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We present studies of structural, magnetic and electrical properties of Zn/1-x-y/Cd/x/Mn/y/GeAs/2/ nanocomposite ferromagnetic semiconductor samples with changeable chemical composition. The presence of MnAs clusters induces in the studied alloy room temperature ferromagnetism with the Curie temperature, TC, around 305 K. The chemical composition of the chalcopyrite matrix controls the geometrical parameters of the clusters inducing different magnetoresistance effects in the crystals. The presence of ferromagnetic clusters in the alloy induces either negative or positive magnetoresistance with different values. The Cd-content allows a change of magnetoresistance sign in our samples from negative (for x = 0.85) to positive (for x = 0.12). The negative magnetoresistance present in the samples with x = 0.85 is observed at temperatures T < 25 K with maximum values of about -32% at T = 1.4 K and B = 13 T, strongly depending on the Mn content, y. The positive magnetoresistance present in the samples with x = 0.12 is observed with maximum values not exceeding 50% at B =13 T and T = 4.3 K, changing with the Mn content, y., Comment: 8 pages, 7 figures

The purpose of this study was to investigate the magnetotransport properties of the Ge(0.743)Pb(0.183)Mn(0.074)Te mixed crystal. The results of magnetization measurements indicated that the compound is a spin-glass-like diluted magnetic semiconductor with critical temperature TSG = 97.5 K. Nanoclusters in the sample are observed. Both, matrix and clusters are magnetically active. Resistivity as a function of temperature has a minimum at 30 K. Below the minimum a variable-range hopping is observed, while above the minimum a metallic-like behavior occurs. The crystal has high hole concentration, p = 6.6E20 cm-3, temperature-independent. Magnetoresistance amplitude changes from -0.78 to 1.18% with increase of temperature. In the magnetotransport measurements we observed the anomalous Hall effect (AHE) with hysteresis loops. Calculated AHE coefficient, RS = 2.0E6 m3/C, is temperature independent. The analysis indicates the extrinsic skew scattering mechanism to be the main physical mechanism responsible for AHE in Ge(0.743)Pb(0.183)Mn(0.074)Te alloy., Comment: 4 pages, 3 figures

Magnetic and transport properties of Ge(1-x-y)Mn(x)Eu(y)Te crystals with chemical compositions 0.041 < x < 0.092 and 0.010 < y < 0.043 are studied. Ferromagnetic order is observed at 150 < T < 160 K. Aggregation of magnetic ions into clusters is found to be the source of almost constant, composition independent Curie temperatures in our samples. Magnetotransport studies show the presence of both negative (at T < 25 K) and linear positive (for 25

Positron annihilation spectroscopy is used to study point defects in Zn/1-x/Mn/x/GeAs/2/ crystals with low Mn content 0 < x < 0.042 with disordered zincblende and chalcopyrite structure. The role of negatively charged vacancies and non-open-volume defects is discussed with respect to the high p-type conductivity with carrier concentration 1E19 < n < 1E21 cm^-3 in our samples. Neutral As vacancies, together with negatively charged Zn vacancies and non-open-volume defects with concentrations around 1E16-1E18 cm^-3, increasing with the amount of Mn in the alloy, are observed. The observed concentrations of defects are not sufficient to be responsible for the strong p-type conductivity of our crystals. Therefore, we suggest that other types of defects, such as extended defects, have a strong influence on the conductivity of Zn1-xMnxGeAs2 crystals., Comment: 8 pages, 3 figures

