Tunneling magnetoresistance in ( Ga , Mn ) As / Al – O / CoFeB hybrid structures

Tunneling magnetoresistance (TMR) in Ga(0.9)2Mn(0.08)As/Al-O/Co40Fe40B20 trilayer hybrid structure as a function of temperature from 10 to 50 K with magnetic field vertical bar H vertical bar <= 2000 Oe has been studied. TMR ratio of 1.6% at low fields at 10 K was achieved with the applied current of 1 mu A. The behavior of junction resistance was well explained by the tunneling resistance across the barrier. Strong bias dependences of magnetoresistance and junction resistance were presented. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3068418]

Efficient electrical injection of spin-polarized electrons is one of the most important technologies for realizing new functional devices based on spins in semiconductors.Transition metal doped III-V compound semiconductor offers an unprecedented opportunity to explore ferromagnetism in semiconductors.Ferromagnetism due to spin-spin interactions is mediated by holes in the valence band and changing the Fermi level using codoping and electric fields or light can directly manipulate the magnetic ordering.A ferromagnetic semiconductor ͑Ga,Mn͒As is one of the promising materials for spin-polarized carrier injector because of its high spin polarization, and Curie temperature T C as high as 110 K was first reported in 1996 by Ohno. 1 High spin polarization predicted by the band structure calculation 2 with support from experimental results showing high spin polarization 3 of over 85% by Andreev reflection in this kind of materials makes them more attractive for further investigation, especially their potential application in spintronic devices.[6][7][8][9][10] Recently, demonstration of spin injection from ferromagnetic metal into ͑Ga,Mn͒As through wide band semiconductor tunnel barrier such as ZnSe 11 and AlAs 12 has been successfully obtained due to the high Curie temperature of ferromagnetic metal and especially the lattice match between epitaxially grown films.Pioneering works in spin injection through interband spin-polarized tunneling in ͑Ga, Mn͒As/ n + -GaAs Easki tunnel structures [13][14][15] and diffusive spin injection 16 from ͑Ga,Mn͒As to adjacent GaAs have been demonstrated by Ohno and co-workers.Tunneling anisotropic magnetoresistance 17 ͑TAMR͒ and tunnel spectroscopy 18 in the system of Au/Al-O/͑Ga,Mn͒As were studied, and TAMR arises from spin-orbital coupling.Al-O has been widely adopted in Magnetic Tunnel Junctions ͑MTJs͒ as an insulating barrier because of the good wetting property of Al and its feasible oxidation by a plasma oxidation technique. 19However, knowledge on the hybrid MTJ structure of ͑Ga,Mn͒As consisting of aluminum oxide as a tunneling barrier is still missing.In this work, we report the fabrication of MTJ based on Ga 0.92 Mn 0.08 As/ Al-O/ Co 40 Fe 40 B 20 hybrid structures grown by MBE and magnetron sputtering together.The results of bias and temperature dependences of TMR ratio have been studied systematically.
The hybrid structure was grown on semi-insulating GaAs substrate, consisting of GaAs͑50͒ / Ga 0.92 Mn 0.08 As ͑400͒ / GaAs͑1͒ / Al-O͑1.2͒/ Co 40 Fe 40 B 20 ͑5͒ / Ir 22 Mn 78 ͑10͒/ Ru͑10͒ ͑thickness in nanometers͒.First, 50 nm thick GaAs buffer layer was deposited with substrate temperature T S = 560 °C in a V80MARKI MBE system in order to grow ͑Ga,Mn͒As on the smooth surface.Then 400 nm thick Ga 1−x Mn x As layer ͑where x = 0.08͒ and 1 nm thick GaAs were grown at T S = 250 °C, where 1 nm GaAs is used to prevent Mn diffusion into alumina layer after growth.The growth details could be found elsewhere. 20After the growth of Ga 0.92 Mn 0.08 As film in the MBE chamber, the sample was clearly packed and transferred into the load lock chamber of the sputtering system when the clean surface was confirmed by atomic force microscope.The amorphous alumina oxide layer was formed by plasma oxidation in the sputtering system, followed by the deposition of Co 40 Fe 40 B 20 / Ir 22 Mn 78 / Ru metallic multilayers, where Ru acts as capping layer.Junctions with size of 8 ϫ 16 m 2 were patterned by conventional UV lithography combined with argon ion milling.A four probe dc transport measurement was performed as a function of temperatures ͑down to 10 K͒ and magnetic field ͑up to 2000 Oe͒ in a physical properties measurement system by Quantum Design.Here, positive bias ͑current͒ refers to current flowing from the bottom electrode to the top electrode.
