Critical exponents of inhomogeneous ferromagnets

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.

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“…Berger et al 17 found that the breadth of the T C distribu- tion in the inhomogeneous ferromagnet La 1/3 Ca 1/3 MnO 3 depends on field as…”

Section: Paramagnetic-ferromagnetic Transitionmentioning

confidence: 99%

“…17 While it is significant that this model can determine critical behavior in systems where the intrinsic broadening due to disorder is comparable to the field broadening, our primary interest is in using it as a means of quantifying disorder.…”

Section: Paramagnetic-ferromagnetic Transitionmentioning

confidence: 99%

Magnetic transitions in disorderedGdAl2

et al. 2003

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“…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] …”

Section: A the Paramagnetic Regimementioning

confidence: 99%

Disorder-induced depression of the Curie temperature in mechanically milledGdAl2

et al. 2004

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Shand, P.M.; Stark, C.; Pekarek, T.M.; Yue, Lanping; Petkov, V.; and Leslie-Pelecky, Diandra, "Disorder-induced depression of the Curie temperature in mechanically milled GdAl 2 " (2004 The effect of disorder on the ferromagnetic transition is investigated in mechanically milled GdAl 2 . GdAl 2 is a ferromagnet when crystalline and a spin glass when amorphous. Mechanical milling progressively disorders the alloy, allowing observation of the change from ferromagnetic to a disordered magnetic state. X-ray diffraction and pair-distribution-function analysis are used to determine the grain size, lattice parameter, and mean-squared atomic displacements. The magnetization as a function of temperature is described by a Gaussian distribution of Curie temperatures. The mean Curie temperature decreases with decreasing lattice parameter, where lattice parameter serves as a measure of defect concentration. Two different rates of change with lattice parameter are observed: the first, slower rate occurs while grain size is changing, and the grain size is constant in the second regime. The breadth of the Curie temperature distribution is linear with lattice parameter. Milled and annealed GdAl 2 shows that the mean Curie temperature has the same general dependence on lattice parameter as milled, but unannealed GdAl 2 , while the breadth of the Curie temperature distribution is different in annealed samples compared to the unannealed materials.

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“…We have studied the correlation between the broadening of the isothermal magnetic entropy change and the Curie temperature (T C ) distribution in nanostructured Pr 2 Fe 17 and Nd 2 Fe 17 alloys produced by high-energy ball-milling after milling times of 10, 20, and 40 h. The changes in the microstructure affect the Fe local environments and as a consequence the magnetic interactions, giving rise to T C distributions centered around 285 K and 330 K for the Pr 2 Fe 17 and Nd 2 Fe 17 alloys, respectively. The width of the distributions enlarges (up to 60 K) as the milling-time increases, and consequently, the isothermal magnetic entropy change curves show an extended full width at half maximum.…”

mentioning

confidence: 99%

“…Hence, we could picture the nanostructured powders having a distribution of Fe-Fe interatomic distances around the value for the Bulk alloys giving rise to a distribution of Curie temperature values. 8,9 Then, we describe the temperature dependence of the magnetization M(T) by a superposition of individual power-law functions, according to 17 …”

mentioning

confidence: 99%