Chih-Wei Cheng

Effect of cap layer thickness on the perpendicular magnetic anisotropy in top MgO/CoFeB/Ta structures

The perpendicular magnetic anisotropy of a series of top MgO/CoFeB/Ta layers is studied. Similar to the bottom Ta/CoFeB/MgO structure, the critical thickness of CoFeB is limited in a range of 1.1–1.7 nm. However, the cap layer shows much sensitive effect. Not only the type of material is crucial, but the thickness of the cap layer also affects the magnetic anisotropy. The perpendicular anisotropy of a 1.2 nm-thick CoFeB can only exist with the capping Ta thickness less than 2 nm. The magnetic characterizations, including the magnetic remanence and coercivity, also show strong dependence on the Ta thickness. The diffusion of Ta into CoFeB layer is considered to play an important role, which could explain changes in perpendicular anisotropy and related magnetic responses. In addition, the asymmetric role of Ta layer in the top structure and bottom structures is also discussed.

Perpendicular magnetic anisotropy induced by a cap layer in ultrathin MgO/CoFeB/Nb

Perpendicular magnetic anisotropy has been observed in MgO/CoFeB/Ta but not in MgO/CoFeB/Ru, indicating that the metal cap layer is crucial. In this study, we grew MgO/CoFeB (1.2 nm)/Nb (1–2.5 nm) by sputtering and found that the magnetic anisotropy depends upon the Na thickness. The easy axis is in-plane when x = 1 nm but changes to perpendicular when x ≥ 1.2, and a transition from in-plane to perpendicular is observed at x = 1.1 nm. Perpendicular magnetic anisotropy gradually decays as the x increases, showing that it only exists in a small window of the cap Nb layer within ∼2 nm.

Perpendicular Magnetic Anisotropy in MgO/CoFeB/Nb and a Comparison of the Cap Layer Effect

High perpendicular magnetic anisotropy (PMA) has been recently revealed in CoFeB/MgO/CoFeB tunnel junctions if the thickness of CoFeB is <;1.5 nm. However, PMA has been observed in MgO/CoFeB/Ta but not in MgO/CoFeB/Ru, indicating a metallic cap layer effect. In this paper, we extend the study to MgO/CoFeB(0.8-1.8 nm)/Nb(1.3 nm) by sputtering and find that the magnetic behavior of MgO/CoFeB/Nb resembles the magnetic behavior of MgO/CoFeB/Ta. The interface anisotropy constant of both MgO/CoFeB/Ta and MgO/CoFeB/Nb is ~2.2 erg/cm2 while MgO/CoFeB/Ru has a much weaker interface anisotropy constant ~0.68 erg/cm2. In addition, the magnetic dead layer (MDL) of MgO/CoFeB/cap (cap = Ta and Nb) is ~0.1 nm while MgO/CoFeB/Ru has a much larger MDL ~0.5 nm. The metallic layer effect on the PMA in CoFeB/MgO-based perpendicular structure is attributed to the interdiffusion during the post-annealing process. The possible correlation between PMA and MDL provides a possible mechanism for the metal layer effect in the CoFeB/MgO-based structures.

“External forces” are indispensable for the occurrence of exchange bias?

We demonstrate here a double-shifted exchange bias which spontaneously exists in the as-deposited [Pt/Co]n/IrMn multilayers without resorting to any “external forces,” such as deposition with field, field annealing, (zero) field cooling with specific procedures. The observed results indicate that the exchange bias might occur spontaneously when a ferromagnetic layer is in contact with an antiferromagnetic layer; herein it is tentatively attributed to interactions among perpendicular anisotropy of [Pt/Co]n multilayers, its intrinsic demagnetization field, and magnetic anisotropy of the IrMn layer. The thickness and the magnitude of magnetic anisotropy of the ferromagnetic multilayer are crucial for the adjustment of asymmetric double-shifted exchange bias which can also be tuned to exist in the in-plane or out-of-plane directions.

Synthetic Antiferromagnetic MgO/CoFeB/Ta(x)/CoFeB/MgO Structures With Perpendicular Magnetic Anisotropy

We have studied the exchange coupling between two ultrathin CoFeB (1.2 nm) layers separated by a Ta spacer with a thickness varying from 0.5 nm to 3.5 nm. These structures show well-defined perpendicular anisotropy due to the MgO stabilization effect after post annealing. The magnetic coupling varies from parallel to antiparallel, depending upon the Ta thickness, and an oscillatory period 1.1 nm with maximum coupling strength (J) ~ 0.02 erg/cm2 is revealed. A simulation, based on coherent rotation, is carried out for a comparison with the magnetic hysteresis curves. The variation of both anisotropy constant and exchange coupling strength are included to analyze the experimental results and the main characteristic of the magnetic reversal shows a spin-flip transition, which corresponded to the antiparallel coupled condition with comparable J and K. The change of the magnetic anisotropy due to thicker cap layer and the influence on the magnetic reversal will be also discussed.

