It is shown that the contribution of the three-spin non-secular dipolar interactions of nuclei to their spin–lattice relaxation in solids can be measured with the modified conventional coherent pulse NMR technique at the laboratory-frame (LF) high frequency. In this method, the measurements are carried out in the specific magic-angle triply rotating frame (TRF), while the first (single) rotating frame (RF) is in the usual resonant coherent conditions and the second (doubly rotating) frame is in the conditions of the standard magic-angle. The procedure is similar to the well-known technique of measuring spin–lattice relaxation in the usual resonant RF. It is realized in the form of continuous spin-locking in the TRF effective field. The NMR signal is then registered in the form of the LF free induction decay (FID) after a sudden turn-off of a special-designed locking high-frequency pulse. The spin–lattice relaxation curve in the TRF is measured point-by-point by multiple repetitions of the experiment with varying the pulse length and recording the FID amplitude. Formulas for the contribution of the three-spin non-secular dipolar interactions into the TRF spin–lattice relaxation rate have been derived and quantitatively analyzed.

Formulas are derived for the contribution of the three-spin non-secular dipolar interactions to the characteristic time $$T_{1\rho \rho }$$ of the nuclear spin–lattice relaxation in solids in the doubly rotating frame (DRF), which are convenient for practical use. Both the singly rotating frame (RF) and the DRF are in their own magic-angle conditions to select such interactions with maximal efficiency. One of the formulas is expressed in particular in terms of motionally averaged parts of the second moment of the absorption RF nuclear magnetic resonance line; another differs mainly from the first in that the second moment parameters are substituted for the corresponding RF local field ones. An analytical interrelation between the formulas is established. The formulas are self-consistent and equivalent, and each of them individually well describes the three-spin dipolar contribution of protons of polycrystalline benzene to their spin–lattice relaxation in the DRF at ultraslow rotary and diffusive motions of its molecules.

Possibilities of magnetic pseudoresonance (a non-resonance peak of magnetic susceptibility) were studied and compared with the ferromagnetic resonance (FMR) in measuring the parameters of thin ferromagnetic films with in-plane uniaxial magnetic anisotropy. The measurements were conducted with two characteristic samples of ferromagnetic films showing this effect. A Q -meter operating at a frequency near 300 MHz (for pseudoresonance) and a standard X -band magnetic resonance spectrometer (for FMR) were used. The Q -meter working at 300 MHz was shown to detect reliably the magnetic pseudoresonance in both epitaxial and polycrystalline films. It was found that the accuracy of determination of the magnetic anisotropy field and orientation of the easy magnetization axis provided by the pseudoresonance method is as good as with FMR, and in some cases the pseudoresonance method gives additional information.

We realized the improvement of a dielectric moisture meter in the form of elevation of its sensitivity to moisture with preservation of the independence of the results of measurement of the mass of the investigated material. In the improved meter, the moisture is found from the ratio of the sum of increments of the reactive and active components of the admittance of the capacitor sensor of moisture meter caused by placing the investigated material inside the capacitor to the difference between these increments or from the ratio of some other combinations of these increments. Both these ratios and the ratio of initial increments are independent of the mass of investigated material but much stronger depend on its moisture content. In practice, the application of these moisture meters can simplify the procedure of measurements and increase their accuracy.

A dielectric moisture meter is described in which moisture content is determined from the phase shift angle between the increments in the total conductivity of the measurement capacitor of the probe of the moisture meter and its reactive component. These increments are produced by introducing sample material into the capacitor. This kind of moisture meter is shown to be insensitive to the mass of the sample, but is quite sensitive to its moisture content. By comparison with other mass-insensitive moisture meters, this meter has a simpler design and has smaller measurement errors because of a reduced number of measurement and computational operations.

