Papers by Keyword: Perpendicular Magnetic Anisotropy

Abstract: We present here a method for measuring the temporal Bakehausen scaling exponent in a two-dimensional thin film. The scaling exponent with long-range interaction is observed to be consistent with theoretical and experimental results. In addition, we demonstrate the characteristic exponent with short-range interaction. Our method contributes to the understanding of DW dynamics and helps to optimize spintronic devices.

Abstract: A serial of [Pt/Co]₄/MgO/[Co/Pt]₂ multilayers were prepared by magnetron sputtering in this study and Antiferromagnetic (AF) interlayer coupling was observed with increasing MgO spacer thickness. Perpendicular magnetic anisotropy of soft layer in [Pt/Co]₄/MgO/[Co/Pt]₂ multilayers were measured by Extraordinary Hall Effect (EHE). The direction of the magnetic field was parallel to the direction of the current. The results revealed that perpendicular magnetic anisotropy constant K_u oscillates with the thickness of MgO layer which is between 1.3 to 3.3 nm. The period of this oscillation is about 0.43 nm. This is very close to the monolayer (100) MgO. With increasing MgO spacer, the periodic variation of interface between [Pt/Co]_n and MgO layer may be one of the reasons for this oscillation behavior of K_u.

Abstract: Fe₅₀(Pt_1–xPd_x)₅₀ (at. %, x = 0, 0.25, 0.5, 0.75, 1) alloy thin films are prepared on MgO(001) substrates by using two methods, high-temperature deposition at 600 °C (one-step method) and low-temperature deposition at 200 °C followed by annealing at 600 °C (two-step method). The influence of formation method on the film structure is investigated. L1₀ crystals epitaxially grow on the substrates when films are deposited at 600 °C. Disordered crystals transform into L1₀ structure when films are annealed at 600 °C. The films with x > 0.5 consist of L1₀(001) crystals with the c-axis normal to the substrate surface, whereas the films with x < 0.25 involve small volumes of L1₀(100) crystals with the c-axis lying in the film plane. Similar final crystallographic orientation is realized for the Fe (Pt,Pd) films prepared by employing the two different methods. The films prepared by one-step method possess island-like surfaces involving side facets, while those prepared by two-step method have very flat surfaces with the arithmetical mean roughness lower than 0.2 nm. The two-step method is effective for preparation of Fe (Pt,Pd) films with flat surfaces.

Abstract: We investigate the perpendicular magnetic anisotropy dependence on the AlO capping layer in Pt/Co/AlO films. AlO was deposited on Pt/Co films by RF magnetron sputtering and atomic layer deposition (ALD) with varying thickness. It is found that the prolonged deposition of thick AlO layers by RF magnetron sputtering causes significant damage to the Pt/Co underneath while AlO layers formed by ALD can be of arbitrary thickness with no damage to the magnetic properties of the films. The decline of the magnetic properties can be attributed to the method of AlO deposition for each process. In the RF magnetron sputtering, AlO atoms with high kinetic energy are ejected from a sputter target resulting in the degradation of Pt/Co films, while the process of deposition of AlO by ALD is governed by a series of chemically reactive condensations allowing for arbitrary deposition thickness of AlO.

Abstract: Magnetic Compton profiles (MCPs) of Co/Au multilayers have been measured and analyzed by DV-Xα cluster model calculations from a viewpoint of perpendicular magnetic anisotropy (PMA). The PMA and the MCPs are discussed for the presently obtained results for Co/Au, along with the previously obtained results for Co/Pd and Co/Pt. A Co/Au multilayer shows a weak PMA which is caused by |m|=1 states of Co 3d electrons at a smooth Co/Au interface. The increase of the interface-to-volume ratio plays the main role in determining the perpendicular anisotropy in Co/Au multilayers. The strain of a Co layer can have a secondary role in determining the magnetic anisotropy in Co/Au multilayers, although the strain dominates PMA in the case of Co/Pd and Co/Pt multilayers.

