R. F. C. Farrow

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.

Magnetic and structural properties of Co/Pt multilayers

Abstract The magnetic properties of Co/Pt multilayers, in particular anisotropy and coercivity, are very sensitive to the Co layer thickness and less dependent on the Pt layer thickness. Such dependence is illustrated and discussed for e-beam evaporated Co/Pt multilayers of various Co and Pt thicknesses. It is shown that the largest perpendicular anisotropy occurs for Co layer thickness of 1–2 monolayers. The magnetic anisotropy of Co/Pt multilayers also strongly depends on the crystallographic orientation. The largest perpendicular magnetic anisotropy was obtained for Co/Pt multilayers with Pt(111) parallel to the film plane. This was demonstrated in both evaporated and sputtered polycrystalline multilayers, as well as in molecular beam epitaxy (MBE) grown Co/Pt(111) superlattices. By contrast, MBE grown 3.7 A Co/16.8 A Pt(100) multilayers show in-plane anisotropy, and 3.7 A Co/16.8 A Pt(110) multilayers exhibit very strong anisotropy within the film plane. These observations suggest that the magneto-crystalline anisotropy plays a key role in the magnetic anisotropy of these structures. Significantly enhanced effective magnetization was observed for Co/Pt multilayers with ultrathin Co layer.

Clusters and magnetism in epitaxial Co-doped TiO2 anatase

We show that under certain conditions, highly Co-enriched TiO2 anatase clusters nucleate on epitaxial TiO2 anatase grown on LaAlO3(001) by oxygen plasma assisted molecular beam epitaxy. In the most extreme cases, virtually all incident Co segregates to the clusters, yielding a nanoscale ferromagnetic phase that is not ferromagnetic in homogeneous films of the same Co concentration. The nucleation of this phase simultaneous with continuous epitaxial film growth must be carefully monitored in order to avoid drawing false conclusions about the film structure.

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

MAGNETIC ANISOTROPY AND MICROSTRUCTURE IN MOLECULAR BEAM EPITAXIAL FEPT (110)/MGO (110)

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 compare the growth, structural, and magnetic properties of FexPt1−x(x≃0.6) films grown by molecular beam epitaxy on MgO (110) substrates held at 300 and 500 °C. Growth at both temperatures yields nearly single orientation epitaxial films with FePt, Pt [001]//MgO[001] and FePt, Pt (110)//MgO (110). For growth at 300 °C, chemical ordering to the L10 (CuAu I) tetragonal phase is incomplete (long-range order parameter, S=0.38) in contrast with growth at 500 °C for which ordering is nearly complete (S=0.81). Despite incomplete chemical ordering, a large two-fold, in-plane magnetic anisotropy (Ku=1.5×107 erg/cm3) is present for the film grown at 300 °C. Transmission electron microscopy for the 300 °C film confirms epitaxial growth with few extended defects but also reveals that chemical ordering is distributed inhomogeneously within the film. The direction of ordering is generally uniform (c axis along MgO [001]) but variations in the degree of ordering are evident on a length scale of 1–10 nm.

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.