Joint optimization of the parameters of the combined multiple-pulse spin-locking sequence has been performed with the aim to maximize the ratio of the 14N nuclear quadrupole resonance (NQR) signal to noise for the best detection of trinitrotoluene (TNT) and hexogen (RDX) over all the operating ranges of temperatures from 30 to −30 °C. It is based on the knowledge of spin–lattice T 1 and effective transversal T 2e relaxation times of 14N nuclei over all these temperature ranges. The problem is solved rigorously, without simplifying assumptions. It is established that the NQR signal-to-noise ratio at any given sequence duration T s has a maximum not only in depending on the number pulses N in its subsequences, but also on the number m of their repetitions. On this basis, the system from two inter-related mutually compatible equations has been derived, each of which contains three unknowns, of which two ones are classified as optimum values of N opt and m opt, whereas the third one t p only corresponds to them, being the correct value t pc (t p is the relaxation pause between the subsequences). This system is most conveniently solved for N opt and t pc if the parameter m is initially preset as an integer and optimum number. The selection of the suitable m opt is dictated by that the calculated correct sequence duration T sc at these N opt, t pc, and m opt is to be equal or near to a scheduled time of explosive detection. In spite of the fact that the T 1 and T 2e times in these explosives essentially differ and strongly change with the temperature, the solution of these equations does not cause any difficulties over all the temperature ranges. Results of the calculations are presented in the graphical form and represent the temperature and m opt dependence of values of N opt, t pc, and T sc, and of the maximum factor of increasing the NQR signal-to-noise ratio determined by these parameters. They are compared with the results obtained previously for RDX in the high-temperature side of the range. It is discussed the influences of errors in the measured T 1 and T 2e times, as well as of rounding off decimal numbers in intermediate calculations on the accuracy of the calculated values of N opt and t pc.

The effect of random electronic paramagnetic impurities on the 14N nuclear quadrupole resonance (NQR) signal intensity and spin–lattice relaxation has been measured in the samples of commercial grade explosives cyclotrimethylene trinitramine (RDX) and trinitrotoluene (TNT). It was found that in some samples the number of impurities measured by the electron paramagnetic resonance method could reach almost 1 % of the number of the host molecules. As a result, a considerable part of the 14N nuclei is removed from the resonance leading to corresponding attenuation of the NQR signal. The temperature dependence of the nuclear spin relaxation times T 1 and T 2e (T 2e is the echo-signal decay time in the multiple pulse spin-locking sequence) was measured in the range from −40 to 40 °C at some frequencies used for detecting this explosives. In the high temperature side of this range, the relaxation times for both the explosives decrease exponentially with increasing temperature but do not achieve a minimum. In this part of the temperature range, T1 and T2e are practically equal in RDX, while T 2e is much shorter than T 1 in TNT. The exponential part of these temperature dependencies is characterized by the same activation energies of molecular motion (reorientation of the NO2 groups) equal to 72 and 60 kJ/mol in the cases of RDX and TNT, respectively. In the low temperature side, the slope of the T1 and T 2e temperature dependences decreases (except for T 2e in TNT). The obtained results for RDX are compared with those reported by other authors for a similar explosive of other manufacturer and discussed in detail.

A strong narrow peak in the field dependence of the radio-frequency absorption (the magnetic pseudoresonance) has been found and investigated in epitaxial thin films of La2/3Sr1/3MnO3 possessing uniaxial magnetic anisotropy in the film plane. The peak is observed when the in-plane external magnetic field H is directed perpendicular to the easy axis and equals to the anisotropy field H u. The frequency dependence of the peak magnitude measured in the frequency range of 10–300 MHz approximately follows the Debye law behavior with the characteristic relaxation time of about 2.2 ns. The physical model of the phenomenon is suggested, based on the giant increase in static transversal susceptibility due to a sharp reorientation of the equilibrium magnetization when approaching the pseudoresonance conditions.

Two versions of the spin-lock inversion mid-echo pulse sequence used for the prompt nuclear quadrupole resonance detection of explosives have been studied experimentally. These sequences are modifications of the conventional multiple pulse spin-locking sequence and differ from it by additional groups of pulses which invert echo signals. It was found that spurious signals produced by some magnetic objects are suppressed by one of these sequences much stronger than by the other. The attenuation of echo signals due to their inversion by the group of the inverting pulses is measured. The number of pulses is optimized in both the sequences to obtain maximum signal-to-noise ratio, taking into account some loss in the magnitude of inverted echoes.

