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Register a new userDirect visualization of the magnetostructural phase transition in nano-scale FeRh thin films using differential phase contrast imaging
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To advance the use of thermally-activated magnetic materials in device applications it is necessary to examine their behaviour on the localised scale in operando conditions. Equi-atomic FeRh undergoes a magnetostructural transition from an antiferromagnetic (AF) to a ferromagnetic (FM) phase above room temperature (~ 75 to 105 {deg}C) and hence is considered a very desirable material for the next generation of novel nanomagnetic or spintronic devices. For this to be realised, we must fully understand the intricate details of AF to FM transition and associated FM domain growth on the scale of their operation. Here we combine in-situ heating with a comprehensive suite of advanced transmission electron microscopy techniques to investigate directly the magnetostructural transition in nano-scale FeRh thin films. Differential phase contrast imaging visualizes the stages of FM domain growth in both cross-sectional and planar FeRh thin films as a function of temperature. Small surface FM signals are also detected due to interfacial strain with the MgO substrate and Fe deficiency after HF etching of the substrate, providing a directional bias for FM domain growth. Our work provides high resolution imaging and quantitative measurements throughout the transition, which were previously inaccessible, and offers new fundamental insight into their potential use in magnetic devices.
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The antiferromagnetic (AFM) to ferromagnetic (FM) first order phase transition of an epitaxial FeRh thin-film has been studied with x-ray magnetic circular dichroism using photoemission electron microscopy. The FM phase is magnetized in-plane due to shape anisotropy, but the magnetocrystalline anisotropy is negligible and there is no preferred in-plane magnetization direction. When heating through the AFM to FM phase transition the nucleation of the FM phase occurs at many independent nucleation sites with random domain orientation. The domains subsequently align to form the final FM domain structure. We observe no pinning of the FM domain structure.
Uncompensated moments in antiferromagnets are responsible for exchange bias in antiferromagnet/ferromagnet heterostructures; however, they are difficult to directly detect because any signal they contribute is typically overwhelmed by the ferromagnetic layer. We use magneto-thermal microscopy to image uncompensated moments in thin films of FeRh, a room-temperature antiferromagnet that exhibits a 1st-order phase transition to a ferromagnetic state near 100~$^circ$C. FeRh provides the unique opportunity to study both uncompensated moments in the antiferromagnetic phase and the interaction of uncompensated moments with emergent ferromagnetism within a relatively broad (10-15~$^circ$C) temperature range near $T_C$. In the AF phase below $T_C$, we image both pinned UMs, which cause local vertical exchange bias, and unpinned UMs, which exhibit an enhanced coercive field that reflects exchange-coupling to the AF bulk. Near $T_C$, where AF and FM order coexist, we find that the emergent FM order is exchange-coupled to the bulk Neel order. This exchange coupling leads to the nucleation of unusual configurations in which different FM domains are pinned parallel, antiparallel, and perpendicular to the applied magnetic field before suddenly collapsing into a state uniformly parallel to the field.
Using a double-pump pulse approach and laser-induced THz emission as an ultrafast amperemeter and magnetometer, we show that a femtosecond laser pulse generates ferromagnetic nuclei in a FeRh/Pt bilayer, i.e. these nuclei acquire a net magnetization and a susceptibility to a magnetic field, but only 20 ps after the initial laser excitation. We argue that this latency is intrinsic to the first-order phase transitions from antiferromagnetic to ferromagnetic states and must be present even in the case when the sign of the exchange interaction changes instantaneously.
It has been well established that both in bulk at ambient pressure and for films under modest strains, cubic SrCoO$_{3-delta}$ ($delta < 0.2$) is a ferromagnetic metal. Recent theoretical work, however, indicates that a magnetic phase transition to an antiferromagnetic structure could occur under large strain accompanied by a metal-insulator transition. We have observed a strain-induced ferromagnetic to antiferromagnetic phase transition in SrCoO$_{3-delta}$ films grown on DyScO$_3$ substrates, which provide a large tensile epitaxial strain, as compared to ferromagnetic films under lower tensile strain on SrTiO$_3$ substrates. Magnetometry results demonstrate the existence of antiferromagnetic spin correlations and neutron diffraction experiments provide a direct evidence for a G-type antiferromagnetic structure with Neel temperatures between $T_N sim 135,pm,10,K$ and $sim 325,pm,10,K$ depending on the oxygen content of the samples. Therefore, our data experimentally confirm the predicted strain-induced magnetic phase transition to an antiferromagnetic state for SrCoO$_{3-delta}$ thin films under large epitaxial strain.
We report a Raman scattering investigation of multiferroic bismuth ferrite BiFeO3 epitaxial (c-axis oriented) thin films from -192 to 1000C. Phonon anomalies have been observed in three temperature regions: in the gamma-phase from 930C to 950C; at ~370C, Neel temperature (TN), and at ~123C, due to a phase transition of unknown type (magnetic or structural). An attempt has been made to understand the origin of the weak phonon-magnon coupling and the dynamics of the phase sequence. The disappearance of several Raman modes at ~820C (Tc) is compatible with the known structural phase transition and the Pbnm orthoferrite space group assigned by Arnold {it et al.} cite{arnold:09}. The spectra also revealed a {it non-cubic} $beta$-phase from 820-930dc and the same {it non-cubic} phase extends through the $gamma$-phase between 930-950dc, in agreement with Arnold {it et al.} cite{arnold2:09}, and an evidence of a cubic $delta$-phase around 1000dc in thin films that is not stable in powder and bulk. Such a cubic phase has been theoretically predicted in cite{vasquez:prb09}. Micro-Raman scattering and X-ray diffraction showed no structural decomposition in thin films during the thermal cycling from 22-1000dc.
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