Controlling ion migration of transition metal oxide(TMO) to recast crystal structure and macroscopic properties is current research emphasis [1, 2], which has potential applications in solid fuel cells, resistance switching etc. Among cobalt-based TMO, B-site atom determines functionality, but oxygen atom is hard to ignored. Electronic structure of B-site atom is profoundly affected by oxygen content due to coupling of lattice, charge, orbit, and spin. Vacuum annealing is a stable method to extract O2--ions [3, 4]. 40nm La0.5Sr0.5CoO3(P-LSCO) film are grown on LSAT(100) substrate. Then films are respectively annealed at 400 centigrade for 2 hours(B-LSCO) and 500 centigrade for 35 hours(S-LSCO) to extract O2--ions. Fig. 1a shows soft X-ray absorption spectrum(XAS) for cobalt L-edge, and an obvious peak shift to low energy (L3-peak of S, B and P-LSCO appear at 782.2eV, 781.5eV and 780.4eV respectively) indicates continuous decrease in average valence states of Co and oxygen content during O2--ions extracting process. For XAS of oxygen K-edge (Fig. 1b), the A and C peaks, representing hybridization of Co-2p/3d and O-2p orbital, gradually decrease in three samples. While B-peak, which represents content of oxygen vacancy, exhibits an increasing trend, consistent with above results. As a result, dramatic magnetism transformation was observed in film upon annealing. Fig. 1c and 1d show magnetic moment-temperature(M-T) curves and magnetic moment- magnetic field (M-H) curves for three samples, demonstrating ferromagnetism(FM) with Tc~220K, antiferromagnetism(AFM) and ferromagnetism with Tc&gt;300K in P-LSCO, B-LSCO and S-LSCO, respectively. In sum, we achieved giant magnetic transformation from FM to AFM to FM accompanying with O2--ion migration in LSCO film by vacuum annealing, which provides idea for regulating physics properties by ion migration engineering.

![](https://assets.underline.io/uploads/markdown_image/1/image/2f9d13eeec602e127c33a77c98f889f5.jpg)

Fig. 1 XAS for a) cobalt L-edge, b) oxygen K-edge, c) M-T curve, d) M-H curve

Oxygen ion migration regulates magnetism in La0.5Sr0.5CoO3 film

Solid-state refrigeration materials have various thermal effects, among which the hydrostatic pressure is an economical and practical method to drive the barocaloric effect (BCE) of brittle materials[1,2]. The BCE is ubiquitous in first-order phase transition materials, and its packaging technology is not limited to the shape of the materials[3,4]. In the brittle magnetocaloric materials MnCoGe0.99In0.01, there is a low-pressure induced large reversible BCE near room temperature, which avoids part of the refrigeration capacity reduction caused by hysteresis loss[5]. Here, we report the BCE of MnCoGe0.99In0.01 at hydrostatic pressure of 1kbar by variable-pressure calorimetry. Fig.1a shows the temperature dependence of the heat capacity dQ/dT of MnCoGe0.99In0.01 at different hydrostatic pressures after baseline subtraction, it can be found that the thermal hysteresis (ΔThys) at 1bar is 8.8K and the phase transition temperature moves to lower temperature with the increase of pressure. The linear fitting relationship between the transformation temperature shifts with the pressure (dT/dP) is obtained according to the heating and cooling transformation temperature under different pressures, as shown in the inset of Fig. 1b. The dT/dP is -10.6K/kbar in the heating process and is -8.7K/kbar in the cooling process. The entropy curve S(T,P) of MnCoGe0.99In0.01 during heating and cooling process was obtained by the calorimetric curves(Fig. 1a) through the pressure-dependent integrations across the phase transition, as shown in Fig.1b. We evaluated the reversible parts (ΔSrev and ΔTrev) of entropy change ΔS and adiabatic temperature change ΔT at 1kbar from the overlapping of the irreversible ΔS and ΔT during application and removal of pressure, as shown in Fig.1c and 1d, where the shaded areas represent the ΔSrev and ΔTrev, respectively. The relative narrow ΔThys and high driving rate dT/dP under pressure are more conducive to achieve large ΔSrev and ΔTrev. The ΔSrev of 19J/kg K and ΔTrev of 2.9K within a temperature span of 56K at 1kbar have been achieved. The refrigeration capacity of MnCoGe0.99In0.01 at 1kbar is as high as 373J/kg K.

![](https://assets.underline.io/uploads/markdown_image/1/image/f5b2d3fe1b51b27d3f3361529defdc74.jpg)

Fig.1.For MnCoGe0.99In0.01, a)dQ/dT, b)S, c) ΔSrev, d)ΔTrev.

Large room-temperature reversible barocaloric effect in MnCoGe0.99In0.01 alloys at low pressure

Conventional vapor compression refrigeration encounters environmental problems. As an alternative, solid-state refrigeration based on magnetocaloric effect (MCE) or barocaloric effect (BCE) has attracted tremendous attentions for its environmental-friendly and energy-saving superiority. The core of solid-state refrigeration lies in the magnitude of the caloric effect of refrigerants. Increasing the caloric effect as much as possible is a long-term pursuit [1-5]. Here, we report a large enhancement of MCE and BCE by hydrostatic pressure in Co-doped La(Fe,Si)13, which has been regarded to be promising refrigerants since its discovery in 2000[1]. The maximal entropy change SMCE almost doubled under 11.31kbar while the SBCE under 9kbar increases more than three times (Figure 1c and 1d). To disclose the essence from the atomic level, neutron powder diffractions were performed (Figure 1a). The results reveal that hydrostatic pressure sharpens the magnetoelastic transition and enlarges the volume change wherein through impacting the intra-icosahedral Fe-Fe bonds (B1, B2, B3) rather than the inter-icosahedral ones (B4, B5) in the NaZn13-type structure [1]. Such result is distinct from the case introducing chemical pressure by H atoms in the La(Fe,Si)13-based compounds, which can adjust phase transition temperature, but not the sharpness of phase transition and the MCE. To understand the mechanism, first-principles calculations are performed (Figure 1b), which offers a theoretical support for the hydrostatic pressure enhanced magnetovolume effect and the evolution from second-order to first-order transition [1]. The present study proves the enormous potential of enhancing caloric effect through influencing the specific atomic environments by pressure, which is also useful to achieve other pressure-related effects, such as controllable negative thermal expansion.

![](https://assets.underline.io/uploads/markdown_image/1/image/192f971b3efb1d85cc2c7296c0334dc7.jpg)

Hydrostatic Pressure Induced Giant Enhancement of the Solid-state Caloric Effect in the Rare-earth-based Giant Magnetocaloric Materials

Hydrostatic Pressure Induced Giant Enhancement of the Solid-state Caloric Effect in the Rare-earth-based Giant Magnetocaloric Materials

**Session chair:** Jing Wang, The Institute of Physics, Chinese Academy of Sciences


BOC - Magnetoelectronic Materials and Phenomena: Complex Oxides

technical paper

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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2022 Joint MMM-INTERMAG

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

**Session chairs:** SN Piramanayagam, Nanyang Technological University and Radhika Barua, Virginia Commonwealth University



Sun Jirong

3

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Presentations

Oxygen ion migration regulates magnetism in La0.5Sr0.5CoO3 film

Large room-temperature reversible barocaloric effect in MnCoGe0.99In0.01 alloys at low pressure

Hydrostatic Pressure Induced Giant Enhancement of the Solid-state Caloric Effect in the Rare-earth-based Giant Magnetocaloric Materials

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