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Electrical switching and memory effects in doped ferromagnetic semiconductors
Volume 41A, number 4
PHYSICS LETTERS
9 October 1972
ELECTRICAL SWITCHING AND MEMORY EFFECTS IN DOPED FERROMAGNETIC SEMICONDUCTORS P. WACHTER Laboratorium f~r Festk?Irperphysik, ETH Z~rich, HSnggerberg, 8049 Ziirich, Switzerland
Received 3 August 1972 We have observed bidirectional threshold and memory type switching in 0.5 to 1.26 at.% Gd-doped EuO single crystals. • Since the industrial fabrication of reversible, bidirectional threshold and memory switches from glassy or amorphous material, field-induced switching has found universal interest [ 1]. Recently electrical switching has also been observed in the liquid state [2]. The physical mechanism of these effects, however, is not yet completely understood. For the first time we report here on electrical switching and memory effects in ferromagnetic semiconductors such as EuO doped with 0.5 to 1.26 atomic percent Gd. The electrical circuit is quite conventional, inasmuch as one uses a current limiting resistor in series with the sample. Voltage and current are displayed on the horizontal and vertical plates of an oscilloscope. The sampies were cleaved single crystals of some mm size. Contacts were made by silver paint and mechanical clamping of the electrodes in a sample holder. In fig. 1 the current-voltage loop is seen in the reversible, bidirectional threshold mode, switched with
Fig. 1. Current-voltage characteristic of EuO+ 0.5% Gd. Frequency 50 c/see. Vertical 0.2 A/div, horizontal 2 V/div.
50 c/sec. The regions of negative conductivity are clearly seen. To assure that we are not dealing with a thermal hysteresis we performed the same type of experiment with incremental steps of voltage, each time waiting for thermal equilibrium before photographing the oscilloscope screen. This is shown in fig. 2 and the critical voltage to flip the switch is obvious. In fig. 3 the memory type of behaviour is shown. At first the voltage is increased stepwise and the switch is in the low conducting region. Beyond the critical voltage the flip into the high conducting phase occurs, however, even with a voltage reversal the high conducting state is pertained. This phase remains stable even with 50 c/sec. Application of an appreciably higher voltage finally flipped the sample back into the low conducting state. No failure of the memory switch has been observed for consecutive operation. The very same sample could also be operated in the reversible bidirectional switching imode.
Fig. 2. Current-voltage characteristic of EuO + 0.5% Gd.Static equilibrium points.
Fig. 3. Current-voltage characteristic in the memory switch mode. EuO + 0.5% Gd II. 391
Volume 41A, number 4
PHYSICS LETTERS
These experiments have been performed at room temperature, however, it is evident that at low temperatures, especially near the Curie temperature, switching by a magnetic field can be achieved. In this case one uses an electric field not sufficient to flip the switch. Application of a magnetic field then causes the transition. In these materials the conduction band and the Gd donor states show an exchange induced splitting [3]. The energy gap (4f 7 -+ 5d) and the donor activation energy are temperature and magnetic field dependent. The appearance of reversible and m e m o r y switch in the same sample and over electrode distances of several mm is a unique feature. The suspicion arises that this might be due to electrode effects. The large electrode distance permitted the use of two additional potential probes (four probe technique). The potential o f the probes was recorded with a 10 M ~ impedance. With the exception o f a few uncorrelated cases it became evident that the contacts were switching and not the bulk. Thus switching occured via Schottky barriers. With this in mind switching should be treated with much greater care. Generally one uses only two con-. tacts at a distance of 10 to 100/am and the Schottky barriers o f both contacts may interact. The fact that
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9 October 1972
our contact switching displays the very same behaviour as "Ovonics" [1] should stimulate the specialists to consider also contact phenomena as a possible explanation. Since pure EuO and also weakly doped EuO and EuTe do not show switching we are convinced that switching is connected with very high doping levels. The doping should bring the sample close to the Molt transition. In this concentration range the dopant forms clusters and conductivity occurs via constriction paths. In certain respects even the crystalline material can then be termed "amorphous". The author wants to thank Dr. P. Wyssmann for helpful discussions and Mr. H.P. Staub for skillfull technical assistance. The crystals have been prepared by Dr. E. Kaldis of this laboratory.
R efeyenc es [l I S.R. Ovshinsky, Phys. Rev. Letters 21 (1968) 1450. 12l G. Busch et al., Phys. Lett. 33A (1970) 64. [31 E. Kaldis, J. Schoened and P. Wachter, AIP Conf. Proc. No. 5, 17th Ann. Conf. Magnetism and Magnetic Materials, 269 (1971 ).
