Combined magnetic, electric, ferroelectric and magnetoelectric characterization of novel multiferroic perovskites obtained by high pressure/temperature synthesis

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.La rivoluzione tecnologica di massa, esplosa a metà del XX secolo, ha invaso profondamente il mercato globale cambiando il modo di fare ricerca scientifica. In questo contesto, le attenzioni della Scienza dei Materiali, della Fisica e dell’ Ingegneria si sono intensamente rivolte alla scoperta di soluzioni alternative di mercato, attraverso lo sviluppo di dispositivi sempre più performanti da proporre in diverse macroaree economiche (elettronica, energia, ambiente, infrastrutture e trasporti). In particolare, un aspetto rilevante è rappresentato dalla continua e crescente richiesta di multifunzionalità tecnologica, ovvero la possibilità di eseguire differenti funzioni attraverso un unico componente fisico. A questa sterminata categoria appartengono di diritto i materiali multiferroici, che rappresentano il cuore della mia attività di ricerca di dottorato. Il multiferroismo è una proprietà fisica definita come la coesistenza di due o più ordini ferroici primari nella materia: ovvero (anti-)ferromagnetismo, ferroelettricità e ferroelasticità. In aggiunta alla coesistenza, un materiale multiferroico che mostri accoppiamento tra i parametri di ordine elettrico e magnetico prende il nome di multiferroico magnetoelettrico. Sebbene la possibilità di sfruttare dispositivi a base di componenti multiferroici magnetoelettrici rappresenti una straordinaria innovazione in diversi settori (memorie magnetiche di nuova generazione, spintronica, sensoristica di campo elettrico e magnetico ed elettronica) ad oggi non si è ancora registrato un loro sostanziale avvento nel mercato. Ciò è dipeso da tre motivi fondamentali: la scarsità di composti naturali che mostrino questa proprietà (ed, insieme, l’enorme difficoltà di produrli artificiale); le basse temperature di ordinamento multiferroico (ben al di sotto di temperatura ambiente) che ne limitano l’applicazione su larga scala; i debolissimi coefficienti di accoppiamento magnetoelettrico finora osservati. Se tuttavia il primo problema può essere in linea di principio sorpassato mediante l’utilizzo di tecniche di sintesi non convenzionali, le altre due criticità appaiono invece fortemente correlate. A tutto questo va aggiunto un’ulteriore problematica rappresentata dalla difficoltà di eseguire un corretto studio delle proprietà magnetoelettriche per vincoli essenzialmente di natura strumentale. Io ho affrontato tutti questi problemi sfruttando un duplice approccio a cavallo di due piani scientifici distinti. Sin da primo mese di attività di ricerca, ho lavorato per sviluppare un laboratorio completamente dedicato alla caratterizzazione di materiali multiferroici magnetoelettrici. La sfida è stata quella di unire le funzionalità di un laboratorio standard di magnetometria a quelle di un laboratorio classico di studio delle proprietà elettriche. Per quanto questa possa sembrare un’operazione semplice, essa non la è affatto, ne sia di prova il fatto che, nonostante il grande di numero di pubblicazioni scientifiche sui materiali multiferroici, sono rari gli articoli in cui sia riportata una convincente caratterizzazione magnetoelettrica. Nella maggior parte dei casi lo studio della magnetoelettricità non viene nemmeno affrontato. Dopo tre anni di lavoro, siamo riusciti nell’intento di assemblare una piattaforma multifunzionale, basata su un “semplice” magnetometro SQuID, che consente di effettuare la caratterizzazione combinata, magnetico, elettrica e magnetoelettrica dei materiali multiferroici. Parallelamente abbiamo implementato il sistema di misura AIXACCT TF-Analyzer 2000, equipaggiandolo con strumenti volti ad estenderne le potenzialità. Attraverso questa apparato è possibile studiare le proprietà ferroelettriche di materiali anche non idealmente dielettrici, quali sono spesso i multiferroici. Per superare le criticità