(Ga,Mn)As/GaAs/(Ga,Mn)As trilayer structures


Tunneling magnetoresistance in (Ga,Mn)As based heterostructures with a GaAs barrier1,2,4,6,8)
We have investigated the properties of tunneling magnetoresistance (TMR) of (Ga,Mn)As trilayer structures with a GaAs intermediary barrier layer. TMR ratio of 290% is observed at 0.39 K around zero applied bias voltage. The bias dependence of TMR ratio as well as the temperature dependent anisotropic behavior are presented.

Sample ASample B
ticknesscomposition
20 nm(Ga1-xMnx)Asx = 0.074
6 nmGaAs
20 nm(Ga1-xMnx)Asx = 0.044
50 nmGaAs:Be5 x 1018 cm-3
p+ GaAs sub.
ticknesscomposition
50 nm(Ga1-xMnx)Asx = 0.064
6 nmGaAs
50 nm(Ga1-xMnx)Asx = 0.039
50 nmGaAs:Be1 x 1019 cm-3
p+ GaAs sub.
Figure.1Sample structures. Both Samples A and B have 6 nm thick GaAs barrier.

Figure.2Magnetoresistance of Sample A and Sample B under B // [100] at small bias voltage. Positive bias voltage V corresponds to the hole injection from the top (Ga,Mn)As layer. Clear increase of TMR is observed (the insets show the magnified views around zero magnetic field) and the ratio reaches 290% at 0.39 K (105% at 4.7 K) and 90% at 15 K for Samples A and B, respectively, in spite of a rather thick barrier layer. Such a high TMR ratio is result of high spin polarization of (Ga,Mn)As and the high quality of crystal and interfaces between (Ga,Mn)As and GaAs.

Figure.3Applied bias voltage V dependence of (a) MR ratio and (b) normalized MR ratio under B // [100]. Positive bias corresponds to carrier flow from the top to the bottom layer. (c) V dependence of junction resistance R0A at zero magnetic field.

Figure.4(a) Temperature dependence of MR ratio (Sample A) under B // [100] (circle) and [110] (triangle). (b) Temperature dependence of magnetization of Sample A under B = 2 mT // [100] (solid line) and [110] (dashed line). The sample was magnetized at 5 K in 1 T before measurement.




Current-induced magnetization reversal in (Ga,Mn)As magnetic tuunel junctions5,7,8)
Current-driven magnetization reversal in a ferromagnetic semiconductor based (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K. Magnetoresistance measurements combined with current pulse application on a rectangular 1.5 x 0.3 μ m2 device revealed that magnetization switching occurs at low critical current densities of 1.1 - 2.2 x 105 A/cm2 despite the presence of spin-orbit interaction in the p-type semiconductor system. Possible mechanisms responsible for the effect are discussed.


Figure.6A schematic cross-section of fabricated device structure and a scanning electron micrograph of a rectangular device (a x b = 1.5 x 0.3 μ m2) after formation of the first Cr/Au electrode.


Figure.7Major (closed symbols) and minor (open symbols) magnetoresistance curves of a 1.5 x 0.3 μ m2 magnetic tunnel junction sample at 30 K taken at a bias of Vd = +10 mV. Magnetic field is applied along a. Three arrows indicate the sweep direction of magnetic field to prepare the different initial configurations A, B, and C.


Figure.8ΔR as a function of Ipulse of the 1.5 x 0.3 μ m2 device at 30 K, where ΔR is the resistance difference between the resistance of MTJ after application of Ipulse (1 ms) and that at parallel magnetization configuration at H = 0. Closed circles show the Ipulse dependence of ΔR for initial configuration A (parallel M), whereas open triangles show the results for initial configuration C (antiparallel M). The inset shows I-V characteristics of the device.




Effect of low-temperature annealing on (Ga,Mn)As trilayer structures3)
Effect of low temperature annealing on (Ga,Mn)As/GaAs/(Ga,Mn)As trilayer structures is studied. Low temperature annealing increases the ferromagnetic transition temperature TC of top (Ga,Mn)As layers significantly, reaching as high as 160 K, whereas no apparent effect is observed on bottom (Ga,Mn)As layers.


Figure.9Temperature, T, dependence of magnetic moments per unit area, μ, under B = 2 mT (applied along [100]) of a trilayer structure with sample A(shown in Fig. 1) before and after annealing. TC of the top layer is 160 K after annealing at 250oC for 30 min. The two as-grown magnetic layers show different TC, and one of TC increases significantly upon low-temperature annealing, whereas the other shows virtually no change. One can identify that the top (Ga,Mn)As layer is the layer that showed significant increase of TC by comparing magnetic moment measurements before and after removal of a top part of the top layer by wet chemical etch, and also from μ - B measurements.




Publications
  1. D. Chiba, N. Akiba, F. Matsukura, Y. Ohno, and H. Ohno, "Magnetoresistance effect and interlayer coupling of (Ga, Mn)As trilayer structures", Applied Physics Letters,Vol. 77, Issue 12, pp. 1873-1875, 18 September, 2000.
  2. D. Chiba, N. Akiba, F. Matsukura, Y. Ohno and H. Ohno,"Properties of (Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As magnetic trilayer structures", Physica E, Vol. 10, Issues 1-3, pp. 278-282, May 2001.
  3. D. Chiba, K. Takamura, F. Matsukura, and H.Ohno, "Effect of low-temperature annealing on (Ga,Mn)As trilayer structures", Applied Physics Letters, Vol. 82, No. 18, pp. 3020-3022, 5 May 2003.
  4. D. Chiba, F. Matsukura, H. Ohno, "Tunnelingmagnetoresistance in (Ga,Mn)As-based heterostructures with a GaAs barrier," Phisica E Vol. 21, pp. 966-969, March 2004.
  5. D. Chiba, Y. Sato, T. Kita, F. Matsukura, and H. Ohno "Current-driven magnetization reversal in a ferromagnetic semiconductor (Ga,Mn)As/GaAs/(Ga,Mn)As tunnel junction," Physical Review Letters, Vol. 93, pp. 216602(1)-(4), November 2004.
  6. Y. Sato, D. Chiba, F. Matsukura, and H. Ohno, hEffect of GaAs Intermediary Layer Thickness on the Properties of (Ga,Mn)As Tri-Layer Structuresh, Journal of Superconductivity: Incorporating Novel Magnetism, Vol. 18, pp. 345-347, 2005.
  7. D. Chiba, T. Kita, F. Matsukura, and H. Ohno, "Pulse-width and Magnetic-field Dependences of Current-induced Magnetization Switching in a (Ga, Mn)As Magnetic Tunnel Junction", Journal of Applied Physics, Vol. 99, pp. 08G514(1)-(3), Apr. 2006.
  8. D. Chiba, F. Matsukura, and H. Ohno, "Electrical Magnetization Reversal in Ferromagnetic III-V Semiconductors", Journal of Physics D: Applied Physics, Vol. 39, pp. R215(1)-(11), Jun. 2006.