We present the studies of structural, electrical, and magnetic properties of bulk Cd$_{1\textrm{-}x}$Mn$_{x}$GeAs$_{2}$ crystals with low Mn content, $x$, varying from 0 to 0.037. The studied samples have excellent crystallographic quality indicated by the presence of diffraction patterns never before observed experimentally for this compound. The electrical transport in our samples is dominated by thermal activation of conducting holes from the impurity states to the valence band with activation energy of about 200$\;$meV. The defect states acting as ionic scattering centers with concentration in the range from 6 to 15$\times$10$^{17}$$\;$cm$^{-3}$ are observed. The effective Mn content in our samples, $\bar{x}_{\theta}$, determined from fit of the susceptibility data to the Curie-Weiss law, is very close to the average chemical content, $x$. It indicates that the Mn ions are distributed randomly, substituting the Cd sites in the host CdGeAs$_{2}$ lattice. We observe a negative Curie-Weiss temperature, $|\theta|$$\,$$\leq$$\,$3.1$\;$K, increasing as a function of $x$. This indicates the significance of the short-range interactions between the Mn ions., Comment: 7 pages, 5 figures

We present the studies of Sn/1-x/Cr/x/Te semimagnetic semiconductors with chemical composition x ranging from 0.004 to 0.012. The structural characterization indicates that even at low average Cr-content x < ?0.012, the aggregation into micrometer size clusters appears in our samples. The magnetic properties are affected by the presence of clusters. In all our samples we observe the transition into the ordered state at temperatures between 130 and 140 K. The analysis of both static and dynamic magnetic susceptibility data indicates that the spin-glass-like state is observed in our samples. The addition of Cr to the alloy seems to shift the spin-glass-like transition from 130 K for x = 0.004 to 140 K for x = 0.012., Comment: 4 pages, 4 figures

We present the studies of electrical transport and magnetic interactions in Zn_{1-x}Mn_{x}GeAs_{2} crystals with low Mn content 0 \leq x \leq 0.043. We show that the ionic-acceptor defects are mainly responsible for the strong p-type conductivity of our samples. We found that the negative magnetoresistance (MR) with maximum values of about -50% is related to the weak localization phenomena. The magnetic properties of Zn1-xMnxGeAs2 samples show that the random Mn-distribution in the cation sites of the host lattice occurs only for the sample with the lowest Mn-content, x=0.003. The samples with higher Mn-content show a high level of magnetic frustration. Nonzero Curie-Weiss temperature observed in all our samples indicates that weak ferromagnetic (for x=0.003) or antiferromagnetic (for x>0.005) interactions with |{\Theta}|<3 K are present in this system. The RKKY model, used to estimate the Mn-hole exchange integral Jpd for the diluted Zn/0.997/Mn/0.003/GeAs/2/ sample, makes possible to estimate the value of Jpd =(0.75+/-0.09) eV., Comment: 10 pages, 7 figures

We present the experimental evidence for the presence of spinodal decomposition of the magnetic ions in the Ge/1-x-y/Cr/x/Eu/y/Te samples with chemical composition varying in the range of 0.015 < x < 0.057 and 0.003 < y < 0.042. The ferromagnetic transition at temperatures 50 < T < 57 K was observed, independent of the chemical composition. The long-range carrier mediated itinerant magnetic interactions seem to be responsible for the observed ferromagnetic order. The magnetic irreversibility with coercive field H/C/ = 5?63 mT and the saturation magnetization M/S/

Magnetotransport properties of spin-glass-like Ge/1-x-y/Sn/x/Mn/y/Te mixed crystals with chemical composition changing in the range of 0.083 < x < 0.142 and 0.012 < y < 0.119 are presented. The observed negative magnetoresistance we attribute to two mechanisms i.e. weak localization occurring at low fields and spin disorder scattering giving contribution mainly at higher magnetic fields. A pronounced hysteretic anomalous Hall effect (AHE) was observed. The estimated AHE coefficient shows a small temperature dependence and is dependent on Mn-content, with changes in the range of 10E-7 < R_S < 10E-6 m^3/C. The scaling law analysis has proven that the AHE in this system is due to the extrinsic mechanisms, mainly due to the skew scattering accompanied with the side jump processes., Comment: 7 pages, 4 figures

