Multiferroic Materials

Multiferroics are materials in which at least two of the ferroelectric, ferro/antiferromagnetic and ferroelastic phases coexist.  Though the mechanisms that allow ferroelectricity and ferromagnetism seem to be incompatible, there are a select few materials in which ferroelectricity and ferromagnetism are both present, namely Cr2O3, yttrium- iron-garnets, boracites, rare-earth ferrites and manganese-based perovskites.  In these materials, the ferroelectric and ferro/antiferromagnetic  phases are coupled in such as way as to produce a cross phenomenon known as the magnetoelectric (ME) effect.  This allows manipulation of the magnetic phase with an external electric field and/or manipulation of the electric phase with external magnetic field.  The integration of the ME effect into device technology would have substantial implications, however the above mentioned single phase materials exhibit prohibitively small ME effect.

In the past decade or so, there has been a strong upsurge in research activities in the field of multiferroics with the discovery of large ME effect in composites laminates and more recently in nanostructured materials.  Our lab has grown multilayers of ferroelectric (Barium Strontium Titanate or  BSTO) and ferromagnetic (Barium Hexaferrite or BaM) materials and explored the change in magnetic properties of BaM that occurs with the presence of the BSTO.  We tested the magnetic properties with our Physical Properties Measurement System (PPMS) and  found that the BSTO causes a change in the magnetic anisotropy of the BaM. 

                                             H_c=2100 Oe                                                                            H_c=1900 Oe

BaM films were grown on Al2O3 using magnetron sputter deposition.  When measured, the resulting films showed a decrease in coercivity with increase in temperature, consistent with other reports of BaM films.

We then grew multilayers of BaM and BSTO using the same technique.  Below is an SEM image.

  H_c = 1750 Oe                                                                                          Hc = 2100 Oe

The multilayer films showed an increase in coercivity with increase in temperature, opposite of the results for pure BaM.  Since the coercivity of BaM is determined by the competition between the magnetocrystalline anisotropy and the shape anisotropy.  The presence of the BSTO has had an influence on the microstructure of the BaM, causing the shape anisotropy and thus the coercivity to be altered.

We are currently in the process of integrating a ferroelectric tester into our PPMS to be able to take ferroelectric data over a wide range of temperatures and external magnetic fields.  We can further examine the behavior of this interesting system as well as many others including ferroelectric polymer/ferromagnetic nanoparticle composites and multilayers. 

We are also trying to determine if our  transverse susceptibility measurement can be used to probe magnetoelectric coupling in multiferroic systems.  When testing bilayers of CrO2/Cr2O3 we observed anomalously large anisotropy coefficients which could not be explained by magnetoelastic or exchange coupling. 

Transverse susceptibility data for CrO2 thin films.

Transverse susceptibility data for CrO2/Cr2O3 bilayers showing anomalous features.

Relevant Publications

“Magnetic anisotropy in CrO₂ and epitaxial CrO₂/Cr₂O₃ bilayers” –N. A. Frey, S. Srinath, H. Srikanth, M. Varela, S. Pennycook, G. Miao and A. Gupta, Physical Review B (submitted, 2006))

"Growth and Characterization of Sputtered BST/BaF Multilayers" -S. Srinath, N.A. Frey, R. Hajndl, H. Srikanth, K.R. Coffey, N.J. Dudney. Journal of Applied Physics. 97 10J115 (2005)

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