A New Property of Ferromagnetic‐Antiferromagnetic Coupling

Abstract

Three systems exhibiting ferromagnetic-antiferromagnetic coupling, Cc&oO, NiFe-NiFeMn, NiFe-Cr,O,, are studied using uniaxial thin films. A new property of such systems is found, i.e., a concentration of the hysteresis loops as a function of the number of cycles traced (iE). This contraction follows a 1 / 1 1 ; law for the first 50 cycles and is related to the rotational hysteresis. This phenomenon is att,qibuted to fluctuations in the ferromagneticantiferromagnetic coupling, which modify the magnetic interactions between differently coupled regions with each reversal. On a btudib s u p des couches minces Zt anisotropie uniaxiale trois s y s t h e s presentant les propriktbs caract6ristiques des couplages ferromagnktiques-antiferromagnbtiques,Co-COO, NiFe-NiFeMn, NiFe-Cr,O,. On a mis en evidence m e propribt-5 nouvelle de contraction des cycles d'hystkresis avec l'ordre n du cycle decrit. Cette contraction suit une loi en 1 / l / i pour les 50 premiers cycles et est libe Zt I'hystkrCsis de rotation. On attribue ce phenomhe B des fluctuations dans l'intensitk du couplage ferromagnbtique-antiferromagnbtique entrainant lors des diffbrents basculements, des modifications des interactions magnbtiques entre des regions Zt couplage diffbrent.

Exchange bias effect in alloys and compounds

“…This is investigated by successive field cycling at low temperature after the field cooling. The evidence of training effect was first reported in a thin film [96]. They further proposed that decrease of H E or M E satisfies following empirical formula Open circles are experimental data while small filled symbols represent the data obtained from least square fit described in equation (2) [reprinted from [99]].…”

Training of the exchange-bias effect in NiO-Fe heterostructures

“…One often finds that the EB field after n loops, H EB n , can be described by the proportionality H EB n ϪH EB ϱ ϰ1/ͱn. 7,8 The training effect in general has its origin in the reorientation of AF domains at the AF/FM interface which takes place during each magnetization reversal of the FM top layer. 3 As pointed out by Nogués and Schuller a pronounced training effect has been found in heterosystems involving polycrystalline AF pinning layers, 9-11 while in single-crystalline pinning systems this effect is expected to be small.…”

“…7,8 In order to evidence the correlation between the training effect for nϾ1 and the decrease of m AF with increasing n we calculate the total saturation magnetization of the heterostructure after subtraction of H.…”

Dynamic enhancement of the exchange bias training effect

“…The identical number of free parameters is required for the power law description first introduced in Ref. 20. However, the physically motivated Eq.…”

“…A gradual degradation of the EB field can take place when cycling the heterostructure through consecutive hysteresis loops. [20][21][22][23][24] This aging phenomenon is known as a training effect and is quantified by the 0 H EB vs n dependence, where n labels the number of loops cycled after initializing the EB via field cooling. EB and the accompanying training effect have been observed in various magnetic systems ranging from core shell magnetic granules 25 and AF/FM thin film heterolayers 14,26 to intrinsic EB taking place at natural interfaces of ferromagnetic nanodomains embedded in an AF matrix of charge ordered manganites.…”