Strain-induced modification of the antiferromagnetic order in epitaxial thin films

 

Robert Gunnarsson

 

There are different types of antiferromagnetic ordering. In type-A antiferromagnets the antiferromagnetism is due to antiparallel order of ferromagnetically aligned atomic planes. Another type of antiferromagnetic order (type-C) occurs when the magnetic moments are ferromagnetically aligned in rods, and the antiferromagnetic coupling is between these rods. In an orthorhombic system, the type-A magnetic order is when the magnetic moments are antiferromagnetically coupled to two (of its six) nearest neighbours, and ferromagnetically coupled to four nearest neighbours. Type-C antiferromagnetism occurs when the magnetic moments are antiferromagnetically ordered to four of its nearest neighbours, and ferromagnetically coupled to only two. What type that best describes the magnetic order in a specific antiferromagnetic material is determined by small differences in free energy. One way to impose such small energy differences in thin films is to modify the lattice unit cell by strain.  As strain can change the free energy of atomic orbitals it can also modify the chemical bonds and thereby also the antiferromagnetic order.

 

This project aims to study the antiferromagnetic coupling in strained lanthanum-strontium-manganese-oxide thin films.  In this material a proper selection of strain can modify the manganese 3d-orbitals such that either the d_x2_-y2 or the d_3z2_-r2 orbital can have the lowest energy [1]. Since in manganese oxides the magnetic order is mediated mainly through these orbitals, the selection of strain is expected to have an impact on the antiferromagnetic order. In practice, the task will be to grow epitaxial thin films of La_0.5Sr_0.5MnO₃ on different substrates, one which will impose a biaxial tensile strain, one which will impose a biaxial compressive strain, and one substrate that will leave the system virtually strain free. By this procedure we expect to produce samples with the same stoichiometry but with different (A-type and C-type) antiferromagnetic order. By measuring the anisotropic charge transport properties the type of antiferromagnetic ordering will be determined.

 

[1] J. Phys. Soc. Jpn 68, 3790 (1999)

Contact:
Robert Gunnarsson <robert.gunnarsson -et- mc2.chalmers.se>