Hall-effect structures with submicrometre linewidths (<0.3pm) have been fabricated in ferromagnetic thin films of Mn[sub 0.60]Al[sub 0.40] which are epitaxially grown on a GaAs substrate. The MnAl thin films exhibit a perpendicular remanent magnetisation and an extraordinary Hall effect with square hysteretic behaviour. The presence of two distinct stable readout states demonstrates the potential of using ultrasmall ferromagnetic volumes for electrically addressable, nonvolatile storage of digital information

Cheeks, T. L.

De Boeck, J.

Gilchrist, H.

Harbison, J. P.

Keramidas, V. G.

Sands, T.

Scherer, A.

Tanaka, M.

English

Caltech Authors - Main

ated, 8, and VI must be equal to n2b2 and VJn, respectively. 
Under these conditions, the voltage provided is 
Ahv, = ( n  - I)V, (2) 
For practical implementations of the circuit of Fig. 1, the 
condition 8, = n’Bz indicates that transistor M1 should be 
composed of n z  parallel connected transistors with (W/L)  
ratio identical to  that of M2. Thus, the matching in trans- 
conductance parameters and threshold voltages of transistors 
is improved. Needless to say, an alternative to the above equa- 
tion consists in using identical M1 and M2 transistors and a 
current-mirror gain of n - * .  On the other hand, to fulfill the 
relationship VI = VJn, voltage dividers made up of either 
passive resistors or diode connected transistors can be imple- 
mented. 
Note that the proposed circuit can maintain a low current 
level in transistors M1 and M2 to reduce their 1 effects, its 
behaviour can be well represented by the ideal quadratic law, 
and, simultaneously, has high output drive capability. 
A -nV, extractor can be derived from Fig. la with minor 
modifications. Practically the negative level shifter M3-M4 
must be substituted by the positive level shifter shown in Fig. 
lb. Following a similar procedure as above, the output 
voltage generated by the resulting circuit extractor is exactly 
equal to (1 - n)V,. 
A very low impedance exists at output node B of any of the 
above circuits, because the low output resistance (l/gma) of the 
source follower appears divided by the gain of the feedback 
loop. Hence, the output resistance of the extractor results in 
the order of [gi/gi]. 
The influence on the circuit performance of transistor 
second order effects such as mobility degradation, channel 
length modulation and device mismatching originate devi- 
ations of the output voltage from the theoretical result given 
by eqn. 2. However, if these effects on equations of I, and I, 
are included separately, it can be demonstrated that they d o  
not impose more than a few millivolts of discrepancy for 
typical parameters values [3]. 
Experimental results: We built the circuit shown in Fig. l a  for 
n = 2 using commercial matched MOS transistor array 
TC4007. The implemented voltage divider consisted of two 
series diode-connected and matched transistors between V,, 
and Vss. Transistor M4 was chosen with an equivalent ( W / L )  
ratio five times larger than that of its M2 transistor counter- 
part. The circuit was measured and Fig. 2 shows the experi- 
mental results obtained for different grounded resistor loads 
connected to node B. The total supply voltage was 8 V .  As 
I .47 5 
observed, an almost constant V, value of 1.472V was provid- 
ed over this load resistor range. In addition, to  examine the 
validity of these results, we have determined the threshold 
voltage by means of the classical linear regression method as 
well. For it, we recorded the rDs of a simple transistor at 
different V,, voltages. Fig. 3 plots the square root of these 
measurements. The extrapolated threshold voltage resulting, 
which would be the crossing of the straight line fit with the 
horizontal axis, was 1.47243 V showing very good agreement 
with the voltage generated by the V, extractor without any 
measurement or calculation. The coefficient of the correlation 
provided by the fitting routine was 0.9997. 
Conclusions: A knV, extractor has been presented. Its high 
accuracy and drive capability has been demonstrated. These 
features of the proposed circuit should be useful not only for 
circuit engineers, but also for process engineers. 
Acknowledgment: This work was supported by CICYT (Spain) 
