Quantum Nucleation in a Single-Chain Magnet

Abstract

The field sweep rate (v = dH/dt) and temperature (T ) dependence of the magnetization reversal of a single-chain magnet (SCM) is studied at low temperatures. As expected for a thermally activated process, the nucleation field (Hn) increases with decreasing T and increasing v. The set of Hn (T, v) data is analyzed with a model of thermally activated nucleation of magnetization reversal. Below 1 K, Hn becomes temperature independent but remains strongly sweep rate dependent. In this temperature range, the reversal of the magnetization is induced by a quantum nucleation of a domain wall that then propagates due to the applied field.PACS numbers: 75.10. Pq, 75.40.Gb, 76.90.+d, 75.45.+j Recent efforts in synthetic chemistry has led to a quickly growing number of magnetic systems that show slow relaxation of magnetization. Apart of interests for applications in, for instant, ultrahigh density magnetic recording, such systems are ideal to test theories. A wellknown example is the single-molecule magnet (SMM) that exhibit slow magnetization relaxation of their spin ground state, which is split by axial zero-field splitting [1,2]. SMMs are among the most promising candidates for observing the limits between classical and quantum physics. A more recent example is the single-chain magnet (SCM) [3,4,5] showing slow relaxation of magnetization as the consequence of the uniaxial anisotropy seen by each spin on the chain and magnetic correlations between spins. Although it seemed that there was a reasonable agreement between the experimental data and Glauber's theory of a 1D Ising spin chain [6], it was shown that several other arguments should be considered to fill the gap between the theory and the experimental results [7]. The most important arguments concerned the introduction of magnetic anisotropy and finite-size effects. Indeed, their influence on the static and dynamic properties of the SCMs was confirmed experimentally [7,8,9,10].Quantum tunneling of domain walls in a 1D mesoscopic ferromagnetic sample was theoretically investigated [11,12,13,14,15,16] and crossover temperatures between the classical and quantum regime were predicted. Domain wall nucleation and depinning were studied in single Ni wires and showed indeed a flattening of the temperature dependence of the mean switching field (H sw ) below about 5 K [17] and 1 K [18]. Because of surface roughness and oxidation, the domain walls of a single wire were trapped at pinning centers. The pinning barrier decreases with an increase of the magnetic field. When the barrier is sufficiently small, thermally activated escape of the wall occurs. This is a stochastic process that can be characterized by a switching (depinning) field distribution. A flattening of the temperature dependence of H sw and a saturation of the width of the switching field distribution were observed. The authors proposed that a domain wall escapes from its pinning site by thermal activation at high temperatures and by quantum tunneling below T c ∼ 5 K [17] and 1 K [18]. However, such cross...

Unified Model of Dynamic Forced Barrier Crossing in Single Molecules

“…22.Dj, 82.37.Np, 87. 15.Aa Continued progress in magnetic [1], optical [2] and mechanical [3] manipulation has enabled experimentalists to study the microscopic states of systems ranging from single molecule magnets to individual biological complexes. A particularly common technique is found by steadily increasing a force field until a change in state is observed (such as a switch in domain polarity [4] or the dissociation of an intermolecular bond [5]).…”

“…Equation (8), and similar approximations, have been explored in the context of field switching in domain-wall junctions [16], magnetic nanoparticles [1,4], and the rupture of single molecule bonds [8,17,18,24]. Eq.…”

“…Figure 3(b), shows the low temperature magnetization reversal of a single-chain magnet taken from Ref. [1]. The spectra at temperatures above T = 0.8 K were globally fit to Eq.…”

Resonant quantum tunneling of spin chains in a three-dimensional magnetically ordered state

“…But for 3D M n 4 networks, no resonance effects were reported in the superparamagnetic blocking regime even though (incoherent) magnetic quantum tunneling could be inferred from low temperature specific heat measurements 5 . For 1D systems, the so called single chain magnet M n 2 N i shows dynamics with a quantum regime below 700 mK but an absence of resonant effects 6 . In the present letter we will show that the well known compound [(CH 3 ) 3 N H]CoCl 3 · 2H 2 O, (CoTAC), a 3D network of exchanged-coupled ferromagnetic chains, does exhibit QTM and resonance effects well below its 3D antiferromagnetic transition temperature (T C = 4.15 K).…”

Local spin dynamics at low temperature in the slowly relaxing molecular chain [Dy(hfac)3{NIT(C6H4OPh)}]: A μ+ spin relaxation study

“…10,13 Subsequently, several other compounds were shown to exhibit SCM behavior, 11,[14][15][16][17][18][19][20][21] with quantum effects influencing the magnetization dynamics at low temperatures. 22 In particular, several works [23][24][25] showed that the requirements necessary to observe Glauber dynamics-a strong Ising-like anisotropy and a very low ratio of interchain/intrachain magnetic exchange interactions-are fulfilled in [Dy(hfac) 3 {NIT(C 6 H 4 OPh)}] (in short DyPhOPh). A previous theoretical and experimental study 25 demonstrated that DyPhOPh consists of two sets of non-interacting parallel chains, with projections mutually tilted by about 90…”