1,720,972 research outputs found
“Production and magnetic characterization of exchange-coupled magnetic nanostructures”
No full textLa crescente richiesta di miniaturizzazione dei dispositivi magnetici ha innescato un crescente interesse nei confronti dello studio delle proprietà magnetiche di elementi confinati lateralmente su scala nanometrica; esempi importanti di queste tecnologie sono le testine di lettura per hard disk e le memorie RAM magnetiche non volatili, entrambi dispositivi che si basano su sistemi a valvola di spin. In questo ambito, questo lavoro di tesi riguarda la preparazione e lo studio delle proprietà dei singoli elettrodi di una valvola di spin, ossia di nanostrutture a bi-strato prodotte mediante “dc-magnetron sputtering”. I campioni preparati consistono di due sottili strati magnetici di Ni80Fe20 (NiFe) e Ir25Mn75 (IrMn), che rappresentano rispettivamente una fase ferromagnetica (FM) ed una antiferromagnetica (AFM), accoppiate alla loro comune interfaccia per interazione di scambio. I bi-strati sono stati studiati sotto forma sia di film continuo che di matrici di nanodot. L'accoppiamento di scambio tra le due fasi implica la comparsa di una caratteristica anisotropia di tipo unidirezionale con verso preferenziale, che si manifesta attraverso il cosiddetto effetto del campo di scambio (EB), ovvero una traslazione orizzontale del ciclo di isteresi, solitamenta accompagnata da un aumento di coercitività del ciclo stesso. L'effetto di EB è stato studiato in bi-strati continui IrMn/NiFe in funzione dello spessore dello strato di IrMn, al variare dell'ordine degli strati, e della temperatura. Un protocollo di misura originale è stato sviluppato per studiare ulteriormente il comportamento magneto termico della fase AFM, rendendo possibile la rilevazione della distribuzione delle barriere energetiche di anisotropia di tale fase. Le nostre osservazioni sperimentali insieme ai risultati ottenuti attraverso analisi di tipo strutturale, ci hanno permesso di proporre una precisa descrizione del meccanismo secondo cui i momenti magnetici (spin) della fase AFM all'interfaccia contribuiscono a determinare l'effetto di EB. Tale meccanismo fonda le sue basi nella descrizione dello strato AFM come costituito sia di nanograni che di una componente magnetica disordinata all'interfaccia. Diverse matrici di dot a bi-strato di forma quadrata e circolare, e taglia variabile da 1 μm fino a 140 nm, sono state realizzate e caratterizzate magneticamente per studiare l'influenza del confinamento spaziale sull'efffetto di EB. In questo caso, i risultati ottenuti ci hanno convinto a suggerire come, per comprendere il comportamento magneto termico dei sistemi accoppiati per interazione di scambio di taglia nanometrica, alla luce del modello proposto, sia necessario prendere in considerazione quale relazione sussista tra la taglia della nanostuttura ed una una caratteristica lunghezza di correlazione magnetica degli spin all'interfaccia.The increasing demand for miniaturization of magnetic devices triggered an increasing interest for the study of the magnetic properties of elements size confined to the nanoscale; read-heads and nonvolatile magnetic random access memories are both important examples of such technological devices based on the spin valve systems. In this context, this thesis work concerns both the preparation and the study of the properties of the single electrodes of a spin valve, namely bilayer nanostructures grown by dc-magnetron sputtering. The prepared samples consist of two thin magnetic layers of Ni80Fe20 (NiFe) and Ir25Mn75 (IrMn), the ferromagnetic (FM) and antiferromagnetic (AFM) phases, respectively, exchange coupled at their common interface, in form of both continuous films and arrays of nanodots. The exchange coupling results in the appearance of a characteristic unidirectional anisotropy, which is revealed by a horizontal shift of the hysteresis loop, called exchange bias (EB), beside a coercivity enhancement. The EB effect has been studied in continuous IrMn/NiFe bilayers as a function of IrMn thickness, stacking sequence and temperature. An original experimental protocol was devised to further study the magnetothermal behavior of the AFM phase allowing us to access its associated distribution of anisotropy energy barriers. Our experimental observations together with the results from structural investigations, allowed us to propose a precise description of the mechanism to which the AFM interfacial spins take part to provide EB, and that founds its basis on the description of the AFM layer as made of both nanograins and a magnetically disordered interfacial component. Arrays of AFM/FM dots with square and circular shape, and different size, that varies from 1 μm down to 140 nm, have been realized and magnetically characterized to study the temperature dependence of EB in conjunction with spatial confinement effects. In this case, the results obtained convinced us to suggest that, to understand the magnetothermal behavior of nanosized exchange-coupled systems in the light of the proposed model, a comparison between the nanostructure size and a characteristic length of correlation of interfacial spins is to be taken into account
Production and magnetic characterization of exchange-coupled NiFe/IrMn and IrMn/NiFe films
No full textNowadays, several advanced magnetic and magnetoelectronic devices rely on interface exchange coupling (EC) between different magnetic phases (for example, magnetoresistive recording read heads, magnetoresistive random access memories, spring magnets, magnetic tunnel junctions, etc). The great challenge for optimizing the performances of such devices is to control the EC strength in order to tune the anisotropy, which represents the key technological parameter for any applications of magnetic materials. In fact, the exchange interaction across the ferromagnetic(FM)/antiferromagnetic(AF) interface gives rise to an additional source of anisotropy (exchange anisotropy), determining the onset of the exchange bias effect and producing significant changes in the coercivity (HC).