under Grant TIC-91-1059. 
I l th  January I993 
J. F. Duque-Carrillo, J. M. Valverde and R. Perez-Aloe (Dept. 
Elecrronica e Ing. Electromed., Universidad de Extremadura, (06071) 
Badajor, Spain) 
Reference 
1 TSIVIDIS, Y.:  ‘Operation and modeling of MOS transistor’ 
(McGraw-Hill, New York, 1987) 
2 LEE, H.-G., OH, s.-Y., and FULLER, G.: ‘A simple and accurate method 
to measure the threshold voltage of an enhancement-mode 
MOSFET, IEEE Trans, 1982, ED-29, pp. 346-348 
WANG, z.: ‘Automatic V, extractor based on an n x n2 MOS tran- 
sistor array and their application’, IEEE J .  Solid-state Circuits, 
1992, SSC-27, pp. 1277-1285 
3 
NON-VOLATl LE MEMORY 
CHARACTERISTICS OF SUBMICROMETRE 
HALL STRUCTURES FABRICATED IN 
EPITAXIAL FERROMAGNETIC MnAl FILMS 
ON GaAs 
J. De Boeck, T. Sands, J. P. Harbison,  A. Scherer, 
H. Gilchrist, T. L. Cheeks, M. Tanaka and 
V. G. Keramidas 
I 
I O  1 5  2 0  2 5  3 0  3 5  4 0  6 5  5 0  
v,,,, -source ,v 
Fig. 3 Square root o/los against V,, 
Indexing terms: Epitaxy and epitaxial growth, Memories, 
Thin-film devices, Magnetic devices and materials 
Hall-effect structures with submicrometre linewidths 
(<0.3pm) have been fabricated in ferromagnetic thin films of 
Mn,.,,AI, 4o which are epitaxially grown on a GaAs sub- 
strate. The MnAl thin films exhibit a perpendicular remanent 
magnetisation and an extraordinary Hall effect with square 
hysteretic behaviour. The presence of two distinct stable 
readout states demonstrates the potential of using ultrasmall 
ferromagnetic volumes for electrically addressable, non- 
volatile storage of digital information. 
Ferromagnetic thin films on semiconductor substrates have 
recently been used to fabricate non-volatile memories based 
on their anisotropic magnetoresistive properties [I]. The idea 
of using the state of the magnetisation in single domain ferro- 
magnetic thin film structures to store binary information is 
very appealing because of, for example, the inherent non- 
volatility and hysteretic behaviour. A ferromagnetic thin film 
with the easy axis of magnetisation normal to the substrate 
surface will exhibit an extraordinary Hall effect (EHE) when 
an in-plane current is applied, caused by anisotropic scat- 
tering at magnetic moments in the film [2]. 
Recently, we have demonstrated the molecular beam epi- 
taxial growth and the resulting perpendicular magnetisation 
ELECTRONICS LETTERS 18th February 1993 Vol. 29 No. 4 421 
of thin ferromagnetic 7 (MnNi),Al, ~I on AlAs/GaAs [3-51. In 
brief, the MBE growth process 16, 71 starts with the room 
temperature deposition of an amorphous template on an 
AlAs/GaAs structure. A solid phase crystallisation of the tem- 
plate takes place on heating to -200°C after which the epi- 
taxial growth of (MnNi)Al, proceeds by conventional 
MBE. After completion of the lOnm thick (MnNi),Al,+, film 
growth, an anneal to  300-450°C is applied to  improve the 
magnetic properties. We have recently discussed the influence 
of the (MnNi),Al,-, alloy composition on the structural and 
magnetic characteristics of the ferromagnetic film [SI. 
Fig. 1 demonstrates the EHE (RzY = V,,/I,, = p,,/t) gener- 
ated at room temperature in a 12pm wide Mno.,,AI,.,, Hall 
bar (thickness I = 1Onm). The external magnetic field is 
-10 
- 8  - 6  - 4  - 2  0 2 4 6 8 
H I  kOe :55211; 
Fig. 1 Room temperature EHE efect in I 2  and 5 p m  wide Hall crosses 
of Mn,  soAl ,  4o epitaxially grown on AIAslGaAs 
applied perpendicular to the sample surface in increments of 
10 mT and a DC current I,, = 1 @A is applied to the Hall bar. 
An extraordinary Hall voltage results from asymmetric scat- 
tering of the carriers at the magnetic sites in the lattice and is 
many orders of magnitude larger than the Hall voltage gener- 
ated by the Lorentz force from the external field. The mea- 
sured pxY (7.3 pQcm) is a materials parameter describing the 
magnitude of the extraordinary Hall effect. The hysteresis 
loop is extremely square because the switching to complete 
saturation occurs within one increment of the applied field, by 
a domain wall sweeping across the intersection. 
The state of the magnetisation in the film can thus be used 
to store a bit of information [8] with infinite retention time 
and the information bit can be read nondestructively by 
