S. V. Petrov

Investigation of a gap in the AFMR spectrum in the quasi-one-dimensional hexagonal antiferromagnet CsMnBr3

The low-frequency part of the magnetic resonance spectrum of the hexagonal six-sublattice quasi-one-dimensional antiferromagnet CsMnBr3 at temperatures which are low compared with TN=8.3 K is investigated experimentally. A temperature-dependent gap Δ(T) is found in the spectrum ωe(H) of the lower AFMR branch; this gap is due to the hyperfine interaction of the nuclear spins with the electronic spins of the (55Mn)2+ ions. The spectrum of the low-lying resonance frequencies of such a system is calculated taking this interaction into acount in the approximation of fluctuationless spin hydrodynamics for the electronic branch of the oscillations. The computational results are in good qualitative and satisfactory quantitative agreement with experiment.

Investigation of NMR in the quasi-one-dimensional antiferromagnetic CsMnBr3

The NMR spectrum of the quasi-one-dimensional easy-plane antiferromagnet CsMnBr3 with a triangular magnetic structure has been studied. The measurements were performed in the decimeter microwave range in a special NMR spectrometer and in a wide range of magnetic fields at temperatures of 1.7 and 3.0 K. Three branches of the NMR spectrum were observed. One branch exhibits a very strong frequency-field dependence. This dependence can be explained, to a first approximation, by the existing theory of the interaction of the electronic and nuclear subsystems [E. A. Turov and M. P. Petrov, Nuclear Magnetic Resonance in Ferro-and Antiferromagnets, Halstead Press, N. Y., 1972]. To explain the behavior of the two other branches, a more complicated theoretical analysis is required. The observed shift of the NMR spectrum with increasing temperature also cannot be explained. The reduction of the spin moment of the magnetic ion Mn2+ due to magnetic fluctuations is determined to be 30%.

NMR in 55Mn2+ nuclei in the quasi-one-dimensional antiferromagnetic CsMnBr3

The NMR spectrum of the quasi-one-dimensional easy-plane antiferromagnetic CsMnBr3, which has trigonal spin lattice, is investigated in detail. The measurements were performed on a wide-band NMR decimeter microwave-band spectrometer over a wide range of magnetic fields at temperatures 1.3–4.2 K. All three branches of the NMR spectrum previously found by us [JETP Lett. 64, 225 (1996)] are severely distorted because of the dynamic interaction with the Goldstone mode in the antiferromagnetic resonance spectrum. The experimental results in fields up to 40 kOe are described satisfactorily by an equation obtained by Zaliznyak et al. [JETP Lett. 64, 473 (1996)]. Formulas are obtained in our work that agree very well with experiment at all fields up to the “collapse” field Hc of all sublattices. The unbiased NMR frequency in CsMnBr3 is determined to be vn0=416 MHz (T=1.3 K) in zero external magnetic field, and in this way the reduction in the spontaneous moment due to the quasi-one-dimensional nature of the system of Mn2+ spins, which according to our data amounts to 28%, is determined more accurately. The field dependences of the directions of the magnetic sublattices with respect to the magnetic field are obtained from the NMR spectra, confirming the equations of Chubukov [J. Phys. Condens. Matter 21, 441 (1988)]. The results on the field dependence of the width and intensities of the NMR lines are discussed, along with three observed anomalies: 1) a strong increase in the NMR frequency for nuclei in sublattices that are perpendicular to the magnetic field; 2) the nonmonotonic temperature dependence of the resonance field for the lower branch of the spectrum; 3) the presence of two branches of the NMR spectrum in large Hc fields, in which the CsMnBr3 must be a quasi-one-dimensional antiferromagnetic.

Mn55 NMR investigation of the suppression of quantum fluctuations in quasi-one-dimensional antiferromagnets by a magnetic field

The NMR of Mn55 nuclei is measured in the quasi-one-dimensional antiferromagnets CsMnBr3, RbMnBr3, and CsMnI3 in magnetic fields upto 8 T attemperatures in the range 1.3–4.2 K. The average moments of the magnetic sublattices and their field dependences, which turned out to be comparatively strong and different for magnetically non-equivalent Mn2+ ions, are determined from the hyperfine-field data obtained. As a result, the magnetizations of separate sublattices in an external magnetic field ∼8 T differ by more than 5%. The results obtained agree qualitatively with the theory of the suppression of quantum fluctuations by a magnetic field.

