Magnetic Oxides

The discovery of tunnelling magnetoresistance (TMR) at room temperature and at low magnetic fields in powder samples of the ferrimagnetic double perovskites Sr2FeMoO6 and Sr2FeReO6 has indicated the potential of compounds from the so-called double perovskite group for magnetic sensor applications. The essential features of materials exhibiting a large TMR effect are

  • halfmetallic spin-polarized conductivity and
  • a ferromagnetic ordering at Curie temperatures as high as possible, ideally above a value of two times room temperature.

From a band electronic point of view the first requirement corresponds to a saddle point (the so-called van Hove instability) close to the Fermi level and a spin density wave instability.

The band model provides a practical tool for a systematic variation of magnetization, Curie temperature and the TMR effect by "doping", i.e. adjusting the electron count of the target compounds to their optimum values. We apply this approach to the double perovskites.

Double perovskites of the general formula A2MM´O6, where M and M´ sites are occupied alternatively by different transition metal cations (M = first row transition metal; M´ = Mo, W, Re) are known since more than forty years. The ideal structure of these compounds can be considered as a regular arrangement of corner sharing MO6 and M´O6 octahedra alternating along the three directions of the crystal with the large A cations situated in the voids between the octahedra. The crystal structure and physical properties of the double perovskite oxides strongly depend on the size and valencies of the cations A, M and M´. An early study of the A = Ba and M´ = Re members by Sleight and collaborators (Inorg. Chem., 1962, 1, 245) revealed that several members of the series (M = Mn, Fe, Ni) are ferrimagnetic (J. Phys. Chem. Solids, 1972, 33, 679), Sr2FeReO6 (Nature, 1998, 395, 677), Ba2FeReO6 (J. Phys.: Condens. Matter, 2000, 12, 965) and Sr2CrReO6 (Appl. Phys. Lett., 2002, 81, 328) are unique in that they are both metallic and ferrimagnetic, Sr2CrReO6 having an exceptionally large Curie temperature of 635 K.

In order to probe the influence of the M site cations on the structure and properties of Sr2FeReO6 we investigate the substitution of Fe by other 3d metals as in Sr2Fe1-xCrxReO6 and Sr2Fe1-xZnxReO6, the substitution of Re by other high valent metals as well as the structures of double perovskites at elevated temperatures and high pressures.

The results of band structure calculations for the double perovskite Sr2FeReO6 indicate that the Fermi level is situated slightly above the van Hove singularity. Therefore, one may achieve a coincidence of the Fermi level and the local maximum of the density of states by a decrease of the valence electron concentration. Superimposed on the electronic are structural effects depending on the radii of the cations involved: The higher the symmetry, the higher the saturation magnetization.

Based on the available theoretical results one may assume that the saturation magnetization - and the TMR effect - increase upon hole doping (M = Cr) or - strictly speaking - increase with the degree of substitution in the solid solution series Sr2Fe1-xCrxReO6, whereas it decreases upon electron doping (M = Zn). The density of states at the Fermi level can be maximized by tuning the electron concentration in the conduction band by means of "doping". On the other hand, there is a correlation between the crystal symmetry (which is dictated on the cation radii) and the saturation magnetization: the higher the symmetry of the system, the larger the saturation magnetization.

can demonstrate the validity of this model by doping with Zn and Cr. As predicted, Cr doping up to the optimal valence electron concentration leads to an enhancement of the magnetic characteristics such as Curie temperature and saturation magnetization. In contrast, Zn doping leads to the opposite effect by an increase of the valence electron concentration. It can be shown that structural factors play an important role by competing with the electronic effects. Quantitative explanation of the structural models by means of the tolerance factor t reveal that the physical properties are influenced by the valence states and ionic radii of the constituting elements. The structural effects may be in line with the electronic effects for the series Sr2Fe1-xZnxReO6 or opposite to the electronic factors as evidenced in the study of the series Sr2Fe1-xCrxReO6. The superposition of structural and electronic effects can be considered the reason for the non-monotonic behaviour of the magnetic properties as a function of the degree of substitution.

Structural and magnetic properties of the solid solution series Sr2Fe1-xMxReO6 (M = Cr, Zn)
A. Jung, I. Bonn, V. Ksenofontov, G. Melnyk, J. Ensling, C. Felser and W. Tremel
J. Mater. Chem. 2005, 1760-1768.