Twisted phase of the orbital-dominant ferromagnet SmN in a GdN/SmN heterostructure

Abstract

The strong spin-orbit interaction in the rare-earth elements ensures that even within a ferromagnetic state there is a substantial orbital contribution to the ferromagnetic moment, in contrast to more familiar transition metal systems, where the orbital moment is usually quenched. The orbital-dominant magnetization that is then possible within rare-earth systems facilitates the fabrication of entirely new magnetic heterostructures, and here we report a study of a particularly striking example comprising interfaces between GdN and SmN. Our investigation reveals a twisted magnetization arising from the large spin-only magnetic moment in GdN and the nearly zero, but orbital-dominant, moment of SmN. The unusual twisted phase is driven by (i) the similar ferromagnetic Gd-Gd, Sm-Sm and Gd-Sm exchange interactions, (ii) a SmN Zeeman interaction 200 times weaker than that of GdN, and (iii) the orbital-dominant SmN magnetic moment. The element specificity of X-ray magnetic circular dichroism (XMCD) is used in seperate modes probing both bulk and surface regions, revealing the depth profile of the twisting magnetization. PACS numbers: 75.25-j, 75.47.-m, 75.50.Pp 1 arXiv:1504.04425v1 [cond-mat.mtrl-sci] 17 Apr 2015 I. INTRODUCTION An inhomogeneous, twisted magnetic ordering commonly occurs near interfaces between ferromagnetic materials, due to competing interactions which favor opposing alignments of the magnetization. These phases are types of engineered domain walls, and thus have important implications for spintronics applications, where current-driven domain wall motion is an active area of research. 1-4 So far, twisted phases are known to manifest in diverse magnetic systems, 5-12 however these all fall under the conventional spin-dominant paradigm of magnetism where the orbital moment plays no significant role. Competing interactions in the presence of a dominant orbital moment have so far remained unexplored, yet the opportunity now exists within the rare-earth nitride (REN) series, where orbital-dominant magnetism is possible due to strong spin-orbit coupling of the 4f electrons. Forming a series of mostly intrinsic ferromagnetic semiconductors, 13-19 the RENs are already integrated within spintronic devices, 20,21 and thus provide a novel system for studying competing interactions. The rare-earth elements, comprising the series across which the 4f shell is filled, have been of interest for nearly a century. They are most commonly found in the trivalent state in a wide range of compounds, including the RENs. The 4f shell, with l = 3, comprises seven distinct orbital states, −3 m l 3, and with the spin degeneracy a total of 14 single-electron states. Gd 3+ has a half filled shell, for which Hund's rules state that the seven electrons fill all of the orbital states with spin-up electrons; L = 0 and S = J = 7/2. It thus has a purely spin moment of 7 µ B . The indirect exchange interaction aligns the spins below a Curie temperature of about 50 K, rising to 70 K under heavy donor doping, 22 but the spherical symmetry of...

On the ferromagnetic ground state of SmN

“…Experimental limitations have so far prevented a quantitative sum rule analysis in SmN, but we may still extract meaningful conclusions by exploring the Sm L 3 -edge XMCD measured in a SmN film and a SmN/GdN superlattice, reported in Ref. [15]. Figure 3 shows the Sm L 3 edge XMCD data taken from Ref.…”

Device Applications of Rare-Earth Nitrides

“…The strongly-correlated nature of the electrons presents an abundant testing ground for theoretical models which are made more tractable by the simple rock-salt structure of these materials. There is also a rich variety of magnetic properties within the series, with varying degrees of orbital contributions making for interesting effects when the spin and orbital contributions of various members are played off against one another [17]. The partially filled 4-f orbitals give rise to a range of magnetic moments within the series which differ both in magnitude and in spin/orbital composition, as described by Hund's rules.…”

“…This orbital dominant magnetism results in interesting phenomena such as twisted magnetisation phases when GdN and SmN are grown in multilayer structures [17].…”

“…The size of samples to be used in the SQUID is constrained by the size of the sample holder to ∼3 mm wide and approximately twice as long. The cut piece is then mounted into a plastic straw using a second straw to keep the sample securely in place 17 . The location of the sample within the straw is important for the measurement as once installed a centring procedure must be undertaken in order to perform precise measurements.…”

Twisted magnetization phases in orbital-dominant rare-earth nitrides

“…The primary experimental work on RENs so far has been on homogeneous RENs, with only two publications on REN heterostructures, by Anton et al [19] and McNulty et al [20], the latter of which is part of this thesis (Chapter 5). The study of these heterostructures is a necessary step for exploring poten-1.1 Outline tial device applications, but more fundamentally allows us to probe unexplored interface effects in REN-based heterostructures.…”