[en] ¹H nuclear magnetic resonance (NMR) measurements have been carried out in Mn₁₂O₁₂-acetate clusters at low temperature in order to investigate microscopically the static and dynamic magnetic properties of the molecule in its high-spin S=10 ground state. Below liquid helium temperature it is found that the local hyperfine fields at the proton sites are static as expected for the very slow superparamagnetic relaxation of Mn₁₂O₁₂ at low temperature. The magnitude and distribution of the hyperfine fields can be reproduced to a good approximation by considering only the dipolar interaction of protons with the local Mn magnetic moments and by assigning the magnitude and orientation of the local moments of the different Mn³⁺ and Mn⁴⁺ ions according to an accepted coupling scheme for the total S=10 ground state. The relaxation time of the macroscopic magnetization of the cluster was measured by monitoring the change of the intensity of the ¹H-NMR shifted lines following inversion of the applied magnetic field. This is possible because the sudden change of the field orientation changes the sign of the shift of the NMR lines in the proton spectrum. Although important differences are noticed, the relaxation time of the magnetization as measured indirectly by the ¹H-NMR method is comparable to the one obtained directly with a superconducting quantum interference device magnetometer. In particular we could reproduce the minima in the relaxation time as a function of magnetic field at the fields for level crossing, minima which are considered to be a signature of the quantum tunneling of the magnetization

[en] The magnetization of the aggregated cores of ferritin was measured as a function of the temperature and the external field. The blocking temperature, the susceptibility, the saturation magnetization, and the effective moments were deduced from the magnetization data and compared with those of natural ferritin. The blocking temperature decreased monotonically with increasing field in contrast to the anomalous field dependence of the blocking temperature of natural ferritin, which showed a broad peak around 0.3 T. The strong temperature dependence of the effective moment of the aggregated cores of ferritin was quite different from the weak temperature dependence of the effective moment of natural ferritin. The experimental results suggest that additional interparticle interactions among the cores may be important in understanding the magnetic behavior of aggregated ferritin cores.

[en] The magnetization of the protein shell (PepA) encapsulated CoPt nanoparticle (average diameter = 2.1 nm) specimen was investigated by implementing non-equilibrium magnetization calculation scheme and Curie-Weiss type formula. In doing that, the magnetization measurement sequence of SQUID (Superconducting Quantum Interference Device) magnetometer was modified to comply with the numerical analysis scheme. Applying the analysis methodology to the CoPt nanoparticles, we extract the values of zero-temperature coercive field and Bloch coefficient, which are in good agreement with the previously reported values. Remarkably, the distribution of the zero-field zero temperature anisotropy energy barrier follows an exponential type function, which is uncorrelated with the known size distribution of the particles. The lacking correlation between them was reported in ferritin, suggesting that it is a generic feature of certain magnetic nanoparticle systems having a range of disorders and imperfections. In addition, we analyze non-saturating behavior of a field dependent magnetization curve in terms of Curie-Weiss type formula, allowing a separation of the surface magnetic moments from the core moments.

[en] The magnetization of the protein shell (PepA) encapsulated CoPt nanoparticle (average diameter = 2.1 nm) specimen was investigated by implementing non-equilibrium magnetization calculation scheme and Curie-Weiss type formula. In doing that, the magnetization measurement sequence of SQUID (Superconducting Quantum Interference Device) magnetometer was modified to comply with the numerical analysis scheme. Applying the analysis methodology to the CoPt nanoparticles, we extract the values of zero-temperature coercive field and Bloch coefficient, which are in good agreement with the previously reported values. Remarkably, the distribution of the zero-field zero-temperature anisotropy energy barrier follows an exponential type function, which is uncorrelated with the known size distribution of the particles. The lacking correlation between them was reported in ferritin, suggesting that it is a generic feature of certain magnetic nanoparticle systems having a range of disorders and imperfections. In addition, we analyze non-saturating behavior of a field dependent magnetization curve in terms of Curie-Weiss type formula, allowing a separation of the surface magnetic moments from the core moments.