We present the studies of magnetic properties of Ge/1-x/Cr/x/Te diluted magnetic semiconductor with changeable chemical composition 0.016 \leq x \leq 0.061. A spin-glass state (at T \leq 35 K) for x = 0.016 and 0.025 and a ferromagnetic phase (at T < 60 K) for x \geq 0.030 are observed. The long range carrier-mediated magnetic interactions are found to be responsible for the observed magnetic ordering for x < 0.045, while for x \geq 0.045 the spinodal decomposition of Cr ions leads to a maximum and decrease of the Curie temperature, TC, with increasing x. The calculations based on spin waves model are able to reproduce the observed magnetic properties at a homogeneous limit of Cr alloying, e.g. x < 0.04, and prove that carrier mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction is responsible for the observed magnetic states. The value of the Cr-hole exchange integral, Jpd, estimated via fitting of the experimental results with the theoretical model, is in the limits 0.77...0.88 eV., Comment: 8 pages, 7 figures

We investigated the nature of the magnetic phase transition in the Ge/1-x-y/Sn/x/Mn/y/Te mixed crystals with chemical composition changing in the range of 0.083 < x < 0.142 and 0.012 < y < 0.119. The DC magnetization measurements performed in the magnetic field up to 90 kOe and temperature range 2-200 K showed that the magnetic ordering at temperatures below T = 50 K exhibits features characteristic for both spin-glass and ferromagnetic phases. The modified Sherrington - Southern model was applied to explain the observed transition temperatures. The calculations showed that the spin-glass state is preferred in the range of the experimental carrier concentrations and Mn content. The value of the Mn hole exchange integral was estimated to be J/pd/ = 0.45+/-0.05 eV. The experimental magnetization vs temperature curves were reproduced satisfactory using the non-interacting spin-wave theory with the exchange constant J/pd/ values consistent with those calculated using modified Sherrington - Southern model. The magnetization vs magnetic field curves showed nonsaturating behavior at magnetic fields B < 90 kOe indicating the presence of strong magnetic frustration in the system. The experimental results were reproduced theoretically with good accuracy using the molecular field approximation-based model of a disordered ferromagnet with long-range RKKY interaction., Comment: 9 pages, 6 figures

We present the study of magnetic and transport properties of Pb(1-x-y-z)Mn(x)Sn(y)Eu(z)Te. AC magnetic susceptibility measurements as well as transport characterization were performed. The obtained results indicate that the presence of two types of magnetic ions influences magnetic properties of investigated IV-VI semimagnetic semiconductor. A qualitative analysis and possible mechanisms of the substantial dependence of the Curie temperature on the Eu content are presented. The most likely reason of the observed Curie temperature behaviour is a strong dependence of the location of a heavy mass Sigma band upon the alloy composition. The theoretical calculations in frame of simple models confirm this point., Comment: 14 pages, 6 figures

The Landau-Lifshitz equation for the magnetization dynamics of a single-domain magnetic system is solved using the methods of self-organization. The description takes into account the torque due to spin transfer. The potential energy of the system includes the uniaxial and easy-plane anisotropies, and the Zeeman energy due to an external magnetic field. The equilibrium and stationary states are investigated as a function of the spin current and external magnetic field. The presented bifurcation diagram allows to determine the margins of a neutral stability mode of the equilibrium and stationary states for different values of the easy-plane anisotropy constant. Using the power spectral density method, the trajectory tracing, Hausdorff dimension, and maximum Lyapunov exponent, the dynamics of the phase states in an external magnetic field is demonstrated. The analytical transcendent equations for switching between different equilibrium states are also obtained, proving the importance of phase averaging., Comment: 15 pages with 15 figures

Thermoelectric power, electrical conductivity, and high field Hall effect were studied over a broad temperature range in ferromagnetic Ga(1-x)Mn(x)As epitaxial layers (0.01570 K) the thermoelectric power in GaMnAs linearly increases with increasing temperature. That indicates the presence of a degenerate hole gas with the Fermi energy EF=220+-25 meV, nearly independent of Mn content (for 0.02

