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An experimental Josephson interferometer memory cell for nondestructive read out
An interferometer memory cell with two unequal Josephson junctions for nondestructive read out of single flux quantum states has been implemen ted and tested successfully. The agreement between the measurements and computer simulations is very satisfactory.
An experimental Josephson interferometer memory cell for nondestructive read out W. Jutzi, E. Crocoll, R. Herwig, H. Kratz, G. Mischke, M. Neuhaus, J. Wunsch, A. Scheidel, and H.J. Wermund Keywords: Superconductingdevices;Josephsonjunction; memory cell
Single flux quantum interferometer memory cells with two Josephson junctions have been investigated experimentally for destructive read out operation.1 Interferometer cells for NDRO (nondestructive read out) with two unequally large Josephson junctions and symmetric gate current insertion have been investigated by computer simulation. 2~ Simulated NDRO margins of an interferometer cell with symmetric gate current insertion and three access lines are -+ 21%; they are -+ 16% for asymmetric gate current insertion and two access lines. 4's A SFQ (single flux quantum) cell for NDRO with asymmetric gate current insertion has been implemented and tested successfully.6 In this paper the implementation and the measured properties of a cell prototype with symmetric gate current insertion are described.
Cell structure and fabrication The interferometer cell is designed to have two Josephson junctions A and B with a ratio of the areas and the corresponding maximum Josephson currents Ia/I b ~ 2. The inductance L between the junctions is chosen to yield a characteristic phase Lib X = 2zr ~ o
- 2rr
to allow for a nonvolatile storage of the two flux quantum states N = 0 and -1 corresponding to the absence or presence of a single flux quantum. 2'4 No bias control line is needed in contrast to the arrangement described by Broom et al. 1 Two control lines are coupled inductively with the inductance of the cell. The word ie the gate current I c is fed symmetrically in the centre of the inductance L. The binary information '1' corresponds to the N = -1 quantum state, the binary '0' to the N = 0 state. Only the N = 0 and -1 quantum states of an implemented interferometer outlined above are represented in Fig. 1 by the maximum and minimum threshold currents IG versus the currents 1~ in a control line. Both quantum states overlap the origin and are therefore nonvolatile without any bias current. The measured solid lines mark The authors are at the Institut fur Elektrotechnische Grundlagen der Informatik, Universita~ Karlsruhe, Hertzstrasse 16, D-7500 Karlsruhe 21, W. Germany. Paper received 20 November 1981.
the thresholds for vortex-to-voltage state transitions. The dashed lines represent vortex-to-vortex transitions. The boundary between solid and dashed line at the critical point PK in Fig. 1 is shifted to higher gate currents with increasing interferometer damping) For the read operation, a cell in a random access memory array or matrix with many cells is selected by currents 1GN and ICXN a t the intersection of an orthogonal word and x-control line. The nominal operating point designated with R01 in Fig. 1 is within the N = -1 state, but outside the N = 0 state. Therefore if a binary '1' has been stored the read operation does not destroy the N = -1 quantum state; no dc voltage appears. On the other hand if a '0' has been stored, the interferometer jumps into the voltage state on trespassing the solid line of the N = 0 state. The interferometer returns into the zero voltage state without trapping a flux quantum if the gate current IGN is removed after the control current I c x n . Thus nondestructive read out of both states is achieved) To write a '1' no gate current is applied. The operating point Wl in Fig. 1 is reached with the nominal control currents ICXN and ICDN- They flow in two separate control lines to select the addressed cell in a memory matrix. One control line is the x-line, the second control line runs diagonally through the matrix, but is parallel to the x-line at each cell. 1'6 The write '0' operating point W0 in Fig. 1 is achieved with the triple coincidence of the currents IGN, ICXN and
ICDN. The SEM micrograph of a cell prototype with one control line is shown in Fig. 2. Although two control lines are needed for cell selection in a memory matrix, all properties of a single cell outside a memory matrix could be tested in applying the currents ICXN and ICDN to the implemented control line. Beneath the control line which is the uppermost metallic layer, a square and a rectangular window junction can be detected. The control line has a width w = 10/am corresponding to the minimum line width. The interferometer areaA ~ 3600 gm 2 contains F' ~ A.w-2 ~ 40 lithographic squares. 7 The cell is arranged on top of a superconducting niobium ground-plate as sketched in Fig. 3. The cross-sectional dimensions, the layer materials and the process parameters
0011-2275/82/003127-04 $03.00 © 1982 Butterworth & Co (Publishers) Ltd. CRYOGENICS. MARCH 1982
127
ie the effective separation is d =tox + X1 + X2 = 0.2 tnn. The corresponding Josephson penetration depth is ~,j = 6.1/am. The capacitance of both junctions in parallel, as deduced by junction resonance, is C " - 9 pF. The corresponding McCumber damping parameter is ~ = R~ Clomax 21r/O o "~ 278.