legate alla deficienza di materiali multiferroici naturali, la scelta è stata quella di ottenere il multiferroismo all’interno della struttura cristallografica della perovksite ABO3, sfruttandone le straordinarie doti tolleranza a sostituzioni chimiche e distorsioni strutturali che questo reticolo cristallino. In questi materiali, il magnetismo e la ferroelettricità originano da diversi meccanismi fisici, indipendenti tra di loro. La ferroelettricità viene indotta mediante l’effetto stereochimico dello ione Bi3+ o Pb2+ .posizionato sul sito A della perovskite che attraverso un’ibridizzazione direzionale dei legami con gli ossigeni circostanti, può causare la rottura di simmetria per inversione spaziale. Parallelamente il magnetismo viene promosso dall’introduzione di due differenti cationi magnetici del III e del IV periodo (Cr, Mn, Fe, Co, Cu; Mo) sul sito B della perovskite. La classe di materiali così ottenuta prende il nome di classe delle perovskiti doppie (A2BB’O6). La decisione di operare una doppia sostituzione sul sito B si spiega proprio a fronte delle criticità fisiche menzionate prima: da un lato la presenza di disordine composizionale, causato dalla presenza di differenti ioni sul sito B della perovskite, può aiutare ad abbassare della simmetria strutturale; dall’altro lato uno schema di differenti interazioni magnetiche di superscambio sono di solito responsabili dell’innalzamento delle temperature critiche ma anche della complessità magnetica. Più di dieci composti a struttura perovskitica doppia sono stati sintetizzati in questi anni di tirocinio sulla base di queste sostituzioni chimiche. Fra tutti questi, BiFe0.5Mn0.5O3 (BFMO) si è rivelato quello di gran lunga più interessante. Lo studio delle proprietà magnetiche, strutturali, ferroelettriche e magnetoelettriche ha consentito di verificare la coesistenza di antiferromagnetismo ambientale (TN = 288 K) con una ferroelettricità irreversibile indotta da un bias di tensione elettrica in continua (al di sotto della transizione semiconduttore-isolante localizzata a TP = 140 K). Ulteriori studi sulle proprietà magnetoelettriche, effettuati attraverso l’utilizzo delle tecniche combinate di caratterizzazione magnetica ed elettrica, hanno permesso di osservare: (a) una trasformazione delle proprietà di trasporto a seguito dello spontaneo ordinamento magnetico del materiale; (b) la presenza di magneto-capacità indotta dalla struttura polare (c) un effetto di tuning della suscettività magnetica in funzione della temperatura per mezzo dell’applicazione di tensione elettrica in continua sul materiale. In particolare quest’ultimo risultato sperimentale ha dimostrato che BFMO, inequivocabilmente, è uno dei primi materiali bulk multiferroici magnetoelettrici. Oltre a questi risultati fondamentali, che rappresentavano l’obbiettivo principale della mia attività di ricerca, BFMO ha consentito di studiare una serie di proprietà esotiche, sussidiarie al multiferroismo e alla magnetoelettricità, ma al contempo incredibilmente affascinanti. Le principali sono: - il meccanismo di inversione spontanea della magnetizzazione termicamente attivato e dipendente dal campo magnetico, - il meccanismo di trasporto elettrico sulla base dell’inusuale modello di Mott’s Variable Range Hopping, caratterizzato da un tipo di conduzione monodimensionale Questi due fenomeni sono stati approfonditamente esplorati soprattutto perché, allo stato dell’arte, mancavano di una chiara e condivisa interpretazione scientifica. Lo studio sperimentale ha mostrato quanto entrambi i fenomeni risultino essere fortemente dipendenti dal disordine composizionale sul sito B della perovskite, individuando il ruolo chiave svolto dalla presenza di disomogeneità nella distribuzione spaziale degli ioni ferro e manganese. Ognuno di questi aspetti, qui soltanto introdotti, e tanti altri di minore rilevanza sono approfonditamente trattati all’interno della Tesi, la quale è stata scritta anche in tributo alle persone che in questi anni hanno collaborato con me