The nanoparticles of the lanthanum-silver manganites with the general formula La1 − x Ag y MnO3 + δ (LAMO) that exhibit the Curie temperature T C in the range 35…50°C, which is important for medical applications, are synthesized and studied. The heating kinetics of the LAMO aqueous suspensions under the action of the high-frequency (HF) magnetic field is experimentally studied and the self-stabilization in the vicinity of the Curie temperature is demonstrated. A sharp increase in the HF-absorption (specific absorption rate) is observed for the materials under study when approaching the Curie temperature from below. This effect is proven by direct measurements of the dynamic magnetic susceptibility in the critical range. The physical interpretations of the HF absorption are based on the hysteresis and oscillations of domain boundaries. It is demonstrated that the materials under study are promising for the medical hyperthermia with controlled heating temperature.

The problem has been solved for the joint optimization of the number N of pulses and the relaxation pause t p in the combined multiple-pulse spin-locking sequence, which is successfully used for detecting some explosives by nuclear quadrupole resonance (NQR). Initially, the formula is derived for the signal-to-noise ratio provided by this sequence. Using the condition that an explosive detection time T det is constant (it is typical in practice), from this expression the set of two interrelated equations is obtained for optimum values of N and t p, which provide the maximum signal-to-noise ratio. These values are determined by both effective transverse T 2e and longitudinal T 1 relaxation times and sequence parameters, and do not depend on the number m of sequence repetitions. The m value is calculated as the quotient of T det by the duration of the onefold sequence with optimum values of N and t p. This set of equations is shown to be mutually compatible at all values of sequence parameters and relaxation times T 1 and T 2e, which occur in actual practice. The results of the experiments, which have been carried out on cyclotrimethylene trinitramine (RDX) at a 14N NQR frequency of 3.41 MHz and temperatures of 26 and −3°C, are in good agreement with the theoretical ones.

Temperature and frequency dependencies of the real (χ′) and imaginary (χ″) parts of the dynamic magnetic susceptibility were studied experimentally in fine particles of La–Ag manganites prepared by various methods. The samples under study have the Curie points in the range TC=42–48°C, which is a medical hyperthermia range of interest. When approaching TC from below, a critical peak of χ″ was revealed, followed by a steep drop while passing to the paramagnetic phase. The experiments on the magnetic radio-frequency (RF) heating of the manganite aqueous suspensions demonstrated good autostabilization of the temperature near TC. Peculiar instability is found in the heating kinetics, caused by the observed critical behavior of the RF losses. The prospects of the La–Ag manganites as candidates for application in the temperature-controlled hyperthermia are discussed.

Performance data of a two-sided shunt-type diode-stabilitron amplitude limiter, in which a stable limiting threshold is specified by the breakdown voltage of the stabilitrons connected in series opposition to the diodes, are measured at 30 MHz in a low-voltage range. It is established that the limiter factor may exceed 100 when the stabilitrons are automatically biased to the breakdown region owning to rectifying of a limited signal by the diodes; it may reach 1000 when they are biased from a power source of the amplifier. The operating conditions for attaining a high limiter factor are determined.

The contribution of ultraslow self-diffusion of polycrystalline benzene molecules to the spin-lattice relaxation of protons is studied as a function of effective magnetic field H 2 in a doubly rotating frame (DRF). Proton relaxation time T 1ρρ is measured by direct recording of NMR in a rotating frame (RF). The effective fields have a “magic” orientation corresponding to angles arccos(1/√3) in the RF and π/2 in the DRF so that the secular part of the dipole-dipole interactions of protons is suppressed in two orders of perturbation theory, while the nonsecular part becomes predominant. It is found that the diffusion contribution of benzene molecules to proton relaxation time T 1ρρ is a linear function of the square of field H 2 and exhibits all peculiarities typical of the model of strong collisions generalized to only fluctuating nonsecular dipole interactions in fields exceeding the local field. This means that the model can also be employed in the given conditions. It is shown that perfect agreement with such a dependence can also be obtained in the model of weak collisions if we take into account the concept of the locally effective quantization field, whose magnitude and direction are controlled by the vector sum of field H 2, and the nonsecular local field perpendicular to it.

A probe with a high-frequency amplifier of a continuous-wave Q-meter-type electron paramagnetic resonance (EPR) spectrometer operating at 300 MHz is described. The probe is inductively connected with both an exciting high-frequency generator and the amplifier. The amplifier is matched to the probe for obtaining a minimum noise figure of 2.9 dB. The single-scan EPR line with a 1.7-G width and 3 × 1018 resonant paramagnetic centers is recorded with a signal-to-noise ratio of about 260.

