown on AlAs/ 
GaAs heterostructures. We have measured the Kerr rota- 
tion and remnant magnetization (hfr) of MnAl with Mn 
concentrations of 40-60 at. 9% and MnAl with up to 17 
at Q& Ni *, . 
“SPressent. address: UNICAMP, Instituts de Fisica, Csmpinas SP, Brail. 
h”Pcrnlanent addrew IMEC, 75 Kapeldrref, B3001 I.euven, Belgium. 
“On leave from Dept. Electron. Eng., The University of Tokyo, 7-3-l 
Nongo, Bunkyo-ku, Tokyo 113, Japan. 
The spectra in Fig. 1 show the polar Kerr rotation of a 
Mno,,Al,, tllm as a function of wavelength from 830 to 225 
nm. The rotation was measured after the samples had been 
poled either parallel or ant.ipar;lllel to the film normal by 
exposure to a 0.5 T magnetic field. The Kerr rotation was 
-0.16” at 800 nm and remained fairly constant between 
500 and 800 nm. This is important, particularly for appli- 
cations, such as magneto-optic recording which require 
wavelengths in this range. The 0.16” value is slightly lower 
than the 0.3”-0.4” values reported for TbFeCo, but similar 
to 0.19-0.2” reported for CoJPt multilayers.’ 
The magnetization measurements were performed us- 
6121 J. Apple Phys. 73 (IO), 15 May 1993 0021-8979/93/l 06121-03$06.00 @  1993 American Institute of Physics 6121 
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04 -- 
I I 
= PtI 1 o, 02 1 ,rp 83 
%%~.-m- e .=-.g_O.. ..~ Q  .Q _-0-^-^---a 
c d .o 0 .‘;’ _-. 
75 
5 0 1 I’ L”-“:/+-- ‘A 8-..a. cc ,, ~ cr. ..i _ I- ,-p-  ^‘* ---3 - .2 
-0.3 
.042L& --... 300 ‘ 400 500 600 -I- 700 .-.._ 600 L  ..^ 900 
Wavelength (nm) 
FIG. 1. hlagnekwptic Kerr rotation of Mn,&l,, as a function of 
wavelength. 
ing a vibrating sample magnetometer with the fields di- 
rected along the easy magnetization axis. Table I summa- 
rizes the remnant magnetization (M,), coercivity (H,), 
and Kerr rotation of MnfiIi --x with 0.4 <x < 0.6. The M, 
values reported here were not compensated with a demag- 
netization factor. The iWr values were similar for films con- 
taining 50-60 at. %  hln and slightly lower for the 40 at. %  
Mn films. The H,. decreased with increasing Mn content. 
The Kerr rotation measured 0.16” for Mn0,tiA10,4 films, 
while the MneGAl,,,, films measured 0.06”. The reduced 
Kerr rotation value measured in the Mn,,Al,,: films may 
be due to a lower magnetization in the film. In general, the 
,Wr values of the epitaxial films were slightly higher than 
the saturation magnetization (iW .Y) of 120 emu/cm3 ob- 
tained from sputter deposited polyc.rystalline r MnAl films, 
but lower than the bulk value of 460 emu/cm3.’ 
The M-H hysteresis loops for the Mn~&l, -.x films were 
nearly square with the exception of Mn,,Al,,, which 
showed a reduced squareness. This may be due to regions 
within the film with misaligned c-axis moments. The 
square hysteresis loops indicate perpendicular magnetiza- 
tion within the films. This is consistent with the extraordi- 
nary Hall effect measurements performed on the same 
films.” 
The effect of Ni additions to MnAl on the remnant 
magnetization is shown in Fig. 2(a). The M, remained 
fairly constant at - 170 emu/cm’ wit.h up to 10 at. %  Ni. 
W ith further Ni additions up to 17 at. %, the M, gradually 
decreased down to 98 emu/cm”. In addition, Fig. 2(b) 
TABLE I. Magnetic properties of 7 hkAl as a function of Mn  concen- 
tration. 
Sample 
8, (deg) iMr 
( 800 nm) (emu/cm’) FTC (Oe) 
Mno 40 ,, 0.06 112.0 3918 
l\;lno 40 s 0.17 224.0 3836 
Mn,, 40 J 0.16 214.5 855 
ml00 
~~~~ y.2: _,..,, ]
8. --” -.. .-. 
;250 7 
5 10 15 20 25 
96 Nickel 
~~:il- -;‘ 
Oh Nickel 
0.4 ~~ ~~ 
EF; 
(4 
fg”3f 
5 
302 
t jO.li 
“A- A-A- A-* 
i, , 
cl! I 
5 .$&& 20 25 
0 
FIG. 2. The effect of Ni additions to MnAl on the (a) magnetization 
(M,) (b) coerrivity (H,) and (c) polar Kerr rotation 0,. (Dotted lines 
added to aid the reader.) 
shows the H, decreasing with increasing Ni content with a 
slight increase at 17 at. %  Ni. 
