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1.
Science ; 370(6518): 797-803, 2020 11 13.
Artículo en Inglés | MEDLINE | ID: mdl-33184207

RESUMEN

Magnetically driven thermal changes in magnetocaloric materials have, for several decades, been exploited to pump heat near room temperature. By contrast, their electrocaloric and mechanocaloric counterparts have only been intensively studied and exploited for little more than a decade. These different caloric strands have recently been unified to yield a single field of research that could help combat climate change by generating better heat pumps for both cooling and heating. Here we outline the timeliness of the present activity and discuss recent advances in caloric measurements, materials, and prototypes.

2.
Nat Commun ; 11(1): 3190, 2020 Jun 24.
Artículo en Inglés | MEDLINE | ID: mdl-32581280

RESUMEN

Epitaxial films may be released from growth substrates and transferred to structurally and chemically incompatible substrates, but epitaxial films of transition metal perovskite oxides have not been transferred to electroactive substrates for voltage control of their myriad functional properties. Here we demonstrate good strain transmission at the incoherent interface between a strain-released film of epitaxially grown ferromagnetic La0.7Sr0.3MnO3 and an electroactive substrate of ferroelectric 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 in a different crystallographic orientation. Our strain-mediated magnetoelectric coupling compares well with respect to epitaxial heterostructures, where the epitaxy responsible for strong coupling can degrade film magnetization via strain and dislocations. Moreover, the electrical switching of magnetic anisotropy is repeatable and non-volatile. High-resolution magnetic vector maps reveal that micromagnetic behaviour is governed by electrically controlled strain and cracks in the film. Our demonstration should inspire others to control the physical/chemical properties in strain-released epitaxial oxide films by using electroactive substrates to impart strain via non-epitaxial interfaces.

3.
Nanoscale ; 12(9): 5652-5657, 2020 Mar 07.
Artículo en Inglés | MEDLINE | ID: mdl-32101212

RESUMEN

Using photoemission electron microscopy (PEEM) to image ferromagnetism in polycrystalline Ni disks, and ferroelectricity in their single-crystal BaTiO3 substrates, we find that voltage-driven 90° ferroelectric domain switching serves to reversibly annihilate each magnetic vortex via uniaxial compressive strain, and that the orientation of the resulting bi-domain reveals the chirality of the annihilated vortex. Micromagnetic simulations reveal that only 60% of this strain is required for annihilation. Voltage control of magnetic vortices is novel, and should be energetically favourable with respect to the use of a magnetic field or an electrical current. In future, stray field from bi-domains could be exploited to read vortex chirality. Given that core polarity can already be read via stray field, our work represents a step towards four-state low-power memory applications.

4.
Nature ; 575(7783): 468-472, 2019 11.
Artículo en Inglés | MEDLINE | ID: mdl-31597164

RESUMEN

Heat pumps based on magnetocaloric and electrocaloric working bodies-in which entropic phase transitions are driven by changes of magnetic and electric field, respectively-use displaceable fluids to establish relatively large temperature spans between loads to be cooled and heat sinks1,2. However, the performance of prototypes is limited because practical magnetocaloric working bodies driven by permanent magnets3-5 and electrocaloric working bodies driven by voltage6-16 display temperature changes of less than 3 kelvin. Here we show that high-quality multilayer capacitors of PbSc0.5Ta0.5O3 display large electrocaloric effects over a wide range of starting temperatures when the first-order ferroelectric phase transition is driven supercritically (as verified by Landau theory) above the Curie temperature of 290 kelvin by electric fields of 29.0 volts per micrometre. Changes of temperature in the large central area of the capacitor peak at 5.5 kelvin near room temperature and exceed 3 kelvin for starting temperatures that span 176 kelvin (complete thermalization would reduce these values from 5.5 to 3.3 kelvin and from 176 to 73 kelvin). If magnetocaloric working bodies were to be replaced with multilayer capacitors of PbSc0.5Ta0.5O3, then the established design principles behind magnetocaloric heat pumps could be repurposed for better performance without bulky and expensive permanent magnets.