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Fig. 1 Threshold curves of the N = 0 and N = -I q u a n t u m states at the vortex-to-voltage-state transitions ( - - ) and at the vortex-tovortex transitions (- - -). T h e nominal drive current amplitudes are m a r k e d by circlesfor N D R O in point R01, for write 'I' and '0' in points W l and W 0 , respectively
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Fig. 2 SEM-micrograph of an interferometer memory cell w i t h t w o w i n d o w Josephson junctions o f different areas. The control line and word line are about 10 p,m wide
are very close to those described by Greiner et al;8 but the resistive layer with Auln 2 has been omitted. The junction b o t t o m electrode consists of AuPbln and the counter electrode of PbAu. 9 The rf-sputter oxidation process proved very useful in achieving reproducible Josephson current densities. The peak to peak voltage used was 300 V and the oxygen pressure 6 Pa. The photoresist stencil lift-off process using chlorobenzene was employed. 8 de-measurements
All measurements have been performed at 4.2 K. The measured gate current I G versus the voltage UI across the interferometer is shown in Fig. 4. The gap voltage is 2A/e = 2.3 inV. The interferometer subgap resistance Rj is 1 ~2 at U[ = 2 mV. The ratio of Rj and the tunnel resistance RNN with normally conducting electrodes is Rj/RNN"" 8. The maximum Josephson current of the interferometer in the N = 0 quantum state is IGmax = 10.2 mA (see Fig. 1). The Josephson current density is calculated jma X = 3.5 kA cm -2 with the junction areas A a = 10 x 10/am 2 a n d A b = 10 x 20/am 2. The sum of the tunnel oxide thickness and the London penetration depths,
128
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C R Y O G E N I C S . M A R C H 1982
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The current thresholds computed with point junctions are represented by dotted lines in Fig. 7. The measured solid lines are nicely approximated by the computed dotted lines around the origin for the N = 0 and -1 modes. Thus, a cell design with point junctions is justified.
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If IGQ is the gate current at the intersection Q of the N = 0 and -1 quantum states the ratio of the gate currents is/GQ#Gmax = 0.79. This ratio is independent of the transformation ratio t of the equivalent control current in the interferometer I c and the current in the control line # I c where t = Ic/I'o The pair of ratios 1GQ/lGmax = 0.79 and Ia/I b = 2.19 corresponds to a characteristic phase X = 27r (1.01) as computer simulations with point junctions (l -+ 0) show. Thus, the transformation ratio t is equal to t = LulL, where the interferometer inductance is L = X qbo(2rr/b) -1 = 0.65 pH and the inductance L u may be calculated from the measured control current shift A/b between the quantum states as L u = q%/2d~ = 0.51 pH. The transformation ratio is
CRYOGENICS.
- 0.8
MARCH
1982
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length 1 = 10 # m parallel to the control line the control magnetic field inside the junction reduces the Josephson current. The first zero is found near I c ~-- 30 mA. The envelope of all quantum state extreme may be approximated by a single junction threshold characteristic with 1/Xj = 1.6. An enlarged representation o f l c versus I t near the origin is shown in Fig. 6. The N = 0 state is characterized by gate current extremes occurring at the same absolute value o f the control current. The current thresholds at the vortex-to-vortex transitions are extrapolated with the tangents o f the measured vortex-to-voltage state branches according to Fig. 6. In Fig. 1 the measured and extrapolated branches are represented with solid and dashed lines, respectively. At point B of the N = 0 state in Fig. 1 the gate current is I G ~- Ia - Ib .4 Using this, since IGmax = Ia + Ib, the current ratio Ia/Ib = 2.2 and the current Ib = 3.2 m A were determined.
