JOURNAL OF APPLIED PHYSICS 99, 08K704 共2006兲
Influence of reference layer stability on the switching performance of sub-micron-sized magnetic tunnel junctions Ulrich Klostermann,a兲 Chanro Park, and Wolfgang Raberg Infineon Technologies, MRAM Joint Development Project, 224 bd John Kennedy, 91105 Corbeil Essonnes, France
Faiz Dahmani Altis Semiconductor, MRAM Joint Development Project, 224 bd John Kennedy, 91105 Corbeil Essonnes, France
Manfred Ruehrig Siemens AG, CT MM 1, Paul-Gossen Strasse 100, D-91052 Erlangen, Germany
共Presented on 2 November 2005; published online 26 April 2006兲 Arrays of sub-micron-sized, magnetic tunnel junctions are patterned and the switching field 共Hc兲 of the free layer 共FL兲 and its distribution 共兲 are measured by Kerr magnetometry. A correlation of Hc and with the magnetic stiffness 共stability against external magnetic fields兲 of the reference system 共RS兲 is observed. For simple pinned RSs the stiffness is varied by different materials 共CoFe vs CoFeB兲 and its thicknesses, while for artificial antiferromagnetic pinned systems the thickness of the exchange coupling layer of Ruthenium is modified. It is shown that less stiffness causes a lower Hc and larger . This effect is assumed to be due to magnetic mirror charges induced in RS by the magnetic dipoles of the patterned FL elements. These mirror charges screen the dipole stray field of the FL resulting in lower effective shape anisotropy, which goes along with reduced switching fields. Micromagnetic simulations performed for different pinning strengths of a single layer RS clearly support this observation. From high field measurements perpendicular to the unidirectional exchange anisotropy of the RS, an order parameter can be extracted quantifying the stiffness 共⬃1 / 兲. is found to be linearly correlated to and Hc. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2176240兴 INTRODUCTION
Magnetoresistive random access memory 共MRAM兲 devices rely on the selective switching of a magnetic free layer 共FL兲 to successfully write the information bit.1 The physical understanding of the switching field distribution is crucial for obtaining high yielding MRAM devices. The switching field distribution is suspected to be impacted by numerous detractors such as shape fidelity, intrinsic anisotropy distributions, interlayer roughness, edge corrosion effects, magnetoelastic effects, and interactions between FL and the reference system 共RS兲.2 It is known that the RS, via the dipole stray field coupling of the patterned reference layer3 and to some extent via the Néel coupling, plays an important role for the switching field offset and switching field distribution of the FL. This article discusses additional influences of the RS, which were neglected so far. Today, most common RSs consist either of a single magnetic layer or an artificial antiferromagnet sandwich both exchange coupled to a thin layer of a natural antiferromagnet 共AFM兲 material. The exchange coupling can be expressed in terms of a unidirectional anisotropy in the RS providing the required stiffness against external fields. In this work the impact of the RS stability on the switching performance of the FL is studied. a兲
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FIG. 1. Switching properties for various RS. 共a兲 For SP systems the thickness 共number in Å兲 and material 共CF vs CFB兲 are varied. 共b兲 For APP systems the Ru thickness is varied. In both cases the configuration of the RS has a strong effect on the Hc, , and offset.
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FIG. 2. Definition of as a control parameter for the stiffness of the RS. For a SP the Iquadrant of a magnetization loop 共no free layer included兲 along the hard axis 共HA兲 is sketched. is a function of the probing field Ho, especially for weak RSs such as SP systems.
FIG. 4. Correlation of FL switching field Hc and its switching field distribution vs the stiffness parameter for the SP series as given in Fig. 1共a兲. The probing field is set to 75 Oe for best correlation.
EXPERIMENT
patterned array sites show that the observed offset field 共Hs兲 is almost identical, confirming the successful SOA process.
Several approaches were used to vary the RS of the magnetic tunnel junction 共MTJ兲 stack. Generally, all stacks were deposited in the following sequence: substrate/seed/AFM/ RS/Al2O3 barrier/FL/cap. For the RS either a simple pinned 共SP兲 system or antiparallel pinned 共APP兲 system is used. In the case of SP the RS consists of CoFe or CoFeB with different thicknesses from 16 to 32 Å, while for an APP system 共CoFe/ Ru/ CoFe兲 the Ru thickness is varied from 6 to 10 Å. After stack deposition in a physical vapor deposition 共PVD兲 sputter system and subsequent thermal annealing in a magnetic field at 1 Tesla to set the pinning direction of the RS along the easy axis, all samples were patterned into large arrays of identical sub-micron-sized elliptical tunnel junctions by using optical lithography and etching of the FL with selective etch stop on the Al2O3 barrier 共SOA兲. Our SOA process is very robust and carefully monitored. On all samples, the switching behavior of the patterned FL is investigated with magneto-optical kerr effect 共MOKE兲, where the switching curve of a large ensemble of identical MTJs 共⬃10 000 junctions兲 is obtained. From curve fitting the switching field 共Hc兲, the switching field distribution 共兲 and offset field 共Hs兲 of the ensemble are extracted. Hysteresis loops obtained on a blanket film area and on neighboring
FIG. 3. 共⬃1 / stiffness兲 vs different SP systems. An APP stack has the best reference layer stability; thicker RS has less stability.
