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Abstract: Understanding magnetism in ferromagnetic metal/semiconductor (FM/SC) hetero-structures is important to the development of the new generation spin field-effect transistor (SpinFET). Here, we report an element-specific x-ray magnetic circularly dichroism study of the interfacial magnetic moments for two FM/SC model systems, namely Co/GaAs and Ni/GaAs, which was enabled using a specially designed FM1/FM2/SC superstructure. We observed a robust room temperature magnetization of the interfacial Co, whilst that of the interfacial Ni was strongly diminished down to 5 K due to hybridization of the Ni d(eg) and GaAs sp3 states. The validity of the selected method was confirmed by first-principles calculations, showing only small deviations (<0.02 and <0.07 μB/atom for Co/GaAs and Ni/GaAs, respectively) compared to the real FM/SC interfaces. Our work proved that the electronic structure and magnetic ground state of the interfacial FM2 is not altered when the topmost FM2 is replaced by FM1 and that this model is applicable generally for probing the buried magnetic interfaces in the advanced spintronic materials.

Open Access

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Abstract: Magnetic skyrmion materials have the great advantage of a robust topological magnetic structure, which makes them stable against the superparamagnetic effect and therefore a candidate for the next-generation of spintronic memory devices. Bulk MnSi, with an ordering temperature of 29.5 K, is a typical skyrmion system with a propagation vector periodicity of ∼18 nm. One crucial prerequisite for any kind of application, however, is the observation and precise control of skyrmions in thin films at room-temperature. Strain in epitaxial MnSi thin films is known to raise the transition temperature to 43 K. Here we show, using magnetometry and x-ray spectroscopy, that the transition temperature can be raised further through proximity coupling to a ferromagnetic layer. Similarly, the external field required to stabilize the helimagnetic phase is lowered. Transmission electron microscopy with element-sensitive detection is used to explore the structural origin of ferromagnetism in these Mn-doped substrates. Our work suggests that an artificial pinning layer, not limited to the MnSi/Si system, may enable room temperature, zero-field skyrmion thin-film systems, thereby opening the door to device applications.

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Abstract: By coupling magnetic elements to metamaterials, hybrid metamolecules can be created with useful properties such as photon-magnon mode mixing. Here, we present results for a split-ring resonator (SRR) placed in close proximity to a thin crystalline film of magnetically hard FeCo. Eddy-current shielding is suppressed by patterning the FeCo into 100-μm disks. At the ferromagnetic resonance (FMR) condition of FeCo, photon-magnon coupling strengths of 5% are observed. Altogether, three distinct features are presented and discussed: (i) remanent magnets allow FMR to be performed in a near-zero field, partially eliminating the need for applied fields; (ii) the anisotropic FMR permits angular control over hybrid SRR and FMR resonances; and (iii) the in-plane and out-of-plane magnetization of FeCo opens the door to “magnetically configurable metamaterials” in real time. Finally, a special study is presented of how best to excite the numerous transverse magnetic and electric modes of the SRR by using near-field excitation from a coplanar waveguide.

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Abstract: We report a study on the transition temperature TC of Cr-doped topological insulator thin films, where an increase in the ferromagnetic onset can provide a pathway towards low-power spintronics in the future. Arrott plots, measured by surface-sensitive x-ray magnetic circular dichroism at the Cr L2,3 edge as a function of field at various low temperatures, give a TC ≈ 7 K for the pristine surface. This is comparable to the bulk value of the film, which means that there is no indication that the spontaneous magnetization is different near the surface. Evaporation of a thin layer of Co onto the pristine surface of the in-situ cleaved sample increases the ordering temperature near the surface to ∼19 K, while in the bulk it rises to ∼10 K. X-ray absorption spectroscopy shows that Cr enters the Bi2Se3 host matrix in a divalent state, and is unchanged by the Co deposition. These results demonstrate a straightforward procedure to increase the transition temperature of doped topological insulators.

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Abstract: Artificial Spin Ice (ASI) have been fabricated from NiFe in the square geometry over areas 1-2mm2. They have small enough volumes to be thermally active within an experimental temperature range. The dynamic hysteresis has been measured using a SQUID-VSM magnetometer. We found that larger dipolar interaction can stabilise the superparamagnetic behaviour and lead to increased order at low temperature. Variation in the observed average blocking temperatures, recorded via hysteresis and zero-field-cooled (ZFC) experiments, is attributed to the distribution of island sizes and interaction effects. A new synchrotron capability at the I10 beamline in Diamond Light Source (UK) has been implemented and used to measure islands as small as 30 × 70 × 8 nm3. At this size, measuring the exact microstates of the individual nanomagnets is challenging. Although, this method is a scattering process it maintains sensitivity to the microstates of the system by using coherent X-rays to produce a speckle pattern. Dynamics were observed in the speckle pattern in the temperature range 180 - 250 K. The characteristic relaxation time was fitted to a Vogel-Fulcher law, with an activation temperature of 40±10 K and freezing temperature of 178±5 K. From magnetometry measurements and simulations we attribute the activation temperature as originating from the non-uniform magnetic structure at the ends of the islands. The freezing temperature relates well to the energy scale of the interaction. Finally, a full-field magnetic microscopy method to probe dynamic ASI has been demonstrated; transmission X-ray microscopy (TXM). The method uses circularly-polarised soft X-rays to probe the magnetic orientation of the individual nanoislands. We have developed an on-membrane heater and thermometer, which is capable of temperatures in excess of  700 K. We have used it to heat the ASI whilst tracking the individual vertex states. We have been able to measure the creation and propagation of emergent monopole excitations and observed increased avalanche velocities and magnetic mobilities at higher temperatures.