In this context, we have studied EC in continuous films with AF/FM and FM/AF configurations, grown by dc-magnetron sputtering in Ar atmosphere. As AF phase we have used Ir25Mn75 (IrMn), whilst we have used Ni80Fe20 (Py) as FM. All the samples have been deposited on a 5 nm thick Cu underlayer to favor a fcc (111) orientation and have been capped with a Cu overlayer (5 nm).
We have studied and compared the effects of EC on the macroscopic magnetic properties (exchange fied Hex, HC and squareness) of a number of samples grown in different values of a magnetic field applied along the film plane (Hdep= 0, 200, 800 Oe) and having different thickness of the AF layer tAF (3, 6 and 10 nm; the thickness of FM was 5 nm in all the samples).
Hysteresis loops have been measured by magneto-optic Kerr effect (MOKE) and SQUID in the 5-300 K temperature range, after cooling in zero-field or in a field Hcool = 100 Oe from T = 400 K.
At T = 300 K, the highest Hex (~150 Oe) has been measured in the Si/Cu/NiFe/IrMn/Cu sample with tAF = 10 nm (HC ~ 25 Oe); Hex goes to zero at TB ~ 390 K. In the same configuration, for tAF = 6 nm, TB ~ 370 K, (at T = 300 K, Hex ~ 100 Oe, HC ~ 60 Oe), whereas for tAF = 3 nm TB ~ 230 K (at T = 300 K, HC ~ 20 Oe).
In all the samples, both Hex and HC increase with decreasing T especially for T < 100 K and, at T = 5 K, the highest Hex (~ 1000 Oe) is measured for tAF = 3 nm (in this sample, HC ~ 640 Oe).
For T 100 K, Hex and Hc assume similar values, irrespective of the configuration.
This research work has been carried out in the framework of the project FIRB2010 “Tailoring the magnetic anisotropy of nanostructures for enhancing the magnetic stability of magnetoresistive devices” – NANOREST
Magnetothermal behavior of the antiferromagnet in exchange-coupled NiFe/IrMn bilayers
No full textThe magnetothermal behavior of antiferromagnetic IrMn layers of different thickness (tAFM = 3, 6, 10 nm) has been studied exploiting the exchange coupling with a ferromagnetic 5nmthick NiFe layer. We have introduced an original protocol for the measurement of NiFe/IrMn sample magnetization as a function of temperature and time at different values of an external magnetic field Hinv, applied antiparallel with respect to the unidirectional exchange anisotropy [1]. This analysis probes the effective distribution of anisotropy energy barriers of the antiferromagnetic phase, as it is sensed by the coupled ferromagnetic layer. The NiFe/IrMn samples have been grown by DC magnetron sputtering at room temperature in a magnetic field of 400 Oe. At low temperature, T 100 K, a large peak is visible, and its position changes with tAFM and Hinv. These results are consistent with the existence of a low-temperature magnetic regime in which the interfacial IrMn spins are frozen in a disordered glassy state and collectively involved in the exchange coupling mechanism. At T ~ 100 K, the collective state breaks up and only the interfacial IrMn spins which are tightly polarized by the IrMn nanograins, forming the bulk of the layer, are effectively involved in the exchange coupling. Therefore, for T > 100 K, the anisotropy energy barriers of bulk IrMn nanograins mainly give rise to the large peak in the distribution. The thermal evolution of the exchange field and of the coercivity in the three samples is discussed and coherently explained in the framework of this description. This research was sponsored by MIUR-‐FIRB2010 project RBFR10E61T/NANOREST
Exchange bias in systems based on glassy ultrafine IrMn layers
No full textThe study of the exchange bias (EB) interaction between ferromagnetic (FM) and antiferromagnetic (AFM) phases plays a crucial role both from the theoretical and from the technological point of view. In fact, although the EB effect is already exploited in emerging technologies, e.g. in non-volatile magnetoresistive magnetic random access memories, some key aspects of the underlying physics are still not completely understood [1]. Recent studies on polycrystalline FM/AFM nanostructures have addressed the issue of establishing the dependence of EB on size confinement, and agree that a comparison between the nanostructure size and an AFM characteristic length of magnetic correlation is to be taken into account [2,3].