Abstract: We have measured magnetic Compton scattering (MCS) for an Fe/MgO multilayer film at several magnetic field applying perpendicular to film plane. A spin specific magnetic hysteresis (SSMH) loop is obtained by the MCS for the Fe/MgO multilayer film. A knickpoint is observed in the SSMH loop around the magnetic field of 0.5 T. Orbital magnetization is enhanced within the magnetic field from-0.5 T to 0.5 T. A decomposition analysis for magnetic Compton profiles shows the suppressed |m|=0 states and enhanced |m|=1 and 2 states within the magnetic field from-0.5 T to 0.5 T. Here m denotes magnetic quantum number. The knickpoint corresponds to a perpendicular magnetic anisotropy, which comes from the enhanced |m|=1 and 2 state and orbital magnetization in the Fe/MgO multilayer.

Abstract: The high magnetic anisotropy may enhance the thermal stability of the magnetic tunnel junction of CoFeB/MgO/CoFeB, which is a promising candidate for the high performance magnetic random access memory (MRAM). However, the interface stabilized perpendicular magnetic anisotropy (PMA) is not completely understood at this moment. In this study, we fabricated separated top and bottom Ta-CoFeB-MgO thin films and found both structures showed strong PMA after a 300 °C post annealing. However, the top structure has thicker magnetic dead layer and much higher coercivity relative to the bottom structures. The asymmetry between the top and bottom Ta-CoFeB-MgO structures may be attributed to the crystalline structure and interface diffusion. X-ray photoelectron spectroscopy depth profile reveals that the boron diffuses mainly toward to the Ta side at CoFeB-Ta interface in both top and bottom structures. It is mainly Ta that appears at the CoFeB-MgO interface of top structure while it is TaO_x that appears at the CoFeB-MgO interface of bottom structure. A detailed discussion about the chemical states at the interface in Ta-CoFeB-MgO thin films will be given.

Abstract: A novel perpendicular magnetic material was obtained in the annealed Co/native oxide/Pd multilayer films. Upon thermal annealing in a vacuum, a sharp transition of magnetic anisotropy from in-plane to out-of-plane is observed for the Co/native oxide/Pd multilayers with 20 Å Co layers. These annealed Co/native oxide/Pd multilayers possess extremely large effect perpendicular anisotropy field (H_eff) and perpendicular anisotropy energy (K_u), with the maximum H_eff of 17 kOe and K_u of 5.1×10⁶ erg/cm³ obtained in the annealed Co(20 Å)/native oxide/Pd(10 Å) multilayers. Moreover, the large perpendicular magnetic anisotropy sustains with annealing temperature varying from 200°C to 400°C, indicative of high thermal stability. The present results open the way to a third alternative for the realization of thermally stable perpendicular magnetic electrodes of tunnel junctions.

Abstract: The effects of strain and interface roughness at the Co/Pd interface are investigated from the viewpoint of perpendicular magnetic anisotropy (PMA) using the DV-Xα cluster model calculation method. It is found that spin projected occupation number ratio of magnetic quantum number |m| = 2 for the Co 3d electrons enhances by expanding the lattice within a close-packed plane of fcc stacking and, hence, enhances the PMA. Rough interface decreases the spin projected occupation number ratio of |m| = 2 and, hence, decreases the PMA. These results explain the PMA properties of Co/Pd multilayers fabricated using molecular beam epitaxy (MBE) technique and RF sputtering techniques.

Abstract: We compare two Co/Pd multilayers with correspondingly smooth and rough interfaces. The first is a Co (1.5 nm)/Pd (2.6 nm) multilayer with a smooth interface deposited by the MBE technique, and the second is a Co (1.6 nm)/Pd (4.0 nm) multilayer with a rough interface deposited by the sputter technique. Both multilayers have almost the same perpendicular magnetic anisotropy energy, 1.15 Merg/cc for the Co (1.5 nm)/Pd (2.6 nm) multilayer and 1.20 Merg/cc for the Co (1.6 nm)/Pd (4.0 nm) multilayer, respectively. The symmetry of the wave function, which is measured using the magnetic Compton profile, is almost the same for both multilayers. This suggests that the smooth interface controls the wave function and enhances the PMA energy even if the Co/Pd multilayer has a thinner Pd layer.