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It is thought that the proposed new family of multi-functional materials, namely, the ferroelectric thermoelectrics may exhibit enhanced functionalities due to the coupling of the thermoelectric parameters with ferroelectric polarization in solids. Therefore, the ferroelectric thermoelectrics are expected to be of immense technological and fundamental significance. As a first step towards this direction, it is most important to identify the existing high performance thermoelectric materials exhibiting ferroelectricity. Herein, through the direct measurement of local polarization switching, we show that the recently discovered thermoelectric semiconductor AgSbSe2has local ferroelectric ordering. Using piezo-response force microscopy, we demonstrate the existence of nanometer scale ferroelectric domains that can be switched by external electric field. These observations are intriguing as AgSbSe2crystalizes in cubic rock-salt structure with centro-symmetric space group (Fm-3m), and therefore, no ferroelectricity is expected. However, from high resolution transmission electron microscopy measurement, we found the evidence of local superstructure formation which, we believe, leads to local distortion of the centro-symmetric arrangement in AgSbSe2and gives rise to the observed ferroelectricity. Stereochemically active 5S2lone-pair of Sb may also give rise to local structural distortion thereby creating ferroelectricity in AgSbSe2. © 2014 AIP Publishing LLC.

World University root node

Department of Metallurgical & Materials Engineering

Applied Physics Letters

Direct evidence of strong local ferroelectric ordering in a thermoelectric semiconductor

In this article, we report on the Co atom incorporation, secondary phase formation and composition-dependent magnetic and optical properties of mixed valence Co(II, III)xZn1-xO epitaxial thin films grown by pulsed laser deposition. The intended total Co concentration is varied between ~6-60 at.% with relatively higher concentration of +3 over +2 charge state. Mixed valence Co(II, III) shows high solubility in ZnO (up to 38 at.%) and ferromagnetism is observed in samples with total Co incorporation of ~29 and 38 at.%. Electron diffraction pattern and high resolution transmission electron microscopy images reveal single crystalline nature of the thin films with wurtzite structure. Co oxide interlayer, with both rock salt and spinel structure, are observed to be formed between the substrate and wurtzite film for total Co concentration at ~17 at.% and above. Magnetization shows composition dependence with a saturation moment value of ~93 emu cm-3 and a coercive field of ~285 Oe observed for ~38 at.% Co:ZnO films. Ferromagnetism was not observed for films with Co concentration 17 and 9 at.%. The Co oxide interlayer does not show any ferromagnetism. All the films are n-type with carrier concentration ~1019 cm-3. The observed magnetism is probably resulting from direct antiferromagntic exchange interaction between Co2+ and Co3+ ions favored by heavy Co alloying giving rise to ferrimagnetism in the system. © 2013 Elsevier B.V.

Journal of magnetism and magnetic materials

Structural and magnetic characterization of mixed valence Co (II, III) xZn1− xO epitaxial thin films

We report here the growth of epitaxial Co-metal thin film on c-plane sapphire by pulsed laser deposition (PLD) using Co:ZnO target utilizing the composition inhomogeneity of the corresponding plasma. Two distinct plasma composition regions have been observed using heavily alloyed Co 0.6Zn0.4O target. The central and intense region of the plasma grows Co:ZnO film; the extreme tail grows only Co metal with no trace of either ZnO or Co-oxide. In between the two extremes, mixed phases (Co+Co-oxides+Co:ZnO) were observed. The Co metal thin film grown in this way shows room temperature ferromagnetism with large in-plane magnetization ~1288 emu cm-3 and a coercivity of ~230 Oe with applied field parallel to the film-substrate interface. Carrier density of the film is ~1022 cm-3. The film is epitaxial single phase Co metal which is confirmed by both X-ray diffraction and transmission electron microscopy characterizations. Planar Hall Effect (PHE) and Magneto Optic Kerr Effect (MOKE) measurements confirm that the film possesses similar attributes of Co metal. The result shows that the epitaxial Co metal thin film can be grown from its oxides in the PLD. © 2014 Elsevier B.V.

