Magnitude and sign control of lithography-induced uniaxial anisotropy in ultra-thin (Ga,Mn)As wires

Abstract

We were able to control the magnitude and sign of the uniaxial anisotropy in 5-nm-thin ͑Ga,Mn͒As wires by changing the crystallographic direction of the lithography-induced strain relaxation. The 1-m-wide ͑Ga,Mn͒As wires, oriented in ͓110͔ and ͓110͔ directions, were fabricated using electron beam lithography. Their magnetic anisotropies were studied by a coherent rotation method at temperatures between 4.5 and 75 K. Depending on the orientation of the wire, the additional uniaxial anisotropy observed along the axis of the 1-m-wide samples either increased or decreased the total uniaxial anisotropy. © 2011 American Institute of Physics. ͓doi:10.1063/1.3556556͔The electrical manipulation of the magnetization vector in ferromagnets is one of the most important areas of focus in spintronics. A material particularly well-suited to such investigations is the ferromagnetic semiconductor ͑Ga,Mn͒As. ͑Ga,Mn͒As exhibits hole-mediated ferromagnetism 1,2 and its magnetic anisotropy depends on hole concentration, Mn concentration, lattice strain, and spin-orbit interaction. [2][3][4][5][6][7] Recently, magnetization vector rotation by an electric field has been demonstrated in ͑Ga,Mn͒As, 8 and both experiments and simulations have shown that the modulation of the uniaxial anisotropy along ͗110͘ plays an important role in magnetization switching. 9 One of the methods for controlling the uniaxial anisotropy is the modulation of the lattice strain in ͑Ga,Mn͒As. 4 Lithography-induced uniaxial anisotropy due to the magnetostriction effect has been observed in relatively thick ͑Ga,Mn͒As wires on GaAs. [10][11][12][13][14][15] Since the lithographyinduced anisotropy can be externally modulated by changing the wire width 15 after the crystal growth, it enables the switching of the magnetization of ͑Ga,Mn͒As by an electric field with adjusted uniaxial anisotropy in combination with lithography-induced uniaxial anisotropies.In this Letter, we prove the presence of the lithographyinduced uniaxial anisotropy in 1-m-wide ultrathin ͑Ga,Mn͒As wires and also propose that this effect can assist in the electrical manipulation of magnetization.Devices were fabricated from a single wafer consisting of 5-nm-thin ͑Ga 0.94 ,Mn 0.06 ͒As grown on a semi-insulating GaAs substrate. Since the lattice constant of ͑Ga,Mn͒As is larger than that of GaAs, a compressive strain is built into ͑Ga,Mn͒As, which induces an in-plane magnetic easy axis. Its Curie temperature of 100 K was determined by a superconducting quantum interferometer device ͑SQUID͒. The wafer was patterned into 40-m-long narrow wires with different wire widths, 1 and 20 m, by electron beam lithography and reactive ion etching. We prepared two sets of 1-m-wide wires oriented along either the ͓110͔ or the ͓110͔ direction and a 20-m-wide wire oriented along ͓110͔. Figure 1 shows a scanning electron micrograph of the geometry of the final device. Magnetoresistance was probed by fourpoint measurements at various temperatures between 4.5 and 75 K. External magnetic fields, 0 H ex = 1.0 and 0.1 T, ...

Quantitative determination of [0 1 0] uniaxial anisotropy and the pinning energy in (Ga,Mn)As films

“…The diluted magnetic semiconductor (Ga,Mn)As provides a fertile ground for the exploration on magnetic anisotropy, making it a promising material for an electrically programmable memory and logic device [1][2][3][4][5][6]. The coexistence of cubic magnetic anisotropy and uniaxial magnetic anisotropy (UMA) in several in-plane directions has long been extensively investigated [7][8][9][10][11][12][13][14]. The cubic anisotropy (K C ) along [0 1 0] (and [1 0 0]) and the uniaxial anisotropy constant of [1 1 0] (K U ) have been clearly determined by magnetization measurements, magnetoresistance, and giant planar Hall effect (PHE) [7,8,15,16].…”

“…A combination of PHE measurements and calculations based on the Stoner-Wohlfarth model has been extensively employed to get K C and K U under a large external field [10][11][12][13][14][15]. When the external field decreases to the same magnitude of the coercivity, the masking effect of the Zeeman energy on the magnetic anisotropy reduces, making the asymmetric reversal of in-plane magnetization arising from the [0 1 0] uniaxial anisotropy detectable [14].…”