Magnetic Relaxation Processes Research Articles

Two tetranuclear 3d–4f complexes: Syntheses, structures and magnetic properties

Abstract The solvothermal reactions of 3,5-bis(trifluoromethyl)benzoic acid (HL) and 2,2′-bipyridine (bipy) with the corresponding metal salts afforded two novel 3d–4f tetranuclear complexes M2Dy2(L)10(bipy)2 (M = Mn 1, Cu 2), which have been characterized by single-crystal X-ray diffraction, infrared spectroscopy, element analyses, powder X-ray diffraction and thermal gravimetric analyses. Complexes 1 crystallizes in the triclinic space group P-1, and the completely deprotonated L ligands possess μ2: η1, η1 and μ2: η2, η1 coordination modes to bridge adjacent metal ions which further connect into 2D layers by hydrogen bonds. Complex 2 crystallizes in the distinct monoclinic space group P21/c, and adjacent metal ions were linked by L ligands with the same coordinated patterns with 1, and supramolecular sheets were formed in the ab plane. Dynamic magnetic susceptibility studies reveal slow magnetic relaxation processes for 2, suggesting SMM type behavior.

Volume fraction dependent magnetic behaviour of ferrofluids for rotating seal applications

Ferrofluid samples consisting of magnetite nanoparticles (NPs) coated with oleic acid and dispersed in a non-polar organic solvent have been synthesized by chemical routes. Different volume fractions, φ, of magnetic NPs were considered. The overall structural characterization of NPs has been performed by x-ray diffractometry, with lattice parameters and average coherence lengths evaluated via Rietveld refinements. The magnetic properties of different samples have been analysed by SQUID magnetometry and temperature-dependent Mössbauer spectroscopy and finally explained by adequate magnetic relaxation mechanisms. Zero field cooling–field cooling protocols provided useful information about specific volume fraction dependent magnetic relaxation and de-freezing processes, the lack of the Verwey transition and stronger dipolar interactions at higher volume fractions. Anisotropy energies as obtained by both temperature dependent Mössbauer spectroscopy and magnetometry data are compared and a new procedure for a quantitative characterization of the dipolar interactions is proposed.

Dy(III) Single-Ion Magnet Showing Extreme Sensitivity to (De)hydration

A new mononuclear dysprosium(III)-cucurbit[6]uril complex has been synthesized and characterized structurally and magnetically. It exhibits single-ion magnet (SIM) behavior with two slow magnetic relaxation processes, which are very sensitive to the solvation degree of the sample. Depending on the amount and type of the solvent in the structure, it is possible to switch the slow magnetic relaxation of this compound between the temperature-independent and temperature-dependent regimes.

Compact Rotating-Sample Magnetometer for Relaxation Phenomenon Measurement Using HTS-SQUID

A compact magnetometer that uses a high-temperature superconductor superconducting quantum interference device (HTS-SQUID) that can measure relaxation phenomena was developed, and its characteristics were evaluated. For measuring magnetic relaxation, a pick-up coil was designed to be fixed at any position in the circle described by a rotating-sample. The pick-up coil was directly connected to an input coil, which is inductively connected to the HTS-SQUID, and the secondary induced magnetic field from the sample was detected with time delay after magnetization. Using the developed system, magnetic signals with time delay from pure water were detected, and the magnetic signal intensity decreased with increasing time delay. This magnetic signal with time delay was not caused by the sample case and the magnetization due to the leaked magnetic field distribution of the permanent magnet used for sample magnetization. Thus, the developed system could detect magnetic signals during the magnetic relaxation process.

Strong spin–phonon coupling between a single-molecule magnet and a carbon nanotube nanoelectromechanical system

Magnetic relaxation processes were first discussed for a crystal of paramagnetic transition ions. It was suggested that mechanical vibrations of the crystal lattice (phonons) modulate the crystal electric field of the magnetic ion, thus inducing a 'direct' relaxation between two different spin states. Direct relaxation has also been predicted for single-molecule magnets with a large spin and a high magnetic anisotropy and was first demonstrated in a Mn12 acetate crystal. The spin-lattice relaxation time for such a direct transition is limited by the phonon density of states at the spin resonance. In a three-dimensional system, such as a single-molecule magnet crystal, the phonon energy spectrum is continuous, but in a one-dimensional system, like a suspended carbon nanotube, the spectrum is discrete and can be engineered to an extremely low density of states. An individual single-molecule magnet, coupled to a suspended carbon nanotube, should therefore exhibit extremely long relaxation times and the system's reduced size should result in a strong spin-phonon coupling. Here, we provide the first experimental evidence for a strong spin-phonon coupling between a single molecule spin and a carbon nanotube resonator, ultimately enabling coherent spin manipulation and quantum entanglement.

