S. A. Cavill

I18 – the microfocus spectroscopy beamline at the Diamond Light Source

The design and performance of the microfocus spectroscopy beamline at the Diamond Light Source are described. The beamline is based on a 27 mm-period undulator to give an operable energy range between 2 and 20.7 keV, enabling it to cover the K-edges of the elements from P to Mo and the L(3)-edges from Sr to Pu. Micro-X-ray fluorescence, micro-EXAFS and micro-X-ray diffraction have all been achieved on the beamline with a spot size of approximately 3 microm. The principal optical elements of the beamline consist of a toroid mirror, a liquid-nitrogen-cooled double-crystal monochromator and a pair of bimorph Kirkpatrick-Baez mirrors. The performance of the optics is compared with theoretical values and a few of the early experimental results are summarized.

Photoinduced Melting of Antiferromagnetic Order in La0.5Sr1.5MnO4 Measured Using Ultrafast Resonant Soft X-Ray Diffraction

We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in La(0.5)Sr(1.5)MnO(4) after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand, the (1/4,1/4,0) diffraction peak, reflecting orbital order, is only partially reduced. We interpret the results as evidence of destabilization in the short-range exchange pattern with no significant relaxation of the long-range Jahn-Teller distortions. Cluster calculations are used to analyze different possible magnetically ordered states in the long-lived metastable phase. Nonthermal coupling between light and magnetism emerges as a primary aspect of photoinduced phase transitions in manganites.

Non-volatile voltage control of magnetization and magnetic domain walls in magnetostrictive epitaxial thin films

We demonstrate reproducible voltage induced non-volatile switching of the magnetization in an epitaxial thin Fe81Ga19 film. Switching is induced at room temperature and without the aid of an external magnetic field. This is achieved by the modification of the magnetic anisotropy by mechanical strain induced by a piezoelectric transducer attached to the layer. Epitaxial Fe81Ga19 is shown to possess the favourable combination of cubic magnetic anisotropy and large magnetostriction necessary to achieve this functionality with experimentally accessible levels of strain. The switching of the magnetization proceeds by the motion of magnetic domain walls, also controlled by the voltage induced strain.

Room temperature ferroelectric and magnetic investigations and detailed phase analysis of Aurivillius phase Bi5Ti3Fe0.7Co0.3O15 thin films

Aurivillius phase Bi5Ti3Fe0.7Co0.3O15 (BTF7C3O) thin films on α-quartz substrates were fabricated by a chemical solution deposition method and the room temperature ferroelectric and magnetic properties of this candidate multiferroic were compared with those of thin films of Mn3+ substituted, Bi5Ti3Fe0.7Mn0.3O15 (BTF7M3O). Vertical and lateral piezoresponse force microscopy (PFM) measurements of the films conclusively demonstrate that BTF7C3O and BTF7M3O thin films are piezoelectric and ferroelectric at room temperature, with the major polarization vector in the lateral plane of the films. No net magnetization was observed for the in-plane superconducting quantum interference device (SQUID) magnetometry measurements of BTF7M3O thin films. In contrast, SQUID measurements of the BTF7C3O films clearly demonstrated ferromagnetic behavior, with a remanent magnetization, Br, of 6.37 emu/cm3 (or 804 memu/g), remanent moment = 4.99 × 10−5 emu. The BTF7C3O films were scrutinized by x-ray diffraction, high resolutio...

Magnetostrictive thin films for microwave spintronics

Multiferroic composite materials, consisting of coupled ferromagnetic and piezoelectric phases, are of great importance in the drive towards creating faster, smaller and more energy efficient devices for information and communications technologies. Such devices require thin ferromagnetic films with large magnetostriction and narrow microwave resonance linewidths. Both properties are often degraded, compared to bulk materials, due to structural imperfections and interface effects in the thin films. We report the development of epitaxial thin films of Galfenol (Fe81Ga19) with magnetostriction as large as the best reported values for bulk material. This allows the magnetic anisotropy and microwave resonant frequency to be tuned by voltage-induced strain, with a larger magnetoelectric response and a narrower linewidth than any previously reported Galfenol thin films. The combination of these properties make epitaxial thin films excellent candidates for developing tunable devices for magnetic information storage, processing and microwave communications.

Picosecond inverse magnetostriction in galfenol thin films

Coherent high-amplitude precession of the magnetization and spin waves with frequencies up to 40 GHz are generated by injecting picosecond compressive and shear acoustic pulses into nanometer-sized galfenol (Fe81Ga19) films. The magnetization modulation is due to the picosecond inverse magnetostrictive effect. The oscillations of the magnetization measured by magneto-optical Kerr rotation last for several nanoseconds, and the maximum modulation of the in-plane effective magnetic field is as high as 40 mT. These results in combination with a comprehensive theoretical analysis show that galfenol films possess excellent properties for ultrafast magnetization control based on the picosecond inverse magnetostrictive effect.

Exchange Bias in Fe@Cr Core-Shell Nanoparticles

We have used X-ray magnetic circular dichroism and magnetometry to study isolated Fe@Cr core-shell nanoparticles with an Fe core diameter of 2.7 nm (850 atoms) and a Cr shell thickness varying between 1 and 2 monolayers. The addition of Cr shells significantly reduces the spin moment but does not change the orbital moment. At least two Cr atomic layers are required to stabilize a ferromagnetic/antiferromagnetic interface and generate the associated exchange bias and increase in coercivity.

Induced magnetic moment of Eu3+ ions in GaN

Magnetic semiconductors with coupled magnetic and electronic properties are of high technological and fundamental importance. Rare-earth elements can be used to introduce magnetic moments associated with the uncompensated spin of 4f-electrons into the semiconductor hosts. The luminescence produced by rare-earth doped semiconductors also attracts considerable interest due to the possibility of electrical excitation of characteristic sharp emission lines from intra 4f-shell transitions. Recently, electroluminescence of Eu-doped GaN in current-injection mode was demonstrated in p-n junction diode structures grown by organometallic vapour phase epitaxy. Unlike most other trivalent rare-earth ions, Eu3+ ions possess no magnetic moment in the ground state. Here we report the detection of an induced magnetic moment of Eu3+ ions in GaN which is associated with the 7F2 final state of 5D0→7F2 optical transitions emitting at 622 nm. The prospect of controlling magnetic moments electrically or optically will lead to the development of novel magneto-optic devices.

Electrical control of magnetic reversal processes in magnetostrictive structures

We demonstrate the interplay between strain-induced, shape-induced, and magnetocrystalline anisotropy energies in a micron scale magnetostrictive device coupled to a piezoelectric transducer. Varying the voltage on the transducer tunes the shape of the magnetic hysteresis loops and the magnetic reversal processes, which involve a single 180° or 90° domain wall, or proceed via the formation of a regular flux closure domain pattern. The flux closure domains can be suppressed or enhanced in the absence of an external magnetic field by sweeping the applied voltage. The functionalities we demonstrate may find applications in magnetic schemes for information storage and logical processing.

Strain dependent defect mediated ferromagnetism in Mn-doped and undoped ZnO thin films

The structural and magnetic properties of pulsed laser deposited zinc oxide thin films have been investigated. Room temperature ferromagnetism is present in undoped as well as Mn-doped films. The saturation magnetization of the thin films reveals a dependence on both the composition and the out of plane lattice parameter. X-ray magnetic circular dichroism down to 2 K reveals a purely paramagnetic contribution from the Mn in Mn:ZnO films. We conclude that the observed ferromagnetism arises entirely from intrinsic defects in the ZnO which can be varied by manipulation of the lattice parameter.