Magnetic clusters in the paramagnetic phase of a high-temperature ferromagnetic metal-organic framework

Abstract

Owing to their exceptional chemical and electronic tunability, metal-organic frameworks can be designed to develop magnetic ground states making a range of applications feasible, from magnetic gas separation to the implementation of lightweight, rare-earth free permanent magnets. However, the typically weak exchange interactions mediated by the diamagnetic organic ligands result in ordering temperatures confined to the cryogenic limit. The itinerant magnetic ground state realized in the chromium-based framework Cr(tri)$_{2}$(CF$_{3}$SO$_{3}$)$_{0.33}$ (Htri, $1H$-$1$,$2$,$3$-triazole) is a remarkable exception to this trend, showing a robust ferromagnetic behavior almost at ambient conditions. Here, we use dc SQUID magnetometry, nuclear magnetic resonance, and ferromagnetic resonance to study the magnetic state realized in this material. We highlight several thermally-activated relaxation mechanisms for the nuclear magnetization due to the tendency of electrons towards localization at low temperatures as well as the rotational dynamics of the charge-balancing triflate ions confined within the pores. Most interestingly, we report the development within the paramagnetic regime of mesoscopic magnetic correlated clusters whose slow dynamics in the MHz range are tracked by the nuclear moments, in agreement with the highly unconventional nature of the magnetic transition detected by dc SQUID magnetometry. We discuss the similarity between the clustered phase in the paramagnetic phase and the magnetoelectronic phase segregation leading to colossal magnetoresistance in manganites and cobaltites. These results demonstrate that high-temperature magnetic metal-organic frameworks can serve as a versatile platform for exploring correlated electron phenomena in low-density, chemically tunable materials.

Figures (6)

• Figure 1: Portion of the crystal structure of Cr(tri)$_{2}$(CF$_{3}$SO$_{3}$)$_{0.33}$ based on the high-temperature powder x-ray diffraction data reported in Ref. Par21. The image highlights a pyrochlore-like lattice made of triazolate-bridged chromium centers, as well as a disordered, charge-balancing trifluoromethanesulfonate anion in the framework pore. Cr, purple; S, yellow; F, green; O, red; N, blue; C, grey; H, white.
• Figure 2: a) Main panel: inverse susceptibility $H$/$M$ as a function of temperature after a zero-field-cooled protocol for three different values of fixed magnetic field. Inset: dependence of the magnetization $M$ on temperature at fixed magnetic field $H = 20$ Oe after a field-cooled protocol. b) Main panel: scaling analysis of the isothermal magnetization (see text). The $M$ vs $H$ curves measured at constant temperature values between $199$ K and $208.5$ K with $0.5$ K steps are reported. Inset: modified Arrott plot for the isothermal magnetization. The curves measured at constant temperature values between $199$ K and $209$ K are reported with $2$ K steps for the aim of clarity. The continuous lines are linear best-fits to the experimental data. The dashed lines are extrapolations of the linear fits.
• Figure 3: Representative frequency-swept, powder-averaged NMR spectra at a fixed magnetic field of $H = 12.0 \; \textrm{kOe}$ at selected temperatures in the paramagnetic regime. The different spectra are offset vertically for clarity. The two well-defined, inhomogeneously-broadened spectral lines are centered around the expected frequency values calculated from the gyromagnetic ratios characteristic of the $^{1}$H and $^{19}$F nuclei. The approach to the ferromagnetic phase ($T_{C} = 203.8$ K) while cooling induces an additional temperature-dependent line broadening which is much more marked than a minor paramagnetic line shift towards lower frequencies for both nuclei. The dashed vertical lines indicate the central resonance frequencies for $^{1}$H and $^{19}$F nuclei at $307$ K.
• Figure 4: Dependence of the spin-lattice relaxation rate for the nuclear magnetization of $^{1}$H and $^{19}$F nuclei (upper and lower panels, respectively) on temperature at two different external magnetic fields. The error bars are the standard deviation of the fit parameters in Eq. \ref{['EqRecoveryFit']} (see Methods). The continuous lines are the results of global fittings based on Eq. \ref{['EqDistrBPP']} (lower panel) and Eq. \ref{['EqThreeBPP']} (upper panel) using the Larmor frequencies at the different magnetic fields as fixed parameters and the remaining quantities as shared parameters (see text).
• Figure 5: X-band FMR spectra at the representative temperatures of $T = 318$ K and $T = 197$ K (left-hand panel and right-hand panel, respectively). For both temperatures, the first-derivative experimental data and their numerical integration are reported in the upper and lower panels, respectively. The continuous lines in the lower panels are based upon a best-fitting function according to Eq. \ref{['EqVoigt']}. The markedly exchange-narrowed, Cauchy-Lorentz character of the high-temperature spectrum is turned to a Gaussian-like shape at low temperatures.