Observation of spin-reduction anisotropy in the quasi-one-dimensional antiferromagnet CsMnI3

The NMR of 55Mn in the quasi-one-dimensional noncollinear antiferromagnet CsMnI3 at T=1.3 K is investigated in magnetic fields up to ∼40 kOe. Six NMR branches corresponding to six manganese spins per magnetic unit cell are observed. The NMR spectra correspond satisfactorily to the well-known magnetic structure of CsMnI3, taking into account the dynamic frequency shift due to the interaction with the low-lying AFMR modes. The average spins 〈SA〉=1.86 and 〈SB〉=1.74 of the magnetically nonequivalent Mn2+ ions are determined from the measured values of the hyperfine fields. The results obtained agree qualitatively with the calculations of spin reduction in quasi-one-dimensional antiferromagnets [Y. Watabe, T. Suzuki, and Y. Natsume, Phys. Rev. B 52, 3400 (1995)].

New phase transition in an easy-axis “triangular” antiferromagnet

The NMR of 55Mn in the quasi-one-dimensional noncollinear anti-ferromagnet CsMnI3 is investigated at T=1.3 K in magnetic fields up to ∼80 kOe and angles between the field and C6 axis ϕ≈ 0.5° and ϕ=7°. A new reorientational magnetic phase transition is observed in a field Hc1≈39.0 kOe. The magnetic structure for H>Hc1 is determined. The average Mn2+ spins of the magnetic sublattices in the new phase are determined from an analysis of the NMR spectrum to be 〈 SC〉=1.63 and 〈SD〉=1.72.

Nuclear magnetic resonance of 55Mn in the antiferromagnet CsMnBr3 in a variable longitudinal magnetic field

The spectrum and intensities of NMR lines are investigated experimentally and theoretically for excitation by an alternating magnetic field h‖ parallel to a static field H in the quasi-one-dimensional, six-sublattice antiferromagnet CsMnBr3. According to theory, two new NMR lines, which are not excited by a transverse magnetic field h⊥, are observed near the phase transition from triangular to collinear structure (H=Hc) [JETP 86, 197 (1998)].

Giant magnetoacoustic effect in KMnF3 due to nuclear spin waves

An explanation is proposed for the gigantic magnetoacoustic effect that we observed in KMnF3 in previous work {Kh. G. Bogdanova, V. A. Golenishchev-Kutuzov, M. I. Kurkin et al., Zh. Éksp. Teor. Fiz. 112, 1830 (1997) [JETP 85, 1001 (1997)]}. The effect entails a tenfold amplitude reduction of an acoustic pulse in a magnetic field that varies over the range 0–8 kOe. It is shown that this effect is due to the interference of two nuclear magnetoelastic waves propagating in the sample under magnetoacoustic resonance conditions, if this resonance occurs in the region of strong spatial dispersion of nuclear spin waves. The effect is said to be gigantic because it exceeds in magnitude the magnetoacoustic effects observed previously in magnetically ordered materials even though it is due to nuclear magnetism, which is 105 times weaker than electronic magnetism. We observe a concomitant anomalous dependence of the dispersion of the velocity of sound on the external magnetic field.

Influence of a nonmagnetic impurity on the properties of the quasi-one-dimensional antiferromagnet CsNiCl3

Various magnetic properties of the diluted quasi-one-dimensional antiferromagnet CsNi1−xMgxCl3 are investigated experimentally for several impurity concentrations. The antiferromagnetic resonance spectrum and the phase diagrams are found to depend significantly on the amount of added Mg. The field and temperature dependences of the static magnetization is measured for crystals with two different contents x. A substantial increase in the magnetization is observed at low temperature, where the additional susceptibility is approximately proportional to the concentration. The physical mechanisms underlying the observed strong influence of magnetic defects formed at breaks in the spin chains in a quasi-one-dimensional antiferromagnet on its magnetic properties in the ordered state and for T

Investigation of the hyperfine interaction in the antiferromagnetic CsMnI3

The lower branch of the resonance spectrum of the quasi-one-dimensional triangular antiferromagnetic CsMnI3 has been investigated experimentally. This branch possesses a gap due to the dynamic hyperfine interaction. The temperature dependence of the energy gap was studied in detail at several frequencies. A theoretical calculation of the corresponding spectrum of coupled electron-nuclear spin oscillations was performed in the “hydrodynamic approximation” with an empirical correction for thermal fluctuations of the antiferromagnetic system. The good agreement between the calculation and experimental data makes it possible to determine the zero-point spin reduction in the antiferromagnetic.