J.‐W. Lee

Investigation of anisotropy by ferromagnetic resonance (FMR) in exchange-coupled bilayer films

We have used ferromagnetic resonance (FMR) to investigate magnetic anisotropy in bilayer exchange-coupled thin films. Experiments, using both our 9 and 33 GHz FMR spectrometers, were done on two systems: NiFe/FeMn and NiFe/α-Fe 2 O 3 . In some of the NiFe/α-Fe 2 O 3 specimens an additional response was detected at 33 GHz. We attribute this additional resonance to a region formed by interdiffusion of NiFe and α-Fe 2 O 3 at their common interface. Effective anisotropy field, g-value and the relative volume have been determined for each ferromagnetic region. Auger profile, TEM and CBED (convergent beam electron diffraction) cross-section examinations were performed for an interdiffused specimen, the results are consistent with the presence of a layer containing Fe and Fe 3 O 4 located between the NiFe and α-Fe 2 O 3 regions. A memory effect, due presumably to irreversible spin reorientation at the interface, was observed in some of the NiFe/α-Fe 2 O 3 specimens.

The effects of deposition conditions on microstructure and magnetic properties of TbFeCo

Transmission electron microscopy (TEM) has been employed to characterize micro/magnetic structural details of magneto‐optical (MO) recording TbFeCo thin films as a function of dc‐magnetron sputtering parameters using bright/dark field imaging, selected area diffraction, convergent beam electron diffraction, and Lorentz electron microscopy. It is found that the preparation conditions have a strong impact on both microstructure and magnetic properties of the films. The microstructures of the films deposited at low argon bleeding pressures ( 20 mTorr) give rise to microvoids surrounding ‘‘honeycomblike’’ networks and in‐plane domains with ‘‘ripple type’’ contrast. The microstructure and magnetic domain structures are related to the films’ magnetic properties as characterized by a MO loop tracer.

Exchange coupled NiFe-TbCo thin films for use in self-biased magnetoresistive heads

It is found that the exchange field produced at the NiFe-TbCo interface has a maximum value at a TbCo layer composition of 27.5%. When proper sputtering conditions are used, this deposited composition produces an exchange field as large as 500 Oe when the layer is coupled to a 370-AA Permalloy layer. Torque and VSM data indicate that the TbCo layers have both in-plane and perpendicular anisotropy components and that the in-plane M-H loops of films that produce the maximum exchange field are square. A linear decrease in exchange field is observed as the NiFe layer thickness increases. The exchange field is independent of the TbCo layer thickness in the range of 1000-4000 AA. Films with a 500-AA layer of TbCo exhibited a greatly reduced exchange effect. A preliminary 500-AA-thick 128- mu m*20- mu m permalloy head biased with 2000 AA of TbCo exhibited at Delta R/R of 1.45%. The hard-axis coercivity is negligibly small ( 1 MHz. >

Effect of sputtering conditions, annealing and the microstructure of Cr underlayer on the magnetic properties of CoNiCr/Cr thin films

The effects of RF power P, substrate heating, and substrate bias V/sub b/ on the structure and magnetic properties of CoNiCr thin films were studied. The films were deposited on glass and Cr/glass substrates by RF diode sputtering. For films deposited at higher P and at elevated substrate temperature T/sub s/, the grain size increased. The texture in the Cr underlayer increased at higher Ts; this was accompanied by an increase in the texture in the CoNiCr film. In-plane coercivity H/sub c/ also increased with P and T/sub s/; this is probably related to the increase in grain size as well as in CoNiCr texture. In the samples deposited with an applied V/sub b/, the grains were considerably larger than in those prepared without bias. The H/sub c/ value and the magnetic domain morphology also changed with V/sub b/; this is probably related to the increase in grain size. The H/sub c/ of CoNiCr thin films also increased following annealing in vacuum. >

OBSERVATION OF PROPOSED FLUX PINNING SITES IN NEUTRON-IRRADIATED YBA2CU3O7-X

Microstructure and magnetic hysteresis have been compared for two samples of the YBa2Cu3O7−δ (δ∼0) high‐temperature superconductor, one unirradiated, and one irradiated with fast neutrons (E≳0.1 MeV) to a fluence of 3×1018 n/cm2. Notable changes in the microstructure include strain‐induced contrast from regions 2–7 nm in size. An intrinsic critical current density (Jc) of 4.6×106 A/cm2 in zero field at 4 K has been determined from magnetic hysteresis measurements for the irradiated sample while 1.2×106 A/cm2 is noted for the unirradiated sample. We propose that the observed defect structure in the irradiated material is responsible for increased pinning and consequently higher Jc’s.

