MnPt Film Research Articles

Voltage-controlled magnetic anisotropy in antiferromagnetic MgO-capped MnPt films

The magnetic anisotropy in MgO-capped MnPt films and its voltage control are studied using first-principles calculations. Sharp variation of the magnetic anisotropy with film thickness, especially in the Pt-terminated film, suggests that it may be widely tuned by adjusting the film thickness. In thick films the linear voltage control coefficient is as large as 1.5 and $\ensuremath{-}0.6$ pJ/Vm for Pt-terminated and Mn-terminated interfaces, respectively. The combination of a widely tunable magnetic anisotropy energy and a large voltage-control coefficient suggest that MgO-capped MnPt films can serve as a versatile platform for magnetic memory and antiferromagnonic applications.

Voltage-controlled magnetic anisotropy in antiferromagnetic L10-MnPt and MnPd thin films

The efficient electrical control of the magnetic states of antiferromagnets is one of the main focus in antiferromagnetic spintronics. In this work, the voltage-controlled magnetic anisotropy (VCMA) effect in two representative antiferromagnets L10-type MnPt and MnPd thin films has been investigated by employing first-principles calculations. Our results indicate that both of the MnPt and MnPd films show in-plane magnetic anisotropy and the magnetic easy axis points either along [1 0 0] or [1 1 0] direction depending on the film thickness. The applied electric field in the range of −0.3 V/Å and + 0.3 V/Å (vacuum as dielectric) leads to the change of magnetocrystalline anisotropy of both films at the order of tens of μJ/m2. Especially, for MnPt film with thickness of two unit cells on Pt(0 0 1), it shows that the moderate electric field is able to switch the in-plane magnetic easy axis between [1 0 0] and [1 1 0] directions. For MnPd thin films, the VCMA effect shows generally linear dependence on the electric field and the VCMA coefficients are estimated in the range of 2.7 to 22.6 fJ/V/m. Our calculation results may stimulate future investigations on the VCMA effect of metallic antiferromagnetic films and pave the possible applications in memory device.

Thickness effect on magnetocrystalline anisotropy of MnPt(0 0 1) film

We investigated the magnetocrystalline anisotropy (MCA) of MnPt(0 0 1) film and MnPt/MgO(0 0 1) using ab initio electronic structure calculations. We found that the magnetic ground state of the MnPt film strongly depends on thickness (n). In bulk and in film with n ≥ 7 monolayers; AFM-II is the magnetic ground state. However, as the film thickness reduces, the magnetic ground state shifts from AFM-II to AFM-III. We employ layer-by-layer Heisenberg model to explain thickness dependent ground state transition. AFM-III state in the Mn-terminated film shows large perpendicular MCA(PMCA), which further enhances on a MgO(0 0 1) substrate. We conclude that the interface plays a key role for the enhancement of PMCA on the substrate.

Epitaxial Growth of Intermetallic MnPt Films on Oxides and Large Exchange Bias.

High-quality epitaxial growth of inter-metallic MnPt films on oxides is achieved, with potential for multiferroic heterostructure applications. Antisite-stabilized spin-flipping induces ferromagnetism in MnPt films, although it is robustly antiferromagnetic in bulk. Moreover, highly ordered antiferromagnetic MnPt films exhibit superiorly large exchange coupling with a ferromagnetic layer.

Chemically ordered MnPt ultrathin films on Pt(001) substrate: Growth, atomic structure, and magnetic properties

Ultrathin MnPt films have been grown on Pt(001) single crystals by alternate deposition of Mn and Pt and studied in situ by grazing incidence x-ray diffraction. The growth mode and chemical order strongly depend on the deposition conditions. At room temperature, a 3D growth mode for Pt leads to a disordered and rough MnPt film. After annealing at 770 K, chemical order develops with the tetragonal axis oriented in the surface plane. At 570 K, Pt grows in a quasi layer-by-layer mode. For all depositions, and even after annealing, Mn remains essentially at the surface with negligible diffusion into bulk. As a consequence, the film deposited at 570 K is chemically ordered, with the tetragonal axis oriented mainly perpendicular to the surface. Coupled to ferromagnetic layers with in-plane (out-of-plane) anisotropy, these ultrathin ($\ensuremath{\approx}$3 nm) MnPt films exhibit exchange bias properties and enhanced coercivity at low temperature.

Magnetic properties of epitaxial Fe films on MnPt/Fe(1 0 0)

Magnetic properties of epitaxial Fe films on MnPt/Fe(1 0 0) were measured using X-ray magnetic circular dichroism (XMCD) and Mott electron spin polarimetry (SP). Surface sensitive secondary electron spin polarization images revealed the magnetization orientation of the Fe overlayers with respect to that of the substrate Fe crystal that was measured with bulk-sensitive XMCD. The magnetic domains of the Fe overlayer are influenced by the substrate magnetic domain structure, giving a clear evidence of the magnetic interaction between the Fe overlayer and the substrate Fe through the antiferromagnetic MnPt film.

