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2009 MAGNETIC SYSTEMSMagnetotransport in a disordered Zn 1-x Mn x GeAs 2 alloyMagnetotransport studies in high magnetic fields are a veryeffective tool for studying the interplay between the electronicand magnetic properties of magnetic materials. Inthis work the results of the magnetoresistance and Hall effectstudies in Zn 1-x Mn x GeAs 2 (0.053 ≤ x ≤ 0.182) are presented.The investigated compound Zn 1-x Mn x GeAs 2 belongsto the II-IV-V 2 group of diluted magnetic semiconductors.Our recent studies of magnetic properties of thisalloy [L. Kilanski, et al., Acta Phys. Pol. A 114, 1151(2008)] showed that the transition from a Curie-Weiss paramagnetinto a ferromagnetic material with Curie temperaturesexceeding 300 K occurs in this system for x ≥ 0.06.The high Curie temperature observed in this alloy is probablyconnected with the presence of nano-sized MnAs grainsin the crystal.The results of Hall effect investigations in the form of magneticfield dependence of the off-diagonal resistivity tensorcomponent ρ xy are presented in figure 106. It is interestingto note that in the Zn 1-x Mn x GeAs 2 crystals we do notobserve the anomalous Hall effect, a phenomenon widelyobserved in ferromagnetic semiconductors. The ρ xy (B)dependence in the case of ferromagnetic Zn 1-x Mn x GeAs 2(x ≥ 0.06) crystals showed the presence of large nonlinearitiesat temperatures lower than 9 K. The bending of theρ xy (B) curve is the most prominent in the case of the samplewith the highest concentration of Mn ions. Such behaviormay indicate the presence of two types of conductingcarriers influencing the carrier transport in this material.It is well established that the magnetic properties of crystalshave a significant influence on the electron transport inZn 1-x Mn x GeAs 2 samples with different Mn content. Apartfrom changes in the basic transport properties such as carrierconcentration and resistivity, we have also observed asignificant differences in the high field magnetoresistanceand the Hall effect. The magnetic field dependence of theresistivity tensor component ρ xx recorded at T ≈ 1.45 K arepresented in figure 105.In the case of the paramagnetic Zn 0.947 Mn 0.053 GeAs 2 samplewe observe a small negative magnetoresistance saturatingat moderate magnetic fields of about B ≈ 50 kOe. Theorigin of this behavior of the magnetoresistance curve is thespin-disorder scattering of the conducting carriers on theMn magnetic moments embedded inside the semiconductorhost. It may be added that the negative magnetoresistance isobserved only at temperatures lower than 5 K. The positivecontribution to the magnetoresistance observed clearly athigher temperatures results from the orbital carrier movementin the presence of the external magnetic field. Thispositive contribution competes at low temperatures with theeffect of spin disorder scattering causing the curves to showa minima.In the case of the two ferromagnetic Zn 1-x Mn x GeAs 2 sampleswith x > 0.06 we observed negative magnetoresistancewith an amplitude much larger than in the case of the paramagneticcrystal. The observed effect is interpreted as agiant magnetoresistance due to the polarization of conductingcarriers inside ferromagnetic grains (similar to that observedin the case of granular ferromagnets). The amplitudeof the giant magnetoresistance is highly compositiondependent due to changes in the concentration of magneticinclusions in the material.Figure 105: Magnetoresistance curves obtained at T ≈1.45 K forZn 1-x Mn x GeAs 2 crystals with different chemical composition x.Figure 106: Magnetic field dependence of the Hall resistivity ρ xymeasured at T ≈ 1.45 K for Zn 1-x Mn x GeAs 2 crystals with differentchemical composition x.D. K. MaudeL. Kilanski, W. Dobrowolski (Institute of Physics Polish Academy of Science, Warsaw, Poland),S. A. Varnavskiy, S. F. Marenkin (Kurnakov Institute of General and Inorganic Chemistry RAS, Moscow, Russia)79