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Switching on Antiferromagnetic Coupled Superparamagnetism By ARTICLE pubs.acs.org/JPCC Switching on Antiferromagnetic Coupled Superparamagnetism by Annealing Ferromagnetic Mn/CdS Nanoparticles ‡ ‡ § § Balaji Sambandam, Nallaiyan Rajendran, Moorthi Kanagaraj, Sonachalam Arumugam, and † || Periakaruppan T. Manoharan*, , † Sophisticated Analytical Instruments Facility and Department of Chemistry, Indian Institute of Technology/Madras, Chennai 600 036, India ‡ Department of Chemistry, Anna University, Chennai 600 025, India § Centre for High Pressure Research, School of Physics, Bharathidasan University, Tiruchirappalli 620 024, India bS Supporting Information ABSTRACT: Mixed phases of metastable cubic and stable hexagonal manganese assisted CdS nanoparticles were synthesized by a colloidal route. These phases were quantified by the Short and Steward method of X-ray diffraction and differential scanning calorimetry analysis. The photoluminescence (PL) emission observed around ∼550 nm for undoped CdS is found þ to be due to cadmium vacancy defects; on increased addition of Mn2 both the intensity and the line width of this emission continuously but disproportionately decrease and a new emission around ∼585 nm, interpreted to be due to manganese dÀd emission, emerges with increased intensity but with no change in line width. The disproportionate decrease in the area and bandwidth of the ∼550 nm emission reveals that the intensity decrease is not a manifestation of the line width change. This decrease in intensity, i.e., the transition probability, is attributed to both the quenching effect by manganese and reduction in Cd þ vacancy defects due to substitution by Mn2 . The enriched 5% Mn/CdS was subjected to annealing in order to find out any possible changes or transformation during the annealing proces. An increase in the electron paramagnetic resonance intensity of preannealed sample on lowering temperature, much more than suggested by the Boltzmann population difference, indicates it to be ferromagnetic. On the other hand, postannealed sample exhibits antiferromagnetic coupled superparamagnetism as revealed by a drastic reduction in intensity down to the Neel temperature (TN) with unaltered line width and a substantial decrease in line width below TN. This conversion from ferromagnetism to antiferromagnetic coupled superparamagnetism after annealing is strongly supported by magnetic measurements, in which the preannealed sample exhibits increased coercivity on lowering the temperature, a case of ferromagnetism, and the postannealed material, however, exhibits low coercivity along with exchange bias below T , representing superparamagnetism. This conversion is þ N due to ionic migration of Mn2 as supported by magnetic and PL measurements. High-resolution transmission electron microscopy confirms the morphology and size of nanoparticles. 1. INTRODUCTION temperature ferromagnetism (FM), ferrimagnetism, superpara- The field of dilute magnetic semiconductors (DMSs) is an magnetism (SPM), or other complex magnetic behaviors be- interesting area as the doping agent can modify or modulate the cause of their potential application for making of devices or just optical, magnetic, and structural behaviors.1,2 Introduction of for pure theoretical/experimental interests. The resultant mag- even a small concentration of magnetic metal ion in DMS could netic properties depend on synthetic methodologies, concentra- play a crucial role by changing the host structure by either tion, site occupation, clustering of ions, thermal treatment, 3 4,5 physical state, size of quantum dots, or cluster, and shape of complete transformation or stepwise transformation as a 9,10 function of its concentration. Complete transformation is possi- the host materials. The ferromagnetism is said to be caused by fi spÀd exchange interactions of the band carriers with localized ble in certain cases on the addition of a de nite concentration of 11 À the magnetic or nonmagnetic ions.3 However, the latter stepwise magnetic ions. By comparison, most of the II VI DMSs show transformation always leads to a mixture of two phases. The diverse magnetic phases, ranging from paramagnetism (PM), spin-glass-like antiferromagnetism (AFM), to ferromagnetism at percentage of such mixed phases can be elucidated by methods 12 ff low or very low temperature. Only a few show ferromagnetism such as powder X-ray di raction through Debye function analysis 13 (DFA),6 the Short and Steward method,7 simulation,8 and by at room temperature. On the other hand, many examples of the differential scanning calorimetry (DSC).5 However, their physi- cal properties depend on the percentage of phases, nature of the Received: February 23, 2011 metal ion, and size and shape of the host matrix. An important Revised: May 12, 2011 tunable property is magnetism as part of a search for room Published: May 13, 2011 r 2011 American Chemical Society 11413 dx.doi.org/10.1021/jp2017902 | J. Phys. Chem. C 2011, 115, 11413–11419 The Journal of Physical Chemistry C ARTICLE Mn/CdS are known to exhibit ferromagnetic properties around cryogenic temperatures.14,5 More