Yingguo Peng

Fe3O4 thin films sputter deposited from iron oxide targets

Fe3O4 thin films have been directly sputter deposited from a target consisting of a mixture of Fe3O4 and Fe2O3 onto Si and glass substrates. The magnetic properties and microstructures of the films have been characterized and correlated. The columnar growth of the Fe3O4 grains was found to be initialized from the substrate surface without any critical thickness. Substrate bias was found to be a very effective means of improving the crystal quality and magnetic properties of the thin films. The crystallographic defects revealed by high resolution transmission electron microscopy seem to be a characteristic of the films prepared by this method.

L10 FePt–MgO perpendicular thin film deposited by alternating sputtering at elevated temperature

FePt–MgO two phase thin films have been obtained by an alternating sputtering method at elevated temperatures. It is found that the L10 FePt grains grow in a columnar manner through the thickness of the film and possess a (001) perpendicular texture promoted by the (200)-textured crystalline MgO underlayer. The amorphous MgO in the two phase mixture forms uniform separation walls between FePt grains. The microstructure is similar to that of Co-alloy/oxide granular media.

Bulk anisotropic composite rare earth magnets

Bulk anisotropic composite Nd13.5Fe80Ga0.5B6∕α-Fe and Nd14Fe79.5Ga0.5B6∕Fe–Co magnets with (BH)max=45–50MGOe have been synthesized by blending a Nd–Fe–Ga–B powder with an α-Fe or Fe–Co powder followed by hot compaction at 600–700 °C and hot deformation (die upsetting) at 850–950 °C with a height reduction of 71%. The composite Nd13.5Fe80Ga0.5B6∕α-Fe and Nd14Fe79.5Ga0.5B6∕Fe–Co magnets show microstructures consisting of a very large soft phase up to ∼50μm, which is more than 1000 times larger than the upper size limit of the soft phase expected from the existing models of interface exchange coupling.

Annealing effects on structural and transport properties of rf-sputtered CoFeB∕MgO∕CoFeB magnetic tunnel junctions

Annealing effects on the structural and transport properties of sputtered CoFeB∕MgO∕CoFeB magnetic tunnel junctions deposited on SiO2∕Si were investigated. At the as-deposited state, the CoFeB was amorphous at the CoFeB∕MgO interface. High-resolution transmission electron microscope image clearly shows that after annealing at 270°C for 1h, crystallization of amorphous CoFeB (three to four monolayers) with lattice matching to MgO (100) occurred locally at the interface between MgO and CoFeB, producing a magnetoresistance (MR) around 35%–40%. After annealing at 360°C for 40min, the MR increased to 102%. The increase in the MR with annealing is attributed to the complete formation of (100) crystalline structure of CoFeB well lattice matched with the (100)-oriented MgO barrier. The bias voltage dependence of the MR shows a consistent correlation with each CoFeB∕MgO interface.

Stress dependence of soft, high moment and nanocrystalline FeCoB films

Soft, high moment materials are crucial for the magnetic data recording industry in applications such as write poles and soft underlayers. Ever increasing areal densities have pushed the requirements for materials exhibiting saturation magnetizations above 2 T. In this work we investigate nanocrystalline FeCoB which exhibits a saturation magnetization of ∼2.2 T. FeCoB exhibits a magnetostriction of >50×10−6 which suggests that the magnetoelastic anisotropy is important. Nanocrystalline FeCoB films are rf diode sputtered as a function of Ar pressure. As the pressure is increased, film stress increases from compressive to tensile with a critical pressure of ∼12 mTorr. TEM and XRD show that pressure has no significant effect on microstructure, however, at pressures above a critical pressure, the FeCoB films change from magnetically isotropic to magnetically uniaxial with low coercivity. This is due entirely to the magnetoelastic anisotropy which must be considered as the industry begins to utilize magnetic a...

Magnetoresistance of polycrystalline Fe3O4 films prepared by reactive sputtering at room temperature

The magnetic, structural, and transport properties of single-layer magnetite (Fe3O4) films prepared by reactive sputtering were investigated. Magnetoresistance (MR) was measured at various thicknesses and temperatures. The increase in MR with thickness is related to grain crystallinity and size, as confirmed by transmission electron microscopy. MR arises from intergranular tunneling, which is supported by the temperature dependence of resistivity (logρ∼T−1∕2). Field-dependent MR correlates with the M curve. Magnetoresistance versus magnetization curves clearly show that the MR effects come from the surface spin arrangement near the grain boundaries. The dependence of MR on the magnetic field observed in polycrystalline Fe3O4 films can be attributed to a surface magnetization near the grain boundary, which will be discussed.

Engineering the microstructure of thin films for perpendicular recording

In this paper we discuss various microstructural features that control the recording properties of thin films used in perpendicular recording. These microstructure features include crystallographic texture, grain size, grain size distribution, grain to grain magnetic isolation, grain to grain composition variation and in the case of L1/sub 0/ materials the grain to grain variation in the degree of atomic order. We discuss recording media comprised of continuous thin films as well as granular thin films. We discuss media composed of either hcp Co alloys or FePt L1/sub 0/ alloys. Methods of controlling the microstructural parameters are discussed as is their effects on recording properties. Examples from our recent research will be used to illustrate these microstructural aspects of perpendicular recording media.

Fabrication, Microstructure, Magnetic, and Recording Properties of Percolated Perpendicular Media

In this paper, a new type of perpendicular magnetic media which we have termed percolated perpendicular media is discussed. We present the method we used to fabricate the media as well as an energetic rationale for the driving force to produce the desired microstructure. The microstructures of samples with various amounts of oxide material are presented and the optimum one for our investigation is presented. We also present some preliminary drag test data that can be used to evaluate the recording properties of this media

Toward an understanding of grain-to-grain anisotropy field variation in thin film media

Grain-to-grain anisotropy field variation has become one of the main causes of medium noise, especially in perpendicular thin film media. In this paper, we present an electron microscopy investigation and theoretical analysis on the grain-to-grain anisotropy field variation in various types of thin film recording media. In alloyed film media, the intrinsic grain-to-grain composition variation would present a lower limit on grain size, thereby limiting area recording density. It is also argued that partial ordering in L1/sub 0/ materials such as FePt would yield large anisotropy field variation, especially for low values of order parameter.

Characterization of interfacial reactions in magnetite tunnel junctions with transmission electron microscopy

To make a uniform AlOx barrier layer in tunnel junctions, a thin layer of Al is often sputtered first and then oxidized. In this study, we sputtered a thick layer of Al onto Fe3O4 and then employed high resolution transmission electron microscopy and x-ray energy dispersive spectroscopy to investigate the interfacial microstructures. Two new layers have been found and investigated at the Al/Fe3O4 interface. The interfacial reaction has been determined to be: Fe3O4+Al⇒Fe+(a)AlOx, where a denotes the amorphous state. The consequence of the interfacial reaction to transport properties is also discussed.