Magnetic field induced switching of the antiferromagnetic order parameter in thin films of magnetoelectric chromia

Abstract

A Landau-theoretical approach is utilized to model the magnetic field-induced reversal of the antiferromagnetic order parameter in thin films of magnetoelectric antiferromagnets. A key ingredient of this peculiar switching phenomenon is the presence of a robust spin polarized state at the surface of the antiferromagnetic films. Surface or boundary magnetization is symmetry allowed in magnetoelectric antiferromagnets and experimentally established for chromia thin films. It couples rigidly to the antiferromagnetic order parameter and its Zeemann energy creates a pathway to switch the antiferromagnet via magnetic field application. In the framework of a minimalist Landau free energy expansion, the temperature dependence of the switching field and the field dependence of the transition width are derived. Least-squares fits to magnetometry data of (0001)-textured chromia thin films strongly support this model of the magnetic reversal mechanism.

Search for the Magnetic Monopole at a Magnetoelectric Surface

“…It is known, however, that Cr 2 O 3 thin films can have stray spins caused by defects at the interface with the Al 2 O 3 substrate, as well as impurity spins at the interface, which have been shown to be susceptible to small magnetic fields in thin film samples 28,30 . (Note that the intrinsic surface spin density resulting from the termination of the antiferromagnetic magnetoelectric [31][32][33] is not reversed under the conditions of our experiment, since this would require the reversal of the full antiferromagnetic domain 33 . In addition, it has an associated field that is negligible compared with that from the monopole effect.)…”

Purely antiferromagnetic magnetoelectric random access memory

“…To realize the memory cell, we use an epitaxial layer stack of Pt(20 nm)/ -Cr 2 O 3 (200 nm)/Pt(2.5 nm) that is prepared on Al 2 O 3 (0001) substrates. Similar stacks with -Cr 2 O 3 have been extensively studied in the scope of traditional MERAM elements with ferromagnetic Co layers 4,5,[14][15][16] . The thicker bottom Pt film serves as the gate electrode and the thin Pt top layer is used to measure the AF order parameter allelectrically via zero-offset anomalous Hall magnetometry 11 (hereafter zero-offset Hall).…”

“…When combining the large body of data on Cr 2 O 3 thin film systems 4,5,7,9,14,16,18,[23][24][25][26][27][28][29][30] , a coherent picture emerges: the total magnetoelectric energy density exerting a selection pressure on the antiferromagnetic order parameter in thin film magnetoelectric antiferromagnets is composed of three effects that act simultaneously:…”

Nanomagnetism of Magnetoelectric Granular Thin-Film Antiferromagnets

“…The temperature dependence of σ z determined by combining ZOHM and NV magnetometry ( Figure 3 a) shows a smooth tapering of σ z through the phase transition, in contrast with the sharp drop to zero expected from the usually observed power-law dependence of σ( T ) for magnetic phase transitions. Such behavior was previously attributed to spatial variations of T crit in thin films 31 , 32 and can be readily accounted for by the convolution where P ( T crit ) is the probability density for T crit and σ(τ) = σ sat (1 – τ) β , with critical exponent, β, and saturation magnetization, σ sat (σ(τ > 1) = 0). Fitting eq 2 to our data (green curve in Figure 3 a) for fixed β = 0.35 allows us to extract P ( T crit ), as depicted in Figure 3 b.…”

“…(1) can be inverted and σ z (x, y) directly obtained from the experimental data together with appropriate filtering [24]. To determine these parameters, we developed an iterative, self-consistent method (see Appendix) based on the data and our minimal model for Cr 2 O 3 's surface magnetisation of T crit in thin films [26,27] and can be readily accounted for by the convolution…”