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For a long time, there were no efficient ways of controlling antiferromagnets. Quite a strong magnetic field was required to manipulate the magnetic moments because of a high molecular field and a small magnetic susceptibility. It was also difficult to detect the orientation of the magnetic moments since the net magnetic moment is effectively zero. For these reasons, research on antiferromagnets has not been progressed as drastically as that on ferromagnets which are the main materials in modern spintronic devices. Here we show that the magnetic moments in NiO, a typical natural antiferromagnet, can indeed be controlled by the spin torque with a relatively small electric current density (~4 × 107 A/cm2) and their orientation is detected by the transverse resistance resulting from the spin Hall magnetoresistance. The demonstrated techniques of controlling and detecting antiferromagnets would outstandingly promote the methodologies in the recently emerged “antiferromagnetic spintronics”. Furthermore, our results essentially lead to a spin torque antiferromagnetic memory.

We report the ac magnetic susceptibility ac and resistivity measurements of EuFe 2 As 2 under high pressure P. By observing nearly 100% superconducting shielding and zero resistivity at P ¼ 28 kbar, we establish that P-induced superconductivity occurs at T c $ 30 K in EuFe 2 As 2 . shows an anomalous nearly linear temperature dependence from room temperature down to T c at the same P. ac indicates that an antiferromagnetic order of Eu 2þ moments with T N $ 20 K persists in the superconducting phase. The temperature dependence of the upper critical field is also determined.KEYWORDS: iron pnictides, pressure-induced superconductivity, susceptibility, upper critical field DOI: 10.1143/JPSJ.78.083701The discovery of superconductivity (SC) at a transition temperature T c ¼ 26 K in LaFeAsO 1Àx F x by Kamihara et al.1) has triggered extensive studies of SC in layered iron pnictides and related compounds. Rotter et al. found that BaFe 2 As 2 with a simpler structure can be made superconducting by doping:2) Perhaps more importantly, it is reported that 122 compounds of the form AFe 2 As 2 (A ¼ Ca, Sr, Ba, and Eu) can be tuned to SC by the application of high pressure P. 3-10)P tuning can provide opportunities to determine the nature of the iron-pnictide high-temperature SC without being adversely affected by disorder due to doping. However, most of these reports are based only on resistivity measurements and hence cannot establish the bulk nature of P-induced SC.11) Even when magnetic measurements are reported, results are not conclusive: In ref. 5, magnetic measurements were performed on SrFe 2 As 2 and BaFe 2 As 2 , but the observed volume fraction was expressed in arbitrary units. In ref. 9, the volume fraction of the P-induced superconducting phase of CaFe 2 As 2 was estimated to be at least 50%, while in ref. 12 CaFe 2 As 2 was reported not to exhibit SC under hydrostatic P produced by the use of helium as a pressure-transmitting medium.EuFe 2 As 2 exhibits two phase transitions, at T o $ 190 K and T N $ 19 K, at ambient P.13-16) The transition at T o is a combined structural and magnetic transition, similar to those in the other 122 compounds: the crystal structure changes from tetragonal to orthorhombic and the Fe 2þ moments order antiferromagnetically. The transition at T N is due to the antiferromagnetic (AFM) ordering of the Eu 2þ moments. The AFM coupling of the Eu 2þ moments is rather weak: the field-induced paramagnetic state with a saturated moment of $7 B /Eu is easily reached by the application of $1 or 2 T in the ab-plane or along the c-axis, respectively. 17) A temperature (T)-P phase diagram has been determined from measurements: 10) while T o decreases with P and is not detected above P ¼ 23 kbar, T N is nearly P-independent up to 26 kbar (the highest P in ref. 10). The authors of ref. 10 state that P-induced SC at T c $ 30 K occurs above 20 kbar. However, their data (at P ¼ 21:6 kbar) shows only a partial drop and approximately half of the normal-state appears to remain as T ! 0. Obviously, further e...

We report resistivity measurements performed on KFe 2 As 2 single crystals down to T = 0.3 K and in magnetic fields up to 17.5 T. The in-plane resistivity vs. T curve has a convex shape down to ∼50 K and shows a T 2 dependence below ∼45 K. The ratio of the c-axis to in-plane resistivities is ∼10 at room temperature and ∼40 at 4.2 K. The superconducting

We report the results of SQUID and torque magnetometry of an organic spin-1/2 triangular-lattice κ-H(3)(Cat-EDT-TTF)(2). Despite antiferromagnetic exchange coupling at 80-100 K, we observed no sign of antiferromagnetic order down to 50 mK owing to spin frustration on the triangular lattice. In addition, we found nearly temperature-independent susceptibility below 3 K associated with Pauli paramagnetism. These observations suggest the development of gapless quantum spin liquid as the ground state. On the basis of a comparative discussion, we point out that the gapless quantum spin liquid states in organic systems share a possible mechanism, namely the formation of a band with a Fermi surface possibly attributed to spinons.