The M,. obtained for epitaxial ( Mn,Ni) Al films differs 
from the results observed in sput.tered han,,, @ ,.&,,a 
films.” In sputtered films, the SfS increased with Ni addi- 
tions up to 4 at. %  and then gradually decreased with 
further additions8 It was suggested that excess Mn atoms 
occupy Al sites and the addition of Ni replaces the Mn 
atoms. Reducing the excess Mn results in decreasing the 
tendency toward antiparallel coupling bet.ween Mn atoms 
located on Mn sites and hiin atoms located on Al sites, 
hence, ultimately increasing the magnetization. Also, the 
addition of Ni causes the c/a ratio to decrease (as in- 
crease) which intluences the Mn-Mn coupling and the 
magnetization.* However in epitaxial films. we found that 
the films maintained an a,, of 0.283 nm which makes them 
coherent with the underlying AlAsiGaAs. The c/u ratio, 
measured by x-ray diffraction, remained the same (CQ re- 
mained constant) with Ni additions of at least 4 at. %, 
however with 18 at. %  Ni the C/U ratio reduced to 1.09 (a, 
increased to 0.284 nm).3 Compared with bulk and sput- 
tered films, the epitaxisl films may have a slightly reduced 
tetragonality due to elastic strain. These results suggest 
that if the mechanism for improved magnetization is solely 
related to Mn-Mn coupling, then a constant a0 value 
should result in a constant M, Since LI” remains constant 
with up to 4 at. %  Ni added to the epitaxial films and 
coherency is maintained, the observed constant M, is a 
reasonable result. Beyond 10 at.. %, mised phase regions or 
loss of coherency may account for the reduct.ion in mag- 
netization, 
The Kerr rotation as a function of Ni concentration is 
shown in Fig. 2(c j. The Kerr rotation for each sample was 
measured in a similar wavelength range as in Fig. 1. The 
wavelength dependence of the rotation remained margin- 
ally constant in all of the samples. The values measured in 
Fig. 2(c) were obtained at 800 nm because of its techno- 
Iogical importance. The Kerr rotation in the figure re- 
mained fairly constant at 0.16” up to - 13 at. %  Ni and 
6122 J. Appl. Phys., Vol. 73, No. 10, 15 May 1993 Cheeks et al, 6122 
Downloaded 18 Dec 2005 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
2  ^-. . . - .  _~” .-  ~ ~ ~  --... . . . . . . . . . .^“--- Fig. 3  was  ob ta ined  at 3 6 0  n m  us ing  a  1 5 0  n m  SisNi,  film . 
t5. 
6  
This  e n h a n c e m e n t  was  at t r ibuted to a n  in ter ference effect 
wi th in the mul t i layer  structure. 
z 1 ~  
2! 
$  
I 
b  In summary ,  w e  h a v e  s tud ied the m a g n e tic a n d  
b, 
E  
a 5  - 
a  & @ = + - 2 : .._ O ~ ~ .~ .~ ~ .~ .~ ~ ~ ~ - ft,;,,-~  
m a g n e to-opt ic proper t ies  of epi tax ia l  T  M n A l  a n d  MnN iA l  
th in film s  g r o w n  o n  A lAs /GaAs  heterostructures.  T h e  
k %  
%  I 
film s  d o  not  revea l  a  s t rong M n  compos i t ion  d e p e n d e n c e  
+ 6  
‘“tf+  \ “.‘<  
c 4  5  ,,’ 
t- 
/ $ .<-  o n  the Ke r r  rotat ion a n d  m a g n e tization. T h e  Ke r r  rotat ion 
1  E l?  
a n d  the m a g n e tizat ion r e m a i n e d  constant  wi th Ni  add i -  
s -1  -’ 
r i 
t ions u p  to - 1 0 - 1 4  at. %  a n d  gradua l l y  dec reased  wi th 
-15  ‘i” fur ther Ni  addi t ions.  This  resul t  dif fers f rom sput tered 
film s  a n d  m a y  b e  re la ted to the coherency  that exists be -  
.~ ,~---..“.&-.  L;-. i- ~ ~  .& II............. ; -&- .... - -.--~ _  
? m  8 0 0  a m  tween  the epi taxiaf  film  a n d  the under l y ing  G a A s  he tero-  %  
Wave leng th  { n m )  structure. W e  h a v e  d iscussed this issue a n d  the possibi l i ty 
of us ing  S i ,N4 over layer  film s  to e n h a n c e  the Ke r r  rotat ion. 