RESULTS
Figures 1共a兲 and 1共b兲 show switching properties 共Hc , , Hs兲 of the SP and APP series, respectively. For SP series a thinner pinned layer results in a higher Hc and the offset field is lower for a CoFeB共=CFB兲 reference layer material compared to CoFe共=CF兲. From the comparison with the APP reference stack 关left of Fig. 1共a兲兴 it is clear that all SP samples show lower Hc, higher , and larger Hs. For the APP series in Fig. 1共b兲 the Hc peaks at a Ru thickness of about 7.5 Å. It is found that Hs is not a suitable order parameter for the switching parameters 共Hc , 兲.
DISCUSSION
A certain correlation to these parameters can be established when examining key control parameters of the RS stability such as the pinning field, coercive field, or the exchange bias field. Here a control parameter to quantify the stiffness or stability of the RS is proposed. This parameter can be extracted from high field measurements perpendicular to the unidirectional exchange anisotropy 共=hard axis, HA兲 of the RS. As depicted in Fig. 2 is a function of the applied probing field Ho and defined by
FIG. 5. Correlation of FL switching field Hc and its switching field distribution vs the stiffness parameter for the APP series as given in Fig. 1共b兲.
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FIG. 6. 共Color online兲 Micromagnetic simulation results for the SP system with different pinning strengths Heb. 共a兲 The higher the exchange bias 共i.e., the stiffer the RS兲, the higher is the Hc of the FL. The magnetization distribution in the FL 共b兲 and RS 共c兲 is shown by a gray code and arrows for fields close to the reversal of the FL.
共Ho兲 ª
M共Ho兲 − M共H = 0 Oe兲 M共Ho兲 = . Ho − 0 Oe Ho
For example, Fig. 3 depicts the RS stiffness versus the SP series for different probing fields Ho. Linear regression analysis for FL switching parameters Hc and showed best correlation for a probing field Ho of about 75 Oe in the case of the SP series 共Fig. 4兲. In other words, when reploting the data from Fig. 1共a兲 versus the stiffness parameter 共from Fig. 3兲, the Hc increases with higher stiffness 共smaller 兲 while decreases. It is noted that higher or lower probing fields than approximately 75 Oe have less good regression correlation. Thus, a best fit is approximately observed for fields close to the FL switching field itself. Figure 5 shows similar correlations for the APP series as given in Fig. 1共b兲. Note that as can be assumed to be approximately constant for the low to medium field range, is evaluated at Ho = 1000 Oe. SIMULATION RESULTS AND MODELING
In order to verify the impact of the RS stability on the switching properties of the FL, micromagnetic simulations have been performed using the commercial Landau-LifshitzGilbert 共LLG兲 micromagnetic simulatorTM. An elliptically shaped FL element 共3 nm, NiFe兲 is separated by 1 nm from a large SP reference layer 共3 nm, CoFe兲, which is varied in its unidirectional pinning strength from Heb = 200 Oe to 2000 Oe 共Fig. 6兲. Although not a continuous layer, the edges of the RS are far enough away from the FL to have an effect on the switching of the MTJ. A clear reduction of Hc is seen for weakly exchange coupled layers 关Fig. 6共a兲兴. This nicely confirms the experimentally observed results. The origin for the reduction can
be attributed to magnetic mirror charges induced by the stray field of the FL 关Fig. 6共b兲兴 in the RS 关Fig. 6共c兲兴. These induced mirror charges partly screen the dipole stray field of the FL resulting in a lower effective shape anisotropy, which goes along with reduced switching fields. The weaker the exchange pinning, the easier these mirror charges are generated, which clearly explains the observed behavior. CONCLUSION
Different magnetic stacks were used to study the impact of the RS on the switching performance of the FL in MTJs. Both experimentally and by micromagnetic simulations, we could show and explain that the switching field of the FL and its distribution are directly correlated to the stiffness of the RS. The stiffness can be quantified by an order parameter , which can be readily extracted from magnetization measurements along the hard axis of the RS. The higher the stiffness, the higher is the FL switching field Hc and the narrower is the switching field distribution . Micromagnetic simulations confirm that the dipole stray field of the FL is partly screened by magnetic charges induced in the RS, resulting in reduced shape anisotropy and thus a lower switching field. ACKNOWLEDGMENTS
The authors wish to thank the team members of the Joint Development Project at Altis Semiconductor and the personnel of the IBM-Infineon MRAM Development Alliance for their contributions and support of this work. A. R. Sitaram et al., VLSI 15 10–12, 共2003兲. T. Schrefl, J. Fidler, J. N. Chapman, and K. J. Kirk, J. Appl. Phys. 89, 7000 共2001兲. 3 U. K. Klostermann, R. Kinder, G. Bayreuther, M. Ruhrig, G. Rupp, and J. Wecker, J. Magn. Magn. Mater. 240, 304 共2002兲. 1 2
