Slow dynamics in mesoscopic magnets and in random magnets
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Slow Dynamics in Mesoscopic Magnets and 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

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Slow Dynamics in Mesoscopic Magnets and in Random Magnets

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Slow dynamics in mesoscopic magnets and in random magnets

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


Introduction

Random materials

Metastable states

Slow dynamics

Mesoscopic materials

Lower barriers

Slow dynamics

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:


Example magnet

Example: Magnet

Ordinary ferromagnets

(usually with pinning centers)

Ferromagnet

with Wandering Axis?

Random Ferromagnet?

Reentrant Spin-Glass?

Superferromagnet?

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.


In this talk

diluted FeN magnetic fluid

and magnetic core of ferritin

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,.


Hystereses

An Ordinary

Ferromagnet

Isolated Nanomagnets

Canonical Spin-Glass

Hystereses

All of them show thermal hystereses.

Can I distinguish them each other

by comparing the field-dependence?


Field dependence

An Ordinary

Ferromagnet

Isolated Nanomagnets

In all of the systems,

the irreversibility appears at lower temperature

as magnetic field increases.

Because their experimental appearances are almost

the same, It is not easy to distinguish them each other.

Field-dependence

Canonical Spin-Glass


Isothermal aging

Ferromagnet

Isolated nanomagnets

Isothermal aging

Canonical Spin-Glass

a kind of aging effects can be widely observed.


Nature of m zfc m fc isolated nanomagnets

Note their time-dependences

Finally

Estimated value at the final convergence

is just on the curve

by the Curie law

Extrapolation

estimated by

Nature of [MZFCMFC]Isolated nanomagnets

Although a remarkable difference exists between MZFC and MFC,

it is temporary behavior.

The equilibrium phase is unique and superparamagnetic.


Nature of m zfc m fc canonical spin glass

Universal curve

independent of W

: Isothermal susceptibility

(W∞, )

Cole-Cole relationship

Nature of [MZFCMFC]Canonical spin-glass

Relaxation curves

after various cooling histories (W=0)

eternity

While memories due to cooling histories disappear fast,

the difference between MZFC(W∞, t) and MFC(W=0, t)survives for a long time,

as predicted by SG theories.


Memory and rejuvenation in the isolated nanomagnets

Ferritin

In contrast with canonical spin-glasses,

we canobserve

neither rejuvenation nor memory effects for MZFC.

Only the memory effects were seen for MFC,

because the population ratio of to can be changed during the halts only on cooling in a field.

Memory and Rejuvenationin the isolated nanomagnets

Ag89Mn11

Mathieu et al. Phys. Rev. B 65 (2002) 092401.


Memory and rejuvenation in the ordinary ferromagnets

In contrast with canonical spin-glasses,

we canobserve

only the rejuvenation effects for AC().

These results are consistent with the previous report

for ferromagnetic thiospinel CdCr2S4.

[ Vincent et al. Europhys. Lett.50 (2000) 674.]

Memory and Rejuvenationin the ordinary ferromagnets

Jonason et al. Phys. Rev. Lett. 81 (1998) 3243.


Features of slow dynamics

As an example,

We shall discuss the experimental results

for a strongly interacted super-spin system

from the viewpoint of

these characteristics of the slow dynamics.

Aging effects are widely observed.

irreversible, rejuvenation, and effects

Features ofSlow dynamics


Strongly interacted super spins ex cofe sio 2 nano granular film

(Co0.95Fe0.05)49 (Pd0.14Si0.27O0.59)51

10nm

Sample

Susceptibility

Critical plots

Above 285 K,

Unhysteretic susceptibility with Curie-Weiss behavior

Super-spins fluctuate with ferromagnetic correlations

Around 285K,

Critical slowing-down and divergences of susceptibilities

A ferromagnetic-like phase transition

We can presume the irreversible phase superferromagnetic.

Strongly interacted super-spins Ex. CoFe-SiO2 nano-granular film


Strongly interacted super spins slow dynamics

Magnetization on reheating

after ZFC with and without the halt

Difference of MZFC with the halt

from the reference

The susceptibility becomes relatively small

only in the vicinity of the aging temperature.

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.


Conclusion

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


Appendix

Appendix


Appendix1

Appendix


Appendix2

Appendix


Appendix3

Appendix

w = 0, h  hFC


Appendix4

Appendix


Appendix5

Appendix

  • MZFC(τw, τ) ≈ MZFC(τw→∞, τ) + MAG(τw, τ),(1)

    MZFC(τw→∞, τ) ≈ χEA·h–a0·[L(τ)]−θ, (2)

    MAG(τw, τ) ≈ a1·[L(τ)/L(τw)]3−θ,(3)

  • MFC(τ)≈ χFC(τ) ·h + Mex(4)

    χFC(τ) ·h ≈ χD·h–a2·[L(τ)]−θ,(5)

    Mex ≈ a3· [L(τ)]−λ,(6)

    ≈χD·h–a2·[ln(τ/τc)]−1 + a3·[ln(τ/τc)]−4λ/3,

    where Mex comes from unknown memories during cooling.

    • L(x) ~ [ln(x/τc)]1/ψ, τc ~τ0·(1−T/Tg)−zυ.


Appendix6

Appendix

(3θ)/ψ ~ 3,

θ/ψ ~ 1,

θ ~ ψ~ 3/4.

χEA·h = 1.01 A/m


Appendix7

Appendix

λ~ 3/2

Dh= 1.18 A/m


Appendix8

Appendix


Appendix9

Appendix


Appendix10

Appendix

heating


Appendix11

Appendix


Appendix12

Appendix


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