Jahresbericht 2005 - IPHT Jena

MAGNETIK & QUANTENELEKTRONIK / MAGNETICS & QUANTUM ELECTRONICS
Fig. 1.7: Linear dependence of the domain wall
velocity v on the field strength H mov in a stripe with
a defect.
Fig. 1.8: Distribution of the field strength necessary
for nucleation of a domain wall in a GMR
stripe without domain wall generator.
In stripes without a domain wall generator the
domain wall nucleates at one end of the stripe at
a field strength H nuc = 18–26 kA/m and moves
through the line within some µs. The peak field is
inversely proportional to the stripe width and has
a very narrow Gaussian distribution (σ ~ 0.8% of
the peak field) as shown in Fig. 1.8. This field is
larger than the field necessary to overcome every
pinning and corresponds to the coercitive field
strength H C.
1.2.7 Measurement of coupling strength
distribution in exchange bias film
systems
(Klaus Steenbeck, Roland Mattheis)
The exchange bias field and the coercitivity of
ferro-/antiferromagnetic (F/AF) coupled polycrystalline
film systems for magnetoelectronics
strongly depend on the coupling strength j of the
individual grains, with their distribution function
P(j) in the grain ensemble. Until now P(j) was not
measurable.
We quantified our proposed method to determine
P(j) with low temperature torquemetry (see IPHT
annual report 2004) and included thermal relaxation
processes in our model used to analyse the
experimental non-equilibrium torque curves L(Φ).
Furthermore, we performed time dependent
measurements which directly show relaxation at
T = 10 K.
L(Φ) is the irreversible contribution to the torque
exerted on the F/AF film by an in-plane rotating
strong magnetic field after a rotation reversal
from the cw to ccw sense. Beginning at the reversal
point (Φ = 0), the film grains of coupling
strength j change their magnetic states from one
equilibrium (cw) to a new one (ccw) at characteristic
angles Φ(j). Thus L(Φ) reflects P(j) and we
derive
P(j) = 1/(S(Kt) 2 ) * G(βS(j),Φ) * d2L(Φ)/dΦ2 ,
where S, K, and t are the area, anisotropy constant,
and thickness of the AF film, respectively.
The function G was calculated in the frame of a
Stoner-Wohlfarth model for 3-axial anisotropy
(inset of Fig. 1.9). This model describes for a single
grain the magnetization angles βS(j/Kt) at
which the coupled AF net moment switches from
one AF anisotropy axis to the next, thus contributing
to the irreversible torque L(Φ). Thermal energy
reduces this switching angle βS(j,T)< βS(j,0) as
demonstrated in Fig. 1.9, because within the
measuring time t > τ =10 –9s * exp(EB/kBT) an
energy barrier EB can be overcome. This must be
considered also at T = 10 K because the grain
volume V � EB is very small. As an example
Fig. 1.9: Calculated critical magnetization angles
β S for switching of the AF net moment µ AF to a
neighbouring easy axis as a function of the grain
coupling (j/Kt). The shown parameter is the ratio
(T/K) of temperature and AF anisotropy constant
K.
Inset: Sketch of the rotating magnetization M F
and the coupled AF net moment µ AF in a film crystallite
with 3 easy axes of the AF.
21