Switching on superferromagnetism

Recent results in electric-field control of magnetism have paved the way for the design of alternative magnetic and spintronic devices with enhanced functionalities and low power consumption. Among the diversity of reported magnetoelectric effects, the possibility of switching on and off long-range ferromagnetic ordering close to room temperature stands out. Its binary character opens up the avenue for its implementation in magnetoelectric data storage devices. Here we show the possibility to locally switch on superferromagnetism in a wedge-shaped polycrystalline Fe thin film deposited on top of a ferroelectric and ferroelastic ${\mathrm{BaTiO}}_3$ substrate. A superparamagnetic to superferromagnetic transition is observed for confined regions for which a voltage applied to the ferroelectric substrate induces a sizable strain. We argue that electric-field-induced changes of magnetic anisotropy lead to an increase of the critical temperature separating the two regimes so that superparamagnetic regions develop collective long-range superferromagnetic behavior.

Figure 1

Space-resolved spectroscopic characterization of the as-grown Fe wedge. XPEEM images of the Fe wedge obtained at the Fe (a) and Ti (b) $L_3$ edges, respectively; both are normalized to pre-edge images. (c) Fe and Ti intensity profiles measured along dotted lines depicted in panels (a),(b). (d) $\mathrm{Fe}{L}_{2,3}$-edge absorption spectra obtained on different wedge for regions [arrows on panel (c)] where the Fe thickness varies. Spectra have been normalized at 720.59 eV corresponding to the $L_2$ peak maximum for the thickest Fe region (3.0 nm). Spectra have been shifted vertically for the sake of comparison. (e) Detail of the Fe $L_2$-edge absorption region showing development of ${\mathrm{FeO}}_x$ characteristic spectral features as $t_{\mathrm{Fe}}$ decreases. Spectra are normalized at 720.59 eV. (f) Magnetic contrast XMCD image obtained at the Fe $L_3$ edge for the same region depicted in panels (a),(b). (g) Comparison of the profile of the XMCD vs that of the Fe $L_3$ XAS showing the onset of long-range FM behavior for $t_{\mathrm{Fe}}=1.3\mathrm{nm}$. All data taken at 320 K.

Figure 2

Polycrystalline nature of the Fe film as observed by TEM. HAADF-STEM image of the heterostructure showing a contiguous nanocrystalline Fe layer. The interplanar distance measured in one of the grains is 2.86 Å, corresponding to the lattice parameter of bcc Fe.

Figure 3

Voltage-dependent evolution of magnetic domains on Fe. XMCD PEEM images obtained at the Fe $L_3$ edge at various voltages: (a) 0 V, (b),(c) 74 V, and (d) 170 V, respectively. Images depicted at panels (b),(c) were obtained with a time difference of ca. 1 h. All images were obtained at RT after a thermal excursion to low temperature. Image region is not the same as that depicted in Fig. 1. $\alpha$-, $\beta$-, and $\gamma$- FM domains are delimited by a dashed line along ${\left[100\right]}_{\mathrm{pc}}$.

Figure 4

Extension of long-range FM ordering towards lower $t_{\mathrm{Fe}}$ regions upon application of $V$. (a)–(c) XMCD PEEM images obtained at the Fe $L_3$ edge after a 45° azimuthal rotation of the sample for decreasing voltages, i.e., (a) $V=170\mathrm{V}$, (b) $V=0\mathrm{V}$, and (c) $V=-170\mathrm{V}$. The azimuthal rotation of the sample allows XMCD to be sensitive to magnetization oriented along both ${\left[100\right]}_{\mathrm{pc}}$ and ${\left[010\right]}_{\mathrm{pc}}$. (d) Line profile of the XMCD along red line shown in panel (c) for $V=-170\mathrm{V}$ (black) and $V=170\mathrm{V}$ (red). Long-range ferromagnetism has extended ca. 1.3 μm along ${\left[100\right]}_{\mathrm{pc}}$ after a localized $\alpha$ to $\beta$ magnetoelastically triggered transition. Such extension corresponds to a decrease of the onset of FM $\left(t_{\mathrm{FM}}\right)$ of ca. 1 Å.

Figure 5

Results of Monte Carlo simulations of an ensemble of dipolar interaction NPs. Thermal dependence of the magnetization for interparticle distances $r=\rho D$ in multiples of the particle diameter $D$ with with $\rho =1.5,1.75,2$ from right to left.

Figure 6

Effect of electric-field-induced stress on transition temperature $T_P$ between SPM and SFM states. Thermal dependence of the magnetization as obtained by simulations of an ensemble of dipolar interacting NP on a triangular lattice with interparticle distance $r=2D$, i.e., $\rho =2$. Black-circles curve corresponds to an ensemble of NPs with random magnetocrystalline anisotropy. Red squares curve takes into account an additional magnetoelastic contribution to the anisotropy $\left(K_{\mathrm{me}}\right)$ resulting from an electric-field-induced compression of 1%. The dashed rectangle marks the region where long-range FM order is induced by the strain-induced anisotropy. Left (right) inset displays representative spin configuration corresponding to circle (square) curves for a temperature inside the rectangular region.