Heikki Holmberg

Magnetotransport properties of a room temperature ferromagnet (Ga, Mn)N

Heavily Mn-doped GaN thin films were fabricated by using solid-state diffusion. The magnetic properties of the samples were determined by Hall effect measurements and by using a superconducting quantum interference device. The results show ferromagnetic behavior even above room temperature, the Curie temperature being 330 K. The samples were of n-type and the estimated electron concentration was /spl ap/1/spl times/10/sup 20/ cm/sup -3/. Due to the spin-disorder scattering, a negative magnetoresistance was observed and it was largest at Curie temperature. The measured sheet resistance versus temperature could also be explained by the spin disorder scattering model for ferromagnetic metals. From the measurement results, the value of the exchange interaction parameter between the electron spins and the magnetic moments of the Mn atoms was estimated to be about 1 eV. The material properties were studied also by using secondary ion mass spectroscopy and X-ray diffraction, which showed some segregation of Ga/sub x/Mn/sub y/.

Magnetic GaAs resonant tunnelling diodes with a Mn-doped emitter

We have fabricated ferromagnetic resonant tunnelling diodes (FRTD’s) based on the AlAs/GaAs/AlAs quantum wells and the p-type Mn-doped GaAs emitter layers. At low temperatures a large magnetic field dependence of the tunnelling current appears in the magnetic RTD’s in rather low fields (B < 1T), which is not found in nonmagnetic RTD’s. The observed decrease of current in the case of the metallic ferromagnetic emitters is explained by a tunnelling anisotropic magnetoresistance effect.

Magnetotransport in ferromagnetic schottky diodes made of Mn-doped GaAs

The Mn-doped GaAs thin films were grown on p-type GaAs substrates by using Molecular Beam Epitaxy technique. The Schottky contacts on top of the (Ga,Mn)As layer were made of Pt metal. In contrast to the non-magnetic GaAs Schottky diodes, a large negative magnetoresistance (MR) in the dc current was observed in the magnetic diodes at low temperatures. The contributions to the observed MR effects from the spin dependent tunnelling through the thin Schottky barrier and the negative MR of the semiconducting layer are discussed in detail.

Magnetotransport in ferromagnetic pn-diode of (Ga,Mn)As and (Ga,Mn)N

Here we have demonstrated a large magnetoresistance in tunnelling (Ga,Mn)As/GaAs diodes at low temperatures.The (Ga,Mn)As and (Ga,Mn)N pn-diodes were fabricated by using Molecular Beam Epitaxy (MBE).Firstly, heavily doped n+-GaAs was grown on top of n-type GaAs substrate. After that the magnetic Mn doped GaAs layer with different Mn concentrations was grown by MBE. The growth temperature was only 230 C in order to inhibit formation of secondary phases and segregation of Mn at the surface. Finally, the ohmic contacts were evaporated by an electro-beam evaporator. The structure of the (Ga,Mn)N pn-diode was similar, except the fact that the n-and p-type areas were in the opposite order. In addition, the n-type magnetic layer (Ga,Mn)N was made by solid state diffusion of Mn inside the MBE chamber. The direct magnetization measurements were performed in order to confirm that the (Ga,Mn)As and (Ga,Mn)N layers are ferromagnetic. However, no magnetic field dependence in the diode current was observed in the I-V characteristics of the pn-diodes, which were made of Mn doped GaAs or GaN and which had a lightly doped n-side.

Magnetotransport in Ferromagnetic (Ga, Mn)As and (Ga, Mn)N Pn-Diodes

GaAs and GaN pn-junctions are fabricated with Mn-doped ferromagnetic layers on top of nonmagnetic substrates. In the case of heavily doped (Ga,Mn)As/GaAs p++n++-junctions (Zener diodes) the tunneling current becomes magnetic field dependent at low temperatures. This can be explained by a model based on the valence band splitting due to the exchange interaction between the hole spins and the localized spins of the Mn atoms. In the pn-junctions with more lightly doped nonmagnetic regions in the (Ga, Mn)N and (Ga, Mn)As diodes, no magnetic field dependence in the I-V characteristics is observed, probably due the absence of ferromagnetism in the depletion region of the p+n-junctions