Heusler compounds for magnetoelectronics and magnetocalorics

October 24, 2018

MPI-CPfS co-workers: M. Nicklas, M. Baenitz, S. Chadov, C. Felser, P. Khuntia, C. Salazar Mejía, C. Shekhar, A. K. Nayak

Figure 1: Regular Heusler structure of Mn3Ga (left panel) and inverse Heusler structure of Mn2PtGa (right panel). Ga-green, Pt-yellow, Mn(I)-blue, and Mn(II)-red spheres. The arrows indicate the direction of the magnetic moments of the Mn. — **Figure 1:** Regular Heusler structure of Mn₃Ga (left panel) and inverse Heusler structure of Mn₂PtGa (right panel). Ga-green, Pt-yellow, Mn(I)-blue, and Mn(II)-red spheres. The arrows indicate the direction of the magnetic moments of the Mn.

**Figure 1:** Regular Heusler structure of Mn₃Ga (left panel) and inverse Heusler structure of Mn₂PtGa (right panel). Ga-green, Pt-yellow, Mn(I)-blue, and Mn(II)-red spheres. The arrows indicate the direction of the magnetic moments of the Mn.

Experiments in high magnetic fields are essential for the understanding of the multi-functional properties found in Heusler alloys. In close cooperation with the Solid State Chemistry Department we investigated the exchange-bias effect in Mn–Pt–Ga Heusler alloys in static and pulsed magnetic fields at the High-Magnetic Field Laboratories in Nijmegen and Dresden (HZDR). Our finding of an exceptionally high exchange-bias in a compensated ferrimagnet may lead to the development of magneto-electronic devices and rare earth-free exchange-biased hard magnets. Pulsed-field experiments, furthermore, permit us to study materials on fast time scales between 10 ms and 1s, which allowed us to probe Ni-Mn-In shape-memory Heusler alloys for the prospective use in magnetocaloric cooling devices. Our results show that the structural and magnetic contributions to the magnetocaloric effect as well as the irreversible behavior observed at the martensitic transition have to be carefully considered for future cooling applications. This work has been carried out in close collaboration with colleagues in the Solid State Chemistry department, ensuring that our findings feed back immediately into material design.

Giant exchange bias in Mn–Pt–Ga Heusler alloys – What can be learned from high-magnetic field studies?

The exchange bias (EB) effect results in a shift of the ferromagnetic magnetization (M) vs. magnetic field (H) hysteresis loop away from the origin along the magnetic field axis. The EB phenomenon originates in the exchange coupling at the interface between a ferromagnet and an antiferromagnet. It is, therefore, usually observed in layered materials and not in bulk samples as we report here. The EB is of high technological relevance, e.g., in the design of new rare-earth free permanent magnets, in the performance of read heads in magnetic storage devices, or in spintronics. Even though future applications will not use magnetic fields in excess of one tesla, in most cases even much smaller fields, pulsed and static high magnetic field experiments are essential for the understanding of the underlying physical principles.

Here, we discuss two closely related findings,

the design of compensated ferrimagnetic Heusler alloys for a giant tunable exchange bias and
how a zero-field cooled (ZFC) exchange bias is induced during the virgin magnetization process in bulk Mn₂PtGa.

Mn₂PtGa possesses the inverse tetragonal Heusler structure. It consists of two non-equivalent types of Mn, one in the Mn-Ga and another in the Mn-Pt planes (see Figure 1). The Mn sitting in the Mn-Ga planes has a higher magnetic moment owing to its more localized nature. It couples antiferromagnetically to the Mn in the Mn-Pt planes. On the other hand, the complete replacement of Pt by Mn to form Mn₃Ga (regular tetragonal Heusler structure) results in one Mn in the Mn-Ga and two Mn in the Mn-Mn planes (former Mn-Pt plane). Thus, Mn₃Ga exhibits a net uncompensated magnetization of opposite sign to that in Mn₂PtGa. This suggests that one can create a fully compensated magnet with a compensation point for a particular Mn/Pt ratio. In Mn_3-xPt_xGa this concentration is x=0.6. Mn_2.4Pt_0.6Ga is a compensated ferrimagnetic Heusler alloy with a net moment of 0 µ_B/f.u.. Random interchange between Pt in the Mn-Pt planes and Mn in the Mn-Ga planes lead to the formation of ferromagnetic clusters embedded in the compensated host and couple to it via exchange bonds at the interface. This provides ideal conditions for the EB, and is the root cause of the giant effects seen in Figure 1.

