摘要:

Exact results are obtained for two‐dimensional Ising models in which periodic conflicting interactions induce frustration effects. A brief review of the remarkable thermodynamic and magnetic properties of these models is presented. For specific choices of the interactions, the main features predicted are: (1) The existence of systems with a highly degenerate level of lowest energy, leading to a residual 0K entropy per spin. (2) According to the distribution of interactions, the existence of a residual 0K entropy is not an impediment to the onset of long range order at finite temperatures. However, if such an order takes place, the corresponding order parameter may not be saturated at 0K, and the associated susceptibility will diverge at 0K as well as at the transition temperatures. (3) Long range spin correlations ⟨S(O&vec;)S(R&vec;) ⟩ at 0K averaged over all ground states are studied in one of the models (which includes the ’’odd model’’) for simple crystallographic directions of R&vec;. They are shown to decrease as f (R&vec;) ⋅R−1/2where f (R&vec;) is an oscillatory factor, proper to each R&vec; direction, which results in marked anisotropy effects.

ISSN:0021-8979    

DOI:10.1063/1.326928

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

We report magnetization measurements on sputter deposited amorphous MnSi. Above 100 K the susceptibility follows a Curie Weiss law, but deviates considerably from it at lower temperatures. At 21 K the alloy becomes a spin‐glass. Comparison with the properties of crystalline MnSi suggests that the two systems have very similar interactions and that amorphous MnSi, like crystalline MnSi, is probably itinerant.

ISSN:0021-8979    

DOI:10.1063/1.326929

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The Eu1−xGdxS system goes from ferromagnetism for small x to antiferromagnetism for large x[1]. We have studied the specific heat and the magnetic properties of that system in the concentration range where the Curie‐Weiss temperature, &Vthgr;, changes sign. Both measurements clearly indicate a spin‐glass freezing for x ∼ 0.5 and a mixed state for x ≳ 0.6. Ferromagnetic clusters have been observed in both the spin‐glass and in the mixed regimes.

ISSN:0021-8979    

DOI:10.1063/1.326930

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Large negative pressure derivatives of Tc, M(0,0) found for Pd1−xMnx(.014 < x < .04) are interpreted in terms of weak itinerant ferromagnetism.

ISSN:0021-8979    

DOI:10.1063/1.326931

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Dilute magnetic alloys of PdFeMn have been investigated up to ∼20 kbar, demonstrating that the mechanism for the kneelike sharp rise in the low‐field ac susceptibility &khgr; on cooling is different from that for the reversed kneelike sharp drop of &khgr;.

ISSN:0021-8979    

DOI:10.1063/1.326932

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Initial susceptibility and thermoremanent magnetization measurements have been made on atomically disordered and weakly ordered Ni3Mn. The blocking of superparamagnetic clusters yields a peak in initial susceptibility near 110 K. Ferromagnetic clusters with Curie temperatures near 265 K cause a break in the susceptibility vs. temperature curves at 265 K. Exchange anisotropy effects appear below 35 K.

ISSN:0021-8979    

DOI:10.1063/1.326933

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Transverse magnetoresistance (TMR) have been measured in two AgMn (1.1 and 5.4 at.%) as well as in one AuFe (6.6 at.%) alloys between 1.5 and 77 K. There is a low‐field positive TMR below the spin‐freezing temperature Todecreasing sharply with temperature. The negative TMR at higher fields is almost temperature‐independent below To. Above To, the magnitude of the negative one increases with field and gradually decreases with temperature. Freshly annealed samples show a definite reduction of the magnitude of the negative TMR for the concentrated alloys. There is no change in the case of the dilute AgMn. The above common features will be qualitatively explained in terms of thermal and magnetic unlocking of spins, magnetic ordering and the spin‐flip scattering.

ISSN:0021-8979    

DOI:10.1063/1.326934

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A SQUID Magnetometer has been used to measure the temperature dependent magnetization of two dilute alloys of Mn in Pd. Results are presented for temperatures T of 0.01 to 1 Kelvin, for applied magnetizing fields Haof 0.5 to 600 Oe, for samples containing 255 and 575 atomic ppm of Mn. This dilute alloy system displays a remarkable variation in its properties. For the concentration region under study, PdMn may be described as agiant‐momentspinglass. (1) At low field and high temperature, the magnetization M follows a Curie‐Weiss law, M = CHa/ (T−&Vthgr;), with positive (ferromagnetic) &Vthgr; values of 31 and 78 mK, respectively. From the Curie constant we obtain a ’’giant’’ effective moment per Mn impurity, p of 11 &mgr;B(&mgr;B = Bohr magneton). With spin S = 5/2, this gives an effective g value of g = 3.7. (2) From the highest field data we estimate the saturation moment per Mn spin gS&mgr;Bto be 9 &mgr;B; with S = 5/2, this gives g = 3.6 in good agreement. (3) As the temperature is lowered in constant applied field the magnetization becomes independent of temperature, a behavior which is characteristic of the total DC magnetization of a spin glass at low temperatures.

ISSN:0021-8979    

DOI:10.1063/1.326935

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Alloys in the system Ni.95S Fe exhibit uniaxial anisotropy which is similar in magnitude to the exchange. Ni.95S is a non‐magnetic d‐band metal and iron a magnetic impurity, so the system bears some resemblance to an Ising spin glass. Anisotropy modifies the magnetic properties as compared with a normal spin glass, giving a susceptibility more like that of an antiferromagnet. The maximum in &khgr;∥at the freezing temperature Tfretains its form but shifts to lower temperature with increasing field. A sharp increase in the parallel magnetization below Tfin a large applied field is identified as spin flop of iron in the weakest exchange fields.

ISSN:0021-8979    

DOI:10.1063/1.326936

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The properties of this dilute insulator have been studied by two groups, either on single crystals or on powders. In single crystals, spin‐glass properties (maximum of the susceptibility at Tf∝ x) are observed down to x = 0.05. They are attributed to an essential role of the dipolar interaction. Below this concentration the blocking of isolated ferromagnetic pairs at Tpbecomes predominant since Tp(concentration independent) becomes larger than Tf. In powders, an identical behavior is observed below x = 0.05. A more complex behavior, described by the blocking of magnetic clusters, is observed for 0.05 < x < 0.13 and spin‐glass properties are only observed for x ≳ 0.13. Thus, the observation of a ’’dipolar spin‐glass’’ in a dilute insulator depends on the sample preparation; it requires confirmation.

ISSN:0021-8979    

DOI:10.1063/1.326945

出版商:AIP    

年代:1979

数据来源: AIP