Download Exchange interactions between soft ferromagnetic thin films and

IMM-Seminar
Title: Exchange interactions between soft ferromagnetic thin films and
multiferroic BiFeO3
Alexandra Mougin
Laboratoire de Physique des Solides
CNRS, Université Paris-Sud – Orsay, France
Controlling the magnetization of a thin ferromagnetic (FM) film using an electric field would
revolutionize the addressing of magnetic memory elements. One strategy is to combine the
magnetoelectric coupling of multiferroic materials like BiFeO3 (BFO) with the exchange coupling (EC)
observed in antiferromagnetic (AFM) / FM systems such as BFO/CoFeB. BFO is a material of choice as
it is one of the very few room-temperature AFM multiferroics. The two types of structures investigated
by MagnetoOptic Kerr Effect (MOKE) consist in FM layers of CoFeB deposited on BFO/STO films, as
well as thick permalloy layers sputtered onto BFO single crystals. A macroscopic shift HE of the FM
loops is one of the signatures of exchange-bias (EB) in coupled AFM/FM bilayers. An analysis of the
magnetic properties of the FM layers – and especially the bias strength – will be done in relation with the
ferroelectric structure of the underlying BFO. In their virgin state, the crystals are in a single ferroelectric
and AFM domain with a cycloidal magnetic structure, whereas thin films, in which the cyloid is
suppressed, are in a highly multidomain state. The single domain BFO crystals show no bias whereas a
significant one is observed in multidomain thin films. This comparative study allows us to assert that the
uncompensated and rigid AFM spins involved in the biasing of the FM loops are related to the density of
the domain walls. I will also show that the complex angular dependences of HE and HC result from the
competition between the anisotropies of the FM and AFM layers. Again, both systems will be compared.
Finally, the electric field effect will be addressed in the single crystals. Previous work on BFO crystals
demonstrated that during electrical testing, any change of the electric polarisation direction induces a spin
flop of the AFM moments. I will show here that a 90° rotation of the anisotropy axes can be obtained in
domains where the polarisation is electrically flipped. Eventually, I will remind some former results
gained by SHMOKE on fully metallic AFM/FM bilayers.
Date:
Time:
Location:
Host:
April 15, 2009
11:00 hours
HG00.068
Prof.dr. Theo Rasing