Exchange Bias

The hysteresis loop of ferromagnetic/antiferromagnetic (FM/AF) thin films exhibits a shift along the magnetic field axis below the Néel temperature (TN) of the AF known as exchange bias field, HEB (Fig. 1). The origin of exchange bias (EB) is the coupling between FM and AF spins at the interface. This effect is a hot topic in Condensed Matter Physics owing mainly to two reasons. First, it has a significant importance in giant magnetoresistance devices where the AF/FM interaction is used to control the reversal of FM multilayers. Second, due to the lack of experimental techniques to investigate the AF/FM interface, the microscopic details of EB are poorly understood. [1]

Fig. 1. Above the Néel temperature of the AF all its spins are disordered. Below TN they become ordered and the exchange coupling between FM and AF spins leads to the shift of the hysteresis loop.

Our experiments aim to shed light on the microscopic origin of EB. We use SQUID and VSM magnetometry, magneto-optical Kerr effect, resonant x-ray scattering, polarized neutron reflectometry, and pulsed laser techniques [2] to investigate the EB effects in FeF2/FM thin films. This system (with TN=78K) shows particular features.

  • Low cooling fields yield negative EB while high cooling fields yield positive EB. [3]
  • Double hysteresis loops are observed at intermediate cooling fields for high quality epitaxial FeF2 (Fig. 2). [4]

Fig. 2. FeF2/FM bilayers exhibit positive exchange bias at high cooling fields. At intermediate cooling fields double hysteresis loops (with the same absolute value of EB) are observed. The remanence goes from negative to positive as the cooling field is increased (Results for FeF2/Ni bilayer).

  • The exchange interaction at the AF/FM interface induces a magnetic depth profile through the thickness of the FM and changes in the magnetization reversal process (Fig. 3) [5-7]

(a)

(b)

Fig. 3. (a) Magneto-optical hysteresis loops of FeF2/Py (Permalloy) bilayers from the top side (Py-air interface) and bottom side (FeF2/Py interface). The difference between both loops demonstrates the existence of a magnetic depth profile through the thickness of the FM. (b) Animation of the magnetization reversal based on MOKE measurements. A domain wall parallel to the AF/FM interface is formed and the magnetic moments rotate in a reversible process (an antiferromagnetic coupling between FeF2 and Py spins is assumed).

[1] J. Nogués, and Ivan K. Schuller, J. Magn. Magn. Mater. 192, 203 (1999)

[2] A. Porat, S. Bar-Ad, and Ivan K. Schuller, Euro. Phys. Lett. 87, 67001 (2009)

[3] I. V. Roshchin et al. Europhys. Lett. 71, 297 (2005)

[4] O. Petracic et al. Appl. Phys. Lett. 87, 222509 (2005)

[5] S. Roy et al. Phys. Rev. Lett. 95, 047201 (2005)

[6] Z-P. Li et al. Phys. Rev. Lett. 96, 217205 (2006)

[7] R. Morales et al. Appl. Phys. Lett. 89, 072504 (2006)

[8] Ivan K. Schuller, Mat. Res. Soc. Bull. 29, 642 (2004)

(c) 2007 Ivan K. Schuller       -       designed by Thomas Gredig