The Performance Paradox: Understanding the Real Drivers that Critically Affect Outcomes
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Arguably, the most important and most nontrivial magnetic property is coercivity H c. Coercivity tends to increase roughly linearly with the first magnetic anisotropy constant K 1 , but anisotropy is not the only consideration. We achieve this nanostructuring by embedding HfCo 7 nanoparticles in a soft Fe 65 Co 35 phase, where the high-magnetization Fe 65 Co 35 phase improves the net magnetization and the overall permanent-magnet performance energy product of the films.
The fabrication of the nanostructures requires the realization of high magnetocrystalline anisotropy, easy-axis alignment, and a suitable nanostructure with high H c and high M s. The magnetic anisotropy is often reduced in nanoparticles due to disorder and surface effects, and this leads to thermally activated magnetization reversal at high temperatures, which subsequently destroys the permanent-magnet properties 18 , 19 , By contrast, thermally activated reversal of individual nanoparticles is unimportant in the present context, because the magnetic nanoparticles are exchange-coupled in both bulk magnets and thin films.
The fabrication of our nanostructures involves two main aspects: the creation of ensembles of easy-axis aligned hard-magnetic nanoparticles and the formation of a soft-magnetic matrix. The single-step cluster-deposition process used for the sample fabrication is described elsewhere 21 , 22 , and also schematically shown as Fig.
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S1 in Supplementary Information. First, randomly-oriented Hf-Co nanoparticles having stoichiometry close to HfCo 7 are produced in a water-cooled gas-aggregation chamber by sputtering a Co-Hf composite target. The high-anisotropy crystal structure in Hf-Co nanoparticles is achieved during the gas-aggregation process without a subsequent high-temperature anneal, often required in the case of wet-chemical and conventional physical vapor-deposition methods 14 , 17 , 18 , 19 , Thus, in the case of the cluster-deposition method, sintering associated with the annealing process is avoided, and thus the size-distribution and easy-axis alignment can be controlled precisely in the nanocluster-assembled nanostructures.
In particular, the accomplishment of easy-axis alignment of nanoparticles in the gas-phase using a magnetic field before combining with a soft phase is an important processing step to obtain a high remanent magnetization M r , close to M s 21 , 22 , 24 , As compared to the magnetic alignment performed in the solid phase, relatively small fields of about 0. The next step is to control the particle size, composition, crystalline ordering, and easy-axis alignment of the Hf-Co nanoparticles to achieve suitable magnetic properties, especially a high coercivity, which is a precondition to use the nanoparticles as building blocks to fabricate exchange-coupled nanocomposites.
Figure 1a shows a transmission-electron-microscope TEM image of the Hf-Co nanoparticles that we have used to fabricate our nanocomposites. The corresponding particle-size histogram, shown in Fig. The Hf-Co nanoparticles are single-crystalline with a high degree of atomic ordering, as can be seen from the high-resolution TEM image in Fig. Figure 1e shows an EDS line scan measured across a single nanoparticle shown in the inset of Fig. The STEM results reveal a uniform distribution of Hf and Co across the nanoparticles and the nanoparticles have a stoichiometry close to the HfCo 7 phase, which crystallizes in an orthorhombic structure see Fig.
S2 in Supplementary Information for phase identification using x-ray diffraction pattern and is also known to be magnetically anisotropic For the magnetic measurements, the Hf-Co nanoparticles were deposited on Si substrates for extended deposition times to form dense and aligned nanoparticle films.
Figure 1f shows the first- and second-quadrant in-plane hysteresis loops, the magnetization M as a function of the applied magnetic field H , measured at room temperature along the easy- and hard-axis directions see Fig. S3 in Supplementary Information for full hysteresis loops. For comparison, the room-temperature hysteresis loop of an isotropic randomly oriented nanoparticle film is also shown in Fig.
This result clearly indicates an effective easy-axis alignment in Hf-Co nanoparticles. Generally a large saturation filed is required in the case of high-anisotropy nanoparticles, and also for nanoparticles that possess spin canting at interface originating from noncollinear spin structure. However, the pronounced noncollinear spin structures, as frequently observed in oxide nanoparticles and some metal nanoparticles, require competing ferromagnetic and antiferromagnetic exchange interactions In the case of strong Co-rich ferromagnetic materials, for the range of measurement field used in this study, the noncollinearity effects are generally smaller by a factor of — as compared to the anisotropy effect 27 , and thus the observed large saturation field only suggests a fairly high HfCo 7 magnetic anisotropy.
S4 in Supplementary Information. Besides having high K 1 and average particles size of about Normally the coercivity of hard magnetic nanoparticles smaller than the critical single-domain size is governed by localized nucleation at particles' surface In the meantime, the exchange-interactions between the nanoparticles also lower H c.
The exchange interactions created by more dense nanoparticles generally lower H c 28 , 29 , so embedded nanoparticles in a non-magnetic matrix will maintain a relatively high H c 30 , For example, in the aligned HfCo 7 nanoparticles, we may have some degree of exchange interactions between the particles, which decreases H c to 8. However, the aim of this study is to fabricate dense hard-soft nanocomposites for obtaining strong exchange coupling to provide a high M s , while maintaining somewhat reasonable H c.
The Co distribution is not visible in the matrix film due to the black background, but the individual and combined color mappings of Hf and Co shows a Co-rich region at the surface as compared to the core due to the soft Fe-Co addition Fig. Also, x-ray diffraction analysis shows that the soft phase exhibits a body-centered cubic structure similar to that of bulk Fe 65 Co 35 Fig.
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S5 in Supplementary Information. For the samples shown in Fig. In this nanostructure, the easy-axis aligned Hf-Co nanoparticles are dispersed in an Fe-Co matrix. Except for fields near H c , the magnetization of the soft phase is parallel to the easy axis of the aligned hard nanoparticles, due to effective exchange coupling. This magnetic structure is confirmed by the room-temperature hysteresis loops measured along the easy-axis direction for the nanocomposite films.