We studied magnetoresistance (MR) of nanoconstrictions created in (Ga,Mn)As epilayers by O+ ion implantation. Original layers exhibit a negative MR that is plausibly caused by weak localization (WL) effects at the lowest temperatures. In constricted samples, additionally, jumps of an enhanced conductance appear on the background of the negative MR, whose positions reflect the hysteresis of magnetization. We argue that they are manifestation of a suppression of WL due to the nucleation of a domain wall in the constriction, Comment: 12 pages, 4 figures

We investigate theoretically the influence of the spin-orbit interaction of Rashba type on the magnetoresistance of a semiconducting ferromagnetic nanostructure with a laterally constrained domain wall. The domain wall is assumed sharp (on the scale of the Fermi wave length of the charge carriers). It is shown that the magnetoresistance in such a case can be considerably large, which is in a qualitative agreement with recent experimental observations. It is also shown that spin-orbit interaction may result in an increase of the magnetoresistance. The role of localization corrections is also briefly discussed., Comment: 5 pages, 2 figures

Structural investigation of as-grown as well as annealed Ga(1-x)Mn(x)As epilayers was carried out using high resolution X-ray diffraction (XRD) measurements for a wide range of Mn concentrations (0.027<=x<=0.083), with special attention on how the interstitial Mn atoms (Mn_I) influence the lattice parameter of this material. We observe a distinct decrease of the lattice parameter after low temperature annealing of Ga(1-x)Mn(x)As, which is known to reduce the Mn_I concentration. The reciprocal space maps measured for all the investigated samples showed that the Ga(1-x)Mn(x)As layers are fully strained - i.e., they remain pseudomorphic to the GaAs (001) substrate - for the entire thickness of the samples used (in the present case over 100 nm). In all cases studied the XRD measurements revealed high crystalline perfection of both as-grown as well as annealed Ga(1-x)Mn(x)As epilayers, Comment: 26 pages

High-field magnetic measurements performed with the use of magnetooptical Kerr effect (MOKE) in the polar configuration as well as high-field and low-field magnetotransport studies were carried out on Ga1-xMnxAs epilayers grown by low temperature molecular beam epitaxy, and subsequently annealed under various conditions. The structural investigations by means of high resolution XRD were also performed. We observe significant changes in magnetoresistivity curves, magnetization and strain introduced by the annealing., Comment: International Conference on Magnetism, ICM 2003, Roma; to be published on Journal of Magnetism and Magnetic Materials

High- and low-field magneto-transport measurements, as well as SQUID measurements of magnetization, were carried out on Ga1-xMnxAs epilayers grown by low temperature molecular beam epitaxy, and subsequently annealed under various conditions. We observe a large enhancement of ferromagnetism when the samples are annealed at an optimal temperature, typically about 280 0C. Such optimal annealing leads to an increase of Curie temperature, accompanied by an increase of both the conductivity and the saturation magnetization. A decrease of the coercive field and of magnetoresistivity is also observed for Ga1-xMnxAs annealed at optimal conditions. We suggest that the experimental results reported in this paper are related to changes in the domain structure of Ga1-xMnxAs., Comment: 2-nd International Conference on Physics and Application of Spin Related Phenomena In Semoconductors, Wurzburg 23-26 July 2002, to be published in a special issue of the Journal of Superconductivity/Novel Magnetism

The Anomalous Hall Effect was investigated in IV-VI ferromagnetic semimagnetic semiconductors of Sn1-xMnxTe codoped with either Eu or Er. The analysis of experimental data: Hall resisitivity and magnetization showed that AHE coefficient RS depends on temperature, its value decreases with the temperature increase. We observe that above ferromagnet-paramagnet transition temperature RS changes sign. We discuss the possible physical mechanisms responsible for observed temperature dependence of RS, particularly change of the sign., Comment: 2nd International Conference on Physics and Application of Spin Related Phenomena In Semiconductors, Wurzburg 23-26 July 2002, to be published in aspecial issue of the Journal of Superconductivity/Novel Magnetism

We present the results of low temperature (LT) annealing studies of Ga1-xMnxAs epilayers grown by low temperature molecular beam epitaxy in a wide range of Mn concentrations (0.01