Magnetism under high pressure studied by 57Fe and 151Eu nuclear scattering of synchrotron radiation

The nuclear forward scattering (NFS) of synchrotron radiation is especially suited for probing magnetism at very high pressure, here in the Mbar range, by the nuclear resonances of ⁵⁷Fe and ¹⁵¹Eu. We report on high pressure (h.p.) NFS studies with the 14.4 keV transition of ⁵7Fe on magnetic RFe₂ Laves phases of cubic C15 structure (YFe₂, GdFe₂) and hexagonal C14 structure (ScFe₂, TiFe₂) at pressures up to 100 GPa (=1 Mbar). We present also h.p. NFS studies performed with the 21.5 keV resonance of ¹⁵¹Eu, probing the magnetism in the CsCl-type h.p. phase of EuTe. This revised version was published online in August 2006 with corrections to the Cover Date.     

A Mössbauer effect study of Y(57Fe:Mn)12

The intermetallic compound, YMn₁₂, doped with 0.5at.%⁵⁷Fe has been investigated using the⁵⁷Fe Mössbauer resonance. The data show that the Fe impurities occupy preferentially the f-site at the expense of the i- and j-sites. Whereas the i- and j-sites are expected to carry identical local magnetic moments in the ordered state, low temperature hyperfine fields of 50.08(±0.05)T and 2.90(±0.03)T are measured for the i- and j-sites respectively. It is proposed that large transferred hyperfine fields are responsible for this disparity.     

Magnetic order observed in Er6Mn23H21 using the Mössbauer effect

The ferrimagnetic compound Er₆Mn₂₃ and its hydride Er₆Mn₂₃H₂₁ have been investigated using the ¹⁶⁶Er and ⁵⁷Fe (as dilute impurity) Mössbauer resonances. For the hydride a small ⁵⁷Fe magnetic hyperfine field is observed below 85 K indicating magnetic ordering. This observation is contrary to the previous assumption that the Mn sublattice must be non-magnetic. The ¹⁶⁶Er results demonstrate that the Er³⁺ magnetic moment is not “quenched” by crystal electric fields on hydrogenation.     

High-Pressure/High-Temperature NFS Study of Magnetism in LuFe 2 and ScFe 2

Using the new technique of nuclear forward scattering (NFS) of synchrotron radiation, we studied the magnetic hyperfine fields B hf and ordering temperatures T M of the Laves phases LuFe 2 (cubic C15) and ScFe 2 (hexagonal C14) at pressures up to 90 GPa and temperatures up to 700 K. For LuFe 2 we find for T M first an increase from 562 K at 0 GPa to 603 K at 10 GPa and then a decrease to 295 K around 75 GPa. The hyperfine fields B hf show at 295 K a continuous decrease with pressure, indicating a reduction of the Fe band moment. A similar behaviour of both T M and B hf was observed in ScFe 2.     

Spin reorientation in Er2Fe17−xMnx - Mössbauer effect study

The polycrystalline samples of Er₂Fe_17–xMn_x compounds have been studied by⁵⁷Fe Mossbauer spectroscopy in the temperature range between 4.2K and 200K. A reorientation of the easy axis of magnetization has been evidenced for compound with x=4 in the measurements on magnetically oriented powdered sample. Spin reorientation from the c-axis to the basal plane is observed when temperature is increased above 105K.     

Magnetic order in Y6(57Fe:Mn)23H26

The ferrimagnetic compound Y₆Mn₂₃ and its hydride Y₆Mn₂₃H₂₆, both doped with 0.5%⁵⁷Fe, have been investigated using the ⁵⁷Fe Mössbauer resonance and dc field magnetization measurements. For the hydride a small ⁵⁷Fe magnetic hyperfine field is observed to increase abruptly below 110 K whereas the bulk magnetization results suggest antiferromagnetic ordering at T_N≈ 180 K.     

