Magnetic and Electrical Properties of the ZnGeAs2 : Mn Chalcopyrite

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2122KOROLEVA et al.M, emu/g43210M, emu/g1.00.80.60.40.2H = 600 OeZFCFCH = 50 Oe0 50 150 250 350T, K100 200 300 400T, KFig. 1. Temperature dependence of the magnetization M forthe composition ZnGeAs 2 with 3.5 wt. % Mn in magneticfields of 50 kOe and 0.6 kOe (inset). The ZFC curve: thesample was zero-field-cooled from 400 to 5 K, after whichits magnetization was measured under heating. The FCcurve: the sample was cooled in a field of 0.6 kOe from 400to 5 K with parallel measurement of its magnetization.and ZnSnAs 2 : Mn [8], compounds with the Curie pointas high as 350 K. We reported on the compoundCdGeAs 2 : Mn, whose Curie temperature T C was stillhigher, 355 K.The II–IV–V 2 ternary semiconductors have beenknown for a long time. These compounds are crystallochemicaland electronic analogs of the III–V semiconductors.Interest in these compounds emerged afterone discovered their unique nonlinear optical properties,namely, the high nonlinear polarizability and birefringence,which opened a way to their use for parametricconversion of laser radiation in the mid-IR range.The most promising for this purpose are high-purityCdGeP 2 , CdGeAs 2 , and ZnGeP 2 crystals. The II–IV–V 2 -type compounds retain the main features characteristicof the III-V materials, namely, the predominantlycovalent bonding, small effective carrier masses, relativelyhigh mobilities of the electrons and holes, andpersistence of the absolute minima and maxima of theconduction and the valence band at Brillouin zone center.We prepared and studied new ZnGeAs 2 : Mn compoundswith the Curie point reaching 367 K, a recordhighvalue for II–IV–V 2 -type compounds. TheZnGeAs 2 compound studied in the present work possessesthe following characteristics: bandgap 0.85 eV;mobility ~10 2 cm 2 /V s (for holes with concentrations of10 18 –5 × 10 19 cm –3 ); effective hole mass 0.4–0.74. Thismakes the II–IV–V 2 : Mn compounds promising forspintronics applications.2. EXPERIMENTAL TECHNIQUEWe report here on a study of the magnetization,electrical resistivity ρ, magnetoresistance ∆ρ/ρ = (ρ H –ρ H = 0 )/ρ H = 0 and the Hall effect of polycrystallineZnGeAs 2 : Mn samples with Mn contents of 1.5 to3.5 wt. %. The magnetization was measured with aSQUID magnetometer. The four-probe technique wasemployed to measure ρ and ∆ρ/ρ. The Hall effect wasstudied by the standard dc method. The contacts to thesamples were secured with current-conducting glue.3. EXPERIMENTAL RESULTSAND DISCUSSIONThe ZnGeAs 2 samples were prepared by directalloying of high-purity powders of ZnAs 2 and Ge takenin stoichiometric ratio. Mn was introduced according tothe hypothetical ZnGeAs 2 –MnGeAs 2 cut. To increasethe Mn solubility, the rate of cooling from 900°C waschosen not lower than 5–10 K/s. X-ray diffraction analysisshowed all samples with the Mn contents of 1.5, 3,and 3.5 wt. % to be single phase, thus identifying themas ZnGeAs 2 . A comparison of the lattice constantsrevealed that the cell volume decreases with increasingMn content, which suggests formation of solid solutionsthrough Mn substitution for Zn. The contents ofthe components were checked by x-ray fluorescence. Astudy of the distribution of elements over the samplelength showed that Zn : Ge : As = 1 : 1 : 2. Within experimentalerror, the Mn was found to be distributed uniformlyover the sample length.Figure 1 displays temperature dependence of themagnetization, M(T), for the composition with 3.5 wt.% Mn in a magnetic field H = 50 kOe. The M(T) curveobtained for T > 60 K is seen to be characteristic of aferromagnet. For T < 60 K, however, magnetization isobserved to grow strongly with decreasing temperature,an effect that can be interpreted as an additional contributiondue to the superparamagnetic phase. The inset toFig. 1 shows the M(T) plot measured in a weak magneticfield of 600 Oe. The M(T) curves obtained in astrong (50 kOe) and a weak (0.6 kOe) field are immediatelyseen to differ substantially. When measuring inthe weak field, the magnetization is observed to dropstrongly as the temperature falls below T S = 86 K, withM decreasing 4.5 times. At T k = 10 K, this drop stops,and here one sees a difference between the magnetizationsof the sample cooled from T > T C in this weakmagnetic field (FC curve) and the one that was zerofieldcooled (ZFC curve) The FC curve exhibits a risewith further decrease in the temperature. Shown inFig. 2 are M(T) curves in the temperature intervalincluding T S , which were measured in magnetic fieldsof 3, 6, 8, 9, and 11 kOe. Examining this figure showsthat as H increases, the decrease in the magnetization atT = T S becomes weaker to disappear altogether at11 kOe, and the value of T S shifts toward lower temper-PHYSICS OF THE SOLID STATE Vol. 49 No. 11 2007
MAGNETIC AND ELECTRICAL PROPERTIES 21232.52.0H = 11 kOe986M, emu/g0.10M, emu/g1.53–0.1ZFCFCZFC–0.2 00.2H, kOe1.00 40 80 120T, KFig. 2. Temperature dependence of the magnetization M forthe composition ZnGeAs 2 with 3.5 wt. % Mn in differentmagnetic fields in the temperature range from 5 to 100 K.Each curve was measured after the sample had been cooledto 5 K with no magnetic field.atures. Indeed, as the field increases from 600 Oe to9 kOe, T S decreases from 86 to 49 K. This is paralleledby the rise in the M(T) curves observed to occur for T
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Magnetic and Electrical Properties of the ZnGeAs2 : Mn Chalcopyrite

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