We report on a band structure calculation and de Haas-van Alphen measurements of KFe 2 As 2 . Three cylindrical Fermi surfaces are found. Effective masses of electrons range from 6 to 18m e , m e being the free electron mass. Remarkable discrepancies between the calculated and observed Fermi surface areas and the large mass enhancement (&3) highlight the importance of electronic correlations in determining the electronic structures of iron pnicitide superconductors. The discovery of superconductivity at T c ¼ 26 K in LaFeAs (O,F) 1) has given rise to intense experimental and theoretical efforts to elucidate the superconducting pairing mechanism and symmetry in iron pnictide superconductors (see ref. 2 for a recent review). Since the development of realistic theories of the mechanism requires detailed knowledge of the Fermi surface (FS), experimental determination of the FS is highly desirable.Accordingly, many angle-resolved photoemission spectroscopy (ARPES) studies have been performed.2) Their results show some level of agreement in the FS and band dispersion with conventional band structure calculations and moderate mass renormalization due to many-body effects. On the other hand, measurements of de Haas-van Alphen (dHvA) or other quantum oscillations, which are bulk probes and allow accurate determination of the FS cross sections and effective masses m à , are rather limited. dHvA measurements performed on the FeP compounds LaFePO 3,4) and SrFe 2 P 2 5) have shown that band shifts of up to $0:1 eV are necessary to bring band structure calculations into agreement with experiments and that the enhancement of effective masses over band ones is about two. Since high T c 's are found only in FeAs compounds, dHvA studies of FeAs compounds are more desired. However, because of the structural/magnetic phase transitions, measurements on the alkaline-earth 122 parent compounds AFe 2 As 2 (A ¼ Ca, Sr, and Ba) [6][7][8] have observed only small FS pockets, which makes it difficult to draw an overall picture of the electronic structures of these compounds. Very recently, dHvA measurements have been performed on BaFe 2 (As 1Àx P x ) 2 for 0:41 x 1. 9) As one goes from x ¼ 1 to 0.41, where T c $ 25 K, the electron FS's shrink and the mass enhancement factor increases from $2 to $4.KFe 2 As 2 is an end member of the high-T c binary alloy (Ba 1Àx K x )Fe 2 As 2 with the ThCr 2 Si 2 structure and has T c $ 3 K.10,11) The low-temperature resistivity exhibits a clear T 2 dependence with a large coefficient of A ¼ 0:026 m cm/K 2 , 12) and specific heat measurements have found correspondingly large Sommerfeld coefficients: exp ¼ 69 or 93 mJ/(K 2 Ámol-f.u.) (f.u. = formula unit) for poly or single crystals, respectively. 13,14) These indicate the existence of moderately large electron correlations.75 As nuclear quadrupole resonance measurements have shown that spin fluctuations (SF's) are much suppressed (compared with the optimally doped compound).13) The first ARPES measurement 15) found and hole FS's at the À point in the Bri...

We have completely determined the Fermi surface in KFe2As2 via de Haas-van Alphen (dHvA) measurements. Fundamental frequencies ǫ, α, ζ, and β are observed in KFe2As2. The first one is attributed to a hole cylinder near the X point of the Brillouin zone, while the others to hole cylinders at the Γ point. We also observe magnetic breakdown frequencies between α and ζ and suggest a plausible explanation for them. The experimental frequencies show deviations from frequencies predicted by band structure calculations. Large effective masses up to 19 me for B c have been found, me being the free electron mass. The carrier number and Sommerfeld coefficient of the specific heat are estimated to be 1.01 -1.03 holes per formula unit and 82 -94 mJmol −1 K −2 , respectively, which are consistent with the chemical stoichiometry and a direct measure of 93 mJmol −1 K −2 [H. Fukazawa et al., J. Phys. Soc. Jpn. 80SA, SA118 (2011)]. The Sommerfeld coefficient is about 9 times enhanced over a band value, suggesting the importance of low-energy spin and/or orbital fluctuations, and places KFe2As2 among strongly correlated metals. We have also performed dHvA measurements on Ba0.07K0.93Fe2As2 and have observed the α and β frequencies.

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