Interstitial light elements play an important role in magnetic materials. Especially, Mn-based compounds with interstitial atoms are important for the easy fabrication of highly functional magnetic devices.
Definition
Interstitial light elements play an important role in magnetic materials by improving the magnetic properties through changes of the unit cell volume or through orbital hybridization between the magnetic and interstitial atoms.
Some crystal structures possess interstitial crystallographic sites, which light elements such as hydrogen, boron, carbon, nitrogen, and oxygen atoms can occupy. There is a rather long history of metallurgical, physical, and chemical research studies on interstitial atoms [1,2]. In past years, domain control of ferromagnets has been studied [1]. Since the 1980s, interstitial atoms have attracted intense attention related to the improvement of magnetic properties of rare-earth Fe and Co-based permanent magnets [2,3,4,5,6].
Interstitial atoms have two major roles—influencing the stability of the crystal structure and in the modification or change in magnetic properties. In the former case, small amounts of interstitial light elements are required to stabilize the desired crystal structure; compounds without interstitial atoms would not exist in thermal equilibrium. In the latter case, interstitial atoms affect the crystal structure parameters, such as the interatomic distances between magnetic atoms or the orbital hybridization between magnetic and interstitial atoms, consequently meaning the magnetic ordering temperature, the magnetic moment, the magnetic structure, and other factors can be altered.
The most well-studied platforms for interstitial atoms are rare-earth Fe-based permanent magnets. The improvements of the magnetic properties have mainly been achieved through the addition of light elements such as boron, carbon, and nitrogen atoms. The Bethe–Slater curve is one of the criteria needed to understand whether metal 3d transition elements of Cr, Mn, Fe, Co, and Ni possess ferromagnetic (FM) or antiferromagnetic (AFM) states (see Figure 1) [7,8,9,10]. This curve exhibits the exchange coupling as a function of the interatomic distance. Fe falls in the FM region near to the border between FM and AFM states. Therefore, in Fe-based compounds, a shorter Fe–Fe distance (shrinkage of the unit cell volume) favors an AFM state, while a longer Fe–Fe distance (an expansion of the unit cell volume) favors the FM state [11,12,13]. With increasing Fe–Fe distance, smaller overlapping of 3d wave functions makes the 3d band narrower, which leads to the FM state, and in most cases the Curie temperature TC is enhanced.
On the other hand, the effects of interstitial atoms in Mn-based compounds are not well researched. As shown in Figure 1, the Mn atom itself shows the AFM ground state, however the expanded Mn–Mn distance in Mn-based compounds leads to the FM state. Mn compounds are indispensable for both FM and AFM materials. For example, MnBi and MnAl have attracted much attention as permanent magnets [14,15,16,17,18]. MnSi has been extensively studied as a magnetic material with a skyrmion state, which is a noncollinear magnetic structure. The skyrmion domains can be driven by the low current density threshold [19]. Recently, Mn3Sn has been intensively studied as a topological antiferromagnet [20] and is a candidate next-generation spintronics material. If the effects of interstitial atoms are well understood, they can be highly useful in the development of spintronics devices or highly functional magnetic devices, which can be developed via easy on-demand control of the magnetic state.
We have surveyed Hydrogen-Absorbed (R or Th)6Mn23, Hydrogen-Absorbed YMn2, Carbon-Added Mn5Si3, (R or Actinide)Mn2Si2 and its germanides, Boron-Added Pd0.75Mn0.25 Alloy, and Boron-Added Sm2Mn8Al9. As in the rare-earth Fe-based compounds, the interstitial atoms give rise to the enhancement of FM interaction in the weak hybridization regime leading to the appearance of room temperature ferromagnetism. However, the Mn compounds surveyed above manifest the change or additional formation of magnetism by the interstitial atoms, while many rare-earth Fe-based parent compounds are already ferromagnets. The change from paramagnetic to FM state is observed in hydrogen-absorbed Th6Mn23, hydrogen-absorbed YMn2 or Sm2Mn8Al9Bx. The result of Mn5Si3Cx thin film may be a rare example of change from the AFM to FM state by the interstitial atoms. In Pd0.75Mn0.25Bx, the room temperature ferromagnetism is induced by a slight addition of boron, while the low-temperature magnetic ground state of the parent compound is unchanged. This can be regarded as an example of the additional formation of magnetism by interstitial atoms. It should be noted that, in some cases, the change or additional formation of a magnetic state seems to abruptly occur, which is valuable for future research. We note here that the magnetic structures have been divided into FM and AFM, although some compounds may show a more complicated state such as canted AFM, and spiral AFM. In the future, discussion taking into account a more microscopic mechanism of the magnetic ordering would be necessary.
3.4. Comments on Critical Behavior
From the fundamental viewpoint, research into critical behavior is interesting. Actually, in strongly correlated electron systems, there have been plenty of studies for seeking a quantum critical point under the suppression of magnetism [132,133,134,135,136]. We speculate that a formation of FM exchange coupling above room temperature would be a discontinuous phenomenon as mentioned in the results of Mn-based compounds. While it is not well investigated for rare-earth Fe-based compounds, we note that RFe11TiCx and RFe11TiCx show a finite change in TC at an infinitely zero value of volume expansion depending on R species [137].
This entry is adapted from the peer-reviewed paper 10.3390/met10121644