Dynamics of novel molecular magnetsV-ring and rare earth compounds

Okayama Univ. H. Nojiri

Introduction

Magnetization step in V-rectangular ring

Short range correlation effect in molecular magnet

Rare earth compounds

Summary

Collaborators

Okayama Univ.

T. Taniguchi and K. Aikawa

Ames Lab., Iowa State Univ.

M. Luban, P. Kögerler

Res. Lab. of Resources Utilization, TITEC

T. Yamase E. Ishikawa

Correlates system and isolated systemCs3Cr2X9:3D coupled dimer

Y. Ajiro, Y. Inagaki et al.

French-Japanese Symposium, Paris 2003

T

H

?

How intra-molecular correlation

is established?

H

H

E

M

E

H

HM

?Crossover

Thermal populationShort range orderSize dependence

T<<J T>>J

High Magnetic Fields in Okayama40 T Single shot pulsed fields

Temperature 0.4-400 KESR 35 GHz-7 THz

30 T Repeating pulsed fields30 T Portable pulsed field

– Complex pulsed field– X-ray, Free Electron Laser

Antiferromagnetic four spin ring:V12

Large rectangle 　 S=1/2(V4+)Small rectangle 　 Mixed valence non-magnetic 　　　　　　　 Effective S=2 below R.T.

(NHEt3)4[VIV8VV

4As8O40H2O]H2O

N.S.=24

Neutron scattering and energy structure

Basler et al. Inorg. Chem. 41(2002)5675

S=0

S=1

S=1

S=2

　 Two triplets

　 Small splitting by exchange anisotropy

Magnetization process V12:two major steps

Two major step for

S =0 to S=1

S =1 to S=2

Intermediate step at 20 T

Small step of ~4 % of full moment 　　

Splitting of large step　 Each major step splits into two steps

　 No orientation dependence, small g-anisotropy

　 Splitting of lowest excited states,

contradicts to neutron result

Step is a very useful means for study of energy level

　

Temperature dependence of large step

Large hysteresis in 4.2 K~1.5 K Second step in down sweep

No hysteresis in low temperature

Hysteresis with thermal effect

Magnetic Fohen effect ？E

H

Intermediate step

No level crossing point at ground state　 Relaxation in excited state　 Non-adiabatic transition?

Sweep velocity~20000 T/s at 10 T

Non adiabatic transition

Sweep velocity ~20000 T/s at 10 T p~0 for infinite v E=0.1 K, v~108 T/s E=3 K, v~105 T/s

Is such large gap is reasonable?

( ) ⎥⎦

⎤⎢⎣

⎡ −−= −12

2exp1 vEp

hπ

E

HE

Small steps

　 4 % of magnetization by isomatic cluster　 Defect driven signal　 Contribution of mixed valence phase NMR-T1, T-dependence

　 Two gaps Large and Small gaps　　 F. Borsa et al. private communication

　　

Temperature dependence of EPR

Small splitting for center peak

-splitting of lowest triplet

Large splitting for side peaks

- higher excited state signals

ESR spectra with short range correlation

T>J Classical regimeconventional paramagnetic resonance

T<J Short range ordercorrelation:~1/T

T< TN Antiferromagnetic order

infinite divergence of

kBT~J

T

Temperature dependence of EPR

Width is nearly temperature independent

Small shift at low temperature

Short range regime is not clear 　　

S=2 to S=0 S=2 to S=1

Field dependence of EPR

Drastic change at steps

At higher fields, where S=2 or 1 is ground state

higher temperature, splitting is observed

low temperature, splitting is averaged

Domination of S=2 ground state

Crossover in infinite chain system

S=1 antiferromagnetic chain

Spiral structure

Formation of Haldan gap at low-T

PbNi2V2O8Uchiyama, Masuda, Uchinokura

N.S.=

Crossover from EPR to triplet resonanceAt high-T EPR split for single ion D

At low-T, Triplet split by effective DTemperature dependence of D

as evolution of quantum ground state

Wide regime of short range order

T. Masuda, K. Uchinokura and H.N.

H

E

Hp =DSz2 +gμBSi ⋅H

Hh = JSiSi+1 +DSiz2 +gμBSi ⋅H( )

i∑

Ht =D'Sz2 +gμBSi ⋅H

EPR of Spin ball V18-N3

Spherical cluster of V4+/V 5 + ions

15 of S=1/2 spins

Strong antiferromagnetic coupling

~5 B at 30 T

K. Aikawa, H.N. and T. Yamase

N.S.=215

EPR of Spin ball V18-N3

Shift of line width below 50 K

Saturation below 3 K

Ground state becomes stable

Short range regime below 20 K

Different from V12

Spin ball and ring (1)Variety of shape and network(2)Topology not available in regular lattice

fine particle 、 atomic spacing no-magic number as C60

(3)Number of state SN replace S=1/2 to S=5/2(4)Common energy structure Mo72Fe30:icosidodecahedron

N.S.=630

ESR of Mo72 Fe30

Increase of line width

Shift of resonance field

　 Development of short range correlation

Broad line width

　 Fe3+:not single ion relaxation

　 Frustration

　 Finite size effect

Decrease of line width at low T

　 No-magnetic ordering

but Saturation of correlation length

　

Rare earth compound Rare Earth compound

Longer spin

Larger magnetization

Easy substitution of ions

Smaller exchange coupling

Na8H18[{Er3O(OH3)(H2O)3}2

Al2(Nb6O19)5]40.5 H2O

Coupled triangles of Er

Magnetization of Er6 Saturate around 2 T Finite slope for anisotropyHysteresis at low fields similar to V15

Na8H18[{Er3O(OH3)(H2O)3}2

Al2(Nb6O19)5]40.5 H2O

Summary(1)Dynamics of V-ringMagnetization is very sensitive and precise probe

of energy level and dynamics

(2)Dynamical crossover and short range correlation

For large-N system, a clear short range order and possibly a quasi order

correlation length>size

(3)Rare earth compound

new candidate of single molecular magnet

1023

101

1015

104

SQUIDMolecule

Atom

630

215

24