PHYSICS LETTERS
9 October 1972
ELECTRICAL SWITCHING AND MEMORY EFFECTS IN DOPED FERROMAGNETIC SEMICONDUCTORS P. WACHTER Laboratorium f~r Festk?Irperphysik, ETH Z~rich, HSnggerberg, 8049 Ziirich, Switzerland
Received 3 August 1972 We have observed bidirectional threshold and memory type switching in 0.5 to 1.26 at.% Gd-doped EuO single crystals. • Since the industrial fabrication of reversible, bidirectional threshold and memory switches from glassy or amorphous material, field-induced switching has found universal interest [ 1]. Recently electrical switching has also been observed in the liquid state [2]. The physical mechanism of these effects, however, is not yet completely understood. For the first time we report here on electrical switching and memory effects in ferromagnetic semiconductors such as EuO doped with 0.5 to 1.26 atomic percent Gd. The electrical circuit is quite conventional, inasmuch as one uses a current limiting resistor in series with the sample. Voltage and current are displayed on the horizontal and vertical plates of an oscilloscope. The sampies were cleaved single crystals of some mm size. Contacts were made by silver paint and mechanical clamping of the electrodes in a sample holder. In fig. 1 the current-voltage loop is seen in the reversible, bidirectional threshold mode, switched with
Fig. 1. Current-voltage characteristic of EuO+ 0.5% Gd. Frequency 50 c/see. Vertical 0.2 A/div, horizontal 2 V/div.
50 c/sec. The regions of negative conductivity are clearly seen. To assure that we are not dealing with a thermal hysteresis we performed the same type of experiment with incremental steps of voltage, each time waiting for thermal equilibrium before photographing the oscilloscope screen. This is shown in fig. 2 and the critical voltage to flip the switch is obvious. In fig. 3 the memory type of behaviour is shown. At first the voltage is increased stepwise and the switch is in the low conducting region. Beyond the critical voltage the flip into the high conducting phase occurs, however, even with a voltage reversal the high conducting state is pertained. This phase remains stable even with 50 c/sec. Application of an appreciably higher voltage finally flipped the sample back into the low conducting state. No failure of the memory switch has been observed for consecutive operation. The very same sample could also be operated in the reversible bidirectional switching imode.
Fig. 2. Current-voltage characteristic of EuO + 0.5% Gd.Static equilibrium points.
Fig. 3. Current-voltage characteristic in the memory switch mode. EuO + 0.5% Gd II. 391
Volume 41A, number 4
PHYSICS LETTERS
These experiments have been performed at room temperature, however, it is evident that at low temperatures, especially near the Curie temperature, switching by a magnetic field can be achieved. In this case one uses an electric field not sufficient to flip the switch. Application of a magnetic field then causes the transition. In these materials the conduction band and the Gd donor states show an exchange induced splitting [3]. The energy gap (4f 7 -+ 5d) and the donor activation energy are temperature and magnetic field dependent. The appearance of reversible and m e m o r y switch in the same sample and over electrode distances of several mm is a unique feature. The suspicion arises that this might be due to electrode effects. The large electrode distance permitted the use of two additional potential probes (four probe technique). The potential o f the probes was recorded with a 10 M ~ impedance. With the exception o f a few uncorrelated cases it became evident that the contacts were switching and not the bulk. Thus switching occured via Schottky barriers. With this in mind switching should be treated with much greater care. Generally one uses only two con-. tacts at a distance of 10 to 100/am and the Schottky barriers o f both contacts may interact. The fact that
392
9 October 1972
our contact switching displays the very same behaviour as "Ovonics" [1] should stimulate the specialists to consider also contact phenomena as a possible explanation. Since pure EuO and also weakly doped EuO and EuTe do not show switching we are convinced that switching is connected with very high doping levels. The doping should bring the sample close to the Molt transition. In this concentration range the dopant forms clusters and conductivity occurs via constriction paths. In certain respects even the crystalline material can then be termed "amorphous". The author wants to thank Dr. P. Wyssmann for helpful discussions and Mr. H.P. Staub for skillfull technical assistance. The crystals have been prepared by Dr. E. Kaldis of this laboratory.
R efeyenc es [l I S.R. Ovshinsky, Phys. Rev. Letters 21 (1968) 1450. 12l G. Busch et al., Phys. Lett. 33A (1970) 64. [31 E. Kaldis, J. Schoened and P. Wachter, AIP Conf. Proc. No. 5, 17th Ann. Conf. Magnetism and Magnetic Materials, 269 (1971 ).