Spin-wave FMR in annealed NiFe/FeMn thin films

Ferromagnetic resonance (FMR) investigations have been made at 33 GHz on as‐deposited and on annealed bilayer NiFe/FeMn thin films. Supplemental investigations were made at 9 GHz. Following a 350 °C anneal, for 1 h or longer in duration, the NiFe FMR spectrum was found to be characterized by a sequence of spin‐wave resonance (SWR) modes. The behavior of the SWR modes is in accord with the presence of a thin ferromagnetic layer at the NiFe/FeMn interface with magnetization different from the bulk. Transmission electron microscopy (TEM) examinations indicate that annealing promotes formation of an interdiffused layer at the NiFe/FeMn interface and favors NiFe grain growth. Magnetic field limitations precluded FMR at 0° (perpendicular) field orientation. At various field orientations below the critical angle, the resonant fields, Hn, were found to vary with mode number n as n2. Also, for n>1, the FMR‐SWR linewidth ΔHn has a component varying as n2. From the 33‐GHz 16° orientation data, the exchange constant ...

Transmission electron microscopy of Co-Cr films for magnetic recording

The microstructure of Co‐Cr thin films for perpendicular magnetic recording has been examined by transmission electron microscopy in both conventional (bright field/dark field and selected area diffraction) and convergent beam electron diffraction modes. Results of as‐deposited samples indicate that the films consist of well‐oriented c axis structures. A range of microstructures (columnar morphology, transition layers, twins, etc.) have been observed. These and other results will be presented.

The effects of the nonabrupt depletion edge on deep‐trap profiles determined by deep‐level transient spectroscopy

The effects of the nonabrupt depletion edge (the Debye tail) on the trap concentrations obtained by deep‐level transient spectroscopy are discussed. In depth profiling of traps it is shown that a dramatic drop in the apparent deep‐trap concentration is obtained near the edge of the total depth probed. This is an artifact of the measurement and is a consequence of the Debye tail in the depletion edge. It is shown that experimentally one can avoid this problem by using sufficiently large reverse biases and forward filling pulses to determine the deep‐trap concentration in any particular region of the semiconductor.

Magnetic‐structural development in Co‐Cr films for perpendicular recording media

Transmission electron microscopy (TEM) was employed to examine the relation between microstructure and the corresponding magnetic domain structure in Co‐Cr films. Emphasis has been placed on the influence of crystallographic orientation on the magnetic domain morphology. Examination of thin samples (7.5–35 nm) reveals that randomly oriented small grains form in‐plane domain structures with cross‐tie walls, magnetic ripple structure, and Bloch lines. As the film thickness increases (40–50 nm), the grains become well oriented along their c axes and the corresponding domain structures are comprised of the stripe‐type characteristic of perpendicular domains. The magnetic domain structures will be discussed in terms of crystallographic orientation and microstructural defects.

Micro/magnetic structural development in Co‐Ni‐Cr and Co‐Ni‐Cr/Cr films for longitudinal recording media

A series of Co‐Ni‐Cr and Co‐Ni‐Cr/Cr films have been examined using transmission electron microscopy in conventional bright/dark‐field imaging, selected‐area diffraction, convergent beam electron diffraction, as well as in Lorentz electron microscopy imaging mode. The crystallographic orientation and magnetic domain structure have been characterized as a function of film thickness. The results indicate that amorphous Co‐Ni‐Cr films form prior to the formation of small randomly oriented equiaxed grains during deposition on amorphous substrates. As the film thickness increases, some of the small grains grow preferentially with their c axis parallel to film plane, but others with their c axis at an angle of 62° from the film plane. Longitudinal domain structures of the films were found to consist of Neel walls associated with cross‐tie walls and Bloch lines. Each domain contains a large amount of the magnetic ripple‐type structure.