Journal of crystal growth

Epitaxial Co metal thin film grown by pulsed laser deposition using oxide target

Co vacancy (VCo) induced robust room temperature ferromagnetism (Ms ∼ 60 emu/cm3 and coercivity ∼ 603 Oe) is experimentally realized in rock-salt CoO epitaxial thin film (∼110 nm) grown by pulsed laser deposition. Co charge state is found to be higher ∼+3.2 (from Co L3/L2 white line ratio) and this is due to the V Co induced charge transfer from the neighboring Co-3d to O-2p states in order to compensate for the hole formation. O-K and cathodoluminescence spectra corroborate the existence of VCo and higher charge state. Temperature dependent magnetization and exchange bias experiments confirm the coexistence of ferromagnetic and antiferromagnetic phases. © 2013 AIP Publishing LLC.

Robust room temperature ferromagnetism in epitaxial CoO thin film

The origin of ferromagnetism in ZnO doped with transition metal impurities has been discussed extensively and appeared to be a highly controversial and challenging topic in today's solid state physics. Magnetism observed in this system is generally weak and soft. We have grown Co:ZnO up to 30 at% Co in single crystal thin film form on c-plane sapphire. A composition dependent coercivity is observed in this system which reaches peak value at 25 at% Co, the values are 860 Oe and 1149 Oe with applied field along parallel and perpendicular to the film substrate interface respectively. This giant coercivity might pave the way to exploit this material as a magnetic semiconductor with novel logic functionalities. The findings are explained based on defect band itinerant ferromagnetism and its partial interaction with localized d electrons of Co through charge transfer. Besides large coercivity, an increase in the band gap with Co concentration has also been observed along with blue emission peak with long tail confirming the formation of extended point defect levels in the host lattice band gap. © 2013 Elsevier B.V. All rights reserved.

Giant coercivity in ferromagnetic Co doped ZnO single crystal thin film

We have investigated the non-equilibrium solubility of Mn and Co in Zn 1-xTMxO (x = 5-30 at.% TM = Co, Mn) single crystal thin film and the role of Co/Mn concentration on the structural and magnetic properties. The films have been grown by pulsed laser deposition and the structural and magnetic characterization has been performed using high resolution transmission electron microscopy and superconducting quantum interference device respectively. Cobalt shows high solubility in single crystalline ZnO films with no tendency to form incoherent secondary precipitates up to 30 at.%. Whereas for Mn:ZnO, secondary precipitate started forming from Mn concentration of 25 at.% and above. HREELS of both Co and Mn L2,3 absorption edges shows that the Co and Mn atoms have substituted the Zn positions in the lattice. ZnO doped with 30 at.% Mn forms single crystalline spinel Mn3O4 phase on the sapphire substrate in the form of an interlayer which is responsible for aligning subsequent Mn:ZnO film with respect to sapphire as [1 1 -2 0]Al2O3 || [1 1 -2 0] ZnO compared to the usual [1 1 -2 0]Al2O3 || [0 1 -1 0] ZnO orientation. Saturation magnetization remained almost the same (∼10 20-1021 μB/cm3) for Co:ZnO but a composition dependent coercivity has been observed. For Zn0.75Co 0.25O film, the coercivity is ∼922 (769) Oe corresponding to applied field perpendicular (parallel) to the c axis, which is the highest value reported in this system. For Mn:ZnO ferromagnetism disappears beyond Mn concentrations of 15 at.%. © 2013 Elsevier Ltd. All rights reserved.

Superlattices and Microstructures

Characterization of structure and magnetism in Zn1− x (Cox/Mnx) O epitaxial thin films as a function of composition

Enhanced electrical transport and ultra low thermal conductivity resulted in a high thermoelectric figure of merit, ZT, of ∼1 and ∼1.15 at ∼680 K in 4 mol% Pb and 2 mol% Bi doped AgSbSe2, which are 150 and 190% higher compared to that of the pristine sample, respectively. With this excellent thermoelectric performance, p-type AgSbSe2, constituting earth abundant Se, offers promise to replace traditional metal tellurides containing expensive and scarce Te for mid temperature (350-700 K) thermoelectric applications. © 2013 The Royal Society of Chemistry.