NMR study of a mixed-metal molecular magnet Yutaka FUJII (University of Fukui) Contents  Introduction (Magnetic properties)  Experimental results  1 H NMR  55 Mn NMR  Summary 4 Dec. Kyoto (International Workshop on “Physics on Nanoscale Magnets”) ~ 1 H and 55 Mn NMR of Mn III Cu II cluster ~

[Mn III Cu II (Br-sap) 2 Cl(MeOH)] Selected Bond Distances (Å) Mn-Cl 2.616(4) Mn-O1S 2.658(9) Other bonds 1.871(5) (6) Axially elongated octahedron for d 4 D = cm -1 g Mn = 1.881, g Cu = 2.21 J = +78 cm -1 Cu 2+ (S=1/2)Mn 3+ (S=2) Ferromagnetic coupling J between Mn 3+ and Cu 2+ S=5/2 ground state Introduction The smallest size of heteronuclear high-spin molecule

Packing diagrams of [Mn III Cu II (Br-sap) 2 Cl(MeOH)] ac projection view bc projection view ab projection view c-axis Crystal parameters monoclinic P21/c a = , b = , c = (Å)  =  = 90º,  = º

High-field ESR of MnCu cluster by Nojiri et al. (Okayama Univ.) Anisotropy parameters from ESR D = cm -1, E = 0.08 cm -1 H m 20 º S z : -5/2 -3/2

 Anisotropy parameters from ESR D = cm -1, E = 0.08 cm -1 → Energy barrier  E = 15 K Energy barrier for S = 5/2 S z =-5/2S z =5/2 S z =-3/2 S z =±1/2 S z =3/2 MnCu cluster as a single molecule magnet  Hysteresis of magnetization curve in the pulsed field Jump of magnetization → Avalanche? Single molecule magnet with mixed-metal core

Magnetization curve for [Mn III Cu II (Br-sap) 2 Cl(MeOH)] Hysteresis T < 0.5 K T-independent Small by Yamaguchi, Ishimoto (ISSP)

Blocking temperature T B ~ 0.5 K Single crystal AC magnetic susceptibility of [Mn III Cu II (Br-sap) 2 Cl(MeOH)] by Yamaguchi, Ishimoto (ISSP)

Motivation In order to study static and dynamic properties... NMR study has just started! Experiments NMR is possible for both nuclei of magnetic ions ( 55 Mn, 63,65 Cu) [Mn III Cu II (Br-sap) 2 Cl(MeOH)] Smallest mixed-metal SMM: S = 5/2 Rather large energy barrier:  E ~ 15 K Core of only two ions → Simpler analysis for nuclear magnetic relaxation (2) 55 Mn-NMR collection of tiny crystals H < 20 kOe T ~ 1.5 K (1) 1 H-NMR single crystal H//c, H < 35 kOe T > 1.5 K

1 H-NMR Experimental results

1 H-NMR spectrum  T 2 becomes short rapidly. → Strong reduction of the intensity above 3 K. Below ~ 3 K  6 (or more) peaks  Little shift  Magnetic moment is almost static from NMR viewpoint. H // c

Proton spin-lattice relaxation rate T 1 -1 Temperature dependence of the relaxation rate of 1 H in MnCu cluster guide for the eyes T 1 -1 enhancement around 4 K H // c Similar enhancement of T ) AF ring clusters with S=0 → scaled by intra-molecular coupling (?) 2) High-spin clusters ex) Mn12, Fe8 → scaled by  E ?

1 H relaxation rate in Mn 12 Bz (= Mn 12 Ph) T 1 -1 of 1 H in Mn12 clusters 1 H relaxation rate in Mn 12 Ac A.Lascialfari et al.: PRB 57 (1998) 514. Y. Fujii, T. Goto, A.Lascialfari, et al.: unpublished. Enhancement of T 1 -1 MnCu cluster: ~ 4 K (~ 0.3  E)Mn12 : ~ 0.7  E

55 Mn-NMR

SS  tet  S Mn ions and their couplings in V c

55 Mn-NMR spectrum for Mn 12 Ac Mn(1) Mn(3) Mn(2) Spin echo intensity Mn 4+ Mn 3+ By T. Kubo et al. H = 0, T < T B

Hyperfine fields for Manganese ions Molecular Magnet Hyperfine Fields H N for Manganese Ions anisotropic isotropic  HNHN Mn 3+ : 3d 4 Mn 4+ : 3d 3 S =3/2 c-axis m Mn3+ c-axis HNHN S =2 α θ m Mn4+ When  is not parallel to S T. Kubo et al.: Phys. Rev. B 65 (2002)

55 Mn-NMR spectrum of MnCu cluster Shift of peak T 2 becomes short Shift of the peak → Hyperfine field opposite to the moment T 2 becomes too short below 4 kOe.  = 0  < 0 H Field-sweep spectrum for powder sample

55 Mn-NMR spectrum 55 Mn-NMR at zero field ~ 240 MHz Peak position and the field range of over 80 % maximum of spectrum H =  55 

Hyperfine Field and 55 Mn NMR frequency Mn12Ac T. Kubo et al.: Phys. Rev. B 65 (2002) For the first approximation, if we use the values of H F and h d for Mn12Ac, zero-field NMR at 240 MHz (H N = 229 kOe) for MnCu cluster is almost reproduced when  = 0 (255 kOe). Magnetic moment is almost parallel to the tetragonal axis. Mn12Ac

Summary 55 Mn-NMR  NMR signal was detected at 1.5 K > T B.  T 2 becomes short below 4 kOe and at higher T.  Hyperfine field at 55 Mn : H N ~ 230 kOe (  ~ 0)  63,65 Cu-NMR  NMR with single crystal or oriented sample  Quadrupole splitting for 55 Mn-NMR  Low temperatures T < T B (~ 0.5 K) → Ordered or not? Future tasks 1 H-NMR  Enhancement of T 1 -1 ~ 4 K (~ 0.3  E) ← Mn12 : ~ 0.7  E  Some effect may reduce characteristic temperature.  Multi-peak spectrum T < 3 K → Static induced moment Similarity and difference from Mn12 and other SMMs

Univ. of Fukui H. Noda, H. Hashimoto, H. Kikuchi, M. Chiba Nara Univ. of Education B. Chen, H. Murakami, T. Kubo Kyoto Univ. T. Goto Univ. of Tsukuba A. Yoshida, M. Nihei, H. Oshio ISSP, Univ. of Tokyo A. Yamaguchi Okayama Univ. H. Nojiri Acknowledgements