title: Slow magnetic relaxation in a dimeric Mn2Ca2complex enabled by the large Mn(iii) rhombicity.
abstract: In this paper we present the characterization of a complex with the formula [Mn2Ca2(hmp)6(H2O)4(CH3CN)2](ClO4)4(1), where hmp-H = 2-(hydroxymethyl)pyridine. Compound 1 crystallizes in the monoclinic space group C2/c with the cation lying on an inversion centre. Static magnetic susceptibility, magnetization and heat capacity measurements reflect a unique Mn(iii) valence state, and single-ion ligand field parameters with remarkable large rhombic distortion (D/kB= -6.4 K, E/kB= -2.1 K), in good agreement with the high-field electron paramagnetic resonance experiments. At low temperature Mn2Ca2cluster behaves as a system of ferromagnetically coupled (J/kB= 1.1 K) Mn dimers with a ST= 4 and mT= ±4 ground state doublet. Frequency dependent ac susceptibility measurements reveal the slow magnetic relaxation characteristic of a single molecule magnet (SMM) below T = 4 K. At zero magnetic field, an Orbach-type spin relaxation process (τ ∼ 10-5s) with an activation energy Ea= 5.6 K is observed, enabled by the large E/D rhombicity of the Mn(iii) ions. Upon the application of a magnetic field, a second, very slow process (τ ∼ 0.2 s) is observed, attributed to a direct relaxation mechanism with enhanced relaxation time owing to the phonon bottleneck effect.
authors.affiliation: Servicio de Medidas Físicas, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain. aarauzo@unizar.es.
authors.forename: Ana
authors.identifier: http://orcid.org/0000-0002-5999-341X
authors.initials: A
authors.lastname: Arauzo
authors.affiliation: Escola Universitària Salesiana de Sarrià (EUSS), Passeig Sant Joan Bosco 74, 08017 Barcelona, Spain.
authors.forename: Elena
authors.initials: E
authors.lastname: Bartolomé
authors.affiliation: Molecular Photonics Laboratory, School of Chemistry, Newcastle University, Newcastle-upon-Tyne, NE1 7RU, UK.
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Benniston
authors.affiliation: Institute of Chemistry, Academy of Sciences of Moldova, Academiei street 3, MD-2028, Chisinau, Moldova.
authors.forename: Silvia
authors.initials: S
authors.lastname: Melnic
authors.affiliation: Institute of Macromolecular Chemistry "Petru Poni" Iasi, Aleea Grigore Ghica Voda, nr. 41A, 700487 Iasi, Romania.
authors.forename: Sergiu
authors.initials: S
authors.lastname: Shova
authors.affiliation: Centro Universitario de la Defensa, Academia General Militar, Zaragoza, Spain and Instituto de Ciencia de Materiales de Aragón and Departamento de Física de la Materia Condensada, CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain.
authors.forename: Javier
authors.initials: J
authors.lastname: Luzón
authors.affiliation: Instituto de Ciencia de Materiales de Aragón and Departamento de Física de la Materia Condensada, CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain.
authors.forename: Pablo J
authors.identifier: http://orcid.org/0000-0003-3449-4929
authors.initials: PJ
authors.lastname: Alonso
authors.affiliation: Laboratoire National des Champs Magnétiques Intenses, CNRS and Université Grenoble Alpes, 25 rue des Martyrs, B.P. 166, 38042 Grenoble Cedex 9, France.
authors.forename: Anne-Laure
authors.initials: AL
authors.lastname: Barra
authors.affiliation: Instituto de Ciencia de Materiales de Aragón and Departamento de Física de la Materia Condensada, CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain.
authors.forename: Juan
authors.initials: J
authors.lastname: Bartolomé