Xavier Moya

Inverse magnetocaloric effect in ferromagnetic Ni-Mn-Sn alloys.

The magnetocaloric effect (MCE) in paramagnetic materials has been widely used for attaining very low temperatures by applying a magnetic field isothermally and removing it adiabatically. The effect can also be exploited for room-temperature refrigeration by using giant MCE materials1,2,3. Here we report on an inverse situation in Ni–Mn–Sn alloys, whereby applying a magnetic field adiabatically, rather than removing it, causes the sample to cool. This has been known to occur in some intermetallic compounds, for which a moderate entropy increase can be induced when a field is applied, thus giving rise to an inverse magnetocaloric effect4,5. However, the entropy change found for some ferromagnetic Ni–Mn–Sn alloys is just as large as that reported for giant MCE materials, but with opposite sign. The giant inverse MCE has its origin in a martensitic phase transformation that modifies the magnetic exchange interactions through the change in the lattice parameters.

Caloric materials near ferroic phase transitions

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.

Ferroelectric Control of Spin Polarization

Spin into Control Spintronics—the use of the spin direction of subatomic particles to control on and off states, instead of electric charge—has the potential to create low-power electronics, because less energy is needed to flip spin states than to flip switches to create voltage barriers. Theoretical work hints that spin-polarized electrons from a ferromagnetic electrode can be controlled by a change in polarization created in a ferroelectric thin film. Garcia et al. (p. 1106, published online 14 January) fabricated an iron-barium titanate junction on a lanthanum strontium manganate substrate that acts as a spin detector. Local control of spin polarization was observed in the ferroelectric layer, which retained its polarization without any applied power. Ferroelectric tunnel junctions control the spin polarization of electrons emitted from iron electrodes. 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.

A ferroelectric memristor

Memristors are continuously tunable resistors that emulate biological synapses. Conceptualized in the 1970s, they traditionally operate by voltage-induced displacements of matter, although the details of the mechanism remain under debate. Purely electronic memristors based on well-established physical phenomena with albeit modest resistance changes have also emerged. Here we demonstrate that voltage-controlled domain configurations in ferroelectric tunnel barriers yield memristive behaviour with resistance variations exceeding two orders of magnitude and a 10 ns operation speed. Using models of ferroelectric-domain nucleation and growth, we explain the quasi-continuous resistance variations and derive a simple analytical expression for the memristive effect. Our results suggest new opportunities for ferroelectrics as the hardware basis of future neuromorphic computational architectures.

Magnetic superelasticity and inverse magnetocaloric effect in Ni-Mn-In

Applying a magnetic field to a ferromagnetic Ni{sub 50}Mn{sub 34}In{sub 16} alloy in the martensitic state induces a structural phase transition to the austenitic state. This is accompanied by a strain which recovers on removing the magnetic field, giving the system a magnetically superelastic character. A further property of this alloy is that it also shows the inverse magnetocaloric effect. The magnetic superelasticity and the inverse magnetocaloric effect in Ni-Mn-In and their association with the first-order structural transition are studied by magnetization, strain, and neutron-diffraction studies under magnetic field.

Interface-induced room-temperature multiferroicity in BaTiO3

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.

Giant Electrocaloric Strength in Single‐Crystal BaTiO3

Over the last fi fteen years, the discovery of giant magnetocaloric effects near room-temperature phase transitions in various magnetic materials [ 1 , 2 ] has led to suggestions of energy-effi cient and environmentally friendly household and industrial refrigeration. However, these large changes in isothermal entropy Δ S and adiabatic temperature Δ T require large changes in magnetic fi eld Δ H , which are challenging to generate economically. In contrast, it is straightforward to generate changes in electric fi eld Δ E in order to drive electrocaloric (EC) effects near ferroelectric phase transitions. Recently, giant EC effects near nominally second-order transitions have been reported in ferroelectric thin fi lms, [ 3 , 4 ] as thin fi lms can support large driving fi elds. However, two issues arise as follows. Firstly, measurements of heat Q and temperature change Δ T are typically indirect [ 3 , 4 ] as the direct measurement of fi lms is challenging. There is thus scope for error (e.g., because the possible role of thermal and electrical hysteresis is typically ignored). Secondly, the EC effects in fi lms are disproportionately small with respect to the large driving fi elds, and so EC strengths |Q |/| E | and | T |/| E | tend to be relatively small. Here we address both of these issues by presenting direct measurements of both Q and Δ T in single-crystal BaTiO 3 (BTO) near the ferroelectric phase transition at Curie temperature T C . We fi nd EC strengths |Q |/| E | and | T |/ | E | that are giant because the fi rst-order ferroelectric phase transition is very sharp. The observed EC effects are reversible at any temperature above the hysteretic transition regime. Giant EC strengths near sharp fi rst-order phase transitions with a large latent heat could therefore contribute to the future development of cooling devices with a high frequency of operation.

Giant and reversible extrinsic magnetocaloric effects in La0.7Ca0.3MnO3 films due to strain

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.

Effect of Co and Fe on the inverse magnetocaloric properties of Ni-Mn-Sn

At certain compositions Ni-Mn-X Heusler alloys (X: group IIIA–VA elements) undergo martensitic transformations, and many of them exhibit inverse magnetocaloric effects. In alloys where X is Sn, the isothermal entropy change is largest among the Heusler alloys, particularly in Ni50Mn37Sn13, where it reaches a value of 20 J kg−1 K−1 for a field of 5 T. We substitute Ni with Fe and Co in this alloy, each in amounts of 1 and 3 at % to perturb the electronic concentration and examine the resulting changes in the magnetocaloric properties. Increasing both Fe and Co concentrations causes the martensitic transition temperature to decrease, whereby the substitution by Co at both compositions or substituting 1 at % Fe leads to a decrease in the magnetocaloric effect. On the other hand, the magnetocaloric effect in the alloy with 3 at % Fe leads to an increase in the value of the entropy change to about 30 J kg−1 K−1 at 5 T.

Cooling and heating by adiabatic magnetization in the Ni50Mn34In16 magnetic shape memory alloy

We report on measurements of the adiabatic temperature change in the inverse magnetocaloric Ni50Mn34In16 alloy. It is shown that this alloy heats up with the application of a magnetic field around the Curie point due to the conventional magnetocaloric effect. In contrast, the inverse magnetocaloric effect associated with the martensitic transition results in the unusual decrease of temperature by adiabatic magnetization. We also provide magnetization and specific heat data which enable to compare the measured temperature changes to the values indirectly computed from thermodynamic relationships. Good agreement is obtained for the conventional effect at the second-order paramagnetic-ferromagnetic phase transition. However, at the first-order structural transition the measured values at high fields are lower than the computed ones. Irreversible thermodynamics arguments are given to show that such a discrepancy is due to the irreversibility of the first-order martensitic transition.