Anisotropic upper critical field of chemically substituted MgB2 single crystals studied by torque magnetometry

The influence of Al, Mn, and Fe ion substitution for Mg and C ion substitution for B on the anisotropy of upper critical field (Hc2) of high quality MgB2 single crystals was studied. The torque investigations of Mg1-xAlxB2 crystals showed the slight increase of Hc2parallelc for the samples with small x and significant reduction of Hc2 anisotropy, γHc2, at lower temperatures, as compared to the value for unsubstituted crystals. In Mg(B0.94C0.06)2 single crystals, Hc2parallelc(0) ∼ 85 kOe is more than twice as large as that for unsubstituted MgB2. Anisotropy of Hc2 decreases to about 4 at low temperatures, i.e., to the value considerably lower than that for MgB2, and its temperature dependence is much less pronounced. The corresponding Hc2parallelab(0) ∼ 340 kOe is close to the maximum possible enhancement of Hc2 due to the chemical substitutions. In addition to the increased scattering of carriers, substitution of magnetic Mn ions for Mg, strongly suppresses Tc and Hc2; however, it is not the case for the substitution of Fe ions for Mg