J. aan de Stegge

Orientational dependence of the exchange biasing in molecular‐beam‐epitaxy‐grown Ni80Fe20/Fe50Mn50 bilayers (invited)

The exchange biasing field (Heb) and coercive field (Hc) of molecular‐beam‐epitaxy‐grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001], and [110] orientations have been investigated by longitudinal Kerr effect measurements. Ni80Fe20 and Fe50Mn50 were deposited as orthogonal wedge‐shaped layers on single‐crystal Cu substrates in a magnetic field, enabling the study of the thickness dependence of Heb and Hc on a single sample for each orientation. A strong dependence of Heb and Hc on the growth orientation is observed. The results are interpreted in terms of the observed noncollinear spin structure of the antiferromagnet and a comparison is given with the predictions from recent theoretical models.

Orientational and structural dependence of magnetic anisotropy of Cu/Ni/Cu sandwiches: Misfit interface anisotropy

Magnetic anisotropies and misfit strain relaxations have been investigated in Cu/Ni‐wedge/Cu (100) and (111) sandwiches deposited by molecular beam epitaxy on single‐crystal Cu substrates. Our results reveal a clear distinction in the nature of the measured anisotropy at Ni thicknesses below and above the critical value tc, where the growth becomes incoherent. Below tc, coherent lattice strain modifies only the volume anisotropy, while interface anisotropy is Neel type; above tc, magnetoelastic effects are found to contribute to the interface anisotropy.

Localized Kerr study of the magnetic properties of an ultrathin epitaxial Co wedge grown on Pt(111)

We have used the magneto‐optical Kerr effect to study the thickness dependence of the perpendicular magnetic anisotropy, coercivity, nucleation field and remanence in an ultrathin Co wedge (0–9 monolayers) grown epitaxially on Pt(111) and capped by a thin Pt overlayer. In addition, the spin‐reversal mechanism in the Co layers has been investigated using Kerr domain imaging. The sample showed perpendicular magnetization up to an extrapolated Co thickness of 15 monolayers (ML), with derived volume and interface anisotropy constants of −0.77 MJ/m3 and 1.15 mJ/m2, respectively. The Kerr rotation and Kerr ellipticity demonstrated linear dependences on the Co thickness (tCo), with sizeable extrapolated offsets at tCo=0, attributed to polarization of the Pt by the Co. The coercivity (Hc) and nucleation field both rose to peak values at tCo=1.5 ML, with a subsequent monotonic fall‐off for higher Co thicknesses. In the case of Hc, this fall‐off did not demonstrate the tCo−5/2 dependence shown by a similar Pd(111)/...

An STM study of Fe3O4 (100) grown by Molecular Beam Epitaxy

Abstract STM imaging of MBE-grown pseudomorphic (100) Fe 3 O 4 surfaces reveals terrace widths that are typically a few hundred angstroms broad, and can be as broad as 1000 A. These terraces are separated by steps that are 1 4 of the spinel lattice constant high, corresponding to the distance (2.1 A) between planes of oxygen (or equivalent iron) atoms. The images show that the p(1 × 1) surface reconstruction is caused by a clustering of atoms in the unit cell. These clusters are aligned along a [110] direction, and change direction on alternate terraces. The reconstruction is driven by the tetrahedral iron atoms, which have dangling bonds that rotate by 90° from one atomic plane to the next. Some regions of the surface also show a high-symmetry close-packed structure with 3 A spacing between atoms. The presence of stacking faults is revealed by the orientation of the unit cells. In one image, the two possible orientations of the unit cells are present on the same terrace, separated by a disordered band, which must contain a stacking fault. In another case, the unit cells are oriented in the same direction on two terraces separated by a 2.1 A step. Again a disordered region appears at the boundary between the two terraces. Single-domain regions are as large as a few hundred angstroms wide, which indicates that the surface diffusion length of the iron atoms during the initiation of growth on the higher symmetry MgO substrate is of this same order.