A method for measuring nuclear magnetic spin-lattice relaxation in solids in the effective field He3 acting in the triply rotating frame (TRF) is described. The method advances the previously described techniques whereby nuclear magnetic resonance and relaxation in the rotating (RF) and doubly rotating frames (DRF) are measured directly. In the present work, the RF and DRF are employed for suppressing the secular part of nuclear dipole-dipole (DD) interactions in the first two orders. As a result, the higher-order DD interactions (four- and five-particle ones) were separated, and their contribution to the nuclear spin-lattice relaxation in the TRF was studied experimentally. The experiments were carried out on protons in polycrystalline benzene. With the introduced technique, an overall spin-lattice relaxation decay in the TRF was recorded continuously during a single radio-frequency pulse with a length not exceeding 1 s. The contribution of multiproton nonsecular DD interactions to the proton spin-lattice relaxation in the TRF was observed selectively as a pronounced local minimum in the temperature dependence of the relaxation timeT 1ϱϱϱ. This contribution corresponds to ultraslow motion of benzene molecules with a rate about γHe3 ≈ 2π · (101-103) s-1 and is determined quantitatively by specific correlation functions corresponding to the multiparticle nonsecular DD interactions of protons. The prospects of using this method for studying ultraslow atomic and molecular dynamics in solids are discussed.

Proton magnetic spin-lattice relaxation in the effective field H2 acting in the doubly rotating frame (DRF) was first applied to the study of slow internal protein dynamics in the submillisecond range of correlation times in the solid state. In this method the local dipolar magnetic field is reduced by the magic-angle rotating-frame method so that the resonance frequency of the relaxation experiment may be set below the value of the local field. As a result, unachievable by the standard nuclear magnetic resonance (NMR) relaxation techniques, slow molecular motions become experimentally accessible. The second effective field H2 is produced by the shallow sine-wave phase modulation of the H1 pulse. The registration of the DRF spin-lattice relaxation signal takes place directly during the continuous H1 pulse by means of an additional low-frequency radio-frequency coil oriented along the H0 field and operating at the rotating-frame NMR frequency of 100 kHz. The measurements of the spin-lattice relaxation time in the DRF within a wide temperature range have been performed in dry and hydrated α-crystallin powders. This is the major protein in the eye lens, which prevents the uncontrolled aggregation of proteins and keeps the lens transparent. The results demonstrate that the protein hydration does not change the amplitude of slow side-chain motions but significantly shortens its correlation time: from about 50 to about 0.5 μs in dry and hydrated samples, respectively. The hydration also decreases the activation energy and restricts the distribution of the correlation times.

A spectrometer is described that ensures observation of NMR and relaxation in the effective field H c2 acting in the doubly rotating frame (DRF). Unlike conventional NMR spectrometers, the presented apparatus allows the investigation of nuclear spin dynamics and relaxation under conditions of the specially transformed effective multispin dipole-dipole interaction Hamiltonian not studied up to now. The setup is a modified version of the previously described spectrometer for direct recording of NMR in the effective field H e1 in the singly rotating frame (RF), all its operating functions being retained. The new version of the apparatus is designed for the study of NMR and, especially, spin relaxation of 1 H and 19 F nuclei in solids under the magic-angle conditions in both the RF and the DRF. It enables one to obtain NMR spectra and longitudinal and transverse nuclear relaxation kinetics in the DRF field H e2 directly in their final form. In all operating modes, the apparatus is coherent with respect to the NMR frequency in the laboratory frame. In transient modes, it is also coherent with respect to the NMR frequencies in both the RF and the DRF. In all cases the NMR signals are recorded continuously during a single-scan experiment, as a rule not exceeding 1 s. The sensitivity and resolution capability of the spectrometer are superior to those of the starting one. In particular modes, its sensitivity is comparable with that of conventional NMR spectrometers. The apparatus enables one to investigate in detail slow molecular motions in solids with rates ≅γH e2 ∽ 10 3 −10 5 s −1 and ≅γH e1 ∽ 10 5 −10 7 s −1 ; various motion parameters, including multiparticle correlations as well as a motion mechanism, can be extracted from the experimental data.