January's Paper of the Month

Rare-earth perovskite transition metals have been shown to have properties ideal for semi-conductor devices which are commonly used in electronic circuits.  

Rare-earth perovskite transition metals have been shown to have properties ideal for semi-conductor devices which are commonly used in electronic circuits.  

As the modern world advances and our reliance on technology increases, it becomes necessary to improve the efficiency of semi-conducting materials. Semi-conductors are commonly used as diodes and transistors in devices such as microprocessors. Research into these materials is one of paramount importance. 

Recent studies have found transition metal oxides to have incredible electric, magnetic and superconducting properties, potentially ideal for semiconductors. LaAlO3 and SrTiO3 are wide band gap insulators with perovskite-based structures which are commonly used as substrates for functional oxide thin films. 

However, thin films of these oxides are not of much use except as high-k dielectrics. They require the addition of ions to tune their electronic band structure and thus improve their magnetic and optical properties. It is the interface between these oxides which prove to be the most interesting, their interaction induces magnetic and conductive properties from otherwise non-magnetic, insulating oxides. 

LaAlO3 is a rare-earth based perovskite transition metal oxide. Naturally such materials are isolated as crystals and it is important to understand the native characteristics if we are to better understand thin film behaviour. 

Due to its high-k dielectric properties, LaAlO3 is a promising material for metal oxide-based semiconductor devices. However, concern has been raised in several studies regarding leakage-current which is caused by structural defects. Understanding these defects theoretically and experimentally is of utmost importance for better use of such materials in optical and electronic applications. 

It has also been discovered that lattice strain affects the role and dynamics of defects. Previous work has also demonstrated that phase transitions can occur when samples are placed under certain temperatures. Current theories also suggest that specific engineering of these defects can provide several different functionalities for transition metal oxides. Raman spectroscopy further provides a useful method of probing these defects. 

January’s Paper of the Month is a collective effort from the National University of Singapore, Nanyang Technological University and Trinity College Dublin. Their paper explored novel magnetic excitations using Raman spectroscopy to probe LaAlO3 and several other polar oxide substrates. 

They built on the idea that a host of robust defects present in LaAlO3 could be promising in providing new functions with controlled engineering. The group conducted magnetic field dependant Raman spectroscopic studies at low temperatures to gain a better understanding of lattice phonons and the functionalities of these defects. 

The low temperature Raman spectroscopy was conducted using a WiTec Raman spectrometer and a Linkam HFS600-PB4 with LNP, allowing a temperature range from -196°C to 600°C.

The HFS set up with liquid nitrogen cooling and a WiTec Raman Spectrometer. 

The HFS set up with liquid nitrogen cooling and a WiTec Raman Spectrometer. 

When discussing the purpose of the Linkam stage, Dr. Surajit Saha said: “The HFS was used to perform temperature dependence of the angular momentum states over a range of 80 to 300 K (-193°C to 26°C). It was useful because we could probe the decay of the angular momentum states with increasing temperature which was not possible to perform with our existing variable temperature setup.”

The low temperature experiments provided evidence for novel transitions which disappear at room temperature. These transitions were found to be magnetically sensitive, suggesting a magnetic degree of freedom caused by the defects. 

They further discovered that the key to magnetic sensitive field states is the presence of a heavy element within the transition metal oxide. These angular momentum states and the magnetic interactions can be tailored for novel optical applications. The magnetic degrees of freedom may potentially be tuned and optimised in rare earth perovskites for optical applications. 

The group’s findings pave the way for further experimentation and testing to better understand the complexities of transition metal oxides. 

By Tabassum Mujtaba

Saha, S. et al. Magnetic Modes in Rare Earth Perovskites: A Magnetic-Field-Dependent Inelastic Light Scattering Study. Sci. Rep. 6, 36859; doi: 10.1038/srep36859 (2016)