Magnetoresistance control in granular Zn1-x-y CdxMnyGeAs2 nanocomposite ferromagnetic semiconductors.

We present studies of structural, magnetic, and electrical properties of Zn_1-x-y Cd_xMn_yGeAs₂ nanocomposite ferromagnetic semiconductor samples with changeable chemical composition. The presence of MnAs clusters induces the studied alloy room temperature ferromagnetism with the Curie temperature, T_C, 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. [ABSTRACT FROM AUTHOR]