sensing the EHE voltage. Writing the information bit then 
requires the reversal of the magnetisation in the a m .  A partic- 
ularly promising aspect of this form of non-volatile storage is 
that it should be scalable to small sizes in all dimensions. As a 
first step in the feasibility study of the application of ferromag- 
netic thin films in future non-volatile memories, we describe 
the down-scaling of the Hall-structure to submicrometre 
dimensions in order to  obtain ultrahigh packing densities. We 
demonstrate the fabrication and EHE characteristics of a sub- 
micrometre Hall structure that can store a bit of information 
in a ferromagnetic volume of 300 x 300 x iOnm3 
(O.oo09 pm3). 
The submicrometre Hall structures are defined by electron 
beam lithography using an IS1 scanning electron microscope. 
A 140nm thick SrF, mask is fabricated by a standard liftoff 
procedure and Xe milling is used to pattern the ferromagnetic 
thin film. The milling is carried out in a UHV etching 
chamber coupled to the growth chamber of a Riber loo0 
MBE system. For passivation purposes, a 20nm amorphous 
GaAs layer is deposited on the freshly etched (MnNi),Al,_,, 
without leaving the vacuum. A multilegged Hall bar with line- 
width of -300nm, fabricated in the ferromagnetic T 
Mn,.,,,Al,.,o, is shown in the SEM picture of Fig. 2. Energy 
dispersive X-ray analysis has been used to  verify the complete 
removal of MnAl around the small structure. 
Fig. 3 shows the EHE hysteresis loop (room temperature, 
I,, = 0.5pA AC) from the Hall cross indicated by the white 
circle in Fig. 2. The EHE loop of Fig. 3 clearly shows that two 
Fig. 2 SEM micrograph of array of multilegged submicrometre Hall 
structuresfabricated in Mn,.,,Al,.,,~AIAs~GaAs heterostructures 
2 9 7  - 
296 - 
295 - 
6. 
> 
n 294 - 
293  - 
- 8  - 6  - 4  - 2  0 2 4 6 8 
ti, k O e  
Fig. 3 Room temperature EHE effect generated in 0.0009pm' ferro- 
magnetic volume indicated by  the white circle in Fig. 2 
distinct levels remain present in the Hall resistance of these 
ultrasmall structures, indicating the two stable saturated states 
of the magnetisation in the 300 x 300 x 10nm3 volume of the 
cross. Compared to the EHE loops of the wider structures 
(Fig. 1) a change in magnetisation reversal and a reduction 
(pxy = 2pSlcm) of the EHE magnitude are observed. Domain 
walls seem to be stable inside this ultrasmall structure and the 
magnetisation reversal proceeds in a number of well defined 
steps. This behaviour is surprising given the fact that a 
domain wall sweeps rapidly through the 12pm wide bar. The 
reduction of the EHE signal, pxy. could be caused in part by 
magnetically inactive material at the edges of the cross as a 
consequence of the processing. Further, the EHE shows an 
offset, - 295 Q in this case, that varies from one Hall cross to  
another in this multilegged structure. The exact origin of the 
offset on the EHE voltage is unclear at present, but geometri- 
cal asymmetries could become important at these reduced 
dimensions and we are currently attempting to model the 
effect of such asymmetries. 
Many properties of the present structures are very appeal- 
ing to high density, semiconductor based non-volatile memory 
applications: good structural quality of the coherent T 
(MnNi)Al, _JAIAs/GaAs epitaxial heterostructures, MBE 
control on uniformity of thickness and alloy composition, 
room temperature EHE signal, infinite retention time on 
power loss, magnetic hardness and square hysteresis loops, 
simplicity and ultrasmall dimensions of the actual memory 
element and finally, compatibility with existing semiconductor 
technology. One of the remaining challenges is to find a suit- 
422 
----- ~ - 
ELECTRONICS LETTERS 18rh February 1993 Voi 29 No. 4 
able on-chip writing mechanism for these magnetic elements 
based on for example, Joule heating in combination with a 
magnetic field from a nearby conductor or current pulse 
induced domain wall movement [SI. 
In summary, we have demonstrated submicrometre ferro- 
magnetic structures that can reveal characteristics that are 
potentially useful for non-volatile memory applications. The 
readout of these structures can be performed by sensing the 
extraordinary Hall effect (EHE) voltage, generated by a sense 
