V. P. Parfenova

Local spin configurations of Fe atoms in the Rh1−xFex (x=0.1, 0.2, and 0.3) system with competing exchange interactions

The local spin configurations of Fe atoms in the magnetically ordered alloys Rh1−xFex (x=0.1, 0.2, and 0.3) have been investigated by Mössbauer spectroscopy. The Mössbauer absorption spectra are measured in the range from 5 K to temperatures of the transition to the paramagnetic state. The measurements in magnetic fields with a strength up to 5 T are carried out at a temperature of 4.2 K. Analysis of the magnetic-hyperfinefield distribution functions demonstrates that Fe atoms form discrete sets of collinear spin configurations corresponding to different net moments of the nearest coordination sphere. The spin structure of the alloys is governed by a random distribution of Fe atoms over the lattice sites and the competition between the Fe-Rh ferromagnetic exchange interaction and the antiferromagnetic interaction of the neighboring Fe atoms. No spin frustration and spin “melting” effects characteristic of spin glasses are revealed in the Rh-Fe alloys.

Origin of the strong influence of rhodium on the Curie temperature of Pd–Fe alloys: the spin reorientation in (Pd100−xRhx)90Fe10 alloys

Abstract The hyperfine field distributions and the local spin configurations for Fe atoms in the (Pd 100− x Rh x ) 90 Fe 10 alloys for x =0, 10 and 20 are investigated by the Mossbauer spectroscopy technique. It was found that the anomalous behavior of T C in these alloys is attributable to the spin reorientation in some part of Fe atoms with the formation of local antiferromagnetic spin configurations.

Local Spin Configurations and Magnetic Ordering in Competing-Exchange System (Rh, Pd)90Fe10

Mossbauer spectroscopy technique has been used to investigate the evolution of the hyperfine field distributions and the local spin configurations of Fe atoms in the magnetically ordered (Rh 1-x Pd x ) 0.9 Fe 0.1 alloys over the concentration range 0 ≤ x ≤ 1. In the whole composition range, the magnetic behavior of the alloys is governed by the competition between two exchange interactions opposite in sign: the direct antiferromagnetic coupling between the nearest-neighbor Fe atoms and the ferromagnetic interaction due to the matrix polarization. For Rh-rich alloys, this leads to a spin-glass-like magnetic structure, where the ferro- and antiferromagnetic local spin configurations are realized with nearly equal probabilities. For the concentration range x ≤ 0.3, the magnetic ordering temperature is equal to about 32 K independent of the Pd concentration. In the same concentration range, a sharp increase of the magnetic moment of Fe atoms was observed. The severe influence of rhodium on the Curie temperature of the ferromagnetic PdFe alloy is explained by the spin reorientation effect, leading to a destruction of the ferromagnetic spin structure. The strong diffuseness of the magnetic transition in the intermediate concentration range 0.4 < x < 0.8 is due to the presence of Fe sites with both Rh-rich and Pd-rich local environments.

Antiferromagnetic spin correlations in palladium-based Pd-Fe, Pd-Fe-Ag, and Pd-Fe-Rh magnetic alloys

The magnetic structure of Pd1−xFex (x=0.03, 0.06, 0.10, 0.15, and 0.20) alloys is investigated using the method of 57Fe-Mössbauer spectroscopy. The distribution functions P(Bhf) of hyperfine magnetic fields have a discrete structure defined by variations of the contribution to Bhf from the magnetic moment of the neighboring Fe atoms. The anomalies of intensities of components of the functions P(Bhf), which increase with the concentration of iron, are indicative of the instability of configurations with a large total spin and of the formation of local spin configurations with the antiferromagnetic orientation of magnetic moments. The probability of formation of such configurations is defined by the competition of the ferromagnetic Fe-Pd exchange interaction with the direct antiferromagnetic exchange between the nearest neighboring atoms of Fe. An Ag or Rh impurity effectively induces the process of spin flipping, which explains the anomalously strong effect of impurities on the magnetic ordering temperature. The results confirm the presence in Pd-Fe alloys of perturbations of long-range ferromagnetic order revealed by neutron diffraction.

On the possibility of accelerating α decay: To the problem of disposing of radioactive nuclear waste

A number of experimental tests of the recent information on the possibility of accelerating α decay by implanting α-emitting isotopes into metals with their further cooling to a low temperature were performed using the nuclear orientation technique. The measurement data on half-lives for 253Es in Fe at 4 K and 50 mK, 224Rn, 225Ra, and 227Ac in Fe at temperatures down to 25 mK, and 221Fr in nonmetal (Si) and metal (Au) at 4 K and ∼20 mK are reviewed. In all cases, no change in half-life was found within an accuracy of 1%.

