Spin-cluster Resonance Research Articles

Resonance properties of the quasi-one-dimensional Ising magnet [(CH3)3NH]CoCl3⋅2H2O in the paramagnetic and magnetically ordered phases

A study is made of the angular, frequency–field, and temperature dependences of the magnetic resonance of the quasi-one-dimensional Ising magnet [(CH3)3NH]CoCl3⋅2H2O in the paramagnetic phase. The experimental results obtained are explained in a model of spin-cluster resonance in a strongly exchange-coupled spin chain. The frequency–field dependences of the ferromagnetic resonance spectrum measured below the Néel temperature are studied for magnetic-field directions along the crystallographic axes a, b, and c. It is shown that for H→0 the spin-wave spectrum of this quasiferromagnet has two gaps (ν1=70.1 GHz and ν2=52.5 GHz).

Magnetic dipolar effects in the spin-cluster resonance spectra of[(CH3)3NH]FeCl3⋅2H2O

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Submillimeter Wave ESR Study of Magnetic Excitations in the Ising Ferromagnetic Chain CoNb2O6

Magnetic excitations in the Ising ferromagnetic chain CoNb 2 O 6 have been studied by submillimeter wave electron spin resonance in strong transverse magnetic fields. A distinct non-linear field dependence of the resonance branches and also zero field splittings have been observed. A large residual intensity of the spin-cluster resonance was found which was caused by the strong mixing among the different n -fold spin-clusters. In addition to a series of n -fold spin-cluster excitations, we also discovered a new set of spin-cluster excitation which can be attributed to non-integer n such as n ∼1.5 or n ∼2.5 for the first time. This effect is explained by the change of the quantization axis of the system in strong transverse magnetic fields. These new aspects of spin-cluster excitation show the significant effect of transverse magnetic fields on the magnetic excitation in an Ising ferromagnetic chain.

The effect of chain length on the temperature dependence of a spin-cluster resonance signal in a one-dimensional Ising ferromagnet

The temperature dependence of the high-field spin cluster resonance observed in the one-dimensional ferromagnet FeTAC at low temperatures is shown to depend on the numbers of ferrous ions in the chains. The longer the chain, the lower is the temperature at which deviations from the previously obtained linear relationship between In(intensity) and 1/T occur. Calculations for the distribution of chain lengths obtained by the random substitution of a nonmagnetic impurity are made for several impurity concentrations and compared with the experimental data points for a pure FeTAC sample. The results indicate that, in this case, other types of crystal defect must limit the chain lengths.

The effect of Co doping on spin cluster resonance in the one-dimensional Ising ferromagnet FeTAC

Electron paramagnetic resonance measurements at 36 GHz and in the range 12–18 GHz were made on a single crystal of the 1D Ising ferromagnet [(CH3)3 NH] FeCl3&amp;sdot;2H2O, known as FeTAC, containing a nominal 10% of the isomorphous compound CoTAC. While the 4.2 K spectra obtained with the external field parallel to the chain axis were similar to the spin cluster resonance spectra previously identified in pure FeTAC, differences in the low-field satellite structure were observed, which are tentatively attributed to differences in the dipolar interaction and to shorter ferrous chains. An appreciable decrease in the strength of the demagnetizing field is associated with the tendency of the Co2+ moments to align perpendicularly to the chain axis.

Spin-cluster resonance at 9 GHz in the one-dimensional Ising ferromagnet

Low-temperature magnetic-resonance spectra at 9.2--9.3 GHz in single crystals of the one-dimensional Ising ferromagnet FeTAC, [(${\mathrm{CH}}_{3}$${)}_{3}$NH] ${\mathrm{FeCl}}_{3}$\ensuremath{\cdot}${2\mathrm{H}}_{2}$O, were strongly anisotropic with minima in the field occurring for H parallel to the easy axis. A ``strong'' signal in this orientation with a g value of 8.8 at 5.5 K (after correction for internal demagnetization and exchange fields) is identified with transitions between neighboring spin-cluster resonances. Possible origins of broad and weak lines, with maximum corrected g values of 13 and 3.7 are discussed. Spectra from samples of different shape are consistent with sample-dependent demagnetization fields.

Microwave magnetic resonances in the one-dimensional ferromagnet FeTAC

Magnetic resonance spectra at 12–36 GHz are reported for Fe2+(3d6, S=2) in the 1D Ising ferromagnet [(CH3)3NH]FeCl3⋅2H2O, known as FeTAC, between 2.5 and 13 K. The main signal was identified as a spin-cluster resonance (SCR) because of its characteristic temperature dependence below 6 K and the linear dependence of the resonance field on the microwave frequency for H parallel to the easy axis at 4.2 K. The temperature dependence of the line intensity was fitted to the function exp[−(2J+hν)/kBT], with 2J/kB=(41±1) K, and the linear field dependence was explained by the value gz=8.26±0.05 in the S=1/2 representation and a demagnetization field of several hundred gauss, which depended on the sample shape. A frequency-dependent satellite structure observed on the low field side of the main line, which appeared as a broad line below 15 GHz, is tentatively associated with energy shifts of the lower-lying spin-cluster states produced by the magnetic dipole interaction and other terms in the Hamiltonian.

