Tuning the spin coherence time of Cu(II)-(bis)oxamato and Cu(II)-(bis)oxamidato complexes by advanced ESR pulse protocols.

Ruslan Zaripov,Francois E Meva,Iskander Khairuzhdinov,Vladislav Kataev,Tobias Rüffer,Mohammad A Abdulmalic,Saddam Weheabby,Bernd Büchner,Kev Salikhov,Violeta Voronkova,Evgeniya Vavilova

doi:10.3762/bjnano.8.96

Abstract

We have investigated with the pulsed ESR technique at X- and Q-band frequencies the coherence and relaxation of Cu spins S = 1/2 in single crystals of diamagnetically diluted mononuclear [n-Bu4N]2[Cu(opba)] (1%) in the host lattice of [n-Bu4N]2[Ni(opba)] (99%, opba = o-phenylenebis(oxamato)) and of diamagnetically diluted mononuclear [n-Bu4N]2[Cu(opbon-Pr2)] (1%) in the host lattice of [n-Bu4N]2[Ni(opbon-Pr2)] (99%, opbon-Pr2 = o-phenylenebis(N(propyl)oxamidato)). For that we have measured the electron spin dephasing time Tm at different temperatures with the two-pulse primary echo and with the special Carr–Purcell–Meiboom–Gill (CPMG) multiple microwave pulse sequence. Application of the CPMG protocol has led to a substantial increase of the spin coherence lifetime in both complexes as compared to the primary echo results. It shows the efficiency of the suppression of the electron spin decoherence channel in the studied complexes arising due to spectral diffusion induced by a random modulation of the hyperfine interaction with the nuclear spins. We argue that this method can be used as a test for the relevance of the spectral diffusion for the electron spin decoherence. Our results have revealed a prominent role of the opba4– and opbon-Pr24– ligands for the dephasing of the Cu spins. The presence of additional 14N nuclei and protons in [Cu(opbon-Pr2)]2– as compared to [Cu(opba)]2– yields significantly shorter Tm times. Such a detrimental effect of the opbon-Pr24− ligands has to be considered when discussing a potential application of the Cu(II)−(bis)oxamato and Cu(II)−(bis)oxamidato complexes as building blocks of more complex molecular structures in prototype spintronic devices. Furthermore, in our work we propose an improved CPMG pulse protocol that enables elimination of unwanted echoes that inevitably appear in the case of inhomogeneously broadened ESR spectra due to the selective excitation of electron spins.

Highlights

Cu(II)−(bis)oxamato and Cu(II)−(bis)oxamidato complexes have attracted in the recent past substantial attention as precursor materials for the synthesis of the corresponding polynuclear complexes which in their turn have been investigated with regard to the magnetic superexchange interactions between the Cu spins mediated by the O and N ligands [1,2,3,4,5,6,7,8]
We have shown that the application of a more sophisticated, so-called Carr–Purcell–Meiboom–Gill (CPMG) multiple microwave pulse sequence [24,25] can boost the Tm time in molecular complexes up to one order of magnitude [20]
We have found that application of the special Carr–Purcell–Meiboom–Gill (CPMG) pulse protocol significantly increase the Tm time of both complexes as compared to the results of the standard two-pulse primary echo measurements

Summary

Introduction

Cu(II)−(bis)oxamato and Cu(II)−(bis)oxamidato complexes have attracted in the recent past substantial attention as precursor materials for the synthesis of the corresponding polynuclear complexes which in their turn have been investigated with regard to the magnetic superexchange interactions between the Cu spins mediated by the O and N ligands [1,2,3,4,5,6,7,8]. Pulse methods of electron spin resonance (ESR) have been shown to be very informative in assessing the magnetically active molecular complexes for the purpose of quantum information processing With these techniques, one can directly measure the electron spin coherence times and, can manipulate the spin states in order to perform quantum logical operations [12,13,14,15,16,17,18,19,20,21,22,23]. We have shown that the CPMG pulse sequence can maintain the spin coherence on the time scale of up to ≈10 μs at low temperatures in the first complex whereas the spin dephasing in the second complex occurs on a shorter time scale We relate this difference with the detrimental influence of the HF interaction with additional 14N nuclei and protons in the Cu(II)−(bis)oxamidato complex whereas this unwanted effect is reduced in the Cu(II)−(bis)oxamato complex containing less 14N nuclei and protons. Echo-detected ESR spectra were recorded by using the standard primary echo method with the subsequent integration of the echo signal during the magnetic field sweep at each field point

Experimental Results

Discussion

Conclusion