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Fig. 6 Measured positive and negative threshold currents I G = f (I t ) at an enlarged control current scale of the interferometer in Fig. 2 w i t h h "- 2Tr, l a / I b "" 2.2, t = 0.8
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Fig. 7 M e a s u r e d t h r e s h o l d c u r r e n t s IG = f ( I c ) a c c o r d i n g t o Fig. 6 (--) and t h r e s h o l d s c o m p u t e d w i t h p o i n t j u n c t i o n s ( . . . . ) f o r h = 2 1 r ( 1 . 0 1 ) , la/Ib = 2 . 1 9 and t = 0.8 RI
WO
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0.5/.¢s Fig. 8 Four out of ten test program time slots of the voltage across the interferometer: a -- R1, W0, R0, R0 (R0, R0, W1, R1, R1, R1); b -- R0, W0, R0, R0 (R0, R0, --, R0, R0, R0). The operations in brackets are not displayed
129
Pulse measurements The basic memory operations of the cell have been tested with a pulse program at a repetition frequency of about 1 MHz and with pulse rise times of about 100 ns. One word of a periodic test program consists of the following operations: 4 x NDRO '1' (R1), 1 x write '0' (W0), 4 × NDRO '0' (R0), 1 x write '1' (Wl). The nominal current amplitudes of the write and read operations IGN, ICXN, ICDN are represented in Fig. 1. The gate current always overlaps the control current at rise and fall to achieve NDRO of the N = 0 quantum state. If the N = -1 mode has been written the interferometer stays in the zero-voltage state since the boundary of the N = -1 state is not trespassed. 2'3 The interferometer voltage picked up by a sampling oscilloscope and averaged is represented in Fig. 8a. The oscillogram shows the expected '0' and '1' signals. Small disturbances at the rise and fall of the overlapping drive pulses are also visible owing to test jig imperfections. In Fig. 8b the Wl-operation has been omitted. Many NDRO-operations of '1' and '0' were performed without any sign of signal degradation. After 10 9 operations the test was discontinued. The critical point Pk in Fig. 1 between the ranges of vortexto-vortex and vortex-to-voltage-state transitions at the mode
130
boundary was measured using the pulse program. Above the critical point proper NDRO operations could be performed within the hatched area in Fig. 1. Hence the tolerances on the gate and control currents are about + 24%. The authors would like to thank the Laboratorium for Elektronenmikroskopie der Universit[t Karlsruhe for making the SEM-rnicrographs. The work of Mrs G. M~iller and E. Mulica in preparing the manuscript is appreciated. This work has been supported in part by the German minister of research and technology under grant no 423-7291-NT 2515 6.
References 1
Broom, R.F., Gueret, P., Kotyczka, W., Mohr, Th. 0., Moser, A., Oosenbrog, A., Wolf, P. IEEE J of Solid-State Qrcuits SC-14 4 (1979) 690
2
Beha,H., Jutzi, W. Offenlegungsschrift 2735133, Feb (1979) Beha, H.IEEETransonMagneticsMAG-15 1 (1979)424 Beha, H. VDE-VerlagGmbH, Berlin (1981) Beha,H. Intermag Conf Grenoble (1981) Yamamoto, M., ishida, A. Digest of technical papers, 12th Conf on Solid State Devices,Tokyo (1980) 109 Jutzi, W. Advances in solid state physics XXI (1981) Vieweg-Verlag, Wiesbaden Greiner, J. etal.IBMJResDevelop 24 2 (1980) 195 Lahari, S.K., Basavaiah,S.dAppIPhys 49 (1978) 2880 Beha, H., Jutzi, W. Patentanmeldung P 3008 926.0, 8.3 (1980)
3 4
5 6 7 8
9 10
CRYOGENICS . MARCH 1982
An experimental Josephson interferometer memory cell for nondestructive read out W. Jutzi, E. Crocoll, R. Herwig, H. Kratz, G. Mischke, M. Neuhaus, J. Wunsch, A. Scheidel, and H.J. Wermund Keywords: Superconductingdevices;Josephsonjunction; memory cell
Single flux quantum interferometer memory cells with two Josephson junctions have been investigated experimentally for destructive read out operation.1 Interferometer cells for NDRO (nondestructive read out) with two unequally large Josephson junctions and symmetric gate current insertion have been investigated by computer simulation. 2~ Simulated NDRO margins of an interferometer cell with symmetric gate current insertion and three access lines are -+ 21%; they are -+ 16% for asymmetric gate current insertion and two access lines. 4's A SFQ (single flux quantum) cell for NDRO with asymmetric gate current insertion has been implemented and tested successfully.6 In this paper the implementation and the measured properties of a cell prototype with symmetric gate current insertion are described.
