1. Mixed valent iridates

Iridates with fractional oxidation state of iridium are relatively unexplored but offer interesting physical properties like charge ordering, magnetic ordering, spin liquid behavior etc. The triple perovskite Ba3MIr2O9 series is rather interesting because a wide range of elements with a variety of oxidation states can be fitted as M. In these compounds, two face sharing IrO6 octahedra form Ir2O9 dimers (see the figure) along the crystallographic c axis. These dimers share 9 electrons and form a molecular orbital resulting in S=1/2 moments per dimer. The dimers form an edge-shared triangular lattice parallel to the ab plane and a buckled honeycomb lattice with dimers of neighboring plane.  

We have carried out detailed bulk and local probe studies on the hexagonal oxides Ba3MIr2O9 (M=Sc,Y, In, Lu and La), where Ir has a fractional oxidation state of +4.5. Our study shows that all these compounds order magnetically except the In compound. While the spins are correlated, our NMR and muSR studies exclude magnetic ordering but shows persistent spin dynamics down to 20mK. We propose a spin liquid ground state for the Ba3InIr2O9 compound. This material is new and interesting in many ways. 

For further studies we have grown single crystal (shown on right) of the materials. 

Our results are published in:

1. Persistent low-temperature spin dynamics in the mixed-valence iridate Ba3InIr2O9, Tusharkanti Dey*, M. Majumder, J. C. Orain, A. Senyshyn, M. Prinz-Zwick, S. Bachus, Y. Tokiwa, F. Bert, P. Khuntia, N. Buettgen, A. A. Tsirlin, and P. Gegenwart, Phys. Rev. B 96, 174411 (2017)

2. Unconventional magnetism in the spin-orbit driven Mott insulators Ba3MIr2O9 (M=Sc,Y), Tusharkanti Dey, R. Kumar, A. V. Mahajan, S. D. Kaushik and V. Siruguri, Phys. Rev. B 89, 205101 (2014)

3. Possible spin-orbit driven spin-liquid ground state in the double perovskite phase of Ba3YIr2O9, Tusharkanti Dey, A.V. Mahajan, R. Kumar, B. Koteswararao, F. C. Chou, A. A. Omrani and H. M. Ronnow, Phys. Rev. B 88, 134425 (2013) 

4. Quasi-molecular electronic structure of the spin-liquid candidate Ba3InIr2O9, A. Revelli, M. Moretti Sala, G. Monaco, M. Magnaterra, J. Attig, L. Peterlini, T. Dey, A. A. Tsirlin, P. Gegenwart, T. Fröhlich, M. Braden, C. Grams, J. Hemberger, P. Becker, P. H. M. van Loosdrecht, D. I. Khomskii, J. van den Brink, M. Hermanns, and M. Grüninger, Phys. Rev. B 106, 155107 (2022)

2. Exitonic magnetism

Materials with 5d4 electronic configuration are generally considered to have a nonmagnetic ground state (J = 0). However recent theoretical studies predict that these materials may exhibit excitonic type of magnetism [G. Khaliullin, PRL-2013]. To understand this interesting phenomenon I have prepared single crystals (shown in pic) of cubic double perovskite material Ba2YIrO6 with Ir5+ (5d4) configuration. We have studied the structural, magnetic, electronic and thermodynamic properties of Ba2YIrO6 in detail. We observed that long-range magnetic order is absent for temperatures down to 0.4 K, but at the same time correlated magnetic moments are present in Ba2YIrO6. Our work instigated a number of experimental and theoretical studies debating the origin of magnetic moments in this material. 

Our results are published in:

1. Ba2YIrO6: A cubic double perovskite material with Ir5+ ions, T. Dey*, A. Maljuk, D.V. Efremov, O. Kataeva, S. Gass, C.G.F. Blum, F. Steckel, D. Gruner, T. Ritschel, A. U. B. Wolter, J. Geck, C. Hess, K. Koepernik, J. van den Brink, S. Wurmehl, B. Buechner, Phys. Rev. B 93, 014434 (2016)

2. The iridium double perovskite Sr2YIrO6 revisited: A combined structural and specific heat study, L.T. Corredor, G. Aslan-Cansever, M. Sturza, K. Manna, A. Maljuk, S. Gass, A. Zimmermann, T. Dey, C. G. F. Blum, M. Geyer, A. U. B. Wolter, S. Wurmehl and B. Buechner, Phys. Rev. B 95, 064418 (2017)

3. Diluted paramagnetic impurities in nonmagnetic Ba2YIrO6, F. Hammerath, R. Sarkar, S. Kamusella, C. Baines, H.-H. Klauss, T. Dey, A. Maljuk, S. Gass, A.U.B. Wolter, H.-J. Grafe, S. Wurmehl, and B. Büchner, Phys. Rev. B 96, 165108 (2017)

4. Observation of heavy spin-orbit excitons propagating in a nonmagnetic background: The case of (Ba,Sr)2YIrO6, M. Kusch, V. M. Katukuri, N. A. Bogdanov, B. Büchner, T. Dey, D. V. Efremov, J. E. Hamann-Borrero, B. H. Kim, M. Krisch, A. Maljuk, M. Moretti Sala, S. Wurmehl, G. Aslan-Cansever, M. Sturza, L. Hozoi, J. van den Brink, and J. Geck, Phys. Rev. B 97, 064421 (2018)

5. Unraveling the nature of magnetism of the 5d4 double perovskite Ba2YIrO6, S. Fuchs, T. Dey, G. Aslan, A. Maljuk, S. Wurmehl, B. Buechner, and V. Kataev, Phys. Rev. Lett. 120, 237204 (2018)

3. Kitaev materials

Kitaev model and subsequently Heisenberg-Kitaev model was first proposed and studied in honeycomb and related lattice geometries. A more generalized form of this model extends to various other lattice geometries. A triangular geometry coupled with strong spin orbit coupling could give rise to interesting phenomenon. 

