The Effect of Interface Texture on Exchange Biasing in Ni80Fe20/Ir20Mn80System
© to the authors 2008
Received: 23 July 2008
Accepted: 11 November 2008
Published: 25 November 2008
Exchange-biasing phenomenon can induce an evident unidirectional hysteresis loop shift by spin coupling effect in the ferromagnetic (FM)/antiferromagnetic (AFM) interface which can be applied in magnetoresistance random access memory (MRAM) and recording-head applications. However, magnetic properties are the most important to AFM texturing. In this work, top-configuration exchange-biasing NiFe/IrMn(x Å) systems have been investigated with three different conditions. From the high-resolution cross-sectional transmission electron microscopy (HR X-TEM) and X-ray diffraction results, we conclude that the IrMn (111) texture plays an important role in exchange-biasing field (H ex) and interfacial exchange energy (J k).H exandJ ktend to saturate when the IrMn thickness increases. Moreover, the coercivity (H c) dependence on IrMn thickness is explained based on the coupling or decoupling effect between the spins of the NiFe and IrMn layers near the NiFe/IrMn interface. In this work, the optimal values forH exandJ kare 115 Oe and 0.062 erg/cm2, respectively.
The exchange-biasing phenomenon using the IrMn basing layer can be applied in magnetoresistance random access memory (MRAM) and recording-head applications extensively because Ir20Mn80 exhibits great characteristics: high interfacial exchange energy (J k) (or exchange-biasing field (H ex)), low coercivity (H c), high blocking temperature (T B), and good thermal stability in device performance [1–5]. Moreover, the NiFe/IrMn also can be applied in the high-frequency ferromagnetic resonance (FMR) . In a ferromagnetic (FM)/antiferromagnetic (AFM) system, the texturing in the AFM layer can have an important impact on the magnetic properties of the system. In the past, a NiO/NiFe system with varied AFM NiO thicknesses was studied . In this paper, we will show how the magnetic properties, such as H ex H c, and J k, of the IrMn/NiFe top-configuration system may vary as a function of the IrMn layer thickness (x). It is found that these magnetic properties are closely related to the degree of the (111) texture in the IrMn layer [8–10]. H ex and J k tend to saturate as x increases beyond 90 Å. H c is inversely proportional to x, which is caused by the spin coupling or decoupling effect near the NiFe/IrMn interface.
The top-configuration NiFe/IrMn system was made by DC magnetron sputtering onto a glass substrate. The deposition sequences were: glass/Ta(30 Å)/NiFe(50 Å)/IrMn(x Å)/Ta(100 Å), where x = 15, 30, 60, 90, 110, and 150 Å. For this system, we have applied three different conditions during and/or after deposition: (a) the substrate temperature (T s) was kept at room temperature (RT) only; (b) T s was at RT with an in-plane external field (h) = 500 Oe during deposition; and (c) T s = RT, with h during deposition and post-deposition annealing in the field at T A = 250 °C for 1 h, and then field-cooling to RT. The seed Ta layer was used in order to induce a stronger (111) texture in the NiFe or IrMn layer . The cap Ta layer was used to protect the IrMn layer from oxidation. The target compositions of the IrMn and NiFe alloy are 20 at.% Ir, 80 at.% Mn and 80 at.% Ni, 20 at.% Fe, respectively. The typical base chamber pressure was better than 1 × 10−7 Torr, and the Ar working chamber pressure was 5 × 10−3 Torr.
The degree of the (111) texture of the Ir20Mn80layer was characterized by the X-ray diffraction method using a CuKα1line. In order to observe the growth texture and the interfacial morphology directly, we performed high-resolution cross-sectional transmission electron microscopy (HR X-TEM). The exchange-biased magnetic hysteresis loop was measured by a LakeShore Model 7300 vibrating sample magnetometer (VSM).
Results and Discussion
In conclusion, under the various conditions (a)–(c) for the top-configuration NiFe/IrMn systems, the magnetic properties, such as H ex J k, and H c, have been investigated. These magnetic properties are closely related to the growth IrMn (111) texturing. From HR X-TEM and X-ray diffraction results, we conclude that the strongest IrMn (111) texture appears in condition (c). Therefore, condition (c) should induce the highest H ex and J k. Furthermore, the H c value first increases and then decreases as x increases from 15 Å to 150 Å. This is due to the spin coupling and decoupling drag effects at the NiFe/IrMn interfaces. The optimal H ex and J k values obtained from this study are 115 Oe and 0.062 erg/cm2, respectively. This H ex value of NiFe/IrMn is larger or equal to the optimal H ex in the NiO/NiFe systems [13, 14].
This work was supported by the National Science Council and I-Shou University, under Grant Nos. (NSC97-2112-M214-001-MY3), (ISU97-S-03), and (ISU97-02-20).
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