5.
Nat Mater ; 18(8): 840-845, 2019 08.
Artículo en Inglés | MEDLINE | ID: mdl-31110346

RESUMEN

Large changes in the magnetization of ferromagnetic films can be electrically driven by non-180° ferroelectric domain switching in underlying substrates, but the shear components of the strains that mediate these magnetoelectric effects have not been considered so far. Here we reveal the presence of these shear strains in a polycrystalline film of Ni on a 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 substrate in the pseudo-cubic (011)pc orientation. Although vibrating sample magnetometry records giant magnetoelectric effects that are consistent with the hitherto expected 90° rotations of a global magnetic easy axis, high-resolution vector maps of magnetization (constructed from photoemission electron microscopy data, with contrast from X-ray magnetic circular dichroism) reveal that the local magnetization typically rotates through smaller angles of 62-84°. This shortfall with respect to 90° is a consequence of the shear strain associated with ferroelectric domain switching. The non-orthogonality represents both a challenge and an opportunity for the development and miniaturization of magnetoelectric devices.

6.
Nat Commun ; 10(1): 1803, 2019 04 18.
Artículo en Inglés | MEDLINE | ID: mdl-31000715

RESUMEN

There is currently great interest in replacing the harmful volatile hydrofluorocarbon fluids used in refrigeration and air-conditioning with solid materials that display magnetocaloric, electrocaloric or mechanocaloric effects. However, the field-driven thermal changes in all of these caloric materials fall short with respect to their fluid counterparts. Here we show that plastic crystals of neopentylglycol (CH3)2C(CH2OH)2 display extremely large pressure-driven thermal changes near room temperature due to molecular reconfiguration, that these changes outperform those observed in any type of caloric material, and that these changes are comparable with those exploited commercially in hydrofluorocarbons. Our discovery of colossal barocaloric effects in a plastic crystal should bring barocaloric materials to the forefront of research and development in order to achieve safe environmentally friendly cooling without compromising performance.

7.
Nat Commun ; 9(1): 1827, 2018 05 08.
Artículo en Inglés | MEDLINE | ID: mdl-29739924

RESUMEN

Materials that show large and reversible electrically driven thermal changes near phase transitions have been proposed for cooling applications, but energy efficiency has barely been explored. Here we reveal that most of the work done to drive representative electrocaloric cycles does not pump heat and may therefore be recovered. Initially, we recover 75-80% of the work done each time BaTiO3-based multilayer capacitors drive electrocaloric effects in each other via an inductor (diodes prevent electrical resonance while heat flows after each charge transfer). For a prototype refrigerator with 24 such capacitors, recovering 65% of the work done to drive electrocaloric effects increases the coefficient of performance by a factor of 2.9. The coefficient of performance is subsequently increased by reducing the pumped heat and recovering more work. Our strategy mitigates the advantage held by magnetocaloric prototypes that exploit automatic energy recovery, and should be mandatory in future electrocaloric cooling devices.

8.
Phys Rev Lett ; 117(14): 147201, 2016 Sep 30.
Artículo en Inglés | MEDLINE | ID: mdl-27740785

RESUMEN

We report a spin valve with a few-layer graphene flake bridging highly spin-polarized La_{0.67}Sr_{0.33}MnO_{3} electrodes, whose surfaces are kept clean during lithographic definition. Sharp magnetic switching is verified using photoemission electron microscopy with x-ray magnetic circular dichroism contrast. A naturally occurring high interfacial resistance ∼12 MΩ facilitates spin injection, and a large resistive switching (0.8 MΩ at 10 K) implies a 70-130 µm spin diffusion length that exceeds previous values obtained with sharp-switching electrodes.

9.
Philos Trans A Math Phys Eng Sci ; 374(2074)2016 Aug 13.
Artículo en Inglés | MEDLINE | ID: mdl-27402930

RESUMEN

Electrocaloric (EC) effects are typically studied near phase transitions in ceramic and polymer materials. Here, we investigate EC effects in an inorganic salt, namely ammonium sulfate (NH4)2SO4, with an order-disorder transition whose onset occurs at 223 K on cooling. For a single crystal thinned to 50 µm, we use a Maxwell relation to find a large isothermal entropy change of 30 J K(-1) kg(-1) in response to a field change of 400 kV cm(-1) The Clausius-Clapeyron equation implies a corresponding adiabatic temperature change of 4.5 K.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