Evidence for in-plane spin-flop orientation at theMnPt∕Fe(100)interface revealed by x-ray magnetic linear dichroism

X-ray magnetic linear and circular dichroism at the Mn ${L}_{2,3}$ edges are used to determine the magnetic properties of epitaxial MnPt films on Fe(100). The good agreement between experimental linear dichroism and multiplet calculations reveals that the Mn spins are aligned in plane but perpendicularly to the underlying in-plane Fe spins. The absence of magnetic circular dichroism rules out the presence of uncompensated Mn spins at the interface.

Exchange bias in ordered antiferromagnets by rapid thermal anneal without magnetic field

Exchange bias of bottom-pinned bilayers and tunnel junctions (TJs) containing MnPt as the antiferromagnet was set by rapid thermal annealing. An annealing duration of 1 min was sufficient to induce the structural phase transition to obtain antiferromagnetic behaviour in MnPt without cool down in a magnetic field. For Mn51.5Pt48.5(20 nm)/Co90Fe10(5 nm) bilayers the exchange coupling was found to be (Jex = 0.4 erg cm−2). Higher rapid thermal annealing temperatures led to increased diffusion in the layers reducing the pinned layer magnetic moment. Structural characterization by x-ray diffraction and transmission electron microscopy clearly showed the phase transition in the MnPt film. Exchange bias, coercive and free layer coupling field of a rapid thermal annealed and a standard annealed TJ were compared.

Blocking temperature in exchange coupled MnPt∕CoFe bilayers and synthetic antiferromagnets

Exchange coupled Mn51.5Pt48.5∕Co90Fe10 bottom-pinned bilayers and synthetic antiferromagnets (SAFs) were prepared by magnetron sputtering. Exchange bias, coercivity, and blocking temperature were determined as a function of the MnPt thickness. Interfacial exchange energy was found to be Jex=0.4erg∕cm2 for a Mn51.5Pt48.5(t⩾20nm)∕Co90Fe10 (5nm) bilayer. Thermal stability of the bilayers was analyzed by separation of the blocking temperature distribution in (a) width ΔTB and (b) center temperature TB,center of the distribution. For thick MnPt films ΔTB is constant but steadily increases for thin films. SAF structures show higher exchange coupling and higher blocking temperatures at thin layer thickness (tMnPt=8.5nm).

Exchange coupling of bilayers and synthetic antiferromagnets pinned to MnPt

Exchange bias and blocking temperature were studied in MnPt based bottom-pinned bilayers and synthetic antiferromagnets (SAF) prepared by magnetron sputtering. The structure and magnetic properties were determined as a function of the MnPt layer thickness. Exchange coupling was found to be (Jex = 0.4 erg/cm2) for a MnPt (t ≤20 nm)/CoFe (5 nm) bilayer. The distribution of the blocking temperature TB was analyzed and its width ΔTB and center point TB,center determined. TB is about 280 ○C for thinner MnPt films, and increases to 330 °C for thick films. ΔT B is constant for thick MnPt but steadily increases as the thickness decreases. SAF structures show higher exchange bias and higher TB,center at thin layer thickness (tMnPt = 8.5 nm) compared to bilayers.

熱処理したＭｎＰｔ膜の結晶配向と膜厚依存性

The crystal structure in MnPt antiferromagnetic films used in spin-valve films was investigated. The as-deposited MnPt film has strongly (111)-oriented disordered face center cubic structure; the (111) orientation decreases significantly with MnPt film thickness increase after thermal annealing, resulting in a CuAu-I ordered antiferromagnetic MnPt film. We found, through cross-sectional TEM, that the MnPt film thickness also affects the grain sturcture in the annealed spin-valve films.

Ferromagnetic behavior of disordered MnPt films produced by rf sputtering

The state of atomic order and the variation of magnetic behavior with atomic order in MnPt are studied. X-ray and neutron diffraction experiments show that quenching still results in appreciable degrees (≳0.8) of order and only through sputtering can completely disordered MnPt be prepared. Magnetization measurements conducted on the quenched powder and on the sputtered films established ferromagnetism in the disordered MnPt. The deduced magnetic moment is 3.25±0.30 μB per Mn atom in the sputtered films at 0 °K. This value is compared with those deduced for Mn in other alloys. The difficulties in invoking the Bethe-Slater criterion or a segregation of constituent atoms to explain the onset of ferromagnetism in the disordered MnPt are discussed.