We studied the temperature dependence of the magnetic properties of VO/Ni bilayers. The Ni films were deposited on either monoclinic or rutile phase VO. The temperature induced VO 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 first order phase transition. Above the transition temperature, Ni deposited on monoclinic VO shows a coercivity enhancement whereas Ni deposited on rutile VO 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 show reversibility, samples with Ni deposited on monoclinic VO shown an irreversibility after the first structural phase transition. This irreversibility can be associated with cracking of the VO layer as it relieves stress due to the transition and has implications for the resistance versus temperature behavior of the VO.
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.
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.
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.
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