The appearance of a magnetic correlation length can be related to the presence of AFM regions showing a magnetic glassy behaviour [3,4], and in this context, we proposed a model for the magnetic structure of the AFM phase based on the existence of a thin structurally and magnetically disordered region in the AFM layer interposed between the FM phase and the bulk of the AFM layer; the latter is supposed to consist of nanograins either magnetically independent from each other or weakly interacting. This model is based on the experimental evidence we found in bilayer systems constituted of Ni80Fe20 (NiFe) as FM phase and Ir25Mn75 (IrMn) as AFM phase.
In this research work, we focus on the IrMn/NiFe system and purposely address the magnetothermal properties of the magnetically disordered IrMn region on its own. To this aim, we produce IrMn/NiFe systems constituted of a very thin IrMn phase, both in form of continuous films and nanodots, so to neglect the contribution of the bulk of the AFM layer to EB and highlight the role of the glassy IrMn region to both the exchange coupling mechanism and size confinement effects. In detail, we report on the magnetic properties of the Cu(3 nm)/IrMn(3 nm)/Py(3 nm) system deposited on a Si substrate by electron beam lithography and lift-off using dc sputtering deposition in presence of a static magnetic field Hdep; we investigate both the continuous reference film and a square array of circular dots with a diameter of ~140 nm, and centre-to-centre distance of ~200 nm
Interface adjustment and exchange coupling in the IrMn/NiFe system
No full textThe exchange bias effect was investigated, in the 5–300 K temperature range, in samples of IrMn [100 Å]/NiFe [50 Å] (set A) and in samples with inverted layer-stacking sequence (set B), produced at room temperature by DC magnetron sputtering in a static magnetic field of 400 Oe. The samples of each set differ for the nominal thickness (tCu) of a Cu spacer, grown at the interface between the antiferromagnetic and ferromagnetic layers, which was varied between 0 and 2 Å. It has been found out that the Cu insertion reduces the values of the exchange field and of the coercivity and can also affect their thermal evolution, depending on the stack configuration. Indeed, the latter also determines a peculiar variation of the exchange bias properties with time, shown and discussed with reference to the samples without Cu of the two sets. The results have been explained considering that, in this system, the exchange coupling mechanism is ruled by the glassy magnetic behavior of the IrMn spins located at the interface with the NiFe layer. Varying the stack configuration and tCu results in a modulation of the structural and magnetic features of the interface, which ultimately affects the spins dynamics of the glassy IrMn interfacial component
Il metodo inverso svela le proprietà dei concentratori solari
No full textScoperto dal ricercatore italiano Antonio Parretta, il metodo “inverso” consente di effettuare velocemente e con mezzi semplici la caratterizzazione ottica completa di un concentratore solare. Basta registrare l’immagine prodotta su uno schermo illuminando il concentratore al contrario per estrarre il profilo dell’efficienza ottica in funzione dell’angolo d’incidenza della luce
per qualunque piano d’incidenza. Adatta per la caratterizzazione di piccoli concentratori solari, questa tecnica può essere estesa, con opportuni accorgimenti, anche ai grandi sistemi
Modeling the exchange bias interaction in ferromagnetic/antiferromagnetic films and nanostructures
No full textA novel approach to model the exchange bias (EB) effect in ferromagnetic
(FM)/antiferromagnetic (AFM) continuous films and nanodots is presented. The aim is to study
both the EB magnetothermal stability and spatial confinement effects, key features for modern
spintronic devices. The EB is due to the exchange coupling at the FM/AFM interface, and is
featured by the exchange bias field (Hex), i.e. the horizontal shift of the hysteresis loop. To
model the exchange coupling we used the three-dimensional Object Oriented MicroMagnetic
Framework. The AFM phase was described as a collection of both fixed and rotatable spins (FSs
and RSs, respectively) both interacting with the FM phase: the FSs have the role of pinning
centers, i.e. they mirror the presence of regions with high anisotropy energy in the AFM phase,
so they increase Hex; the RSs change their orientation following the FM magnetization, so they
do not contribute to Hex. The calculations were performed for a continuous film and for
squared dots with size (D) ranging from 1200 nm down to 300 nm. By changing the FSs to RSs
relative fraction we accounted for confinement effects, confirming a reduction of the
anisotropy energy for the AFM grains at the dot border. Moreover, by introducing spatial
correlations among FSs we modeled the proven existence of a low temperature frozen