Specific features of the magnetic and electric properties and the magnetically inhomogeneous state of Nd0.5Sr0.5MnO3 single crystal

The hysteretic behavioral features of magnetization and resistance upon remagnetization in quasistatic (up to 9 T) and pulse (up to 14 T) magnetic fields have been investigated. The relaxation processes of magnetization and resistance after the effect of a 9 T magnetic field have also been studied. A mechanism of the remagnetization of antiferromagnetic insulating-ferromagnetic metallic (AFM/I-FM/M) phases and the existence of high-conductivity state of the sample after removal of the magnetizing field is proposed for low temperatures. This mechanism is due to slow relaxation of the nonequilibrium lattice corresponding to the FM phase (of larger volume) toward the equilibrium AFM lattice (of lesser volume).

A Study of Structural and Physical Properties of Heavily Co-doped LaSr2Mn2O7 Bi-layered Manganite

In this paper, we report the influence of cobalt doping on the structure and magnetic properties of bilayered-perovskite LaSr2Mn2−xCoxO7 (x=0.25,0.3). Parent and doped samples were synthesized through the sol-gel method under optimized conditions. X-ray diffraction studies coupled with the Rietveld refinement of xrd data, show that both doped and parent samples crystallize in a Sr3Ti2O7-type layered perovskite structure. Temperature dependent magnetic behavior of the parent sample exhibits an antiferromagnetic and a charge-ordering state around 220 K. Magnetic field dependence of magnetization M(H) for doped samples at 20 K show well-defined M–H loops confirming the formation of intrinsic ferromagnetic ordering due to the Co doping. Additionally, we studied the ac–magnetic susceptibility and magnetization relaxation process in our samples. Dynamic scaling analysis hints an existence of glassy magnetic states in doped samples. Electrical and magnetic properties of our samples further demonstrate that Co doping suppresses charge ordering in bilayered LaSr2Mn2O7.

A single-molecule magnet featuring a parallelogram [Dy4(OCH2–)4] core and two magnetic relaxation processes

An alkoxido-bridged tetranuclear Dy(iii) complex, [Dy4(H3L)2(OAc)6]·2EtOH {, H6L = 1,3-bis[tris(hydroxymethyl)methylamino]propane}, has been solvothermally synthesized and characterized. An X-ray crystallographic study revealed that complex possesses a novel "parallelogram" [Dy4(OCH2-)4] core, and a new binding mode η(3):η(3):η(1):η(1):η(1):η(2):μ(4) of the Bis-tris propane ligand was observed. Magnetic investigations indicated that it is a single-molecule magnet (SMM), showing two distinct magnetic relaxation processes with the energy barriers of 44 K and 107 K, respectively. Such a two-step magnetic relaxation process could be well described by the sum of two modified Debye functions.

Magnetic Relaxation in Bismuth Ferrite Micro-Cubes

The process of magnetic relaxation was studied in bismuth ferrite BiFeO3 multiferroic micro-cubes obtained by means of microwave assisted Pechini process. Two different mechanisms of relaxation were found. The first one is a rapid magnetic relaxation driven by the domain reorientations and/or pinning and motion of domain walls. This mechanism is also responsible for the irreversible properties at low temperatures. The power-law decay of the magnetic moment confirms that this relaxation takes place in the system of weakly interacting ferromagnetic or superferromagnetic domains. The second mechanism is a long-term weak magnetic relaxation due to spin glass-phase.

Cumulative growth of minor hysteresis loops in the Kolmogorov model

The phenomenon of nonrepeatability of successive remagnetization cycles in Co/M (M = Pt, Pd, Au) multilayer film structures is explained in the framework of the Kolmogorov crystallization model. It is shown that this model of phase transitions can be adapted so as to adequately describe the process of magnetic relaxation in the indicated systems with “memory.” For this purpose, it is necessary to introduce some additional elements into the model, in particular, (i) to take into account the fact that every cycle starts from a state “inherited” from the preceding cycle and (ii) to assume that the rate of growth of a new magnetic phase depends on the cycle number. This modified model provides a quite satisfactory qualitative and quantitative description of all features of successive magnetic relaxation cycles in the system under consideration, including the surprising phenomenon of cumulative growth of minor hysteresis loops.