Mössbauer studies of Dy2Fe17−yAly hydrides

The x-ray and Mössbauer measurements of both⁵⁷Fe and¹⁶¹Dy in Dy₂Fe_17–y Al (y=0, 1,5 and 3) compounds and their hydrides are reported.Hydrogenation slightly increases the lattice parameters. An appreciable increase of the isomer shift and the hyperfine field at⁵⁷Fe nuclei is observed after hydrogenation. The hyperfine field for both the parent compound and its hydride decreases with increasing Al content across the series. Only a small variation of the hyperfine field at¹⁶¹Dy nuclei is noticeable after hydrogenation.     

Mössbauer effect studies of the Dy12Fe82B6 system

The Mössbauer measurements of both⁵⁷Fe and¹⁶¹Dy in Dy₁₂Fe₈₂B₆ system are reported. The presence of B exceeds the magnetic hyperfine fields at both⁵⁷Fe and¹⁶¹Dy nuclei as compared to corresponding pure metals Fe and Dy respectively. The only small excess of the hyperfine field at⁵⁷Fe nucleus is observed. The excess at¹⁶¹Dy nucleus is about 12% of the free ion Dy³⁺ value.     

Mössbauer effect study of charge and spin transfer in Fe-Cr

The influence of temperature and time of annealing on hyperfine fields and isomer shifts has been studied for a range of Fe–Cr alloys containing 1–45 at% Cr. It has been revealed that up to 15at% Cr neither time nor temperature of annealing practically does affect the hyperfine parameters. For more concentrated samples, however, both temperature and time of annealing are important. In particular, the Mössbauer spectrum of Fe–45.5 at% Cr annealed at 700°C for 5 h was a single-line indicating that the sample was paramagnetic. The observed changes of the hyperfine fields and the isomer shifts have been interpreted in terms of a spin and charge trasfer, respectively.Strong linear correlations between the following quantities have been revealed: the hyperfine field H(0,0) and the isomer shift IS(0,0); the average hyperfine field $\text{H}$ and the average isomer shift $\text{IS}$; the average hyperfine field $\text{H}$ and the average number of Cr atoms in the first two coordination spheres, $\text{N}$. It has been calculated from the first two correlations that a) a change of polarization of itinerant s-like electrons of one electron is equivalent to a change of the hyperfine field of 1602 kOe, and b) on average, a unit change of s-like electron polarization is equivalent to 3277 kOe. The two constant are very close to theoretical estimations, which can be found in literature. Correlation between the hyperfine field and the isomer shift led to a conclusion that the substitution of Fe atoms by Cr ones decreases the density of spin-up electrons on average by 0.026 electrons per one Cr atom in a unit cell. These electrons are most likely trapped by Cr atoms, because the hyperfine field at neighbouring Fe nuclei decreases and the density of charge at those nuclei increases at the rate of 0.029 electrons per one Cr atom in a unit cell.Based on the results obtained, a formula describing the dependence of the average hyperfine field on Cr contents and on the heat treatment has been postulated for Fe–Cr alloys.     

Magnetic ordering in semiconducting Tl0.53K0.47Fe1.64Se2 single crystals studied by M?ssbauer spectroscopy

The results of a 57Fe M?ssbauer spectroscopy study between 4.5 and 523.2 K and in external magnetic fields (up to 90 kOe) of semiconducting Tl0.53K0.47Fe1.64Se2 single crystals are reported. Evidence is provided for a possible phase separation into the magnetic majority and minority phases. It is demonstrated that the magnetic moments of the divalent Fe atoms located at the 16i site (space group I4/m) of the majority phase and of the minority phase are antiferromagnetically ordered, with the Néel temperature T(N) = 518.0(3.6) K. The magnetic moments at 5.0 K of 2.09(1) and 2.28(2) μ(B) in these two phases are tilted from the crystallographic c axis by 18(1)° and 32(2)°, respectively. The Debye temperature of Tl0.53K0.47Fe1.64Se2 is found to be 228(4) K.