The far-infrared magnetic excitation spectrum of CoCl2(NC5H5)2

The quasi-one-dimensional Ising-like system CoCl2 dipyridine has been studied using a Fourier spectrometer over the range 10-60 cm-1. Spin-cluster excitations from the ground state and spin-cluster resonance between bound states have been observed at fields up to 8 T and temperatures in the range 1.5<T<30 K. The excitation energies show the influence of (small) transverse elements in the intra-chain exchange interaction and a transverse magnetic field component. The field and temperature dependence of the intensity have been interpreted on the basis of a pure Ising model, implemented-to finite order-with the effect of transverse elements. The zero-field limit is discussed in detail. The linewidth is only slightly dependent on temperature, in contrast with earlier observations for CoCl2.2H2O.

Far infrared absorptions in CoCl 2 dipyridine

The far infrared transmission spectra of the quasi one-dimensional Ising-like system CoCl 2 dipyridine have been measured in the range of 10–60 cm -1. Spincluster excitations as well as spincluster resonance have been observed as a function of field and temperature. The field- and temperature dependence of frequency and intensity have been interpreted on the basis of a pure Ising model, implemented, in first order, with the effect of transverse exchange elements.

Quasi-one-dimensional magnetic behaviour of the Ising system CsFeCl3.2aq

The magnetic behaviour of the quasi-one-dimensional system CsFeCl3.2aq(aq=H2O, D2O) has been investigated by heat capacity measurements, quasi-elastic neutron scattering and spin-cluster resonance. the experiments demonstrate that below 25K the compound is a very good realisation of an (S=1/2) Ising system. The temperature dependence of the spin correlations within the chains above TN can also be described in terms of a classical spin model, based upon the full spin (S=2) of the Fe2+ ions.

Fe2+Localized Excitations in Fe(1-x)CoxCl2·2H2O

Electron spin resonance experiments in pulsed high magnetic fields have been performed on the randomly mixed antiferromagnet with competing spin anisotropies, Fe (1- x ) Co x Cl 2 ·2H 2 O. When the external field is applied along the b-axis (the easy axis of CoCl 2 ·2H 2 O) of the mixed crystals, new resonance lines appear together with the spin-cluster resonances. We assign these new lines to Fe 2+ localized excitations in this mixed system. A quantitative agreement between the theory and the experiments is attained using only one adjustable parameter.

ESR study of magnetic excitations in one-dimensional magnets

ESR measurements are performed in order to study spin dynamics in selected one-dimensional systems. In an Ising-like antiferromagnet CsCoCl3, a new kind of ESR due to the propagating domain walls is observed, while in Ising-like ferromagnets CoCl2·2NC5H5 and (CH3)3NHCoCl3·2H2O, spin-cluster resonances are observed. The resonance in the XY ferromagnet (CH3)4NNiBr3, is also associated with the thermallyactivated excitations.

ESR Study of Mn2+in the Impure Ising Linear Chain [(CH3)3NH]CoCl3·2H2O: Mn

ESR investigation was carried out on the impure Ising linear-chain compound [(CH 3 ) 4 NH]CoCl 3 ·2H 2 O: Mn in the temperature range 1.8–300 K at 24 GHz. Only single lines with g =2 for Mn 2+ are observed within the temperature range 10–80 K. Below about 10 K five lines corresponding to the fine structure of Mn 2+ and two spin-cluster resonances for the Co 2+ linear-chain system appear for orientations H // c and H // b , respectively. The anisotropy of the fine structure spectrum can be explained only by recognizing the canceling in the diagonal part of the Mn–Co interaction tensor.

ESR Study of Quasi-One-Dimensional Ising-Like Antiferromagnet CsCoCl3

Electron spin resonance was measured at 9.40 GHz as a function of temperature in a quasi-one-dimensional Ising-like antiferromagnet CsCoCl 3 . Observed resonance in the partially disordered phase, \(T_{N2} \lesssim T<t_{N1}\) and its neighborhood is interpreted in terms of ESR due to spins at propagating domain walls in one-dimensional Ising-like antiferromagnet. The theory developed by Shiba and the present author gives a more consistent interpretation of the temperature dependence of the absorption intensity and the angular dependences of the resonance field and the linewidth than the existing theories. Discussion is given in connection with spin-cluster resonance.

