M. I. Kurkin

Magnetoresistance of lanthanum manganites with activation-type conductivity

The temperature dependence of the resistivity and magnetic moment of La0.85Ba0.15MnO3 and La0.85Sr0.15MnO3 manganite single crystals in magnetic fields up to 90 kOe is investigated. Analysis of the experimental results shows that the magnetoresistance of lanthanum manganites far from the Curie temperature TC can be described quantitatively by the s-d model normally used for ferromagnets and taking into account only the exchange interaction between the spins of charge carriers and magnetic moments. These data also show that the features of lanthanum manganites responsible for colossal magnetoresistance (CMR) are manifested in a narrow temperature interval δT ≈ 20 K near TC. Our results suggest a CMR mechanism analogous to the mechanism of giant magnetoresistance (GMR) observed in Fe/Cr-type multilayers with nanometer layer thickness. The nanostratification observed in lanthanum manganites and required for GMR can be described taking into account the spread in TC in the CMR range δT.

A model for quantitative description of magnetoresistance of the La0.85Ba0.15MnO3 manganite below the curie temperature TC in the presence of colossal magnetoresistance effect near TC

Precision resistive and magnetic measurements of the La0.85Ba0.15MnO3 manganite with activation-type conductivity have been performed. Based on the experimental data, a mechanism has been proposed, which is sufficient for a quantitative description of the magnetoresistance below the Curie temperature TC. The proposed mechanism predicts a considerable enhancement in the magnetoresistance near TC owing to an increase in the magnetic susceptibility. This implies that this mechanism can at least in part participate in the formation of colossal magnetoresistance. It has been concluded that all other mechanisms responsible for the effect of colossal magnetoresistance occur only near TC. This restriction determines the specificity of the new approach to the problems of colossal magnetoresistance in lanthanum manganites.

Parametric magnetoelectric effect in an AC magnetic field

Electric polarization is predicted to occur under the conditions of parametric instability of vibrations of magnetization in a longitudinal high-frequency magnetic field (parametric magnetoelectric effect). The requirements on materials are indicated favoring the observation of this effect. An example of such materials is the easy-plane antiferromagnet Cr2TeO6.

Evaluation of the effect of various mechanisms on the magnetoresistance of lanthanum manganites La0.85Sr0.15MnO3 with activation-type conductivity

A method is proposed that allows one to divide the magnetoresistance (MR) observed in manganites into three mechanisms: dimensional, orientational, and magnetic. The first two mechanisms are associated with the stratification of a substance into ferromagnetic and nonferromagnetic phases, which significantly differ in electric resistivity. The dimensional mechanism of MR is attributed to the effect of a magnetic field on the size of magnetic inclusions. The orientational mechanism of MR is determined by the dependence of electric resistivity on the mutual orientation of the magnetizations of magnetic inclusions. The magnetic mechanism of MR is determined by the properties of the magnetization of a ferromagnet, in particular, by the Curie–Weiss singularity on the temperature dependence of magnetic susceptibility at the Curie point. This mechanism exists in homogeneous substances, although its value may depend on the magnetic properties of inhomogeneities. The method is developed for substances with activation-type conductivity and is applied to the analysis of MR of La0.85Sr0.15MnO3 manganite near the Curie point, where the MR attains its maximum. The dimensional mechanism turns out to be dominant in magnetic fields H greater than the saturation field Hs (H > Hs). The orientational, dimensional, and magnetic mechanisms have a comparable effect on the MR for H < Hs. The effect of the orientational mechanism on MR is relatively weak (does not exceed the third part of the total MR), although this mechanism determines the giant MR in multilayered metal films. The possibility of application of the method to the analysis of MR near the insulator–metal transition is analyzed.

First-order phase transitions in magnetization of Fe/Cr/Fe three-layer films

The possibility of appearing jumps in the magnetization curves of Fe/Cr multilayers has been predicted under the assumption that magnetic ordering in chromium interlayers has the form of a linearly polarized spin-density wave. This possibility has been analyz ed for an Fe/Cr/Fe three4ayer film with a conventional quality of Fe/Cr interfaces, which does not ensure a change in the ferromagnetic and antiferromagnetic orientations of the magnetizations of neighboring iron layers with a variation in the thickness of the chromium interlayer by one atomic layer (short oscillation period). The model used suggests that the wave vector of the spin-density wave is responsible for the experime ntally observed long period of these oscillations. Relationships have been derived for the range of thicknesses of chromium interlayers in which the appearance of the predicted effect can be expected, and the magnitude of the effect has been evaluated.

Magnetoelastic mechanism of magnetoelectric interaction

The magnetoelastic mechanism of the magnetoelectric interaction characterized by the energy ϕmu is analyzed theoretically. This interaction is described in terms of the expansion of the magnetic energy ϕm in powers of components of the vectors uj (displacements of crystal lattice sites from equilibrium positions) rather than the vectors ∇u, which determine the magnetostriction component ϕms of the magnetoelastic energy. One more difference between the energies ϕmu and ϕms lies in the fact that the magnetoelectric energy ϕmu corresponds to the interaction of magnetic excitations with optical phonons, whereas the magnetostriction energy ϕms corresponds to the interaction of magnetic excitations only with acoustic phonons. It is established that the magnetoelectric interaction is affected by the electroactive optical phonons (interacting with the electric field), which, as a rule, make a small contribution to the total number of optical branches of the phonon spectrum. The magnetoelectric energy ϕmu is described within the approach in which all nonelectroactive branches of optical phonons are automatically excluded from consideration. This approach is based on the concept of the electric sublattice formed by all chemically identical ions with equal valences. Analysis is performed using the Fe2TeO6 compound as an example.

Possible use of nuclear magnetic resonance for studying magnetoelectric effects in the Mn2Sb ferromagnet

The possibilities of resonance excitation of nuclear spins by an alternating electric field (nuclear magnetoelectric resonance) in the Mn2Sb ferromagnet are analyzed as applied to the studying of magnetoelectric effects in this compound.

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.

Magnetoacoustic resonance on nuclear spin waves in an easy-plane antiferromagnet KMnF3

A theoretical model of magnetoacoustic resonance on nuclear spin waves in an easy-plane antiferromagnet KMnF3 with large dynamic frequency shift is developed. Experimental results on resonance decay of ultrasound, splitting of the ultrasound pulse into two component and anomalous dispersion of the sound velocity depending on the value of the external magnetic field confirm this theory.