R. F. Marks

Epitaxial growth and properties of ferromagnetic co-doped TiO2 anatase

We have used oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) to grow CoxTi1−xO2 anatase on SrTiO3(001) for x=∼0.01–0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or polycrystalline, these CoxTi1−xO2 films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the Co distribution, which varies widely with growth conditions. Co is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic CoxTi1−xO2. The magnetic properties of OPA-MBE grown material are significantly better than those of analogous pulsed laser deposition-grown material.

Control of the axis of chemical ordering and magnetic anisotropy in epitaxial FePt films

Growth of epitaxial films of the L10 phase of FePt, with the tetragonal c axis along either the film normal or in‐plane, is described. Films were grown by coevaporation of Fe and Pt, under ultrahigh vacuum conditions, onto a seed film of Pt grown on MgO or SrTiO3 substrates. The perpendicular or in‐plane orientation of the c axis was controlled by selecting the (001) or (110) substrate plane, respectively. Nearly complete chemical ordering was achieved for growth at 500 °C for both orientations. Magnetic and magneto‐optical characterization of these films confirmed the huge magnetic anisotropy expected for this phase. In the most highly ordered films, anisotropy fields in excess of 120 kOe were measured.

Growth temperature dependence of long‐range alloy order and magnetic properties of epitaxial FexPt1−x (x≂0.5) films

The structural and magnetic properties of 1000‐A‐thick epitaxial FexPt1−x(001) alloy films, grown by molecular beam epitaxy with x≂0.5, have been studied as a function of growth temperature. X‐ray diffraction analysis showed that the long‐range order parameter increased from near zero for films grown at 100 °C to a maximum of 0.93 in films grown at 500 °C. Over this range the magnetic easy axis changed from in‐plane to perpendicular and the polar Kerr rotation increased strongly. Spontaneous long‐range ordering in these films has an activation energy ∼0.2 eV, consistent with an energy barrier for surface and step down diffusion.

MnxPt1−x: A new exchange bias material for Permalloy

We report exchange biasing of Permalloy in as-grown MnxPt1−x/Permalloy structures grown by coevaporation in ultra-high vacuum. Polycrystalline and epitaxial structures with x between 0.35 and 0.75 were studied. X-ray diffraction confirms the presence of chemical ordering in the MnxPt1−x films (grown at 200 °C) in as-grown polycrystalline and epitaxial structures. An interfacial coupling energy of 0.032 erg/cm2 is found for structures where the MnxPt1−x (x=0.56) is grown at 200 °C onto epitaxial Permalloy (001). Polycrystalline structures have a smaller (0.027 erg/cm2) interfacial coupling energy. No exchange bias is observed for films with x≲0.4. The blocking temperature, for a sample with x=0.56, is >500 K.

Epitaxial growth of Pt on basal-plane sapphire: a seed film for artificially layered magnetic metal structures☆

The epitaxial growth of Pt on basal-plane sapphire, i.e. sapphire (0001), by molecular beam epitaxy is described. Growth is monitored in situ using X-ray photoelectron diffraction (XPD), reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED). The structural perfection of thin ( < 250 A) films of Pt is studied using X-ray diffraction and electron microscopy. We find that Pt nucleates on the sapphire surface at 600°C as islands which are rotationally twinned about the Pt[111] axis. The epitaxial relationship is: Pt[111]‖Al2O3[0001] and Pt(110)‖Al2O3 (1010). Island coalescence occurs at a thickness of

In situ contacts to GaAs based on InAs

15 A and X-ray diffraction shows that the Pt films have a high structural perfection. Such films are nearly ideal as seed films for a variety of epitaxial magnetic multilayers and alloys.

Off-axis electron holography of epitaxial FePt films

We report preliminary electrical results on n+‐InAs/n‐GaAs contact structures grown by molecular beam epitaxy. The data indicate that the conduction‐band discontinuity is sufficiently small to allow the formation of an ohmic contact to n‐type GaAs for very heavily doped InAs layers. The structures require a short‐term anneal to obtain a low resistance contact. An InAs layer which is only 200 A thick is sufficient to provide a specific contact resistance of 10−6 Ω cm2. The contacts appear to be thermally stable for short‐term anneals up to 900 °C.

Giant magnetoresistance in as-grown epitaxial films of phase-separated Co-Cu and Co-Ag

Off-axis electron holography at the nanometer level has been used to investigate the magnetic microstructure of thin epitaxial FexPt1−x (x∼0.5) ordered alloy films. High-resolution electron microscopy in cross section showed high quality epitaxial growth but also revealed some widely spaced regions with structural defects. Lorentz microscopy and off-axis electron holography in field-free conditions established conclusively that similar defective areas were associated with local perturbations of the in-plane magnetic field within the thin films. Further holographic observations with the FePt[001] axis parallel to the film normal revealed variations in phase shifts in the vacuum outside the sample which indicated flux leakage along the film normal. Overall, the results demonstrate that off-axis electron holography is a highly useful technique for mapping local variations of the in-plane magnetic structure associated with defective thin films.

Spin valves using insulating cobalt ferrite exchange-spring pinning layers

Giant magnetoresistance is reported in As-grown, epitaxial Co-Cu and Co-Ag alloy layers. The (111)- and (001)-oriented films were deposited onto (0001) sapphire and (001) rocksalt substrates, respectively, by co-evaporation in ultrahigh vacuum. Phase separation of the Co and Cu or Ag was obtained without any annealing after growth. The magnetoresistance at 4.2 K exceeded 50% and 70% in Co-Cu and Co-Ag films, respectively, and for films deposited at 250 °C a significant perpendicular magnetic anisotropy was observed in both magnetoresistance and magnetization studies. Grazing-incidence small-angle X-ray scattering indicates that the characteristic diameter and separation of Co clusters are approximately 25 A and 76 A, respectively, in (111) Co-Ag films grown at 250 °C.

Magnetic anisotropy and structural characterization of Co/Pt superlattices grown along selected orientations by molecular‐beam epitaxy

Cobalt ferrite, CoFe2O4, thin films are explored as pinning layers for read sensor applications. High ΔR/R values, 12.8%, and high pinning fields, 1500 Oe, are observed. Unlike other coercivity based pinning layers, the soft properties of the free layer are not compromised. The properties of the cobalt ferrite layers are strongly dependent on the microstructure, which, in turn, depends on the reactive sputtering process.