Michael Fricke

Comparative evaluation of Fe−Ni-Invar spectra with hyperfine field distributions and relaxation theories

In order to decide what is the origin of the broad lines of Mössbauer spectra of some INVAR alloys, we evaluate a set of spectra under the aspects of hyperfine field distribution and hyperfine field fluctuation.

Structure and magnetic properties of FeMnAl alloys investigated by Mössbauer spectroscopy and X-ray diffraction

The system (Fe0.88Mn0.121−xAlx has been investigated in a concentration range from 5 to 14 at.% Al. We applied Mössbauer spectroscopy in the temperature range from 4 up to 900 K and X-ray diffractometry at room temperature. The as-cast samples show a bcc phase for all concentrations and exhibit broadened six-line Mössbauer spectra typical for disordered alloys. The Mössbauer spectra during a high temperature treatment show dramatic changes. These are due to ordering processes appearing at temperatures above 700 K. As an example of the observed changes, we present results obtained for the alloy withx= 14 at.% Al.

The influence of Cu and Nb on the crystallization products of annealed FeCuNbSiB

Depending on different Nb and Cu concentration, we studied the formation of nano- and microcrystalline phases in differently annealed samples of FeCuNbSiB. First, the kinetics of crystallization was explored by measuring the temperature-dependent electrical resistivity and magnetization. After this, we collected Mössbauer spectra and did an X-ray diffraction analysis of annealed samples at chosen temperatures to discover the different crystalline phases. It was found how the onset of crystallization had been shifted towards other temperatures. The analysis of Mössbauer spectra shows that building up the well-known DO3 structure of Fe3Si is disturbed in the alloy without Cu. At higher Nb content, we resolve two different crystallization steps in the formation of Fe3Si and a new crystalline phase, probably being a structure like Fe23B6.

Mössbauer study indicating magnetic clusters in FeNiMn-Reentrant spin glass alloys

Abstract57Fe Mössbauer measurements on the reentrant spin glass system (Fe.65Ni.35)1−xMnx are analysed under the extreme assumption that only relaxation determines the shape of the spectra. A plain model of a fluctuating hyperfine field based on the Blume-Tjon-theory /1/ is used for the evaluation. As a result the increase of the fluctuation frequencies with increasing temperature yields in lowering of the relative magnetization. Noticeable are some typical kinks of the magnetization at the reentrant temperatures. Our evaluation results in fluctuation frequencies from 107 Hz to 109 Hz, which indicates, if true, that the reason of the fluctuations seems not to be single spins but larger collective spin groups (clusters).

Magnetism of an Fe65Mn35 alloy in the fcc/hcp phase region

Among the three possible phase structures (bcc, fcc and hep) of iron rich Fe100-cMnc alloys (cMn<40 at%), the two closed packed structures, fcc and hep, produce poor resolved and similar Mössbauer spectra. Therefore the evaluation of spectra collected on a sample (nominalcMn=35 at%) with coexisting fcc and hep phases only was possible with the help of the Afans’ev/Tsymbal method of “Mössbauer lines sharpening” introduced recently/1,2,3/. A comparison of the hyperfine field vs. temperature with an FeNi-Invar alloy (cNi=35 at%) shows a surprising similarity.

Investigation of the re-entrant transition from ferromagnetism to a spin glass by means of Mössbauer spectroscopy in successively polarized (Fe0.65Ni0.35)1−x Mn x alloys

To investigate the mechanism of the re-entrant transition, we chose the alloy system (Fe0.65Ni0.351−xMnx. Mössbauer spectra were collected as a function of temperature in different weak external magnetic fields. From these spectra, we extracted the mean hyperfine fields and intensity ratios of the absorption lines. Both parameters represent the local states of the magnetization. We performed measurements for three alloys withx1=0.091;x2=0.102;x3=0.1136 Mn concentration.

An unambiguous procedure for discovering relaxation influence on Mössbauer spectra

A mathematical procedure which gives an opportunity to distinguish between relaxation and distribution mechanisms of the Mössbauer line broadening is proposed. It is based on the method of Mössbauer line sharpening developed recently [1] and allows one to answer unambiguously the question whether or not relaxation manifests itself in Mössbauer measurements by examining the wings of the spectrum. The procedure is applied to the spectra of high spin ferric metmyoglobin and (Fe0.65Ni0.35)1−xMnx alloys. Analysis of the metmyoglobin spectra manifests the presence of the relaxation influence in the temperature range 10 K≦T≦100 K. Analysis of the alloy spectra shows that for the temperatures 4 K≦T≦300 K and Mn concentrations 0≦x≦0.245 no relaxation is observed and the line broadening is mainly caused by the distributions of magnetic hyperfine fields. A possible explanation of this result is given.

Spin dynamics in the reentrant spin glass (Fe 0:65 Ni 0:35 ) 1 x Mn x

The spin dynamics in the reentrant spin glass ( Fe0.65 Ni0.35)1-x Mn_x has been studied by zero, longitudinal and transverse field μSR. In the ferromagnetic reentrant and pure spin glass regimes (x\leqslant 0.175), zero field experiments reveal a stretched exponential muon relaxation with a universal behaviour of the dynamic exponent \beta above the spin glass transition. There are no qualitative differences between the ferromagnetic and paramagnetic phases. In transversal field μSR experiments the divergence of the relaxation rate close to the spin glass transition is suppressed for manganese doping up to x=0.113 but enhanced for slightly higher doping (x\geqslant 0.12). We understand this behaviour as a crossover from an itinerant to a more localized state of the 3d electron system. This is also supported by the fact that in the highly doped regime with dominant antiferromagnetic interactions the muon relaxation rate diverges above the antiferromagnetic transition temperature.

Test for the influence of relaxation on Mössbauer spectra of (Fe0.65Ni0.35 1−x Mn x alloys by the sharpening method

A mathematical procedure which gives an opportunity to distinguish between relaxation and static mechanisms of the Mössbauer line broadening is developed. It is based on the method of Mössbauer line sharpening developed recently and allows one to answer unambiguously the question whether or not relaxation manifests itself in Mössbauer measurements by examinign the wings of the spectrum. The procedure was applied to the spectra of (Fe0.65Ni0.35)1−xMnx alloys withx=0 (INVAR) andx=0.047, where the mechanism of the line broadening has not been clarified so far. An analysis of the spectra shows that in the temperature range 4 to 300 K no relaxation is observed and the line broadening is mainly caused by the distributions of magnetic hyperfine fields.