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.

Comparative Magnetic Studies in the Solid State and Solution of Two Isostructural 1D Coordination Polymers Containing CoII/NiII-Curcuminoid Moieties

“…Therefore, system 1, as the related mononuclear system, presents a large and positive D value [22]. These results also agree with other Co II ions coordinated to β-diketone groups [13]. The intermediate value of the ZFS parameter D in 1 (44 cm´1), compared with its mononuclear predecessors (75 and 25 cm´1), may indicate that even tiny intramolecular effects (from the 4,4'-bpy between the Co II centres) and/or intermolecular interactions among the chains have a great effect on the anisotropy.…”

“…Regarding SMMs, a fast-growing number of mononuclear 3d/4f compounds that display slow relaxation of the magnetization have recently been introduced [7][8][9]. In this regard, the use of external magnetic fields to cancel an often-present fast zero-field relaxation of the magnetization [10,11], as well as the observation www.mdpi.com/journal/magnetochemistry of frequency dependence in 3D mononuclear species with negative but also positive [12,13] zero-field splitting (D) values, have been key factors toward the development of the subject. Indeed, the SMM character of compounds with a positive D parameter has shaken up a well-established requirement for the occurrence of this unique behaviour.…”

“…Indeed, nowadays there are models that predict mononuclear 3d-SMMs in a rationalized way [10], and therefore, using such information with the right organic connectors, we may be able to tune SCM behaviour as well. Nevertheless, the proper progress of this subject requires a deep insight regarding extra factors that can affect the final properties [3][4][5][6]13,14].…”

Evidence of Slow Magnetic Relaxation in Co(AcO)2(py)2(H2O)2

“…Indeed, slow relaxation in positive-D compounds is a phenomenon that is not yet fully understood-although attempts have been made to explain it [23,24]. The apparent unimportance of D as a design parameter in general has also been noted by others [25].…”

Five mononuclear pentacoordinate Co(II) complexes with field-induced slow magnetic relaxation

“…However, the LF relaxation branch is much slower: (LF, 1.9 K) ~ 0.5 s for 2 and 5. The possible mechanism of the spin relaxation of easy plane systems based on mononuclear Co(II) complexes with large magnetic anisotropy has been proposed by considering the nuclear spin I(Co) = 7/2 [23]. According to this analysis, a direct term that includes the hyperfine interaction dominates at low temperatures and a Raman term is predominant for temperatures above 4 K. Table 6 serves for the comparison of structural and magnetic parameters of 1 through 5.…”