Po-Hsiang Huang

Room-temperature fabricated ZnCoO diluted magnetic semiconductors

(0002) textured and epitaxial ZnCo0.07O films were fabricated at room temperature by ion beam deposition on Si substrates. Hall measurement revealed that ZnCo0.07O films were n-type semiconductors with carrier concentrations higher than 1019∕cm3. The carrier concentration of ZnCo0.07O can be manipulated by controlling the oxygen flow rate during deposition or by postannealing. The saturation magnetization and magnetoresistance ratios strongly depended on the carrier concentration. In addition, epitaxial (0002) ZnCo0.07O films, grown on Cu underlayers, showed room-temperature ferromagnetism, which may be potentially used for spintronic devices.

Coexistence of exchange-bias fields and vertical magnetization shifts in ZnCoO∕NiO system

Exchange fields accompanying vertical magnetization shifts were observed in the epitaxial ZnCo0.07O (dilute magnetic semiconductor)/NiO system after field cooling. Transitions of exchange fields and magnetization shifts were observed at 50K, above which the magnetization shift disappeared and the exchange field was significantly reduced. Both the exchange field and the magnetization shift increased with increasing cooling-field strength at temperatures below 50K, which might be attributed to the existence of “frozen” spins in ZnCoO. The observed linear dependence of the exchange field on the magnetization shift may directly elucidate the role of pinned spins on the exchange fields.

Room-temperature growth of epitaxial Fe3O4 films by ion beam deposition

Epitaxial and polycrystalline Fe3O4 films were grown on MgO (100) and Si (100) substrates, respectively, at room temperature by using reactive ion beam deposition. The MS value of epitaxial Fe3O4 films was around 310 emu/cm3, and was almost independent of thickness from 45 to 195 nm. The MS value of polycrystalline films showed significant thickness dependence, which might be attributed to the formation of the initial layer. The Verwey transition at 110 K was observed on 195 nm epitaxial films, and decreased significantly with decreasing thickness. The reduction of the Verwey temperature may be related to the residual strain in the film.

Room-temperature growth of epitaxial (111) Fe3O4 films with conductive Cu underlayer

Epitaxial (111) Fe3O4 film on the Cu underlayer was obtained by using reactive ion-beam sputtering with high incident energy at room temperature. X-ray φ scans and transmission electron microscope diffraction pattern revealed unusual 12-fold symmetry of the (111) Fe3O4 films on Cu (001) underlayers due to the presence of two kinds of epitaxial (111) grains in Fe3O4 films. The saturation magnetization of the 70nm thick (111) Fe3O4 films is 291emu∕cm3. A clear Verwey transition of Fe3O4 films was observed around 116K. The root-mean-square roughness of the Fe3O4 surface is only 3.5A.

Enhancement of exchange coupling between GaMnAs and IrMn with self-organized Mn(Ga)As at the interface

An atomically flat and uniform reaction layer of Mn(Ga)As was found to self-organize at the (Ga,Mn)As∕IrMn interface by postannealing. The Mn(Ga)As layer exhibits strong ferromagnetic characteristics up to the measured 300K. In particular, the manifested horizontal shift of field-cooled hysteresis loops shows a clear signature of exchange bias attributable to the exchange coupling between IrMn and Mn(Ga)As. Implication from composition analyses, exchange-bias effect, and thickness dependence of the Mn(Ga)As layer versus annealing conditions is also discussed.

Self-assembly and magnetic properties of MnAs nanowires on GaAs(001) substrate

The in-plane aligned MnAs nanowires have been grown by molecular-beam epitaxy on GaAs(001) substrates at high growth temperature (≥450 °C). A discontinuous growth with break intervals (50 s’ interval per 10 s’ growth) was employed. The obtained nanowires were identified to be mainly type-B hexagonal MnAs. The influences of growth temperature and As4/Mn flux ratio on the nanowires’ morphology were investigated. Both high growth temperature and high As4/Mn flux ratio are necessary for the growth of uniaxially aligned MnAs nanowires with high aspect ratio. The magnetic anisotropy of the nanowires and their multimodal size distributions contribute to the large coercivity and special shape of the M-H loops along the magnetic easy axis, which is [11¯02]MnAs∥[110]GaAs. However, the longer growth time would lead to the both azimuthal alignments of the MnAs wires and the weakening of the magnetic anisotropy.

Uniaxial to unidirectional transition of perpendicular interlayer coupling in IrMn/CoFe/NiFeO/CoFe quadrilayers

We studied the interlayer coupling in the quadrilayer consisting of IrMn∕CoFe (bottom layer)/NiFeOx∕CoFe (top layer). An in-plane perpendicular interlayer coupling is observed between CoFe layers at room temperature. An anisotropy transition from uniaxial to unidirectional in a perpendicular direction is observed around Tt=55K. The nano-oxide layer NiFeOx shows no distinguishable ferromagnetic signal in the high-temperature (uniaxial) phase, while a strong signal appeared in the low-temperature (unidirectional) phase. A possible two-component scenario, in which the nano-oxide layer may contain both amorphous short-range antiferromagnetic domains and superparamagneitc clusters, is proposed to explain the phase transition.

Morphology and magnetic properties of InMnAs nanodots and nanowires with ultrahigh Mn concentrations

In1−xMnxAs (0.22 ≤ x ≤ 0.55) nanostructures with ultrahigh Mn concentration were grown on GaAs(001) substrates by molecular beam epitaxy. When the growth is performed at 380 °C, nanodots are obtained. The M(T) relation of InMnAs nanodots is highly dependent on the morphology which is affected by Mn concentration. When the growth temperature is higher up to 550 °C, the shape transition from nanodots to nanowires takes place and well-shaped nanowires are obtained at high Mn concentrations. The formation of InMnAs nanowires brings about the in-plane uniaxial magnetic anisotropy, with the easy axis along the self-alignment orientation, namely, [1−10] GaAs.

Effect of substrate orientation on magnetic properties of (Ga, Mn)As

Structural and magnetic properties of Ga0.93Mn0.07As layers grown on (001) and (311)A GaAs substrates by molecular-bean epitaxy are investigated. The as-grown (001) and (311)A Ga0.93Mn0.07As layers exhibit the same Curie temperature (TC) of 80K. However, upon annealing, the TC’s of the (001) and (311)A Ga0.93Mn0.07As layers are enhanced by 80 and 60K, respectively. X-ray diffraction studies reveal that the AsGa defects cannot be removed by low-temperature annealing, and a higher concentration of AsGa defects exist in the (311)A layers than in the (100) reference layers. The less enhancement in TC by annealing for the (311)A Ga0.93Mn0.07As layer can be ascribed to the larger amount of AsGa defects in the material.

Anisotropy transition of Co in IrMn∕Co∕FeOx∕Co by field cooling

The temperature dependence of Co anisotropy on a nano-FeOx layer was studied in the structure of IrMn∕Co(FM1)∕FeOx∕Co(FM2). An anisotropy transition of the FM2 was observed from a combination of uniaxial and unidirectional anisotropies at room temperature (RT) to unidirectional anisotropy at temperature below 80K through field cooling process. Various ferromagnetic (FM) and antiferromagnetic (AFM) components existing in the FeOx layer were attributable to the observed anisotropy of FM2. AFM domains with TN higher than room temperature were responsible for the observed uniaxial anisotropy at RT and AFM domains with TN of 80K were accountable for the anisotropy transition, below which the unidirectional anisotropy became dominant. In addition, the direction of the shifted loop could be determined by the cooling field direction.