Cell structure and fabrication The interferometer cell is designed to have two Josephson junctions A and B with a ratio of the areas and the corresponding maximum Josephson currents Ia/I b ~ 2. The inductance L between the junctions is chosen to yield a characteristic phase Lib X = 2zr ~ o
- 2rr
to allow for a nonvolatile storage of the two flux quantum states N = 0 and -1 corresponding to the absence or presence of a single flux quantum. 2'4 No bias control line is needed in contrast to the arrangement described by Broom et al. 1 Two control lines are coupled inductively with the inductance of the cell. The word ie the gate current I c is fed symmetrically in the centre of the inductance L. The binary information '1' corresponds to the N = -1 quantum state, the binary '0' to the N = 0 state. Only the N = 0 and -1 quantum states of an implemented interferometer outlined above are represented in Fig. 1 by the maximum and minimum threshold currents IG versus the currents 1~ in a control line. Both quantum states overlap the origin and are therefore nonvolatile without any bias current. The measured solid lines mark The authors are at the Institut fur Elektrotechnische Grundlagen der Informatik, Universita~ Karlsruhe, Hertzstrasse 16, D-7500 Karlsruhe 21, W. Germany. Paper received 20 November 1981.
the thresholds for vortex-to-voltage state transitions. The dashed lines represent vortex-to-vortex transitions. The boundary between solid and dashed line at the critical point PK in Fig. 1 is shifted to higher gate currents with increasing interferometer damping) For the read operation, a cell in a random access memory array or matrix with many cells is selected by currents 1GN and ICXN a t the intersection of an orthogonal word and x-control line. The nominal operating point designated with R01 in Fig. 1 is within the N = -1 state, but outside the N = 0 state. Therefore if a binary '1' has been stored the read operation does not destroy the N = -1 quantum state; no dc voltage appears. On the other hand if a '0' has been stored, the interferometer jumps into the voltage state on trespassing the solid line of the N = 0 state. The interferometer returns into the zero voltage state without trapping a flux quantum if the gate current IGN is removed after the control current I c x n . Thus nondestructive read out of both states is achieved) To write a '1' no gate current is applied. The operating point Wl in Fig. 1 is reached with the nominal control currents ICXN and ICDN- They flow in two separate control lines to select the addressed cell in a memory matrix. One control line is the x-line, the second control line runs diagonally through the matrix, but is parallel to the x-line at each cell. 1'6 The write '0' operating point W0 in Fig. 1 is achieved with the triple coincidence of the currents IGN, ICXN and
ICDN. The SEM micrograph of a cell prototype with one control line is shown in Fig. 2. Although two control lines are needed for cell selection in a memory matrix, all properties of a single cell outside a memory matrix could be tested in applying the currents ICXN and ICDN to the implemented control line. Beneath the control line which is the uppermost metallic layer, a square and a rectangular window junction can be detected. The control line has a width w = 10/am corresponding to the minimum line width. The interferometer areaA ~ 3600 gm 2 contains F' ~ A.w-2 ~ 40 lithographic squares. 7 The cell is arranged on top of a superconducting niobium ground-plate as sketched in Fig. 3. The cross-sectional dimensions, the layer materials and the process parameters
0011-2275/82/003127-04 $03.00 © 1982 Butterworth & Co (Publishers) Ltd. CRYOGENICS. MARCH 1982
127
ie the effective separation is d =tox + X1 + X2 = 0.2 tnn. The corresponding Josephson penetration depth is ~,j = 6.1/am. The capacitance of both junctions in parallel, as deduced by junction resonance, is C " - 9 pF. The corresponding McCumber damping parameter is ~ = R~ Clomax 21r/O o "~ 278.
