G. Chern

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.

Thermal growth and magnetic characterization of α-Fe2O3 nanowires

Single-crystalline hexagonal ?-Fe2O3 nanowires about 15?nm in diameter were synthesized on a 50?nm iron thin film by thermal oxidation within 30?min at 250??C in air. Scanning electron microscopy images show that the diameters of ?-Fe2O3 nanowires were about 8?25?nm, and the lengths were up to a few micrometres. Transmission electron microscopy analysis revealed that the nanowires had a hexagonal crystal structure with a growth direction of [1 1 0]. Magnetic force microscopy analysis of a single ?-Fe2O3 nanowire revealed that the direction of the magnetic domains was along the wire axis at room temperature. In addition, the results of magnetoresistance revealed that the anisotropic magnetoresistance of ?-Fe2O3 nanowires reached 0.5% at 300?K.

Characterization of the structural and magnetic ordering of Fe3O4/NiO superlattices grown by oxygen-plasma-assisted molecular-beam epitaxy

Oriented single‐crystalline thin films of NiO and Fe3O4 and Fe3O4/NiO superlattices have been grown on cleaved and polished substrates of MgO(001) using oxygen‐plasma‐assisted molecular‐beam epitaxy (MBE). We report the growth mode and structural characterization of these films using in situ RHEED and ex situ scanning electron microscopy and x‐ray diffraction, and their magnetic characterization using SQUID magnetometry. Also reported are preliminary results of magnetotransport measurements. MgO has a very small lattice mismatch to the cubic rocksalt structure of NiO and to the half‐unit‐cell dimension of the spinel structure of Fe3O4. Pseudomorphic growth of superlattices consisting of alternating layers of NiO and Fe3O4 with repeat wavelength down to 17 A and of single thick layers of either of these materials are readily obtained. The grown films exhibit cubic single‐crystalline symmetry in registry with the substrate, with sharp interfaces and strongly layer‐thickness‐dependent strain. RHEED pattern e...

Observation of parallel-antiparallel magnetic coupling in ultrathin CoFeB-MgO based structures with perpendicular magnetic anisotropy

A series of MgO/CoFeB/Ta(x)/CoFeB/MgO multilayered structures is fabricated by sputtering. Magnetic parallel-antiparallel oscillatory behavior is observed as a function of Ta thickness, while perpendicular magnetic anisotropy (PMA) also exists due to the MgO stabilization. The oscillatory period is ∼1.3 nm with a maximum interlayer exchange coupling (IEC) of ∼0.02 erg/cm2. The Ta spacer can be replaced by a layer of other metals to form a general perpendicular synthetic antiferromagnetic structure. The tuning of IEC and PMA by insertion of Ru is discussed.

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.

Electronic states and structural characterization in single-crystal Fe–Ni–O alloy thin films grown by molecular beam epitaxy

High quality epitaxial Fe3O4, NiO and a series of Fe1−xNixOy (0<x<1) thin films have been fabricated by molecular beam epitaxy. In situ reflection high energy electron diffraction (RHEED), ex situ X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS) studies have been carried out. It was observed that the crystal structure of all the Fe1−xNixOy films studied resemble that of the inverse spinel Fe3O4. The lattice spacing along the perpendicular direction as a function of x shows a minimum at x=0.5 instead of a linear variation indicating that the structures are different from a bulk ferrite Fe2NiO4-like phase. The XAS results are consistent with XRD results and further identify the cation distribution in Fe1−xNixOy system as x varies. The mechanism of the formation of the metastable phase and its implication on the magnetic properties of these Fe–Ni–O films are briefly discussed.

Magnetization changes with modulation period in Fe3O4/NiO superlattices

We have magnetically characterized single crystal Fe3O4/NiO superlattices grown on MgO(001), where the layer thickness has been varied over a wide range. Hysteresis for modulation wavelength Λmod = 258 A is similar to bulk Fe3O4, while loops for Λmod < 80 A are nearly linear with applied field with minimal hysteresis and remanence. The magnetization slopes of the latter are roughly proportional to Λmod, and increase from 5 to 300 K. Measurements above 300 K suggest that both exchange coupling across NiO/Fe3O4 interfaces and strain-induced variations in anisotropy, rather than paramagnetism, lead to this linear behavior.

Incommensurate growth and interface stability in MgOSrTiO3(001)

Abstract High quality MgO film has been grown on SrTiO 3 (001) by an oxygen-plasma-assisted molecular beam epitaxy system. Although the bulk mismatch of the two cubic structures is 7.8|X%, the surface lattice mismatch is as large as 23.7|X%. Reflection high energy electron diffraction (RHEED) provides direct evidence indicating that single crystalline MgO forms with nearly relaxed lattice constant from the first monolayer. The orientation of the MgO film is, however, rotated 45° relative to the SrTiO 3 (001) surface structure. This incommensurate growth behavior, different from the inverted system SrTiO 3 MgO (001) , is believed due to the compression-expansion reversing effect. There is also indirect evidence showing that the mismatch interface energy can be removed through the relaxation along the perpendicular direction in a large misfit rocksalt/perovskite system.

Fabrication and Characterization of Microstructured Magnetic Tunnel Junction Rings

Microstructured magnetic tunnel junction rings have been fabricated by a top-down technique combining electron beam lithography and ion milling process. Four-terminal magnetoresistance measurements and magnetic force microscopy were used to successfully explore a four-transition process within the free layer throughout the magnetization reversal. Various magnetization configurations were identified to be the onion state, vortex-pair state, vortex state, vortex-core state, and reverse onion state. In addition, the various durations of each magnetic state observed in the magnetoresistance curve can be utilized for the study of a coupling effect between the pinned layer and the free layer