New ferromagnetic superconductor—CsEuFe4As4

July 8, 2016, Science China Press
Temperature dependence of direct-current magnetic susceptibility, which is a measure of superconducting volume fraction, for CsEuFe4As4. (b) Temperature dependence of the specific-heat capacity, confirming bulk nature of the two transitions at 15.5 and 35.2 K. (c) Crystal structure of (Rb/Cs)EuFe4As4 consisting of alternate Eu, FeAs, Rb/Cs and FeAs layers along the c axis. (b) Field-dependent magnetization curves indicating coexistence of superconductivity and ferromagnetism in CsEuFe4As4. Credit: ©Science China Press

Superconductivity (SC) and ferromagnetism (FM) are mutually antagonistic collective phenomena in solids. Macroscopically, a superconductor expels magnetic fluxes from its interior below the superconducting critical temperature TSC. By contrast, a ferromagnet magnetizes itself (for a single magnetic domain) spontaneously below the ferromagnetic transition temperature TFM. Although SC and FM are incompatible in most cases, there is still the possibility of their coexistence, which, since the 1950s, has become a long-standing issue for scientists who work in the field of superconductivity. Since the late 1970s, a few material systems called "magnetic superconductors" have been considered as the candidates of ferromagnetic superconductors (FMSC). However, bulk SC and full FM were rarely observed simultaneously.

One of the iron-based superconductors, P-doped EuFe4As4, shows evidence of both SC and FM. However, the specific heat jump at TSC (which demonstrates bulk SC) is not obvious. Furthermore, the details of ferromagnetism are still debated. Very recently, researchers in Zhejiang University succeeded in replacing every alternate Eu layer by a non-magnetic Rb/Cs layer, yielding twin compounds RbEuFe4As4 and CsEuFe4As4 (see the upper-right panel of Fig. 1). In the twin materials, researchers observed both bulk superconductivity and full ferromagnetism simultaneously.

Fig. 1(a) and 1(b) are the dependence of direct-current magnetic susceptibility and specific heat, respectively, for sister compound CsEuFe4As4. The two transitions at TFM = 15.5 K and TSC =35.2 K can be clearly seen. In general, the DC magnetic susceptibility at a small magnetic field is a measure of superconducting volume fraction. One sees that the superconducting magnetic-shielding fraction, represented by the zero-field cooling data, is almost 100 percent at the lowest temperature. This indicates bulk SC, which is further confirmed by the obvious specific-heat jump at TSC. Surprisingly, the field-cooling data lose the diamagnetism below TFM, which is due to the Eu-spin ferromagnetic ordering that is demonstrated by the field-dependent magnetization shown in Fig. 1(d). Note that SC persists at the low temperature region, as seen by the initial magnetization (see the inset) as well as the non-convergence of magnetization at high fields at 2 K. The researchers also notice that there is no specific heat jump (only a kink instead) at TFM. They point out that it is a rare third-order ferromagnetic transition that has not been experimentally found up to date.

The observation of robust SC and FM in (Rb/Cs)EuFe4As4 strongly suggests that the expected mutual suppression between SC and FM can be minimized via a certain mechanism, which may shed light on the mechanism of iron-based SC, as the team leader Guang-Han Cao said. These authors also address the intriguing issue of how SC can coexist with FM.

Explore further: Researchers synthesize ferromagnetic superconducting compound amenable to chemical modification

More information: Yi Liu et al, A new ferromagnetic superconductor: CsEuFe4As4, Science Bulletin (2016). DOI: 10.1007/s11434-016-1139-2

Related Stories

Superconductivity trained to promote magnetization

October 6, 2015

Under certain conditions, superconductivity, which is basically incompatible with magnetism, can promote magnetization. Russian scientist Natalya Pugach from the Skobeltsyn Institute of Nuclear Physics at the Lomonosov Moscow ...

Scientists explain the theory behind Ising superconductivity

November 23, 2015

Superconductivity is a fascinating quantum phenomenon in which electrons form pairs and flow with zero resistance. However, strong enough magnetic field can break electron pairs and destroy superconductivity. Surprisingly, ...

Recommended for you

Coffee-based colloids for direct solar absorption

March 22, 2019

Solar energy is one of the most promising resources to help reduce fossil fuel consumption and mitigate greenhouse gas emissions to power a sustainable future. Devices presently in use to convert solar energy into thermal ...

Physicists reveal why matter dominates universe

March 21, 2019

Physicists in the College of Arts and Sciences at Syracuse University have confirmed that matter and antimatter decay differently for elementary particles containing charmed quarks.

ATLAS experiment observes light scattering off light

March 20, 2019

Light-by-light scattering is a very rare phenomenon in which two photons interact, producing another pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of ...

How heavy elements come about in the universe

March 19, 2019

Heavy elements are produced during stellar explosion or on the surfaces of neutron stars through the capture of hydrogen nuclei (protons). This occurs at extremely high temperatures, but at relatively low energies. An international ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.