Control of the inhomogeneity degree by magnetic dilution in crystals of antiferromagnetic molecular rings

J. J. Henderson, C. M. Ramsey, E. del Barco, T. C. Stamatatos, and G. Christou

Phys. Rev. B 78, 214413 – Published 9 December 2008

Abstract

Employing the electron spin in solid-state qubits is considered a promising method for quantum information processing. In this context, we present a microwave spectroscopic study on magnetically dilute crystals of ${Fe}_{18}$ antiferromagnetic molecular rings. Doping with ${Ga}^{3 +}$ , at concentrations ranging from 0.5% down to 0.005%, results in magnetic ${Fe}_{17} Ga$ $(S = 5 / 2)$ molecules dispersed throughout the lattice, with 2–8 nm average intermolecular separation. The results reveal a significant decrease in the degree of inhomogeneity of the system, hinting at a controlled reduction in decoherence mechanisms inherent to an ensemble of anisotropic molecular magnets while maintaining the crystalline order.

Received 12 November 2008

DOI:https://doi.org/10.1103/PhysRevB.78.214413

Authors & Affiliations

J. J. Henderson, C. M. Ramsey, and E. del Barco^*

Department of Physics, University of Central Florida, Orlando, Florida 32816, USA

T. C. Stamatatos and G. Christou

Department of Chemistry, University of Florida, Gainesville, Florida 32611, USA

^*delbarco@physics.ucf.edu

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Vol. 78, Iss. 21 — 1 December 2008

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Images

Figure 1
(Color online) (a) Molecular structure of a ${Fe}_{18}$ ( ${Fe}^{3 +}$ , $S = 5 / 2$ ) antiferromagnetic wheel doped with a single ${Ga}^{3 +}$ $(S = 0)$ . This doping leads to spin noncompensation and a magnetic ground state $S = 5 / 2$ at low temperature. (b) Representation of the magnetic dilution achieved in a single crystal of ${Fe}_{18}$ molecular wheels by controlling the average separation between magnetic molecules through Ga doping. (c) Optical micrograph of a microstrip resonator. A typical ${Fe}_{18} : Ga$ crystal can be seen mounted on the 0.4-mm-wide microstrip line resonator with vacuum grease. (d) Room-temperature transmission $(S_{21})$ and reflection $(S_{11})$ parameters of the microstrip line resonator used in the experiments.Reuse & Permissions
Figure 2
(Color online) EPR spectra of 9.7 GHz as a function of the longitudinal magnetic field, $H_{L}$ , recorded at different temperatures on a single crystal of ${Fe}_{18}$ molecular wheels doped with Ga at a concentration of 0.005% (Ga:Fe). Inset: energy level diagram calculated by direct diagonalization of the Hamiltonian given in Eq. (1) (see text for parameter values) with the magnetic field aligned along the wheel $z$ axis. All indicated transitions correspond to the peaks observed in the main figure.Reuse & Permissions
Figure 3
(Color online) EPR spectra of 9.7 GHz as a function of the transverse magnetic field, $H_{T}$ , recorded at different temperatures on a single crystal of ${Fe}_{18}$ molecular wheels doped with Ga at a concentration of 0.005% (Ga:Fe). Inset: energy level diagram calculated by direct diagonalization of the Hamiltonian given in Eq. (1) (see text) with the magnetic field perpendicular to the $z$ axis of the wheel. The blue arrows indicate the transitions corresponding to the peaks observed in the main figure.Reuse & Permissions
Figure 4
(Color online) Position of the 9.7 GHz EPR absorption peaks as a function of the angle of application of the transverse field within the plane of the wheel. The experiments have been performed at $T = 300 mK$ on a single crystal of ${Fe}_{18}$ molecular wheels doped with Ga at a concentration of 0.005% (Ga:Fe).Reuse & Permissions
Figure 5
(Color online) (a) Behavior of the EPR peak corresponding to the $S_{z} = \pm 5 / 2$ transition at 9.7 GHz as a function of the Ga:Fe doping concentration (0.005%–0.5%). (b) Peak’s width versus concentration. The dephasing times extracted from the peak’s width are shown below some of the data. (c) Calculated average distance between magnetic molecules $({Fe}_{17} Ga)$ as a function of the concentration. The calculated dipolar dephasing time associated to the average intermolecular distance is provided for some of the data.Reuse & Permissions

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