Magnetic Double Transition in Au-Fe near the Percolation Threshold

Abstract

The ^^Fe Mossbauer effect on 16.8-at.% Fe-Au displays the magnetic double transition from paramagnetism to ferromagnetism at Tc == 165 and from ferromagnetism to spinglass-like behavior at Tf^ 45 K. Below T/ the magnetic state is characterized byfreezing of Fe spin components v^^hich are transverse with respect to a strong applied dc field.PACS numbers: 75.30. Kz, 76.80.+y, 75.25. + Z One of the most interesting phenomena of Az^Fe alloys is the suggested magnetic double transition for alloys near the percolation concentration (-16 at.% Fe): a transition from paramagnetism to ferromagnetism at the Curie temperature T^ followed by a transition at some lower temperature Tf which has been interpreted as a disappearance of ferromagnetism and the appearance of a spinglass-like state/"^ This suggestion is based principally on the anomalous temperature behavior of the low-field ac and dc susceptibilities. However, magnetic measurements in these alloys are difficult to interpret.'* Thus the magnetic double transition in Au-Fe alloys has been questioned recently.^ Moreover it has been found in a neutron diffraction study^ that ferromagnetism persists even down to the lowest temperatures.In this Letter we report ^'^Fe Mossbauer-effect results on AwFe which give new information on Fe spin correlations and hyperfine interactions in this alloy. Our 16.8-at.% Fe-Au samples were foils of ~ 6 /im thickness, solution-annealed at 820 °C in vacuum for 49 h, and rapidly quenched in water.The transition from paramagnetism to ferromagnetism on decrease of temperature is indicated by a sudden spectral broadening near T^ due to the development of magnetic hyperfine splitting (Fig. 1). The paramagnetic spectra obtained above 165 K are in agreement with earlier Mossbauer investigations.'^'^ Taking the onset of spectral line broadening for the transition yields T^ = 165± 5 K (see inset in Fig. 1). At 77 K our alloy shows typicsil ferromagnetic Fe spin alignment in an applied (longitudinal) field //ex t of 20 kOe. The corresponding spectrum (Fig. 2, top) indicates that essentially all of the Fe spins at 77 K were aligned parallel to i/ex t, since the absorption lines No. 2 and No. 5 (i.e., the Am=0 nuclear transitions) disappear as compared to the zero-field spectrum at 77 K (Fig. 1).The spectra shown in Fig. 2 were obtained by field cooling the sample from 77 to 4.2 K in a (longitudinal) field of H^^ , =20 kOe. The striking effect is the reappearance of the Am=0 lines at ~ 45 K and the gradual increase in their relative intensity as the temperature is lowered to 4.2 K. This means that the Fe spins (starting at ~ 45 K) gradually rotate away from the applied field direction as the temperature decreases, the ensemble of Fe spins now forming an average angle 6 =5^0 relative to the applied field. Thus below 45 K the magnetic state is characterized by a freezing of Fe spin components which are transverse with respect to the applied field and which increase in magnitude if the temperature is lowered. When 100 CO 1.001 < cr LU a95( > LU cr n \ \ r ...

Microscopic coexistence of antiferromagnetic and spin-glass states

“…The increase of mean magnetic hyperfine fields on cooling below 50 K (Fig. 2c, green line), is reminiscent of precursor phenomena in ferromagnetic re-entrant spin-glass systems like AuFe 52,53 or the mixed spinel Mg 1+t Fe 2−2t Ti t O 4 54 . It represents a gradual freezing of transverse spin components within Mössbauer frequency window.…”

Hyperfine interactions on iron nuclei in the BCC and fractally decomposed BCC/FCC mixed phase iron–gold alloys

“…It is about 25 at.% of iron at room temperature [3]. A competition between spin glass and ferromagnetic order was found close to the percolation limit and at low temperature [8]. Actually the majority of papers concentrated on the alloys rich in gold, while there are far less reports concerned with the iron-rich side of the system.…”

Magnetic short- and long-range order inPbFe0.5Ta0.5O3

“…Coexistence of long-range and spinglass orders have been studied in disordered ferromagnetic materials both theoretically and experimentally 2,[10][11][12][13][14][15][16] . For example, for reentrant spinglasses in amorphous metallic compounds Fe-Mn 10,11 , Fe-Zr 12,13 , Au-Fe 15,16 a spin canting along with transverse spin freezing was considered as a mechanism of coexistence.…”