Tuhin Maity

The coexistence of conventional and inverse magnetocaloric effects (MCEs) at low magnetic fields is highly desirable for broadening the operational temperature window and facilitates self-regulated magnetic refrigeration. Beyond practical implications, the coexistence of MCE provides insights to elucidate the magnetic ground state. However, in most perovskites reported to exhibit such MCE switching, the response is either weak at low fields, suffers from significant thermal hysteresis, or occurs only at high fields, owing to a comparatively strong internal exchange field. Here, we report that with suitable Gd substitution in A-site of La ₂ FeMnO ₆ effectively tunes the competing 3d–3d, 3d–4f and 4f –4f exchange interactions, realizing a robust low-field concurrence of conventional and inverse MCE. The field-tunable entropy compensation point further suggests the potential for intrinsic thermal regulation. Combined DC magnetization, specific-heat, electron-spin-resonance, and first-principles calculations provide a consistent picture in which the 3d–4f exchange interaction generates an internal exchange field that competes with the external Zeeman energy, resulting in the observed entropy-sign switching. These results identify A-site substitution as an effective strategy for engineering the low-field coexistence of conventional and inverse MCE in rare-earth double perovskites.

Quantum spin-liquid (QSL) ground state emerges in frustrated magnetic systems where competing interactions suppress long-range magnetic ordering. TbInO3, a hexagonal perovskite, shows QSL behaviour due to geometrical frustration and strong spin-orbit coupling present in it. Here, we report the strain-driven modification of the magnetic ground state and the enhancement of magnetic frustration at milli-Kelvin (mK) temperature (<1 K) in the TbInO3 epitaxial thin film, grown on MgO (100) substrate. We observe a Curie-Weiss crossover at 1-30 K temperature, strong antiferromagnetic interactions, along with the absence of long-range ordering down to 400 mK, consistent with a QSL ground state. Notably, the thin film exhibits a significantly enhanced effective moment in the mK range compared to the bulk, attributed to strain-induced modification of the exchange pathway among Tb 3+ ions. This impact broadens the QSL state's temperature range to lower temperatures and enhances magnetic frustration in the temperature-field phase space. Further, the first-principles calculation also supports the enhancement of frustration in thin film. These findings demonstrate that strain-induced tuning of exchange interactions can enhance the magnetic frustration, offering a route to stabilise QSL phases down to lower temperatures in hexagonal perovskite thin films.

Thermal conductivity in semiconductors typically decreases at high temperatures due to enhanced phonon-phonon interactions. In contrast, certain magnetic semiconductors, such as chromium nitride (CrN), exhibit an increased thermal conductivity above its Néel temperature, predicted to be caused by strong spin-phonon coupling. However, this hypothesis lacks experimental verification and detailed understanding. Here, we present conclusive experimental evidence showing that the enhanced thermal conductivity in CrN arises from anomalous temperature-dependent acoustic phonon lifetimes, driven by a dynamic coupling between spin fluctuations and acoustic phonons in the system. Temperature-dependent inelastic x-ray scattering measurements reveal that acoustic phonon lifetimes in CrN are strongly suppressed near Néel temperature but anomalously increase at higher temperatures. In contrast, the optical phonon lifetime decreases with rising temperature, akin to a nonmagnetic semiconductor, as it remains unaffected by spin-phonon coupling. This finding unveils the microscopic origin of anomalous heat transport behavior in magnetostructurally coupled materials, offering a pathway for effective thermal management in magnetic material–based devices.

