title: Origin of slow magnetic relaxation in Kramers ions with non-uniaxial anisotropy.
abstract: Transition metal ions with long-lived spin states represent minimum size magnetic bits. Magnetic memory has often been associated with the combination of high spin and strong uniaxial magnetic anisotropy. Yet, slow magnetic relaxation has also been observed in some Kramers ions with dominant easy-plane magnetic anisotropy, albeit only under an external magnetic field. Here we study the spin dynamics of cobalt(II) ions in a model molecular complex. We show, by means of quantitative first-principles calculations, that the slow relaxation in this and other similar systems is a general consequence of time-reversal symmetry that hinders direct spin-phonon processes regardless of the sign of the magnetic anisotropy. Its magnetic field dependence is a subtle manifestation of electronuclear spin entanglement, which opens relaxation channels that would otherwise be forbidden but, at the same time, masks the relaxation phenomenon at zero field. These results provide a promising strategy to synthesize atom-size magnetic memories.
authors: Departament de Química Inorgànica and Institut de Recerca de Química Teòrica i Computacional, Universitat de Barcelona, Diagonal 645, E-08028 Barcelona, Spain.
authors: Silvia
authors: S
authors: Gómez-Coca
authors: Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain.
authors: Ainhoa
authors: A
authors: Urtizberea
authors: Departament de Química Inorgànica and Institut de Recerca de Química Teòrica i Computacional, Universitat de Barcelona, Diagonal 645, E-08028 Barcelona, Spain.
authors: Eduard
authors: E
authors: Cremades
authors: 1] Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain [2] Departamento de Física de la Materia Condensada, Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain.
authors: Pablo J
authors: PJ
authors: Alonso
authors: 1] Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain [2] Departamento de Física de la Materia Condensada, Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain.
authors: Agustín
authors: A
authors: Camón
authors: Departament de Química Inorgànica and Institut de Recerca de Química Teòrica i Computacional, Universitat de Barcelona, Diagonal 645, E-08028 Barcelona, Spain.
authors: Eliseo
authors: E
authors: Ruiz
authors: 1] Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain [2] Departamento de Física de la Materia Condensada, Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain.
authors: Fernando
authors: F
authors: Luis