T h e  m a g n e to-opt ic a n d  m a g n e tic proper t ies  descr ibed  h e r e  
FIG. 3. Magneto-op t ic  K e r r  rotat ion for 1 5 0  n m  of S iJNi l  depos i ted  o n  
M n A I. 
m a y  b e  su i tab le for dev ice  appl icat ions wh ich  inc lude m a g -  
net ical ly cont ro l lab le  optoelectronics.  
then  gradua l l y  dec reased  to -0 .09” with fur ther Ni  add i -  
t ions. S ince  the squareness  of the m a g n e tizat ion loops  (i.e., 
the componen t  of m a g n e tizat ion pe rpend icu la r  to the film  
p l a n e >  r e m a i n e d  squa re  for al l  N i  composi t ions,  o n e  m a y  
conc lude  that the l oop  squareness  makes  on ly  a  smal l  con-  
tr ibut ion. Therefore ,  the dec rease  in  the Ke r r  rotat ion wi th 
increas ing  Ni  content  m a y  b e  m o r e  st rongly re la ted to the 
dec rease  in  m a g n e tizat ion of the films.  
T h e  Ke r r  rotat ion m e a s u r e d  for bo th  M n A .1 a n d  M n -  
N iA l  w e r e  lower  than  the pred ic ted  va lues  for M n A 1 7  a n d  
MnB i .h  This  m a y  b e  re la ted to the IO  n m  film  thickness. It 
is expec ted  that thicker film s  wil l  h a v e  h ighe r  rotat ions. 
However ,  p repa r ing  thicker film s  rema ins  to b e  a  cha l l enge  
to M B E  growth.  O n e  possibi l i ty for p repa r ing  samp les  
wi th h ighe r  Ke r r  rotat ions is depos i t ing  th in film s  of Si,N, 
o n  the m e tal surface. A  Ke r r  rotat ion of l-s”, as  s h o w n  in  
‘T. Sands ,  J. P . Harb ison,  M . L. Leadbeater ,  S . J. A l len,  Jr., G . W . Hul l ,  
R. Ramesh ,  a n d  V . G . Ke ramidas ,  Appt .  Phys .  Lett. 57,  2 6 0 9  (  1990) .  
‘J. P . Harb ison,  T. Sands ,  R. Ramesh ,  L. T. F lorez,  B . J. W & e n s ,  a n d  
V . G . Ke ramidas ,  J. Cryst. G rowth  111,  9 7 8  (1991) .  
‘J. P . Harb ison,  T. Sands ,  J. D e  R o e &  T. L. Cheeks,  P . Micel i ,  M . 
Tanaka,  L. T. F lorez,  B . J. Wi l kem,  H. L. Gi lchr ist ,  a n d  V . G . K e r a -  
midas ,  J. Cryst. G rowth  (to b e  pub l ished) .  
‘T. L. Cheeks,  M . J. S . P . Bmsi l ,  T. Sands ,  J. P . Harb ison,  S . J. A l len,  Jr., 
D. E . Aspnes ,  a n d  V . G . Ke ramidas ,  App l .  Phys .  Lett. 60,  1 3 9 3  (1992) .  
‘T. L. Cheeks,  R. E . Nahory ,  T. Sands ,  J. P . Harb ison,  M . J. S . P . B r a s &  
H. L. Gi lchr ist ,  S . A . Schwarz ,  M . A . Pudens i ,  S . J. A l len,  Jr., L. T. 
P lorez ,  a n d  V . G . Ko ramidas ,  Inst. Phys .  Conf.  Ser .  No.  120,  1 0 1  
(1992) .  
“M . H. Kryder ,  J. App l .  Phys .  57,  3 9 1 3  (1985) .  
‘5. X . Shen .  R. D  Ki rby,  a n d  D. J. Se l lmyer ,  J. Magn .  Mater .  81,  1 0 7  
11989) .  
“A . Mor isako,  N. Kohsh i ro ,  a n d  M . Matsumoto ,  J. App l .  Phys .  67,  5 6 5 5  
(1990) .  
‘F. J. A . M . G r i e d a n u s  a n d  W . B . Zeber ,  Mater .  Res.  Bul l .  X V , 3 1  
(1990) .  
6 1 2 3  J. App l .  Phys. ,  Vo l .  73,  No.  10,  1 5  M a y  1 9 9 3  Cheeks  et a/. 6 1 2 3  
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Epitaxial-tau(Mn,Ni)Al/(Al,Ga)As heterostructures: Magnetic and magneto-optic properties

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Epitaxial-tau(Mn,Ni)Al/(Al,Ga)As heterostructures: Magnetic and magneto-optic properties

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