Figure 3b shows single-phase in-plane hysteresis loops for all considered volume fractions of the soft phase. S6 in Supplementary Information. Note that the room-temperature initial magnetization curves also were measured along the easy-axis direction for Hf-Co and Hf-Co:Fe-Co nanocomposite films and the results reveal a nucleation-type coercivity mechanism Fig. S7 in Supplementary Information. As expected from the volume fractions of the two phases, J s continuously increases from On the other hand, H c initially increases from 8.
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From Fig. The mode obeys where K 1 r and M r are the local anisotropy and local magnetization, respectively. The corresponding nucleation field coercivity is where L , K 1 , M s are the average center-to-center distance, magnetic anisotropy constant, and saturation magnetization of the hard particles, respectively. In order to understand the magnetization reversal, we have also used equation 1 to estimate the nucleation mode numerically for an ideal thin-film model structure, shown in Fig.
The model structure bottom image in Fig. If the nanocomposite films were macroscopic rather than nanoscale, then the nucleation mode would be localized almost entirely in the soft regions and lead to very low H c. However, in small-scale nanostructures, such as the present ones, the mode is largely delocalized—extending into the hard phase—and therefore benefits from the admixture of the anisotropy of the hard grains. This penetration means that the hard phase micromagnetically stabilizes the soft phase and realizes a nearly uniform nucleation mode.
From a simplistic viewpoint, the high anisotropy always wins. However, in reality, Brown's paradox states in effect that H c is usually much smaller than H A. The same applies to the energy product, a key figure of merit that describes the magnet's ability to store magnetostatic energy in free space For example, Fig. The Hf-Co nanoparticles exhibit appreciable coercivities from 8. Note that J r shows a slight increase to 9. This result can be attributed to the presence of about Similar trends are found in the nanocomposites, such as high coercivities of The low temperature coefficients translate into a favorable temperature dependence of the nominal energy product corresponding to the mass of the magnetic materials in the films Fig.
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In addition, the energy product, However, to the best of our knowledge, high-temperature energy products have not been reported for FePt. The excellent high-temperature performance in our Co-rich nanostructures is achieved in spite of the fact that the anisotropy of the hard HfCo 7 nanoparticles is relatively modest, by overcoming Brown's paradox through nanostructuring.
Also, for the first time, the potential of nanoparticles for fabricating high-temperature permanent-magnet materials is demonstrated. Naturally, for bulk magnetic applications, scale-up methods to create such nanostructures remain a significant challenge, but our approach provides useful insights for developing future rare-earth-free permanent magnets.
The aligned Hf-Co nanoparticles and the Hf-Co:Fe-Co nanocomposites were produced using a cluster-deposition method described elsewhere also see the supplementary information S1 22 , Some of the nanoparticles were deposited on carbon-coated copper grids with low coverage densities to measure the average particle size and size distribution using a FEI Tecnai Osiris Scanning transmission electron microscope.
The mass or nominal thickness of the films was measured using a quartz-crystal thickness monitor. Kuch, W. Magnetic nanostructures: Edge atoms do all the work. Nature Mater. Reiss, G. Magnetic nanoparticles: Applications beyond data storage. Sellmyer, D. Applied physics: Strong magnets by self-assembly. Nature , — Zhang, Z. Evidence of intrinsic ferromagnetism in individual dilute magnetic semiconducting nanostructures. Nature Nanotech. Lee, J. Exchange-coupled magnetic nanoparticles for efficient heat induction. Brown, W. Micromagnetics Wiley, New York, Aharoni, A.
Theoretical search for domain nucleation. Theory of nucleation fields in inhomogeneous ferromagnets. Givord, D.
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Skomski, R. Matter 15 , R— Permanent Magnetism Institute of Physics, Bristol, Kneller, E. The exchange-spring magnet: a new material principle for permanent magnets. IEEE Trans. Giant energy product in nanostructured two-phase magnets.
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B 48 , — Poudyal, N. Advances in nanostructured permanent magnet research. D: Appl. Coey, J. Permanent magnets: Plugging the gap. Scripta Materialia 67 , — Jones, N. Paradoxical engagement strategies cause tensions, and those targeted by these engagement strategies need to be supported and cared for. Our study explores these issues in an investigation of the enactment of an engagement strategy at a UK Health charity pseudonym of HealthOrg ; infused with tensions arising from paradoxical demands to raise service quality and cut costs.
We highlight the importance of an ethic of care Gilligan in shaping the explicit resourcing of structures and practices that enable workers and managers to cope with corresponding tensions. It was important for them to have opportunities to talk about and engage with tensions at play—providing psychological safety to express doubts and anxieties with peers and managers, and to agree on ways to manage these.
These discursive practices are shown to positively frame experiences of engagement, consistent with earlier research Kahn In contrast, we also observe in our data, the negative implications for workers and managers confronted with recasting of priorities, when they do not feel their managers are listening or providing opportunities for dialogue in order to help them work through the consequences of paradoxical demands. Our paper is structured into four sections. In section one, we present the theoretical background to our study, leading to a set of research questions.
In section two, we explain the context of our study and methods. In section three, we detail the results of our investigation, followed by two sections that provide a discussion of the theoretical and practical significance of our findings including limitations. In the final section, we conclude the paper and provide suggestions for future research. Each of the strands of our theoretical basis for the study is now presented.
Moments of engagement are characterized by the simultaneous investment of physical, emotional and intellectual energies, shaped by three psychological conditions: the extent to which employees feel a sense of meaningfulness , psychological safety and availability of resources to engage in work-related tasks.