# Presentation: 279772

## DMR 02-45227: Exchange bias in (Ga,Mn)As/MnO Bilayers

- K. Dziatkowski, *et.al*., Appl. Phys. Lett. **88**, 142513 (2006);
- Z. Ge, *et al*., (submitted to Phys. Rev. B).

- Application:
- spin valve and sensor applications combined with GMR effect

- Exchange bias (HEB):
- the shift of the M-H loop

- Spin valve

- We found:
-  _Unidirectional_ anisotropy in (Ga,Mn)As/MnO bilayers revealed by microwave FMR.
-  An antiferromagnetic (AF) layer can effectively pin magnetic moments in (Ga,MN)As layer; and the rotation (angular dependence) of magnetization during field reversal can be described by the _single-domain model_.
-  Dependence of coercivity (HC) on temperature in (Ga,Mn)As is changed by the coupling to the AF layer.
-  The exchange coupling is _very robust with respect to temperature_. Once established, it survives beyond the Nèel temperature of the AF layer (MnO).

**DMR 02-45227:** Exchange bias in (Ga,Mn)As/MnO Bilayers
**M. Dobrowolska**, *University of Notre Dame*

- B (T)

- R ()

**GMR**

- Angular dependence

**Notes:**
Exchange coupling between a ferromagnetic (FM) and an antiferromagnetic (AF) thin film is known to displace the hysteresis loop of the FM layer from zero magnetic field (in comparison to that same layer in the absence of the AF film) by an amount referred to as the exchange bias field, HEB. This FM/AF interaction (called “exchange coupling”) has attracted intense interest due to its application to giant magnetoresistance (GMR) spin-valve heads used for high density recording.
Ferromagnetic resonance (FMR) is a powerful technique for the study of magnetic properties, e.g., magnetic anisotropy, interlayer exchange coupling, and relaxation of magnetization, both of single-layer and multi-layer FM structures. Our FMR studies on exchange-biased structures comprised of a (Ga,Mn)As FM layer interfaced with an MnO AF layer revealed the presence of a unidirectional anisotropy (exchange anisotropy) existing in these systems.
We should note that the mechanisms of exchange bias are still not well understood. Our present emphasis is therefore to use the angular dependence of planar Hall effect (PHE) in (Ga,Mn)As/MnO exchange-biased layer combinations in order to understand the mechanisms that occur during magnetization reversal in these systems and, by extension, also in other exchange-biased systems involving FM semiconductors. (Note that the PHE experiment is sensitive to the details of magnetization reversal that cannot be obtained from standard magnetization measurements, e.g., how exactly the direction of magnetization rotates to change its direction, thus shedding new light on the process that is of key importance in the operation of spin valves.)  Our results show that in exchange-biased systems magnetization reversal can be asymmetric, i.e., that the mechanisms of magnetization reversal can be different on the two sides of the same hysteresis loop, depending on the orientation of the applied magnetic field. The data which we obtain in the laboratory also serve to verify the assumptions used by theorists in modeling exchange bias; e.g., our results are found to be in agreement with the prediction of a single-domain model, thus verifying the applicability of the single-domain assumption to FM semiconductor films.

## Education:

- Two grad students, Mr. (now Dr.) W.L. Lim and Mr. Z. Ge; and two undergraduate students, Miss Carissa Reynolds and  Miss Leanna Steier, contributed to this work. Miss Reynolds was an REU student from Wittenberg University, and Miss Steier is a Notre Dame undergraduate whom we employ part-time in our research group. In addition, the Ph.D. research of Mr. (now Dr.) Brian Kirby at the
- University of Missouri has benefited directly from our work on exchange bias. Dr. Kirby is now employed at NIST, where he and his colleagues are continuing this work on exchange bias.

**DMR 02-45227:**** **Exchange bias in (Ga,Mn)As/MnO Bilayers
**M. Dobrowolska**, *University of Notre Dame*

_**Societal Impact**_**:**
- Our group has been continually active as a resource of materials for other groups.  We are currently interacting with _at least 30 other institutions_ by providing them with magnetic semiconductor specimens for their research. The  understanding of these materials and their heterostructures, as well as of devices based upon them developed in our laboratory, is thus automatically of benefit to our collaborators (including graduate students in their Ph.D. research), who depend not only on the specimens that we provide, but on the intellectual input from our group in the form of characterization and general understanding of the properties of these materials.