Design for a multifrequency high magnetic field superconducting quantum interference device-detected quantitative electron paramagnetic resonance probe: Spin-lattice relaxation of cupric sulfate pentahydrate (CuSO4⋅5H2O)

Abstract

We have designed a spectrometer for the quantitative determination of electron paramagnetic resonance (EPR) at high magnetic fields and frequencies. It uses a superconducting quantum interference device (SQUID) for measuring the magnetic moment as a function of the applied magnetic field and microwave frequency. We used powdered 2,2-diphenyl-1-picrylhydrazyl to demonstrate resolution of g-tensor anisotropy to 1 mT in a magnetic field of 3 T with a sensitivity of 1014 spins per 0.1 mT. We demonstrate multifrequency operation at 95 and 141 GHz. By use of an aligned single crystal of cupric sulfate pentahydrate (chalcanthite) CuSO4⋅5H2O, we show that the spectrometer is capable of EPR line shape analysis from 4 to 200 K with a satisfactory fit to a Lorentzian line shape at 100 K. Below 100 K, we observed line-broadening, g shifts, and spectral splittings, all consistent with a known low-dimensional phase transition. Using SQUID magnetometry and a superconducting magnet, we improve by an order of magnitude the sensitivity and magnetic field range of earlier power saturation studies of CuSO4⋅5H2O. We were able to saturate up to 70% of the magnetic moment with power transfer saturation studies at 95 GHz, 3.3 T, and 4 K and obtained the spin-lattice relaxation time, T1=1.8 ms, of CuSO4⋅5H2O at 3.3 T and 4 K. We found an inverse linear dependence of T1, in units of seconds (s) at 3.3 T between 4 and 2.3 K, such that T1=0.016⋅K⋅s⋅τ−1−0.0022⋅s, where τ is the absolute bath temperature. The quantitative determination of EPR is difficult with standard EPR techniques, especially at high frequencies or fields. Therefore this technique is of considerable value.

Thermomagnetic properties of the Finland trityl radical

“…The exact nature of the broadening is not clear; however, temperature dependent broadening can be a function of internal demagnetization fields, or short range order approaching a phase transition. 16,22,27 These results are in contrast to recent EPR work on diluted trityl systems which exhibited little to no linewidth dependence on temperature down to 1 K. 25,26 This indicates that at low temperatures, the EPR linewidth is highly dependent on the trityl concentration, whereas at temperatures above about 10 K, there is little dependence. It would be of interest to examine the linewidth as a function of concentration, as well the orientationdependent single crystal linewidth.…”

“…The beginning of saturation occurs at 40% suppression. In contrast, in single crystals of copper sulfate, 16,28 and the molecular magnet Fe 8 , 17,18 over 80% of the magnetization was suppressed. The very low suppression at saturation for the trityl is attributed to the g-anisotropy present in a powder spectrum.…”

“…[16][17][18]. This commercial magnetometer was also used for conventional magnetization as a function field and temperature.…”

“…16,28 Hence, for an S =1/ 2 system, by measuring the magnetic susceptibility and on is the susceptibility in the presence of microwaves in the presence and absence of microwave irradiation and a knowledge of P a , one can explicitly determine the energy transfer time under continuouswave irradiation. 16,28 At low temperatures the energy transfer and hence the sample and spin temperature are complicated by the possible presence of a phonon blockade, which limits the flow of heat from the phonons to the cryostat, as well as nonresonant microwave heating. In the context of this paper T 1 refers to the time constant of the heat flow of the combined system of electron spins and phonons to the cryostat.…”

“…[16][17][18][19] This technique can quantify the spin moment of nanomagnetic materials as a function of microwave irradiation ͑50-141 GHz͒, magnetic field ͑0-7 T͒, and temperature ͑1.8-400 K͒. Here, we apply the SQUID-EPR technique to characterize tris͑8-carboxy-2,2,6,6-tetramethylbenzo͓1,2-d:4,5-dЈ͔-bis͑1,3dithiol-4-yl͒ methyl sodium salt ͑"Finland trityl"͒.…”