Manipulation of magnetic anisotropy in nanostructures

Of late, the magnetic properties of micro/nano-structures have attracted intense research interest both fundamentally and technologically particularly to address the question that how the manipulation in the different layers of nanostructures, geometry of a patterned structure can affect the overall magnetic properties, while generating novel applications such as in magnetic sensors, storage devices, integrated inductive components and spintronic devices. Depending on the applications, materials with high, medium or low magnetic anisotropy and their possible manipulation are required. The most dramatic manifestation in this respect is the chance to manipulate the magnetic anisotropy over the intrinsic preferential direction of the magnetization, which can open up more functionality particularly for device applications. Types of magnetic anisotropies of different nanostructured materials and their manipulation techniques are investigated in this work. Detail experimental methods for the quantitative determination of magnetic anisotropy in nanomodulated Ni45Fe55 thin film are studied. Magnetic field induced in-plane rotations within the nanomodulated Ni45Fe55 continuous films revealed various rotational symmetries of magnetic anisotropy due to dipolar interactions showing a crossover from lower to higher fold of symmetry as a function of modulation geometry. In a second approach, the control of exchange anisotropy at ferromagnetic (FM) – aniferomagnetic (AFM) interface in multifferoic nanocomposite materials, where two different phase/types of materials were simultaneously synthesized, was investigated. The third part of this work was to study the electroplated thin films of metal alloy nanocomposite for enhanced exchange anisotropy. In this work a unique observation of an anti-clock wise as well as a clock wise hysteresis loop formation in the Ni,Fe solid solution with very low coercivity and large positive exchange anisotropy/exchange bias have been investigated. Hence, controllable positive and negative exchange anisotropy has been observed for the first time which has high potential applications such as in MRAM devices

Temporal order of interaction directs native assembly of the mammalian signal recognition particle

Assembly of most ribonucleoproteins (RNPs) proceeds via multi-step structural reorganization in both protein and RNA components. To date, essentially all assembly models for RNP have tacitly assumed that structural interaction between any two given components during an RNP formation either facilitates later steps in the assembly or has no effect on continued assembly. In contrast to this implicit hypothesis, this work reveals that untimely interaction between components may, in fact, disrupt native formation of an RNP. In the assembly of the mammalian signal recognition particle (SRP), SRP54 interacts with the preformed SRP19-SRP RNA complex to form the native SRP ternary complex. In contrast, if SRP54 interacts with the RNA prior to formation of the native SRP19-SRP RNA complex, these three components interact to form an alternate ternary complex. In this complex, two extended loops in SRP19 cannot natively interact with the RNA. It has been shown that the premature RNA binding by SRP54 alters the SRP19- RNA assembly energy landscape in such a way that an alternate SRP19-RNA folding pathway leading to non-native complex formation becomes kinetically prevalent. In a different instance, it has been found that prior RNA binding by SRP68/72 negatively affects assembly of SRP19 with the SRP RNA, and vice versa. Experimental results support a model in which structural tension between two similar but slightly distinct RNA structures induced by SRP19 and SRP68/72 binding is the origin of such anticooperativity. Overall, this work suggests that the order by which each constituent in an RNP interacts with others during assembly may have a decisive role in formation of the native complex. It has been inferred in this work that idiosyncratic assembly mechanisms, such as cellular compartmentalization and preferred early and late assembly phases, may play critical regulatory roles in preventing order-of-interaction driven misassembly for many multi-component RNPs in the cell

Ordered magnetic dipoles: controlling anisotropy in nanomodulated continuous ferromagnetic films

In this paper, the research focus is how to entangle magnetic dipoles to control/engineer magnetic properties of different devices at a submicron/nano scale. Here, we report the generation of synthetic arrays of tunable magnetic dipoles in a nanomodulated continuous ferromagnetic film. In-plane magnetic field rotations in modulated Ni 45Fe 55 revealed various rotational symmetries of magnetic anisotropy due to dipolar interaction with a crossover from lower to higher fold as a function of modulation geometry. Additionally, the effect of aspect ratio on symmetry shows a novel phase shift of anisotropy, which could be critical to manipulate the overall magnetic properties of the patterned film. The tendency to form vortex is in fact found to be very small, which highlights that the strong coupling between metastable dipoles is more favorable than vortex formation to minimize energy in this nanomodulated structure. This has further been corroborated by the observation of step hysteresis, magnetic force microscopy images of tunable magnetic dipoles, and quantitative micromagnetic simulations. An analytical expression has been derived to estimate the overall anisotropy accurately for nanomodulated film having low magnetocrystaline anisotropy. Derived mathematical expressions based on magnetic dipolar interaction are found to be in good agreement with our results

