The interest towards antiferromagnetic (AFM) materials is continuously increasing mainly because of their crucial role in the operating principle of modern, miniaturized spintronic devices. In magnetoresistive spin valves and tunnel junctions, the effective magnetic anisotropy of the ferromagnetic (FM) electrodes can be finely controlled through the interface exchange coupling with an AFM layer: the torque action exerted by the interfacial AFM spins on the FM ones brings about the insurgence of an unidirectional exchange anisotropy for the FM magnetization, and then of the exchange bias (EB) effect (quantitatively expressed by the exchange field Hex). In order to make progress in this field, achieving a fundamental comprehension of magnetism of nanosized AFM elements is mandatory. A possible approach to gain an insight into the magnetic properties of AFM materials, circumventing the difficulty represented by the inherent weakness of the magnetic response, is just to exploit the exchange coupling with a soft ferromagnet. In fact, although the magnetic response of the AFM is overwhelmed by that of the FM, the magnetothermal behavior of the coupled system is strongly determined by that of the AFM and the real problem is of how to disentangle it. In this context, we have studied the EB effect, in the 5-300 K temperature range, both in IrMn/NiFe bilayers [1] and in a Ni/NiO nanogranular system [2]. In the first case, we have investigated a set of samples differing for the thickness of the AFM IrMn component (3, 6 and 10 nm, whereas the NiFe layer was 5 nm thick); as for the nanogranular material, it consisted of Ni nanoparticles (mean size of the order of 10 nm) dispersed in a NiO nanocrystalline matrix and we report about samples with Ni content varying between 4 wt.% and 69 wt.%. In spite of the evident compositional and structural dissimilarity between the two systems, the magnetothermal evolution of the EB properties can be coherently explained in the framework of a general phenomenological model based on the existence of a structurally disordered AFM region, showing a glassy magnetic behavior, at the interface between the FM phase and the bulk of the AFM component (the latter consists of nanograins, supposed to be magnetically independent or weakly interacting). At low temperature, the spins of the disordered AFM region are frozen in a magnetic disordered state and are collectively involved in the exchange coupling with the FM moments, which results in high Hex values. With increasing temperature, the frozen collective regime breaks up (the critical temperature, marking such a passage, usually does not exceed 100 K). However, the AFM interfacial spins do not enter the paramagnetic regime - this event would definitely decouple the FM and AFM phases as regards exchange interaction, whereas only a marked reduction of Hex is observed – because of the polarizing action of the adjacent FM and bulk AFM spins. In particular, we propose that only the AFM interfacial spins tightly anchored to the spin lattice of the bulk AFM nanograins are able to exert a strong torque action on the FM spins and, therefore, are effectively involved in the EB mechanism. We can say that these interfacial AFM spins re-enter into the AFM state from the frozen regime, namely feature re-entrant antiferromagnetism. Thus, at high temperature, the FM/AFM exchange coupling is governed by a fraction of interfacial AFM spins sustained by magnetically uncorrelated AFM nanograins and it critically depends on the thermal stability of the latter ones. In the case of the IrMn/NiFe system, this description of the FM/AFM exchange coupling mechanism has been extended so as to include spatial confinement effects, namely what happens passing from a continuous bilayer to nanodots [3]. For this purpose, the EB effect has been studied in arrays of IrMn[10 nm]/NiFe[5 nm] square dots with different size (D = 1000, 500 and 300 nm). We have observed that at T = 300 K Hex decreases with reducing the dot size (it is absent in the smallest ones), whereas the opposite trend is visible at T = 10 K. In particular, we will discuss, also by the support of micromagnetic calculations, how the low temperature value of Hex depends on the ratio between the dot size D and the length of magnetic correlation among the frozen AFM interfacial spins. This research work has been sponsored by MIUR under project FIRB2010-NANOREST.

Re-entrant antiferromagnetism in exchange biased systems

DEL BIANCO, Lucia
2015

Abstract

The interest towards antiferromagnetic (AFM) materials is continuously increasing mainly because of their crucial role in the operating principle of modern, miniaturized spintronic devices. In magnetoresistive spin valves and tunnel junctions, the effective magnetic anisotropy of the ferromagnetic (FM) electrodes can be finely controlled through the interface exchange coupling with an AFM layer: the torque action exerted by the interfacial AFM spins on the FM ones brings about the insurgence of an unidirectional exchange anisotropy for the FM magnetization, and then of the exchange bias (EB) effect (quantitatively expressed by the exchange field Hex). In order to make progress in this field, achieving a fundamental comprehension of magnetism of nanosized AFM elements is mandatory. A possible approach to gain an insight into the magnetic properties of AFM materials, circumventing the difficulty represented by the inherent weakness of the magnetic response, is just to exploit the exchange coupling with a soft ferromagnet. In fact, although the magnetic response of the AFM is overwhelmed by that of the FM, the magnetothermal behavior of the coupled system is strongly determined by that of the AFM and the real problem is of how to disentangle it. In this context, we have studied the EB effect, in the 5-300 K temperature range, both in IrMn/NiFe bilayers [1] and in a Ni/NiO nanogranular system [2]. In the first case, we have investigated a set of samples differing for the thickness of the AFM IrMn component (3, 6 and 10 nm, whereas the NiFe layer was 5 nm thick); as for the nanogranular material, it consisted of Ni nanoparticles (mean size of the order of 10 nm) dispersed in a NiO nanocrystalline matrix and we report about samples with Ni content varying between 4 wt.% and 69 wt.%. In spite of the evident compositional and structural dissimilarity between the two systems, the magnetothermal evolution of the EB properties can be coherently explained in the framework of a general phenomenological model based on the existence of a structurally disordered AFM region, showing a glassy magnetic behavior, at the interface between the FM phase and the bulk of the AFM component (the latter consists of nanograins, supposed to be magnetically independent or weakly interacting). At low temperature, the spins of the disordered AFM region are frozen in a magnetic disordered state and are collectively involved in the exchange coupling with the FM moments, which results in high Hex values. With increasing temperature, the frozen collective regime breaks up (the critical temperature, marking such a passage, usually does not exceed 100 K). However, the AFM interfacial spins do not enter the paramagnetic regime - this event would definitely decouple the FM and AFM phases as regards exchange interaction, whereas only a marked reduction of Hex is observed – because of the polarizing action of the adjacent FM and bulk AFM spins. In particular, we propose that only the AFM interfacial spins tightly anchored to the spin lattice of the bulk AFM nanograins are able to exert a strong torque action on the FM spins and, therefore, are effectively involved in the EB mechanism. We can say that these interfacial AFM spins re-enter into the AFM state from the frozen regime, namely feature re-entrant antiferromagnetism. Thus, at high temperature, the FM/AFM exchange coupling is governed by a fraction of interfacial AFM spins sustained by magnetically uncorrelated AFM nanograins and it critically depends on the thermal stability of the latter ones. In the case of the IrMn/NiFe system, this description of the FM/AFM exchange coupling mechanism has been extended so as to include spatial confinement effects, namely what happens passing from a continuous bilayer to nanodots [3]. For this purpose, the EB effect has been studied in arrays of IrMn[10 nm]/NiFe[5 nm] square dots with different size (D = 1000, 500 and 300 nm). We have observed that at T = 300 K Hex decreases with reducing the dot size (it is absent in the smallest ones), whereas the opposite trend is visible at T = 10 K. In particular, we will discuss, also by the support of micromagnetic calculations, how the low temperature value of Hex depends on the ratio between the dot size D and the length of magnetic correlation among the frozen AFM interfacial spins. This research work has been sponsored by MIUR under project FIRB2010-NANOREST.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2338903
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