Interlayer Exchange Coupling and Perpendicular Magnetic Anisotropy in Co

We fabricated a series of Co40Fe40B20 (1.2 nm)/MgO(x nm)/Co20Fe60B20 (1.2 nm), x = 0.9-1.5, by sputtering and found the presence of the interlayer-exchange-coupling (IEC), as a function of the thickness of MgO spacer, and perpendicular magnetic anisotropy (PMA) after post annealing at 225°C. The coupling strength and sign (corresponded to parallel or antiparallel coupling), which were characterized by the minor loop shift, varied depending upon MgO layer thickness. In addition, the anisotropy energy constant (Ku) and the exchange coupling energy (J) show inverse relation as the annealing temperature varies in the range of 225°C-370°. The anisotropy constant increases as temperature increases while the exchange coupling decreases from ferromagnetic to antiferromagnetic as temperature increases. This effect seems to be consistent with the recent report of a theoretical calculation based on the orange peel model. A comparison between the experimental results and the theoretical model will be given.

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The annealing effects of perpendicular MgO/CoFeB/Ta structure are investigated with ferromagnetic resonance technique. The data of angular dependent resonance field, linewidth, and intensity are obtained for both as-grown and annealed samples. An increment of magnetic out-of-plane anisotropy field from 1800 to 3300 Oe with annealing is observed, which should be related to the improvement of crystalline at the interface. Ferromagnetic resonance linewidth significantly broadens after annealing, and this broadening is attributed to the enhancement of inhomogeneity of the internal field.The annealing effects of perpendicular MgO/CoFeB/Ta structure are investigated with ferromagnetic resonance technique. The data of angular dependent resonance field, linewidth, and intensity are obtained for both as-grown and annealed samples. An increment of magnetic out-of-plane anisotropy field from 1800 to 3300 Oe with annealing is observed, which should be related to the improvement of crystalline at the interface. Ferromagnetic resonance linewidth significantly broadens after annealing, and this broadening is attributed to the enhancement of inhomogeneity of the internal field.

Fe

High perpendicular magnetic anisotropy (PMA) has been observed in MgO/CoFeB/Ta thin films if the thickness of CoFeB is in the range between 1.1 and 1.5 nm. However, both the coercivity and remanence vanish when the thickness of CoFeB is less than 1.1 nm, indicating a characteristic of superparamagnetic state. The magnetization versus external field of these thinner films shows a fit with a Langevin model. In addition, a temperature dependent study identifies a blocking temperature of 160 K by field cool and zero field cool measurements. The particle size of ~ 23 nm in diameter is estimated for a pancake shape cluster and the small particles are probably formed at the CoFeB/MgO interface at the beginning of the growth. Moreover, even the thicker films, which show ferromagnetic response at room temperature, also exhibit the temperature-dependent characteristic of superparamagnetic state due to the pinning at the edge of the small particles.

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Ultrathin CoFeB/MgO/CoFeB system with perpendicular magnetic anisotropy is a promising candidate for the high density magnetic random access memory. However, a dipolar interaction between the CoFeB layers may introduce a minor loop shift (Hs) and causes uncertainty during the operation. In this report, we systematically studied the dipolar effect in these structures and found that the coupling may be either ferromagnetic or antiferromagnetic (15 Oe > Hs > −15 Oe) depending upon the CoFeB thickness (0.9–1.4 nm). A modified Fabry-Perot model, which accounts the Bloch wave interference, may explain the present observations of the dipolar effect in the perpendicular junctions of CoFeB/MgO/CoFeB.

B

A collapse of tunnel magneto resistance (TMR) in the perpendicular magnetic configuration of CoFeB/MgO/CoFeB tunnel junction at high annealing temperatures has been reported recently [Ikeda et al., Nature Mater. 9, 721 (2010)]. This observation indicates that not only the temperature-dependent magnetic characterization is important in a pseudo-spin valve type devices but also implies an asymmetrical dependence on the magnetic behavior between the top and bottom CoFeB layers. In this report, we have measured a series of MgO/CoFeB/Ta with different thicknesses of CoFeB (1.0−1.7 nm) and Ta cap layer (1–5 nm) and found a intrinsic dependence of magnetic coercivity of Hc = Hco[1−(T/TB)1/2], where Hco is Hc at 0 K and TB is the blocking temperature, for all films. A systematic study shows that Hco varies in the range of 2500 Oe−250 Oe with a rough inverse linear dependence on CoFeB layer thickness. The TB for all films except the thinnest one (1.0 nm) is in a smaller range of 280–300 K, but drops to 150 K for t...