In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

Gan : processing, defects, and devices

Ordered magnetic nanostructures: fabrication and properties

▪ Abstract Continuing increases in the areal density of hard disk drives will be limited by thermal instability of the thin film medium. Patterned media, in which data are stored in an array of single-domain magnetic particles, have been suggested as a means to overcome this limitation and to enable recording densities of up to 150 Gbit cm−2 (1 Tbit inch−2) to be achieved. However, the implementation of patterned media requires fabrication of sub-50-nm features over large areas and the design of recording systems that differ substantially from those used in conventional hard drives. This review describes patterned media, including the fabrication of arrays of small magnetic particles and their magnetic properties, such as domain structure, reversal mechanisms, thermal stability, and interactions. The practical implementation of patterned media recording schemes is assessed.

Patterned Magnetic Recording Media

We study the origin of the gap and the role of chemical composition in the half-ferromagnetic Heusler alloys using the full-potential screened Korringa-Kohn-Rostoker method. In the paramagnetic phase the ${C1}_{b}$ compounds, like NiMnSb, present a gap. Systems with 18 valence electrons, ${Z}_{t},$ per unit cell, like CoTiSb, are semiconductors, but when ${Z}_{t}g18,$ antibonding states are also populated, thus the paramagnetic phase becomes unstable and the half-ferromagnetic one is stabilized. The minority occupied bands accommodate a total of nine electrons and the total magnetic moment per unit cell in ${\ensuremath{\mu}}_{B}$ is just the difference between ${Z}_{t}$ and $2\ifmmode\times\else\texttimes\fi{}9.$ While the substitution of the transition metal atoms may preserve the half-ferromagnetic character, substituting the sp atom results in a practically rigid shift of the bands and the loss of half-metallicity. Finally we show that expanding or contracting the lattice parameter by 2% preserves the minority-spin gap.

/pdf/origin-and-properties-of-the-gap-in-the-half-ferromagnetic-1d93np84qf.pdf

Origin and properties of the gap in the half-ferromagnetic Heusler alloys

We report magnetic and magnetoresistance (MR) measurements in the current-in-plane (CIP) and current-perpendicular-to-the-plane (CPP) geometries of multilayers and spin-valve structures based on the predicted half-metallic (100% spin-polarized) ferromagnetic Heusler alloy NiMnSb. Multilayers of the form [NiMnSb/Cu]×10, as well as a trilayer spin valve of the form [NiMnSb/Cu/Y] (Y=Ni80Fe20 and NiMnSb/FeMn) have been grown using a low-rate, low-pressure sputtering deposition technique. The multilayers do not show evidence of antiferromagnetic coupling between the NiMnSb layers, and the measured CPP magnetoresistance is 4.5% at 4.2 K. NiMnSb/Cu/NiFe spin valves and NiMnSb/Cu/NiMnSb/FeMn quasispin-valves adopt more closely antiparallel alignments of adjacent magnetic layer magnetizations, and CPP MR ratios as high as 7.2% are measured at 4.2 K. Although these CPP MRs are larger than the CIP MRs for similar samples, the largest CPP MR observed is still smaller than expected for a 100% spin-polarized material.

Magnetoresistance of NiMnSb-based multilayers and spin valves

Wet and dry etching of thin metallic multilayer structures is necessary for the development of sensitive magnetic field sensors and memory devices based on spin–valve giant magnetoresistance elements. While it is well established that Cu, Co, and Fe are soluble in HNO3 and H3PO4 at room temperature, little effort has been made to investigate selective wet and dry etch chemistries. For example, we find Ag is not etched in H2SO4, HCl, or H3PO4 under conditions where etch rates for the other metals are in the range of 2000–60 000 A/min. Electron cyclotron resonance (ECR) SF6/Ar plasmas provide etch selectivities of ⩾5:1 for Ag over Cu, Co, and Fe, while lower selectivities are obtained with CH4/H2/Ar. Cl2-based plasma chemistries leave significant metal–chlorine surface residues, which can be removed in situ by low ion energy H2 or Ar plasma treatments that eliminate corrosion problems. Cu etch rates in excess of 3000 A/min at 25 °C can be obtained in ECR Cl2/Ar discharges because the high ion flux prevents ...

Patterning of Cu, Co, Fe, and Ag for magnetic nanostructures

A remarkable increase in InGaP etch rate in electron cyclotron resonance BCl3 discharges is observed as the microwave power is increased from 250 W (etch rate ∼500 A/min) to 1000 W (etch rate ∼8000 A/min). The surface roughness measured by atomic force microscopy decreases from 36 nm at 250 W to 2 nm at 1000 W. The high ion flux incident on the InGaP at high microwave powers appears to remove InClx species by sputter‐assisted desorption and prevents formation of the nonstoichiometric In‐rich surfaces generally observed with Cl2‐based dry etching using conventional reactive ion etching.

High rate dry etching of InGaP in BCl3 plasma chemistries

http://www.physics.wm.edu/~reilly/pdf/jmmm198-55.pdf

Effect of deposition parameters on the CPP-GMR of NiMnSb-based spin-valve structures

Thin films of the ferromagnetic Heusler alloy NiMnSb, of interest for magnetic multilayer devices because of their predicted half-metallic (i.e., 100% spin-polarized) transport properties, have been successfully deposited by rf magnetron sputtering from a single composite target. A novel combination of low argon gas pressure, low deposition rates, and moderate substrate temperatures (250–350 °C) are shown to result in high-quality, low-roughness polycrystalline films of the C1b-type crystal structure, with thicknesses as low as 100 A, without the need for any post-deposition annealing. The structural properties of these films, determined by x-ray diffraction and atomic force microscopy are presented as a function of deposition conditions. The magnetic properties and resistivity are consistent with bulk MiMnSb, which suggests that they will be effective as spin-polarized conducting layers in multilayer thin-film structures.

J. A. Caballero

Papers

Magnetoresistance of NiMnSb-based multilayers and spin valves

Patterning of Cu, Co, Fe, and Ag for magnetic nanostructures

High rate dry etching of InGaP in BCl3 plasma chemistries

Effect of deposition parameters on the CPP-GMR of NiMnSb-based spin-valve structures

Deposition of high-quality NiMnSb magnetic thin films at moderate temperatures