Magnetization Process of Molecular Magnet in ultra-fast sweeping magnetic fields 1. Introduction to High Field Lab. in Okayama 2. Ultra-fast sweeping magnetic fields 3. Asymmetric Magnetization in V Summary H. Nojiri, Department of Physics, Okayama University

High Field Lab. in Japan High Magnetic Field Forum of Japan IMR, Tohoku NRIM, Tsukuba ISSP, Tokyo Osaka Okayama Kobe Osaka-p Hiroshima Fukui Toyama-p Akita Yamaguchi All Japan group for high magnetic field researchers Bridge between facilities and users Exchange of science and technology Future Plan

Collaborators Okayama University T. Taniguchi Humboldt UniversityA. Kirste, M. von Ortenberg Kyushu University Y. Ajiro Nagoya University K. Awaga Hokkaido University K. Takeda LLN B. Barbara U. G.A. Mueller Acknowledgment Y. Matsuda, Y. Ueda for “ mini-dog ”

Pulsed High Field ESR System High Resolution Excitation at q=0 High Sensitive High Resolution Excitation at q=0 High Sensitive Faraday H//k Voigt H  k Source Optical pumped FIR laser Gunn Oscillator BWO tube Source Optical pumped FIR laser Gunn Oscillator BWO tube

Pulsed High Field ESR System at Okayama 35 GHz-7 THz – meV K 40 T pulsed fields (Single shot)

 -fixed Transmission T(H) Magnetic dipole transition (  S z =±1)  -fixed Transmission T(H) Magnetic dipole transition (  S z =±1) Mn12 Cluster S=10 Splitting by Anisotropy 20 transitions(  S z =±1) ⇩ Mn12 Cluster S=10 Splitting by Anisotropy 20 transitions(  S z =±1) ⇩

Repeating Pulsed Field Generators Pulse width~1 msec Repetition rate 0.5 Hz Water cool Bitter Magnet Bore 30 mm (R.T.) Solenoid Coil 30 T Split Coil 20 T developed and used for  SR and neutron diffraction

Repeating Pulsed Fields THz-ESR S/N improves more than one-order Compatible with tunability Broad signal can be measured

Mini pulsed field system Small energy (1 W for 1Hz-repetition) Cooling with a mechanical cooler Low voltage<1000 V Mobile Customized for each port

Mobile pulsed field-High Field Circus Prototype for 40 T generation 1 M ￥ (1Euro~125 ￥ )

Pulsed Field Control for Spin Dynamics Spin Relaxation in nano-scale magnet –Quantum Tunneling for Small Ferro-magnet –Discrete Level –Non-adiabatic transition –H is in the Hamiltonian and tunable! Control of Time Structure H

Pulse field- Sinusoidal C=8 mF~800  F L=50  H~ 10 mH Velocity 0~10 5 T/s at 1 Tesla Charge voltage (Fine control) + Combination of L&C B (T) time (s) Method I:Combination of L and C

Initial field +fast sweep –Polarize spin in equilibrium state Crossing at zero field No switching noise –No polarization change of current Sweep around a critical field Method II:Two coils

Experimental set up Capacitor Bank 1 90 kJ ODOGONE Compensation Ａｍｐ． Digitizer Capacitor Bank 2 2 kJ Minidog Control 1 Delay Control 2 Trigger Ａ Ｂ Ｃ Ａ： Magnet coil 1 B ： Magnet coil 2C ： pick up coil dH/dt dM/dt Cryostat

Capacitor banks and Magnets Capacitor Bank 2 (Minidog) Capacitor Bank 1 (ODOGONE) Magnet (Mini) Magnet (Normal)

Mini Magnet / pickup coil 部分 5 mm 4.5 mm 2.75 mm 20 mm2 mm or 5mm ステンレ ス ケーブル Magnet(Mini) 銅線 φ １ｍｍ 4 layer Pick up coil Cu wire φ0.07 ｍｍ sample Pickup coil カプトンチューブ 磁化検出コイル 補償コイル 磁場コイル

Example of Magnetization Mn12bz//a T~2 K ~0.5 mg

Ultra-fast discharge 5×10 8 T/s Ultra-strong magnetic field ~300 T Limited sensitivity and large noise Method III:Single turn coils

Fast sweep ~10 7 T/s Limited Sensitivity Enhancement in dM/dH Low-temperature Test with Ni powder (~0.2 emu)

Basic properties of Mn 12 Bz [Mn 12 O 12 (O 2 CC 6 H 5 ) 16 (H 2 O) 4 ] 2C 6 H 5 CO 2 H No tetragonal symmetry of molecule Mn(7)-O(6) 1.98 Mn(3)-O(3) 1.90 Mn(7)-O(14) 2.08 Mn(3)-O(7) 1.93 Mn(7)-O(18) 1.93 Mn(3)-O(19) 2.10 Energy Barrier  E=38 K  E ~66 K

3 mg(~0.03 emu) of Mn12 bz (M=3204) c.f. V15 10 mg Reproducible data for high dM/dH Mn 12 bz at 4.2 K

3 mg(~0.03 emu) of Mn12 bz (M=3204) c.f. V15 10 mg Large hysteresis for the first sweep Mn 12 bz at 2 K

Pulsed Field Magnetization:Sweep speed-dependence Hysteresis and the jump of magnetization decrease in cycling probably for temperature increase H

Asymmetric Magnetization for Hidden double degeneracy in V15 Y. Ajiro et al.

V15:K 6 [V 15 As 6 O 42 (H 2 O)] 8H 2 O Saito and Miyashita Chiorescu et al.

Double degeneracy for time reversal symmetry Hexagon site, singlet, large gap Triangle site, S=1/2+S=3/2 S=1/2 is doubly degenerated  ~3 K Tunneling gap~0.1 K  S=3/2 S=1/2 Doubly degenerated for time reversal symmetry

Temperature dependence Decrease of saturation magnetization below 3 K Hysteresis appears below 1 K Jump occurs at level crossing point

Magnetization Hysteresis Small jump in up-sweep Large jump in down-sweep Thermalization after the jump

Sweep field and Maximum field dependence Maximum Field Magnetization curve changes Thermalization time ~ 1 msec Jump is bigger for faster sweep

Sweep field dependence of magnetization jump For low sweep, low maximum field thermalization is insufficient Around 10 4 T/s, the ratio is about 2:1

Summary and Future Time structure control becomes possible by using pulsed fields Sweep rate 10 7 T/s Zero field cross without noise Mn12 avalanche magnetization reversal Asymmetric Magnetization in V 15 3 He and dilution in ultra-fast sweep in progress