Single-chain magnets (SCMs) are potential candidates for ultra-high magnetic storage devices. Furthermore, 1D structures have proven to be extremely good candidates for probing and understanding exchange interactions in extended systems. Uniaxial anisotropic spin centers paired with strong magnetic correlations at first neighbors within the chain result in slow relaxation of the magnetization in these one-dimensional (1D) systems. The magnetic dynamics in SCMs at a finite temperature follows the Arrhenius law, where the relaxation rate depends on the activation barrier associated to the creation of a magnetic wall, which will then propagate within the chain leading to its full magnetization reversal. This barrier is determined by: a) the energy to reverse a single spin over the anisotropy barrier resulting from spin-orbit interaction; and, b) the correlation energy, which is the energy necessary to overcome the intra-chain exchange interactions, and, in the Ising limit (for which |D/J| > 4/3) and assuming a 2JS1S2 type exchange interaction between first neighbors, is given by 4|J|S. Note that the energy required to create a magnetic domain is larger in the case of infinite chains, since the formation of two domain walls is necessary. However, in real samples, chains present finite lengths due to the present presence of crystalline defects, whose concentration ultimately determines the maximum average length of the magnetic domains. In the latter case, a smaller activation energy is required, since a single nucleation occurs at the end of the chain. In fact, the two scenarios can be found in the same sample at different temperatures, since the correlation length (length of a domain) depends exponentially on temperature. For high temperatures, where the correlation length is smaller than the average distance between defects, the system behaves as composed of infinite chains. As the temperature is decreased, the increasing correlation length will eventually be limited by defects and the system will behave as a set of finite chains. The nucleation and motion of the domain walls dictate the magnetic dynamics of a SCM. Once created, the domain walls move within the chain at no energy cost.
In this project we have tuned the interactions between chains in a crystal controlled manner by covering the inner magnetic core with non-magnetic organic ligands with suitable functionalities that can to modify the inter-chain metal connections. The characterization of the samples has been performed by EPR and detailed ac/dc susceptibility and specific heat studies of two CoII-based SCM systems that confirm the SCM nature of complexes purposely synthesized with different ligands in order to vary the interactions between neighboring chains within the crystal. Magnetic measurements detect a phase transition to 3D ordering occurring at temperatures below 0.45 K in sample the compound with stronger halogen bonding (1), while no ordering is observed in sample (2) down to the lowest temperature achieved in the experiments (34 mK). Interestingly, a crossover between 1D and 3D magnetic dynamics can be induced by varying the sweeping rate of the applied magnetic field. The exclusive “visco-magnetic” (in a parallelism to visco-elastic) behavior observed in this SCM compound may lead to novel applications in where the magnetic response of a device changes attending to the characteristic time of the input.
Group members involved:
A. Amjad, G. Mínguez- Espallargas, E. Coronado, F. Luis, M. Evangelisti and E. del Barco.
“Three-Leaf Quantum Interference Clovers in a Trigonal Single-Molecule Magnet”
In preparation (2015)
A. Amjad, G. Minguez Espallargas, J. Liu, J. M. Clemente-Juan, E. Coronado, S. Hill and E. del Barco.
“Single-crystal EPR spectroscopy of a Co(II) Single-Chain Magnet”
Polyhedron 66, 218-221 (2013).
Collaborators in this project:
Stephen Hill (FSU, Tallahassee, USA)
Fernando Luis (UZ, Zaragoza, Spain)
Marco Evangelisti (UZ, Zaragoza, Spain)
Eugenio Coronado (UV, Valencia, Spain)