Spin density wave instability in a ferromagnet

Abstract

Due to its cooperative nature, magnetic ordering involves a complex interplay between spin, charge, and lattice degrees of freedom, which can lead to strong competition between magnetic states. Binary Fe3Ga4 is one such material that exhibits competing orders having a ferromagnetic (FM) ground state, an antiferromagnetic (AFM) behavior at intermediate temperatures, and a conspicuous re-entrance of the FM state at high temperature. Through a combination of neutron diffraction experiments and simulations, we have discovered that the AFM state is an incommensurate spin-density wave (ISDW) ordering generated by nesting in the spin polarized Fermi surface. These two magnetic states, FM and ISDW, are seldom observed in the same material without application of a polarizing magnetic field. To date, this unusual mechanism has never been observed and its elemental origins could have far reaching implications in many other magnetic systems that contain strong competition between these types of magnetic order. Furthermore, the competition between magnetic states results in a susceptibility to external perturbations allowing the magnetic transitions in Fe3Ga4 to be controlled via temperature, magnetic field, disorder, and pressure. Thus, Fe3Ga4 has potential for application in novel magnetic memory devices, such as the magnetic components of tunneling magnetoresistance spintronics devices.

Spin Spiral and Topological Hall Effect in a Metamagnet Fe$_{3}$Ga$_{4}$

“…The lattice parameters are a = 10.0979 Å, b = 7.6670 Å, and c = 7.8733 Å with an obtuse angle of β = 106.298 • [9]. While crystallographically and electronically, as will be discussed in more details later, it is rather 3D, magnetically it can be viewed as a stack of ferromagnetically ordered planes with complex, but, presumably, weaker interplanar coupling [14].…”

“…The material is known to have two metamagnetic tran- sitions [9,14], from a ferromagnetic (FM) to a spin density wave (SDW) at T 1 = 60 K, and back to a ferromagnetic state at T 2 ≈ 360 K (in this paper we apply the term SDW to any phase where spin polarization varies periodically in space; thus defined SDW can be either a spiral, or an amplitude SDW, wherein the magnitude of the magnetic moment varies continuously, or a combination of both). The long-range order is lost at T 3 = 420 K. The nature of the SDW phase will be discussed later, we will just mentioned that the neutron data can be fit equally well [14,15] by an amplitude SDW, where the spins are mostly aligned along c, or by a spin spiral, with the helical orientation, i.e., with the spins rotating in the ab plane. Either way, the spiral wave vector appears to be (0, 0, 0.29).…”

“…Experimentally, the low-temperature ferromagnetic phase shows the lowest susceptibility in the fields below 0.3 T for the field direction along a, and the highest along c, but the c and b directions are nearly the same. This was interpreted [14,15] as if a is the easy axis, and b is the hard one. On the first glance, this implies that the ground state in the SDW phase can be either amplitude wave with polarization along a, or a cycloidal spiral with the spins rotating in the ac plane.…”

“…However, the latter is not compatible with the neutron data, therefore the authors of Ref. [14,15] argued it must be an amplitude wave (note that in text of Ref. [14] it was incorrectly stated that the suggested SDW is linearly polarized along c, but the figure correctly shows the one polarized along a [15]).…”

Theoretical and experimental study of high-pressure synthesized B20-type compounds Mn1− x (Co,Rh) x Ge

“…The design, fabrication and study of novel compounds not found in nature, with an emphasis on interrelation between their electronic structure and physico-chemical properties, provide a promising basis for the development of technologically important materials [1][2][3][4][5][6][7]. Examples of such systems are transition-metal (TM) monogermanides with a cubic B20-type crystal structure, which are attractive for researchers in view of their exotic properties and hence, a variety of possible applications.…”