M. Tselepi

Single crystal Fe films grown on InAs(100) by molecular beam epitaxy

Thin Fe films have been grown on InAs(100) by molecular beam epitaxy, and studied using in situ magneto-optical Kerr effect (MOKE), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Despite the large lattice mismatch between Fe and InAs, the growth of Fe on InAs at 175 °C was found to be epitaxial with the orientation relationship Fe(100)〈001〉‖InAs(100)〈001〉, as evidenced by LEED. STM images indicate that growth proceeds via a 3D Volmer–Weber mode. The magnetic hysteresis loops measured using in situ MOKE show a distinct cubic anisotropy with the easy axis along 〈001〉, the easy axis of bulk bcc Fe, which further confirms that well ordered single crystal Fe films have been stabilized on the InAs(100) substrate. Current–voltage measurements in the temperature range of 2.5–304 K show that Fe forms an ohmic contact on InAs. We propose that Fe/InAs is a suitable heterostructure for magnetoelectronic devices as, unlike Fe/GaAs, there is no Schottky barrier to electron transport.

Uniaxial magnetic anisotropy of epitaxial Fe films on InAs(100)-4×2 and GaAs(100)-4×2

The evolution of the uniaxial magnetic anisotropy of ultrathin epitaxial Fe films grown on InAs(100)-4×2 and GaAs(100)-4×2 has been studied in situ by means of the magneto-optical Kerr effect. In Fe/InAs(100)-4×2, the uniaxial magnetic anisotropy easy axis direction along [011] was found to be rotated 90° compared with that of Fe/GaAs(100)-4×2 along [011]. Real-time reflection high energy electron diffraction measurements of Fe/InAs(100)-4×2 show that the lattice constant of the epitaxial Fe films relaxes remarkedly faster along the [011] direction than along the [011] direction in the same thickness range where the uniaxial magnetic anisotropy occurs. These results suggest that the symmetry-breaking atomic scale structure of the reconstructed semiconductor surface gives rise to the uniaxial magnetic anisotropy in a ferromagnetic metal/semiconductor heterostructure via surface magneto-elastic interactions.

Giant enhancement of orbital moments and perpendicular anisotropy in epitaxial Fe/GaAs(100)

The spin and orbital magnetic moments and the perpendicular magnetic anisotropy of 8 and 33 monolayer epitaxial bcc Fe films grown on GaAs(100)-4×6 have been measured using x-ray magnetic circular dichronism and polar magneto-optical Kerr effect. Both the films have approximately the same spin moments of about 2.0μB close to that of the bulk value. The ultrathin film shows a giant orbital moment enhancement of about 300% with respect to the bulk value and a perpendicular interface anisotropy field HsFe–GaAs of the order of −5×104 Oe. This may be partially due to an increased degree of localization of electronic states at the Fe/GaAs interface associated with the atomic scale interface structure.The spin and orbital magnetic moments and the perpendicular magnetic anisotropy of 8 and 33 monolayer epitaxial bcc Fe films grown on GaAs(100)-4×6 have been measured using x-ray magnetic circular dichronism and polar magneto-optical Kerr effect. Both the films have approximately the same spin moments of about 2.0μB close to that of the bulk value. The ultrathin film shows a giant orbital moment enhancement of about 300% with respect to the bulk value and a perpendicular interface anisotropy field HsFe–GaAs of the order of −5×104 Oe. This may be partially due to an increased degree of localization of electronic states at the Fe/GaAs interface associated with the atomic scale interface structure.

Micromagnetism in mesoscopic epitaxial Fe dot arrays

The domain structures of epitaxial Fe (20 nm)/GaAs(100) circular dot arrays (diameters from 50 to 1 μm) were studied with magnetic force microscopy. A transition from a single domain to a multidomain remanent state was observed upon reducing the dot diameter beneath 10 μm in dot arrays with the separation twice the dot diameter. When the separation is reduced to half the dot diameter, the single domain states were found to “collapse” into stripe-like multidomain states due to local dipole coupling between dots. Micromagnetic simulations further suggest that for ultrathin Fe dots of less than about 2 nm thickness the diameter does not have a significant influence on the domain structures due to a dramatic reduction of the dipole energy.

