Relaxation in the 3D ordered CoTAC spin chain by quantum nucleation of 0D domain walls

We have shown that resonant quantum tunnelling of the magnetisation (QTM), until now observed only in 0D cluster systems (SMMs), occurs in the molecular Ising spin chain, CoTAC ([(CH3)3NH]CoCl3 2H2O) which orders as a canted 3D-antiferromagnet at TC = 4.15 K. This effect was observed around a resonant like field value of 1025 Oe. We present here measurements of the relaxation of the magnetisation as a function of time, from the zero field cooled (ZFC) antiferromagnet state and from the saturated ferromagnet state. We show that, at the resonant field, the relaxation from the saturated state occurs in a complicated process, whereas, surprisingly, in the case of the ZFC state, the relaxation is exponential. Synthetic chemistry has succeeded in designing a wide range of molecular magnets which exhibit original magnetic properties. Single molecule magnets (SMMs), which are zerodimensional systems (0D), are made of well isolated clusters with a well-defined macro-spin S, a large Ising-like anisotropy, and a weak magnetic coupling with its neighbours. These SMMs have been shown to be model systems for investigating quantum tunnelling of the magnetisation (QTM) [1, 2]. Recently, we have reported the observation of resonant QTM in a three-dimensional (3D) ordered system, [(CH3)3NH]CoCl3·2H2O (CoTAC) [3]. CoTAC is a 3D network of exchange-coupled ferromagnetic chains which orders antiferromagnetically below 4.15 K [4–7] (See Figure 1). The magnetic structure is shown in the right hand side of Figure 1: The ferromagnetic Co chains run along the b axis. A weak ferromagnetism develops along the a-axis. The main component of the magnetisation is parallel to the c-axis (which is the easy magnetisation axis) and is antiferromagnetic from one chain to the other in the a direction. Due to the presence of 3D long-range order, this system is essentially different from SMMs because there is no well isolated entity, such as a macro spin, which can tunnel. We proposed that the observed phenomena are caused by the nucleation of 0D domain-walls along the chains. Based on exchange couplings and anisotropy measurements, we first showed that the domain walls are made up of a very small number (8 or so) of spins. We proposed that the resonant effects are due to the level crossing between a state containing one of these walls, and one which has not. We could then show with a crude model that the Zeeman energy at the resonant field is of the order of the energy of a domain-wall nucleation. The energy barrier is caused by the anisotropy and exchange coupling between the chains. In this paper, we present measurements of the relaxation of the magnetisation, with the field applied along the c-axis, and at the resonant field (1025 Oe), starting from the saturated state and from the zero field cooled (ZFC) state. We show that the relaxation behavior is essentially ar X iv :0 90 4. 29 84 v1 [ co nd -m at .m tr lsc i] 2 0 A pr 2 00 9 Figure 1. Left: Representation of the crystallographic structure of CoTAC with Co (blue), Cl (green), O (red), H (pink), C and N (black)atoms. Right: Representation of the magnetic structure. The dashed lines represent the chains direction. The canting angle θ is equal to 10◦. different from what is observed in the SMMs and that we can describe the main features by considering the different possible magnetic configurations. The measurements were carried out with a SQUID magnetometer developed at the Institut Néel equipped with a miniature 3He-4He dilution refrigerator allowing measurements down to 70 mK. Absolute values of the magnetisation are made by the extraction method. The sample is a CoTAC single crystal of 12.2 mg. -60 -40 -20 0 20 40 60 10 10