H. Asada

Carrier-induced ferromagnetism in Ge0.92Mn0.08Te epilayers with a Curie temperature up to 190 K

IV-VI diluted magnetic semiconductor Ge0.92Mn0.08Te epilayers are grown on BaF2 substrates by molecular beam epitaxy. The ferromagnetic behaviors, such as the spontaneous magnetization, the coercive field, and the Curie temperature TC, are altered by the hole concentration p. In the Ge0.92Mn0.08Te layer with high p, strong magnetic anisotropy and the temperature dependence of the magnetization expected for homogeneous ferromagnets are observed, implying that long-range ordering is induced by the holes. The maximum TC reaches 190 K for 1.57×1021 cm−3.

Magnetic properties of IV–VI compound GeTe based diluted magnetic semiconductors

Magnetic properties of IV–VI compound GeTe based diluted magnetic semiconductors with 3d transition metals from Ti to Ni have been investigated. Ferromagnetic order is observed for the Cr, Mn, and Fe doped GeTe films, whereas the Ti, V, Co, and Ni doped films are paramagnetic. The ferromagnetic order could give rise to p–d exchange interaction because amplitudes of negative magnetoresistance and the anomalous Hall effect are proportional to that of spontaneous magnetization. The Curie temperatures determined by extrapolating the steep linear part of the temperature dependence of the squared residual magnetization for the Cr, Mn, and Fe doped GeTe films are 12, 47, and 100 K, respectively.

Carrier-enhanced ferromagnetism in Ge1−xMnxTe

We have studied the dependence of magnetic properties on the carrier concentration of IV–VI diluted magnetic semiconductor Ge1−xMnxTe prepared by ionized-cluster beam technique. With increasing carrier concentration, the magnetic properties drastically change; saturation magnetization increases and coercive field decreases in hysteresis loop, and the Curie temperature increases. These results obviously show carrier-enhanced ferromagnetic order. The similar effect is also observed in magnetoresistance behavior.

Ferromagnetic properties of IV–VI diluted magnetic semiconductor Ge1−xMnxTe films prepared by radio frequency sputtering

IV–VI diluted magnetic semiconductor Ge1−xMnxTe films with various Mn compositions up to x=0.53 were prepared on glass substrates by a rf sputtering method. Magnetic measurements indicate that all the Ge1−xMnxTe films (0.07⩽x⩽053) show ferromagnetic order at low temperatures since hysteretic behavior is clearly observed. The effective spin Seff’s of Mn ions deduced from the spontaneous magnetization at 4.2 K are less than 1.0, indicating that Mn ions are partially aligned ferromagnetically. The value of the Seff depends on the carrier concentration as well as the Mn composition. The magnetic order of the Ge1−xMnxTe films is thus caused by the competition between the ferromagnetic Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction and the antiferromagnetic superexchange mechanism. The Curie temperature based on the mean field approximation increases linearly with increasing Mn composition up to x=0.3 and above this point tends to saturate around 80 K. On the basis of a simple RKKY mechanism, the spin exchang...

Ferromagnetism in Ge1−xCrxTe epilayers grown by molecular beam epitaxy

IV-VI ferromagnetic semiconductor Ge1−xCrxTe has been grown on BaF2 (111) by molecular beam epitaxy. The ferromagnetism was clearly established by direct magnetization and Hall measurements. The experimental correlation between the anomalous Hall resistivity ρxy and the resistivity ρxx, ρxy∝ρxx1.76, is understood from the semiclassical nature of the charge carrier dynamics, suggesting that the ferromagnetism gives rise to p-d exchange interaction. The Curie temperature increases systematically from the substrate temperature TS of 300to250to200°C and with increasing the Cr composition along with each TS.

Ferromagnetic semiconductor Ge1−xCrxTe with a Curie temperature of 180K

A IV-VI ferromagnetic semiconductor Ge1−xCrxTe (x∼0.06) with Curie temperature TC up to 180K is grown by molecular-beam epitaxy. The magnetization is well reproduced from anomalous Hall effect. As the Te∕Cr flux ratio increases during the growth of Ge1−xCrxTe, the spontaneous magnetization and the magnetic anisotropy are decreased and TC is increased. On the other hand, the Te∕Cr flux ratio over 3.6 leads to formation of Cr–Te precipitations. The magnetoresistance measurements reveal that the increase of TC is attributed to the decrease of nonstoichiometric defects.

Large magnetic circular dichroism of Co clusters in Co-doped ZnO

Large magnetic circular dichroism (MCD) in Co-doped ZnO films is reported. The Co-(∼20%) doped ZnO films with oxygen deficiencies show ferromagnetic behavior at room temperature. While paramagnetic films exhibit a small MCD signal, broad MCD spectra with large amplitude of ∼20kdeg∕cm are observed for the ferromagnetic films. In the ferromagnetic films, Co cluster formation occurs, especially near the interface between the substrate and the film. The amount of the Co clusters is consistent with the spontaneous magnetization and the amplitude of the MCD.

IV-VI diluted magnetic semiconductor Ge1-xmnxTe epilayer grown by molecular beam epitaxy

Growth of the IV-VI diluted magnetic semiconductor Ge1−xMnxTe by molecular beam epitaxy is reported. The epitaxial growth of Ge1−xMnxTe (x=0.13) on BaF2 (111) with a GeTe buffer layer is confirmed by x-ray diffraction and reflection high-energy electron diffraction. The ferromagnetic order is clearly established by the magnetization and magnetotransport measurements. The Curie temperature of 100K is obtained for the hole concentration of 7.86×1020cm−3. The existence of the strong p-d exchange which gives rise to the ferromagnetic order is revealed by the hard x-ray photoemission measurements.

Correlation between magnetic properties and carrier concentration in Ge1−xMnxTe

The dependence of magnetic properties on the carrier concentration for Ge1−xMnxTe films prepared by ionized-cluster beam technique has been studied to clarify the mechanism of carrier-induced ferromagnetism. The experimental results suggest that the cluster of spins aligned by short-range ferromagnetic interaction tends to be formed for samples with the lower carrier concentration than the critical value for the occurrence of homogeneous ferromagnetic order. With increasing carrier concentration, the long-range ferromagnetic interaction grows. The magnetotransport properties of Ge1−xMnxTe films are found closely related to its magnetic properties, indicating carrier-enhanced ferromagnetic effects.

Ferromagnetic order in Ge1−xCrxTe

The ferromagnetic properties of IV-VI diluted magnetic semiconductor Ge1−xCrxTe films prepared by sputtering are investigated. The ferromagnetism such as the spontaneous magnetization and the Curie temperature is significantly affected by the stoichiometry, while the ferromagnetism of Ge1−xMnxTe films was hardly affected by the defects and depended strongly on the hole concentration [Y. Fukuma, H. Asada, N. Nishimura, and T. Koyanagui, J. Appl. Phys. 93, 4034 (2003)]. The difference could be ascribed to the interaction of the ferromagnetic order: short-range order such as a superexchange mechanism plays more an important role than long-range order such as a Ruderman-Kittel-Kasuya-Yosida mechanism in the ferromagnetism of the Ge1−xCrxTe films.