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Probing Strain-Induced Changes in Electronic Structure with XMCD Print
Wednesday, 30 March 2011 00:00

The difference in the lattice constant of a substrate compared to a thin film deposited on top of it or a matrix compared to a nanocolumn embedded in it induces a lattice distortion—i.e., strain—in the nanostructure as compared to the bulk. The strong coupling of electronic properties with structural parameters in transition-metal oxides then allows tuning and ultimately controlling the physical characteristics of nanoarchitectures through strain. A research team from the UK and Berkeley has now demonstrated that soft x-ray magnetic circular dichroism (XMCD) techniques are uniquely suited to provide detailed information about the impact of strain on the electronic properties of magnetic oxide nanoarchitectures.

Magnetism in a Pinch

Scientists have long known that if you squeeze certain materials, their properties change. One phenomenon, called the piezoelectric effect, is the basis for many useful technologies that convert mechanical force into an electrical signal and vice versa. Another is magnetostriction, where applying a magnetic field changes the length of a magnetostrictive material, which influences its magnetic properties. Going a step beyond simply exploiting such properties in a given material, materials scientists have improved device performance by "strain engineering," selectively stretching or compressing materials as needed to optimize their properties in desired ways. At today's cutting edge, scientists are experimenting with engineering novel materials by growing them in thin-film form. Would it be possible to control magnetism in such materials by deforming their lattices in various ways (e.g. folds, trenches, substrates with different thermal expansion rates)? To help answer such questions, van der Laan et al. have demonstrated how a well-known technique involving polarized light can be used to detect changes in charge or spin induced by distortions of the crystal lattice.

 

Pinched lattice

Schematic depiction of two simple materials with different lattice constants (distance between unit cells along a given direction in the crystal lattice).

 

The delicate balance between charge, spin, orbital, and lattice degrees of freedom in transition-metal oxides leads to unique phenomena such as high-temperature superconductivity and colossal magnetoresistance, as well as a remarkable diversity of charge-, spin-, and orbital-ordered phases. The rich phase diagrams are determined by the strong local interactions of electrons in transition-metal d orbitals. Subtle changes in d occupancy and overlap—and therefore phase transitions—can be induced by variations in temperature, by external fields, through doping, and through lattice distortions. In particular, the strong coupling of electronic properties with structural parameters allows us to control the physical characteristics of nanoarchitectures through strain at interfaces of layered and nanocomposite heterostructures.

A research team from Diamond Light Source in the UK and Berkeley have now shown, at ALS Beamline 4.0.2, that XMCD is uniquely suited to provide detailed information on the impact of strain on the electronic properties of magnetic oxide nanoarchitectures in an element-, valence-, and site-specific way.

In the case of an isotropic system magnetically saturated by an external field, the XMCD signal scales with the angle, θ, between the field and the x-ray beam as cosθ. Consequently, for a perpendicular orientation of x rays and field, i.e., θ = 90°, the XMCD signal vanishes completely.  In systems with cubic anisotropy, the XMCD spectrum is slightly different along different lattice directions, but the XMCD signal along principal directions still disappears for θ = 90°. This is no longer the case for systems having axial anisotropy, such as in uniaxial, tetragonal, or trigonal symmetry of the lattice.

 

XMCD data for Cr and Mn

Angular dependence of the Cr (left) and Mn (right) L3,2 XMCD signals with varying angles θ between the x-ray beam and external field. For sufficiently small lattice distortions, the size of the XMCD spectrum obtained for a perpendicular orientation of the field and x-ray beam scales linearly with the distortion. This provides a unique means to determine the lattice distortion.

 

The research team determined the strain-induced changes in the electronic structure of ferrimagnetic spinel MnCr2O4 films by monitoring the angular dependence of the Mn and Cr L3,2 XMCD signals. The MnCr2O4 films were deposited on Nb-doped SrTiO3 substrates, leading to approximately 1% compressively strained MnCr2O4 films. The experiments were performed using an eight-pole electromagnet installed at ALS Beamline 4.0.2. This device provides magnetic fields of up to 0.9 T in arbitrary directions, making possible this first study of the angular dependence of the XMCD signal in any system. Using the vector magnet, the researchers observed a pronounced angular dependence of the Mn2+ and Cr3+ L3,2 XMCD spectra as well as nonvanishing XMCD signals with distinct spectral features in transverse geometry, i.e., for perpendicular alignment of magnetic moment and x-ray beam (θ = 90°). The experimental XMCD results can be well reproduced using atomic multiplet calculations taking into account the reduced symmetry of the crystal lattice induced by the substrate.

 

Experiment vs theory

Comparison of experimental (black) and theoretical (red) results for the Cr (left) and Mn (right) L3,2 edges. The x-ray absorption (XA) spectrum (top) and the XMCD spectrum for parallel (middle) and perpendicular (bottom) orientations of the magnetic field and x-ray beam are shown. The atomic multiplet theory used for the calculations takes into account the details of the local symmetry around the absorbing Cr and Mn atoms.

 

It is of great practical interest for the strain engineering of novel materials—i.e. for tuning and controlling materials properties through lattice distortions—that a very small axial distortion completely determines the angular dependence of the XMCD. The angular dependence of XMCD is a general phenomenon that will occur in any strained or distorted lattice of magnetic transition-metal oxides. This opens the way to studying strain using soft x-ray spectroscopy and microscopy techniques on ultrafast time scales with nanometer spatial resolution.

 


 

Research conducted by G. van der Laan (Diamond Light Source, UK), R.V. Chopdekar and Y. Suzuki (University of California, Berkeley), and E. Arenholz (ALS).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Operation of the ALS is supported by DOE BES.

Publication about this research: G. van der Laan, R.V. Chopdekar, Y. Suzuki, and E. Arenholz, "Strain-induced changes in the electronic structure of MnCr2O4 thin films probed by x-ray magnetic circular dichroism," Phys. Rev. Lett. 105, 067405 (2010).

ALS Science Highlight #223

 

ALSNews Vol. 318