Sahoo, Balaram:

Spin structure of exchange biased heterostructures : Fe/MnF 2 and Fe/FeF 2

Duisburg (2006), II, 180 S.
Dissertation / Fach: Physik
Fakultät für Physik
Duisburg, Essen, Univ., Diss., 2006
In this work, the 57Fe probe layer technique is used in order to investigate the
depth- and temperature-dependent Fe-layer spin structure of exchange biased Fe/MnF2
and Fe/FeF2 (pseudo-twinned) antiferromagnetic (AFM) systems by conversion electron
M¨ossbauer spectroscopy (CEMS) and nuclear resonant scattering (NRS) of synchrotron
Two kinds of samples with a 10 °A 57Fe probe layer directly at or 35 °A away from the
interface, labeled as interface and center sample, respectively, were studied in this work.
The spin structure was explained by considering two different models, unidirectional and
step-shaped distribution (fanning) model. The results obtained by CEMS for Fe/MnF2
suggests that, at 80 K, i.e., above TN = 67 K of MnF2, the remanent state Fe-layer
spin structure of the two studied samples are slightly different due to their different
microstructure. In the temperature range from 300 K to 80 K, the Fe-layer spin structure
does not change just by zero-field cooling the sample in remanence. By zero-field cooling
the samples in remanence to 18 K, i.e., below TN, the Fe spins rotate towards the (± 45±)- easy axes of MnF2 twins. This rotation results in the same spin structure for both
the interface and center samples at 18 K. By field cooling the interface sample in a field
of 0.35 T to 18 K and measuring in remanence, a smaller rotation (or fanning angle) of
the Fe-spins in comparison to the case of zero-field cooling in remanence from 300 K to
18 K was observed. When the interface sample was zero-field cooled or field cooled to
18 K, and subsequently zero-field heated to 80 K (T > TN), the CEMS results indicate
that the Fe-layer keeps the memory of its low temperature spin structure.
For Fe/FeF2, a continuous non-monotonic change of the remanent-state Fe spin structure
was observed by cooling from 300 K to 18 K. This effect can be related to the peculiar
T-dependence of magnetic anisotropy of FeF2 and short-range-ordered magnetic correlations
in the AFM induced by Fe above TN = 78 K. The high temperature Fe spin
structure of the two different samples (interface and center) is different due to their different
microstructure, but at 18 K (T < TN) the spin structures of both samples are the
same, and the Fe spins are oriented close to the easy axes of the FeF2 twins, similar to
the case of Fe/MnF2 at 18 K.
NRS of synchrotron radiation was used to investigate the temperature- and depthdependent
Fe - layer spin structure during magnetization reversal in pseudo-twinned
Fe/MnF2. A 57Fe-probe layer was embedded in the 56Fe layer in a wedge-type manner,
so that the distance of the 57Fe layer from the Fe/MnF2 interface varies when the
synchrotron beam is scanned from one end of the sample to the other end. A depthdependent
Fe spin structure in an applied magnetic field (applied along the bisector of
the twin domains) was observed at 10 K, where the Fe spins closer to the interface are
not aligned along the field direction. During magnetization reversal the spins of the
top Fe layer rotate at a smaller field than the Fe spins closer to the interface. Upon
decreasing the field from the fully aligned state in a strong positive magnetic field, the
Fe spins coherently rotate up to the easy direction of MnF2 (at ± 45± from the applied
field), then ”jump” to the opposite direction of the easy axes (i.e., ¨ 45±), and then
further rotate towards the negative applied field direction. The depth-dependence of the
spin structure in an applied field and the rotation via the jump disappear at 150 K, i.e.,
above TN of MnF2.