The mass technological revolution, exploded in the mid XX century, has invaded the global market and completely changed the way of making scientific research. In this contest, the efforts of Materials Science, Physics and Engineering are today prevalently devoted to seek innovative solutions in the new generation devices scalability within different economical macro-areas (i.e. electronics, energy, transports etc.). A main issue is related to the increasing request of multi-functionality, or rather the possibility of joining different and independent "functions" in a sole physical component. To this wide category also belong multiferroic materials, which are principal actors of my Ph.D activity. Multiferroism is defined as the coexistence of two or more primary ferroic orders in the matter: namely (anti-)ferromagnetism, ferroelectricity, ferroelasticity. Particularly, if a material shows superposition and interplay between the magnetic and the electric order parameters, it is specifically called magnetoelectric multiferroic. Although magnetoelectric multiferroic-based devices are considered innovative solutions for different technological fields (namely data storage, spintronics, electric and magnetic field multi-sensing, electronics), nowadays a large-scale application is not started yet. This is due to three fundamental reasons: the lack of natural compounds combined with the difficulty to obtain them artificially; the low temperature occurrence of multiferroic behavior (well below room temperature); and the extremely weak magnetoelectric coupling. If the first drawback can be in principle overtaken by means of unconventional synthesis techniques, the last two are complementary criticalities, since an improvement of the former usually implies a worsening of the latter and vice versa. Moreover, another fundamental problem adds to the previous ones, i.e., the huge experimental complexity of performing specific combined electric and magnetic characterization. I engaged this intricate situation using a double approach. Since the first months of activity I worked to establish a self-standing laboratory committed to the experimental characterization of multiferroic magnetoelectric properties. In particular, standard magnetometric techniques and standard electrical measurements techniques have been assembled in a unique platform (based on a "simple" SQuID magnetometer), to properly perform combined electric and magnetic investigations. Moreover, a high-voltage setup for ferroelectric characterization, equipped with the AIXACCT TF-Analyzer 2000, has been optimized and tested, allowing to study also bulk samples with non-dielectric properties (just like many multiferroics). Beside this experimental work, an equal effort was devoted to the production and characterization of novel bulk systems with potential multiferroic magnetoelectric character, specifically by means of HP/HT solid state reactions In four years, I stabilized more than ten single-phase compounds belonging to the perovskite ABO3 class. Due to its large tolerance, perovskite lattice enables a variegate number of chemical substitutions and structural distortions. In these materials, magnetism and ferroelectricity derive from independent mechanisms; ferroelectricity is induced exploiting the stereochemical effect of Bi3+ or Pb2+ ions on the A-site of perovskite, while magnetism is promoted by the introduction of two different III-IV period metal cations (i.e. Cr, Mn, Fe, Co, Cu; Mo) on the B-site of perovskite. Systems obtained by these chemical substitutions, are usually called double perovskites, with general formula A2BB'O6. The choice of a double substitution on the B and B'-site can be explained considering that it may allow a lowering of the space and time-symmetry (operation that in some cases contributes to the coexistence of magnetic and electric order); on the other hand the presence of different magnetic interactions usually promotes high Curie temperatures despite an enhancement of the system complexity. BFMO in particular revealed intriguing, although unusual, properties requiring magnetometric, structural, ferroelectric and magnetoelectric characterization to investigate its overall physical behavior. BFMO displayed a highly distorted cell with a strong compositional inhomogeneity involving the spatial distribution of iron and manganese; it showed coexistence of a RT antiferromagnetic order (TN = 288 K) and ferroelectricity, which is irreversibly induced by an external DC electric bias (just below the semiconductor-to-insulator onset, occurring at TP = 140 K). In addition, some interesting evidences of magnetoelectric coupling were highlighted by means of our combined magnetic/electric techniques, such as the observation of magnetic ordering-induced changes of the transport properties, the occurrence of magnetocapacitance effects and the detection of a tuning of the magnetization thermal dependence under a DC electric bias. Especially the latter experimental outcome unequivocally promotes BFMO as a possible bulk multiferroic magnetoelectric compound. Despite such preeminent results, BFMO gave also the chance to study exotic phenomenologies subsidiary to multiferroism and magnetoelectricity but incredibly fascinating, specifically: - the thermal activated field-dependent spontaneous magnetization reversal process; - the Mott's Variable Range Hopping transport mechanism characterized by 1D conductance. These two mechanisms were deeply investigated since nowadays a general consensus on their interpretation is still lacking. The presented data allowed to describe them as disorder-related phenomena, pointing out the crucial role played by composition inhomogeneity in the spatial distribution of iron and manganese ions. All these aspects, together with many others less relevant, are deeply treated in my Ph.D thesis, whose writing want to be a tribute to me and to my scientific effort, but mainly to all the people who collaborate with me during these years.
Combined magnetic, electric, ferroelectric and magnetoelectric characterization of novel multiferroic perovskites obtained by high pressure/temperature synthesis / Delmonte, D. - (18/03/2015).