10.
Nat Commun ; 6: 8801, 2015 Nov 26.
Artículo en Inglés | MEDLINE | ID: mdl-26607989

RESUMEN

Caloric effects are currently under intense study due to the prospect of environment-friendly cooling applications. Most of the research is centred on large magnetocaloric effects and large electrocaloric effects, but the former require large magnetic fields that are challenging to generate economically and the latter require large electric fields that can only be applied without breakdown in thin samples. Here we use small changes in hydrostatic pressure to drive giant inverse barocaloric effects near the ferrielectric phase transition in ammonium sulphate. We find barocaloric effects and strengths that exceed those previously observed near magnetostructural phase transitions in magnetic materials. Our findings should therefore inspire the discovery of giant barocaloric effects in a wide range of unexplored ferroelectric materials, ultimately leading to barocaloric cooling devices.

11.
Nanoscale ; 7(17): 8058-61, 2015 May 07.
Artículo en Inglés | MEDLINE | ID: mdl-25871851

RESUMEN

Magnetoelectric 0-1 composites comprising CoFe2O4 (CFO) nanoparticles in a polyvinylidene fluoride (PVDF) polymer-fibre matrix have been prepared by electrospinning. The average diameter of the electrospun composite fibres is ∼325 nm, independent of the nanoparticle content, and the amount of the crystalline polar ß phase is strongly enhanced when compared to pure PVDF polymer fibres. The piezoelectric response of these electroactive nanofibres is modified by an applied magnetic field, thus evidencing the magnetoelectric character of the CFO/PVDF 0-1 composites.

12.
Nat Mater ; 13(5): 439-50, 2014 May.
Artículo en Inglés | MEDLINE | ID: mdl-24751772

RESUMEN

A magnetically, electrically or mechanically responsive material can undergo significant thermal changes near a ferroic phase transition when its order parameter is modified by the conjugate applied field. The resulting magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects are compared here in terms of history, experimental method, performance and prospective cooling applications.


Asunto(s)
Compuestos de Hierro/química , Calor , Fenómenos Magnéticos , Transición de Fase , Termodinámica
13.
Nat Commun ; 5: 3439, 2014 Mar 12.
Artículo en Inglés | MEDLINE | ID: mdl-24619137

RESUMEN

A characteristic feature of spin ice is its apparent violation of the third law of thermodynamics. This leads to a number of interesting properties including the emergence of an effective vacuum for magnetic monopoles and their currents - magnetricity. Here we add a new dimension to the experimental study of spin ice by fabricating thin epitaxial films of Dy2Ti2O7, varying between 5 and 60 monolayers on an inert substrate. The films show the distinctive characteristics of spin ice at temperatures >2 K, but at lower temperature we find evidence of a zero entropy state. This restoration of the third law in spin ice thin films is consistent with a predicted strain-induced ordering of a very unusual type, previously discussed for analogous electrical systems. Our results show how the physics of frustrated pyrochlore magnets such as spin ice may be significantly modified in thin-film samples.

14.
Nat Commun ; 4: 1453, 2013.
Artículo en Inglés | MEDLINE | ID: mdl-23385597

RESUMEN

Repeatable magnetization reversal under purely electrical control remains the outstanding goal in magnetoelectrics. Here we use magnetic force microscopy to study a commercially manufactured multilayer capacitor that displays strain-mediated coupling between magnetostrictive Ni electrodes and piezoelectric BaTiO(3)-based dielectric layers. In an electrode exposed by polishing approximately normal to the layers, we find a perpendicularly magnetized feature that exhibits non-volatile electrically driven repeatable magnetization reversal with no applied magnetic field. Using micromagnetic modelling, we interpret this nominally full magnetization reversal in terms of a dynamic precession that is triggered by strain from voltage-driven ferroelectric switching that is fast and reversible. The anisotropy field responsible for the perpendicular magnetization is reversed by the electrically driven magnetic switching, which is, therefore, repeatable. Our demonstration of non-volatile magnetic switching via volatile ferroelectric switching may inspire the design of fatigue-free devices for electric-write magnetic-read data storage.

15.
Nat Mater ; 12(1): 52-8, 2013 Jan.
Artículo en Inglés | MEDLINE | ID: mdl-23104152

RESUMEN

Large thermal changes driven by a magnetic field have been proposed for environmentally friendly energy-efficient refrigeration, but only a few materials that suffer hysteresis show these giant magnetocaloric effects. Here we create giant and reversible extrinsic magnetocaloric effects in epitaxial films of the ferromagnetic manganite La(0.7)Ca(0.3)MnO(3) using strain-mediated feedback from BaTiO(3) substrates near a first-order structural phase transition. Our findings should inspire the discovery of giant magnetocaloric effects in a wide range of magnetic materials, and the parallel development of nanostructured bulk samples for practical applications.

