Spin transition in Gd3N@C80, detected by low-temperature on-chip SQUID technique

Abstract

We present a magnetic study of the Gd 3 N@C 80 molecule, consisting of a Gd-trimer via a Nitrogen atom, encapsulated in a C 80 cage. This molecular system can be an efficient contrast agent for Magnetic Resonance Imaging (MRI) applications. We used a low-temperature technique able to detect small magnetic signals by placing the sample in the vicinity of an on-chip SQUID. The technique implemented at NHMFL has the particularity to operate in high magnetic fields of up to 7 T. The Gd 3 N@C 80 shows a paramagnetic behavior and we find a spin transition of the Gd 3 N structure at 1.2 K. We perform quantum mechanical simulations, which indicate that one of the Gd ions changes from a 8 S 7/2 state (L = 0, S = 7/2) to a 7 F 6 state (L = S = 3, J = 6), likely due to a charge transfer between the C 80 cage and the ion.

Magnetic Coupling ofGd3N@C80Endohedral Fullerenes to a Substrate

“…A magnetic coupling between the Gd atoms inside one fullerene has already been discussed theoretically for isolated Gd 3 N@C 80 molecules [21,22]. Experimentally, a ferromagnetic coupling of about 62 eV has been deduced from ESR measurements on bulk samples [11], while micro-SQUID experiments pointed towards a weak antiferromagnetic interaction between the Gd ions [23]. We note that hence both essential ingredients for singlemolecule-magnet behavior, anisotropy and coupling, are present in Gd 3 N@C 80 =Cuð001Þ.…”

Molecular and Spin Dynamics in the Paramagnetic Endohedral Fullerene Gd3N@C80

“…A previous study of the magnetization using a special technique 13 was limited to very low temperatures. Static magnetization provides information on the absolute value of the average magnetic moment of the molecules; this is not available by ESR at high frequencies.…”

Metallofullerenes: A New Class of MRI Agents and More?

“…Two studies using density functional theory showed the possibility of two low-energy spin states, a 21 µB ferromagnetically coupled state and a canted antiferromagnetic state; the two studies did differ on which was the lowest energy and most plausible state at room temperature [29,30]. Experimental studies were then performed using a SQUID magnetometer (Quantum Design, CA, USA), which resulted in a negative Curie temperature, thus, verifying that the canted antiferromagnetic state was predominant at room temperature [31]. Further modeling and micro-SQUID analysis determined a magnetic transition did occur at 1.2 K, above which the best modeling fit would suggest that one of the Gd atoms in the cluster exists as Gd +2 as opposed to Gd +3 , thus, transferring an electron to the cage [31].…”

“…Experimental studies were then performed using a SQUID magnetometer (Quantum Design, CA, USA), which resulted in a negative Curie temperature, thus, verifying that the canted antiferromagnetic state was predominant at room temperature [31]. Further modeling and micro-SQUID analysis determined a magnetic transition did occur at 1.2 K, above which the best modeling fit would suggest that one of the Gd atoms in the cluster exists as Gd +2 as opposed to Gd +3 , thus, transferring an electron to the cage [31]. This electron transfer process is a critical feature of the metallofullerene that yields many of their interesting electronic properties, and is obviously important in the high relaxivity and clustering properties.…”