Rong-Rong Wu

Giant negative thermal expansion in bonded MnCoGe-based compounds with Ni2In-type hexagonal structure.

MnCoGe-based compounds undergo a giant negative thermal expansion (NTE) during the martensitic structural transition from Ni2In-type hexagonal to TiNiSi-type orthorhombic structure. High-resolution neutron diffraction experiments revealed that the expansion of unit cell volume can be as large as ΔV/V ∼ 3.9%. The optimized compositions with concurrent magnetic and structural transitions have been studied for magnetocaloric effect. However, these materials have not been considered as NTE materials partially due to the limited temperature window of phase transition. The as-prepared MnCoGe-based compounds are quite brittle and naturally collapse into powders. By using a few percents (3-4%) of epoxy to bond the powders, we introduced residual stress in the bonded samples and thus realized the broadening of structural transition by utilizing the specific characteristics of lattice softening enforced by the stress. As a result, giant NTE (not only the linear NTE coefficient α but also the operation-temperature window) has been achieved. For example, the average α̅ as much as -51.5 × 10(-6)/K with an operating temperature window as wide as 210 K from 122 to 332 K has been observed in a bonded MnCo0.98Cr0.02Ge compound. Moreover, in the region between 250 and 305 K near room temperature, the α value (-119 × 10(-6)/K) remains nearly independent of temperature. Such an excellent performance exceeds that of most other materials reported previously, suggesting it can potentially be used as a NTE material, particularly for compensating the materials with large positive thermal expansions.

Evolution of magnetostructural transition and magnetocaloric effect with Al doping in MnCoGe1?xAlx compounds

The effect of Al doping in MnCoGe1−xAlx compounds has been investigated. The substitution of Al for Ge enhances Mn–Mn covalent bonding by shortening the distance of nearest Mn atom layers, and thus stabilizes the hexagonal structure. As a result, first-order magnetostructural transition between ferromagnetic martensite and paramagnetic austenite takes place for the optimized compositions (x = 0.01, 0.02). Accompanied with the magnetostructural transition, large magnetocaloric effect (MCE) is observed. More doping of Al(x = 0.03, 0.04) leads to the separation of magnetic and structural transitions and remarkable reduction of MCE.

Giant barocaloric effect in hexagonal Ni2In-type Mn-Co-Ge-In compounds around room temperature.

The most widespread cooling techniques based on gas compression/expansion encounter environmental problems. Thus, tremendous effort has been dedicated to develop alternative cooling technique and search for solid state materials that show large caloric effects. An application of pressure to a material can cause a change in temperature, which is called the barocaloric effect. Here we report the giant barocaloric effect in a hexagonal Ni2In-type MnCoGe0.99In0.01 compound involving magnetostructural transformation, Tmstr, which is accompanied with a big difference in the internal energy due to a great negative lattice expansion(ΔV/Vu2009~u20093.9%). High resolution neutron diffraction experiments reveal that the hydrostatic pressure can push the Tmstr to a lower temperature at a rate of 7.7u2009K/kbar, resulting in a giant barocaloric effect. The entropy change under a moderate pressure of 3u2009kbar reaches 52u2009Jkg−1K−1, which exceeds that of most materials, including the reported giant magnetocaloric effect driven by 5u2009T magnetic field that is available only by superconducting magnets.

Abnormal percolative transport and colossal electroresistance induced by anisotropic strain in (011)-Pr 0.7 (Ca 0.6 Sr 0.4 ) 0.3 MnO 3 / PMN-PT heterostructure

Abnormal percolative transport in inhomogeneous systems has drawn increasing interests due to its deviation from the conventional percolation picture. However, its nature is still ambiguous partly due to the difficulty in obtaining controllable abnormal percolative transport behaviors. Here, we report the first observation of electric-field-controlled abnormal percolative transport in (011)-Pr0.7(Ca0.6Sr0.4)0.3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 heterostructure. By introducing an electric-field-induced in-plane anisotropic strain-field in a phase separated PCSMO film, we stimulate a significant inverse thermal hysteresis (~ -17.5u2005K) and positive colossal electroresistance (~11460%), which is found to be crucially orientation-dependent and completely inconsistent with the well accepted conventional percolation picture. Further investigations reveal that such abnormal inverse hysteresis is strongly related to the preferential formation of ferromagnetic metallic domains caused by in-plane anisotropic strain-field. Meanwhile, it is found that the positive colossal electroresistance should be ascribed to the coactions between the anisotropic strain and the polarization effect from the poling of the substrate which leads to orientation and bias-polarity dependencies for the colossal electroresistance. This work unambiguously evidences the indispensable role of the anisotropic strain-field in driving the abnormal percolative transport and provides a new perspective for well understanding the percolation mechanism in inhomogeneous systems.

Anisotropic modulation of magnetic properties and the memory effect in a wide-band (011)-Pr 0.7 Sr 0.3 MnO 3 /PMN-PT heterostructure

Memory effect of electric-field control on magnetic behavior in magnetoelectric composite heterostructures has been a topic of interest for a long time. Although the piezostrain and its transfer across the interface of ferroelectric/ferromagnetic films are known to be important in realizing magnetoelectric coupling, the underlying mechanism for nonvolatile modulation of magnetic behaviors remains a challenge. Here, we report on the electric-field control of magnetic properties in wide-band (011)-Pr0.7Sr0.3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 heterostructures. By introducing an electric-field-induced in-plane anisotropic strain field during the cooling process from room temperature, we observe an in-plane anisotropic, nonvolatile modulation of magnetic properties in a wide-band Pr0.7Sr0.3MnO3 film at low temperatures. We attribute this anisotropic memory effect to the preferential seeding and growth of ferromagnetic (FM) domains under the anisotropic strain field. In addition, we find that the anisotropic, nonvolatile modulation of magnetic properties gradually diminishes as the temperature approaches FM transition, indicating that the nonvolatile memory effect is temperature dependent. By taking into account the competition between thermal energy and the potential barrier of the metastable magnetic state induced by the anisotropic strain field, this distinct memory effect is well explained, which provides a promising approach for designing novel electric-writing magnetic memories.

