Nickelates

Rare-earth (R) perovskite nickelates RNiO₃ are a fascinating family of compounds, known for their bandwidth controlled metal-insulator (MI) transition with changes in resistance of several orders of magnitude. At high temperatures the nickelates are paramagnetic metals, while at low temperatures they are semiconducting, exhibiting charge disproportionation and unique antiferromagnetic ordering. The transition temperature is structurally correlated to the size of the rare earth and can lie anywhere between 100 and 600 K depending on the choice of the rare earth. The origin of the MI transition is still under debate [1,2].
Our efforts have focused on the understanding and manipulation of the MI transition in nickelates by strain and electric field-effect. At the same time we are investigating new interface phenomena in artificially layered heterostructures composed of nickelates.

[1] G. Catalan, Phase Transitions 81, 729 (2008).
[2] M. Medarde, J. Phys: Cond.-Matt. 9, 1679 (1997).

Phase Diagram of RNiO₃

Metal-Insulator transition in ultrathin LaNiO₃

LaNiO₃ is the only member in the nickelate series that does not display a MI transition or any type of ordering phenomena and remains a metal at all temperatures with an enhanced effective mass and Pauli paramagnetism.
However, this material has become the subject of tremendous interest, mainly due to theoretical predictions of orbital ordering and high-Tc superconductivity in LaNiO3 based heterostructures [3].
We investigated the transport properties of ultrathin films of LaNiO₃. A metal-insulator transition was observed as the film thickness was decreased to only a few unit cells and can be associated with a dimensional crossover from 3D to 2D. In addition, a transition region between the insulating and metallic region was detected where quantum corrections to the conductivity appears and are accurately described by weak localization in the presence of magnetic fluctuations.
The conductivity and the MI transition were found to be tunable by electric field, raising the question of whether field-effect modulation of the intrinsic MI transition in other nickelates could also be achieved and opening new possibilities for applications of these materials.

[3] J. Chaloupka and G. Khaliulin, Phys. Rev. Lett. 100, 016404 (2008).

Sheet Resistance as a function of temperature for LaNiO₃ films of different thickness. Arrows indicate upturns in resistivity.

Related papers :

• Metal-Insulator Transition in Ultrathin LaNiO₃ Films
  R. Scherwitzl, S. Gariglio, M. Gabay, P. Zubko, M. Gibert, and J.-M. Triscone
  Physical Review Letters 106, 246403 (2011).



• Electric-field tuning of the metal-insulator transition in ultrathin films of LaNiO₃
  R. Scherwitzl, P. Zubko, C. Lichtensteiger and J.-M. Triscone
  Applied Physics Letters 95, 222114 (2009).

Electric-field and strain control of the MI transition in NdNiO₃

We are studying NdNiO₃ thin films, grown by off-axis rf magnetron sputtering on a variety of different substrates. By changing the substrate and thereby the strain, the structure of NdNiO3 is modified and its influence on the transport properties can be investigated. The main discovery was that the MI transition can be completely suppressed by increasing the compressive strain.
Field-effect devices were then fabricated in order to investigate the effect of doping on the MI transition. The field-effect technique has the advantage that purely electronic effects can be studied, unhindered by structural modifications, sample to sample variation or induced changes in the disorder landscape. In this study, an ionic liquid was used as a gate dielectric.
The results of applying this technique to an 8 u.c. film of NdNiO₃ are shown in Figure 3. The transition temperature could be reversibly tuned by more than 50 K and the conductivity could be modulated by up to 60 000%. By simultaneously performing Hall measurements, the change in carrier density was monitored, reaching up to 3x1015 cm-2, while the mobility remained unaffected by the field-effect.
In conclusion, the MI transition of NdNiO3 is strongly dependent on the carrier concentration.

Electric field control of the MI transition in 8 u.c. NdNiO₃ film.

Related papers :

• Electric-Field Control of the Metal-Insulator Transition in Ultrathin NdNiO₃ Films
  Raoul Scherwitzl, Pavlo Zubko, I. Gutierrez Lezama, Shimpei Ono, Alberto F. Morpurgo, Gustau Catalan and Jean-Marc Triscone
  Advanced Materials 22, 5517 (2010).

LaNiO₃-LaMnO₃ superlattices

Building on our studies of LNO thin films, we are now also focusing on the electronic transport and magnetic phenomena in LNO/LaMnO₃ (LMO) superlattices. This system constitutes a rich platform to investigate the role of artificially induced charge ordering on functional properties.
We synthesize high crystalline quality LNO/LMO superlattices with a range of stacking periodicities on SrTiO₃ single crystal substrates by means of off-axis magnetron sputtering.
Transport and magnetic properties are investigated.

X-ray diffraction pattern of a 6/3 LNO/LMO superlattice with 6 bilayers grown on a (001)STO substrate.

Related papers :

• Exchange bias in LaNiO₃–LaMnO₃ superlattices
  Marta Gibert, Pavlo Zubko, Raoul Scherwitzl, Jorge Íñiguez & Jean-Marc Triscone
  Nature Materials (2012).

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