Synthesis, structures and magnetic characterizations of isostructural tetranuclear Ln4 clusters (Ln = Dy, Ho, and Eu)

Isostructural tetranuclear clusters [Ln4(salen)6]·xH2O [Ln = Dy, 1·5.5H2O; Ho, 2·4.6H2O; Eu, 3·5.2H2O; salen = N,N'-ethylenebis(salicylideneiminato)dianion] were prepared by self-assembling the corresponding lanthanide ions and a quadridentate Schiff base. Interestingly, the Ln ions are surrounded by the N- and O-donors of the Schiff bases, leading to seven- and eight-coordinate environments. The heptacoordinated Ln centers adopt a distorted capped trigonal prism, while the octacoordinated Ln atoms are in a dodecahedral disposition. The Dy complex exhibits slow magnetic relaxation, characteristic of a single-molecule magnet. Two relaxation modes in the Dy system are evident when H(dc) = 1.4 kG is applied, which is attributable to the existence of the dissimilar coordination geometries around Dy. In comparison, the Ho and Eu analogues were magnetically inspected, displaying no typical slow magnetic relaxation. These findings elucidate that the metal component in the system plays a central role in the occurrence of the magnetization relaxation process. The pronounced long-range order may contribute to intrachain exchange couplings and through-space dipolar interactions between adjacent chains.

Observation of Coherent Helimagnons and Gilbert Damping in an Itinerant Magnet

We study the magnetic excitations of itinerant helimagnets by applying time-resolved optical spectroscopy to Fe(0.8)Co(0.2)Si. Optically excited oscillations of the magnetization in the helical state are found to disperse to lower frequency as the applied magnetic field is increased; the fingerprint of collective modes unique to helimagnets, known as helimagnons. The use of time-resolved spectroscopy allows us to address the fundamental magnetic relaxation processes by directly measuring the Gilbert damping, revealing the versatility of spin dynamics in chiral magnets.

Modulation of Homochiral DyIII Complexes: Single‐Molecule Magnets with Ferroelectric Properties

Homochiral Dy(III) complexes: by changing the ligand-to-metal ratio, enantiomeric pairs of a Dy(III) complex of different nuclearity could be obtained. The mono- and dinuclear complexes exhibit characteristics of single-molecule magnets and different slow magnetic relaxation processes. In addition, the dinuclear complexes exhibit ferroelectric behavior, thus representing the first chiral polynuclear lanthanide-based single-molecule magnets with ferroelectric properties.

Creeping of minor hysteresis loops in Co thin films

In the framework of the Kolmogorov crystallization model, the non-repeatability of successive remagnetization cycles for ultrathin Co/M (M = Pt, Pd, Au) films is considered. That model has been shown to be suitable for describing magnetic relaxation processes in such systems with a “memory.” To this end some additional elements should be included in the model: it is necessary (i) to take into account that every cycle starts from the state left “heritably” by the preceding state and (ii) to suppose that the velocity of the new magnetic phase expansion depends on the cycle number. In the framework of such a model, we have succeeded to outline adequately (including quantitative description) all features of the successive magnetic relaxation cycles along with the astonishing effect of the cumulative growth of minor hysteresis loops.

Modeling the magnetization kinetics of ferromagnetic particles by the Monte Carlo method

The Monte Carlo method, which is a powerful tool for finding equilibrium states (and/or their corresponding characteristics) in systems of a different nature in many cases allows one to also describe the course of kinetic processes. This work presents the Monte Carlo modeling of magnetic relaxation and dynamics of induced magnetization of an ensemble of ferromagnetic nanoparticles. It is shown by comparison with the exact solution that in both problems there is a proportionality between the number of calculation steps and the physical duration of the transition or periodic process. On this basis, relationships are proposed making it possible to estimate the time interval corresponding to a single Monte Carlo step. For the process of magnetic relaxation the obtained result is a generalization of the well-known Nowak-Chantrell-Kennedy formula for the case of particles with finite magnetic anisotropy.

Static field induced magnetic relaxations in dinuclear lanthanide compounds of [phen<sub>2</sub>Ln<sub>2</sub>(HCOO)<sub>4</sub>(HCOO)<sub>2-2<italic>x</italic>(NO<sub>3</sub>)<sub>2<italic>x</italic>] (1, Ln = Gd and <italic>x</italic> = 0.52; 2, Ln = Er and <italic>x</italic> = 0.90; phen = 1,10-phenanthroline)</sub></sub>

Two dinuclear lanthanide (Ln) complexes, formulated as [phen 2 Ln 2 (HCOO) 4 (HCOO) 2-2 x (NO 3 ) 2 x ] (1, Ln = Gd and x = 0.52; 2, Ln = Er and x = 0.90; phen = 1,10-phenanthroline), were synthesized and characterized. They are isostructural. The dinuclear molecule consists of two Ln 3+ bridged by four formate groups and chelated by phen and formate/nitrate ligands, and the Ln 3+ possesses a coordination environment of distorted tri-capped trigonal prism of LnO 7 N 2 . Both compounds behave as paramagnets between 2 and 300 K, but display two static field induced magnetic relaxation processes. One is slow and of spin-lattice type, and it results from the lifting of Kramer's degeneracy of the ground-states of both Gd 3+ and Er 3+ , and the other is fast, and it might be spin-spin type.