Figure 2: a) M(H) isotherms measured up to 60 T at 4.2 K. The data for successive samples are shifted by 0.3µB along the magnetization axis for clarity. a) Field-cooled (FC) M(H) loops measured up to 32 T after field cooling the samples in 15 T (closed symbols) and 25 T (open symbols). The loop for x=0.7 and for 0.5 are shifted along the magnetization axis for clarity. The vertical and horizontal black lines represent the ordinate and the corresponding abscissa, respectively. Inset: dependence of exchange-bias field (HEB) on the cooling field (HCF). The red line is a guide to the eye. — **Figure 2:** a) M(H) isotherms measured up to 60 T at 4.2 K. The data for successive samples are shifted by 0.3µ_B along the magnetization axis for clarity. a) Field-cooled (FC) M(H) loops measured up to 32 T after field cooling the samples in 15 T (closed symbols) and 25 T (open symbols). The loop for x=0.7 and for 0.5 are shifted along the magnetization axis for clarity. The vertical and horizontal black lines represent the ordinate and the corresponding abscissa, respectively. Inset: dependence of exchange-bias field (H_EB) on the cooling field (HCF). The red line is a guide to the eye.

**Figure 2:** a) M(H) isotherms measured up to 60 T at 4.2 K. The data for successive samples are shifted by 0.3µ_B along the magnetization axis for clarity. a) Field-cooled (FC) M(H) loops measured up to 32 T after field cooling the samples in 15 T (closed symbols) and 25 T (open symbols). The loop for x=0.7 and for 0.5 are shifted along the magnetization axis for clarity. The vertical and horizontal black lines represent the ordinate and the corresponding abscissa, respectively. Inset: dependence of exchange-bias field (H_EB) on the cooling field (HCF). The red line is a guide to the eye.

Even though the effect of the giant exchange bias is also visible in low field experiments, the hysteresis loop is shifted, but it does not close. This shift of a minor hysteresis loop could have different origins than the EB. Therefore we carried out high field magnetization experiments. They show closed hysteresis loops, which are shifted by more than 3 T from the origin after field cooling in 15 and 25 T for Mn_2.4Pt_0.6Ga (see Figure 2). This clearly proofs that the EB effect is the origin of the observed giant shift and demonstrates the importance of experiments in high magnetic fields.

Figure 3: Magnetization vs. magnetic field hysteresis curves for Mn2PtGa taken at 1.9 K after zero-field cooling and field cooling as indicated. All curves shown an exchange bias. Left panel: ZFC and FC (7 T) M(H) loops performed as 0 → 7 → -7 → 7 T; right panel ZFC M(H) loop performed as 0 → -7 → 7 →-7 T. The insets show a magnified view around H = 0. — **Figure 3:** Magnetization vs. magnetic field hysteresis curves for Mn₂PtGa taken at 1.9 K after zero-field cooling and field cooling as indicated. All curves shown an exchange bias. Left panel: ZFC and FC (7 T) M(H) loops performed as 0 → 7 → -7 → 7 T; right panel ZFC M(H) loop performed as 0 → -7 → 7 →-7 T. The insets show a magnified view around H = 0.

**Figure 3:** Magnetization vs. magnetic field hysteresis curves for Mn₂PtGa taken at 1.9 K after zero-field cooling and field cooling as indicated. All curves shown an exchange bias. Left panel: ZFC and FC (7 T) M(H) loops performed as 0 → 7 → -7 → 7 T; right panel ZFC M(H) loop performed as 0 → -7 → 7 →-7 T. The insets show a magnified view around H = 0.

Mn₂PtGa exhibits the peculiar effect of a zero-field cooled exchange bias. In this material no external field is needed to induce the exchange anisotropy at the interface between the ferrimagnetic host and the embedded ferromagnetic clusters, which arise from the intrinsic antisite disorder similar to that described above. The zero-field cooled M(H) loops measured with different protocols are displayed in Figure 3. They evidence the intrinsic character of the ZFC EB. We note that the field cooled M(H) loop gives an almost identical EB.

The observation of the ZFC EB can be explained in a simple model assuming that upon applying field the virgin magnetization process will try to align the moments in the AFM and FM phases along the field direction, which implies that the interface spins of the AFM and FM phases will align in the same direction to minimize the energy. This sets up the exchange interaction between the magnetically soft FM phase and the magnetically hard AFM phase.

Our studies exemplify that high magnetic field experiments are indispensable for the understanding of basic physical principles in functional magnetic materials even when the ultimate goal of the research is future application in the low field range.

