Exchange Bias Manifesto


(9-25-2002)

Ivan K. Schuller and G. Guntherodt

There is a time in the life of scientists when it is important to consider:

What is the Grand Purpose of it all?.

In this case, we started wondering, why all papers in Exchange Bias start by stating something like:

“Although exchange bias was discovered more than 40 years ago, the origin of this phenomenon is still not clear”.

This brief document attempts to put some order into this issue and raise some relevant questions. We would appreciate any corrections, comments and additions. However we must insist on one requirement: Be brief!.

Figure 1. Status of the Exchange Bias field, based on the introduction of literally hundreds of papers.

Figure 2. One possible good (i.e.brief) way to summarize the status of research for a particular system. In this case for FeF2 (110)-Fe (polycrystalline). From M. Fitzsimmons et al, Phys Rev. B 65, 134436(2002).
 

“Philosophical” Considerations

  • There are many apparently confusing, contradictory (?) facts. See figure 1.
  • There are very diverse systems, with various degrees of control on the structure.
  • Because of this it is important to have some brief concise way of classifying the main results, as indicated in figure 2 above. This should be amplified and other similar tables be constructed.
  • The purpose is to find out which facts are essential, which are just side issues
  • It is probably useful (?) to categorize the systems:
    1. Crystalline AF – Interface - Crystalline F
    2. Complex AF- Interface – Complex F
    3. Disordered Magnet – Interface – F
    4. Hard F – Interface – Soft F (Spring Magnet)
     
  • The various parameters needed to characterize an exchange biased system are:
    1. Growth method (thin film, natural oxidation, …)
    2. Characterization tools (quantitative, qualitative?)
    3. AF crystal structure
    4. F crystal structure
    5. Interfacial roughness
    6. Neel temperature
    7. Curie Temperature
    8. Thin film crystal structures
    9. Bulk and thin film anisotropies
    10. Bulk and thin film spin structure
    11. Blocking temperature
    12. Cooling field
    13. Cooling procedure
    14. Exchange Bias
    15. Coercivity
    16. Left and Right hand dependence of M-H as a function of T
    17. Reversal Mode
    18. Training?

Definitions


  1. “Parallel”, “perpendicular” to the interface.
  2. “90°”, “collinear” with respect to the AF spins.

 

ESTABLISHED(?) EXPERIMENTAL FACTS TO BE EXPLAINED
(More or less ordered systems)


 


Experiment
System
1) Spin orientation FeF2-Fe, Co/Pt-CoO (?), FeF2-Co/Pt
Parallel (F-AF) Spins Favored
2) Compensated vs uncompensated depends on system? maximized
Compensated (FeF2, MnF2, FeMn) 
Uncompensated (CoO)
3) Positive He FeF2-, MnF2-Fe, Co-CoO
4) Coupling: A) 90°
B) Collinear
LaFeO3- Co, FeMn- Co
5) Roughness non-monotonic MnF2-Fe
6) Ion irradiation initial increase NiFe/FeMn
7) AF dilution increase CoO-Co, FeF2-Fe
Metallic (Py-IrMn+Rh,Pt..)
8) Loose spins  ??? Py-CoO
9) Crystallinity
Single Xtals small
Epitaxial varied
Twin large
Poly intermediate
CoO, NiO, FeF2 ???
 
FeF2
FeF2, CoO
10) He maximized (w/r AF anisotropy)
    Along  ?
FeF2-Co?, MnF2-Fe
Close
CoO-FeNi
11) AF anisotropy MnF2<<<<FeF2<<CoO 
12) 3rd order anisotropy MnF2-Fe, NiFe-CoO
13) Asymmetric loop FeF2-, MnF2-Fe, CoO-Co, 
NiMn-NiFe, PtMn-NiFe
14) Asymmetric Reversal FeF2 (twinned), CoO
15) Training Fe, Co, Ni, Ni50Fe50, Ni81Fe19 - FeMn
La(2/3)Ca(1/3)MnO,La(1/3)Ca(2/3)MnO3
Co, -CoO,Ni, NiFe - NiO, NiFe - aFe2O3
Co - LaFeO3,Co – IrMn
                            No
FeF2-, MnF2- Fe
16) Rotation in  F, some FeF2-, MnF2-Fe, CoO-Co
17) Vertical shift MnF2-,FeF2-Fe, CoO-Co
18) Coercivity  enhancement FeF2-,MnF2-Fe,CoO-Co, NiFe-FeMn
La(2/3)Ca(1/3)MnO3 -
La(1/3)Ca(2/3)MnO3 -, all(?)
19) Reversible measurements bigger Co-CoO (AMR, BLS)
20) Thickness dependence
              i)   AF (up+1/d) FeF2-Fe, CoO-Co, FeMn-FeNi
                   AF (up+flat) CoO-Co, FeMn-FeNi
             ii)   F (1/d) FeF2-Fe
21) Cooling field FeF2-, MnF2-Fe, CoO-Co
22) Magnetic history, cooling from T

RANDOM QUESTIONS


1) Is more than one theory possible? This is the case after all for the resistivity.

2) Is it possible that different structural parameters are responsible for He and Hc? The angular symmetries are different apparently?

3) Is there a correlation between He and asymmetry in reversal? Does large asymmetry imply large Heb?

4) Is the correlation between asymmetry and 3rd order anisotropy important?

5) What about the crystalline orientation?

6) Are domain walls important for He, Hc, both or one of them?

7) Is crystallinity (rocking curve width) important?

8) Are uncompensated spins needed, do they exist, ...?

9) Is the phenomenology of Exchange Bias the same independent of system: bilayer, multilayer, disordered AF, spring magnet,…?

10) Will we ever solve this or we will get tired before and die?. This is more relevant to me than some of you young lads.

Other Considerations

1. Numerical simulations show anisotropic reversal in M either from:

a) Third order anisotropy.
b) Stoner-Wohlfart + Domains.
2. Most CoO on Co is done by oxidizing Co in air.
 



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