Combined magnetic, electric, ferroelectric and magnetoelectric characterization of novel multiferroic perovskites obtained by high pressure/temperature synthesis
Delmonte D
18/03/2015
Abstract
The mass technological revolution, exploded in the mid XX century, has invaded the global market and completely changed the way of making scientific research. In this contest, the efforts of Materials Science, Physics and Engineering are today prevalently devoted to seek innovative solutions in the new generation devices scalability within different economical macro-areas (i.e. electronics, energy, transports etc.). A main issue is related to the increasing request of multi-functionality, or rather the possibility of joining different and independent "functions" in a sole physical component. To this wide category also belong multiferroic materials, which are principal actors of my Ph.D activity. Multiferroism is defined as the coexistence of two or more primary ferroic orders in the matter: namely (anti-)ferromagnetism, ferroelectricity, ferroelasticity. Particularly, if a material shows superposition and interplay between the magnetic and the electric order parameters, it is specifically called magnetoelectric multiferroic. Although magnetoelectric multiferroic-based devices are considered innovative solutions for different technological fields (namely data storage, spintronics, electric and magnetic field multi-sensing, electronics), nowadays a large-scale application is not started yet. This is due to three fundamental reasons: the lack of natural compounds combined with the difficulty to obtain them artificially; the low temperature occurrence of multiferroic behavior (well below room temperature); and the extremely weak magnetoelectric coupling. If the first drawback can be in principle overtaken by means of unconventional synthesis techniques, the last two are complementary criticalities, since an improvement of the former usually implies a worsening of the latter and vice versa. Moreover, another fundamental problem adds to the previous ones, i.e., the huge experimental complexity of performing specific combined electric and magnetic characterization. I engaged this intricate situation using a double approach. Since the first months of activity I worked to establish a self-standing laboratory committed to the experimental characterization of multiferroic magnetoelectric properties. In particular, standard magnetometric techniques and standard electrical measurements techniques have been assembled in a unique platform (based on a "simple" SQuID magnetometer), to properly perform combined electric and magnetic investigations. Moreover, a high-voltage setup for ferroelectric characterization, equipped with the AIXACCT TF-Analyzer 2000, has been optimized and tested, allowing to study also bulk samples with non-dielectric properties (just like many multiferroics). Beside this experimental work, an equal effort was devoted to the production and characterization of novel bulk systems with potential multiferroic magnetoelectric character, specifically by means of HP/HT solid state reactions In four years, I stabilized more than ten single-phase compounds belonging to the perovskite ABO3 class. Due to its large tolerance, perovskite lattice enables a variegate number of chemical substitutions and structural distortions. In these materials, magnetism and ferroelectricity derive from independent mechanisms; ferroelectricity is induced exploiting the stereochemical effect of Bi3+ or Pb2+ ions on the A-site of perovskite, while magnetism is promoted by the introduction of two different III-IV period metal cations (i.e. Cr, Mn, Fe, Co, Cu; Mo) on the B-site of perovskite. Systems obtained by these chemical substitutions, are usually called double perovskites, with general formula A2BB'O6. The choice of a double substitution on the B and B'-site can be explained considering that it may allow a lowering of the space and time-symmetry (operation that in some cases contributes to the coexistence of magnetic and electric order); on the other hand the presence of different magnetic interactions usually promotes high Curie temperatures despite an enhancement of the system complexity. BFMO in particular revealed intriguing, although unusual, properties requiring magnetometric, structural, ferroelectric and magnetoelectric characterization to investigate its overall physical behavior. BFMO displayed a highly distorted cell with a strong compositional inhomogeneity involving the spatial distribution of iron and manganese; it showed coexistence of a RT antiferromagnetic order (TN = 288 K) and ferroelectricity, which is irreversibly induced by an external DC electric bias (just below the semiconductor-to-insulator onset, occurring at TP = 140 K). In addition, some interesting evidences of magnetoelectric coupling were highlighted by means of our combined magnetic/electric techniques, such as the observation of magnetic ordering-induced changes of the transport properties, the occurrence of magnetocapacitance effects and the detection of a tuning of the magnetization thermal dependence under a DC electric bias. Especially the latter experimental outcome unequivocally promotes BFMO as a possible bulk multiferroic magnetoelectric compound. Despite such preeminent results, BFMO gave also the chance to study exotic phenomenologies subsidiary to multiferroism and magnetoelectricity but incredibly fascinating, specifically: - the thermal activated field-dependent spontaneous magnetization reversal process; - the Mott's Variable Range Hopping transport mechanism characterized by 1D conductance. These two mechanisms were deeply investigated since nowadays a general consensus on their interpretation is still lacking. The presented data allowed to describe them as disorder-related phenomena, pointing out the crucial role played by composition inhomogeneity in the spatial distribution of iron and manganese ions. All these aspects, together with many others less relevant, are deeply treated in my Ph.D thesis, whose writing want to be a tribute to me and to my scientific effort, but mainly to all the people who collaborate with me during these years.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
