IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
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Temperature Dependence of Electrical Transport and Magnetization Reversal in Magnetic Tunnel Junction Chien-Tu Chao1 , Che-Chin Chen1 , Cheng-Yi Kuo1 , Cen-Shawn Wu1 , Lance Horng1 , Shinji Isogami2 , Masakiyo Tsunoda2 , Migaku Takahashi2 , and Jong-Ching Wu1 Department of Physics, National Changhua University of Education, Taiwan Department of Electronic Engineering, Tohoku University, Sendai 980-8579, Japan A series of tunneling magnetoresistance (TMR) has been measured at various temperatures ranging from 4 K to 360 K for characterizing the electrical transport and magnetization reversal of nanostructured magnetic tunnel junctions (MTJs) with thin effective MgO 2 . MTJs with 2 elliptical shape were fabricated by barrier of 1 nm thickness and resistance-area (RA) product of 10 using electron beam lithography in combination with ion beam milling. Typical TMR curves were observed at temperature above 70 K, below which there was no significant anti-parallel (AP) state revealed. A linear relationship is found between resistance and temperature 4 and 4 , respectively. The TMR ratio in both parallel (P) and AP states, having linear coefficients of 3 2 . The negative temperature coefficients and TMR tendency with temperature indicated that was found to be proportional to electrical transport is dominated mainly by tunneling mechanism. In addition, the biasing field of pinned CoFeB layer due to RKKY coupling increased with decreasing temperature until a maximum biasing field reached at 200 K, after which the biasing field decreased with decreasing temperature.
1
m
150 250 nm
4 15 10
8 07 10 ( K)
Index Terms—Magnetic tunnel junction, magnetization reversal, temperature dependence.
I. INTRODUCTION
II. EXPERIMENT
LECTRICAL transport properties in magnetic tunnel junction (MTJ) have been comprehensively studied [1]–[4] because of their scientific interests and attractive applications, such as magnetic random access memory (MRAM) and read-heads for hard disk drives. For optimizing the performance of these devices, achieving low resistance-area (RA) product while maintaining high tunneling magnetoresistance (TMR) ratio is a key issue for ultra-high density memory and storage applications. Many investigations have been focused on adopting thinner MgO barrier layer, in which low RA value was achieved but sacrificing TMR ratio [5]–[7]. Existence of pinholes in MgO barrier layer with only few monolayers might be an important issue to cause the reduction of TMR ratio. Therefore, an additional metallic transport mechanism due to pinholes was proposed to be in parallel with tunneling effect for the junction resistance [8], [9]. These two transport channels, tunneling and metallic transport, reveal opposite behaviors in resistance levels with respect to temperature. Hence, temperature dependence of magnetoresistance (MR) has been employed to identify the dominative contribution between these two transport channels in MTJs having ultrathin barrier [8]–[10]. In this study, a series of temperature dependent TMR measurements of nanostructured MTJs, having a thin MgO barrier layer with in-situ heat treatment [11], was performed at various temperatures ranging from 4 K to 360 K. The results show that the resistance level of parallel (P) and anti-parallel (AP) states as well as TMR ratio are strongly temperature dependent, with which the transport mechanism is thus identified.
E
Manuscript received October 31, 2009; revised January 14, 2010; accepted February 27, 2010. Current version published May 19, 2010. Corresponding author: J. C. Wu (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2010.2045354
The MTJ multilayered structures consist of . The effective thickness is of 1 nm and resistance-area (RA) . The stack films were subjected to a product is of 10 for 1 hour under an planar field cooling treatment at 360 external magnetic field of 3000 Oe to improve the film quality and set the biasing direction of pinned layer. Notice that the CoFeB/Ru/CoFe/IrMn layers perform as a synthetic artificial antiferromagnet (SAF) structure, in which the CoFeB is pinned due to RKKY coupling through Ru and is used to reduce stray field coupling to the free layer (CoFeB under Mg/MgO). Two ferromagnetic CoFeB layers, sandwich the Mg/MgO layers, play an important role in electrical transport, i.e. the TMR is mainly dominated by the relative angle between the magnetizations of two ferromagnetic CoFeB layers. elliptical The MTJ films were patterned into pillar devices by using a standard electron beam lithography in conjunction with an ion beam milling through a self-aligned technique. Temperature dependent TMR behaviors were measured in a standard ac four-terminal arrangement in a Physical Property Measurement System (PPMS) under various temperatures ranging from 4 K to 360 K. The external magnetic field was applied along the biasing direction of the pinned layer. III. RESULTS AND DISCUSSION Figs. 1(a) and (b) show the major and minor loops measured at various temperatures under the external magnetic field ranges of 2000 Oe and 300 Oe, respectively. Note that the major and minor loops were measured in two different measurements. The resistance levels of P and AP states both increases with decreasing temperature. Typical TMR curves exhibiting obvious plateau of AP state resistance level are present at temperature above 70 K. Therefore, two distinct peaks appear in major loop that was measured at 4 K, as shown in the inset of Fig. 1(a).