Static field induced magnetic relaxations in dinuclear lanthanide compounds of [phen2Ln2(HCOO)4(HCOO)2−2x (NO3)2x ] (1, Ln = Gd and x = 0.52; 2, Ln = Er and x = 0.90; phen = 1,10-phenanthroline)

Two dinuclear lanthanide (Ln) complexes, formulated as [phen 2 Ln 2 (HCOO) 4 (HCOO) 2-2 x (NO 3 ) 2 x ] (1, Ln = Gd and x = 0.52; 2, Ln = Er and x = 0.90; phen = 1,10-phenanthroline), were synthesized and characterized. They are isostructural. The dinuclear molecule consists of two Ln 3+ bridged by four formate groups and chelated by phen and formate/nitrate ligands, and the Ln 3+ possesses a coordination environment of distorted tri-capped trigonal prism of LnO 7 N 2 . Both compounds behave as paramagnets between 2 and 300 K, but display two static field induced magnetic relaxation processes. One is slow and of spin-lattice type, and it results from the lifting of Kramer's degeneracy of the ground-states of both Gd 3+ and Er 3+ , and the other is fast, and it might be spin-spin type.

Magnetic phase segregation and glass nature of Pr0.4Bi0.3Ca0.3MnO3 manganite

The results of detailed investigations of magnetic properties of Pr0.4Bi0.3Ca0.3MnO3 manganites at low temperatures are presented. The performed investigations of temperature, field and frequency dependences of magnetization and susceptibility as well as the magnetization relaxation processes indicate that below TC = 44 K the manganite Pr0.4Bi0.3Ca0.3MnO3 is in the magnetic phase-segregated state with ferromagnetic inclusions forming a cluster glass.

Multiple-decker phthalocyaninato dinuclear lanthanoid(iii) single-molecule magnets with dual-magnetic relaxation processes

The SMM behaviour of dinuclear Ln(III)-Pc multiple-decker complexes (Ln = Tb(3+) and Dy(3+)) with energy barriers and slow-relaxation behaviour were explained by using X-ray crystallography and static and dynamic susceptibility measurements. In particular, interactions among the 4f electrons of several dinuclear Ln(III)-Pc type SMMs have never been discussed on the basis of the crystal structure. For dinuclear Tb(III)-Pc complexes, a dual magnetic relaxation process was observed. The relaxation processes are due to the anisotropic centres. Our results clearly show that the two Tb(3+) ion sites are equivalent and are consistent with the crystal structure. On the other hand, the mononuclear Tb(III)-Pc complex exhibited only a single magnetic relaxation process. This is clear evidence that the magnetic relaxation mechanism depends heavily on the dipole-dipole (f-f) interactions between the Tb(3+) ions in the dinuclear systems. Furthermore, the SMM behaviour of dinuclear Dy(III)-Pc type SMMs with smaller energy barriers compared with that of Tb(III)-Pc and slow-relaxation behaviour was explained. Dinuclear Dy(III)-Pc SMMs exhibited single-component magnetic relaxation behaviour. The results indicate that the magnetic relaxation properties of dinuclear Ln(III)-Pc multiple-decker complexes are affected by the local molecular symmetry and are extremely sensitive to tiny distortions in the coordination geometry. In other words, the spatial arrangement of the Ln(3+) ions (f-f interactions) in the crystal is important. Our work shows that the SMM properties can be fine-tuned by introducing weak intermolecular magnetic interactions in a controlled SMM spatial arrangement.

A bifunctional luminescent single-ion magnet: towards correlation between luminescence studies and magnetic slow relaxation processes

A new heterodinuclear [Zn(L)Dy](3+) complex, with L being a compartmental Schiff base ligand, exhibits the characteristic Dy(3+) luminescence associated with a single-ion field induced slow relaxation of the magnetisation. This complex may be considered as one of the rare examples of a bifunctional luminescent single-ion magnet.