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?
,'/'~",
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i
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The threshold gate currents I c at the vortex-to voltage state transitions versus current in the control line I~ has been measured at low frequency for the flux quantum states N = -+ 7 as shown in Fig. 5. Due to the finite junction
%
li
,
'
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l
,
.~
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,
,
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,
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Fig. 1 Threshold curves of the N = 0 and N = -I q u a n t u m states at the vortex-to-voltage-state transitions ( - - ) and at the vortex-tovortex transitions (- - -). T h e nominal drive current amplitudes are m a r k e d by circlesfor N D R O in point R01, for write 'I' and '0' in points W l and W 0 , respectively
~=
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Fig. 3 Sketch o f an interferometer cross section w i t h t w o Josephson junctions
~ 9 0 ~ rn ~brd line
20
E
dosephson junction IO x 20/~m 2
Interferometer inductance
dosephsonjunction I0 x IOFm 2
Fig. 2 SEM-micrograph of an interferometer memory cell w i t h t w o w i n d o w Josephson junctions o f different areas. The control line and word line are about 10 p,m wide
are very close to those described by Greiner et al;8 but the resistive layer with Auln 2 has been omitted. The junction b o t t o m electrode consists of AuPbln and the counter electrode of PbAu. 9 The rf-sputter oxidation process proved very useful in achieving reproducible Josephson current densities. The peak to peak voltage used was 300 V and the oxygen pressure 6 Pa. The photoresist stencil lift-off process using chlorobenzene was employed. 8 de-measurements
All measurements have been performed at 4.2 K. The measured gate current I G versus the voltage UI across the interferometer is shown in Fig. 4. The gap voltage is 2A/e = 2.3 inV. The interferometer subgap resistance Rj is 1 ~2 at U[ = 2 mV. The ratio of Rj and the tunnel resistance RNN with normally conducting electrodes is Rj/RNN"" 8. The maximum Josephson current of the interferometer in the N = 0 quantum state is IGmax = 10.2 mA (see Fig. 1). The Josephson current density is calculated jma X = 3.5 kA cm -2 with the junction areas A a = 10 x 10/am 2 a n d A b = 10 x 20/am 2. The sum of the tunnel oxide thickness and the London penetration depths,
128
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Fig. 4 Interferometer gate current I G versus interferometer voltage U I without magnetic field
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C R Y O G E N I C S . M A R C H 1982
N=
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-I
+1
.
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8 l
The current thresholds computed with point junctions are represented by dotted lines in Fig. 7. The measured solid lines are nicely approximated by the computed dotted lines around the origin for the N = 0 and -1 modes. Thus, a cell design with point junctions is justified.
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If IGQ is the gate current at the intersection Q of the N = 0 and -1 quantum states the ratio of the gate currents is/GQ#Gmax = 0.79. This ratio is independent of the transformation ratio t of the equivalent control current in the interferometer I c and the current in the control line # I c where t = Ic/I'o The pair of ratios 1GQ/lGmax = 0.79 and Ia/I b = 2.19 corresponds to a characteristic phase X = 27r (1.01) as computer simulations with point junctions (l -+ 0) show. Thus, the transformation ratio t is equal to t = LulL, where the interferometer inductance is L = X qbo(2rr/b) -1 = 0.65 pH and the inductance L u may be calculated from the measured control current shift A/b between the quantum states as L u = q%/2d~ = 0.51 pH. The transformation ratio is
CRYOGENICS.
- 0.8
MARCH
1982
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length 1 = 10 # m parallel to the control line the control magnetic field inside the junction reduces the Josephson current. The first zero is found near I c ~-- 30 mA. The envelope of all quantum state extreme may be approximated by a single junction threshold characteristic with 1/Xj = 1.6. An enlarged representation o f l c versus I t near the origin is shown in Fig. 6. The N = 0 state is characterized by gate current extremes occurring at the same absolute value o f the control current. The current thresholds at the vortex-to-vortex transitions are extrapolated with the tangents o f the measured vortex-to-voltage state branches according to Fig. 6. In Fig. 1 the measured and extrapolated branches are represented with solid and dashed lines, respectively. At point B of the N = 0 state in Fig. 1 the gate current is I G ~- Ia - Ib .4 Using this, since IGmax = Ia + Ib, the current ratio Ia/Ib = 2.2 and the current Ib = 3.2 m A were determined.
x /b
...