often, manganese-doped CdS and CdSe have been reported to exhibit superparamagnetism15 or antiferromagnetism coupled superparamagnetism16 at low temperature. We report here the magnetic properties observed in mixed phases of zinc blende and wurtzite structure of manganese-doped CdS nanoparticles which have been confirmed and quantified by both powder XRD (Short and Steward method7) and DSC þ measurements.5 Though the nanoparticles of 5 mol % Mn2 / CdS samples prepared in the medium of CTAB surfactant and propylene diamine (PD) ligand prior to annealing exhibits low temperature ferromagnetic behavior the same after annealing reveals antiferromagnetism with a Neel temperature (TN)of þ 32 K, followed by superparamagnetism below this temperature Figure 1. DSC profile for CTAB/BD without Mn2 assisted CdS and fi þ due to uncompensated surface spins. This nding has been CTAB/PD with and without Mn2 assisted CdS nanoparticles. supported by electron paramagnetic resonance (EPR) measure- ments as it can reveal AFM and SPM nature at lower temperature over the same temperature range and measuring field using a through a measurement of temperature-dependent intensity and field-cooled (FC) protocol in a cooling field of 100 Oe. The field- line width. dependent dc magnetic susceptibility was measured over a range of þ3toÀ3 T at temperatures ranging from 8 to 300 K. The 2. MATERIALS AND METHODS electron paramagnetic resonance (EPR) measurements were 2þ made by using a Bruker X-band continuous wave EPR both at 2.1. Preparation of CTAB/PD Assisted CdS and Mn /CdS room temperature (300 K) and at liquid nitrogen temperature Nanoparticles. The chemicals used in our experiments, cetyl (77 K) and also by a Bruker ELEXSYS 500 (Bruker BioSpin) trimethylammonium bromide (Loba Chemie, 99.5%), propyle- down to 8 K. JEOL JEM 3010 operating at 300 kV was used for nediamine (Alfa Aesar, 98%), cadmium nitrate tetrahydrate high-resolution TEM studies for sample imaging. Samples were (Merck, 99%), manganese acetate terahydrate (Merck, 99.5%), dispersed in ethanol, and a drop of the resultant colloid was and thiourea (Rankem, 99%), were used as purchased. Milli-Q placed on a carbon-coated copper grid and allowed to dry at Millipore ultrapure water was used for all reactions. room temperature under ambient conditions. EDS patterns were Three grams of CTAB was dissolved in very minimum amount recorded using the attached Philips CM12 TEM analyzer. The of ultrapure distilled water. To this 0.308 g of cadmium nitrate atomic percentages have been calculated using the software in the and 10 mL of 1, 3-diaminopropane (propylenediamine, PD) instrument, which takes into consideration all parameters for fl were added and the resultant solution was kept for re uxing with correct measurement. constant stirring in nitrogen atmosphere. During the refluxing, 0.228 g of thiourea was added and the refluxing continued for a few hours at 120 °C. The appearance of a pale yellow precipitate 3. RESULTS AND DISCUSSION indicates the formation CdS nanoparticles. The precipitate was washed with distilled water and methanol several times. Finally 3.1. Identification of Phases before and after Annealing. the product was thoroughly dried in the oven at 80 °Cfor3h. The percentage ratio of cubic to hexagonal phases has been The same procedure was adopted for other preparations of elucidated by DSC measurement. Figure 1 depicts the quantita- 0.5, 1, and 5% with appropriate manganese and cadmium tive comparison of the DSC curves for CTAB/BD/0% Mn-CdS concentrations. and CTAB/PD assisted with and without manganese-doped CdS 2.2. Measurement Details. A Bruker Discover D8 diffract- nanoparticles. Here PD and BD stand for propylenediamine and ometer with a Cu KR rotating anode source was used for X-ray butylenediamine, respectively. Quantitative calculation of the powder diffraction analysis. The powdered Mn/CdS nanoparti- cubic phase present in the mixed phases of CTAB/PD with and cles were placed in a glass plate and measured. The X-ray pattern without manganese-assisted CdS nanoparticles is made possible was identified by comparison to phases in the powder diffraction by comparing them with 100% cubic nature of CTAB/BD/0% file (PDF) database (ICDD, release 2000). DSC measurements Mn-CdS5 as the base or standard. The measurements on all have been done by using a DSC 204 Netzsch Thermal Analyzer. CTAB/PD/X% Mn-CdS (X =0À5) samples were performed In order to quantify the percentages of the phases, we have done a under the same conditions of CTAB/BD/0% Mn-CdS systematic measurement with accurately weighed samples. All nanocrystals5 using the same quantity of samples in all experi- measurements were made under a nitrogen atmosphere with a ments. In the case of CTAB/BD assisted 0% Mn-CdS, the cubic heat flow rate of 5 °C/min. The magnetic susceptibility was to hexagonal transition (phase conversion) is 100% due to the measured using Quantum Design 9T PPMS with VSM and presence of only the cubic component prior to DSC measure- helium reliquifier magnetometer and Lakeshore VSM 7410. For ment, a case of 100% conversion from cubic to hexagonal.
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