Maity et al. reply to the comment on “Superspin glass mediated giant spontaneous exchange bias in a nanocomposite of BiFeO3−Bi2Fe4O9”, A. Harres, J. Geshev, and V. Skumryev, Physical Review Letters, 114, 099703 (2015)

In this article we reply to the concerns raised by Harres et al. [Phys. Rev. Lett. 114, 099703 (2015)] about some of the results reported in our original paper [T. Maity et al. Phys. Rev. Lett. 110, 107201 (2013)]. We show that the magnetic hysteresis loops are not minor and both path dependency of exchange bias and presence of superspin glass phase in the nanocomposite are indisputable

Origin of the asymmetric exchange bias in BiFeO3/Bi2Fe4O9 nanocomposite

We show from detailed magnetometry across 2-300 K that the BiFeO3-Bi2Fe4O9 nanocomposite offers a unique spin morphology where superspin glass (SSG) and dilute antiferromagnet in a field (DAFF) coexist at the interface between ferromagnetic Bi2Fe4O9 and antiferromagnetic BiFeO3. The coexisting SSG and DAFF combine to form a local spin texture, which gives rise to a path- dependent exchange bias below the spin freezing temperature (similar to 29 K). The exchange bias varies depending on the protocol or path followed in tracing the hysteresis loop. The exchange bias has been observed below the blocking temperature (T-B) 60 K of the superparamagnetic Bi2Fe4O9. The conventional exchange bias (CEB) increases nonmonotonically as temperature decreases. The magnitude of both exchange bias (H-E) and coercivity (H-C) increase with decrease in temperature and are found to be asymmetric below 20 K depending on the path followed in tracing the hysteresis loop and bias field. The local spin texture at the interface between ferromagnetic and antiferromagnetic particles generates a nonswitchable unidirectional anisotropy along the negative direction of the applied field. The influence of this texture also shows up in " asymmetric" jumps in the hysteresis loop at 2 K, which smears off at higher temperature. The role of the interface spin texture in yielding the path dependency of exchange bias is thus clearly delineated

Asymmetric ascending and descending loop shift exchange bias in Bi2Fe4O9-BiFeO3 nanocomposites

We show detail study of asymmetric exchange bias originating from asymmetric behaviour of ascending and descending loops of the magnetic hysteresis of Bi2Fe4O9-BiFeO3 multiferroics nanocomposites. Detail magnetometry study reveals the co-existence of super spin glass (SSG) and dilute antiferromagnet in a field (DAFF) at the interface between antiferromagnetic (AFM) BiFeO3 and ferromagnetic (FM) Bi2Fe4O9 in nanocomposite particles. The interfacial spins behave differently for positive and negative fields and result into asymmetric exchange bias, which has been precisely identified by several critical magnetic measurements such as training effect, stop & wait protocol, isothermal remanence (IRM) & thermoremanence (TRM) measurements, and high field relaxation measurement. The DAFF spins, which generate non-switchable unidirectional anisotropy at the complex FM-SSG-DAFF-AFM interface below Vogel-Fulcher freezing temperature of BiFeO3 at 29.4 K, are solely responsible for such asymmetry in exchange bias

Superspin Glass Mediated Giant Spontaneous Exchange Bias in a Nanocomposite of BiFeO3_3-Bi2_2Fe4_4O9_9