:,"
.~/ ...........
,
Fig. 6 Measured positive and negative threshold currents I G = f (I t ) at an enlarged control current scale of the interferometer in Fig. 2 w i t h h "- 2Tr, l a / I b "" 2.2, t = 0.8
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Fig. 7 M e a s u r e d t h r e s h o l d c u r r e n t s IG = f ( I c ) a c c o r d i n g t o Fig. 6 (--) and t h r e s h o l d s c o m p u t e d w i t h p o i n t j u n c t i o n s ( . . . . ) f o r h = 2 1 r ( 1 . 0 1 ) , la/Ib = 2 . 1 9 and t = 0.8 RI
WO
RO
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RO
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WO
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tO
O
0.5/.¢s Fig. 8 Four out of ten test program time slots of the voltage across the interferometer: a -- R1, W0, R0, R0 (R0, R0, W1, R1, R1, R1); b -- R0, W0, R0, R0 (R0, R0, --, R0, R0, R0). The operations in brackets are not displayed
129
Pulse measurements The basic memory operations of the cell have been tested with a pulse program at a repetition frequency of about 1 MHz and with pulse rise times of about 100 ns. One word of a periodic test program consists of the following operations: 4 x NDRO '1' (R1), 1 x write '0' (W0), 4 × NDRO '0' (R0), 1 x write '1' (Wl). The nominal current amplitudes of the write and read operations IGN, ICXN, ICDN are represented in Fig. 1. The gate current always overlaps the control current at rise and fall to achieve NDRO of the N = 0 quantum state. If the N = -1 mode has been written the interferometer stays in the zero-voltage state since the boundary of the N = -1 state is not trespassed. 2'3 The interferometer voltage picked up by a sampling oscilloscope and averaged is represented in Fig. 8a. The oscillogram shows the expected '0' and '1' signals. Small disturbances at the rise and fall of the overlapping drive pulses are also visible owing to test jig imperfections. In Fig. 8b the Wl-operation has been omitted. Many NDRO-operations of '1' and '0' were performed without any sign of signal degradation. After 10 9 operations the test was discontinued. The critical point Pk in Fig. 1 between the ranges of vortexto-vortex and vortex-to-voltage-state transitions at the mode
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boundary was measured using the pulse program. Above the critical point proper NDRO operations could be performed within the hatched area in Fig. 1. Hence the tolerances on the gate and control currents are about + 24%. The authors would like to thank the Laboratorium for Elektronenmikroskopie der Universit[t Karlsruhe for making the SEM-rnicrographs. The work of Mrs G. M~iller and E. Mulica in preparing the manuscript is appreciated. This work has been supported in part by the German minister of research and technology under grant no 423-7291-NT 2515 6.
References 1
Broom, R.F., Gueret, P., Kotyczka, W., Mohr, Th. 0., Moser, A., Oosenbrog, A., Wolf, P. IEEE J of Solid-State Qrcuits SC-14 4 (1979) 690
2
Beha,H., Jutzi, W. Offenlegungsschrift 2735133, Feb (1979) Beha, H.IEEETransonMagneticsMAG-15 1 (1979)424 Beha, H. VDE-VerlagGmbH, Berlin (1981) Beha,H. Intermag Conf Grenoble (1981) Yamamoto, M., ishida, A. Digest of technical papers, 12th Conf on Solid State Devices,Tokyo (1980) 109 Jutzi, W. Advances in solid state physics XXI (1981) Vieweg-Verlag, Wiesbaden Greiner, J. etal.IBMJResDevelop 24 2 (1980) 195 Lahari, S.K., Basavaiah,S.dAppIPhys 49 (1978) 2880 Beha, H., Jutzi, W. Patentanmeldung P 3008 926.0, 8.3 (1980)
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CRYOGENICS . MARCH 1982