Jose de la Venta

Storing information in magnetic recording technologies requires careful optimization of the recording media’s magnetic properties. For example, heat-assisted magnetic recording (HAMR) relies on a prerecording heating step that momentarily lowers the coercivity of the ferromagnetic recording media, and thereby decreases the energy expenditure for each writing operation. However, this process currently requires local temperature increases of several hundred Kelvins, which in turn can cause heat spreading, damage the write head, and limit recording rates. Here, we describe a general mechanism for dramatically tuning the coercivity of ferromagnetic films over small temperature ranges, by coupling them to an adjacent layer that undergoes a structural phase transition with large volume changes. The method is demonstrated in Ni/FeRh bilayers where the Ni layer was deposited at 300 K and 523 K, above and below the FeRh metamagnetic transition at 370 K. When the Ni layer is grown at high temperatures, the 1% FeRh lattice expansion relative to room temperature alters the Ni’s crystallographic texture during growth and leads to a 500% increase in coercivity upon cooling through the FeRh’s metamagnetic transition. Our analysis suggests this effect is related to domain wall pinning across grain boundaries with different orientations and strain states. This work highlights the promise of thermally tuning the coercivity of ferromagnetic materials through structural coupling to underlying films that could enable simplified heatsink designs and expand the selection of materials compatible with HAMR.

We studied the temperature dependence of the magnetic properties of VO ₂ /Ni bilayers. The Ni films were deposited on either monoclinic or rutile phase VO ₂ . The temperature induced VO ₂ transformation from a monoclinic to a rutile structure induces strain in the Ni film. Due to an inverse magnetoelastic effect the coercivity of the Ni films is strongly modified. Both Ni films show strong enhancement of the coercivity near the transition temperature. The coercivity enhancement of Ni is associated with the phase coexistence observed in the VO ₂ first order phase transition. Above the transition temperature, Ni deposited on monoclinic VO ₂ shows a coercivity enhancement whereas Ni deposited on rutile VO ₂ shows suppression of the coercivity. The samples were cycled several times to check if the changes in coercivity were reversible. While samples with Ni deposited on rutile VO ₂ show reversibility, samples with Ni deposited on monoclinic VO ₂ shown an irreversibility after the first structural phase transition. This irreversibility can be associated with cracking of the VO ₂ layer as it relieves stress due to the transition and has implications for the resistance versus temperature behavior of the VO ₂ .

The magnetic properties of bulk hybrid V₂O₃/Ni composites were studied as a function of composition and synthesis conditions. We find a sharp increase in the coercivity and a sharp decrease of the magnetization as the temperature of the bulk materials passes through the V₂O₃ structural phase transition. The magnitude of the effect of the V₂O₃ phase transformation on the magnetic properties of the Ni is strongly dependent on sintering temperature. The V₂O₃ crystallite size is also dependent on the sintering conditions. Stress on the Ni particles due to the V₂O₃ structural transformation produces an inverse magnetostrictive effect which is responsible for the changes in the magnetic properties.

We studied the temperature dependence of the magnetic properties of VO₂/Ni bilayers deposited on three different substrates. The temperature induced VO₂ transformation from a monoclinic to a rutile structure induces strain in the Ni film. Due to an inverse magnetostrictive effect, the coercivity of the Ni films is strongly modified. The morphology of the films is influenced by the substrate choice and has a strong impact on the magnetic properties. Ni films grown on top of rutile VO₂ show a reversible change in the coercivity and a strong enhancement of the coercivity near the transition temperature. The coercivity enhancement of Ni is associated with the phase coexistence observed in the VO₂ first order phase transition.

We report on a strain-induced and temperature dependent uniaxial anisotropy in V₂O₃/Ni hybrid thin films, manifested through the interfacial strain and sample microstructure, and its consequences on the angular dependent magnetization reversal. X-ray diffraction and reciprocal space maps identify the in-plane crystalline axes of the V₂O₃; atomic force and scanning electron microscopy reveal oriented rips in the film microstructure. Quasi-static magnetometry and dynamic ferromagnetic resonance measurements identify a uniaxial magnetic easy axis along the rips. Comparison with films grown on sapphire without rips shows a combined contribution from strain and microstructure in the V₂O₃/Ni films. Magnetization reversal characteristics captured by angular-dependent first order reversal curve measurements indicate a strong domain wall pinning along the direction orthogonal to the rips, inducing an angular-dependent change in the reversal mechanism. The resultant anisotropy is tunable with temperature and is most pronounced at room temperature, which is beneficial for potential device applications.

We report on a strain-induced and temperature dependent uniaxial anisotropy in V$_{2}$O$_{3}$/Ni hybrid thin films, manifested through the interfacial strain and sample microstructure, and its consequences on the angular dependent magnetization reversal. X-ray diffraction and reciprocal space maps identify the in-plane crystalline axes of the V$_{2}$O$_{3}$; atomic force and scanning electron microscopy reveal oriented rips in the film microstructure. Quasi-static magnetometry and dynamic ferromagnetic resonance measurements identify a uniaxial magnetic easy axis along the rips. Comparison with films grown on sapphire without rips shows a combined contribution from strain and microstructure in the V$_{2}$O$_{3}$/Ni films. Magnetization reversal characteristics captured by angular-dependent first order reversal curve measurements indicate a strong domain wall pinning along the direction orthogonal to the rips, inducing an angular-dependent change in the reversal mechanism. The resultant anisotropy is tunable with temperature and is most pronounced at room temperature, which is beneficial for potential device applications.

This article introduces magnetic field modulated microwave spectroscopy (MFMMS) as a unique and high-sensitivity technique for use in the search for new superconductors. MFMMS measures reflected microwave power as a function of temperature. The modulation induced by the external ac magnetic field enables the use of phase locked detection with the consequent sensitivity enhancement. The MFMMS signal across several prototypical structural, magnetic, and electronic transitions is investigated. A literature review on microwave absorption across superconducting transitions is included. We show that MFMMS can be used to detect superconducting transitions selectively with very high sensitivity.