Peak In Ac Susceptibility Research Articles

Thermomagnetic data in iron-rare-earth-boron alloys

The magnetic phases present in iron-rare-earth-boron alloys were examined with ac and dc susceptibility and hysteresis loops measurements in the temperature range of 4.2 K to 700°C. In FeNdB a shoulder has been observed on the demagnetization curve below 150 K and a peak in ac susceptibility around 140 K. A similar shoulder has been found in FePrB but no peak in ac susceptibility, indicating that its origin is quite different. The relation between the microstructure and magnetic properties is discussed.

Disappearance of the magnetic relaxation in spin glasses

We have studied the relaxation of the magnetization in the spin-glass phase of Fe10Ni70P20 and CuMn0.243%. Following zero field cooling the magnetization M(H,T,t) of spin-glasses increases quasilogarithmically with time t after applying a field H at t=0: M(H,T,t)=M(H,T,t=0)+S(H,T) ln t. The quantity S(H,T) has been studied for 30 s≤t≤1200 s below Tg, temperature of the ac susceptibility peak. For both systems S(H) increases almost linearly with H for a given temperature, reaches a maximum value Sm for Hm, then decreases and tends to zero for a field HM at least four times larger than Hm. For a given field, S(T) exhibits a maximum near Tg in low field. This maximum becomes higher, sharper and is displaced to lower temperatures for increasing field values. A line Hm∝(Tg-T)γ with γ=1.5 and 1.8 for CuMn and Fe70Ni10P20, respectively, is obtained. This line has a form similar to the de Almeida-Thouless line Hc=(Tg-T)1.5. However, Hm or HM are smaller by a factor 16 or 4 than the corresponding Hc value. The data for each sample, when scaled according to S/M(t=30 s), collapse onto similar curves when plotted as a function of H/Hm. This suggests some universal behavior of the magnetic relaxation in quite different spin-glasses.

The low temperature magnetic properties of austenitic FeCrNi alloys. 3. The effect of chromium concentration in FeCr-17wt% Ni alloys

The magnetic properties of ternary FeCrNi alloys containing 17 wt% Ni and 18–24% Cr have been studied over a range of temperature from 4.2–35 K by both ac and dc techniques in fields from 5 Am −1 − 6 MA m −1. At room temperature, the matrix of all the alloys was the paramagnetic fcc austenite phase (with an unavoidable 0.1 – 0.5 wt% δ ferrite) and on cooling all the alloys showed distinct peaks in ac susceptibility, though equivalent peaks in dc susceptibility were only observed at low fields in the alloy containing 18 wt% Cr. An analysis of the magnetization/field/temperature results showed that all the alloys were superparamagnetic at low temperature, due to the formation of ferromagnetic clusters in the disordered antiferromagnetic matrix, and that the chromium atoms make no measureable contribution to the magnetization of these clusters. Appropriate susceptibility values are tabulated for use as design data.

Magnetic anomaly in superconducting TmRh4B4

We have investigated the magnetic and superconducting properties of TmRh4B4 (which becomes superconducting at 9.6 K) by means of ac and dc magnetic susceptibility and specific heat measurements. At 10.7 K, an ac susceptibility peak similar to those found in spin glasses has been observed. In addition, a pronounced specific heat peak has been observed at 11.4 K. The susceptibility peak is essentially unaffected by substitution of 1% Lu or Er for the Tm, but it diminishes when much larger amounts of Er are substituted. The physical origin of this anomalous peak will be discussed.