Crystal structure and ferroelectric properties of ϵ-Ga2O3 films grown on (0001)-sapphire

The crystal structure and ferroelectric properties of ϵ-Ga2O3 deposited by low-temperature MOCVD on (0001)-sapphire were investigated by single-crystal X-ray diffraction and the dynamic hysteresis measurement technique. A thorough investigation of this relatively unknown polymorph of Ga2O3 showed that it is composed of layers of both octahedrally and tetrahedrally coordinated Ga3+ sites, which appear to be occupied with a 66% probability. The refinement of the crystal structure in the noncentrosymmetric space group P63mc pointed out the presence of uncompensated electrical dipoles suggesting ferroelectric properties, which were finally demonstrated by independent measurements of the ferroelectric hysteresis. A clear epitaxial relation is observed with respect to the c-oriented sapphire substrate, with the Ga2O3 [10-10] direction being parallel to the Al2O3 direction [11-20], yielding a lattice mismatch of about 4.1%

Weak ferromagnetism and spin reorientation in antiferroelectric BiCrO3

BiCrO3 is an antiferroelectric perovskite known to exhibit an unconventional spin reorientation transition between antiferromagnetic structures, accompanied by a large jump in weak ferromagnetism. Using a combination of neutron powder diffraction, magnetometry, and symmetry analysis, we confirm the dominant G-type antiferromagnetic order below TN = 111 K and identify the magnetic phase transition with a spontaneous rotation of Cr3+ moments from the b axis to a particular direction in the ac plane. We demonstrate the role of antiferroelectric displacements produced by the Bi3+ lone-pair electrons and octahedral rotations in establishing spin canting via the antisymmetric Dzyaloshinskii-Moriya interaction. This mechanism results in weak ferromagnetism above and below the spin-reorientation and explains the dramatic increase in net magnetization on cooling

Thermally activated magnetization reversal in bulk BiFe0.5Mn0.5O3

We report on the synthesis and characterization of BiFe0.5Mn0.5O3, a potential type-I multiferroic compound displaying temperature induced magnetization reversal. Bulk samples were obtained by means of solid state reaction carried out under the application of hydrostatic pressure at 6 GPa and 1100{\deg}C. The crystal structure is an highly distorted perovskite with no cation order on the B site, where, besides a complex scheme of tilt and rotations of the TM-O6 octahedra, large off-centering of the bismuth ions is detected. Below T1 = 420 K the compound undergoes a first weak ferromagnetic transition related to the ordering of iron rich clusters. At lower temperatures (just below RT) two distinct thermally activated mechanisms are superimposed, inducing at first an enhancement of the magnetization at T2 = 288 K, then a spontaneous reversal process centered at T3 = 250 K, finally giving rise to a negative response. The application of fields higher than 1500 Oe suppresses the process, yielding a ferromagnetic like behaviour. The complementary use of SQuID magnetometry and M\"ossbauer spectroscopy allowed the interpretation of the overall magnetic behaviour in terms of an uncompensated weak competitive coupling between non-equivalent clusters of interactions characterized by different critical temperatures and resultant magnetizations. PACS numbers: 75.85.+t, 75.60.Jk, 76.80.+y, 75.30.Et, 75.30.KzComment: 30 pages, 13 figure

Action Mechanisms of the Secondary Metabolite Euplotin C: Signaling and Functional Role in Euplotes

Among secondary metabolites, the acetylated hemiacetal sesquiterpene euplotin C has been isolated from the marine, ciliated protist Euplotes crassus, and provides an effective mechanism for reducing populations of potential competitors through its cytotoxic properties. However, intracellular signaling mechanisms and their functional correlates mediating the ecological role of euplotin C are largely unknown. We report here that, in E. vannus (an Euplotes morphospecies which does not produce euplotin C and shares with E. crasssus the same interstitial habitat), euplotin C rapidly increases the intracellular concentration of both Ca2+ and Na+, suggesting a generalized effect of this metabolite on cation transport systems. In addition, euplotin C does not induce oxidative stress, but modulates the electrical properties of E. vannus through an increase of the amplitude of graded action potentials. These events parallel the disassembling of the ciliary structures, the inhibition of cell motility, the occurrence of aberrant cytoplasmic vacuoles, and the rapid inhibition of phagocytic activity. Euplotin C also increases lysosomal pH and decreases lysosomal membrane stability of E. vannus. These results suggest that euplotin C exerts a marked disruption of those homeostatic mechanisms whose efficiency represents the essential prerequisite to face the challenges of the interstitial environmental.L'articolo è disponibile sul sito dell'editore http://onlinelibrary.wiley.com