Heusler materials for magnetocaloric cooling – Taking advantage of the short time scales of pulsed magnetic fields.

Heusler materials are promising materials for magnetocaloric cooling applications, based on non-toxic, rare-earth free, cheap materials, which have the potential to replace compressor based cooling appliances. Here, we want to understand the correlation between magnetism and structure on different time scales and its consequences on the hysteresis and reversibility/irreversibility in magnetocaloric Heusler alloy. In particular, we look for the

Correlation between the crystal structure and reversibility of the phase transitions
Response time and relaxation effects
Coupling between different degrees of freedom

in order to identify new Heusler materials with a large and almost reversible magnetocaloric effect (MCE), which are suitable for magnetic cooling applications. Therefore, we study the MCE in Heusler compounds on realistic (fast) time scales in pulsed-field experiments, which include magnetocaloric, magnetization, and magnetostriction measurements. The magnetostiction set up was developed in our group. The experiments will contribute to understanding the different origins of the MCE, the contribution due to the alignment of the magnetic moments and that of the structural transition from a low magnetization twinned martensitic phase to a high magnetization austenitic phase

In the following we will summarize the major results of our case study on Ni₅₀Mn₃₅In₁₅, which exhibits a large magnetocaloric effect and had been suggested as promising material for magnetic cooling [Liu et al., Nature Mat. 11, 620 (2012)].

Figure 4: Time dependences of the adiabatic temperature change ∆T^t_ad(T) at 6 T (top) and 20 T (bottom), measured in pulsed magnetic fields. Each measurement was preceded by heating up the sample to the fully austenitic state and then cooling down to the completely martensitic state (100 K) before approaching the measurement temperature. The field profile is also indicated (right axis). The inset shows ∆T^t_ad(T) with pulsed fields of 20 T and 20 T at 240 K and the average of the two.

We have investigated the MCE in the shape memory Heusler alloy Ni₅₀Mn₃₅In₁₅ by direct magnetocaloric measurements in pulsed magnetic fields. The results are depicted in Figure 4. The conventional MCE around the Curie temperature in the austenitic phase exhibits a strong magnetic-field dependence. For a field change of 20 T we find a maximum ∆T_ad= 11 K. In this region of the phase diagram, we do not find any hysteresis or irreversible behavior in the magnetocaloric experiments in pulsed-magnetic fields. ∆T_ad exhibits a strong effect on the applied magnetic field. This changes below the martensitic phase transition. Here, we observe an inverse MCE, which originates from structural and magnetic contributions. In the direct magnetocaloric experiments, we find a saturating MCE with a maximum negative temperature change of ∆T_ad= -7K. This value is significantly larger than the results of the specific-heat analysis. The cause for this lies in the hysteresis observed at the martensitic phase transition, which leads to a strongly irreversible time dependence of ∆T^t_ad below the martensitic phase transition. The irreversibility leads to a much smaller MCE in the following cycles (see Figure 5).

Figure 5: Time dependence of ∆T^t_ad(T) measured at 250 K for a magnetic-field pulse of 20 T. Before measuring the initial curve (black circles), the sample was heated up to the fully austenite state and then cooled down to the completely martensitic state (100 K). The second curve (red diamonds) was taken 1 h after the first pulse. The field profile is also shown (right axis). The inset shows the corresponding M(H) curves following the same protocol.

Figure 6: Longitudinal magnetostriction, ∆l/l0, measurements at 100 K for Ni50Mn35In15. The experiments were performed in two steps. In pulse-1 (blue line) the measurements were performed after cooling the sample from room temperature to the measurement temperature. In pulse-2 (red line) the measurement was repeated at the same temperature one hour after the first measurement. — **Figure 6:** Longitudinal magnetostriction, ∆*l/l*₀, measurements at 100 K for Ni₅₀Mn₃₅In₁₅. The experiments were performed in two steps. In pulse-1 (blue line) the measurements were performed after cooling the sample from room temperature to the measurement temperature. In pulse-2 (red line) the measurement was repeated at the same temperature one hour after the first measurement.

**Figure 6:** Longitudinal magnetostriction, ∆*l/l*₀, measurements at 100 K for Ni₅₀Mn₃₅In₁₅. The experiments were performed in two steps. In pulse-1 (blue line) the measurements were performed after cooling the sample from room temperature to the measurement temperature. In pulse-2 (red line) the measurement was repeated at the same temperature one hour after the first measurement.

