55Mn NMR in Mn12 acetate: Quantum tunneling and magnetic relaxation of Mn12 cluster
Kakeji Kuboa, Bunro Imanaria, Takao Gotob, Keiji Takedac, Kunio Awagad
aPhysics Department, Faculty of Education, Nara University of Education, Nara 630-8528, Japan
bGraduate School of Human and Environmental studies, Kyoto University, Sakyo-ku, Kyoto 606-8501
cDepartment of Chemistry, Faculty of Science, Sapporo 060-0810, Hokkaido University
dDepartment of Chemistry, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan
For studying the relaxation of Mn12 cluster (S=10) associated with quantum tunneling, we have investigated 55Mn NMR on Mn12 acetate in the external fields. With increasing longitudinal fields, the relaxation time decreased following significant dips at every 0.45 T, which is due to the effects of phonon-assisted quantum tunneling between the spin states at magnetic level crossings. The relaxation has been also shown to depend on the reversing-rate of the fields.
Magnetic Properties of V15 Molecular Magnet
Yoshitami Ajiroa, Hiromasa Itoha, Yuji Inagakia, Tadashi Wakisakaa, Takayuki Asanoa, Takuo Sakonb, Mitsuhiro Motokawab, Yasuo Narumic, Koichi Kindoc, Bernard Barbarad
aDepartment of Physics, Kyushu University, Fukuoka 812-8581, Japan
bInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan
cKYOKUGEN, Osaka University, Toyonaka, Osaka 560-8530, Japan
dLaboratoire de Magnetisme Louis Neel, CNRS, BP 166, 38042, Grenoble, France
We apply the ESR and magnetization techniques to reveal the magnetic properties of a facinating molecular magnet, K6[V15As6O42(H2O)]·8H2O (so-called V15). The results are discussed in terms of quasi-noninteresting sub-units of a weakely antiferromagnetically coupled triangle sandwiched with two strongly AF coupled hexagons. A profound ``layer structure'' has paramount importance in determining the unusual magnetic properties of this molecular magnet.
Coherent spin quantum dynamics in antiferromagnetic molecular rings
Florian Meier, Daniel Loss
Department of Physics and Astronomy, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
Molecular magnetic clusters with antiferromagnetic exchange interaction and easy axis anisotropy belong to the most promising candidate systems for the observation of coherent spin quantum tunneling on the mesoscopic scale. Various antiferromagnetic molecular rings of this type have been synthesized to date, the most prominent examples being the ferric wheels. The experimental detection of coherent spin quantum tunneling in antiferromagnetic molecular clusters requires probes which allow one to trace the dynamics of single spins within the cluster. We point out that both nuclear magnetic resonance and electron spin resonance on doped rings are adequate experimental techniques. Although challenging, the detection of coherent spin quantum tunneling in antiferromagnetic molecular rings is feasible with present day techniques. [Phys. Rev. Lett. 86, 5373 (2001); Phys. Rev. B 64, 224411 (2001).]
Real time scale simulation for quantum processes in dissipative environments
Seiji Miyashita
Department of Applied Physics, University of Tokyo, Bunkyo-ku, Tokyo, Japan
Nanoscale magnets show dynamics of magnetization responding time dependent external fields. If the field changes faster than the time scale of the system, the system can not follow the change of the field and it shows so-called non-adiabatic process. On the other hand, if the change of the field is slow, the system follows the change adiabatically. However, if the change is too slow, the environment cause the thermaization and the system shows additional characteristics, e.g. the phonon-bottleneck process in V15 and magnetization process of Fe-rings, such as Fe12, etc. In order to study such cases numerically, we have to simulate very long time comparing with the precession period of the magnetization. We will present a new method to study dissipative processes for long time. With this method we will clarify characteristics of dynamics of nanoscale molecular magnets.
Quantum Tunneling of Magnetization in Molecular Nanomagnet Fe8 Studied by NMR
Satoru Maegawa, Miki Ueda
Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan
Magnetization and NMR measurements have been performed for single crystals of molecular magnet Fe8. The temperature and field dependences of magnetization below 25 K are well described in terms of the isolated clusters with the total spin S=10 composed of eight iron ions with spin s=[ 5/2]. The proton spin relaxation rates 1/T1 were measured down to 20 mK. The results above 400 mK show that the magnetic fluctuation is caused by the thermal transition of the electron states between the discrete energy levels separated by the single-ion-type anisotropy. Below 400 mK, the relaxation rates are temperature independent. In this temperature region the stepwise recoveries of NMR signals at the level crossing fields caused by the resonant quantum tunneling of magnetization were observed.
NMR studies on spin dynamics of the molecular nanomagnet Mn12
Yuji Furukawaa, Koji Watanabea, Ken-ichi Kumagaia, Ferdinado Borsab, Dante Gatteschic
aDivision of Physics, Graduate School of Sciences, Hokkaido University, Sapporo 060-0810, Japan
bDepartment of Physics and Astronomy, Ames Lab., ISU, Ames, Iowa 50011, USA and Dipartimento di Fisica Ä Volta" e Unita'INFM di Pavia, Via Bassi 6, 27100 Pavia, Italy
cDepartment of Chemistry, University of Florence, Via Maragliano 77, 50144 Firenze, Italy
Inner spin structure and spin dynamics of the Mn12 cluster in its high spin S=10 ground state has been studied by 55Mn nuclear magnetic resonance (NMR) under both external magnetic fields parallel and perpendicular to the magnetic easy-axis. The external field dependence of 55Mn-NMR spectrum gives a direct confirmation of the internal spin structure of the Mn12 cluster, in which spin moments of Mn4+ (S=3/2) ions are polarized antiparallel to that of Mn3+ (S=2) ions. It is proved that the microscopic spin configuration rotates rigidly when the external magnetic field is applied perpendicular to the easy-axis.