First experimental evidences of the ferroelectric nature of struvite

Struvite (MgNH4PO4·6H2O) is a mineral first identified in 1845. It is tested for several reasons: (1) it is a problem in liquid wastewater treatment plants; (2) on the other hand, it is recovered from this wastewater because of phosphorus, magnesium, and nitrogen; (3) it is the main component of infectious urinary stones. In this paper, we present the first experimental evidences of the ferroelectric nature of struvite at room temperature. Struvite shows a hysteresis loop and spontaneous electric polarization that can be reversed by an application of an external electric field. The measured value of residual polarization of struvite is equal to 0.95 μC/cm2. We also report observations of the ferroelectric domains in struvite using birefringence imaging technique. The non-centrosymmetry of the crystal lattice is confirmed with the use of the Kurtz−Perry powder test. The second harmonic generation response for struvite in relation to that of potassium dihydrogen phosphate is 0.36. We suggest that ferroelectric properties for struvite, in particular, spontaneous polarization, can have a significant impact on the behavior of struvite in aqueous solutions, such as liquid wastewater or urine

A novel primary human immunodeficiency due to deficiency in the WASP-interacting protein WIP

A female offspring of consanguineous parents, showed features of Wiskott-Aldrich syndrome (WAS), including recurrent infections, eczema, thrombocytopenia, defective T cell proliferation and chemotaxis, and impaired natural killer cell function. Cells from this patient had undetectable WAS protein (WASP), but normal WAS sequence and messenger RNA levels. WASP interacting protein (WIP), which stabilizes WASP, was also undetectable. A homozygous c.1301C>G stop codon mutation was found in the WIPF1 gene, which encodes WIP. Introduction of WIP into the patient’s T cells restored WASP expression. These findings indicate that WIP deficiency should be suspected in patients with features of WAS in whom WAS sequence and mRNA levels are normal

Nationwide consensus on the clinical management of treatment-resistant depression in Italy: a Delphi panel

Background: Treatment-resistant depression (TRD) is defined by the European Medicines Agency as a lack of clinically meaningful improvement after treatment, with at least two different antidepressants. Individual, familiar, and socio-economic burden of TRD is huge. Given the lack of clear guidelines, the large variability of TRD approaches across different countries and the availability of new medications to meet the need of effective and rapid acting therapeutic strategies, it is important to understand the consensus regarding the clinical characteristics and treatment pathways of patients with TRD in Italian routine clinical practice, particularly in view of the recent availability of esketamine nasal spray. Methods: A Delphi questionnaire with 17 statements (with a 7 points Likert scale for agreement) was administered via a customized web-based platform to Italian psychiatrists with at least 5 years of experience and specific expertise in the field of depression. In the second-round physicians were asked to answer the same statements considering the interquartile range of each question as an index of their colleagues' responses. Stata 16.1 software was used for the analyses. Results: Sixty panellists, representative of the Italian territory, answered the questionnaire at the first round. For 8/17 statements more than 75% of panellists reached agreement and a high consensus as they assigned similar scores; for 4 statements the panellists assigned similar scores but in the middle of the Likert scale showing a moderate agreement with the statement, while for 5 statements there was indecision in the agreement and low consensus with the statement. Conclusions: This Delphi Panel showed that there is a wide heterogeneity in Italy in the management of TRD patients, and a compelling need of standardised strategies and treatments specifically approved for TRD. A high level of consensus and agreement was obtained about the importance of adding lithium and/or antipsychotics as augmentation therapies and in the meantime about the need for long-term maintenance therapy. A high level of consensus and agreement was equally reached for the identification of esketamine nasal spray as the best option for TRD patients and for the possibility to administrate without difficulties esketamine in a community outpatient setting, highlighting the benefit of an appropriate educational support for patients

High Pressure Induced Insulator-to-Semimetal Transition through Intersite Charge Transfer in NaMn7O12

The pressure-dependent behaviour of NaMn7O12 (up to 40 GPa) is studied and discussed by means of single-crystal X-ray diffraction and resistance measurements carried out on powdered samples. A transition from thermally activated transport mechanism to semimetal takes place above 18 GPa, accompanied by a change in the compressibility of the system. On the other hand, the crystallographic determinations rule out a symmetry change to be at the origin of the transition, despite all the structural parameters pointing to a symmetrizing effect of pressure. Bond valence sum calculations indicate a charge transfer from the octahedrally coordinated manganese ions to the square planar ones, likely favouring the delocalization of the carriers