16.
Nat Mater ; 10(10): 753-8, 2011 Oct.
Artículo en Inglés | MEDLINE | ID: mdl-21857674

RESUMEN

Multiferroic materials possess two or more ferroic orders but have not been exploited in devices owing to the scarcity of room-temperature examples. Those that are ferromagnetic and ferroelectric have potential applications in multi-state data storage if the ferroic orders switch independently, or in electric-field controlled spintronics if the magnetoelectric coupling is strong. Future applications could also exploit toroidal moments and optical effects that arise from the simultaneous breaking of time-reversal and space-inversion symmetries. Here, we use soft X-ray resonant magnetic scattering and piezoresponse force microscopy to reveal that, at the interface with Fe or Co, ultrathin films of the archetypal ferroelectric BaTiO3 simultaneously possess a magnetization and a polarization that are both spontaneous and hysteretic at room temperature. Ab initio calculations of realistic interface structures provide insight into the origin of the induced moments and bring support to this new approach for creating room-temperature multiferroics.

17.
Nat Mater ; 9(9): 688; author reply 689, 2010 Sep.
Artículo en Inglés | MEDLINE | ID: mdl-20733597
18.
Science ; 327(5969): 1106-10, 2010 Feb 26.
Artículo en Inglés | MEDLINE | ID: mdl-20075211

RESUMEN

A current drawback of spintronics is the large power that is usually required for magnetic writing, in contrast with nanoelectronics, which relies on "zero-current," gate-controlled operations. Efforts have been made to control the spin-relaxation rate, the Curie temperature, or the magnetic anisotropy with a gate voltage, but these effects are usually small and volatile. We used ferroelectric tunnel junctions with ferromagnetic electrodes to demonstrate local, large, and nonvolatile control of carrier spin polarization by electrically switching ferroelectric polarization. Our results represent a giant type of interfacial magnetoelectric coupling and suggest a low-power approach for spin-based information control.

19.
J Phys Condens Matter ; 22(27): 275602, 2010 Jul 14.
Artículo en Inglés | MEDLINE | ID: mdl-21399261

RESUMEN

For an epitaxial Pr(0.48)Ca(0.52)MnO(3) film on NdGaO(3), we use transmission electron microscopy to observe a 'charge-ordered' superlattice along the in-plane direction a. The same film shows no electrical signatures of charge order. The in-plane electrical anisotropy ρ(a)/ρ(c) = 28 is constant, and there is no evidence of sliding charge density waves up to the large field of ∼10(3) V cm(-1).


Asunto(s)
Calcio/química , Compuestos de Manganeso/química , Óxidos/química , Praseodimio/química , Anisotropía , Electroquímica/métodos , Electrónica , Microscopía de Fuerza Atómica/métodos , Microscopía Electrónica de Transmisión/métodos , Conformación Molecular , Física/métodos , Presión , Propiedades de Superficie
20.
Enferm Infecc Microbiol Clin ; 8(10): 602-5, 1990 Dec.
Artículo en Español | MEDLINE | ID: mdl-2098117

RESUMEN

Clinical aspects of 10 consecutive patients hospitalized for acute human leptospirosis, confirmed by serology using the microscopic agglutination test (MAT), between 1975 and 1988 in a 1000 beds Teaching Hospital are retrospectively analyzed. All of them were male, mean age of 55, presenting a suggestive epidemiological data to be at risk for Leptospira infection. Leptospira icterohaemorrhagiae was responsible for 8 cases and Leptospira canicola for 2. 7 patients were treated, after 3-4 days of admittance, with penicillin. 5 patients died. Cause of death was gastrointestinal haemorrhage in 3 and cardiogenic shock in 2. We discuss the clinical, biochemical, pathological and therapeutic aspects of the acute human leptospirosis.


Asunto(s)
Leptospirosis/diagnóstico , Anciano , Humanos , Leptospirosis/tratamiento farmacológico , Masculino , Persona de Mediana Edad , Estudios Retrospectivos
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