Exchange biasing in MBE-grown Ni80Fe20/Fe50Mn50 bilayers

The exchange biasing field (Heb) and coercive field (Hc) of molecular-beam-epitaxy (MBE) grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001] and [110] orientations were investigated by Kerr effect measurements. A strong dependence of Heb and Hc on the growth orientation is observed. A strong uniaxial in-plane anisotropy introduced by the Fe50Mn50 layer was found for the [110]-oriented sample. Conversion electron Mossbauer studies revealed a roughness of 2–3 A and no significant moment reduction at the Ni80Fe20/Fe50Mn50 interface.

Perpendicular giant magnetoresistance of microstructures in Fe/Cr and Co/Cu multilayers (invited)

We discuss the fabrication and microstructuring techniques of pillar structures made of high vacuum sputtered Fe/Cr multilayers and of molecular beam epitaxy evaporated Co/Cu multilayers, for which we measured the giant magnetoresistance effect with the current perpendicular to the multilayer plane from 4 K to 300 K. Using optical lithography and reactive ion etching techniques we obtained structures with a typical height of 0.5 μm and a width ranging between 3 and 10 μm. For both Fe/Cr and Co/Cu multilayers we find an enhanced magnetoresistance with respect to the in‐plane case. The perpendicular magnetoresistance of the Fe/Cr pillars strongly decreases with temperature, while for the Co/Cu samples the temperature dependence is weaker, indicating electron‐magnon scattering processes of different strength.

Short period oscillation of the interlayer exchange coupling in the ferromagnetic regime in Co/Cu/Co(100)

A unique sample was prepared on a Cu(100) single crystal, consisting of three Co layers separated by two Cu layers in the form of wedges oriented perpendicular to each other: Cu(100)/80 A Co/Cu wedge A/30 A Co/Cu wedge B/30 A Co/7 A Cu/30 A Au. Position‐sensitive magneto‐optical Kerr effect measurements along Cu wedge B, at a fixed position on Cu wedge A corresponding to maximum antiferromagnetic (AF) coupling, enabled us to investigate not only the AF but also the ferromagnetic (F) coupling between the two 30 A Co layers as a function of the Cu thickness. The measurements confirmed both long and short period coupling oscillations in the AF regime, and revealed the predicted extension of the short period through the F regime.

Antiferromagnetic coupling between Co/Ni multilayers with perpendicular magnetization across MBE-grown Cu(111) layers

A cleat oscillatory exchange coupling has been observed for an epitaxial (111) M1/Cu/M2 structure, grown by Molecular Beam Epitaxy (MBE) on a single crystal Cu(111) substrate. The magnetic layers M1 and M2 were designed to have perpendicular easy axis by using Co/Ni multilayers. The Cu layer was deposited in the form of a wedge. The magneto-optically measured hysteresis loops show, in the case of antiferromagnetic (AF) coupling, clear first-order spin-flip transitions between the saturated and antiparallel states. Signatures of a ferromagnetic or biquadratic component, observed earlier in loops of an AF coupled Co/Cu/Co(111) sandwich, were absent in the present loops. The absence of these signatures is believed to be a consequence of the relatively strong (perpendicular) uniaxial anisotropy of the present samples.

Oscillation of the interlayer coupling in Co/Au(111)/Co

An oscillatory interlayer coupling in Co/Au(111)/Co has been obtained from polar Kerr hysteresis loop measurements on a molecular‐beam‐epitaxy‐grown 50 A Au/2 A Co/0–40 A Au wedge/7 A Co/20 A Au sample deposited on a Cu(111) single crystal. The period of the coupling is seen to increase from 11 to 14 A with increasing Au thickness, indicating a preasymptotic behavior. The asymptotic period of 14±1 A is larger than the theoretically expected period of 11.4 A. The observation of coupling up to 40 A Au will be discussed in relation to the general absence of coupling in evaporated Co/Cu(111)/Co samples.

Perpendicular anisotropy of epitaxial “wedge-shaped” Co/Ni sandwiches grown on single crystal Cu substrates

Abstract The magneto-optical Kerr effect has been used to probe three ultrathin Ni/Co-wedge/Ni sandwiches deposited by molecular beam epitaxy on single-crystal Cu substrates. Results show a marked orientational dependence of both the volume and interface anisotropy terms. The Co thickness dependence of the Kerr effect and coercivity is also discussed.