Effect of substitution of In for Co on magnetostructural coupling and magnetocaloric effect in MnCo1-xInxGe compounds

Effect of replacement of Co by In with larger atomic radius but fewer valence numbers on magnetostructural coupling and magnetocaloric effect is studied in MnCo1-xInxGe compounds. The substitution of Co by a small amount of In (1.5%–2%) can shift martensitic transformation Tstru to lower temperature and make it overlap with Curie temperature TC. As a result, magnetostructural coupling is created and large entropy change (ΔS) takes place. Further increasing In content to xu2009=u20090.03 leads to decoupling, but the martensitic transition (Tstru ∼ 249u2009K) is still close to the magnetic transition (TcAu2009∼u2009269u2009K). As a result, two close ΔS peaks appear. Mechanism related to different large entropy change in the coupled and decoupled samples are discussed.

Corrigendum: Critical dependence of magnetostructural coupling and magnetocaloric effect on particle size in Mn-Fe-Ni-Ge compounds.

Magnetostructural coupling, which is the coincidence of crystallographic and magnetic transition, has obtained intense attention for its abundant magnetoresponse effects and promising technological applications, such as solid-state refrigeration, magnetic actuators and sensors. The hexagonal Ni2In-type compounds have attracted much attraction due to the strong magnetostructural coupling and the resulted giant negative thermal expansion and magnetocaloric effect. However, the as-prepared samples are quite brittle and naturally collapse into powders. Here, we report the effect of particle size on the magnetostructural coupling and magnetocaloric effect in the Ni2In-type Mn-Fe-Ni-Ge compound, which undergoes a large lattice change across the transformation from paramagnetic austenite to ferromagnetic martensite. The disappearance of martensitic transformation in a large amount of austenitic phase with reducing particle size, to our best knowledge, has not been reported up to now. The ratio can be as high as 40.6% when the MnNi0.8Fe0.2Ge bulk was broken into particles in the size range of 5~15u2009μm. Meanwhile, the remained magnetostructural transition gets wider and the magnetic hysteresis becomes smaller. As a result, the entropy change drops, but the effective cooling power RCeffe increases and attains to the maximum at particles in the range of 20~40u2009μm. These observations provide constructive information and highly benefit practical applications for this class of novel magnetoresponse materials.

Magnetic properties and magnetocaloric effects of GdxEr1-xGa (0 <= x <= 1) compounds

We carefully studied the magnetic properties and magnetocaloric effect of GdxEr1-xGa (0u2009≤u2009xu2009≤u20091) compounds. The GdxEr1-xGa compounds undergo two magnetic transitions with temperature increasing: spin-reorientation or antiferromagnetic-to-ferromagnetic (FM) transition and FM-to-paramagnetic transition. As the content of Gd increases from 0 to 1, the transition temperature in low temperature region changes from 15u2009K to 66u2009K and the Curie temperature increases obviously from 30u2009K to 181.9u2009K. Although the maximum value of magnetic entropy change (ΔSM) for GdxEr1−xGa decreases with the increase of x, the refrigerant capacity (RC) improves remarkably compared with that of ErGa compound. Table-like ΔSM curves are observed for the compounds with xu2009=u20090.1, 0.2, 0.3, and 0.4, which are very useful for real cooling applications. And Gd0.2Er0.8Ga and Gd0.3Er0.7Ga compounds show better magnetocaloric features than others in this series under considerations of both ΔSM and RC. The results of this series of compounds sho...

Effect of epitaxial strain on small-polaron hopping conduction in Pr0.7(Ca0.6Sr0.4)0.3MnO3 thin films

We investigated the epitaxial strain effect on the small-polaron hopping conduction properties in Pr0.7(Ca,Sr)0.3MnO3 (PCSMO) films. An increase in the carrier localization, as evidenced by the enhancement of the small-polaron activation energy EA in the high temperature region, was obtained by increasing the epitaxial lattice strain in either the tensile or compressive strained film. Furthermore, it was found that the magnitude of EA, and thus the carrier localization, strongly depends on the sign of the lattice strain, which explains the diverse percolative transport behaviors in PCSMO films with different types of strains. Meanwhile, similar dependencies on the strain of the films were also obtained for the electron-phonon interaction, characterized by the calculated small-polaron coupling constant. Our results reveal that the type of lattice strain plays a crucial role in determining the degree of localization of charge carriers and the electron-phonon coupling strength, which is important for underst...

Textured PrCo5 nanoflakes with large coercivity prepared by low power surfactant-assisted ball milling

The effect of the milling time on the structure, morphology, coercivity, and remanence ratio of textured PrCo5 nanoflakes produced by low power surfactant-assisted ball milling (SABM) was investigated. The X-ray powder diffraction (XRD) patterns indicate that the SABM PrCo5 samples are all CaCu5-type hexagonal structure. The average grain size is smaller than 10u2009nm when the SABM time is equal to or longer than 5.5u2009h. The thickness of nanoflakes is mainly in the range of 50−100 nm while the length is 0.5−5u2009μm when the SABM time reaches 8u2009h. For the field-aligned PrCo5 nanoflakes, the out-of-plane texture is indicated from the increasing (0 0 l) peaks in the XRD patterns, and the easy magnetization direction is perpendicular to the flake surface. The strong texture of PrCo5 nanoflakes leads to a large coercivity Hc (7.8 kOe) and obvious anisotropic magnetic behaviors for the aligned samples.