Aging dynamics in a reentrant ferromagnet:Cu0.2Co0.8Cl2−FeCl3

Abstract

Aging dynamics of a reentrant ferromagnet Cu0.2Co0.8Cl2-FeCl3 graphite bi-intercalation compound has been studied using AC and DC magnetic susceptibility. This compound undergoes successive transitions at the transition temperatures Tc (= 9.7 K) and TRSG (= 3.5 K). The relaxation rate S(t) exhibits a characteristic peak at tcr close to a wait time tw below Tc, indicating that the aging phenomena occur in both the reentrant spin glass (RSG) phase below TRSG and the ferromagnetic (FM) phase between TRSG and Tc. The relaxation rate S(t) (= dχZF C (t)/d ln t) in the FM phase exhibits two peaks around tw and a time much shorter than tw under the positive T -shift aging, indicating a partial rejuvenation of domains. The aging state in the FM phase is fragile against a weak magnetic-field perturbation. The time (t) dependence of χZF C (t) around t ≈ tcr is well approximated by a stretched exponential relaxation: χZF C (t) ≈ exp[−(t/τ ) 1−n ]. The exponent n depends on tw, T , and H. The relaxation time τ (≈ tcr) exhibits a local maximum around 5 K, reflecting a chaotic nature of the FM phase. It drastically increases with decreasing temperature below TRSG.

Ageing dynamics of ferromagnetic and re-entrant spin glass phases in a stage-2 Cu0.80Co0.20Cl2graphite intercalation compound

“…It drastically increases with further decreasing T below T RSG . Note that very similar behaviour of t cr versus T (τ versus T ) is also observed in the re-entrant ferromagnet Cu 0.2 Co 0.8 Cl 2 -FeCl 3 GBIC [9]. The existence of the broad peak around 5.5 K suggests the chaotic nature of the FM phase in our system (see section 4).…”

“…We will also show that the t dependence of S ZFC (t) around t t w is well described by a stretched exponential relaxation (SER), S ZFC (t) ≈ (t/τ ) 1−n exp[−(t/τ ) 1−n ], where t w is a wait time, n is an SER exponent and τ is an SER relaxation time which is of the same order as t cr (experimentally). The temperature dependence of t cr and τ shows a very characteristic behaviour, which is very similar to that observed in Cu 0.2 Co 0.8 Cl 2 -FeCl 3 GBIC [9]. We will also report two kinds of the memory phenomena in the measurement of thermoremnant magnetization (TRM) and the field-cooled (FC) magnetization.…”

“…The re-entrant ferromagnet undergoes two phase transitions at the critical temperatures T RSG and T c (T c > T RSG ): the re-entrant spin glass (RSG) phase below T RSG and the ferromagnetic (FM) phase between T RSG and T c . The ageing behaviour in the RSG phase has been reported in many re-entrant ferromagnets, including (Fe 0.20 Ni 0.80 ) 75 P 16 B 6 Al 3 [1][2][3][4], CdCr 2x In 2(1−x) S 4 (x = 0.90, 0.95, and 1.00) [5][6][7], and the Cu 0.2 Co 0.8 Cl 2 -FeCl 3 graphite bi-intercalation compound (GBIC) [8,9]. The ageing behaviour of the RSG phase is similar to that of the spin glass (SG) phase of spin glass systems.…”

“…Recently, the slow dynamics of re-entrant ferromagnets has been extensively studied from the time evolution of the magnetization M(t) and the absorption of AC magnetic susceptibility χ (t) after the appropriate ageing protocol [1][2][3][4][5][6][7][8][9]. The re-entrant ferromagnet undergoes two phase transitions at the critical temperatures T RSG and T c (T c > T RSG ): the re-entrant spin glass (RSG) phase below T RSG and the ferromagnetic (FM) phase between T RSG and T c .…”

“…In contrast, there have been few reports on the observation of the ageing behaviour in the FM phase. The measurement of the relaxation rate S(t) (=(1/H )dM(t)/d ln t) for (Fe 0.20 Ni 0.80 ) 75 P 16 B 6 Al 3 [1][2][3][4] and Cu 0.2 Co 0.8 Cl 2 -FeCl 3 GBIC [8,9] has revealed that not only the RSG phase but also the FM phase exhibit ageing phenomena. For CdCr 2x In 2(1−x) S 4 with x = 0.90, 0.95, and 1.0 [5][6][7], the ageing behaviour of the absorption χ (ω, t) is observed both in the FM and RSG phases.…”