current passed through the perpendicularly magnetised 
(MnNi),Al,-, layer. The actual storage volume is as small as 
04009gm3. 
Acknowledgment: J. De Boeck acknowledges partial financial 
support by NATO. 
7th January 1993 
J. De Boeck,’ T. Sands, J. P. Harbison, A. Scherer, H. Gilchrist, T. L. 
Cheeks, M. Tanaka,t and V. G. Keramidas (Bellcore, 331 Newman 
Springs Rd., Red Bank, NJ 07701, U S A )  
Present Address: lnteruniversitary Microelectronics Center, Kapel- 
dreef 75, B 3001 Leuven, Belgium 
t On leave from: Dept. Electr. Eng., The University of Tokyo, 7-3-1 
Hongo, Bunkyo-ku Tokyo 113, Japan 
References 
DAUGHION, J .  M.:  ‘Magnetoresistive memory technology’, Thin 
Solid Films, 1992, 216, pp. 162-168 
DAHLBERG, E. D., RIGGS, K., and PRINZ, G.  A.: ‘Magnetotransport: an 
ideal probe of anisotropy energies in epitaxial films’, J .  Appl. Phys., 
1988,63, pp. 4270-4275 
ti. w., RAMFSH, R., and KFRAMIDAS, v. G.: ‘Epitaxial ferromagnetic T 
MnAl films on GaAs’, Appl. Phys. Lett., 1990,97, pp. 2609-261 I 
I., RAMESH, R., FLOREZ, L. I., and KERAMIOAS, v. G.:  ‘Galvano- 
magnetic properties of epitaxial MnAl films on GaAs’, J.  Appl. 
Phys., 1991, pp. 4689-4691 
TANAKA, M., FLOREZ, L. I., WILKENS, B. I., GILCHRIST, H. L., and 
KERAMIDAS, v. G.: ‘MBE growth of ferromagnetic (Mn, Ni)AI on 
AlAs/GaAs heterostructures’. MBE VI1 Conf., Schwabisch 
Gmiind, Germany, August 24th-28th, 1992 (also to be published 
in J .  Cryst. Growth) 
and KERAMIDAS, v. G . :  ‘MBE growth of ferromagnetic metastable 
epitaxial MnAl thin films on AIAs/GaAs heterostructures’, J .  
Crysr. Growth 1991, 111, pp. 978-981 
I. L., BRASIL, M. I. s. P., CHANG, c .  c., RAMESH, R., FLOREZ, L. T., 
DEROSA, F., and KERAMIDAS, v. G.:  ‘Epitaxial T Mn AI/AlAs/GaAs 
heterostructures with perpendicular magnetization’. Mat. Res. 
Symp. Proc., 1992,231, pp. 341-346 
SANDS, I., HARBISON, J. P., LEADBEATER, U .  L., ALLEN, S. I., JUN., HULL, 
LEADBEATER, M., ALLEN, S. I., JUN., DEROSA, F., HARBISON, J .  P., SANDS, 
HARBISON, J .  P., SANDS, T., DE BOECK, 1.. CHEEKS, I. L., MICELI, P. F., 
HARBISON, J. P., SANDS, T., RAMESH, R., FLOREZ, L. T., WILKENS, B. J., 
SANDS, T., HARBISON, J. P., ALLEN, S. J., JUN., LEADBEATER, M., CHEEKS, 
8 SAND$ T., DE BOECK, J., HARBISON, 1. P., SCHERER, A., GILCHRIST, H. L., 
CHEEKS, T. L., MICELI, P. F., RAMESH, R., and KERAMIDAS, v. G.: ‘The 
extraordinary Hall effect in coherent epitaxial 7 (Mn, Ni)AI thin 
films on GaAs’. 37th Annual Conf. on Magnetism and Magnetic 
Materials in Houston, Texas, USA, December Ist-4th, 1992, (also 
to be published in J.  Appl. Phys., 1993) 
B F R G ~ ,  L.: ‘Motion of a magnetic domain wall traversed by fast- 
rising current pulses’, J.  Appl. Phys., 1992, 71, pp. 2721-2726 (and 
references therein) 
9 
ERRATA 
ASI, A., and SHAFAI, L.: ‘Dispersion analysis of anisotropic 
inhomogeneous waveguides using compact 2D-FDTD, 1992, 
28, (15), pp. 1451-1452 
Authors’ corrections 
In the third line of the righthand column of p. 1451, the words 
‘forward difference for’ should be deleted 
The inline equation after eqn. 3 should read: 
C6l 
Additional reference 
6 CANGELLARIS, A. c . :  ‘Numerical stability and numerical dispersion 
of a compact ZD-FDTD method used for the dispersion analysis 
of waveguides’, I E E E  Microw. Guid. Waue Lett., 1993, 3, (l), pp. 
3-5 
Printers’ corrections 
In eqns. 2 and 3, EE(i,j + $)should read ‘E:(i, j + 1)’ 
ELECTRONICS LETTERS 18th February 1993 Vo/ 29 N o  4 423 
- -  - - ~ 


Non-Volatile Memory Characteristics of Submicrometre Hall Structures Fabricated in Epitaxial Ferromagnetic MnAl Films on GaAs

Crysr. Growth 1991, 111, pp. 978-981 I. L., BRASIL, M. I. s. P., CHANG, c . c., RAMESH, R., FLOREZ, L. T., DEROSA, F., and KERAMIDAS, v. G.: ‘Epitaxial T Mn AI/AlAs/GaAs heterostructures with perpendicular magnetization’. Mat. Res.

Epitaxial T Mn AI/AlAs/GaAs heterostructures with perpendicular magnetization’.

Magnetotransport: an ideal probe of anisotropy energies in epitaxial films’,

The extraordinary Hall effect in coherent epitaxial 7 (Mn, Ni)AI thin films on GaAs’.

http://authors.library.caltech.edu/477/1/DEBel93.pdf

Non-Volatile Memory Characteristics of Submicrometre Hall Structures Fabricated in Epitaxial Ferromagnetic MnAl Films on GaAs

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main