UME KALSOOM ROLL#13(M.PHIL)

2. UME KALSOOMROLL#13(M.PHIL)

3. Interfacial spin order in exchange biased systemsL. E. Fernandez-Outon, G. ValPublished by the AIP Publishinglejo-Fernandez, SadiaManzoor, B. Hillebrands, and K. O’GradyPublished by……………………..The AIP publishing

4. Objectives of paper • Exchange biased phenomena occurs when layer of ferromagnetic is in contact with anti ferromagnetic leads to shift of hysteresis loop. • Description of series of experiments are given to explain the dependence of interfacial spin values. These factors are • Temperature • Field We will discuss these experiments and their results one by one.

5. EXCHANGE BIASE • Exchange bias or exchange anisotropy occurs in bilayers (or multilayer) of magnetic materials where the anti ferromagnetic spin effected by the magnetization of ferromagnetic material.

6. NEEL TEMPERATURE • The Néel temperature or magnetic ordering temperature, TN, is the temperatureabove ananti ferromagneticmaterial becomes paramagnetic-that is, the thermal energy becomes large enough to destroy the macroscopic magnetic ordering within the material.

7. CURIE TEMPERATURE The Curie Temperature is the critical point where a material's intrinsic magnetic moments change direction.  Below the Curie temperature, neighbouring magnetic spins align in a ferromagnet in the absence of an appliedmagnetic field.  Above the Curie temperature, the magnetic spins are randomly aligned in a paramagnet unless a magnetic field is applied.

8. WORKING OF EXCHANGE BIASE • In ferromagnetic/anti ferromagnetbilayers, the Curie temperature of the ferromagnetic is larger than the NéeltemperatureTN of the anti ferromagnetic. This inequality means that the direction of the exchange bias can be set by cooling through TN in the presence of an applied magnetic field. The moment of the magnetically ordered ferromagnetic will apply an effective field to the anti ferromagnetic as it orders, breaking the symmetry and influencing the formation of domains.

9. Schematics Exchange Bias

10. CONTRIBUTION OF DIFFERENT SCIENTISTS • MEIKLEJOHN AND BEAN • They use Co/CoO to observe hysteresis loop shift when AF layer is cooled at Neel temperature. • Exchange biased is due to domain formation in AF layer. • But there were too much difference in numerical model an experimental values. Numerical model means,value of Hex,Hc,Hr

11. LIMITATION OF M-BEAN WORK • Maurie suggested that Meiklejhon and Bean model was good for single crystal layered structure. • Its value of Hex is agreed with numerical values but it didn’t give any information about correctives of materials and information about the reduction of Hex with field cycling.

12. CONTRIBUTION OF OTHER SCIENTISTS • Malozemoff suggested that Hex is due to presence of roughness in interface which leads to the formation of domain formation in the direction perpendicular to interface.And domain motion is the reason of reduction of Hc. • Fulcomer and Charapsuggested that the value of Hex and Hc is frequency and temperature dependence. • They concluded that the exchange field from F to AF would result in thermal activated change in orientation of AF sublattics leading to the variation of observed value of Hex.

13. CONTRIBUTION OF OTHER SCIENTISTS • Nowak proposed that domain wall state model. • He proposed that dilution of AF layer enhanced the exchange biasing. Dilution means the introduction of non magnetic atoms in AF layer.

14. CONTRIBUTION OF OTHER SCIENTISTS • Takona suggested the interfacial spin effect in NiFe/CoOsystem.He observed that enhancement of Hex due to field cooling effect. • Stamps suggested that the existence of two different type of walls. • He suggested that the shift of hysteresis loop is due to formation of reversible and irreversible domain wall formation.

15. GIVEN PAPER APPROCH • In this paper they described number of experiments to study the effect of thermal activations that allows them to differentiate the bulk and interfacial contribution. And also discuss the dependency of Hex on temperature. • One most important thing is volume of given layers under observation. • They discuss the observation of Takano but for IrMn/CoFe and FeMn/NiFe system.

16. WORKING MECHANISM Two systems are discussed here in this paper. • IrMn/CoFe • FeMn/NiFe • IrMn an FeMn layers are thermally un stable. So to produce reproduce able data following are the steps. • AF layers are heated at their Neel temperature.(Activation temperature) • The saturation of F layer. • Under this thermal activation AF layers themselves at field cool temperature of F layers. • Thus hysteresis loop obtained due to this mechanism has its hysteresis loop shifted at thermal activation free temperature.(Thermal active free temperature) • Then cycling in the negative field is done to remove training effects. .

17. An Intuitive Picture of Exchange Bias Markus Kirschner

18. SHIFT OF HYSETERSIS LOOP AFTER THERMAL ACTIVATION

19. MATHEMATICAL EXPLAINATION • The dependency of temperature on Hex can be mathematically represented by an equation

20. GRAPHICAL EXPLAINATION

21. MEDIAN BLOCKING TEMPERATURE • The temperature at which Hex=0 is the median blocking temperature. • Accordingly is the temperature at which half distribution of energy barrier to reversal in AF is in the original state, while the other half has reversed its state due to thermal activation.

22. TEMPERATURE EFFECT ON Hex

23. LIMITATION In FeMn system below 50K,there is a dramatic increase in value of Hex which doesn’t follow the behavior we cannot find the reason of this abnormal behavior

24. CONCLUSION • Through different measurements they gave procedure to differentiate between bulk and interface effects using exchange biased system. • They observed the temperature dependence of the degree of order of interface spin.

25. ANY QUESTION?