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