Slow Dynamics in Mesoscopic Magnets and in Random Magnets

Download Presentation

Slow Dynamics in Mesoscopic Magnets and in Random Magnets

Loading in 2 Seconds...

- 121 Views
- Uploaded on
- Presentation posted in: General

Slow Dynamics in Mesoscopic Magnets and in Random Magnets

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

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

Random materials

Metastable states

Slow dynamics

Mesoscopic materials

Lower barriers

Slow dynamics

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:

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.

diluted FeN magnetic fluid

and magnetic core of ferritin

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 (100m)

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,.

An Ordinary

Ferromagnet

Isolated Nanomagnets

Canonical Spin-Glass

All of them show thermal hystereses.

Can I distinguish them each other

by comparing the 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.

Canonical Spin-Glass

Ferromagnet

Isolated nanomagnets

Canonical Spin-Glass

a kind of aging effects can be widely observed.

Note their time-dependences

Finally

Estimated value at the final convergence

is just on the curve

by the Curie law

Extrapolation

estimated by

Although a remarkable difference exists between MZFC and MFC,

it is temporary behavior.

The equilibrium phase is unique and superparamagnetic.

Universal curve

independent of W

: Isothermal susceptibility

(W∞, )

Cole-Cole relationship

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.

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.

Ag89Mn11

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

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.]

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

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

(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.

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.

The irreversible phase below Tc has

both the memory and rejuvenation effects,

although it is presumed to be superferromagnetic.

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

w = 0, h hFC

- 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.

(3θ)/ψ ~ 3,

θ/ψ ~ 1,

θ ~ ψ~ 3/4.

χEA·h = 1.01 A/m

λ~ 3/2

Ｄh= 1.18 A/m

heating