Magnetization and EPR studies of the single molecule magnet Ni4 with integrated sensors

Abstract

Integrated magnetic sensors that allow simultaneous EPR and magnetization measurements have been developed to study single molecule magnets. A high frequency microstrip resonator has been integrated with a micro-Hall effect magnetometer. EPR spectroscopy is used to determine the energy splitting between the low lying spin-states of a Ni4 single crystal, with an S = 4 ground state, as a function of applied fields, both longitudinal and transverse to the easy axis at 0.4 K. Concurrent magnetization measurements show changes in spin-population associated with microwave absorption. Such studies enable determination of the energy relaxation time of the spin system.An understanding of decoherence and energy relaxation mechanisms in single molecule magnets (SMMs) is both of fundamental interest [1] and important for the use of SMMs in quantum computing [2]. Quantum tunneling of magnetization (QTM) has been widely studied in SMMs [3,4,5]. A recent focus is on coherent QTM in which the tunneling rates may be faster than the rate of decoherence [6,7]. Experiments on SMMs [7] and doped antiferromagnetic rings [8,9] have been reported. However, the basic relaxation mechanisms in SMMs are still under active investigation both experimentally and theoretically [10].We have developed sensors allowing simultaneous EPR and magnetization measurements at low temperatures (typically below 1 K) to study SMMs. A microstrip resonator [11] with resonance frequency between 25 and 30 GHz has been integrated on a chip with a micro-Hall effect magnetometer [12]. The high filling factor in such resonators allows measurement of photon absorption in very small crystals as well as the application of large microwave magnetic fields, needed for Rabi experiments. The fast response of the Hall sensor (> 1 MHz) also enables time-resolved measurements of the magnetization In this paper we show EPR spectroscopy of the two lowest energy levels associated with high spin states (S = 4) of a Ni 4 single crystal. We present simultaneous measurements of associated photon-induced changes in the magnetization. This represents an important advance from the experiments in refs. [7,8], in which only photon induced magnetization changes were measured, and EPR studies were not possible. In particular, it enables a direct determination of the energy relaxation time of the spin system.A schematic of our integrated sensor is presented in Fig. 1.The Hall sensor is fabricated from a GaAs/AlGaAs heterostructure 2D electron gas to form a cross of width 50 µm, chosen to optimize the coupling with a SMM crystal with lateral dimensions of about 100 µm. Our magnetometer has a Hall coefficient of SMM crystal Hall sensor microstrip H T H L resonator h ac Figure 1: Schematic of the sensor developed for this study.1780 Ω/T and a noise level of 2 µT/ √ Hz with a current of 10 µA. This permits detection of changes in the magnetization smaller than 5 × 10 −4 of the saturation magnetization of our SMM crystals (≈ 5×10 5 µm 3 ), using a lock-in detection bandwith of 300 ms. The micros...

Energy level lifetimes in the single-molecule magnetFe8: Experiments and simulations

“…Such experiments employing continuous-wave electron spin resonance 4,19,20,21 and pulsed microwaves time-resolved magnetometry 11,22,23,24,25,26,27,28,29,30,31,32 were performed during last years in order to get a better understanding of the spin relaxation time T 1 and decoherence time T 2 . These parameters were determined mainly by the use of detailed linewidth analysis of absorption peaks or direct relaxation measurements in the time domain 11,22,23,24,25,26,27,28,29,30,31,32 . A possibility to overcome various experimental difficulties (like extensive heating of the sample by long microwave pulses, phonon-bottleneck effects, and thermal relaxation) can be the use of very short microwave pulses or sequences of pulses on a nanosecond scale.…”

Magnetization relaxation in the single-molecule magnet Ni 4 under continuous microwave irradiation

“…(4) Γ * 2 is an upper bound on the dephasing rate, and can be extracted from the line width, found to be about 4 GHz [30]. The only fitting parameter for the continuous lines displayed in fig.…”

“…A constant transverse field H ⊥ is applied 1 perpendicularly to z, while the longitudinal field H z is swept [8,30]. In this configuration, an avoided crossing between the two lowest energy levels |a and |b occurs, with a tunnel splitting ∆ ( fig.…”

On-chip integration of high-frequency electron paramagnetic resonance spectroscopy and Hall-effect magnetometry

“…Only recently, have experimental groups [16][17][18][19][20][21] attempted to study magnetization dynamics upon application of electromagnetic radiation in the microwave range. Although these experiments provided very exciting results and have become the milestones in the study of relaxation phenomena in molecular magnets, they lacked the experimental conditions required to perform timeresolved studies of decoherence of molecular magnets in their solid state form.…”