Transient Enhancement Studies of Domain-Wall Pinning in Ferromagnetic Metals

Abstract

The technique of transient enhancement of initial ac susceptibility is introduced. The time dependence of the low-field ac susceptibility of ferromagnets is observed in the presence of a second time-varying field. The technique enables the diffusion of domain-wall pinning centers to be studied and also acts as a sensitive indicator of domain nucleation for magnetic critical-phenomena studies near the Curie temperature. Preliminary results for ferromagnetic gadolinium over the temperature range 240-292 K yield an activation energy for the pinning centers of ~0.7 eV.

Enhancement technique in the measurement of alternating current susceptibility

“…10 We have calculated the percentage enhancement of the ac susceptibility of Gd for a biasing ͑en-hancing͒ field of 2.5 Oe ͑433 Hz͒ and a measuring field of 0.25 Oe ͑137 Hz͒. It varies from 11.4 to 18.02 in the temperature region 277-283.5 K and is comparable with the magnitude as measured by Wantenaar et al 5 at 279.5 K although the frequencies of the measuring field ͑2 kHz͒ and the biasing field ͑450 Hz͒ are different.…”

“…5 Here, the modification is done basically to observe the effect of a second field on the ac susceptibility, measured by the conventional method. The second biasing field may be either a pulsed dc field, or a sinusoidal field with a frequency different from that of the measuring field ͑which has frequency and amplitude given by 1 , h 1 ).…”

“…Wilson et al 5,6 have earlier studied the effect of enhancement using pulsed and sinusoidal enhancing fields. Since their main interest was to determine the temperature of nucleation of domains, all they really needed to determine was whether or not there was any enhancement at all.…”

Reentrance in theMn(tetracyanoethylene)x⋅y(CH2Wynn1, Gîrţu2, Zhang3 et al. 1998Phys. Rev. B475340View full textAdd to dashboardCite

“…For ferromagnets the pinning of the domain walls causes frequency dependence of the microwave frequency susceptibility, the few cases in which such f dependence is seen in ac susceptibility measurements being primarily related to the mobility of pinning centers in systems with annealed disorder. 16,17 As we expect quenched disorder in our system, we take the absence of frequency dependence of the ac susceptibility near 75 K as evidence that this transition is to a nonglassy state with a spontaneous moment. In contrast, a SG is characterized by long relaxation times and is expected to exhibit frequency dependence of the ac susceptibility.…”

The Effects of Broadening on the High Temperature Critical Susceptibility Exponent g

“…Neglecting the effects of internal demagnetizing fields, and assuming that each homogeneous region of the sample contributes a paramagnetic susceptibility of the form of equation 1, we have for the total susceptibility for a distribution P(Te) of Curie temperatures 2This assumes that the data are taken over a range of temperatures T in which no region of the sample becomes ferromagnetic. The experimental technique of transient enhancement (Wantenaar et al 1976) enables one to determine precisely the temperature at which thermal nucleation of ferromagnetic domains begins and hence to avoid the inclusion of data from below this temperature in the analysis.…”