INTERMAG 2015ES-02 Analysis of magnetic relaxation with pre-existing nucleation sites based on the Fatuzzo-Labrune model.. Department of Physics, Chungbuk National University, Cheongju,
Trang 1INTERMAG 2015
ES-02 Analysis of magnetic relaxation with pre-existing nucleation sites based on the
Fatuzzo-Labrune model.
D QUACH1, D HANDOKO1, S LEE1, J SHIM1, T PHAN1, D KIM1, D PHAM2, K LEE3,
J JEONG3, D NGO4
1 Department of Physics, Chungbuk National University, Cheongju, Korea; 2 Faculty of
Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam
National University, Hanoi, Viet Nam; 3 Department of Material Science and Engineering and
Graduate School of Energy Science and Technology, Chungnam National University, Daejeon,
Korea; 4 Department of Micro- and Nanotechnology, Technical University of Denmark, Kgs
Lyngby 2800, Denmark
Time-resolved magnetic domain patterns of (Co/Pt) and (CoFeB/Pd) multilayers with
perpendicu-lar magnetic anisotropy are observed by means of magneto-optical microscopy, from which
mag-netic relaxation curves are determined for Interestingly, it has been observed that the relaxation
processes not only from the saturated state, but also with pre-existing domains, are well explained
based on the Fatuzzo-Labrune model [1, 2] Full details of the relaxation behavior and subsequent
microscopic domain patterns evolving from the pre-existing nucleation sites originated from the
sub-structured magnetic domains are discussed
It has been well known that the Fatuzzo-Labrune model is applicable in explaining magnetic
relax-ation processes involved with a thermally activated magnetizrelax-ation reversal in various magnetic thin
films [3, 4] According to the model, the magnetic relaxation is quantied by a reversal parameter k
= v/Rr c , where v is the domain wall velocity, R the nucleation rate, and r c the initial domain radius
The shape of the curve and the value of k reflect a competition between domain wall motion and
domain nucleation during the magnetic relaxation process For k >> 1, the magnetization reversal
mechanism is known to be dominated by domain wall propagation and the curve has a so-called
S-shaped one, while, for k << 1, the reversal mechanism is considered to be dominated by
nucle-ation with the curve of so-called L-shape meaning an exponential decay However, so far, no study
has been addressed to explore the validity of the Fatuzzo-Labrune model to relaxation behavior
starting not from the saturated state but from the partially saturated state with non-reversed
pre-ex-isting sub-structured domains, which corresponds to the minor loop field cycling
In this work, we explore the validity of the Fatuzzo-Labrune model even for the case of relaxation
behavior with pre-existing sub-structured magnetic domains using magneto-optical microscopy,
which is capable of quantitatively characterizing magnetization reversal behaviors directly
deter-mined from the real-time domain observation We have investigated magnetic relaxation behaviors
of (3.1-Å Co/7.7-Å Pt)10 and (4-Å CoFeB/10-Å Pd)4 multilayers with a perpendicular magnetic
anisotropy to explore how the reversal mechanism changes itself even for the same sample when
beginning with pre-existing nucleation sites
After saturated the films by a very high negative field -H sat about of 1 kOe, the field is switched to
a positive holding field +H hold in 5s to let magnetization appear to reach the reversed state of +M S
Then the field is switched again to a negative reversing field -H rev and relaxation curve is
moni-tored As an example, relaxation processes under a constant negative reversing field of H rev = 220
Oe with variation of holding field H hold, in case of (3.1-Å Co/7.7-Å Pt)10 film, are presented in Fig
1(a) With increasing H hold, the relaxation process becomes slower, which is characterized by a
decrease of half-reversal time as seen in the inset of Fig 1(b) In addition, the curve changes the
shape from L- to S-shaped one Fig 1(b) shows fitting values of k and R for relaxation processes
plotted in Fig 1(a) As seen in Fig 1(b), k increases with increasing H hold, which means the reversal
mechanism changes from nucleation dominance to domain wall motion dominance Fig 2 presents
domain patterns of pre-existing non-reversed domains corresponding to the initial relaxation
pro-cesses showed in Fig 1(a), which is considered to be consistent with k values.
In summary, from the direct domain observation, we have demonstrated that non-revered domains
serving as pre-existing nuclei strongly affecting on the relaxation behavior of (Co/Pt) and (CoFeB/
Pd) multilayers with a perpendicular anisotropy, where our systematic analysis reveals that the
Fatuzzo-Labrune model becomes still valid and could be readily extended to the case of relaxation behaviors starting from the pre-existing sub-structured domains
[1] E Fatuzzo, “Theoretical Considerations on the Switching Transient in Ferroelectrics”, Phys Rev 127, 1999 (1962) [2] M Labrune, S Andrieu, F Rio, and P Bernstein,” Time dependence of the magnetization process of RE-TM alloys”, J Magn Magn Mater 80, 211 (1989).
[3] Y W Windsor, A Gerber, and M Karpovski, “Dynamics of successive minor hysteresis loops”, Phys Rev B 85, 064409
(2012).
[4] Qiong Wu, Wei He, Hao-Liang Liu, Yi-fan Liu, Jian-Wang Cai, and Zhao-Hua Cheng, “Magnetization reversal asymmetry
in [Co/Pt]/CoFe/IrMn multilayers with enhanced perpendicular exchange bias”, J Appl Phys 113, 033901 (2013).
Fig 1 (a) Time-dependent relaxation curves of (3.1-Å Co/7.7-Å Pt) 10 film under negative reversing
field of H rev = 220 Oe after exposing at different
holding fields H hold between 260 and 400 Oe, fitted
by Fatuzzo-Labrune model (solid lines) (b) k and
R values with respect to the holding field The inset
shows a holding field dependence of half-reversal
time (t1/2 ) corresponding to the case of (a).
Fig 2 Different domain patterns of (3.1-Å Co/7.7-Å Pt) 10 film at a nucleation phase with ferent holding field, leading to a contrastingly dif-ferent relaxation mechanism even in the same film.