Slow Dynamics in Mesoscopic Magnets and in Random Magnets

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Slow Dynamics in Mesoscopic Magnetsand in
Random Magnets

• H. Mamiya
• National Institute for Materials Science
• Tsukuba 305-0047, Japan
• Collaboration
• M. Ohnuma, NIMS, Japan
• T. Furubayashi, NIMS, Japan
• I. Nakatani, NIMS, Japan
• S. Nimori, NIMS, Japan
• M. Sasaki, Tohoku University, Japan
• P. E. Jönsson, RIKEN, Japan
• H. Takayama, University of Tokyo, Japan

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Introduction
Well-clarified Bulky materials with periodic
structures Permanently stable ground
states Ultra-fast excitations
Central objects of future researches
Experimental understanding of slow dynamics
Issue
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Example Magnet
Ordinary ferromagnets (usually with pinning
centers)
Ferromagnet with Wandering Axis?
Random Ferromagnet?
Superferromagnet?
Reentrant Spin-Glass?
Correlated superspin glasses?
Speromagnet?
Cluster-Glass?
Canonical spin-glasses
Isolated nanomagnets (ideal superparamagnets)
Super-Spin-Glass?
Too many models have been proposed. Experimental
studies have been confused them.
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In this talk,
We show experimental features of the slow
dynamics in ordinary ferromagnets, in a
canonical spin-glass, and in isolated
nanomagnets,
pure Tb and Ni3Al foils,
Cu0.97Mn0.03 wires (100?m)
from the point of view of irreversible, aging,
rejuvenation, and memory effects.
Then, we will discuss strongly interacted
super-spin systems using the knowledge of the
feature,.
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Hystereses
All of them show thermal hystereses.
Can I distinguish them each other
by comparing the field-dependence?
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Field-dependence
Canonical Spin-Glass
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Isothermal aging
Canonical Spin-Glass
ZFC or AC TRM
Canonical spin-glasses Remarkable Remarkable
Ordinary ferromagnets Observable
Isolated nanomagnets No Observable
a kind of aging effects can be widely observed.
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Nature of MZFC ? MFCIsolated nanomagnets
Although a remarkable difference exists between
MZFC and MFC, it is temporary behavior. The
equilibrium phase is unique and superparamagnetic.
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Nature of MZFC ? MFC Canonical spin-glass
Relaxation curves after various cooling histories
(?W0)
eternity
While memories due to cooling histories disappear
fast, the difference between MZFC (?W?8, t) and
MFC (?W0, t) survives for a long time, as
predicted by SG theories.
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Memory and Rejuvenation in the isolated
nanomagnets
Ag89Mn11
Mathieu et al. Phys. Rev. B 65 (2002) 092401.
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Memory and Rejuvenation in the ordinary
ferromagnets
Jonason et al. Phys. Rev. Lett. 81 (1998) 3243.
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Features ofSlow dynamics
Ordinary ferromagnets Canonical spin-glasses Isolated nanomagnets
Hysteresis MZFC ? MFC (To be temporary) MZFC ? MFC (Semi-)permanent MZFC ? MFC Temporary
Aging Observable Remarkable Only TRM
Rejuvenation Observable Remarkable None
Memory Little Remarkable Only MFC
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Strongly interacted super-spins Ex. CoFe-SiO2
nano-granular film
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Strongly interacted super-spins Slow dynamics
The irreversible phase below Tc has both the
memory and rejuvenation effects, although it is
presumed to be superferromagnetic.
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Conclusion
As shown for an example of interacted super-spin
systems,
Ordinary ferromagnets
Superferromagnet?
Random Ferromagnet?
Reentrant Spin-Glass?
Ferromagnet with Wandering Axis?
Correlated superspin glasses?
Speromagnet?
Cluster-Glass?
Super-Spin-Glass?
spin-glasses
Superparamagnets
The characteristics of the slow dynamics can be a
key to experimental understanding of the confused
systems
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Appendix
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Appendix
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Appendix
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Appendix
?w 0, h ? hFC
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Appendix
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Appendix

• MZFC(tw, t) MZFC(tw?8, t) MAG(tw,
  t), (1)
• MZFC(tw?8, t) ?EAh a0L(t)-?, (2)
• MAG(tw, t) a1L(t)/L(tw)3-?, (3)
• MFC(t) ?FC (t) h Mex (4)
• ?FC (t) h ?Dh a2L(t)-?, (5)
• Mex a3 L(t)-?, (6)
• ?Dh a2ln(t/tc)-1 a3ln(t/tc)-4?/3,
• where Mex comes from unknown memories during
  cooling.
• L(x) ln(x/tc)1/?, tc t0(1-T/Tg)-z?.

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Appendix
? (3??)/? 3, ??/? ?1, ? ? 3/4. ?
?EAh 1.01 A/m
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Appendix
? ? 3/2 ? ?D?h 1.18 A/m
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Appendix
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Appendix
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Appendix
heating
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Appendix
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Appendix