Magnetic transitions in disorderedGdAl2

Abstract

The role of disorder in magnetic ordering transitions is investigated using mechanically milled GdAl 2 . Crystalline GdAl 2 is a ferromagnet while amorphous GdAl 2 is a spin glass. Nanostructured GdAl 2 shows a paramagnetic-to-ferromagnetic transition and glassy behavior, with the temperature and magnitude of each transition dependent on the degree and type of disorder. Disorder is parametrized by a Gaussian distribution of Curie temperatures T C with mean T C and breadth ⌬T C . A nonzero coercivity is observed at temperatures more than 20 K above the highest T C of any known Gd-Al phase; however, the coercivity decreases with decreasing temperature over the same temperature range where the GdAl 2 grains ferromagnetically order. Models for the anomalous coercivity behavior are proposed and evaluated for their ability to explain the origin of the lowtemperature glassy magnetization peak.

Disorder-induced depression of the Curie temperature in mechanically milledGdAl2

“…20 T C is essentially independent of the measuring field; however, ⌬T C depends on measuring field as H 1/ where ϳ 1 -1.5 for GdAl 2 . 20 Two effects shown in Fig. 5 must be explained: the shift in T C and the broadening of the transition with milling time.…”

“…The broadening is attributed to the distribution of local environments created by disorder. T C cannot be determined accurately from the minimum of dM͑T͒ / dT, and instead is found by fitting M͑T͒ to a distribution of Curie temperatures: [20][21][22] …”

“…There is no significant difference between values of T C obtained at these two fields. 20 The critical exponent ␤ can vary between 0.3 and 0.6 with almost no change in T C . The error bars indicate the range of T C for ␤ from 0.3 to 0.6.…”

“…Excluding the open symbols, which represent data from annealed samples, the dependence of ⌬T C on percent change of lattice parameter is roughly linear with a slope of 34 K / % and an intercept of 2.9 K. The nonzero intercept is likely due to field broadening, which is nonnegligible at 100 Oe. 20 ac magnetic susceptibility measurements were made to investigate how disorder affects the time-dependent magnetic response. Figure 10 shows a slight frequency dependence in the real part of the ac susceptibility Ј and a more noticeable frequency dependence in the imaginary part of the ac susceptibility Љ.…”

Reduction of the Yb valence inYbAl3nanoparticles

“…In this case, the relationship between the structural and magnetic changes induced by the milling process results in the evolution from a ferromagnetic behavior to a spin-glass phase, which emerges at lower temperatures. [4][5][6][7][8] More recently, a study of the magnetic properties of TbAl 2 milled alloys has revealed that not only disorder but also size effects are important in crucial understanding of the magnetic behavior, with a pronounced variation of coercivity with the grain size. 9 The relevance of the extension of these studies of nanocrystalline particle systems to Ce and Yb intermetallics, the latter being known as strongly correlated electron materials, requires some understanding of how macroscopic properties in these systems become modified at the nanoscale.…”