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010
Fig. 1. The TMR major (a) and minor (b) loops at specific temperatures 70 K, 120 K, 200 K, 280 K, and 360 K. The resistance in both P and AP states decreased with increasing temperature. The inset in Fig. 1(a) is the major loop measured at 4 K. The minor loops show shifts with respect to zero magnetic field, this influence may be resulted from coupling which is induced by pinned layer.
Similar results are found in major loops from 4 K to 60 K as well (not shown). As the temperature decreased, the switching fields for the magnetization of two ferromagnetic CoFeB layers get closed to the same value, resulting in the duration of AP state reduces temporally. According to theoretical prediction, the total conductance can be described as [12] (1) where is the angle between the magnetization directions of and indicate that the efthe two ferromagnetic layers. fective tunneling electron spin polarizations of the ferromagis contributed by elastic direct tunnel and varies netic layers. is not dependent on the relative slightly with temperature. orientation of the electrode magnetizations. Therefore, the total term, which is proportional conductance is dominated by to . Total conductance increases with increasing temperature, i.e., the resistance increases with decreasing temperature. A linear relationship is found between resistance and temperature in both P and AP states, as shown in Fig. 2, having linear
Fig. 2. (a) The resistances in P and AP states as a function of temperature T. The curves showed linear relationship between resistance and temperature in both P and AP states. The fitting equations were showed in insets. (b) TMR ratio as a function of T. The inset is the fitting equation and indicates that TMR . ratio is proportional to a factor of 1 BT
0
coefficients of and , respectively. The fitting equations of P and AP state resistances are written in inset of Fig. 2. The negative coefficients in both P and AP states meant that temperature dependence of resistance is dominated mainly by tunneling mechanism [8], [9]. In our measurement results, TMR versus temperature curve can be fit by a power factor of 3/2 and written as following equation: (2) This proportional factor is also corresponding to previous study [2]. The tendency of TMR ratio with respect to temperature also shows that the tunneling mechanism dominates the resistance of MTJ device. In Fig. 3, the coercive field of free layer diminishes with increasing temperature due to thermal assisted effect, the tendency is the same as theoretical model [13], [14]. The resistance variation in high field is employed to distinguish the switching field of pinned CoFeB layer, and then the strength of biasing field due to RKKY coupling can be identified. The temperature dependent biasing fields of pinned CoFeB layer increases with decreasing temperature until a maximum biasing field reaches at 200 K, after which the biasing field decreases with decreasing
CHAO et al.: TEMPERATURE DEPENDENCE OF ELECTRICAL TRANSPORT AND MAGNETIZATION REVERSAL
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ACKNOWLEDGMENT This work was supported in part by the Ministry of Economic Affairs under Grant 97-EC-17-A-01-S1-026 and in part by the National Science Council under Grant NSC 98-2112-M-018004-MY3. REFERENCES
Fig. 3. The coercive field of free layer and biasing fields of pinned layer versus temperature. The coercive field of free layer diminishes with increasing temperature due to thermal assisted effect. The biasing field of pinned layer shows a maximum at specific temperature 200 K. The fitting curve is plotted according to (3).
temperature. The biasing field is resulted from RKKY coupling of two magnetic layers separated by a nonmagnetic metal layer. The temperature dependence of coupling strength is predicted theoretically and described as following equation [15]–[17]: (3) is the coupling strength at 0 K, and is a characwhere teristic temperature. In (3), with decreasing temperature, the coupling strength increases. This trend is the same as our experimental results above 200 K. Below that, the biasing field with respect to temperature is inconsistent with the equation. For this case, the decreasing tendency of biasing field below 200 K may be investigated completely in the near future. Note that the exchange coupling field between CoFe and IrMn bi-layer cannot be observed through TMR loop, because the magnetization switching of CoFe layer had no contribution to resistance variation. The details of magnetization switching needs to be identified by temperature dependent hysteresis loop further. IV. CONCLUSIONS In summary, the electrical transport and magnetization switching properties of MTJ device were characterized by using temperature dependent TMR measurement. Resistances in P and AP states increase with decreasing temperature; TMR . These two ratio is proportional to a factor of behaviors are believed that the electrical transport is dominated by tunneling mechanism. In this case, the in-situ heat treatment of MgO barrier can achieve the goals of pinhole free and low RA value. The coercive field of free layer diminished with increasing temperature due to thermal assisted effect. In addition, the biasing field of pinned CoFeB layer increased with decreasing temperature until a maximum biasing field reached at 200 K, after which the biasing field decreased with decreasing temperature. This decreasing tendency of biasing field below 200 K needs to be investigated further.
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