Ferromagnetic metal/semiconductor hybrid structures for magnetoelectronics

We report on the following new ferromagnetic metal/semiconductor heterostructure material systems: (1) Fe/InAs(100)-4×2, (2) Fe/InAs(graded)/GaAs(100), and (3) Fe/InAs/AlSb/GaSb/AlSb/InAs/GaAs resonant tunneling diodes. Single crystal Fe films have been stabilized in these structures using molecular beam epitaxy growth, as evidenced by low energy electron diffraction. The magnetic and electrical properties have been studied using in situ (and focused) magneto-optical Kerr effect, alternating gradient field magnetometry, and current–voltage measurements. The results show that Fe/InAs based heterostructures are very promising systems for use in future magnetoelectronic devices as they have well defined magnetic properties as well as favorable electrical properties.

Interface magnetic properties of epitaxial Fe-InAs heterostructures

The growth and interface magnetic properties of epitaxial Fe films grown on InAs [100]-4/spl times/2 have been studied using low-energy electron diffraction, in situ magneto-optical Kerr effect, and X-ray magnetic circular dichroism. The magnetic properties at room temperature were found to proceed via three phases with thickness; a nonmagnetic phase, a superparamagnetic phase, and a ferromagnetic phase. The initial ferromagnetic phase might be stabilized by interparticle interactions. The films show bulk-like spin moments of 1.90 /spl plusmn/ 0.15 /spl mu//sub B/ with the thickness above about 20 ML and a large enhancement /spl sim/ 260% of the ratio of orbital versus spin moment in the ultrathin region.

Effects of interdot dipole coupling in mesoscopic epitaxial Fe[100] dot arrays

The domain structure and the coercivity of epitaxial Fe[100] circular dot arrays of different diameters and separations have been studied using magnetic force microscopy (MFM) and focused magneto-optical Kerr effect (MOKE). The MFM images of the 1 /spl mu/m diameter single domain dot arrays show direct evidence of strong interdot dipole coupling when the separation is reduced down to 0.1 /spl mu/m. The coercivity of the dots is also found to be dependent on the separation, indicating the effect of the interdot dipole coupling on the magnetization reversal process.

Spin-polarized electron transport in ferromagnet/semiconductor hybrid structures (invited)

Two major problems in spin electronics remain to be solved: room temperature spin injection at a source and spin detection at a drain electrode. The lateral size of magnetic contacts and the presence of a potential barrier at the interface are believed to have a key influence on the efficiency of both of these processes. We therefore aimed to clarify these issues by studying spin-polarized transport across epitaxially grown single crystal Fe (001)/GaAs nanoclusters and at the Schottky barrier formed at Ni80Fe20/GaAs interfaces. We observed a negative contribution to the magnetoresistance of an ultrathin (2.5 ML) discontinuous epitaxial Fe film as occurs in tunnel magnetoresistance. This result suggests that spin transport via GaAs is possible on the nanoscale. In the continuous NiFe/GaAs structures, circularly polarized light was used to create a population of spin-polarized electrons in the GaAs substrate and spin-polarized electron transport across the interface at room temperature was detected as an el...

Scaling behaviour and evolution of ferromagnetism in epitaxial Fe/GaAs(100) and Fe/InAs(100)

Abstract The scaling behaviour of the thickness evolution of ferromagnetism in single crystal Fe films on GaAs(100)-4 × 2 and InAs(100)-4 × 2, grown by molecular beam epitaxy, have been studied. In both systems there is a superparamagnetic-ferromagnetic transition whose onset occurs around 4.5 monolayers (ML) Fe. The susceptibility around the transition follows a scaling behaviour; x ~ (1 − d/dc)−gg with γ = 2.13 ± 0.30b for Fe/GaAs and γ = 2.21 ± 0.25 for Fe/InAs, suggesting the 2D percolation of clusters.

Spin dynamics in ultrathin Fe/GaAs (100)-(4×6) films at the ferromagnetic/superparamagnetic phase transition by in situ Brillouin light scattering

Epitaxially grown bcc Fe films on GaAs(100)-(4×6) have been studied with in situ magneto-optical Kerr effect (MOKE), low energy electron diffraction, and in situ Brillouin light scattering (BLS) measurements. It has been reported earlier by Xu et al. [Phys. Rev. B 58, 890 (1998)] that this system exhibits at room temperature a thickness dependent superparamagnetic to ferromagnetic phase transition at a thickness dc of ∼4 ML. Combined MOKE and BLS measurements were carried out in the thickness range of 3.8–20 ML of Fe in order to investigate the spin dynamics close to dc. Two effects were observed in the vicinity of the transition thickness. First, the spin wave frequency is significantly decreased. Second, a strong broadening of the spin wave peaks takes place.