Temperature Dependence of Spin Cluster Resonance Intensity

A consideration is given to understand the temperature dependence of spin cluster resonance intensity by simple extention of the Date and Motokawa model. The calculated result shows that the intensity increases sharply with increasing temperature, and decreases at higher temperatures after reaching a maximum at a certain temperature. The experimental check is mainly done by the measurements in a quasi-one-dimensional Ising ferromagnet, [(CH 3 ) 3 NH]CoCl 3 ·2H 2 O. The proposed model gives a good agreement with the observed behavior over the whole temperature range studied.

Excitations and magnetic ground state in Co(pyr)2Cl2

The magnetic ground state and field induced magnetic arrays are derived by the method of Kudō and Katsura. From the critical fields a set of exchange parameters can be deduced. Spin cluster resonances are detected in the various phases as well as in the paramagnetic phase at T≳TN, where evidence for the existence of local two-dimensional arrays can be found.

Spin-cluster excitations in RbFeCl3·2H2O

Spin-cluster excitations (SCE) from the ground state are observed in the pseudo-one-dimensional canted Ising antiferromagnet RbFe${\mathrm{Cl}}_{3}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O. Since the moments are canted with respect to each other, SCE were observed which show characteristics of a ferromagnetic chain and an antiferromagnetic chain when the external field is along the $c$ or $a$ axis, respectively. The results cannot be interpreted with the simple Ising model as in the case of spin-cluster resonance (SCR) reported earlier. For the small spin clusters observed in the present experiment, it is necessary to include effects from the higher single-ion states. Together with SCR the data provide rather detailed information on the magnetic ground state and the interactions.

Spin-cluster resonance in the pseudo-one-dimensional canted Ising antiferromagnet RbFeCl3·2H2O

Spin-cluster resonance (SCR) is observed in the pseudo-one-dimensional canted Ising antiferromagnet RbFe${\mathrm{Cl}}_{3}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O. Transitions between clusters up to 9 spins ($|m&gt;\ensuremath{\Rightarrow}|m+9&gt;$) could be detected. Since the moments in the chain are canted with respect to each other, SCR were observed which show characteristics of a ferromagnetic chain and an antiferromagnetic chain for the external magnetic field parallel to the $c$ and $a$ axis, respectively. The experimental data for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}\ensuremath{\parallel}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ could excellently be described by a pure Ising model, with $\frac{{J}_{b}}{k}=0.76$ K, $\frac{{J}_{c}}{k}=\ensuremath{-}0.21$ K, $\frac{{J}_{\mathrm{bc}}}{k}=\ensuremath{-}0.13$ K, $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}\ifmmode\cdot\else\textperiodcentered\fi{}\frac{\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}}{|\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}|}=1.50$ ${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$, and a correction for a demagnetizing field. In order to describe the SCR for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}\ensuremath{\parallel}\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ the influence of the crystal-field splitting of the pseudo-ground-doublet of the ${\mathrm{Fe}}^{2+}$ ion had to be taken into account.

Influence of Impurities on the Temperature Dependence of Spin Cluster Resonance Intensity

The temperature dependence of spin cluster resonance signals can be used to calculate the dominant exchange interaction in chainlike structures. It is shown that impurities in the sample can result in a drastic change of the calculated value of the exchange. The results are applied to the case of CsCoCl 3 ·2H 2 O.

Magnetic Resonance in Antiferromagnetic FeCl2·4H2O

The electronic magnetic resonance spectrum of single crystals of Fe${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}4${\mathrm{H}}_{2}$O has been studied in the microwave region from 9 to 76 GHz at temperatures between 0.38 and 4.2 K. Two distinct types of resonance characteristic of the antiferromagnetic state were observed, each of which is associated with a pair of highly anisotropic $g$ tensors (${g}_{\ensuremath{\perp}}\ensuremath{\simeq}0$) with principal axes\ifmmode\pm\else\textpm\fi{}30\ifmmode^\circ\else\textdegree\fi{} from the $b$ axis in the ${a}^{\ensuremath{'}}\ensuremath{-}b$ plane. One of these resonance pairs is easily distinguishable from the other, since its integrated resonance intensity is some three orders of magnitude larger. It is an unusual antiferromagnetic resonance in that the frequency-field relation is temperature-independent, whereas the resonance intensity is strongly temperature-dependent. The intensity is greatest at the lowest temperatures and decreases rapidly as the N\'eel temperature is approached, with a small remanent signal observable even above ${T}_{N}$. The zero-field resonance was observed at 72.5 GHz. No spin-flop resonance was observed. The weak-intensity resonances, which we denote as Ising spin resonances because of the very large $g$ anisotropy, are similar in some respects to the spin-cluster resonances observed in Co${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O and Fe${\mathrm{Cl}}_{2}$, but are characterized by a strong local antiferromagnetic interaction rather than the dominant ferromagnetic interaction of those other materials. Resonances corresponding to many of the possible local spin configurations were measured, and the temperature dependence of the resonance intensities was determined. In addition, resonances associated with a small concentration of Mn impurities were observed.