We observe an enormous $\textit{spontaneous}$ exchange bias ($\sim$300-600 Oe) - measured in an unmagnetized state following zero-field cooling - in a nanocomposite of BiFeO$_3$ ($\sim$94%)-Bi$_2$Fe$_4$O$_9$ ($\sim$6%) over a temperature range 5-300 K. Depending on the path followed in tracing the hysteresis loop - positive (p) or negative (n) - as well as the maximum field applied, the exchange bias ($H_E$) varies significantly with $\mid-H_{Ep}\mid$ $>$ $\mid H_{En}\mid$. The temperature dependence of $H_E$ is nonmonotonic. It increases, initially, till $\sim$150 K and then decreases as the blocking temperature $T_B$ is approched. All these rich features appear to be originating from the spontaneous symmetry breaking and consequent onset of unidirectional anisotropy driven by "superinteraction bias coupling" between ferromagnetic core of Bi$_2$Fe$_4$O$_9$ (of average size $\sim$19 nm) and canted antiferromagnetic structure of BiFeO$_3$ (of average size $\sim$112 nm) via superspin glass moments at the shell.Comment: 5 pages with 4 figures; published in Phys. Rev. Let

Asymmetric shift of exchange bias loop in Ni-Ni(OH)2 core-shell nanoparticles

We report the observation of the asymmetric shift of exchange bias loop in Ni-Ni(OH)2 core-shell nanoparticles where the average size of the ferromagnetic (FM) Ni nanoparticles is ∼30 nm and the thickness of antiferromagnetic (AFM) Ni(OH)2 shell is ∼5nm. The exchange bias (EB) found below Néel temperature (TN∼22 K) of Ni(OH)2 is path dependent, while the coercivity (HC) increases and decreases for positive and negative bias field respectively. In the present case, we found that the inversion symmetry of hysteresis loop is broken and the shift in EB loop is only observed in descending part of the hysteresis loop, which is conspicuous. We demonstrate that the asymmetric shift of EBs in these core-shell nanoparticles is due to the presence of frustrated super spin glass (SSG) at the interface which influences the reversal mechanism of the hysteresis loop. It is argued that the net interface moment from the SSG at the interface of core-shell nanoparticles sets a unidirectional anisotropy after field cooling, which is thought to be the origin of this path dependency of the EB and observed via descending part of the hysteresis loop, ushering potential for novel spin based applications

A Threefold RNA–Protein Interface in the Signal Recognition Particle Gates Native Complex Assembly

Intermediate states play well established roles in the folding and misfolding reactions of individual RNA and protein molecules. In contrast, the roles of transient structural intermediates in multi-component ribonucleoprotein (RNP) assembly processes and their potential for misassembly are largely unexplored. The mammalian signal recognition particle SRP19 protein is unstructured but forms a compact core domain and two extended RNA-binding loops upon binding the SRP RNA. The SRP54 protein subsequently binds to the fully assembled SRP19-RNA complex to form an intimate three-fold interface with both SRP19 and the SRP RNA and without significantly altering the structure of SRP19. We show, however, that the presence of SRP54 during SRP19-SRP RNA assembly dramatically alters the folding energy landscape to create a non-native folding pathway that leads to an aberrant SRP19-RNA conformation. The misassembled complex arises from the surprising ability of SRP54 to bind rapidly to an SRP19-RNA assembly intermediate and to interfere with subsequent folding of one of the SRP19 RNA-binding loops at the three-way protein-RNA interface. An incorrect temporal order of assembly thus readily yields a non-native three-component ribonucleoprotein particle. We propose there may exist a general requirement to regulate the order of interaction in multi-component RNP assembly by spatial or temporal compartmentalization of individual constituents in the cell

Observation of complete inversion of the hysteresis loop in a bimodal magnetic thin film

The existence of inverted hysteresis loops (IHLs) in magnetic materials is still in debate due to the lack of direct evidence and convincing theoretical explanations. Here we report the direct observation and physical interpretation of complete IHL in Ni45Fe55 films with 1 to 2 nm thin Ni3Fe secondary phases at the grain boundaries. The origin of the inverted loop, however, is shown to be due to the exchange bias coupling between Ni45Fe55 and Ni3Fe, which can be broken by the application of a high magnetic field. A large positive exchange bias (HEB=14×HC) is observed in the NiFe composite material giving novel insight into the formation of a noninverted hysteresis loop (non-IHL) and IHL, which depend on the loop tracing field range (HR). The crossover from non-IHL to IHL is found to be at 688 Oe