Dynamics of novel molecular magnets V-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 system Cs 3 Cr 2 X 9 :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 H M ? Crossover Thermal population Short range order Size dependence T<<J T>>J

High Magnetic Fields in Okayama 40 T Single shot pulsed fields Temperature K ESR 35 GHz-7 THz 30 T Repeating pulsed fields 30 T Portable pulsed field –Complex pulsed field –X-ray, Free Electron Laser

Antiferromagnetic four spin ring:V 12 Large rectangle S=1/2(V 4+ ) Small rectangle Mixed valence non-magnetic Effective S=2 below R.T. (NHEt 3 ) 4 [V IV 8 V V 4 As 8 O 40 H 2 O]H 2 O N.S.=2 4

Neutron scattering and energy structure Basler et al. Inorg. Chem. 41(2002)5675 S=0 S=1 S=2 Two triplets Small splitting by exchange anisotropy

Magnetization process V 12 :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~10 8 T/s  E=3 K, v~10 5 T/s Is such large gap is reasonable? E H

Small steps 4 % of magnetization by isomatic cluster Defect driven signal Contribution of mixed valence phase NMR-T 1, 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>JClassical regime conventional paramagnetic resonance T<J Short range order correlation:  ~1/T T< T N Antiferromagnetic order infinite divergence of  k B T~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 PbNi 2 V 2 O 8 Uchiyama, Masuda, Uchinokura N.S.= 

Crossover from EPR to triplet resonance At high-T EPR split for single ion D At low-T, Triplet split by effective D Temperature dependence of D as evolution of quantum ground state Wide regime of short range order T. Masuda, K. Uchinokura and H.N. H E

EPR of Spin ball V 18 -N 3 Spherical cluster of V 4+ /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.=2 15

EPR of Spin ball V 18 -N 3 Shift of line width below 50 K Saturation below 3 K Ground state becomes stable Short range regime below 20 K Different from V 12

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 C 60 (3)Number of state S N replace S=1/2 to S=5/2 (4)Common energy structure Mo 72 Fe 30 :icosidodecahedron N.S.=6 30

ESR of Mo 72 Fe 30 Increase of line width Shift of resonance field Development of short range correlation Broad line width Fe 3+ :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 Na 8 H 18 [{Er 3 O(OH 3 )(H 2 O) 3 } 2 Al 2 (Nb 6 O 19 ) 5 ]40.5 H 2 O Coupled triangles of Er

Magnetization of Er 6 Saturate around 2 T Finite slope for anisotropy Hysteresis at low fields similar to V 15 Na 8 H 18 [{Er 3 O(OH 3 )(H 2 O) 3 } 2 Al 2 (Nb 6 O 19 ) 5 ]40.5 H 2 O

Summary (1)Dynamics of V-ring Magnetization 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 SQUID Molecule Atom  2424