Neodymium-based permanent magnets are fundamental to modern technologies, underpinning high-performance applications in electronics, renewable energy, and advanced medical systems. Among emerging neodymium compounds, neodymium nitride (NdN) has attracted significant attention due to its unique electronic structure, where strongly localized 4f orbitals and strong spin-orbit coupling are anticipated to drive exceptional magnetic behavior. Here, we show conclusive experimental evidence of orbital angular momentum-driven ferromagnetic ordering and prominent magnetic anisotropy in epitaxial, near-stoichiometric NdN thin films synthesized using ultrahigh vacuum deposition techniques. Magnetization and X-ray magnetic circular dichroism measurements reveal a dominant 4f orbital moment of 5.14 μ_B, contributing to a total magnetic moment of 2.43 μ_B per formula unit at 4 K, close to the first-principles density functional theory calculated values. Complementary synchrotron-radiation photoelectron spectroscopy, along with the theoretical calculations, uncovers occupied 4f states ∼6.4 eV below the Fermi level, contributing to the orbital-driven ferromagnetism in NdN. Moreover, the high crystalline quality of the NdN films is further supported by the structural characterization and vibrational properties. The intrinsic orbital angular momentum-driven magnetism of NdN positions it as a promising platform for next-generation orbitronic devices beyond conventional spintronics.

Tuning spin and charge degrees of freedom of complex oxide materials can enable significant advancements in future spintronics. In this study, by three dimensional strain engineering, we demonstrate room temperature ferroelectricity and magnetoelectricity in a vertically aligned nanocomposite thin film structure, composed of vertical nanopillars of SmFeO3 (SFO) embedded within the NiFeO4 (NFO) matrix. A three-dimensional tensile strain is induced in the SFO as a result of the unique film architecture. The tensile strain in SFO produces strong room temperature ferroelectric response instead of the normally very weak ferroelectricity of unstrained SFO, which is an improper ferroelectric. The induced ferroelectricity in SFO enables self-biased magnetoelectric coupling to be achieved between the two phases (magnetoelectric coupling coefficient ∼4 × 10−11 sm−1 at room temperature). The magnetoelectric coupling is facilitated by strain transfer across the vertical interfaces of the two phases. We additionally observe an exchange bias of ∼200 Oe (at 2 K) surviving up to the room temperature, indicating strongly coupled interfaces of SFO and NFO. These findings represent a step forward in future magnetoelectric RAM devices.

In epitaxially grown spinel thin films, the anti-phase boundaries (APBs) are formed due to cationic rearrangements within the spinels on the lattice-mismatched substrates. We demonstrate that such an APB formation could significantly influence the magnetocaloric effect (MCE) and suggest a model on how to avoid their formation and detrimental effects on MCE. We fabricate the geometrically frustrated epitaxial MnCr2O4 on SrTiO3 substrate with two different concentrations of APBs. The film with largely dense APB shows an entropy change, −ΔSM of ∼ 1.3 J/Kg-K, whereas the film with reduced APB shows a value of −ΔSM∼ 5.2 J/Kg-K for a change of field of 50 kOe. Furthermore, density functional theoretical calculations show that if there are more Jahn–Teller active A-site ions compared to B-site in an AB2O4 spinel, then such an APB formation is more probable. Hence, by creating a comparable fraction of Jahn–Teller active B-site cations, such an APB formation can be reduced. Moreover, avoiding the formation of APB and, thus, unwanted antiferromagnetic coupling at the boundaries could intrigue the future scientific community to design highly efficient magnetocaloric cooling devices using geometrically frustrated spinel systems.

Hexagonal perovskite materials are emerging quantum spin liquid (QSL) systems providing a fertile ground to realize novel quantum phenomena. The epitaxially grown thin films of such materials offer a compelling approach to utilize exotic quantum phases for device applications with better control over the structure. We fabricate the intriguing QSL triple perovskite Ba₃CuSb₂O₉ epitaxially onto a MgO (100) substrate by pulsed laser deposition technique as well as in bulk form for comparison. The presence of only (00l) parallel planes of Ba₃CuSb₂O₉ in x-ray diffraction validates the epitaxial growth of the thin film. Temperature-dependent magnetization of thin film reveals no magnetic ordering down to 400 mK, with a large antiferromagnetic Curie–Weiss temperature ( θ CW ≈ − 11.68 K). This indicates strong magnetic frustration and QSL behaviour, similar to bulk Ba₃CuSb₂O₉. The presence of magnetic correlations at low temperature (in the quantum spin liquid state) is further confirmed by analysing the low temperature magnetic isotherms. These experimental findings underscore the potential of this quantum material for its use in quantum technologies.