2.4.1 Sample preparation and characterization
DMS samples of the three different subgroups of the group II-V semiconductors, investigated in this work, were obtained as follows.
Single crystals of (Zn1-xMnx)3As2, belonging to the II3-V2 subgroup, were grown with the modified Bridgman method or slow cooling of a melt in the presence of a temperature gradient [11]. Stoichiomentric amounts of Zn3As2 and Mn3As2 were synthesized, sealed in a carbon-coated evacuated quarts ampoule and put into a vertical furnace. The crystal growth temperature was adjusted between ~ 780 − 8000 C and the speed of cooling was kept at ~ 10 C/h in the region of crystallization temperatures and near the to phase transition temperature [56]. Further preparation details of materials of the II3-V2 subgroup of the II-V semiconductors, doped with Mn, one can find in [8, 10, 57]. Single crystals of (Zn1-xMnx)3As2 obtained with the method above were investigated with X-ray diffraction techniques, supporting the composition, homogeneity and crystal quality of the low-temperature-phase Cd3As2-type crystal structure observed in all samples with x up to ~ 0.135 [11]. Abovex ~ 0.15 inhomogeneities of a secondary phase of the compound ZnMn2As2 were found to segregate in the (Zn1-xMnx)3As2 alloys [8, 11, 58].
Zn1-xMnxAs2 belonging to subgroup II-V2 was neither obtained nor investigated earlier. Main features of the preparation method of this material are given below [Publication 1].
On the first stage formation of Zn1-xMnxAs2 solid solutions within the system of ZnAs2− MnAs2 was investigated by synthesizing samples in the interval of 0 ≤x≤ 0.5 with the modified Bridgman method. A mixture of stoichiometric amounts of Zn, Mn and As was put into a carbonized quartz ampoule to synthesize polycrystalline ingots in vacuum at 10000 C. Then the material was cooled down to 8000 C at a speed of 500 C/h and then to 7000 C with cooling rate of 100 C/h. Analysis with the X-ray powder diffraction method revealed a
monoclinic structure with space groupC2h5 and the lattice parametersa = 9.28 Å,b = 7.68 Å and c = 8.03 Å. The upper boundary of Zn1-xMnxAs2 solid solutions at x ~ 0.2 − 0.3 was established, as well. A good coincidence of positions of the diffraction peaks with those of ZnAs2 was observed. Clear X-ray fluorescence background and changes in intensities of the X-ray diffraction lines indicated presence of Mn in the compound. However, no variation of the lattice parameters with the composition was observed [Publication 1], all of them were found close to those of ZnAs2 [ 56, 59].
To obtain single crystals of Zn1-xMnxAs2 withx between 0 − 0.2 the amount of the starting materials was enlarged, the time of the synthesis at 10000 C was increased to 20 h and the cooling rate in the crystallization region was decreased down to ~ 1.5 − 20 C /h. The results of X-ray investigations of the Zn1-xMnxAs2 single crystals exhibited good agreement with those of the polycrystalline material, with evidence of negligible distortions of the ZnAs2
type structure by substitution of Zn for Mn up tox = 0.2. A secondary phase was not detected in these investigations, too. X-ray phase analysis yielded the same picture of diffraction lines for samples obtained from different parts of ingots [Publication 1].
Single crystals of CdSb doped with 2 at % of Ni were obtained with the modified Bridgman method, as well, using a following two-stage procedure [Publication 3]. At first Ni was dissolved in Cd by thermal annealing of the melt at 7000 C for 8 h. On the second stage stoichiometric amounts of Cd:Ni and Sb were put into a quartz ampoule with a thin graphite layer, evacuated and filled with Ar gas to pressure of 0.1 atm. The material was kept at 4600 C for 12 h and then cooled down at the speed of 0.50 C/h. The X-ray diffraction analysis of the ingots with volume of ~ 1 cm3 revealed the single-phase material, having the orthorhombic structure, the space group D152h and the lattice parameters same as in undoped CdSb [Publication 3]. The direction of growth of the ingots had the angle of 50 0 ± 50 with the crystallographic axis [100].
2.4.2 Magnetic measurements
Unoriented single crystals of (Zn1-xMnx)3As2 withx = 0.08 − 0.13 and of Zn1-xMnxAs2 withx
= 0.01 − 0.1 were used for investigations of the magnetization, as well as an oriented sample of the latter compound withx = 0.05. Samples of p-CdSb:Ni with the shape of rectangular prisms of dimensions 6.0×2.0×2.2 mm3, 6.0×1.8×2.4 mm3 and 6.0×2.1×2.0 mm3 were cut
from single-crystalline ingots to have the longest edge parallel to the crystallographic axes [100], [010] and [001], respectively, to be oriented along the magnetic field.
The dc magnetization,M, was investigated with a SQUID magnetometer separately between 3 − 310 K and 250 − 500 K in fieldsB ≤ 6 T after annealing the sample for 0.5 − 2 h at temperatures ~ 150 − 2000 C to avoid any influence of a remanent magnetization. The dependence ofM (T) was measured at Bbetween 2 G and 15 kG after cooling the sample from 300 K to 2 − 5 K in zero field (B < 0.1 G ) or in the field of the measurements, yielding the data in the zero-field cooled (MZFC) and the field-cooled (MFC) regimes, respectively.
Investigations of the thermoremanent magnetization (TRM) were performed after cooling the sample from 300 K down to 5 K in a field with subsequent reducing it to zero. Control of the sample temperature with flowing helium gas was realized to an accuracy of 0.2 %, as measured using a carbon-glass thermometer.
2.4.3 Transport measurements
Investigations of the longitudinal and the Hall resistivities were made using crystalline samples of p-CdSb:Ni shaped to rectangular prisms of sizes 5.5×1.9×2.3 mm3, 5.5×1.5×2.3 mm3 and 5.5×1.9×2.0 mm3. The samples were cut from the ingots so that their longest edges were oriented along the [100] (sample 1), [010] (sample 2) and [001] (sample 3) axes, respectively. The measurements of (T) andH (T) were performed by recording the signals from two different pairs of the potential contacts and one pair of the Hall contacts, respectively. For measurement in zero field or in pulsed magnetic fields up to 25 T the sample was put in a He exchange gas Dewar and temperature was varied between 1.5 and 300 K with accuracy of 0.5 %. MR was investigated in the transversal magnetic field configuration ofj ||
[100] andB || [001] (# 1), j || [010] andB || [100] (# 2) andj || [001] andB || [010] (# 3). The pulse length of the magnetic field was 8 ms, the error of B did not exceed 5 % and the inhomogeneity was less than 0.3 %.
3 Magnetic properties of II-V semiconductors doped with Mn and Ni