collective regime of the interfacial AFM spins and its interplay with D. A good agreement was
found between the results of the calculations and the experimental data [1]. Finally, we tested
our model for the description of the dynamical properties of the FM/AFM system; we will
present our results regarding the spin wave frequency dependence on the external magnetic
field
Tuning the magnetic exchange coupling at the IrMn/NiFe interface by Cu insertion
No full textThe exchange bias effect was investigated in a set of IrMn[100Å]/Cu[tCu]/NiFe[50Å] samples differing for the nominal thickness (tCu) of the Cu spacer layer (tCu = 0, 0.5, 1, 2 Å). The Cu insertion affects the exchange coupling mechanism that, in this system, is ruled by the glassy magnetic behavior of the IrMn spins located at the interface with the NiFe phase. Varying tCu provides a tool to tune the exchange field and the coercivity and their thermal dependence
Magnetic effects of growth induced stress in FeCo thin films
No full textThin Fe50Co50 (FeCo) layers have recently attracted great attention due to their high saturation magnetization, spin polarization factor [1] and possible application in magnetic devices, showing perpendicular magnetization, as well. Thanks to the high FeCo magnetostrictive coefficient, growth induced stress (s) may represent an additional parameter useful to tailor the thin films magnetic properties, in particular the magnetization reversal process. Indeed, a compressive or tensile stress, namely a negative or positive s, may induce an additional in-plane or out-of-plane magnetic anisotropy, respectively, whose strength is proportional to s [2].
We present FeCo layers grown by dc-magnetron sputtering in Ar atmosphere on Si substrates and having a thickness, t, ranging from 5 nm up to 100 nm. The layers were covered with a 5 nm Cr layer to protect them from oxidation. Room temperature magnetization (M) measurements were performed using a SQUID magnetometer with a magnetic field (H) applied in the plane of the film. The dependence of the three reduced M, m=M/Msat, components (in plane and parallel to H; in plane and perpendicular to H (mt); out of plane) as a function of H was accessed at room temperature, for different in-plane directions, d ̂, of the applied field, using a MOKE apparatus for vector magnetometry [3]. Samples resistance was measured with the four probes method at room temperature whilst s was measured using an optical profilometer with 1μm lateral and 1 nm vertical resolution.
For t ≤ 20 nm, the shape of the in-plane M loops is squared and the coercivity increases with t for t up to 15 nm. This increase is possibly ascribable to the change in grain size; however, the coercivity values are larger than those expected for the bcc FeCo alloy. For t = 5 nm mt is about 0.1, and it decreases with increasing t, indicating that in-plane anisotropy changes slightly with d ̂. For t > 20 nm, coercivity smoothly decreases and the shape of the loops changes, as the approach to saturation is slower and the shape of the whole loop gets less and less squared. The large coercivity values suggest the presence of an in-plane tensile stress, and profilometry measurements support this hypothesis. s is found to be always positive, and its value monotonically decreases with t. This result supports the fact that for large t values coercivity decreases. The s decrease for large t values, besides being due to thickness increase, could be also due to the partial development of a compressive stress due to structural defects [4]. Indeed, resistivity measurements show that samples resistivity increases with t (the resistivity of the sample with t = 50 nm is nearly four times larger than that of the sample with t = 5 nm), thus supporting the conclusion that the presence of structural defects may increases with t. The partial development of a compressive stress could induce an out-of-plane magnetic anisotropy, thus explaining the slower approach to saturation observed for t > 20 nm. These results will be discussed and compared to MOKE vector magnetometry data.
[1] L. Platt et al, J. Appl. Phys. 88 (2000) 2058.
[2] P. Zou et al., J. Appl. Phys. 91 (2002) 7830.
[3] P. Vavassori, App. Phys. Lett. 77 (2000) 1605.
[4] T. Pienkos et al., Microel. Eng. 70 (2003) 442
Re-entrant antiferromagnetism in the exchange-coupled IrMn/NiFe system
No full textWe have studied the magnetic exchange coupling in an antiferromagnetic(AFM)/ferromagnetic(FM) IrMn[10 nm]/NiFe[5 nm] bilayer and in arrays of dots with different size (1000, 500, 300 nm). The coupling mechanism is governed by the AFM spins at the interface between the FM phase and the bulk of the AFM layer. They exhibit a sort of re-entrant antiferromagnetic behavior --- passing from a disordered frozen collective regime (T<100 K) to a high-temperature one, where they are polarized by the bulk AFM spins --- determining the strength of their torque action on the FM spins
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