The martensitic transition in Ni-Mn based shape-memory alloys is connected to a change in the material’s length and/or volume, due to the different unit-cell volumes of the martensite and austenite phases. We found a strong field-induced magnetic irreversibility far below the martensitic transition temperature, which is in contrast to the usual irreversible behavior, only observed close to the transition temperature. The pulsed-field magnetostriction measurements display a peculiar phenomenon, where both irreversible and reversible behavior is realized depending on the history of the sample (see Figure 6).

The irreversible behavior that we have observed in these experiments has to be carefully taken into consideration for the potential use of shape-memory Heusler alloys in magnetocaloric-cooling applications; especially since the length of the magnetic field pulse is comparable to the inverse of useful operation frequencies of cooling devices. Based on our findings, we aim to design Heusler materials with a small thermal/magnetic hysteresis.

Our publications:

Experimental techniques

High resolution magnetostriction measurements in pulsed magnetic fields using fiber Bragg gratings; R. Daou, F. Weickert, M. Nicklas, F. Steglich, A. Haase, and M. Doerr, Rev. Sci. Inst. 81, 033909 (2010). DOI:10.1063/1.3356980. MPG.PuRe
Implementation of specific-heat and NMR experiments in the 1500 ms long-pulse magnet at the Hochfeld-Magnetlabor Dresden; F. Weickert, B. Meier, S. Zherlitsyn, T. Herrmannsdörfer, R. Daou, M. Nicklas, J. Haase, F. Steglich, and J. Wosnitza, Meas. Sci. Technol. 23, 105001 (2012). DOI:10.1088/0957-0233/23/10/105001. MPG.PuRe

Scientific results

-Exchange bias-

Large Zero-Field Cooled Exchange-Bias in Bulk Mn₂PtGa; A. K. Nayak, M. Nicklas, S. Chadov, C. Shekhar, Y. Skourski, J. Winterlik, and C. Felser, Phys. Rev. Lett. 110, 127204 (2013). DOI:10.1103/PhysRevLett.110.127204. MPG.PuRe
Kinetic arrest related to a first-order ferrimagnetic to antiferromagnetic transition in the Heusler compound Mn₂PtGa; A. K. Nayak, M. Nicklas, C. Shekhar, and C. Felser, J. Appl. Phys. 113, 17E308 (2013). DOI:10.1063/1.4800687. MPG.PuRe
Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias; A. K. Nayak, M. Nicklas, S. Chadov, P. Khuntia, C. Shekhar, A. Kalache, M. Baenitz, Y. Skourski, V. K. Guduru, A. Puri, U. Zeitler, J. M. D. Coey and C. Felser, Nature Mater. 14, 679 (2015). DOI:10.1038/nmat4248. MPG.PuRe
Magnetic phase coexistence and metastability caused by the first-order magnetic phase transition in the Heusler compound Mn₂PtGa; A. K. Nayak, R. Sahoo, C. Salazar Mejia, M. Nicklas, and C. Felser, J. Appl. Phys. 117, 17D715 (2015). DOI:10.1063/1.4916757. MPG.PuRe

-Magnetic cooling-

Large field-induced irreversibility in Ni-Mn based Heusler shape-memory alloys: A pulsed magnetic field study; A. K. Nayak, C. Salazar Mejia, S. W. D’Souza, S. Chadov, Y. Skourski, C. Felser, and M. Nicklas, Phys. Rev. B 90, 220408(R) (2014). DOI:10.1103/PhysRevB.90.220408. MPG.PuRe
Direct measurements of the magnetocaloric effect in pulsed magnetic fields: The example of the Heusler alloy Ni₅₀Mn₃₅In₁₅; M. Ghorbani Zavareh, C. Salazar Mejía, A. K. Nayak, Y. Skourski, J. Wosnitza, C. Felser, and M. Nicklas, Appl. Phys. Lett. 106, 071904 (2015). DOI:10.1063/1.4913446. MPG.PuRe
Strain behavior and lattice dynamics in Ni₅₀Mn₃₅In₁₅; C. Salazar Mejía, A. K. Nayak, J. A. Schiemer, C. Felser, M. Nicklas, and M. A. Carpenter, J. Phys.: Cond. Mat. 27, 415402 (2015). DOI:10.1088/0953-8984/27/41/415402. MPG.PuRe
Pulsed high-magnetic-field experiments: New insights into the magnetocaloric effect in Ni-Mn-In Heusler alloys; C. Salazar Mejía, M. Ghorbani Zavareh, A. K. Nayak, Y. Skourski, J. Wosnitza, C. Felser, and M. Nicklas, J. Appl. Phys. 117, 17E710 (2015). DOI:10.1063/1.4916556. MPG.PuRe