Angular distributions of α particles emitted by oriented nuclei and their relation to nuclear deformation

Low-temperature spin orientation of radioactive nuclei is a nuclear spectroscopic method that allows us to obtain experimental data on nuclei and extranuclear fields. We present the results from measuring the angular anisotropy of α radiation emitted by transuranium nuclei of 253,254Es, 255Fm, 241,243Am, along with that of γ radiation from 250Bk nuclei oriented in an iron matrix at temperatures of 10–300 mK. The data allow us to establish the relation between the mechanism of α decay and nuclear deformation and to compare them to the theoretical data. We also measure the energy of magnetic hyperfine splitting for the investigated nuclei, and find the magnetic hyperfine field value for Es in Fe to be |Bhf| = 396(32) T. The nuclear magnetic moment for 254Es was determined, and its value was |μ(254Es)| = 4.35(41)μN.

Transition from the ferromagnetic state to the spin glass state in ordered Fe0.75−xAl0.25+x alloys and the temperature evolution of the magnetic structure of the Fe0.70Al0.30 alloy

The magnetic structure of ordered alloys Fe0.75−xAl0.25+x (x = 0, 0.025, and 0.05) is studied by Mössbauer 57Fe spectroscopy in a temperature range of 5–295 K. An increase in the Al concentration at T = 5 K induces a transition from collinear ferromagnetism (x = 0) to a magnetic structure of the cluster-spin-glass type (x = 0.05). The unexpectedly strong effect of aluminum on the magnetic structure is explained by the anomalously high probability of formation of frustrated magnetic configurations with a large number of Al atoms in the nearest neighborhood of Fe atoms. This anomaly is associated with the establishment of a short-range order, which is a key factor determining the radical change in the magnetic structure in a narrow range of Al concentration. The “intermediate” phase of the Fe0.70Al0.30 alloy (100 K < T < 200 K) is a mixed-type magnetically ordered phase whose magnetic structure is determined by the competition of opposite exchange interactions. The nominally “ferromagnetic” phase of this alloy (T > 200 K) is characterized by strong violation of the long-range ferromagnetic order, which is due to the effect of the antiferromagnetic superexchange interaction.

First measurement of the nuclear magnetic moment of 254Es

Angular distributions of α particles and γ rays emitted by 253,254Es, 255Fm, and 250Bk nuclei were studied using the low-temperature nuclear orientation method. Information on the hyperfine interactions of these actinide impurities in an iron host lattice is derived from experimental data and the value of magnetic moment of 254Es nucleus is determined.

Thermal destruction of antiferromagnetic Fe-Fe exchange bonds and mechanism of the transition to the spin-glass state in (Fe0.65Ni0.35)1−xMnx systems with antiferromagnetic competing exchange interactions

The thermal evolution of the competition between the ferro-and antiferromagnetic exchange interactions in (Fe0.65Ni0.35)1−xMnx alloys, which display different magnetic properties, depending on composition and temperature, is investigated. The distribution functions of the magnetic hyperfine fields P(Bhf) for 57Fe are determined by Mössbauer spectroscopy in the temperature range 5–300 K for the alloys with x=0, 0.024, 0.082, 0.136, 0.195, and 0.252. The temperature dependence of the integrated intensity Is(T) is analyzed for the low-and high-field portions of P(Bhf). The features found in the behavior of Is(T) are interpreted as results of variation of the ratio between the competing exchange interactions of different signs as a result of the thermal destruction of antiferromagnetic Fe-Fe exchange bonds. It is shown that the changes in the spin structure in the low-temperature range are due to the thermal destruction of Fe-Fe exchange bonds. One of the consequences of this destruction is “reentrance” (an increase in the hyperfine field with increasing temperature for some of the Fe atoms). The relationship between the thermal destruction of Fe-Fe exchange bonds and the magnetic transitions of the Fe-Ni-Mn system to the spin-glass state is considered.

Temperature evolution of frustrated spin states in the system with competing exchange interactions. Fe0.65Ni0.35)1−xMnx (x=0, 0.024, 0.034)

The influence of temperature on the distribution function P(Bhf) of the magnetic hyperfine fields for 57Fe in (Fe0.65Ni0.35)1−xMnx alloys (x=0, 0.024, 0.034) are investigated by Mössbauer spectroscopy. The Mössbauer absorption spectra are measured in the temperature interval 5–300 K; in the interval 5–80 K the measurements are performed in a magnetic field of 0.2 T. Anomalies are found in the temperature curves of the intensity of the principal maximum of the functions P(Bhf)[Bhf=30–38 T] and the total (integrated) intensities of the low-field components [Bhf=(4–13) T]. The detected anomalies in the behavior of the total intensities are interpreted as resulting from a change in the balance of competing exchange interactions due to the thermal annihilation of antiferromagnetic Fe-Fe exchange interaction. The emergence of strong satellite lines in the interval Bhf=20–29 T in Mn-doped alloys is attributed to reorientation of the spins of Fe atoms under the influence of strong antiferromagnetic Mn-Fe exchange interaction.