Figure 1 shows the temperature dependence of the junction resistance and ͑Ga,Mn͒As single layer resistance at zero fields.The right upper inset is the schematic of the junction a͒ Authors to whom correspondence should be addressed.Electronic addresses: xfhan@aphy.iphy.ac.cn and jhzhao@red.semi.ac.cn.pattern and four probe geometry used for this measurement.The top electrode of the junction is directly connected to Cu through a relatively large pad with size of 300ϫ 300 m 2 .The top and bottom electrodes were isolated by SiO 2 .Compared to the junction size of 8 ϫ 16 m 2 , the pad resistance is only 0.1% and is neglected.͑Ga,Mn͒As single layer resistance was measured by two probe sensing measurement through this pad.The as-grown ͑Ga,Mn͒As is a highly selfcompensated ferromagnetic semiconductor which exhibits semiconducting behavior at low temperatures with Curie temperature of 35 K, shown in Fig. 1͑b͒.A hump was observed at 35 K, which has been confirmed by several groups in the range of x = 0.015-0.071Mn concentrations. 21Figure 1͑a͒ shows typical junction resistance as a function of temperature which shows very strong temperature dependence and increases with decreasing temperature.
R-H loops of the typical spin-valve-type junction at T = 10 K with sensing current of +1 A are shown in Fig. 2. The free layer ͑Ga,Mn͒As and the reference layer CoFeB are magnetically parallel at the magnetic field H = −2000 Oe.As field is swept from Ϫ2000 to 2000 Oe, ͑Ga,Mn͒As first switches at around 100 Oe, while the magnetization of the reference layer CoFeB exchange biased by antiferromagnet IrMn sustained, leading to an antiparallel configuration; then CoFeB switches at around 1000 Oe, resulting in a parallel state.The almost zero-hysteretic negative MR behavior in the parallel state ͑tilted part of R-H loop͒ was similar to our previous report, 22 probably due to the coherent magnetization rotation of both the two magnetic layers toward the applied magnetic field and the suppression of magnetic disorder at the ͑Ga,Mn͒As/Al-O interface as well.In addition, slight asymmetry from current polarity was observed in resistance measurement which can be attributed to the different barrier heights on the two sides of the insulator.
The temperature dependence of the TMR is shown in Fig. 3. Clear resistance change with sensing current of +1 A associated with magnetization switching of ͑Ga,Mn͒As has been observed.Coercivity ͑H c ͒ of ͑Ga,Mn͒As decreases with increasing temperature, where H c are 30, 20, 10, and 7.5 Oe at 10, 15, 20, and 25 K, respectively.The relatively small H c is expected for thick ͑Ga,Mn͒As layer 23 where the thickness of the bottom ͑Ga,Mn͒As layer was 400 nm.TMR ratios, 1.6%, 1.4%, 1.2%,and 1.0% at 10, 15, 20, and 25 K, respectively, were obtained which decrease faster with increasing temperature and TMR almost drops to zero as temperature approaches T C .Bias dependences of junction resistance and TMR ratio are shown in Fig. 4. Junction resistance decreases rapidly with increasing dc bias, showing much stronger bias dependence than that in conventional 3d transition metal MTJs, e.g., CoFe/Al-O/CoFe.The strong bias dependence of parallel state resistance at 10 K as shown in Fig. 4͑a͒ in terms of zero bias anomaly is widely believed to be due to the corre-   23 TMR ratio dependence of direct sensing current is shown in Fig. 4͑b͒; TMR ratio decreases rapidly with increasing sensing current.The inset of Fig. 4͑b͒ shows the dc bias voltage dependence of TMR ratio in the low bias range.V half at which TMR drops to one-half of its maximum is 2.7 mV.This is due to the relatively low Fermi energy of ͑Ga,Mn͒As and large increase in the empty states even with small bias ͑see Ref. 6͒.In our case, the Fermi energy of the as-grown semiconducting ͑Ga,Mn͒As is even smaller, and hence an even stronger bias dependence can be expected.
In summary, we have observed TMR in Ga 0.92 Mn 0.08 As/ Al-O / Co 40 Fe 40 B 20 hybrid structures at low temperatures from 10 to 25 K. Clear switching behavior of R-H minor loop in the low field range shows TMR ratio of 1.6% at 10 K. Strong bias dependences of both resistance and TMR ratio were presented, probably due to the correlation gap in ͑Ga,Mn͒As and its low Fermi energy, similar to that observed in ͑Ga,Mn͒As/AlAs/MnAs hybrid structures.
V half at which TMR ratio drops to one-half of its maximum is 2.7 mV.

FIG. 1 .
FIG. 1. Temperature dependence of the junction resistance and ͑Ga,Mn͒As single layer resistance in zero field.Inset shows the schematic of the junction pattern and measurement geometry.

FIG. 2 .
FIG. 2. Typical junction resistance vs magnetic field in the field range of 2000 Oe at 10 K; forward corresponds to sweeping field from negative to positive and backward vice versa.
This project was supported by the State Key Project of Fundamental Research of Ministry of Science and Technology ͑MOST, Grant Nos.2006CB932200 and 2007CB924903͒ and the National Natural Science Foundation ͑NSFC, Grant No. 0574156͒.X. F. Han gratefully thanks the partial support of the Outstanding Young Researcher Foundation ͑NSFC, Grant Nos.50721001 and 50528101͒ and K. C. Wong Education Foundation, Hong Kong.