Absence of an Ir4+ based triangular lattice material motivated us to synthesize Ba3IrTi2O9 which forms a triangular lattice of Ir4+ ions (Jeff=1/2). Our study suggested that the material is highly frustrated and has a spin liquid ground state. Ba3IrTi2O9 has been used as a model system to study generalized Heisenberg-Kitaev model on a triangular lattice. Discovery of Ba3IrTi2O9 motivated a number of theoretical and experimental studies suggesting very interesting magnetic structures like vortex crystal (shown in picture taken from Phys. Rev. B 91, 155135 (2015)) which looks similar to skyrmions. However, these interesting phases are yet to be experimentally realized. 

We have also studied magnetic properties of the related materials Ba3RuTi2O9 and Ba3RhTi2O9.

Our results are published in:

1. Frustration induced disordered magnetism in Ba3RuTi2O9, T. Dey* and A. V. Mahajan, Eur. Phys. J. B 86, 247 (2013)

2. Spin liquid behavior in Jeff=1/2 triangular lattice compound Ba3IrTi2O9, Tusharkanti Dey, A. V. Mahajan, P. Khuntia, M. Baenitz, B. Koteswararao and F. C. Chou, Phys. Rev. B (R) 86, 140405 (2012)

3. Ba3MxTi3−xO9 (M = Ir, Rh): A family of 5d/4d-based, diluted quantum spin liquids, R. Kumar, D. Sheptyakov, P. Khuntia, K. Rolfs, P. G. Freeman, H. M. Rønnow, Tusharkanti Dey, M. Baenitz and A. V. Mahajan, Phys. Rev. B 94, 174410 (2016)

4. Freeman P., Dey T.; Mahajan A., Prsa K., Rolfs K., Rols S., Ronnow H., Sharma R. K. and Yang L., (2013). Magnetic Correlations of Ba3MTi2O9 with M = Ru, Rh and Ir., Institut Laue-Langevin (ILL) doi:10.5291/ILL-DATA.4-01-1321

4. 75As NMR studies of iron pnictide superconductors

The interest in high temperature superconductivity experienced a resurgence after the discovery of superconductivity in LaFeAs(O1−xFx) with Tc = 26K in 2008. Within a short span of 4-5 years, hundreds of new materials in the iron pnictide family have been reported to be superconducting. However the mechanism of superconductivity in these materials is not yet clear.

We have studied the evolution of 75As NMR parameters with composition and temperature in the Ba(Fe1-xRux)2As2 system where Fe is replaced by isovalent Ru. Our study suggests possible role of AFM fluctuations in driving superconductivity. Also we have performed 75As NMR measurements as a function of temperature in Eu1−xKxFe2As2 (x = 0.5, 0.7) system. The large Eu2+ moments and their fluctuations are found to dominate the 75As NMR properties.

Our results are published in:

1. 75As NMR local probe study of magnetism in (Eu1-xKx)Fe2As2, Tusharkanti Dey*, Panchanana Khuntia, Avinash V. Mahajan, Anupam and Zakir Hossain, Eur. Phys. J. B 86, 458 (2013)

2. 75As NMR study of antiferromagnetic fluctuations in Ba(Fe1-xRux)2As2, Tusharkanti Dey, P. Khuntia, A. V. Mahajan, Shilpam Sharma and A. Bharathi, J. Phys.: Condens. Matter 23, 475701 (2011)

5. Magnetism in metal oxide nanoparticles

Observation of room temperature ferromagnetism in nanoparticles of undoped, wide band gap, semiconductor metal oxides such without any unpaired “d” or “f” electron have questioned the basic understanding of the origin of magnetism. Sundaresan et al., [PRB-2006] claimed that oxygen vacancies at the surfaces are responsible for ferromagnetism in CeO2, Al2O3, ZnO, In2O3, and SnO2 nanoparticles. 

To verify this claim, we have performed 27Al and 119Sn NMR measurements as function of temperature on Al2O3 and SnO2 nanoparticles, respectively. A 15% loss of 119Sn NMR signal intensity for the nano sample compared to the bulk sample was observed. This is indicative of ferromagnetism from a small fraction of the sample. Further, the 119Sn 1/T1 at room temperature is found to be much higher for the nano sample than for its bulk counterpart. These results indicate the presence of magnetic fluctuations in SnO2 nanoparticles in contrast to the bulk (non-nano) which is diamagnetic. We conclude that these local moments could arise from surface defects in the nanoparticles.

In this context, we have also performed zero field 89Y NMR measurements to estimate the internal magnetic field in YCo2 nanoparticles. The 89Y spectra is very broad (~30MHz). This indicates that the internal field strongly depend on the size of the particles. 

Our results are published in:

1. 119Sn NMR probe of magnetic fluctuations in SnO2 nanoparticles, Tusharkanti Dey*, P. Khuntia, A. V. Mahajan, Nitesh Kumar and A. Sundaresan, Europhys. Lett. 96, 67008 (2011) 

2. 27Al and 119Sn NMR Study of Magnetism in Nanoparticles of Al2O3 and SnO2, T. Dey et. al., at ICMM-2010, SINP, Kolkata, India, Nov 2010 (poster presentation)