[en] We report a magnetization and electron spin resonance (ESR) study of the clinoatacamite Cu₂(OH)₃Cl, which is known to have a S = 1/2 distorted pyrochlore lattice. The static magnetic susceptibility shows a strong increase at temperatures below T_N2 = 6.4 K without any appreciable anomalies at T_N1 = 18.1 K. The magnetization vs. field curve exhibits the presence of a weak ferromagnetic moment of 0.1 μ_B/Cu at T = 2 K, possibly due to an antisymmetric Dzyaloshinsky-Moriya (DM) interaction and/or weak ferromagnetic interplane interactions. From the ESR linewidth, we estimate the magnitude of the DM interaction as 10% of the leading isotropic exchange interaction, J ∼ 170 K. The temperature dependence of the linewidth is characterized by three distinct temperature regions: (i) a high-temperature exchange narrowing region at temperatures above 170 K, (ii) a two-dimensional short-range ordered region at temperatures between 170 K and 36 K, and (iii) a low-temperature region at temperatures below 36 K where an antiferromagnetic-like resonance mode develops. The evolution of the ESR linewidth is comparable to that of the S = 1/2 kagome lattice in Cu₃Zn(OH)₆Cl₂ [Zorko et al., Phys. Rev. Lett. 101, 02640 (2008)]. This suggests that the magnetic properties of Cu₂(OH)₃Cl may be described in terms of a stack of kagome lattices.

[en] We report ¹H nuclear magnetic resonance spin-lattice relaxation rate (NSLR) measurements as a function of temperature (1.5-4.2 K) and as a function of applied magnetic field (0.2-8.2 T) in the molecular magnet [(PhSiO₂)₆Cu₆(O₂SiPh)₆] in short Cu6. The results are explained in terms of a simple model whereby the NSLR is driven by the fluctuations of the local hyperfine field due to the reorientation of the total spin of the molecule in its ground state. From the analysis of the data, we infer the temperature and field dependence of the characteristic rate of the fluctuations of the total magnetization of the Cu6 ring in its ground state. (c) 2000 American Institute of Physics

[en] We present a novel method to measure the relaxation rate W of the magnetization of Mn₁₂O ₁₂ -acetate (Mn12) magnetic molecular cluster in its S=10 ground state at low T . It is based on the observation of an exponential growth in time of the proton NMR signal during the thermal equilibration of the magnetization of the molecules. We can explain the novel effect with a simple model which relates the intensity of the proton echo signal to the microscopic reversal of the magnetization of each individual Mn12 molecule during the equilibration process. The method should find wide application in the study of magnetic molecular clusters in off-equilibrium conditions. (c) 2000 The American Physical Society

[en] We report the synthesis and magnetic properties of the molecular cluster Cu₃(${\mathrm{\mu <!--\; ??\; -->}}_3$−OH)(μ-OH)(μ-O₂Ar${}^{4F-<!--\; ???\; -->\mathrm{P}\mathrm{h}}$)₂(py)₃(OTf)₂, abbreviated as (Cu₃(OH)). Using magnetization, electron paramagnetic resonance and spin dimer analysis, we derive a microscopic magnetic model of (Cu₃(OH)) and measure the electron T₁ and T₂ relaxation times. The Cu²⁺ ions are arranged to form a distorted triangular structure with the three different exchange coupling constants $J_1=-<!--\; ???\; -->43.5$ K, $J_2=-<!--\; ???\; -->53.0$ K, and $J_3=-<!--\; ???\; -->37.7$ K. At T = 1.5 K T₁ is of the order of 10⁻⁴ s and T₂ is evaluated to be $0.26\mathrm{\mu <!--\; ??\; -->}\mathrm{s}$. We find that the temperature dependence of $1/T_1$ and $1/T_2$ is governed by Orbach process and spin bath fluctuations, respectively. We discuss the role of spin–phonon mechanism in determining a spin decoherence time in a class of spin triangular clusters. (paper)

[en] Magnetic susceptibility, ¹H NMR and ⁶³Cu NMR-NQR experiments on two slightly different species of the molecular S=1/2 antiferromagnetic (AF) ring Cu8, [Cu₈(dmpz)₈(OH)₈]·2C₅H₅N (Cu8P) and [Cu₈(dmpz)₈(OH)₈]·2C₅H₅NO₂ (Cu8N), are presented. The magnetic energy levels are calculated exactly for an isotropic Heisenberg model Hamiltonian in zero magnetic field. From the magnetic susceptibility measurements we estimate the AF exchange coupling constant J∼1000 K and the resulting gap Δ∼500 K between the S_T=0 ground state and the S_T=1 first excited state. The ^63,65Cu NQR spectra indicate the presence of four crystallographically inequivalent copper nuclei in each ring. From the combination of the ⁶³Cu NQR spectra and of the ⁶³Cu NMR spectra at high magnetic field, we estimate the quadrupole coupling constant v_Q of each site and the average asymmetry parameter η of the electric-field gradient tensor. The nuclear spin-lattice relaxation rate (NSLR) decreases exponentially on decreasing temperature for all nuclei investigated. The gap parameter extracted from ⁶³Cu NQR-NSLR is the same as for the susceptibility while a smaller value is obtained from the ⁶³Cu NMR-NSLR in an external magnetic field of 8.2 T. (c) 2000 The American Physical Society